Thread: On Einstein's explanation of the invariance of c

  1. #861
    BenTheMan:

    Surely this thread has run its course.
    Motor Daddy and I aren't quite sick of it yet. You don't need to participate. Please don't close it.


    Motor Daddy:

    I know exactly what a reference frame is, and I know what "at rest" means (not accelerating).
    Wrong. "At rest" means "Not moving". "At rest" means zero velocity. Also, you can be at rest and still be accelerating. That's exactly what happens (momentarily) every time you put your foot on the accelerator to start your car moving from the traffic lights.

    I am measuring the velocity of the ship in space. The ship has its own velocity in space, which is not relative to any other object.
    You're merely repeating an unproven assertion.

    You don't seem to get that an object can be in motion in space. Do you not understand that an object can traverse space? Why do you insist on saying an object can't be in motion in space? I can measure the motion of an object in space, using light. The ship has a velocity in space just as light has a velocity in space.
    Yes, objects can traverse "space". Part of what you are saying there is that objects can move - something I obviously don't deny. The other part of what you're saying is that there is a mysterious stationary substance called "space" that is absolutely at rest, which is wrong.

    You can't measure the motion of an object in your "space". The most you ever actually manage in your thought experiments is to stand outside the thing you're measuring and measure light travel times in that external frame of reference. I know that's not what you think you're doing, but that's what you're doing. When your method works, that is.

    No I'm not viewing it from outside. I am not measuring my velocity compared to the ship, I am measuring the ship's velocity in space from inside the ship using light. Do you not understand what I am saying?
    I understand perfectly what you are saying. It is based on the false imagining that the light travel times measured inside the ship can be different in the two directions. That's error number one. Error number two is your belief that if you measure the times using clocks in another reference frame for the same light, you'll measure the same times.

    Your answers are only correct if the ship is not moving in space, which means it has a zero velocity.
    A spaceship always has zero velocity in its own rest frame, by definition. And no experiment can contradict that. If you see a spaceship moving, then you're watching it fly past. You're not sitting in it.

    By the way, if you're sitting in it and you look out the window at the stars and planets passing by, then you say the stars and planets are moving, not the spaceship. That's what reference frames are all about. People sitting on those planets say the spaceship is moving and the planets are stationary. Change of reference frame - see?

    Your idea that some things are "really" stationary and others are "really" moving with respect to "space" is false. There's no standard of absolute rest. It just doesn't exist.

    Yes they were measured from within the ship.
    I set the problem; you're telling me how I constructed it?

    All my numbers add up in and outside the ship, compared to every frame in the universe.
    No, (a) because you assume that the one-way light travel times measured in the ship will be diffferent, and (b) because you assume that anyone in any frame will measure the same one-way travel times. Both of those assumptions are wrong.

    Light measures distance in space.
    No. Light just travels the same distance in the same time in any reference frame. And if you think this statement and your statement are saying the same thing, then you don't know what a reference frame is.

    You want to try and say an object has a zero velocity if there is nothing to measure it against.
    No. I want to say that an object has zero velocity if you're sitting on it and measuring its speed in its own reference frame.

    I am telling you that I can measure the velocity of the ship in space, which means I can tell you the distance the ship traversed "empty space" in a given amount of time.
    But you can't tell me that. You always in fact are implicitly introducing another external reference frame. You call it "space" and you think it's special, but it's just one more frame and has no special status.

    Space has distance, and the ship can travel a distance in space in x amount of time, no other object or frame required to measure the distance or time.
    You can't measure distance or time in the absence of a reference frame. A reference frame is what enables these measurements - otherwise quoted times and distances are meaningless.

    Do you understand that volume has distance, and objects can travel in a volume?
    Yes.

    The speed of light is in fact the same in all frames.
    So, repeat after me: "I agree with Einstein that the speed of light is the same in all frames."

    Go on, let me see you write that. Can you bring yourself to do it?

    The part you fail to realize is that objects also have speed, and the object's speed changes the amount of time the light takes to travel from point a to point b on the ship.
    I understand that if an object has speed relative to a particular reference frame, then what you say here is perfectly true. In that case I agree with you completely. Where we disagree is in your imagining an absolute standard for speed which you call "space". Space is a nothing. It is not a substance.

    Do you not understand that the ship's velocity changes the distance light has to travel from one end of the ship to the other? The ship could be 1 meter in length, and it could take light .5 seconds to travel from end to end of the ship, because the ship has a velocity in space too!
    I agree, but it's not a velocity "in space". It's a velocity relative to some reference frame which is implied and not absolute.

    When I say it has a rest length of 1 metre, I need to measure that. For example, use light.
    But first you must know if the meter stick is in motion in space.
    NO! That's where you're absolutely and utterly wrong.

    The speed of light is the same IN ALL REFERENCE FRAMES. Therefore, I can make the measurement in ANY reference frame I like and find the correct answer.

    Quote Originally Posted by Motor Daddy View Post
    As Neddy Bate previously mentioned, the formula for my theory of finding the absolute velocity and distance between clocks is:

    v = (cT - ct) / (T + t)
    d = T(c - v)
    where:
    v is the absolute speed along the line between the clocks
    d is the distance between the clocks
    T is the greater time
    t is the lesser time
    I have no argument with that at all. As long as both times are measured in the same reference frame, the relative speed v calculated for the object will be correct for that frame, as will the distance d.

    There's nothing absolute about this calculational method, except for the incorrect assumption that the c refers to the speed of light relative to an absolute "space", and the assumption that v is the speed of the object relative to that "space".

    And, of course, there's your incorrect assumption that if we make the measurement in the frame of the ship then T and t will be different.

    There is no time dilation or length contraction involved.
    Correct. Time dilation and length contraction are never an issue if you stick to one reference frame. They only ever come into play when you want to translate from one frame to another.

    You measure one-way light travel times and find the absolute velocity of the ship.
    No. The best you can do with light travel times is to find the relative velocity of the ship - relative to some specified frame of reference, that is.

  2. #862
    Valued Senior Member Motor Daddy's Avatar
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    4,547
    Quote Originally Posted by James R View Post

    Motor Daddy:



    Wrong. "At rest" means "Not moving". "At rest" means zero velocity. Also, you can be at rest and still be accelerating.
    No, at rest means "not accelerating." You say an object can't have an absolute velocity in space, so how can you say "at rest" means "not moving?" Not moving compared to what? In your universe, your ship never moves so why bother saying when it is "at rest?". You will never admit that the ship has a motion in space, so why even bother saying when it's "at rest" with the meaning of "not moving?" At rest simply means not accelerating. Are you implying that an object could be in motion? How would you determine that motion? I mean, how do you determine if an object is "moving" or not? At rest is simply when the velocity is not changing. Acceleration is the rate of change of velocity, so "not accelerating"means the velocity isn't changing, which is considered at rest.

    Quote Originally Posted by James R View Post
    That's exactly what happens (momentarily) every time you put your foot on the accelerator to start your car moving from the traffic lights.
    Acceleration is the rate of change of velocity. When your car's speed is zero at the red light, when the light turns green you accelerate, which means the velocity is changing. If you are traveling down the road at a constant 60 MPH, not accelerating, you are at rest.

    Quote Originally Posted by James R View Post
    You're merely repeating an unproven assertion.
    It's not an assertion, it's a fact of distance and time. It is the very nature of distance and time. You run away from a street light and the light has to travel a greater distance to get to you. It's not even debatable! Can you dispute that if a car is on a 1/4 mile drag strip and the car starts heading towards the finish line, that if the finish line is moved away from the start line that the car has to travel a greater distance to reach the finish line? You seem to imply you can dispute that fact?



    Quote Originally Posted by James R View Post
    Yes, objects can traverse "space". Part of what you are saying there is that objects can move - something I obviously don't deny. The other part of what you're saying is that there is a mysterious stationary substance called "space" that is absolutely at rest, which is wrong.
    I never once implied space is a substance. Space is volume. We measure distance in that volume using light. You need to understand what space is. Space is simply volume, nothing more. Space is not a substance and I never implied such an absurdity.

    Quote Originally Posted by James R View Post
    You can't measure the motion of an object in your "space".

    I can and I did!

    Quote Originally Posted by James R View Post
    The most you ever actually manage in your thought experiments is to stand outside the thing you're measuring and measure light travel times in that external frame of reference. I know that's not what you think you're doing, but that's what you're doing. When your method works, that is.
    I measure the motion of the object in space. Do you understand that light traverses space and has a velocity? Light has a velocity in space, do you agree with that? I presume you do, so then you must also understand that since light travels at a constant velocity in space, and we use light travel time to measure distance, that when I measure the time it takes for light to go from point a to point b on a ship, the elapsed time represents the distance that light traveled in space. The elapsed time does not tell you the distance between point a and point b on the ship, as the ship could have had a velocity in space. I know I keep repeating that, but it is a fact that you must understand. If you move the finish line further from the start line, the car has to travel a greater distance in a greater amount of time. Indisputable fact! It is the EXACT same thing with light. If the ship has a velocity, you are moving the finish line towards or away from the light, altering the amount of distance and time the light travels in space to reach the finish line.



    Quote Originally Posted by James R View Post
    I understand perfectly what you are saying. It is based on the false imagining that the light travel times measured inside the ship can be different in the two directions. That's error number one. Error number two is your belief that if you measure the times using clocks in another reference frame for the same light, you'll measure the same times.
    I'm not imagining different one-way times, it is a simple fact of distance and time. I'm not guessing that the times could be different, I'm telling you they HAVE to be if the ship is in motion. They MUST be different if the ship is in motion. There is no question about it.



    Quote Originally Posted by James R View Post
    A spaceship always has zero velocity in its own rest frame, by definition. And no experiment can contradict that. If you see a spaceship moving, then you're watching it fly past. You're not sitting in it.
    Wrong. I am not talking about observing motion, I am talking about measuring motion in space using light. I am not observing the ship's motion. There is no way I could sit inside the ship and observe its own motion. Therefore, since I know I can't observe the ship's motion from within, and the ship is at rest (not accelerating) so I can't feel any change in velocity, I must use light from within the ship to measure the distance the ship travels relative to light travel times in each direction.

    Quote Originally Posted by James R View Post
    By the way, if you're sitting in it and you look out the window at the stars and planets passing by, then you say the stars and planets are moving, not the spaceship. That's what reference frames are all about. People sitting on those planets say the spaceship is moving and the planets are stationary. Change of reference frame - see?
    I fully understand that concept. I also understand that my perception of motion is not valid. Case in point, It appears the sun rises and sets and I am stationary. We both know that the sun doesn't orbit the Earth, correct? You have to understand that if you look out a window that you can't tell which object is moving. That is an illusion, nothing more. I don't rely on illusions to base my facts on, do you?



    Quote Originally Posted by James R View Post
    Your idea that some things are "really" stationary and others are "really" moving with respect to "space" is false. There's no standard of absolute rest. It just doesn't exist.
    An object is at an absolute zero velocity if it is not traversing space. You can measure the velocity by using light in each direction. Light travels in space so you can determine if an object is also traveling in space by timing light in each direction. If the times are the same in all direction the object is not in motion, period!



    Quote Originally Posted by James R View Post
    I set the problem; you're telling me how I constructed it?
    Do you understand that light travels the same distance in the same time inside the ship as well as outside the ship?



    Quote Originally Posted by James R View Post
    No, (a) because you assume that the one-way light travel times measured in the ship will be diffferent, and (b) because you assume that anyone in any frame will measure the same one-way travel times. Both of those assumptions are wrong.
    Those are NOT assumptions, they are rock solid facts! Those are facts of distance and time. I didn't dream those up in order for my theory to work properly. In order for objects to be in motion they must travel in space, and light travels in space, so objects must travel relative to light.



    Quote Originally Posted by James R View Post
    No. Light just travels the same distance in the same time in any reference frame. And if you think this statement and your statement are saying the same thing, then you don't know what a reference frame is.
    According to you, at rest means a zero velocity, so according to you, all frames at rest are a zero velocity.



    Quote Originally Posted by James R View Post
    No. I want to say that an object has zero velocity if you're sitting on it and measuring its speed in its own reference frame.
    You mean YOU have a zero velocity in reference to the object if you are not moving relative to the object. Correct. But in order to speak of the object's velocity, you must have something to measure against. You do have something to measure against in order to find an object's velocity, that is called light. Light traverses space, so when you relate an object's motion to light, you are relating an object's motion to space (which is NOT a substance!)!



    Quote Originally Posted by James R View Post
    But you can't tell me that. You always in fact are implicitly introducing another external reference frame. You call it "space" and you think it's special, but it's just one more frame and has no special status.
    Space is volume. Volume has no velocity, it just is. We measure distance in the volume using light. If you use light to also measure an object you are measuring the object relative to the volume. The object traverses the volume a specific distance in a specific time.


    Quote Originally Posted by James R View Post
    You can't measure distance or time in the absence of a reference frame. A reference frame is what enables these measurements - otherwise quoted times and distances are meaningless.
    You are measuring using light which travels in the space frame. Light defines the space frame.







    Quote Originally Posted by James R View Post
    So, repeat after me: "I agree with Einstein that the speed of light is the same in all frames."


    I agree that light has a constant velocity in space. If an object is traveling towards light traveling towards the object, the light will traverse the object in less time than if the object is traveling in the same direction and light is coming towards it. The velocity of the object doesn't change the velocity of light, it changes the amount of time it takes light to traverse the object.

    Quote Originally Posted by James R View Post
    Go on, let me see you write that. Can you bring yourself to do it?
    Never!!!


    Quote Originally Posted by James R View Post
    I understand that if an object has speed relative to a particular reference frame, then what you say here is perfectly true. In that case I agree with you completely. Where we disagree is in your imagining an absolute standard for speed which you call "space". Space is a nothing. It is not a substance.
    I'm not saying space is a substance, I consider space an infinite volume, period.




    Quote Originally Posted by James R View Post
    I agree, but it's not a velocity "in space". It's a velocity relative to some reference frame which is implied and not absolute.
    When using light to measure velocity, the velocity is relative to space.




