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

1. Originally Posted by arfa brane
No, light travels independently at a constant velocity. It takes the same time to reach the front clock as it takes for light reflected from that clock to reach the rear clock.

This is easily demonstrated: set up a source of light that 'sends' a beam simultaneously towards both clocks, after positioning it midway between the clocks. The light will reach both the clocks simultaneously, regardless of the train's velocity.
If the train has a velocity, and you send light from the midpoint in the train towards each end, the light will certainly NOT reach each end in the same amount of light travel time. Certainly NOT! Light has to travel a greater distance to reach the front than it does the rear, if the train has a true velocity towards the front. How can you say light has to travel the same distance? If you are 200' from a lamp post, and you start running away from the lamp when the lamp starts to emit light towards you, the light has to travel greater than 200' before it reaches you. Likewise, if you are 200' from a lamp post and you start running towards the lamp when the lamp emits light, the light doesn't have to travel 200' to reach you. Do you deny that?

Originally Posted by arfa brane
Or use a device that sends two balls in opposite directions simultaneously and at the same speed. The balls will also reach the clocks at the same time after traveling the same distance, equal to half the distance between the clocks, regardless of the train's velocity.
So you release the balls at the same time towards each end. Do the balls travel the same distance in space to reach each end? One end is traveling towards the ball and the other end is traveling away. You are very confused about measurements.

2. md #761

This argument is confusing propagation speed in space and apparent speed as determined by an observer.

arfa brane
No, light travels independently at a constant velocity. It takes the same time to reach the front clock as it takes for light reflected from that clock to reach the rear clock.

This is easily demonstrated: set up a source of light that 'sends' a beam simultaneously towards both clocks, after positioning it midway between the clocks. The light will reach both the clocks simultaneously, regardless of the train's velocity.
Correct , but only because the clocks are synched giving the perception of equal transit times, i.e., an equivalent fixed frame.

md
If the train has a velocity, and you send light from the midpoint in the train towards each end, the light will certainly NOT reach each end in the same amount of light travel time....
Correct because propagation speed is constant.

arfa brane
Or use a device that sends two balls in opposite directions simultaneously and at the same speed. The balls will also reach the clocks at the same time after traveling the same distance, equal to half the distance between the clocks, regardless of the train's velocity.
md
So you release the balls at the same time towards each end. Do the balls travel the same distance in space to reach each end? One end is traveling towards the ball and the other end is traveling away. You are very confused about measurements.
Wrong, the balls aquire the speed of the train, in contrast to light
propagation.

3. I feel a strange mixture of pride and shame for starting such a wayward monster thread.

4. Originally Posted by phyti
Correct , but only because the clocks are synched giving the perception of equal transit times, i.e., an equivalent fixed frame.
What if the clocks aren't synchronized? Or if instead of clocks you have two photodetectors and electronic timers?

What happens is, the beams of light take the same amount of time to reach both detectors from a midpoint. This is because the path-lengths don't change. If the speed of light is constant and the laws of physics are the same at the rear and the front of the train, this is the only possible outcome when the source and the receivers are in a "stationary"frame. It isn't the outcome for an observer outside the train unless they have the same velocity as the train.

5. Originally Posted by Motor Daddy
If the train has a velocity, and you send light from the midpoint in the train towards each end, the light will certainly NOT reach each end in the same amount of light travel time.
False. In the frame of the train, it will. In any other frame in motion wrt the train, it won't.
Since you don't understand the basic notion of "frame" you will never understand the above.

6. Originally Posted by RJBeery
I feel a strange mixture of pride and shame for starting such a wayward monster thread.
To see the humor in it all.

It seems like after you started it, there was a hijacking followed by a train wreck!

7. Originally Posted by arfa brane
What if the clocks aren't synchronized? Or if instead of clocks you have two photodetectors and electronic timers?
The clocks will remain earth synched clocks, and despite arriving simultaneously, the signals will show different times.

What happens is, the beams of light take the same amount of time to reach both detectors from a midpoint. This is because the path-lengths don't change. If the speed of light is constant and the laws of physics are the same at the rear and the front of the train, this is the only possible outcome when the source and the receivers are in a "stationary"frame. It isn't the outcome for an observer outside the train unless they have the same velocity as the train.
The hermann (spacetime diagram) will show a parallelogram, 1 long segment + 1 short segment in one direction, and the reverse order in the other direction. Obviously the total path lengths are equal, which is not in question. The uncertainty is where and when the reflection event occurs because there is no known method for determining this. Einstein defines the inbound and outbound paths as equal for three reasons.
1. The round trip times will be equal, thus no need to know the details of the reflection event.
2. The observer can calculate the distance as if he is not moving, thus maintaining his perception of being in a rest frame.
3. He knows using the round trip time will inflate the distance and by how much.
[x'=g(x-vt) with (x-vt)=distance to clock]

8. Originally Posted by Tach
False. In the frame of the train, it will. In any other frame in motion wrt the train, it won't.
Since you don't understand the basic notion of "frame" you will never understand the above.

