# Measuring Light Velocity Correctly: Thought Experiment

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• Magatron
In summary, the experiment demonstrated that it is possible to perceive light as travelling faster than 300,000 km/s but that it's an illusion, it never really traveled that fast.
Magatron
TL;DR Summary
A way of viewing light velocity measure which does not require time and/or length manipulation but has limitations
Let there be a track 450,000 km long and a rocket 300,000 km long with a laser attached to the bottom of it's back end with a clock beside it, and a second synchronized clock attached to bottom of its front end. Both clocks were also synchronized with a track clock while the rocket was parked there before the experiment. All observers agreed that the track was 1.5 light seconds long and the rocket 1 light second long by those three clocks.

The rocket travels left to right at a velocity of 150,000 km/s along a path which is directly vertically above and parallel to the track and when it reaches the point where its back end is at the left end of the track it fires the laser while recording the time on the back clock. When the rocket has traveled 150,000 km to the right, its front end will be at the right end of the track, its back end at the 150,000 km mark, and the light beam will have reached the front clock, stopping it at that time and also triggering a device at the right end of the track to show what the time on that clock was and that the front end of the rocket had been at the right end of the track at the same time.

From the track frame, it would seem as if the light beam had traveled 450,000 km, the full length of the track, in 1 second, while from the rocket frame it would seem that the same beam had only traveled 300,000 km, the length of the rocket, in 1 second. It would appear to an observer in the track frame that the light beam had traveled at 450,000 km/s, 50% faster than normal light speed.

Plugging the numbers into a Lorentz transformation calculator indicates that to prevent that from being the perceived outcome the length of the rocket, with v= 0.5 c, should be contracted to 0.8660254037844386 of 300,000 km= 259807.62113533158 km, and that its time should be dilated to 1.1547005383792515 normal. We know that the time of 1 second had been recorded on the rocket clock and it copied that figure on the right end of the track, so we know its time wasn't dilated. We also know that the rocket's front end had been traveling for 1 second at 150,000 km/s so we know it didn't travel farther than 150,000 km of track, so its length hadn't been contracted.

The Lorentz transformation isn't fooling us, we know what actually happened and that neither time nor length had really been altered in any way, therefore, we know that it is in fact possible to perceive light as traveling faster than 300,000 km/s but we also know that it's an illusion, it never really traveled that fast, we just had to measure the speed of light in the same frame that it was generated in and not from another frame in relative motion to it, because the result would not be the true velocity, it would be a trick of perspective.

From the perspective of the rocket, we saw the track move 150,000 km under us in 1 second, exactly as we should. Light velocity seemed normal, because we didn't measure it relative to the track, we measured it from inside the rocket, based on things also inside the rocket, and found that it traveled 300,000 km in 1 second, exactly as it should. We did the correct thing, we measured the velocity of light in the same frame which it had been generated in because we could not have measured it from the track frame, or based on things in the track frame, and obtained the correct reading. You could calculate the correct reading from the track perspective but you would need to use only things located inside the rocket for the calculation, which would be complicated and you would need information about the things in the rocket so why even attempt it? Why would you ever need to know the speed of light in another frame?

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I can't help thinking there are some practical difficulties with that experimental setup.

Magatron said:
second synchronized clock
Synchronised in which frame? You appear to be implicitly assuming that it's both, because otherwise your claim that "[f]rom the track frame, it would seem as if the light beam had traveled 450,000 km, the full length of the track, in 1 second" makes no sense. But clocks can't be synchronised in both frames.

FactChecker
I was going to draw a Minkowski diagram of this when I noticed that there's another confusion in that the track is defined to be 1.5 light seconds long, and you seem to assume that this is true in both the track and rocket frames.

I think you really need to get hold of a decent book on relativity.

Ibix said:
Synchronised in which frame? You appear to be implicitly assuming that it's both, because otherwise your claim that "[f]rom the track frame, it would seem as if the light beam had traveled 450,000 km, the full length of the track, in 1 second" makes no sense. But clocks can't be synchronised in both frames.
They were synchronized with the track clock while the rocket was parked near the track before the experiment.

Ibix said:
I was going to draw a Minkowski diagram of this when I noticed that there's another confusion in that the track is defined to be 1.5 light seconds long, and you seem to assume that this is true in both the track and rocket frames.