    Quote Originally Posted by James R View Post
    NO! That's where you're absolutely and utterly wrong.
    The speed of light is the same IN ALL REFERENCE FRAMES. Therefore, I can make the measurement in ANY reference frame I like and find the correct answer.

    The speed of light is constant, the speed of objects is not constant.




    Quote Originally Posted by James R View Post
    There's nothing absolute about this calculational method, except for the incorrect assumption that the c refers to the speed of light relative to an absolute "space", and the assumption that v is the speed of the object relative to that "space".
    The meter is the distance light travels in a specific amount of time, IN A VACUUM! It says nothing about a reference frame!


    Quote Originally Posted by James R View Post
    And, of course, there's your incorrect assumption that if we make the measurement in the frame of the ship then T and t will be different.
    That is an undeniable fact! If an object is in motion in space, light will travel different distances in each direction.




    Quote Originally Posted by James R View Post
    Correct. Time dilation and length contraction are never an issue if you stick to one reference frame. They only ever come into play when you want to translate from one frame to another.
    No such animals. Simply a result of bad theory.


    Quote Originally Posted by James R View Post
    No. The best you can do with light travel times is to find the relative velocity of the ship - relative to some specified frame of reference, that is.
    Relative to light, because that is how we measure distance.
    Last edited by Motor Daddy; 01-01-11 at 09:50 AM.

  3. #863
    Registered Senior Member
    Posts
    208
    For frik and frak;

    An object/observer cannot move relative to itself, therefore there is no motion to detect.

    In an inertial frame the transit time for light to a distant clock equals the return time, on the condition that the clocks are synchronized according to the SR convention.

    If light did not travel a longer distance in space as it oscillates within a moving light clock, there would be no time dilation.

    thanks for the entertainment

  4. #864
    Motor Daddy:

    I'm going to reply to you in two posts. In this one, I'm going back to basics for a minute, just to make sure we're on the same page regarding reference frames, definitions of velocity and so on. In the next post I'll respond to your previous post.

    Here's what a reference frame is, approximately. We'll work in one spatial dimension for simplicity. Imagine you have constructed 100 identical straight sticks of equal length. The exact method of construction and the exact length is not important, as long as the lengths are the same when you lay one stick on top of another.

    Now, to constuct a reference frame, you lay out your 100 sticks end-to-end along a straight line. You now have a distance scale. If you want the distance from the end of the row of sticks to some other point on the line, you just count how many sticks there are between the end and the point you're interested in.

    Now, as well as a distance scale, we need some clocks. At every intersection of two sticks along the line, we place a clock. All of the clocks along the line are adjusted so that they read the same time always. Again, we won't worry about the exact method used to synchronise them for now; we just assume they are synchronised.

    So, a reference frame is this set of sticks and clocks.

    Now, suppose we number the intersections between any two sticks. We start at the end of the row (let's say) and call that position zero. The next intersection is position 1, the one after that position 2 etc. We now have a definition of position relative to our reference frame.

    Imagine a ball located at some particular intersection, say number 57. Then we say the position of the ball is at x=57 (sticks).

    Next we define velocity. If an object moves from one position to another along our line of sticks, then while it is moving it has a velocity equal to the number of intersections crossed divided by the time taken (as measured on the clocks). For example, if the ball moves from location x=57 to x=58 in 2 seconds, then it has a velocity of 1 stick per 2 seconds or 0.5 sticks per second.

    Finally, we have acceleration, although that is not important to our discussion. Acceleration is the rate of change of velocity. For example, if the ball changes speed from 3 sticks per second to 5 sticks per second in a time of 1 second, then its acceleration is 2 sticks per second per second, or 2 sticks per second squared.

    Now, let's be clear about what is meant by the term "at rest". An object, such as the ball, is at rest in our reference frame if its position is not changing with time. In other words, if it stays at x=57 then it is at rest with respect to this reference frame. It's velocity in this case is the change in position over time, and since its position is not changing, the velocity is zero when it is at rest. "At rest" says nothing about acceleration. "At rest" simply tells you velocity is zero.

    Now, consider a different type of motion. Imagine the ball is "stationary" (whatever that means), and we move our whole assembly of sticks and clocks past the ball. Can you see that, in the reference frame we have constructed, the ball now has a velocity? The reason is that the stick intersections (57,56, 55, 54...) are moving past the ball, and so its position in the reference frame is changing over time. Therefore, in the reference frame of the sticks/clocks, the ball has a velocity, because of the way we defined velocity.

    We could achieve the same velocity for the ball in a different way, of course. We could hold the stick and clocks "stationary" and move the ball along the sticks, instead of holding the ball stationary and moving the reference frame. In either case, the speed of the ball would be the same in the reference frame.

    Now, suppose we put our whole assemblage of sticks and clocks in an aeroplane which is flying above the Earth. And suppose we place our ball at x=57 and leave it there.

    Now, relative to our reference frame of sticks and clocks, the ball is at rest. Recall that we defined "at rest" to mean that an object does not change position, and position is defined by the intersections of the sticks. So, our ball is at rest in this reference frame. Note that the motion of the plane is completely irrelevant to whether the ball is at rest in the reference frame. As long as the ball stays at x=57 it is at rest, by definition.

    Ok, so now we have a definition of "reference frame", "position", "velocity" (and "acceleration", though we needn't worry about that right now).

    So, let's now consider TWO reference frames, by which I mean two identical sets of sticks and clocks laid out parallel to each other. Obviously, we can slide one entire set of sticks along the other set of sticks, so that each set of sticks has a velocity relative to the other set. For definiteness, let's assume on set of sticks is laid out on the Earth (attached to it) and the other set of sticks is laid out in our aeroplane, which is flying parallel to the Earth's surface, in the direction that both sets of sticks are laid out.

    Now consider the ball on the plane. Suppose it just sits in the aisle at location x=57 relative to the sticks on the plane. Then, the ball is at rest relative to the plane's reference frame. But the ball obviously has some velocity relative to the ground's set of sticks, its position on those sticks is changing as the plane flies past.

    So, is the velocity of the ball "really" zero, as indicated by the plane's reference frame, or is it "really" 800 km/hr (say), as indicated by the Earth's reference frame? Answer: it is BOTH.

    Or, to put it another way, it makes no sense to say "The ball's velocity is zero" unless you specify which reference frame you're using. You ought to say "The ball's velocity is zero in the reference frame of the plane", or equivalently "The ball's velocity is zero relative to the plane." Obviously, at the same time the ball's velocity is NOT zero relative to the ground.

    More points to appreciate: EVERY reference frame - every set of sticks and clocks - is attached somewhere. The position of any given object may be different in different reference frame, as may its velocity and acceleration. Also, it makes no sense to define "position", "velocity" or "acceleration" in the absence of a reference frame.

    When it comes to light, we can consider a light photon as just another ball moving along our lines of sticks. If light covers a certain number of sticks in a certain amount of time (as measured by the clocks at the intersections) then light has a certain speed determined by the reference frame. Up to this point I have talked about lengths in terms of stick lengths. Obviously, if we can somehow ensure that each stick has length 1 metre, then we can get velocities in metres per second.

    Ok, that's enough background.

    What I need from you, Motor Daddy, is some indication of whether you agree with the above description of what a reference frame is, as well as definitions of position and velocity. Because if we can't agree on these basics then we can't possibly begin to discuss relativity.

    So, please let me know if you think any of the above definitions or explanations I have given here are unworkable.

  5. #865
    Motor Daddy:

    In this post I'll reply to your previous post.

    No, at rest means "not accelerating." You say an object can't have an absolute velocity in space, so how can you say "at rest" means "not moving?" Not moving compared to what? In your universe, your ship never moves so why bother saying when it is "at rest?". You will never admit that the ship has a motion in space, so why even bother saying when it's "at rest" with the meaning of "not moving?" At rest simply means not accelerating. Are you implying that an object could be in motion? How would you determine that motion? I mean, how do you determine if an object is "moving" or not? At rest is simply when the velocity is not changing. Acceleration is the rate of change of velocity, so "not accelerating"means the velocity isn't changing, which is considered at rest.
    This is Physics 101. "At rest" means "zero velocity in a particular reference frame". Look it up if you don't believe me. The same definition is used in every physics textbook and on every reputable physics site on the internet.

    Referring to my previous post, you will see that "at rest" means only "not moving with respect to some reference frame". So when you ask me "compared to what", I say "compared to the reference frame I'm using". Think of sticks and clocks if you want a concrete picture.

    You say "in your universe the ship never moves". Clearly that is not true. Read the previous post. The ship will never move relative to a bunch of sticks that are nailed to its floor, but it can certainly move relative to a bunch of sticks nailed to the Earth. How do we tell whether it is moving or not? Step 1: specify which reference frame we want to use. Step 2: See if the ship's position changes over time by looking at its position along the line of sticks. Easy.

    If you are traveling down the road at a constant 60 MPH, not accelerating, you are at rest.
    Wrong. 60 mph means you're moving across sixty mile-long sticks every hour. Since you have a velocity, you're not at rest. BUT, where are these sticks attached to? They are attached to the road. So, to say it properly "You're travelling at 60 mph relative to the road."

    Suppose you have another line of sticks inside the car, nailed to car's floor. The car's position never changes relative to them, so in that reference frame the car is at rest.

    Suppose a plane is flying above the road. Then, the car is moving relative to the sticks that are nailed to the floor of the plane, but not at 60 mph, because obviously the car moves along the plane's sticks at a different rate than it moves along the road's sticks.

    Also, notice that the ROAD is moving at 60 mph relative to the car. As measured on the car's set of sticks, a fixed point on the road moves along those sticks. So, in the car's frame the road is not "at rest". Nor is the road "at rest" in the plane's frame.

    It's not an assertion, it's a fact of distance and time. It is the very nature of distance and time. You run away from a street light and the light has to travel a greater distance to get to you. It's not even debatable!
    I agree. Here you're using sticks nailed to the road, which is fine.

    I never once implied space is a substance. Space is volume. We measure distance in that volume using light. You need to understand what space is. Space is simply volume, nothing more. Space is not a substance and I never implied such an absurdity.
    What are your "space" sticks nailed to?

    I measure the motion of the object in space. Do you understand that light traverses space and has a velocity?
    I understand that you have a set of sticks nailed somewhere and light travels along those sticks. Therefore it has a velocity. Your problem is that you don't know where your sticks are nailed.

    If you move the finish line further from the start line, the car has to travel a greater distance in a greater amount of time. Indisputable fact! It is the EXACT same thing with light.
    Let's go back to your original thought experiment with one-way travel times in the spaceship. First, consider a set of sticks nailed to the floor of the spaceship. What I am telling you is that light travels along that set of sticks at the same rate in both directions, always. This is completely against what you might expect, but also completely compatible with your claim in this thread that the speed of light is always the same in every frame. Right?

    Now, perhaps at this point you want to change your mind and say that the speed of light is NOT always the same after all, with respect to the sticks nailed to the floor of the spaceship. If so, please let me know, because that would be a very significant change to your argument.

    Next, consider a set of sticks nailed to the Earth. The spaceship has a velocity relative to that frame, and so does the light going in both directions along the spaceship. For that frame, those sticks, I agree with you that light takes a different time in the two directions, for exactly the reasons you say it does. What I have been trying to point out to you up to this point is that you are still using a set of sticks to make this argument. Whether those sticks are nailed to ground, or strung up between Earth and the Sun or whatever, they must be there (notionally).

    I'm not imagining different one-way times, it is a simple fact of distance and time. I'm not guessing that the times could be different, I'm telling you they HAVE to be if the ship is in motion. They MUST be different if the ship is in motion. There is no question about it.
    What you say is true when you're using the sticks located outside the spaceship, strung up between Earth and the Sun, that the spaceship flies along.

    What you say is false when you're using the sticks located inside the spaceship, nailed to the floor. You say you agree that light travels at the same speed in all reference frames. Therefore it must cover the same number of sticks in the same amount of time inside the ship. Right?

    The reason the times are different outside the ship is that the points of emission and reception of the light are moving relative to those sticks outside. But they are at rest relative to the sticks inside. See?

    I am not talking about observing motion, I am talking about measuring motion in space using light. I am not observing the ship's motion. There is no way I could sit inside the ship and observe its own motion.
    Correct. The ship can never have motion with respect to sticks nailed to its floor. Agree?

    Therefore, since I know I can't observe the ship's motion from within, and the ship is at rest (not accelerating) so I can't feel any change in velocity, I must use light from within the ship to measure the distance the ship travels relative to light travel times in each direction.
    The only way the times can be different inside the ship (relative to the sticks on the floor) is if the speed of the light is different in the two directions inside the ship. Do you agree? i.e. the light has to cover more sticks in one direction in a certain time than in the other direction. Because inside the emitter and detector both sit at fixed points on the sticks nailed to the floor of the ship. The number of sticks between the emitter and detector doesn't change, so the only way the time taken for the light can change is if the speed of the light is different depending on the direction.

    But you say the speed of light is the same in both directions. Can you now see the contradiction?

    So, do you still maintain that the speed of light is the same in both directions relative to the sticks inside the ship, or do you want to modify your argument that the speed of light is only the same in both directions relative to sticks that are nailed down to "space"?

    Something has to give in your argument.

    Case in point, It appears the sun rises and sets and I am stationary. We both know that the sun doesn't orbit the Earth, correct? You have to understand that if you look out a window that you can't tell which object is moving. That is an illusion, nothing more. I don't rely on illusions to base my facts on, do you?
    What I am telling you is that a set of sticks nailed to the Earth is just as good as one nailed up between the Sun the and the Earth. In the frame of the sticks nailed to the Earth, the sun really does move. In the frame of the sticks nailed up between Earth and the Sun, the Sun doesn't move. And there's no way you can say what is real and what isn't, because you always need to use a set of sticks somewhere to measure positions, velocities and so on.

    An object is at an absolute zero velocity if it is not traversing space.
    So, in my terms, the object is sitting at a fixed particular position on some set of sticks that are nailed to "space".

    Where is this "space". How can I nail my sticks to it? And how can I be sure my sticks aren't moving with respect to "space"?