Light doesn't travel in the train frame, light travel is independent of frames.

Say there is a constant light traveling in space. Without measuring that light I can tell you how far it traveled in 1 sec, because by definition it always travels the same distance in the same time.

Now, you come along and say that a train has a velocity. Let's stop right there. How did you determine that the train has a velocity?

You are making the same mistake Einstein did. You assume the train has a velocity compared to the tracks, but you don't know the velocity of the tracks compared to light travel. Then you go on to try and prove there is no absolute simultaneity using light travel. You don't know the track's velocity compared to light, so you don't know the train's velocity compared to light, so how can you say the train has a velocity compared to light?

Who cares if the train has a velocity compared to the tracks? Not me, as I am going to tell you if the train has a velocity compared to LIGHT, which is an absolute velocity. The only way the train's absolute velocity will be the same as the train's velocity compared to the tracks is if the tracks have an absolute zero velocity compared to light. You are mixing apples and oranges and then claiming I am wrong. You don't understand what measuring a velocity using light means, because you have never done that, because it has never been done before.

9. Originally Posted by Motor Daddy
Light doesn't travel in the train frame, light travel is independent of frames.
I told you that you don't understand the notion of "frame", thank you for proving it so promptly.

You are making the same mistake Einstein did.
Thank you, I am flattered.

You still haven't answered a couple of my questions.

Originally Posted by James R
Also, while you're answering this, please clarify that you do in fact agree with Einstein's speed-of-light postulate, because you seem to flip-flop back and forth on that one in your posts.
I've repeated over and over, the speed of light is a constant because we define it that way. It is a constant by definition. You can not change the speed of light unless you change the definition of a meter to reflect a different amount of light travel time, or, you change the definition of a second. It is as simple as that, the speed of light is DEFINED, not measured.
Note that this doesn't answer my question, which was:

Do you agree with Einstein's speed-of-light posulate?

It says that the speed of light is the same in all inertial reference frames.

Do you agree with it or not?

Originally Posted by James R
The other postulate of special relativity is that the laws of physics take the same form in all inertial frames. I assume you understand what that postulate means. If not, I can explain it to you.

Do you agree with the second postulate? If not, which laws of physics change between different reference frames, and how do they need to be changed?

I'd like you to consider a particular scenario I came up with you to illustrate the problem with your idea of absolute velocity.

Consider the universe's longest spaceship - a spaceship that is 1 light-second long. (Note: one light-second is the distance light travels in 1 second, or 299792458 metres.)

This spaceship happens to be travelling through space at a speed of c/2, relative to Motor Daddy's posulated "absolute zero speed" reference frame. Relative to other frames, its speed would be different, but I am assuming that it just happens to have this speed relative to the absolute-zero frame. Note that c is the "absolute" speed of light = 299792458 m/s.

Now, somebody on the spaceship sends a signal from the rear end of the spaceship to the front end.

According to Motor Daddy's theory of physics, that signal must travel at speed c in ALL reference frames, because Motor Daddy says the speed of light, c, is DEFINED to be constant, and that is true in ALL frames.

How long does it take the light pulse to go from one end of the spaceship to the other?

Well, if I understand Motor Daddy's methods of calculation correctly, we must take into account the spaceship's absolute speed in order to work out the travel time of the light. The light travels at speed c along the spaceship, but the spaceship itself is travelling at c/2. So, we substract c/2 from c to get the "closing speed", as Motor Daddy would describe it, which is c/2.

Now, the spaceship has a length of 1 light-second. To calculate the travel time of the light we divide the length by the "closing speed", which gives a travel time of 2 seconds for the light.

So far so good. We appreciate that all these calculations were done correctly in the best available reference frame - Motor Daddy's "absolute zero" reference frame.

So, now let's translate the results to the reference frame of a person moving with the spaceship. According to Motor Daddy, in that frame the spaceship still has length of 1 light-second. From the previous calculation we know that light takes 2 second to travel the length of the spaceship. So, we are now in a position to calculate the speed of light in the frame of the spaceship.

The speed of light in the spaceship frame is the distance it travels divided by the time taken, which is 1 light-second divided by 2 seconds, which gives a speed of light equal to c/2 in the spaceship frame.

