# Faster or Slower?

Discussion in 'Physics & Math' started by Motor Daddy, Apr 25, 2022.

1. ### Motor DaddyValued Senior Member

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5,425
Sssssssss,

According to your (Einstein's) concept of measurement, how much faster is B than A in that post?

What is the closing speed between B and A?

How much distance did A travel in 1 second?

How much distance did B travel in 1 second?

3. ### Motor DaddyValued Senior Member

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5,425
The box is a cube at all times in all frames. Look at the coordinates of the z receiver at t=.65. Look at the coordinates of the x receiver at t=1.38 when the light sphere contacts the x receiver.

Now look at the distance the center of the cube traveled at those times.

The coordinates of the receivers at those times verify that the cube is in fact a cube. The receivers are always .5 light seconds in distance from the center of the cube to the reciever, verified by the coordinates and the distance the center traveled in that time period.

Your numbers DO NOT add up. You can not make that diagram with your concept of measurement and have all the distances, times, speeds, coordinates add up. You have to FUDGE FACTOR some BS to make the numbers add up. You have to claim the cube is not a cube in order for the numbers to add up.

If you think you can make the numbers add up, with the coordinates, distances, and times, then SHOW ME!

5. ### James RJust this guy, you know?Staff Member

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38,026

What have you been doing for the past 8 years, since you left sciforums? Not studying any relativity, obviously. Perhaps you could have used your time away from here more productively.

It seems like you're still stuck with all the same misconceptions and misunderstandings you had 8 years ago. What a shame.

Relativity shows us that moving objects contract in length in the direction of motion, as seen from a frame other than their rest frame. Your statement that a cube stays a cube in all frames of reference, even when it is moving at different speeds in different frames is just wrong. It is only a cube in its rest frame.

If x is the direction of motion of the cube and z is the perpendicular direction, and you fire a light pulse at the centre of the cube, then even in a Newtonian/Galilean universe the light will not hit the detectors on the x and z walls of your cube simultaneously. Light moving in the x direction has to "catch up" with the front wall of the cube, which is moving away from the emission point. Light moving in the z direction won't even hit the centre of the wall of the cube that is in the z direction from the centre of the cube, because the centre of that wall will have moved forwards in the x direction as the light travelled.

For a person inside your Newtonian-universe cube, the speed of light would appear to be different in the x and z directions. The observed speed of light for the person inside your Newtonian cube would be $c-v$ in the x direction, where $v$ is the speed of the cube relative to whatever "absolute" frame you imagine that light travels in.

Meanwhile, in our real world, which uses Einsteinian physics, a person inside the cube would see light travel at c towards all 6 faces of the cube. Moreover, the light emitted at the centre of the cube would travel straight towards the centres of those faces, as long as the cube was not accelerating. The light would hit the x and z detectors simultaneously in the cube frame. However, an observer outside the cube, watching it move at constant speed in the x direction, would see the z detector receive light before the x detector. That observer would also see that the edge length of the cube (no longer a cube, but a rectangular prism) is shorter in the x direction than in the z direction, and he or she would measure the speed of light as being c in all directions.

7. ### James RJust this guy, you know?Staff Member

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38,026
You're right that if we use an incorrect diagram, like you do, then we can't make the numbers add up. We have to use a correct diagram that takes into account real-world physics (i.e. relativity).
We have to use correct equations, if that's what you mean. The Lorentz transformations are the correct equations that apply in our relativistic universe. Galileo and Newton's equations work well enough for things that are moving at speeds that are small compared to the speed of light, but in your example the cube is supposed to be moving at a reasonable fraction of light speed, is it not?

Yes, when you're examining the situation from a reference frame other than the rest frame of the cube.

It would certainly be easier for all of us if our universe was nice and Newtonian, with absolute time and space and a nice ether to carry light for us and all that. Unfortunately for you, 100+ years of experiments have shown that our universe is not like that. While you might be content to shut your eyes and pretend it's still the 16th century, the rest of us are realists. We want an accurate description of physical reality, not a convenient fantasy. So we use Einstein.

8. ### Motor DaddyValued Senior Member

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5,425
What direction of motion? You are IN THE CUBE, and you send a light signal from the center to the receivers. The light sphere contacts the receivers at different times. From those times you can CALCULATE the motion of the cube in space.

In the cube there is NO MOTION. The motion is only the cube changing position in space, and the coordinates reflect that change in space.

What you are claiming is that a ball changes shape if you travel real fast past it. That is NONSENSE! The ball does not change shape!