    According to you, at rest means a zero velocity, so according to you, all frames at rest are a zero velocity.
    A frame (set of sticks) is always at rest with respect to itself, obviously. The sticks that make up a frame never move relative to each other. Other objects may or may not be at rest relative to a given frame.

    I agree that light has a constant velocity in space.
    So, does it have a constant velocity inside the spaceship? Yes or no? i.e. relative to the sticks nailed to the floor of the ship?

    Or does it only have a constant absolute velocity, by which you mean velocity relative to sticks nailed to "space"?

    The meter is the distance light travels in a specific amount of time, IN A VACUUM! It says nothing about a reference frame!
    Actually, the metre is defined to be the distance that light travels in 1/299792458 seconds relative to ANY frame. The definition is based on Einstein's postulate that the speed of light is the same in every frame.

    The reason the definition doesn't mention a particular frame is because any frame will do in this special case.

  6. #866
    Valued Senior Member Motor Daddy's Avatar
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    Quote Originally Posted by James R View Post
    Motor Daddy:

    I'm going to reply to you in two posts. In this one, I'm going back to basics for a minute, just to make sure we're on the same page regarding reference frames, definitions of velocity and so on. In the next post I'll respond to your previous post.

    Here's what a reference frame is, approximately. We'll work in one spatial dimension for simplicity. Imagine you have constructed 100 identical straight sticks of equal length. The exact method of construction and the exact length is not important, as long as the lengths are the same when you lay one stick on top of another.

    Now, to constuct a reference frame, you lay out your 100 sticks end-to-end along a straight line. You now have a distance scale. If you want the distance from the end of the row of sticks to some other point on the line, you just count how many sticks there are between the end and the point you're interested in.

    Now, as well as a distance scale, we need some clocks. At every intersection of two sticks along the line, we place a clock. All of the clocks along the line are adjusted so that they read the same time always. Again, we won't worry about the exact method used to synchronise them for now; we just assume they are synchronised.

    So, a reference frame is this set of sticks and clocks.

    Now, suppose we number the intersections between any two sticks. We start at the end of the row (let's say) and call that position zero. The next intersection is position 1, the one after that position 2 etc. We now have a definition of position relative to our reference frame.

    Imagine a ball located at some particular intersection, say number 57. Then we say the position of the ball is at x=57 (sticks).

    Next we define velocity. If an object moves from one position to another along our line of sticks, then while it is moving it has a velocity equal to the number of intersections crossed divided by the time taken (as measured on the clocks). For example, if the ball moves from location x=57 to x=58 in 2 seconds, then it has a velocity of 1 stick per 2 seconds or 0.5 sticks per second.

    Finally, we have acceleration, although that is not important to our discussion. Acceleration is the rate of change of velocity. For example, if the ball changes speed from 3 sticks per second to 5 sticks per second in a time of 1 second, then its acceleration is 2 sticks per second per second, or 2 sticks per second squared.
    I agree with all that. You basically have meter sticks from end to end and synchronous clocks at every point along the line of sticks. I agree 100%.

    Quote Originally Posted by James R View Post
    Now, let's be clear about what is meant by the term "at rest". An object, such as the ball, is at rest in our reference frame if its position is not changing with time. In other words, if it stays at x=57 then it is at rest with respect to this reference frame. It's velocity in this case is the change in position over time, and since its position is not changing, the velocity is zero when it is at rest. "At rest" says nothing about acceleration. "At rest" simply tells you velocity is zero.
    I agree the ball is at rest in that frame.


    Quote Originally Posted by James R View Post
    Now, consider a different type of motion. Imagine the ball is "stationary" (whatever that means), and we move our whole assembly of sticks and clocks past the ball. Can you see that, in the reference frame we have constructed, the ball now has a velocity? The reason is that the stick intersections (57,56, 55, 54...) are moving past the ball, and so its position in the reference frame is changing over time. Therefore, in the reference frame of the sticks/clocks, the ball has a velocity, because of the way we defined velocity.
    Certainly you can measure the change in position of the ball compared to the sticks, and talk about how much the ball moved compared to the sticks. I agree 100%.


    Quote Originally Posted by James R View Post
    We could achieve the same velocity for the ball in a different way, of course. We could hold the stick and clocks "stationary" and move the ball along the sticks, instead of holding the ball stationary and moving the reference frame. In either case, the speed of the ball would be the same in the reference frame.
    Correct. You are talking about the distance and time the ball changed positions relative to the sticks. No different than measuring how far a car travels down a road in x amount of time. I am in 100% agreement so far.


    Quote Originally Posted by James R View Post
    Now, suppose we put our whole assemblage of sticks and clocks in an aeroplane which is flying above the Earth. And suppose we place our ball at x=57 and leave it there.

    Now, relative to our reference frame of sticks and clocks, the ball is at rest. Recall that we defined "at rest" to mean that an object does not change position, and position is defined by the intersections of the sticks. So, our ball is at rest in this reference frame. Note that the motion of the plane is completely irrelevant to whether the ball is at rest in the reference frame. As long as the ball stays at x=57 it is at rest, by definition.
    I'm good with that. I agree.


    Quote Originally Posted by James R View Post
    Ok, so now we have a definition of "reference frame", "position", "velocity" (and "acceleration", though we needn't worry about that right now).
    I understand and agree.


    Quote Originally Posted by James R View Post
    So, let's now consider TWO reference frames, by which I mean two identical sets of sticks and clocks laid out parallel to each other. Obviously, we can slide one entire set of sticks along the other set of sticks, so that each set of sticks has a velocity relative to the other set. For definiteness, let's assume on set of sticks is laid out on the Earth (attached to it) and the other set of sticks is laid out in our aeroplane, which is flying parallel to the Earth's surface, in the direction that both sets of sticks are laid out.

    Now consider the ball on the plane. Suppose it just sits in the aisle at location x=57 relative to the sticks on the plane. Then, the ball is at rest relative to the plane's reference frame. But the ball obviously has some velocity relative to the ground's set of sticks, its position on those sticks is changing as the plane flies past.

    So, is the velocity of the ball "really" zero, as indicated by the plane's reference frame, or is it "really" 800 km/hr (say), as indicated by the Earth's reference frame? Answer: it is BOTH.



    Or, to put it another way, it makes no sense to say "The ball's velocity is zero" unless you specify which reference frame you're using. You ought to say "The ball's velocity is zero in the reference frame of the plane", or equivalently "The ball's velocity is zero relative to the plane." Obviously, at the same time the ball's velocity is NOT zero relative to the ground.

    I understand the concept, and I agree, you can measure the distance the ball travels in the plane or compared to the Earth. I have no problems understanding that concept and I agree 100%.



    Quote Originally Posted by James R View Post
    More points to appreciate: EVERY reference frame - every set of sticks and clocks - is attached somewhere. The position of any given object may be different in different reference frame, as may its velocity and acceleration. Also, it makes no sense to define "position", "velocity" or "acceleration" in the absence of a reference frame.
    In the context of the previous explanations, I agree 100%. The ball can be measured to have a different velocity in each frame. In order to make it clear which velocity you are speaking, you must talk about which frame you are measuring the velocity in, ie, in the plane frame, or the Earth frame...etc. I understand the concept of relative motion and agree 100%.


    Quote Originally Posted by James R View Post
    When it comes to light, we can consider a light photon as just another ball moving along our lines of sticks. If light covers a certain number of sticks in a certain amount of time (as measured by the clocks at the intersections) then light has a certain speed determined by the reference frame. Up to this point I have talked about lengths in terms of stick lengths. Obviously, if we can somehow ensure that each stick has length 1 metre, then we can get velocities in metres per second.

    Here's where you start to lose me, and I disagree.

    Light is not just another ball moving along the sticks. When light travels, light lays sticks out consecutively in space. Totally different concept. Light always lays sticks out at the rate of 1 per 1⁄299,792,458 of a second. Since light lays the sticks in space, and objects travel in space, objects move along space's sticks that light has laid.




    Quote Originally Posted by James R View Post
    Ok, that's enough background.

    What I need from you, Motor Daddy, is some indication of whether you agree with the above description of what a reference frame is, as well as definitions of position and velocity. Because if we can't agree on these basics then we can't possibly begin to discuss relativity.

    So, please let me know if you think any of the above definitions or explanations I have given here are unworkable.
    I concur to all except the last statement about light. I will explain in detail in the following responses to your next post, as I see you have mentioned some things that bring up the issue and I will address the issue in full. I will continue my response in the morning as it's late and I'm going to bed, but wanted to reply to your first post tonight. I appreciate your patience with me. I hope I can convey my understanding very clear to you, so you too can understand it.
    Last edited by Motor Daddy; 01-01-11 at 10:24 PM.

  7. #867
    Motor Daddy:

    I'm glad we agree on the basics of reference frames, and I think we now agree on what is meant by "at rest". I'll wait for your next post.

  8. #868
    Valued Senior Member Motor Daddy's Avatar
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    Quote Originally Posted by James R View Post
    Motor Daddy:

    I'm glad we agree on the basics of reference frames, and I think we now agree on what is meant by "at rest". I'll wait for your next post.
    We'll put "at rest" on the back burner for a while. I have issues with it meaning a zero velocity but I know what you mean when you say at rest, so it won't impact the discussion.

  9. #869
    Valued Senior Member Motor Daddy's Avatar
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    Quote Originally Posted by James R View Post
    Motor Daddy:

    In this post I'll reply to your previous post.



    This is Physics 101. "At rest" means "zero velocity in a particular reference frame". Look it up if you don't believe me. The same definition is used in every physics textbook and on every reputable physics site on the internet.

    Referring to my previous post, you will see that "at rest" means only "not moving with respect to some reference frame". So when you ask me "compared to what", I say "compared to the reference frame I'm using". Think of sticks and clocks if you want a concrete picture.
    I'm good with that for now. Like I said, I have some issues with "at rest" but for now it doesn't impact this discussion. I know what you mean when you say "at rest."

    Quote Originally Posted by James R View Post
    You say "in your universe the ship never moves". Clearly that is not true. Read the previous post. The ship will never move relative to a bunch of sticks that are nailed to its floor, but it can certainly move relative to a bunch of sticks nailed to the Earth. How do we tell whether it is moving or not? Step 1: specify which reference frame we want to use. Step 2: See if the ship's position changes over time by looking at its position along the line of sticks. Easy.
    Again, I agree that you can relate the motion of the ship to some object like the Earth, just as you can relate the speed of a car to a road on Earth. Certainly the ship never moves compared to sticks nailed to its floor. What I'm saying is that the ship can be in motion in space, without regards to any other set of sticks nailed to the object. The ship can be in motion in space just as the ship can be in motion compared to Earth. You can know the motion of the ship in space by using light to measure the motion.



    Quote Originally Posted by James R View Post
    Wrong. 60 mph means you're moving across sixty mile-long sticks every hour. Since you have a velocity, you're not at rest. BUT, where are these sticks attached to? They are attached to the road. So, to say it properly "You're travelling at 60 mph relative to the road."
    Again, I'll agree to your definition of "at rest." Since you've defined it to be that way, I know what you are talking about. I don't agree that is a proper definition and I'll explain why later. It has to do with force and acceleration. We'll keep that on the back burner for now.

    Quote Originally Posted by James R View Post
    Suppose you have another line of sticks inside the car, nailed to car's floor. The car's position never changes relative to them, so in that reference frame the car is at rest.
    Certainly.

    Quote Originally Posted by James R View Post
    Suppose a plane is flying above the road. Then, the car is moving relative to the sticks that are nailed to the floor of the plane, but not at 60 mph, because obviously the car moves along the plane's sticks at a different rate than it moves along the road's sticks.

    Also, notice that the ROAD is moving at 60 mph relative to the car. As measured on the car's set of sticks, a fixed point on the road moves along those sticks. So, in the car's frame the road is not "at rest". Nor is the road "at rest" in the plane's frame.
    Certainly. I agree 100%. You are simply stating that the motion of an object can be measured compared to different objects, all of which could have different speeds.




    Quote Originally Posted by James R View Post
    What are your "space" sticks nailed to?
    They aren't nailed to space, light lays them out one right after the other as it travels. All those sticks you previously mentioned that are nailed to the floor, or ground etc are in reference to the sticks light lays as it travels. We define the length of all the sticks in reference to light's sticks. Every stick light lays in space is exactly the same length as the sticks in all the frames you previously mentioned. The problems arise when light is laying sticks as it travels the length of the ship. The ship has sticks nailed to the floor. The ships sticks are the same length as the sticks the light is laying. If the ship is in motion in space, and light is inside the ship traveling from one end to the other, there is.......conflict.

    What you are telling me, and what Einstein says, is that when you send light from one end of the ship to the other, that light will lay an equal number of sticks as are nailed to the floor of the ship in the time it takes light to travel from one end of the ship to the other. That only happens if the ship has a zero velocity in space. IF the ship has a velocity in space in the direction from say the rear to the front, light will lay more sticks from rear to front than are nailed tot he floor. If light is then sent in the opposite direction, from front to rear, light will lay less sticks than are nailed to the floor. Are you with me so far??

    Quote Originally Posted by James R View Post
    I understand that you have a set of sticks nailed somewhere and light travels along those sticks. Therefore it has a velocity. Your problem is that you don't know where your sticks are nailed.
    The sticks aren't nailed, light lays them as it travels. See above.



    Quote Originally Posted by James R View Post
    Let's go back to your original thought experiment with one-way travel times in the spaceship. First, consider a set of sticks nailed to the floor of the spaceship. What I am telling you is that light travels along that set of sticks at the same rate in both directions, always. This is completely against what you might expect, but also completely compatible with your claim in this thread that the speed of light is always the same in every frame. Right?
    The only way the amount of sticks that light lays is the same as the amount of sticks that are nailed to the floor is if the ship is not traveling in space. By definition, light always lays the same number of sticks per time interval, so light speed is constant. Issues arise from the fact that sticks are nailed to floors of planes, trains, and roads, and the object the sticks are nailed to can be in motion relative to light. With me so far??

    Quote Originally Posted by James R View Post
    Now, perhaps at this point you want to change your mind and say that the speed of light is NOT always the same after all, with respect to the sticks nailed to the floor of the spaceship. If so, please let me know, because that would be a very significant change to your argument.
    No chance, light ALWAYS lays the same sticks in the same amount of time.