To summarise, we used the "real" speed of light in the absolute frame, and is was c. In the moving frame of the spaceship we found that the relative speed of light was c/2.

Notice that the speed of light is different in the two frames.

Now, Motor Daddy has repeatedly claimed that the speed of light is a defined constant that is the same in all reference frames. You can't change the speed of light, Motor Daddy tells us. It is always 299792458 m/s, in every frame.

But the above example shows that Motor Daddy's methods of calculation are in conflict with his claim that the speed of light doesn't ever change in different reference frames.

----

So, Motor Daddy, here's what I want to know. Which of the following explanations solves the apparent problem:

1. The speed of light is really different in different frames after all.
2. I've made a mistake in my calculations because there is some kind of time dilation or length contraction effect after all.
3. Motor Daddy doesn't really understand what a reference frame is and so insists that we can only ever work in the "absolute zero" frame, and that all other frames give incorrect results. This means we can't do any physics in frames other than the unidentified "absolute zero speed" frame.

12. Originally Posted by phyti
The uncertainty is where and when the reflection event occurs because there is no known method for determining this.
This sentence really is what sums it all up.

My experiment using photodetectors and timers doesn't get past this uncertainty problem because obviously, using timers then means the synchronization of clocks (as timers). Really what SR tells us is that you can only synchronize clocks in a stationary frame, or conversely, clocks are only synchronized if they're in a stationary frame.

There is no way around this; assuming the path-lengths are constant equals assuming a stationary frame exists, determining path-lengths of light using clocks or timers means you have to assume the path-lengths are constant. To get out of the 'circle' you transform coordinates to a non-stationary frame and retain the above as a kind of corollary of c being constant, and that stationary frames exist.

13. Originally Posted by James R

I'd like you to consider a particular scenario I came up with you to illustrate the problem with your idea of absolute velocity.

Consider the universe's longest spaceship - a spaceship that is 1 light-second long. (Note: one light-second is the distance light travels in 1 second, or 299792458 metres.)
How did you determine the length of the ship before you determined the velocity of the ship? What are the one-way times?

Originally Posted by James R
This spaceship happens to be travelling through space at a speed of c/2, relative to Motor Daddy's posulated "absolute zero speed" reference frame. Relative to other frames, its speed would be different, but I am assuming that it just happens to have this speed relative to the absolute-zero frame. Note that c is the "absolute" speed of light = 299792458 m/s.
So if the velocity is .5c, I assume the one way times are not the same in each direction, correct? Since the one-way times are not the same, do you acknowledge that measuring the round trip time of light and dividing by two gets the length wrong?

Originally Posted by James R
Now, somebody on the spaceship sends a signal from the rear end of the spaceship to the front end.
OK, but just remember, you have to measure the one-way times BEFORE you can tell me the speed of the ship or the length between the clocks. You can not tell me any information until you measure the one-way times. Yet you started this scenario with telling me how long the ship is and the velocity of the ship without knowing the one-way times. So again, how did you arrive at those figures without first having measured the one-way times?

Originally Posted by James R
According to Motor Daddy's theory of physics, that signal must travel at speed c in ALL reference frames, because Motor Daddy says the speed of light, c, is DEFINED to be constant, and that is true in ALL frames.
Light travels at c, period. Light always travels 299,792,458 meters per second, by definition. That's how far light travels in space, which may or may not be the same distance between clocks. If the clocks have a zero velocity then the distance between the clocks will match the time it takes light to travel from one clock to the other, and the one-way times will be the same in each direction. If the clocks have a velocity, the distance light travels in space will be more or less than the distance between the clocks. So, if you are measuring light travel time between clocks and you don't know the velocity of the clocks, there is NO way you can accurately determine the distance between the clocks.

Originally Posted by James R
How long does it take the light pulse to go from one end of the spaceship to the other?
You do not calculate light travel times from the length and velocity of the ship, you determine the velocity and the length of the ship by knowing the one-way times. You are basing your idea on a false assumption of the length and velocity of the ship. You can not know those first, you need the one-way times and THEN you can know those.

Originally Posted by James R
Well, if I understand Motor Daddy's methods of calculation correctly, we must take into account the spaceship's absolute speed in order to work out the travel time of the light. The light travels at speed c along the spaceship, but the spaceship itself is travelling at c/2. So, we substract c/2 from c to get the "closing speed", as Motor Daddy would describe it, which is c/2.
Again, what are the one-way times that you measured and determined the ship's velocity and length? You need to know those FIRST.