9. ### Motor DaddyValued Senior Member

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5,425
That is a circular argument claiming that we have to use length contraction and time dilation to make the numbers add up, because that is the real world is length contraction and time dilation.

NONSENSE!

There is distance and there is time. Period!

10. ### originHeading towards oblivionValued Senior Member

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11,713
Nope, that just ain't so.
I know (for what ever reason) you do not want relativity to be an accurate model, but the universe doesn't care what you want to be true.

11. ### originHeading towards oblivionValued Senior Member

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11,713
That's nice you have convinced yourself of that, not very insightful for educated people though.

12. ### Motor DaddyValued Senior Member

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5,425
...and James, how about you tell me your numbers in post #56, according to your Einstein method of calculating distance and time?

How much faster is B than A in the treadmill frame in post #56? What is the closing speed? How far did A travel in 1 second? How far did B travel in 1 second? Do your numbers add up?

13. ### James RJust this guy, you know?Staff Member

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38,026
The one you specified in your thought experiment!

The image you linked to had your cube moving. I assumed you were interested in examining the situation from a reference frame that was watching the cube fly past. Otherwise, what's the point of your thought experiment?
In which reference frame?

Your scenario has the light being sent to the 6 faces of the cube from the exact centre of the cube, right? If the receivers are in the centres of all the faces, then they will all detect the light at the same time, in the reference frame of the cube. Why would they not?

In a frame that sees the cube moving, of course the light will reach different faces at different times. Some faces will be moving away from the light emission point while others move towards it.
If you're outside the cube, yes.
The light is in motion. The cube walls don't move.
Which coordinates? The ones in which the cube is moving, it seems.

You're very lax about specifying whose coordinates you're using, and about specifying which reference frame you're talking about. If you want to talk about relativity, you need to be specific, because observers in different frames use different coordinate systems.
100+ years of experiments prove you wrong, I'm sorry to tell you. Apparently, the universe doesn't care what you consider nonsense. It just does what it does.
It would be circular if we were talking about an imaginary theory that hasn't been tested. But we've thoroughly tested relativity over 100+ years. Relativity is the only theory that can explain the experimental results we see. Newtonian physics is dead. Sorry to break it to you.

14. ### Motor DaddyValued Senior Member

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5,425
The cube moving was CALCULATED from the times to the receivers. It was not known before those times were measured.

I am in a box.
In the box there is no motion.
I send a light signal from the center of the box.
The light signal contacts the receivers in a measured time in the box.
From the measured times in the box I calculate the motion of the box in space.

There is no outside observer. There is simply me in a box. My meter stick measures the distance from center to all the receivers as being the same amount of meters.
In the box the distance is EXACTLY the same from the center to the z receiver, and the center to the x receiver. There is no difference in distance. There is no "length contraction", there is simply the same distance from the center to each receiver.

In order for Einstein's second postulate to be correct, the light from the center of the box must hit the receivers in the same amount of time, IN THE BOX. The receivers show that the time is DIFFERENT. So the measured speed of light is different IN THE BOX!

OUTSIDE the box the light sphere has a radius, and that radius increases at the rate of 299,792,458 m/s in ALL directions!

15. ### James RJust this guy, you know?Staff Member

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38,026
Ssssssss calculated th numbers for you in post #77. There's no need for me to repeat the calculations.
Well, let's see.
What do you mean by "closing speed"? It looks like A and B are getting further apart in your diagram. They are further apart at t=1 than at t=0, on your diagram.

It looks like A and B separated by 8 ft in one second, which would make the speed of A relative to B 8 ft/sec. The speed of B relative to A is the same.
Okay.

Speed of A relative to treadmill = 2 feet/sec.
Speed of B relative to treadmill = 10 feet/sec.

I assume that A and B are both travelling in the same direction relative to the treadmill. Is that correct?

Then the Speed of B relative to A is 8 feet/sec, and the speed of A relative to B is 8 feet/sec, too.

It might be clearer to talk about velocities instead of speeds, because that way we can use signs to denote the directions of travel. Maybe you should do that in future posts.
Obviously not me. Maybe somebody else. I haven't seen this until now.
You just told me the speed of A compared to the treadmill frame. You said it is 2 feet per second.

What's this "closing speed" you're referring to?
Yes. It looks that way. So what?
That all seems fine. Is there some problem?

16. ### originHeading towards oblivionValued Senior Member

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11,713
No matter how many times you say that it will still be wrong. Do you think that there is some magic number of times that repeating a falsehood will make it true?