    Quote Originally Posted by James R View Post
    Next, consider a set of sticks nailed to the Earth. The spaceship has a velocity relative to that frame, and so does the light going in both directions along the spaceship. For that frame, those sticks, I agree with you that light takes a different time in the two directions, for exactly the reasons you say it does. What I have been trying to point out to you up to this point is that you are still using a set of sticks to make this argument. Whether those sticks are nailed to ground, or strung up between Earth and the Sun or whatever, they must be there (notionally).
    You make the mistake in thinking that light lays the same number of sticks as the ship's floor has when it travels one-way. That is a fatal mistake. Do you acknowledge that if the ship is in motion in space that light will lay more or less sticks than the floor has nailed to it?





    Quote Originally Posted by James R View Post
    What you say is false when you're using the sticks located inside the spaceship, nailed to the floor. You say you agree that light travels at the same speed in all reference frames. Therefore it must cover the same number of sticks in the same amount of time inside the ship. Right?
    Saying light speed is constant by counting the number of sticks it always lays is something very different than saying light always lays the same number of sticks as it traverses traveling the length of the ship. Do you understand the difference between how many sticks light lays and how many of the ship's sticks light traverses?

    Quote Originally Posted by James R View Post
    The reason the times are different outside the ship is that the points of emission and reception of the light are moving relative to those sticks outside. But they are at rest relative to the sticks inside. See?
    The times are different inside as well, as light travels in space, and it lays sticks as it travels. Objects also travel in space. If you compare the amount of sticks light lays to the amount of sticks nailed to the floor of the object, the only way the sticks will be the same as if the object is not traveling in space. See?



    Quote Originally Posted by James R View Post
    Correct. The ship can never have motion with respect to sticks nailed to its floor. Agree?
    Agree, it goes without saying.



    Quote Originally Posted by James R View Post
    The only way the times can be different inside the ship (relative to the sticks on the floor) is if the speed of the light is different in the two directions inside the ship. Do you agree? i.e. the light has to cover more sticks in one direction in a certain time than in the other direction. Because inside the emitter and detector both sit at fixed points on the sticks nailed to the floor of the ship. The number of sticks between the emitter and detector doesn't change, so the only way the time taken for the light can change is if the speed of the light is different depending on the direction.
    No, I don't agree. Light lays its own sticks. Light does not travel along sticks nailed to any object. The speed of light is always the same because light always lays sticks at the same rate. That is something very different than saying light always travels along nailed down sticks at the same rate. Why would anyone think that light would always take the same amount of time to travel x number of sticks nailed to the floor of an object that is capable of different motions?

    Quote Originally Posted by James R View Post
    But you say the speed of light is the same in both directions. Can you now see the contradiction?
    No contradiction on my part. Light always lays the same number of sticks per a specific amount of time. Your misunderstanding seems to be that you think light always traverses the same number of sticks in the same amount of time, regardless of the motion of the object the sticks are nailed to. The speed of light is constant because light always lays the same number of sticks per time interval, not because it always traverses the same amount of sticks nailed to an object that is capable of an infinite amount of different motions.

    Quote Originally Posted by James R View Post
    So, do you still maintain that the speed of light is the same in both directions relative to the sticks inside the ship, or do you want to modify your argument that the speed of light is only the same in both directions relative to sticks that are nailed down to "space"?

    Something has to give in your argument.
    My argument is as sound as sound gets. Yours however has some serious flaws that need to be addressed. I think I've addressed those flaws, and now you need to reconsider what you think is real and what is an illusion in Einstein's world.



    Quote Originally Posted by James R View Post
    What I am telling you is that a set of sticks nailed to the Earth is just as good as one nailed up between the Sun the and the Earth. In the frame of the sticks nailed to the Earth, the sun really does move. In the frame of the sticks nailed up between Earth and the Sun, the Sun doesn't move. And there's no way you can say what is real and what isn't, because you always need to use a set of sticks somewhere to measure positions, velocities and so on.
    Light lays its own sticks in space.



    Quote Originally Posted by James R View Post
    So, in my terms, the object is sitting at a fixed particular position on some set of sticks that are nailed to "space".

    Where is this "space". How can I nail my sticks to it? And how can I be sure my sticks aren't moving with respect to "space"?
    Space doesn't have sticks, light defines space by laying sticks in space as it travels. Objects travel relative to light's sticks, by definition.

    Quote Originally Posted by James R View Post
    A frame (set of sticks) is always at rest with respect to itself, obviously. The sticks that make up a frame never move relative to each other. Other objects may or may not be at rest relative to a given frame.
    It goes without saying.



    Quote Originally Posted by James R View Post
    So, does it have a constant velocity inside the spaceship? Yes or no? i.e. relative to the sticks nailed to the floor of the ship?


    Or does it only have a constant absolute velocity, by which you mean velocity relative to sticks nailed to "space"?
    Light always lays the same number of sticks per time interval. Light may or may not lay the same number of sticks as are nailed to the floor as it travels from one end to the other. Light may lay 10 sticks in the time it takes light to travel the length of an object that is 1 stick long. You good now??

    Quote Originally Posted by James R View Post
    Actually, the metre is defined to be the distance that light travels in 1/299792458 seconds relative to ANY frame. The definition is based on Einstein's postulate that the speed of light is the same in every frame.

    The reason the definition doesn't mention a particular frame is because any frame will do in this special case.
    Light doesn't travel relative to frames, light travels in space and it lays sticks at a constant rate.
    Last edited by Motor Daddy; 01-02-11 at 08:16 AM.

  10. #870
    Motor Daddy:

    Thanks for your post. I think I understand where you're coming from, but I have some issues with your approach. This post may be a little repetitive, but if I say the same thing in a few different ways then maybe you'll start to see where I'm coming from.

    What you are telling me, and what Einstein says, is that when you send light from one end of the ship to the other, that light will lay an equal number of sticks as are nailed to the floor of the ship in the time it takes light to travel from one end of the ship to the other. That only happens if the ship has a zero velocity in space.
    What you're saying is that light lays out a certain number of sticks per unit time, full stop. The sticks always have the same length in every reference frame, and time is the same in every reference frame.

    But there's a conflict here. If I measure up some metre sticks using light, then nail them to the floor of the spaceship, then they should indicate the correct length of the spaceship, regardless of whether it is moving or not. It doesn't change length just because it is moving. So, if there's an emitter and detector of light at opposite ends of the ship, inside, then when light is emitted it should start laying down sticks of length equal in length to the sticks nailed to the floor of the ship, one metre every 1/299792458 seconds. If you're inside the ship and you count up the number of sticks layed by light between the emitter and the detector, then there must be an equal number to the number nailed to the floor.

    If this was not the case, then the speed of light as measured using the sticks nailed to the floor would necessarily be different from the speed of light determined by the sticks layed by the light.

    And yet, you claim that the speed of light is the same whether you measure it inside the spaceship or outside.

    Now, as far as I can see, you have a number of options for solving this apparent problem:

    (a) you can say that when the spaceship is moving, we're not allowed to use the spaceship's sticks to measure distances, because the distances we obtain will be wrong. In that case, the speed of light measured using the spaceship's sticks is definitely NOT the same as the speed measured using light's sticks, so you need to give up your claim that light has a constant speed in all reference frames. OR

    (b) you can maintain a constant speed of light by adjusting the "apparent" length of the ship's metre sticks according to its velocity, so that when we count sticks in the spaceship as light travels and count the sticks laid by the light we end up with the same number. This maintains the constancy of the speed of light in both frames, but introduces a "length contraction" effect when light travels in one direction, and a "length expansion" effect when light travels in the other direction. These effects would not be "real", but apparent. When we laid out the sticks laid down by light next to the ones nailed to the ship, we'd see that their lengths did not match. Moreover, in one direction the ship's sticks would be shorter than light's sticks, and in the opposite direction the ship's sticks would be longer than light's sticks. OR

    (c) You could maintain the constancy of the speed of light by playing with the times inside and outside the spaceship, pretending in effect that spaceship clocks run slower than outside clocks when light travels in one direction and faster when it travels in the other direction. OR

    (d) You can adopt Einstein's approach to the problem and alter both distances and time so as to maintain the constancy of the speed of light in all frames.

    These are the only options I can see to resolve your problem. Maybe you can think of something else; I'm not sure. But as far as I can see, you're forced to choose one of these.

    Quote Originally Posted by Motor Daddy
    IF the ship has a velocity in space in the direction from say the rear to the front, light will lay more sticks from rear to front than are nailed tot he floor. If light is then sent in the opposite direction, from front to rear, light will lay less sticks than are nailed to the floor. Are you with me so far??
    Yes. So, suppose light is travelling from the rear to the front. According you you, it lays more sticks than are nailed to the floor of the ship. Let's say 27 metre sticks are nailed to the floor between the attachment points of the source and detector of the light. And suppose the light takes 54/299792458 seconds to go from the source to the detector.

    Now, according to you, the light ought to lay down 54 sticks in total over that time. The speed of light relative to its own sticks is the distance covered (54 sticks) divided by the time taken (54/299792458 seconds), which gives 299792458 metres per second.

    But in the frame of the spaceship, the light covered 27 sticks nailed to the floor in a time of 54/299792458 seconds, which gives a calculated speed of light of 149896229 metres per second for the light.

    To repeat myself, I can only see four ways to deal with the discrepancy:

    (a) you can say that when the spaceship is moving, we're not allowed to use the spaceship's sticks to measure distances, because the distances we obtain will be wrong. We MUST use only light's sticks. In that case, we can never make a reliable measurement using metre sticks, because we never know in advance whether an object is moving with respect to "space" or not. OR

    (b) you can maintain a constant speed of light by adjusting the "apparent" length of the ship's metre sticks according to its velocity, so that when we count sticks in the spaceship as light travels and count the sticks laid by the light we end up with the same number. For example, you could say in this example that, given the speed of the ship through space, the ship's sticks have an "effective" length of only half a metre instead of one metre. This maintains the constancy of the speed of light in both frames. OR

    (c) You could maintain the constancy of the speed of light by saying that the clocks on the spaceship tick only half as fast as clocks in "space" as the light travels from the rear to the front. Again this would maintain the speed of light in both frames. OR

    (d) You can adopt Einstein's approach to the problem and alter both distances and time so as to maintain the constancy of the speed of light in all frames.

    I can't see any other options.

    The only way the amount of sticks that light lays is the same as the amount of sticks that are nailed to the floor is if the ship is not traveling in space. By definition, light always lays the same number of sticks per time interval, so light speed is constant.
    But do you agree that if I measure the speed of light using the spaceship's sticks then I'll get the wrong answer?

    So, am I not allowed to use the spaceship's frame for measurements? Should I always measure things using light relative to "space"? Or can you fix your system some other way so that I can accurately measure lengths using metre sticks inside the spaceship?

    Quote Originally Posted by Motor Daddy
    Quote Originally Posted by James R
    Now, perhaps at this point you want to change your mind and say that the speed of light is NOT always the same after all, with respect to the sticks nailed to the floor of the spaceship. If so, please let me know, because that would be a very significant change to your argument.
    No chance, light ALWAYS lays the same sticks in the same amount of time.
    It's important to realise that you haven't answered the question I asked here. If we accept your premise that light always lays the same number of sticks in the same amount of time, something has to give in the spaceship frame. Either the speed of light is not the same in that frame, or else we need to adjust lengths, times or both to make sure that the speed of light stays constant in that frame. Or, we give up on using any frame but that of "space", and say we must use light for every length measurement, because we can never rely on the accuracy of metre sticks laid by anything but light.

    You make the mistake in thinking that light lays the same number of sticks as the ship's floor has when it travels one-way. That is a fatal mistake. Do you acknowledge that if the ship is in motion in space that light will lay more or less sticks than the floor has nailed to it?
    It is no mistake. Einstein's postulate is that the speed of light is the same in all frames. That is, if you measure the speed using sticks nailed to the ground and again using sticks nailed to the spaceship you get the same answer for the speed of light. The way that happens in relativity is that metre sticks inside the spaceship actually have different lengths to metre sticks on the ground, and clocks inside the spaceship actually tick at a different rate to clocks on the ground.

    I agree with you that if measurements are made in a frame in which the ship is moving (such as the ground frame), then light will lay more or less ground-length sticks in each direction. This is because light must travel at the same speed in both directions in the ground frame.

    Similarly, in the spaceship (and we do not agree on this), light must lay equal numbers of sticks in each direction in order to have the same speed in both directions in the spaceship frame. That's Einstein's picture.

    Saying light speed is constant by counting the number of sticks it always lays is something very different than saying light always lays the same number of sticks as it traverses traveling the length of the ship. Do you understand the difference between how many sticks light lays and how many of the ship's sticks light traverses?
    Yes, I understand what you're saying. I just know that your picture is inconsistent.

    My argument is as sound as sound gets. Yours however has some serious flaws that need to be addressed. I think I've addressed those flaws, and now you need to reconsider what you think is real and what is an illusion in Einstein's world.
    I'm not actually clear about what your argument is with Einstein's picture. The discussion about what is "real" and what is "illusion" needs fleshing out. What matters to a physicist is that we have a workable model that gives the right answers so we can do physics. The model should match what we measure in reality. All tests of Einstein's theory give a good match to what is measured. So, in a sense it's a philosophical question about whether space "really" contracts in different frames, or whether time "really" dilates, or whether these are "illusions". What we do know is that there's nothing illusory about the results of twin-paradox experiments, for example. Things really do seem to experience time at a different rate when in motion. Either that, or something else is going on that gives an outcome that makes it look exactly like that is happening.

    Anyway, what I'm saying is that I'm not sure how you think that Einstein's theory is flawed. Again, I see at least two possible arguments:

    (a) Einstein's theory is self-inconsistent (i.e. has mathematical errors).
    (b) Einstein's theory is consistent but does not match real-world observations (in which case you need to point out which real-world experiments diagree with Einstein's predictions).

    Light doesn't travel relative to frames, light travels in space and it lays sticks at a constant rate.
    But surely "space" is a frame just like every other?