Originally Posted by James R
Now, the spaceship has a length of 1 light-second. To calculate the travel time of the light we divide the length by the "closing speed", which gives a travel time of 2 seconds for the light.
You do NOT calculate the travel time of light, you measure it. From those times you can calculate the velocity and length. That is why you consistently get these problems wrong, you assume the velocity and length of a ship , and then go on to use light times. Dead wrong!!!

Originally Posted by James R
So far so good. We appreciate that all these calculations were done correctly in the best available reference frame - Motor Daddy's "absolute zero" reference frame.
Anything but good. You made a fatal mistake at the very beginning. The first and only thing you needed to tell me is the one-way times. From that I can tell you everything, and the numbers would add up perfectly.

Originally Posted by James R
So, now let's translate the results to the reference frame of a person moving with the spaceship. According to Motor Daddy, in that frame the spaceship still has length of 1 light-second. From the previous calculation we know that light takes 2 second to travel the length of the spaceship. So, we are now in a position to calculate the speed of light in the frame of the spaceship.
Wrong again. You do not calculate the speed of light, it is defined!

Originally Posted by James R
The speed of light in the spaceship frame is the distance it travels divided by the time taken, which is 1 light-second divided by 2 seconds, which gives a speed of light equal to c/2 in the spaceship frame.
The speed of light is always a constant. The speed of light is not c/2 for any frame. The speed of light is c, by definition, period!

Originally Posted by James R
To summarise, we used the "real" speed of light in the absolute frame, and is was c. In the moving frame of the spaceship we found that the relative speed of light was c/2.
There is no "relative speed of light." The speed of light is the distance light travels in space in a specific time, by definition.

Originally Posted by James R
Notice that the speed of light is different in the two frames.
No, the speed of light is always the same. When you understand that you measure the one-way times FIRST, and then figure out the velocity and length, then you will not make such a mess of an example. Einstein made the same mistake!

Originally Posted by James R
Now, Motor Daddy has repeatedly claimed that the speed of light is a defined constant that is the same in all reference frames. You can't change the speed of light, Motor Daddy tells us. It is always 299792458 m/s, in every frame.
Correct, the speed of light is a constant by definition.

Originally Posted by James R
But the above example shows that Motor Daddy's methods of calculation are in conflict with his claim that the speed of light doesn't ever change in different reference frames.
Wrong again, the only conflict is the mess you have created by ASSUMING the speed of the ship. By ASSUMING the length of the ship, and then going on to try to use light to measure with. Why don't you take my advice and MEASURE the one-way times FIRST, BEFORE you tell me the length and speed of the ship?? If you use my method correctly, you will not make such a mess of a thought experiment. I've given you step by step instructions multiple times in this thread, and yet you continue to start you thought experiments with the speed of the ship (or train). You CAN'T know that until you measure the one-way times!!!!!!

Originally Posted by James R
So, Motor Daddy, here's what I want to know. Which of the following explanations solves the apparent problem:

1. The speed of light is really different in different frames after all.
No, the speed of light is a constant, by definition.

Originally Posted by James R
2. I've made a mistake in my calculations because there is some kind of time dilation or length contraction effect after all.
Absolutely not. YOU are the one that started the thought experiment assuming speed and length, and then got your numbers all mixed up. As I said, tell me the one-way times. That's all I need!

Originally Posted by James R
3. Motor Daddy doesn't really understand what a reference frame is and so insists that we can only ever work in the "absolute zero" frame, and that all other frames give incorrect results. This means we can't do any physics in frames other than the unidentified "absolute zero speed" frame.
I fully understand what a reference frame is. I also understand how to measure one-way times and find the true velocity and length of the ship. Listen to what I am telling you, measure the one-way times and find the velocity and length from those numbers. That is how you measure with light. You don;t assume a velocity and length and then try to use light to calculate light travel time. That's ridiculous!

Originally Posted by James R
I've specified YOUR problem in my response multiple times. If you fail to acknowledge the problem with your methods you will never be able to measure the absolute velocity of an object.

Please answer my questions directly. Stick to this experiment and tell me the one-way times. That's all the information I need. I will give you the actual velocity and length and the numbers will add up with every other object's motion and length in the universe. Guaranteed!

14. Originally Posted by Motor Daddy
How did you determine the length of the ship before you determined the velocity of the ship? What are the one-way times?
Do you know how to use a ruler? How much time does it take you?
Do you know how to look in an owner's manual to find out the length of a car? How do car makers determine this?

You do not calculate the speed of light, it is defined!
?
How is it defined, who or what defines it?? Why don't we calculate the speed of light? What happens if someone does?