17. ### James RJust this guy, you know?Staff Member

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38,026
Okay. So the times were measured by whom, exactly? The person travelling with the cube, or the person watching the cube fly past? I get the impression is was the person watching the cube fly past. Am I right?
Apart from the motion of the light, you mean? Okay.
Okay. All those times, measuring in the box by an observer who is stationary in the box, are the same.
How could you do that, given a set of identical times measured in the box?
Okay.
Yes.
Yes. So the light travels equal distances in equal times and hits the x and z receivers at exactly the same time. How could it not?
Yes. Good. We agree!
Why would they show that the times are different? You just walked me through it and explained correctly why they would be the same.

What's all this about different times, measured by "simply you", in the box with no outside observer? Where are you getting the different times from?
Do you agree that both an observer inside the box and an observer outside the box see the same speed of light in all direction, then?

Because that's Einstein's postulate: the speed of light is independent of the motion of the source.

If you are now accepting that postulate, then you and I are in complete agreement.

Great!

18. ### Motor DaddyValued Senior Member

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5,425
That is closing speed, which is the change in distance between two objects over a duration of time. There is simply 2 different objects with a distance between them at different points in time. At t=0 the distance was 2 ft. At t=1 the distance was 10 feet, so the distance changed 8 feet in 1 second. There is no "length contraction", regardless of the rate of change, and there is no "time dilation" regardless of the rate of change. The closing speed is NECESSARILY 8 ft/sec.

So we've established that there is a change in distance between A and B over a duration of time. The distance is THE SAME for both objects, and the time is THE SAME for both objects. Good so far?

19. ### Motor DaddyValued Senior Member

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5,425
No you are not right. So your whole response is wrong.

Again, I am in the box. I measured the times in the box with my one clock. I measured the distance to each receiver with my one meter stick. The distances were the same to each receiver from the center. The times were DIFFERENT to the different receivers. So the speed of light is different to the different receivers.

There is NO outside observer. It is simply me doing measurements in a box that resides in space. There is no outside observer.

20. ### James RJust this guy, you know?Staff Member

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38,026
It's called "closing speed" even if they are getting further apart, rather than "closing in" on one another? (??) I'd be more inclined to just call it "relative speed".
That all seems fine. I don't know why you introduced length contraction and time dilation there. You're only talking about a relative speed, aren't you?
All good.
Which distance? And which time?

A and B in your example are each travelling at different speeds. Therefore, they won't measure equal the same distance between, say, two fixed lines drawn on the ground. That's due to length contraction. Also, clocks that they carry with them will run at different speeds, as seen by an observer on the ground/treadmill. That's time dilation.

Is there a problem?

21. ### Motor DaddyValued Senior Member

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5,425
No! You have just mixed frames! There is simply 2 different objects, and there is a distance between them. The objects themselves do not have a speed. There is simply the changing distance between the two objects, and a measure of time of change of that distance change.

You are trying to claim that A has a speed compared to B, as if B was motionless and A has all the speed? You are trying to claim B has a speed compared to A, as if A is motionless and B has all the speed?

You are claiming the red car can claim to be doing 0 MPH and the blue car doing 150 MPH. That was proven FALSE by the radar gun!

22. ### James RJust this guy, you know?Staff Member

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38,026
I'm sorry to hear that. Where exactly did I go wrong? I thought we were in agreement.
That's all fine, like I said previously. We agree.
How could they be different? You just said the distances were the same. Are you saying you think the speed of light in the box is not the same towards each receiver? Because the calculation is simple: distance = velocity * time. If the distance and velocity are both the same, then the time must be the same as well. The only way you can get different times for the same distance is if the velocity somehow changes.
Yes. Fine. That would be a correct analysis of your thought experiment.

Unfortunately, though, your thought experiment makes no sense. Why would the speed of light in your box be different in different directions? Is there something else going on with your box that you didn't tell me about?
Yes, you said that. So we have an isolated box in its own little universe somewhere, and you inside doing some measurements. For some unexplained reason, you're telling me that in this universe of yours, light doesn't have the same speed in all directions. Why is that?

23. ### Motor DaddyValued Senior Member

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5,425
You went wrong by claiming the measurements were taken outside the box. They were not. The measured distances and times are from inside the box. I simply measured the distance and times, calculated the speed of the box in space, and drew a diagram to reflect my findings. There is no other person with a different watch and different ruler making measurements. There is ME in the box, taking measurements and reporting my findings.

They are different because the box is in motion in space, and light travels INDEPENDENTLY of the box. Light travels in space. The box also travels in space. They travel independently of each other in space.

The velocity of the box in space is independent of the speed of light. Your mistake is to claim that light always takes the same amount of time to travel from the center to the receivers in the box. That is not the case, because light speed is independent of the velocity of the box in space.