  11. #871
    Valued Senior Member Motor Daddy's Avatar
    Posts
    4,547
    Quote Originally Posted by James R View Post


    What you're saying is that light lays out a certain number of sticks per unit time, full stop. The sticks always have the same length in every reference frame, and time is the same in every reference frame.
    Correct.

    Quote Originally Posted by James R View Post
    But there's a conflict here. If I measure up some metre sticks using light, then nail them to the floor of the spaceship, then they should indicate the correct length of the spaceship, regardless of whether it is moving or not.
    They do indicate the correct length of the spaceship, they are the same length as the sticks light lays.


    Quote Originally Posted by James R View Post
    It doesn't change length just because it is moving.
    I agree 100%

    Quote Originally Posted by James R View Post
    So, if there's an emitter and detector of light at opposite ends of the ship, inside, then when light is emitted it should start laying down sticks of length equal in length to the sticks nailed to the floor of the ship, one metre every 1/299792458 seconds. If you're inside the ship and you count up the number of sticks layed by light between the emitter and the detector, then there must be an equal number to the number nailed to the floor.

    If light laid the same amount of sticks as the floor has in the time light traveled from emitter to receiver than the ship has a zero velocity. Cut and dry.

    Quote Originally Posted by James R View Post
    If this was not the case, then the speed of light as measured using the sticks nailed to the floor would necessarily be different from the speed of light determined by the sticks layed by the light.
    The speed of light is not measured against the ships sticks. Light travel time defines the length of the sticks. The ship moves relative to light, not the other way around. The length of the ship is defined by light travel time. If light takes a different amount of time to traverse the length of the ship, by default the ship has a velocity.

    Quote Originally Posted by James R View Post
    And yet, you claim that the speed of light is the same whether you measure it inside the spaceship or outside.
    Again, the speed of light is not measured, it is defined. The length of the ship IS a specific light travel time. No question about it. If light takes a different amount of time to traverse the ship, by default the ship has a velocity, as the ship's length is a light travel time.

    Quote Originally Posted by James R View Post
    Now, as far as I can see, you have a number of options for solving this apparent problem:

    (a) you can say that when the spaceship is moving, we're not allowed to use the spaceship's sticks to measure distances, because the distances we obtain will be wrong. In that case, the speed of light measured using the spaceship's sticks is definitely NOT the same as the speed measured using light's sticks, so you need to give up your claim that light has a constant speed in all reference frames. OR
    If you are measuring the motion of the ship, you do not use the sticks nailed to the floor of the ship, you compare the motion of the ship to the motion of light and arrive at a velocity of the ship. You can not use the sticks nailed to the ship to measure the speed of light unless you know the velocity of the ship. I have given you a way of knowing the velocity of the ship and a way of knowing the length of the ship.

    I repeat, you do not measure light speed by measuring the time it takes light to travel the ship's sticks. Light defines the length of the sticks!

    Quote Originally Posted by James R View Post
    (b) you can maintain a constant speed of light by adjusting the "apparent" length of the ship's metre sticks according to its velocity, so that when we count sticks in the spaceship as light travels and count the sticks laid by the light we end up with the same number. This maintains the constancy of the speed of light in both frames, but introduces a "length contraction" effect when light travels in one direction, and a "length expansion" effect when light travels in the other direction. These effects would not be "real", but apparent. When we laid out the sticks laid down by light next to the ones nailed to the ship, we'd see that their lengths did not match. Moreover, in one direction the ship's sticks would be shorter than light's sticks, and in the opposite direction the ship's sticks would be longer than light's sticks. OR
    No, the sticks are all the same length. There is no length contraction in reality. That is a result of bad theory.

    Quote Originally Posted by James R View Post
    (c) You could maintain the constancy of the speed of light by playing with the times inside and outside the spaceship, pretending in effect that spaceship clocks run slower than outside clocks when light travels in one direction and faster when it travels in the other direction. OR

    No, a specific duration is just that, you do not change the duration of a second, nor do you contract the length of sticks. Those are simply band-aids to repair bad theory!


    Quote Originally Posted by James R View Post
    (d) You can adopt Einstein's approach to the problem and alter both distances and time so as to maintain the constancy of the speed of light in all frames.
    That is going to require a big box of band-aids. I'm not willing to do that. My theory needs no band-aids.

    Quote Originally Posted by James R View Post
    These are the only options I can see to resolve your problem. Maybe you can think of something else; I'm not sure. But as far as I can see, you're forced to choose one of these.

    1. The speed of light is not measured, it is defined. You do not measure the time it takes light to traverse the ship's sticks.

    2. The length of the sticks that light lays are the same length of every stick in every frame in the universe. There is no need to start changing stick lengths and dilating time to put a band-aid on bad theory when I can perform the measurements without using band-aids.

    3. I've given you the equations (with the help of Neddy Bate's math skills) to measure the absolute velocity of the ship. The equations are built from rock solid facts as are observed in the universe, ie the sticks are the same length etc..





    Quote Originally Posted by James R View Post
    Yes. So, suppose light is travelling from the rear to the front. According you you, it lays more sticks than are nailed to the floor of the ship. Let's say 27 metre sticks are nailed to the floor between the attachment points of the source and detector of the light. And suppose the light takes 54/299792458 seconds to go from the source to the detector.

    Now, according to you, the light ought to lay down 54 sticks in total over that time. The speed of light relative to its own sticks is the distance covered (54 sticks) divided by the time taken (54/299792458 seconds), which gives 299792458 metres per second.
    Correct. I want to emphasize that it isn't that light "ought to" lay down 54 sticks, it is that light HAS TO lay down 54 sticks, BY DEFINITION!

    Quote Originally Posted by James R View Post
    But in the frame of the spaceship, the light covered 27 sticks nailed to the floor in a time of 54/299792458 seconds, which gives a calculated speed of light of 149896229 metres per second for the light.
    By definition, the ship is 27/299792458 light seconds in length. If light takes more or less time than 27/299792458 seconds to travel the length of the ship, The ship MUST have a velocity. MUST HAVE A VELOCITY!!!


    Quote Originally Posted by James R View Post
    To repeat myself, I can only see four ways to deal with the discrepancy:

    (a) you can say that when the spaceship is moving, we're not allowed to use the spaceship's sticks to measure distances, because the distances we obtain will be wrong. We MUST use only light's sticks. In that case, we can never make a reliable measurement using metre sticks, because we never know in advance whether an object is moving with respect to "space" or not. OR
    1. I gave you the equations to find the velocity of the ship and know the length of the ship. Why are you trying to measure the speed of light on a ship of unknown velocity?

    2. You can use meter sticks in the ship as usual. What is the problem? Are you trying to measure the motion of the ship in space using the meter sticks that are nailed to the ship's floor? That's ridiculous. In order to measure the ship's motion in space you measure the ship compared to light's sticks. Do you not understand that all object's motion in space can be measured relative to light's sticks?

    3. The speed of light is defined, why are you continuously trying to measure the speed of light? Light ALWAYS lays the same amount of sticks in the same amount of time, by definition. It doesn't change, it can't, by definition!

    Quote Originally Posted by James R View Post
    (b) you can maintain a constant speed of light by adjusting the "apparent" length of the ship's metre sticks according to its velocity, so that when we count sticks in the spaceship as light travels and count the sticks laid by the light we end up with the same number. For example, you could say in this example that, given the speed of the ship through space, the ship's sticks have an "effective" length of only half a metre instead of one metre. This maintains the constancy of the speed of light in both frames. OR
    No, I refuse to use band-aids. My theory needs no band-aids!

    Quote Originally Posted by James R View Post
    (c) You could maintain the constancy of the speed of light by saying that the clocks on the spaceship tick only half as fast as clocks in "space" as the light travels from the rear to the front. Again this would maintain the speed of light in both frames. OR
    No, I refuse to use band-aids. My theory needs no band-aids!


    Quote Originally Posted by James R View Post
    (d) You can adopt Einstein's approach to the problem and alter both distances and time so as to maintain the constancy of the speed of light in all frames.

    No, I refuse to use band-aids. My theory needs no band-aids!





    Quote Originally Posted by James R View Post
    But do you agree that if I measure the speed of light using the spaceship's sticks then I'll get the wrong answer?
    Yes, unless the ship has a zero velocity. You will get the wrong answer for one reason only. You fail to acknowledge the ship's velocity in space!

    Quote Originally Posted by James R View Post
    So, am I not allowed to use the spaceship's frame for measurements? Should I always measure things using light relative to "space"? Or can you fix your system some other way so that I can accurately measure lengths using metre sticks inside the spaceship?
    You can measure things in the ship using the ship's sticks. If you move a table 3 feet in the ship, the sticks are accurate. But why would you try to use the ship's sticks to measure the velocity of the ship? You admittedly, along with every other scientist in the past, said that it was impossible to measure the ship's velocity in space. I have given you a way to do that, and you are denying it. The ships sticks can not be used to measure the ship's velocity in space. If you want to compare other objects motion compared to the ship's sticks, be my guest. However, if you use light you must use the proper procedures, and that entails knowing and acknowledging the velocity of the ship. Einstein doesn't know, and fails to acknowledge the velocity of the ship. I don't! He uses band-aids to cover up his boo-boos. I don't have boo-boos so I need no band-aids.



    Quote Originally Posted by James R View Post
    It's important to realise that you haven't answered the question I asked here. If we accept your premise that light always lays the same number of sticks in the same amount of time, something has to give in the spaceship frame. Either the speed of light is not the same in that frame, or else we need to adjust lengths, times or both to make sure that the speed of light stays constant in that frame. Or, we give up on using any frame but that of "space", and say we must use light for every length measurement, because we can never rely on the accuracy of metre sticks laid by anything but light.
    The speed of light is defined, and light always lays the same amount of sticks in the same amount of time. The speed of light is not measured compared to the ship's sticks. You are mistaking if you think you measure the speed of light against the sticks without first knowing the velocity and taking that into account. I've given you a way to do that. Use it!



    Quote Originally Posted by James R View Post
    It is no mistake. Einstein's postulate is that the speed of light is the same in all frames.
    The speed of light is always the same, but he is wrong to assume you measure the speed of light compared to a ship of unknown velocity. He therefore needs to use band-aids to cover his bad theory.

    Quote Originally Posted by James R View Post
    That is, if you measure the speed using sticks nailed to the ground and again using sticks nailed to the spaceship you get the same answer for the speed of light. The way that happens in relativity is that metre sticks inside the spaceship actually have different lengths to metre sticks on the ground, and clocks inside the spaceship actually tick at a different rate to clocks on the ground.
    All objects have the same length sticks and a specific duration is just that. You do not use light to measure length unless you know the velocity of the object being measured. It's as simple as that!

    Quote Originally Posted by James R View Post
    Similarly, in the spaceship (and we do not agree on this), light must lay equal numbers of sticks in each direction in order to have the same speed in both directions in the spaceship frame. That's Einstein's picture.
    If the ship has a velocity, light will NOT lay the same number of sticks as it traverses. That is due to the ship's velocity.



    Quote Originally Posted by James R View Post
    Yes, I understand what you're saying. I just know that your picture is inconsistent.
    My picture is not inconsistent, Einstein's is, as is evident by his band-aid factory.



    Quote Originally Posted by James R View Post
    I'm not actually clear about what your argument is with Einstein's picture. The discussion about what is "real" and what is "illusion" needs fleshing out. What matters to a physicist is that we have a workable model that gives the right answers so we can do physics. The model should match what we measure in reality.
    Mine not only gets the measurements correct, but it also reflects reality, which his does not. In reality, all the sticks are the same length.





    Quote Originally Posted by James R View Post
    Anyway, what I'm saying is that I'm not sure how you think that Einstein's theory is flawed. Again, I see at least two possible arguments:

    (a) Einstein's theory is self-inconsistent (i.e. has mathematical errors).
    (b) Einstein's theory is consistent but does not match real-world observations (in which case you need to point out which real-world experiments diagree with Einstein's predictions).
    (c) Einstein can't tell you the velocity of the ship in space, I can. Einstein uses band-aids, which don't reflect reality. I don't.



    Quote Originally Posted by James R View Post
    But surely "space" is a frame just like every other?
    Space is the ultimate frame. All objects are in motion in space. I can tell you that motion, Einstein can't!

  12. #872
    Motor Daddy:

    Quote Originally Posted by Motor Daddy
    The speed of light is not measured against the ships sticks. Light travel time defines the length of the sticks. The ship moves relative to light, not the other way around. The length of the ship is defined by light travel time. If light takes a different amount of time to traverse the length of the ship, by default the ship has a velocity.
    When you say "the ship moves relative to light", you're not using the reference frame terminology that I thought we agreed on earlier. To say that the ship moves "relative to light" introduces a picture by which a bunch of sticks is stationary with respect to the light, and the ship's velocity is measured relative to those sticks.

    As soon as you say "light moves at speed c", you're automatically talking about a frame other than one in which the individual photons of light maintain constant positions relative to the measuring sticks.

    Apart from this point, I understand what you're saying, though.

    Again, the speed of light is not measured, it is defined. The length of the ship IS a specific light travel time. No question about it. If light takes a different amount of time to traverse the ship, by default the ship has a velocity, as the ship's length is a light travel time.
    Obviously, in general the length of the ship is NOT determined by the light travel time. The ONLY time that the length of the ship is correctly recorded by light, according to you, is when the ship has zero velocity. If the ship has any other speed, light will have to travel a longer or shorter distance from one end of the ship to the other. Moreover, the ONLY time the rulers nailed to the ship agree with the light travel time is when the ship has zero velocity, according to you. Those rulers were calibrated correctly using light, we assume, so the rulers always measure the correct length of the ship. Light gets it wrong if the ship has a velocity, because we need to factor in the velocity of the ship in order to "correct" the length of the ship when we measure it with light.

    But this isn't the main problem. I'll get to the main problem shortly.