15. Originally Posted by arfa brane
Do you know how to use a ruler?
Yes, you make sure it isn't moving in relation to the object you are measuring, and then see what the distance is from point a to point b. Do you understand that if you measured the one-way light travel times from one end of the meter stick to the other, each way, that the times might be different, depending on the motion of the meter stick? You do believe the meter stick can be in motion, correct? Such a simple concept to understand and yet you fail to grasp it.

Originally Posted by arfa brane
How much time does it take you?
It depends, sometimes it takes 3.7 minutes to perform a measurement using a ruler, sometimes it takes 2.9 seconds. How much time does it take you to measure something?

Originally Posted by arfa brane
Do you know how to look in an owner's manual to find out the length of a car? How do car makers determine this?
Yes, do you? They take measurements using meter sticks.

Originally Posted by arfa brane
How is it defined, who or what defines it?? Why don't we calculate the speed of light? What happens if someone does?
It is defined because a meter is defined by light travel time. It is impossible to separate the distance light travels from the time it travels. But you need to understand, it is the distance light travels in space, not the distance between clocks, as the clocks could be in motion.

16. Originally Posted by Motor Daddy
They take measurements using meter sticks.
They would have to be sure though, as you've pointed out, that the "meter sticks" aren't moving, right (or left)? How do they "make sure it isn't moving"?
Do you mean they assume they're in a frame where rulers and car parts are at rest relative to each other, or in car-maker lingo "the parts and the rulers are stationary during measurement"?

It is defined because a meter is defined by light travel time.
The speed of light is metres per second, so it's defined in "light travel time" (in seconds), per second. . .

17. Originally Posted by arfa brane
They would have to be sure though, as you've pointed out, that the "meter sticks" aren't moving right? How do they "make sure it isn't moving"?

Do you mean they assume they're in a frame where rulers and car parts are at rest relative to each other, or in car-maker lingo "the parts and the rulers are stationary during measurement"?

The speed of light is metres per second, so it's defined in "light travel time" (in seconds), per second. . .
Once you've defined a meter you can make a meter stick.

Simply find a good stick, as straight as possible. Place the stick on the ground and measure the one-way times in each direction from end to end. Use my method to find the velocity of the stick and the length of the stick. Once the length is known, hopefully the length is longer than a meter, or exactly a meter (highly doubtful unless you picked a really good stick to start with). If the length is too short you need a longer stick of which you can estimate by knowing the length of the first stick. Say the first stick is only .5 meters long. Now you know you need a stick twice as long as the first stick. So find a stick twice as long, or a little longer. Measure the one-way times again on the new stick. If it is 1.1 meters long, simply chew some off a little at a time, and continue to retest the one-way times. Caution, if you chew off too much at a time you may end up repeating this procedure several times until you finally end up with a true meter, and of course, you know a true meter when you see one using my method, because when you know the one-way times, and know the velocity, you know the true length.

Once you have a true meter stick, make several more by cutting other sticks to the same exact length as the first one. Make a bunch and give some to your friends!!

Now, since you have a meter stick you can measure things without using light!! Caution, if you bring your meter stick on a plane, and measure one meter from point A to point B, and then you use light to check the times between A and B, if the plane is not at an absolute zero velocity, the light travel times will be different each way, because the plane has a velocity.

18. Originally Posted by Motor Daddy
measure the one-way times in each direction from end to end.
If you attach a source of light to one end of the stick and a mirror to the other end, would that work?
How would you measure the times for each end-to-end transition of the light if you did this? Would it still work if you only measured the total transition time, or would that mean assuming the stick's length doesn't change?

If it's a metre, which is a small distance, you will need a fairly accurate clock, won't you? In fact, if you want to be sure about having a stick which is "as close as possble" to a metre length you will need a clock that measures time intervals as accurately as possible.

p.s. a "meter" is something you find inside power distribution boxes on the side of your house, or on gas supply lines.

19. Originally Posted by arfa brane
If you attach a source of light to one end of the stick and a mirror at the other end, would that work?
How would you measure the times for each end-to-end transition of the light if you did this? Would it still work if you only measured the total transition time, or would that mean assuming the stick's length doesn't change?
No, it would absolutely NOT work for anything other than a Zero velocity.

If the object has a velocity, it might take light 2 seconds to travel one way and 6 seconds to travel the other way, resulting in a round trip time of 8 seconds. If the object had a true zero velocity, it could take 4 seconds each way, 8 seconds round trip.

Dividing the round trip time by two averages the two one way times, and gets the length wrong for everything except a true zero velocity. We both know objects can have motion, right?

20. Ok, let's ASSUME the stick has a "true" zero velocity.
Would you measure the LENGTH of the stick if you measured the total time for light to travel from the source to the mirror, and back?