    Quote Originally Posted by Motor Daddy
    Quote Originally Posted by James R
    Now, as far as I can see, you have a number of options for solving this apparent problem:

    (a) you can say that when the spaceship is moving, we're not allowed to use the spaceship's sticks to measure distances, because the distances we obtain will be wrong. In that case, the speed of light measured using the spaceship's sticks is definitely NOT the same as the speed measured using light's sticks, so you need to give up your claim that light has a constant speed in all reference frames.
    If you are measuring the motion of the ship, you do not use the sticks nailed to the floor of the ship, you compare the motion of the ship to the motion of light and arrive at a velocity of the ship. You can not use the sticks nailed to the ship to measure the speed of light unless you know the velocity of the ship. I have given you a way of knowing the velocity of the ship and a way of knowing the length of the ship.

    I repeat, you do not measure light speed by measuring the time it takes light to travel the ship's sticks. Light defines the length of the sticks!
    I think you misunderstand. I am not using the sticks nailed to the ship to measure the velocity of the ship; I'm using them to measure the velocity of light inside the ship. The velocity of the ship is always zero with respect to the sticks nailed to its floor; I believe we have already agreed on that.

    You claim I cannot use the sticks nailed to floor of the ship to measure the speed of light. But if those sticks have been properly calibrated using light previously, before they were nailed to the floor and while they had zero velocity, then those sticks all have the same length as the sticks that light lays down. Also, the clocks in the ship all keep time just the same as clocks everywhere else in the universe, according to you.

    So why can't I measure the speed of light in the reference frame of the ship? Surely I should be able to measure how far light travels in a given time using my correctly-calibrated sticks on the ship and correctly calibrated clocks on the ship, and get the correct answer for the speed of light inside the ship. Shouldn't I? And that measured speed should naturally match the unchanging, defined speed of light, always, according to you. How could I use correct sticks and clocks and yet calculate a speed of light that is wrong?

    The ONLY way this could happen is if I'm (a) just not allowed, by fiat of Motor Daddy, to measure the speed of light at all, OR (b) the speed of light is ONLY 299792458 m/s in a frame that has zero velocity, so I should expect a "wrong" answer in the ship's frame when it has a velocity.

    If (a), then I'm not clear why I can't measure the speed of light. Apparently I can measure the length of any object other than light, so why can't I measure light? Why does light have to be merely defined? Wouldn't measuring the speed of light be a good way to confirm that I'm using correctly-calibrated rulers and clocks?

    If (b), then you have to give up on your claim that the velocity of light is the same in all reference frames, because as I have clearly shown, under your assumptions the speed of light MUST be different in the reference frame of the travelling ship compared to its speed in a ship that has zero velocity.

    This is currently the number 1 thing that is making your theory self-inconsistent. And it could be easily fixed, as far as I can tell. All you have to do is to say that light ONLY has a measured speed of 299792458 m/s when measured in a frame that has zero absolute velocity. Why use Einstein's postulate when (a) you say you disagree with Einstein, and (b) it makes your theory inconsistent?

    Quote Originally Posted by Motor Daddy
    1. The speed of light is not measured, it is defined. You do not measure the time it takes light to traverse the ship's sticks.
    What stops me from measuring it? Nothing. But if I do, I'll calculate a different speed for the light depending on the velocity of the ship through "space". So, what you're really saying is you don't want me to go off and measure the speed of light. You'd rather pretend that it has the same speed in all frames. If I actually measured it in the ship's frame, I'd discover that light doesn't have the "correct" velocity in that frame, so your solution is to forbid me from doing the experiment that proves this aspect of your theory false.

    2. The length of the sticks that light lays are the same length of every stick in every frame in the universe. There is no need to start changing stick lengths and dilating time to put a band-aid on bad theory when I can perform the measurements without using band-aids.
    Only at the expense of dispensing with the constancy of the speed of light in all reference frames. As I said, I don't know why you're so keen to hold onto that postulate, which after all is one Einstein introduced, and you say you disagree with him about just about everything else.

    3. I've given you the equations (with the help of Neddy Bate's math skills) to measure the absolute velocity of the ship. The equations are built from rock solid facts as are observed in the universe, ie the sticks are the same length etc..
    Those equations are correct provided that (1) the light travel times are different in the two directions, and provided that (2) the light travel times are measurably the same in whatever frame you measure them in (i.e. no matter whether you measure the times using clocks on the ground, on the spaceship or on a different spaceship flying parallel to the first at a different speed).

    Your error is that you assume both of these provisos, completely without any actual evidence to back up your assumptions. In fact, assumption (2) turns out to be completely wrong. Assumption (1) is true in every frame except for the frame of the ship (i.e. its rest frame). In the frame of the ship the light travel times are in fact always equal.

    Therefore, if you actually used your equations in a real-world experiment that measures the light travel time using clocks on the ship, then your equations would tell you that the ship always has zero velocity. And this would happen no matter how fast the ship was flying with respect to anything else you care to name.

    We can go back and forth on this forever and get nowhere: "The travel times will be different." "No they won't". "Yes they will". Repeat ad nauseam. I'm just telling you, in the full expectation that you'll continue to make the assertion regardless.

    Quote Originally Posted by MD
    Quote Originally Posted by JR
    Yes. So, suppose light is travelling from the rear to the front. According you you, it lays more sticks than are nailed to the floor of the ship. Let's say 27 metre sticks are nailed to the floor between the attachment points of the source and detector of the light. And suppose the light takes 54/299792458 seconds to go from the source to the detector.

    Now, according to you, the light ought to lay down 54 sticks in total over that time. The speed of light relative to its own sticks is the distance covered (54 sticks) divided by the time taken (54/299792458 seconds), which gives 299792458 metres per second.

    But in the frame of the spaceship, the light covered 27 sticks nailed to the floor in a time of 54/299792458 seconds, which gives a calculated speed of light of 149896229 metres per second for the light.
    By definition, the ship is 27/299792458 light seconds in length. If light takes more or less time than 27/299792458 seconds to travel the length of the ship, The ship MUST have a velocity. MUST HAVE A VELOCITY!!!
    Then you MUST agree that the naive calculation of the speed of light using the ship's sticks gives a different value for the speed of light than the "correct" value.

    To use a reference frame, I should not need to add in "hidden" information about the ship's speed in order to calculate the "correct" result for the speed of light. Something is wrong here. Either the ship's frame is not correctly calibrated, or else it does give correct answers but the speed of light is not the same in the ship's frame as it it outside.

    You often complain that Einstein needs to know the velocity of a frame before he can start doing anything, but it seems to me that in this instance it is you who needs to know the ship's speed in advance in order to "correct" the measurements to give the correct speed of light in the ship's frame.

    1. I gave you the equations to find the velocity of the ship and know the length of the ship. Why are you trying to measure the speed of light on a ship of unknown velocity?
    If you like, you can assume I'm doing it as a check to make sure that the rulers I nailed to the floor and the clocks I'm carrying on the ship are correctly calibrated so that I can use them to perform correct measurements on other objects.

    You're telling me that before I can do this check, I first need to determine the velocity of the ship using your methods, so I can "correct" all my subsequent measurements made using light.

    Taking that point to its logical conclusion, before I can measure anything, anywhere, I must always first use your method to determine the velocity of the thing I'm standing on. So, I need to measure the velocity of the Earth before constructing any metre sticks using light to measure teh correct length in a metre stick factory, for example.

    What is the velocity of the Earth, by the way? I assume you have this important piece of information to hand.

    2. You can use meter sticks in the ship as usual.
    But all speeds of objects in the ship that I calculate using the ship's sticks will be wrong, won't they? To get the correct speeds I need to add or subtract the ship's velocity to or from all measured speeds. More generally, I'll need to perform a vector addition or subtraction, because the ship may be moving in a different direction than the object I'm trying to measure the speed of inside the ship.

    What is the problem? Are you trying to measure the motion of the ship in space using the meter sticks that are nailed to the ship's floor? That's ridiculous.
    No, and I never said I was trying to do that. Doing that will always give an answer of zero speed. I think we agree on that, don't we? And to get the true speed of the ship, I need to use light to determine it, as per your recipe. That's the first step before I can measure anything else inside the ship.

    Quote Originally Posted by Motor Daddy
    Quote Originally Posted by James R
    But do you agree that if I measure the speed of light using the spaceship's sticks then I'll get the wrong answer?
    Yes, unless the ship has a zero velocity. You will get the wrong answer for one reason only. You fail to acknowledge the ship's velocity in space!
    This is the current crux of our disagreement. You're saying to get the right answer for the speed of light in the ship's frame, I need not only the measurements of distance covered and time taken, but ALSO I need to know the ship's velocity in advance. Whereas Einstein says you can just make the measurements and calculate the result. Done.

    Call me perverse for daring to want to measure the speed of light rather than accept a definition, but I wonder what people did with such problems prior to the redefinition of the metre in the 1960s. One wonders how physicists ever got any useful work done, since they couldn't accurately measure anything without having access to your measurement procedure. And since they still don't correct for the Earth's absolute velocity through space when they do experiments in their labs today, all the currect results of physics must be wrong, too.

    You can measure things in the ship using the ship's sticks. If you move a table 3 feet in the ship, the sticks are accurate.
    Well, not really, because the ship is really moving too. The real distance moved by that table can only be determined by light. Right? Metre sticks are mostly useless, because they are invariably used in frames that are themselves moving.

    But why would you try to use the ship's sticks to measure the velocity of the ship?
    I've never said I could do that, or rather I said that the result of such a measurement will always be zero. You're got a wrong idea here.

    You admittedly, along with every other scientist in the past, said that it was impossible to measure the ship's velocity in space. I have given you a way to do that, and you are denying it.
    No. I haven't denied that your method works just fine if you apply it in any given reference frame. Inside the ship, it gives the correct speed - zero - because the light travel times are the same in both directions in that frame. Outside the ship the travel times will be different and you can calculate the speed of the ship relative to whatever outside frame you want to use.

    You have never managed to identify the "space" frame in an unambiguous way. You can't say that is it the frame in which light travels at 299792458 m/s, because you say that light travels at that speed in every frame. But the only way you can ensure that is to introduce a correction factor based on the speed of the given frame relative to "space", and yet the speed of any frame in "space" is supposedly determined using light. Therefore, attempting to define the "space" frame using light is circular if you also want to maintain the constancy of the speed of light in all frames.

    Einstein doesn't know, and fails to acknowledge the velocity of the ship. I don't! He uses band-aids to cover up his boo-boos. I don't have boo-boos so I need no band-aids.
    Einstein denies that the ship has a single "true" velocity relative to "space" (or "in space", if you prefer). He says that in its own frame it always has zero velocity. In the Earth frame it has some other velocity. In the frame attached to the Andromeda galaxy it has some third velocity.

    Einstein asserts that the speed of light will have the same measured speed in all frames, which is why we are free to define that speed in the first place.

    Importantly, Einstein's assertions are in accordance with all available experimental evidence. Your assertions are not.

    The speed of light is not measured compared to the ship's sticks. You are mistaking if you think you measure the speed of light against the sticks without first knowing the velocity and taking that into account. I've given you a way to do that. Use it!
    So to measure the speed of light I need to measure (1) how far light travels in a frame, (2) how long it takes to do so, and (3) how fast the frame is moving and in what direction. And to work out (3) I need to measure at least 6 one-way light travel times (2 for each orthogonal direction in space).

    Einstein, on the other hand, says if I want to measure the speed of light I simply measure (1) how far it travels and (2) how long it takes, and we're done.

    Quote Originally Posted by Motor Daddy
    The speed of light is always the same, but [Einstein] is wrong to assume you measure the speed of light compared to a ship of unknown velocity. He therefore needs to use band-aids to cover his bad theory.
    Einstein says that to measure the speed of anything in a given frame you use the same method (1) see how far it goes, relative to the sticks in the frame, (2) see how long it takes, according to clocks in the frame, (3) divide (1) by (2) to get the answer. The same method is applied to object speeds and light speeds.

    You, on the other hand, say you can't measure the speed of light in a particular frame without first knowing in advance the speed of the frame. And to know that you first have to assume a defined speed of light and make a bunch of other measurements. I'm not sure if you have different rules for measuring the speeds of other objects in a given frame. It seems to me that previously we agreed that speeds of other objects can be measured using Einstein's method in any frame. So maybe only light is special for you. That introduces an asymmetry in your theory that Einstein does not have in his.

    Quote Originally Posted by Motor Daddy
    Quote Originally Posted by James R
    But surely "space" is a frame just like every other?
    Space is the ultimate frame. All objects are in motion in space. I can tell you that motion, Einstein can't!
    Ok. What's the velocity of the Sun through "space"? Or the Earth, if you prefer. Or the Milky Way?

    Can you tell me any of these things?

    Or is it only in principle that you can tell me these things, and not in reality?

  13. #873
    Valued Senior Member Motor Daddy's Avatar
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    Quote Originally Posted by James R View Post
    Motor Daddy:



    When you say "the ship moves relative to light", you're not using the reference frame terminology that I thought we agreed on earlier. To say that the ship moves "relative to light" introduces a picture by which a bunch of sticks is stationary with respect to the light, and the ship's velocity is measured relative to those sticks.



    As soon as you say "light moves at speed c", you're automatically talking about a frame other than one in which the individual photons of light maintain constant positions relative to the measuring sticks.

    Apart from this point, I understand what you're saying, though.


    When you send a light signal from the rear of the ship to the front, you are not trying to measure the speed of light in reference to the ship's sticks, you are measuring the TIME it takes light to get from the rear to the front. From that time you know everything, including the velocity of the ship and length of the ship. The speed of light is already defined, so all you are doing is measuring the TIME of light travel. Since time is money.... I mean distance , when speaking of light travel, when you time light you automatically know the distance light traveled. The distance light traveled is not in reference to the ship's sticks. The ship's sticks have nothing to do with the measurement of light travel time.



    Quote Originally Posted by James R View Post
    Obviously, in general the length of the ship is NOT determined by the light travel time. The ONLY time that the length of the ship is correctly recorded by light, according to you, is when the ship has zero velocity. If the ship has any other speed, light will have to travel a longer or shorter distance from one end of the ship to the other. Moreover, the ONLY time the rulers nailed to the ship agree with the light travel time is when the ship has zero velocity, according to you. Those rulers were calibrated correctly using light, we assume, so the rulers always measure the correct length of the ship. Light gets it wrong if the ship has a velocity, because we need to factor in the velocity of the ship in order to "correct" the length of the ship when we measure it with light.
    Correct, that is my position on the matter.





    Quote Originally Posted by James R View Post
    I think you misunderstand. I am not using the sticks nailed to the ship to measure the velocity of the ship; I'm using them to measure the velocity of light inside the ship. The velocity of the ship is always zero with respect to the sticks nailed to its floor; I believe we have already agreed on that.
    Yes we already agreed to that, and I stand by that. I think YOU misunderstand. Light has a velocity by definition. If it didn't you wouldn't have the sticks in the first place. You do NOT use the sticks to measure the velocity of light inside the ship. You measure the TIME of light travel from point a to point b, that is all you are measuring. The ship's sticks are irrelevant. The speed of light is not in reference to the sticks nailed to objects, the speed of light is defined, hence you can make a meter stick. You then don't make a stick and nail it to the floor and try to measure the speed of light, not knowing the ship's velocity, get a different answer than the definition of the speed of light, and conclude you need to invent length contraction and or time dilation. That's ridiculous. The only thing you are measuring when you send a light signal is the time of light travel.

    Quote Originally Posted by James R View Post
    You claim I cannot use the sticks nailed to floor of the ship to measure the speed of light. But if those sticks have been properly calibrated using light previously, before they were nailed to the floor and while they had zero velocity, then those sticks all have the same length as the sticks that light lays down. Also, the clocks in the ship all keep time just the same as clocks everywhere else in the universe, according to you.
    Correct.

    Quote Originally Posted by James R View Post
    So why can't I measure the speed of light in the reference frame of the ship? Surely I should be able to measure how far light travels in a given time using my correctly-calibrated sticks on the ship and correctly calibrated clocks on the ship, and get the correct answer for the speed of light inside the ship. Shouldn't I? And that measured speed should naturally match the unchanging, defined speed of light, always, according to you. How could I use correct sticks and clocks and yet calculate a speed of light that is wrong?
    As long as you take into account the ship's velocity, be my guest! Measure the speed of light all you want to. You say you are measuring how far light travels in a given time? No, you are measuring the time of light travel. How far light travels in a given time is defined. Why would you define a meter using light time, then make a meter stick and accelerate it to half the speed of light, and expect light to travel the stick in the same amount of time?

    Quote Originally Posted by James R View Post
    The ONLY way this could happen is if I'm (a) just not allowed, by fiat of Motor Daddy, to measure the speed of light at all, OR (b) the speed of light is ONLY 299792458 m/s in a frame that has zero velocity, so I should expect a "wrong" answer in the ship's frame when it has a velocity.
    B is correct.

    Quote Originally Posted by James R View Post
    If (a), then I'm not clear why I can't measure the speed of light. Apparently I can measure the length of any object other than light, so why can't I measure light? Why does light have to be merely defined? Wouldn't measuring the speed of light be a good way to confirm that I'm using correctly-calibrated rulers and clocks?
    Measure the speed of light all you want, but do it properly. That entails knowing the velocity of the ship. No two ways about it. You must know the velocity of the meter stick before you measure the time it takes light to traverse the stick. You can expect a wrong speed of light for anything other than a zero velocity stick.

    Quote Originally Posted by James R View Post
    If (b), then you have to give up on your claim that the velocity of light is the same in all reference frames, because as I have clearly shown, under your assumptions the speed of light MUST be different in the reference frame of the travelling ship compared to its speed in a ship that has zero velocity.

    This is currently the number 1 thing that is making your theory self-inconsistent. And it could be easily fixed, as far as I can tell. All you have to do is to say that light ONLY has a measured speed of 299792458 m/s when measured in a frame that has zero absolute velocity. Why use Einstein's postulate when (a) you say you disagree with Einstein, and (b) it makes your theory inconsistent?

    B is correct. But do not twist my meaning. Light speed is constant in space. Light always lays the same number of sticks in the same amount of time. However, if one nails sticks to the floor of a ship, accelerates the ship to an unknown velocity, and then proceeds to try to measure the speed of light against the ship's sticks and arrives at something different than 299792458 m/s, and then goes on to claim the speed of light is different in his frame, he is sadly mistaking. The speed of light is always 299792458 m/s. If the velocity of the ship is taken into account, the numbers will add up perfectly, at any velocity. So in short, I do not agree with Einstein's postulate. If one disregards the velocity of the frame and simply measures the time it takes light to traverse the ship, the numbers will not always add up to the speed of light being 299792458 m/s. The error is ignoring the velocity of the ship.



    Quote Originally Posted by James R View Post
    What stops me from measuring it? Nothing. But if I do, I'll calculate a different speed for the light depending on the velocity of the ship through "space". So, what you're really saying is you don't want me to go off and measure the speed of light. You'd rather pretend that it has the same speed in all frames. If I actually measured it in the ship's frame, I'd discover that light doesn't have the "correct" velocity in that frame, so your solution is to forbid me from doing the experiment that proves this aspect of your theory false.
    I'm not forbidding you to do what you want, but don;t take faulty measurements and come back and say light doesn't always travel the same speed. Yes, light always travels the same speed, by definition. If you get some different answer you've done something wrong!



    Quote Originally Posted by James R View Post
    Only at the expense of dispensing with the constancy of the speed of light in all reference frames. As I said, I don't know why you're so keen to hold onto that postulate, which after all is one Einstein introduced, and you say you disagree with him about just about everything else.
    I don't agree with that postulate, but don't twist my words. Light always travels the same speed, by definition. Ignoring the velocity of the ship is a fatal mistake and doesn't mean light doesn't always travel the same speed in space. It means you've calculated improperly.











    Quote Originally Posted by James R View Post
    To use a reference frame, I should not need to add in "hidden" information about the ship's speed in order to calculate the "correct" result for the speed of light. Something is wrong here. Either the ship's frame is not correctly calibrated, or else it does give correct answers but the speed of light is not the same in the ship's frame as it it outside.
    So what you are saying is that you should be able to ignore an object's velocity in space and take measurements using light and arrive at the same speed of light. That you should be able to ignore an object's velocity in space and pretend it never has a velocity in space, effectively saying that you should be able to ignore motion to measure motion. That's absurd!


    Quote Originally Posted by James R View Post
    You often complain that Einstein needs to know the velocity of a frame before he can start doing anything, but it seems to me that in this instance it is you who needs to know the ship's speed in advance in order to "correct" the measurements to give the correct speed of light in the ship's frame.
    Let's back up a minute here. I defined the speed of light by defining the meter using light travel time. I made a meter stick taking into account the velocity of the stick so as to ensure the stick is calibrated properly. You nailed it to the floor of a ship, accelerated the ship to an unknown velocity, and timed light from one end to the other. You arrived at some different answer for the speed of light, and then contrived a length contraction method so that your answer would be correct. You totally ignore your mistake of not knowing the velocity of the stick, and but a band-aid on it and then claim you shouldn't have to know the velocity of the stick, that you should be able to ignore the motion of the stick when measuring the speed of light against the stick. You are out of your mind (figuratively speaking, of course, not you personally)!



    Quote Originally Posted by James R View Post
    If you like, you can assume I'm doing it as a check to make sure that the rulers I nailed to the floor and the clocks I'm carrying on the ship are correctly calibrated so that I can use them to perform correct measurements on other objects.

    You're telling me that before I can do this check, I first need to determine the velocity of the ship using your methods, so I can "correct" all my subsequent measurements made using light.

    Taking that point to its logical conclusion, before I can measure anything, anywhere, I must always first use your method to determine the velocity of the thing I'm standing on. So, I need to measure the velocity of the Earth before constructing any metre sticks using light to measure teh correct length in a metre stick factory, for example.
    If you measure using light, you must take into account the velocity of the object. You can not ignore an object's motion when measuring the speed of light.

    Quote Originally Posted by James R View Post
    What is the velocity of the Earth, by the way? I assume you have this important piece of information to hand.
    I never measured it using my method, have you? What are the one way times? If you give me the one-way times I can tell you.



    Quote Originally Posted by James R View Post
    But all speeds of objects in the ship that I calculate using the ship's sticks will be wrong, won't they? To get the correct speeds I need to add or subtract the ship's velocity to or from all measured speeds. More generally, I'll need to perform a vector addition or subtraction, because the ship may be moving in a different direction than the object I'm trying to measure the speed of inside the ship.
    If you measure the speed of a ball in the ship compared to the sticks on the ship, the measurements will be correct, as the sticks are not moving relative to the ship and you are talking about how far the ball moved compared to the ship.



    Quote Originally Posted by James R View Post
    This is the current crux of our disagreement. You're saying to get the right answer for the speed of light in the ship's frame, I need not only the measurements of distance covered and time taken, but ALSO I need to know the ship's velocity in advance. Whereas Einstein says you can just make the measurements and calculate the result. Done.
    You can not ignore the ship's motion and expect to measure the correct speed of light. Einstein doesn't know, and has no way of knowing the speed of the ship in space. I do! He needs to use band-aids because he can't know the speed of the ship. I don't. Do you not understand the significance of knowing the speed of the ship in space?


    Quote Originally Posted by James R View Post
    Call me perverse for daring to want to measure the speed of light rather than accept a definition, but I wonder what people did with such problems prior to the redefinition of the metre in the 1960s. One wonders how physicists ever got any useful work done, since they couldn't accurately measure anything without having access to your measurement procedure. And since they still don't correct for the Earth's absolute velocity through space when they do experiments in their labs today, all the currect results of physics must be wrong, too.
    They must be!

    You can measure the speed of light all you want, if you get an answer other than 299792458 m/s you are wrong! So why bother, you know what the speed of light is. It doesn't change, by definition.



    Quote Originally Posted by James R View Post
    Well, not really, because the ship is really moving too. The real distance moved by that table can only be determined by light. Right? Metre sticks are mostly useless, because they are invariably used in frames that are themselves moving.
    You can measure the table's velocity in the ship's frame using the clocks and rulers in the ship, or you can measure the real velocity of the table in space using light. If the table is bolted to the floor of the ship, and you measure the table to be traveling in space at x velocity, then the ship is also traveling at x velocity in space.


    Quote Originally Posted by James R View Post
    You have never managed to identify the "space" frame in an unambiguous way. You can't say that is it the frame in which light travels at 299792458 m/s, because you say that light travels at that speed in every frame.
    Light always travels that speed in every frame when the velocity of the frame is taken into account.



    Quote Originally Posted by James R View Post
    But the only way you can ensure that is to introduce a correction factor based on the speed of the given frame relative to "space", and yet the speed of any frame in "space" is supposedly determined using light. Therefore, attempting to define the "space" frame using light is circular if you also want to maintain the constancy of the speed of light in all frames.
    Light defines distance in space. Objects travel in space. How is that circular?



    Quote Originally Posted by James R View Post
    Einstein denies that the ship has a single "true" velocity relative to "space" (or "in space", if you prefer). He says that in its own frame it always has zero velocity. In the Earth frame it has some other velocity. In the frame attached to the Andromeda galaxy it has some third velocity.
    What a genius he was, eh? That's so profound of him to say that a ship has a zero velocity in its own frame! He has no clue how to know the ship's velocity in space, so he has no choice but to say it is zero.

    Quote Originally Posted by James R View Post
    Einstein asserts that the speed of light will have the same measured speed in all frames, which is why we are free to define that speed in the first place.
    The measured speed of light in all frames is not the same if the velocity of the frame isn't taking into account. He uses band-aids to make the speed of light the same in all frames because he doesn't know and has no way of knowing the velocity of the frame. The speed of light is defined prior to taking measurements, as the meter is defined by light, so you have it backwards and your argument is circular..

    Quote Originally Posted by James R View Post
    Importantly, Einstein's assertions are in accordance with all available experimental evidence. Your assertions are not.
    By default, if you take measurements from the Earth disregarding the earth's velocity, your measurements are wrong. Einstein;s method matches perfectly with the incorrect data, as his method fails to acknowledge the velocity of the frame too. A perfect match! Bad theory to match the bad measurements. They both disregard the velocity of the frame so why wouldn't they match?



    Quote Originally Posted by James R View Post
    So to measure the speed of light I need to measure (1) how far light travels in a frame, (2) how long it takes to do so, and (3) how fast the frame is moving and in what direction. And to work out (3) I need to measure at least 6 one-way light travel times (2 for each orthogonal direction in space).
    The only thing you need to measure is the one-way times. That's it! Everything else is a calculation.


    Quote Originally Posted by James R View Post
    Einstein, on the other hand, says if I want to measure the speed of light I simply measure (1) how far it travels and (2) how long it takes, and we're done.
    How does he know how far light travels when he disregards the ship's velocity and uses a faulty sync method?









    Quote Originally Posted by James R View Post
    Ok. What's the velocity of the Sun through "space"? Or the Earth, if you prefer. Or the Milky Way?

    Can you tell me any of these things?

    No, I never measured them. Can you tell me the length of my living room? Certainly not, as you've never measured it.

    Quote Originally Posted by James R View Post
    Or is it only in principle that you can tell me these things, and not in reality?
    In reality, my living room doesn't have a length because you've never measured it.
    Last edited by Motor Daddy; 01-03-11 at 09:21 AM.

  14. #874
    Valued Senior Member Motor Daddy's Avatar
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    James, I think I finally figured out how to nail jello to a wall. Funny thing about force is, the more force you apply to me the more I force back. It's the whole "equal and opposite" thing.

    Do you agree that if a ship has 20 meter sticks nailed from one end of the ship to the other that the ship is 20 meters in length?

  15. #875

  16. #876
    Valued Senior Member Neddy Bate's Avatar
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    Quote Originally Posted by Motor Daddy View Post
    James,

    Do you agree that if a ship has 20 meter sticks nailed from one end of the ship to the other that the ship is 20 meters in length?



    How can you even ask such a basic question? James R has already explained this to you in great detail:


    Quote Originally Posted by James R View Post
    Now, to constuct a reference frame, you lay out your 100 sticks end-to-end along a straight line. You now have a distance scale. If you want the distance from the end of the row of sticks to some other point on the line, you just count how many sticks there are between the end and the point you're interested in.

  17. #877
    Valued Senior Member Neddy Bate's Avatar
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    Quote Originally Posted by Motor Daddy View Post
    By default, if you take measurements from the Earth disregarding the earth's velocity, your measurements are wrong. Einstein;s method matches perfectly with the incorrect data, as his method fails to acknowledge the velocity of the frame too. A perfect match! Bad theory to match the bad measurements. They both disregard the velocity of the frame so why wouldn't they match?
    It sounds like you're saying we should discard a working system in favor of your far more complicated approach. With your approach, we would need to mechanically synchronize clocks before every experiment, because the earth might have moved since the last time we did that experiment. We would have to take "absolute speed measurements" in at least three directions, to try to ascertain the "absolute velocity" of the earth. Then we would have to do the experiment very quickly, lest the earth's "absolute velocity" change too much before we get the experiment done! It sounds like it would be much easier to keep using the existing system which has worked for over 100 years.

    Oh, and here's another reason why we should keep using the existing system. Because as soon as we test your ideas in a laboratory, we find that the one-way times for light signals are always the same in both directions. So there is no need for Motor Daddy physics anyway.

  18. #878
    Valued Senior Member Motor Daddy's Avatar
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    Quote Originally Posted by Neddy Bate View Post
    It sounds like you're saying we should discard a working system in favor of your far more complicated approach. With your approach, we would need to mechanically synchronize clocks before every experiment, because the earth might have moved since the last time we did that experiment. We would have to take "absolute speed measurements" in at least three directions, to try to ascertain the "absolute velocity" of the earth. Then we would have to do the experiment very quickly, lest the earth's "absolute velocity" change too much before we get the experiment done! It sounds like it would be much easier to keep using the existing system which has worked for over 100 years.



    It's a matter of principle. If you admit his principles are wrong and mine are correct then I will admit his method is easier.

    Plus, he can't tell you the velocity of a box in space, no matter how hard he tries. I can!

    Quote Originally Posted by Neddy Bate View Post
    Oh, and here's another reason why we should keep using the existing system. Because as soon as we test your ideas in a laboratory, we find that the one-way times for light signals are always the same in both directions. So there is no need for Motor Daddy physics anyway.
    Simply impossible! You are saying that it doesn't matter how close or far away from a light source you are, it always takes light the same time to reach you. Simply impossible.
    Last edited by Motor Daddy; 01-03-11 at 03:45 PM.

  19. #879
    Valued Senior Member Motor Daddy's Avatar
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    Quote Originally Posted by Neddy Bate View Post


    How can you even ask such a basic question? James R has already explained this to you in great detail:
    I know he agrees but I'd like to hear it from him just for the record, that way he can't say later he never agreed the ship is 20 meters long.

  20. #880
    Motor Daddy:

    When you send a light signal from the rear of the ship to the front, you are not trying to measure the speed of light in reference to the ship's sticks, you are measuring the TIME it takes light to get from the rear to the front. From that time you know everything, including the velocity of the ship and length of the ship.
    Wait! Didn't you say you need TWO measurements of the light travel time to determine the velocity of the ship and its length? You can't do it with just one, can you?

    Quote Originally Posted by Motor Daddy
    Quote Originally Posted by James R
    Obviously, in general the length of the ship is NOT determined by the light travel time. The ONLY time that the length of the ship is correctly recorded by light, according to you, is when the ship has zero velocity. If the ship has any other speed, light will have to travel a longer or shorter distance from one end of the ship to the other. Moreover, the ONLY time the rulers nailed to the ship agree with the light travel time is when the ship has zero velocity, according to you. Those rulers were calibrated correctly using light, we assume, so the rulers always measure the correct length of the ship. Light gets it wrong if the ship has a velocity, because we need to factor in the velocity of the ship in order to "correct" the length of the ship when we measure it with light.
    Correct, that is my position on the matter.
    If that is your position on the matter, then you have given up the notion that light travels at the same speed in every reference frame. What your real position is is that light has a constant speed in the reference frame of "space". In every other frame its measured speed is different. Sure, you can reconcile the "wrong" measurements if you happen to know the speed of the frame you measured in, but that's a correction to the basic measurements.

    Einstein, on the other hand, gets the numbers right without any mucking around with correction factors.

    Yes we already agreed to that, and I stand by that. I think YOU misunderstand. Light has a velocity by definition.
    EVERY velocity is with respect to some reference frame, so as soon as you say "light has a velocity" you've implicitly defined a reference frame of some kind. I say there is only one reference frame in which you believe that light has its defined speed: the reference frame of "space". In any other frame, the measured speed will not equal the defined speed.

    The speed of light is not in reference to the sticks nailed to objects, the speed of light is defined, hence you can make a meter stick. You then don't make a stick and nail it to the floor and try to measure the speed of light, not knowing the ship's velocity, get a different answer than the definition of the speed of light, and conclude you need to invent length contraction and or time dilation. That's ridiculous. The only thing you are measuring when you send a light signal is the time of light travel.
    In fact, you can make a set of sticks, nail it to the floor of your spaceship, make your spaceship go any speed you want, then measure the speed of light either inside or outside the ship and you'll find it has the same speed. That's in Einstein's universe, not Motor Daddy's. It's a completely counter-intuitive idea, but it's correct, as verified by countless experiments.

    Quote Originally Posted by MD
    Quote Originally Posted by JR
    The ONLY way this could happen is if I'm (a) just not allowed, by fiat of Motor Daddy, to measure the speed of light at all, OR (b) the speed of light is ONLY 299792458 m/s in a frame that has zero velocity, so I should expect a "wrong" answer in the ship's frame when it has a velocity.
    B is correct.
    I note that you have now given up on your previous claim that light has the same speed in all reference frames. You have now altered that claim to read "Light has the same speed when you adjust the measured speed according to the absolute velocity of the frame you're measuring it in."

    Given this amended claim, your theory is now clearly distinguished from Einstein's. I'm glad we have got to this point at least. Previously you were using Einstein's language but also arguing against the results that would be implied by that language.

    At some point, if you wish to construct a truly consistent theory of physics, you'll need to somehow reconcile your speed-of-light postulate with the predictions of Maxwell's equations that light actually travels at the same speed in all frames, without any necessity for correcting the speed of the frame. Nobody else has ever managed to do that without using Einstein's spacetime, but maybe you can/have. If you've done this, I'd be very interested to see your derivations.

    B is correct. But do not twist my meaning. Light speed is constant in space. Light always lays the same number of sticks in the same amount of time.
    When you talk about light "laying sticks", those sticks are obviously "left behind" as the light travels. So, those sticks form a reference frame and the photons of light necessarily travel relative to that frame. Your contention, I assume, is that those sticks left behind by the light are absolutely at rest in "space".

    So in short, I do not agree with Einstein's postulate. If one disregards the velocity of the frame and simply measures the time it takes light to traverse the ship, the numbers will not always add up to the speed of light being 299792458 m/s. The error is ignoring the velocity of the ship.
    That's all well and good for you to make that claim. If your theory was correct, then it would be a true claim. However, countless real-world experiments prove that the numbers do "add up" to give the same speed of light in every frame. This is actual experimental evidence that disproves your theory.

    To continue to maintain that you are correct in the face of reams of actual evidence that proves you wrong is rather perverse, wouldn't you say?

    Yes, light always travels the same speed, by definition. If you get some different answer you've done something wrong!
    Your light always travels at the same speed only in the frame of the sticks it leaves behind - the frame of "space". In every other frame, measurements should get the answer "wrong", like you say. You need to correct for the speed of every other frame when you make a measurement, in your system.

    Einstein requires no correction. In Einstein's universe (i.e. the actual one) the measurements always get the speed of light correct in any frame, with nothing extra needed.

    Quote Originally Posted by Motor Daddy
    Quote Originally Posted by James R
    To use a reference frame, I should not need to add in "hidden" information about the ship's speed in order to calculate the "correct" result for the speed of light. Something is wrong here. Either the ship's frame is not correctly calibrated, or else it does give correct answers but the speed of light is not the same in the ship's frame as it it outside.
    So what you are saying is that you should be able to ignore an object's velocity in space and take measurements using light and arrive at the same speed of light. That you should be able to ignore an object's velocity in space and pretend it never has a velocity in space, effectively saying that you should be able to ignore motion to measure motion. That's absurd!
    Yes, that's exactly what I'm saying. And yes, it sounds absurd. I keep saying: it's completely counter-intuitive. It goes against every instinct you have from your observations of motion in your everyday, low-speed life. But countless actual physics experiments confirm that it is true nonetheless.

    Quote Originally Posted by MD
    Let's back up a minute here. I defined the speed of light by defining the meter using light travel time. I made a meter stick taking into account the velocity of the stick so as to ensure the stick is calibrated properly. You nailed it to the floor of a ship, accelerated the ship to an unknown velocity, and timed light from one end to the other. You arrived at some different answer for the speed of light, and then contrived a length contraction method so that your answer would be correct.
    No. In my analysis up to this point, I have been working with your spacetime, not Einstein's. In getting the "wrong" answer for the speed of light in the spaceship frame, I used your assumptions that rods in the spaceship have the same length as ones outside, and that clocks in the spaceship keep time with clocks outside. In the real world, in Einstein's universe, neither of your assumptions is true. Using Einstein's assumptions, when we measure the distance travelled by light and the time taken, then divide one by the other, in any frame, we get the same result for the speed of light, regardless of the "speed of the frame in space".

    If you measure using light, you must take into account the velocity of the object. You can not ignore an object's motion when measuring the speed of light.
    i.e. in the Motor Daddy universe I need to add in correction factors, whereas in Einstein's universe the answers are correct automatically.

    What is the velocity of the Earth, by the way? I assume you have this important piece of information to hand.
    I never measured it using my method, have you? What are the one way times? If you give me the one-way times I can tell you.
    I can't give you an answer, because I don't believe in absolute velocities through "space". You do.

    I can tell you the relative velocity of the Earth, relative to myself, the Sun, the centre of the Milky Way etc.

    I would have thought that it was supremely important, before you do anything else with your theory, to identify the zero-velocity "space" frame. That would be especially important because you need to know the speed of every other frame relative to that before you can do any useful physics in the Motor Daddy universe.

    If you measure the speed of a ball in the ship compared to the sticks on the ship, the measurements will be correct, as the sticks are not moving relative to the ship and you are talking about how far the ball moved compared to the ship.
    So what makes light special? Why don't photons of light behave just like little balls? Do you have a theoretical answer to that question?

    Don't just repeat yourself and say "Light lays it's own sticks; balls do not." I'm asking you what is the physical reason why light lays its own sticks and balls do not. Do you know?

    You can not ignore the ship's motion and expect to measure the correct speed of light.
    Yes, I can. Because I know from Einstein (supported by experiments) that light will have the same measured speed in every frame of reference.

    Call me perverse for daring to want to measure the speed of light rather than accept a definition, but I wonder what people did with such problems prior to the redefinition of the metre in the 1960s. One wonders how physicists ever got any useful work done, since they couldn't accurately measure anything without having access to your measurement procedure. And since they still don't correct for the Earth's absolute velocity through space when they do experiments in their labs today, all the currect results of physics must be wrong, too.
    They must be!
    All I can say to this is that it is funny that physics just seems to ... work! You've got a CD player that plays CDs. You have a microwave oven that works. You may even have a GPS receiver that works. And all of these devices were invented by physicists using Einstein's theories. They all work rather well, and yet the physics they are based on turns out to be completely wrong at its very foundations, you tell us.

    I guess those physicists just hada lot of dumb luck or something.

    Quote Originally Posted by Motor Daddy
    You have never managed to identify the "space" frame in an unambiguous way. You can't say that is it the frame in which light travels at 299792458 m/s, because you say that light travels at that speed in every frame.
    Light always travels that speed in every frame when the velocity of the frame is taken into account.
    But without correction for the velocity of the frame, your theory gets it wrong.

    Quote Originally Posted by MD
    What a genius [Einstein] was, eh? That's so profound of him to say that a ship has a zero velocity in its own frame! He has no clue how to know the ship's velocity in space, so he has no choice but to say it is zero.
    You previously agreed with me (and Einstein) that a ship always has zero velocity in its own frame.

    Do you want to renege on that agreement now?

    By default, if you take measurements from the Earth disregarding the earth's velocity, your measurements are wrong. Einstein;s method matches perfectly with the incorrect data, as his method fails to acknowledge the velocity of the frame too. A perfect match! Bad theory to match the bad measurements. They both disregard the velocity of the frame so why wouldn't they match?
    Interesting. Now you are saying that even though Einstein gets the wrong answers, the answers "match perfectly" with everything else we know. So maybe we can do physics with Einstein, even though his theories are 100% wrong. Is that what you're saying?

    If so, then I'll be quite happy to go on using Einstein's theories, because they are so much simpler than yours. I can make valid measurements of anything in any frame that work just fine using Einstein. If I use Motor Daddy physics, I'll need to constantly introduce correction factors to correct for the "real" velocities of frames. It would be unnecessarily complicated, even if it wasn't just wrong.

    Einstein, on the other hand, says if I want to measure the speed of light I simply measure (1) how far it travels and (2) how long it takes, and we're done.
    How does he know how far light travels when he disregards the ship's velocity and uses a faulty sync method?
    He uses the sticks nailed to the floor to measure the distance in the ship's frame. He uses sticks nailed to the Earht to meaure the distance in the Earth frame. Simple!

    As to "faulty sync method", you haven't mentioned that so far, so I'm not sure what you're talking about.

    ---

    By the way, I've been playing around with your method for calculating lengths using light. I've come up with the following nice formula:



    Here, are the measured light travel times in the two directions, is the speed of light, and is the length between the detectors.

    Notice that this formula gives the length in terms of ONLY the light travel times, and the velocity of the reference frame is not needed.

    This formula is correct both in the Motor Daddy universe and in Einstein's universe. The only difference is that in Einstein's universe, the measured values of and will vary depending on the reference frame of the clocks doing the measuring, whereas in the Motor Daddy universe those values are supposedly fixed.

    In particular, if we do the experiment in the spaceship's frame, then Einstein (and experiment in the real world) tells us that . Therefore, in that frame (ONLY) the formula becomes:


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