I think you really need to get hold of a decent book on relativity.
In the track frame when the rocket was parked there before the experiment and its clocks were synchronized with the track clock, the track was agreed by all to be 1.5 light seconds long and the rocket 1 light second long.

Magatron said:
The Lorentz transformation isn't fooling us, we know what actually happened and that neither time nor length had really been altered in any way, therefore, we know that it is in fact possible to perceive light as traveling faster than 300,000 km/s but we also know that it's an illusion, it never really traveled that fast, we just had to measure the speed of light in the same frame that it was generated in and not from another frame in relative motion to it, because the result would not be the true velocity, it would be a trick of perspective.
This is completely wrong. The speed of light in vacuum is independent of the source (second postulate of SR).

PeroK said:
This is completely wrong. The speed of light in vacuum is independent of the source (second postulate of SR).
Or IS it? That was just Einstein's postulate, he merely postulated it. Definition of "postulate": suggest or assume the existence, fact, or truth of (something) as a basis for reasoning, discussion, or belief.

Motore, Dale and weirdoguy
Magatron said:
Or IS it?
Yes. Yours is a common error when a student first encounters SR. We've seen the same error on a hundred different posts on here! It's nothing new.

PeroK said:
Yes. Yours is a common error when a student first encounters SR. We've seen the same error on a hundred different posts on here! It's nothing new.
So explain how it's in error. All I saw was you saying it's an error, I could say that about anything.

Magatron said:
So explain how it's in error. All I saw was you saying it's an error, I could say that about anything.
Simultaneity is relative. SR is not just time dilation and length contraction. When the rocket reaches a significant speed relative to the ground, then the ground clocks are no longer synchronised in the rocket's frame of reference.

That was the point that @Ibix was making.

Dale
Magatron said:
(...)we measured the velocity of light in the same frame which it had been generated(...)

Light was genereated in all frames. Everything happens in all frames - that's the basic fact about frames of reference built in their definition. I guess you have a misunderstanding about that, so tell us what do you think frame of reference is?

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Dale
weirdoguy said:
Light was genereated in all frames. Everything happens in all frames - that's the basic fact about frames of reference built in their definition. I guess you have a misunderstanding about that, so tell us what do you think frame of reference is?
Light was not generated in all frames, the laser was only in the rocket's frame. You could isolate the rocket frame and the beam would still be seen to exist. If you instead isolated the track frame, the beam would not be seen to exist, that's the difference. From the track frame, it appears that the rocket frame is dragging the beam with it, adding its velocity to it, but it's an illusion because in reality it is simply acting exactly as it would if the rocket were not moving at all? Why? Because uniform motion is exactly the same as no motion at all, as far as light is concerned.

weirdoguy
Magatron said:
Light was not generated in all frames

It was, from the very definition of reference frame. You really should work with some textbook, because you have a lot of misunderstandings.

PeroK
Magatron said:
Light was not generated in all frames, the laser was only in the rocket's frame. You could isolate the rocket frame and the beam would still be seen to exist. If you instead isolated the track frame, the beam would not be seen to exist, that's the difference. From the track frame, it appears that the rocket frame is dragging the beam with it, adding its velocity to it, but it's an illusion because in reality it is simply acting exactly as it would if the rocket were not moving at all? Why? Because uniform motion is exactly the same as no motion at all, as far as light is concerned.
Sadly, this is all wrong. Your misunderstanding of reference frames and the basis of physics is quite fundamental. There is no point in posting this complete misunderstanding of SR.

You may ask questions about SR if you want to study it.

weirdoguy said:
It was, from the very definition of reference frame. You really should work with some textbook, because you have a lot of misunderstandings.
Whichever frame the light beam can be seen exist in if you remove all other frames is the only one that the speed of light is SEEN to be constant in, Einstein was simply wrong. For all intents, the light beam does not even exist in any other frame, it's like having a video of a light beam taken in another frame and moving the video player around, the light beam is not really moving faster because the video player is moving, it just looks like it's moving faster.

Dale and weirdoguy
Sigh... Again - can you give us your definition of reference frame? It's wrong, but it can be a basis for further discussion.

weirdoguy said:
Sigh... Again - can you give us your definition of reference frame? It's wrong, but it can be a basis for further discussion.
I wouldn't worry about it. The OP is living on borrowed time!

PeroK said:
I wouldn't worry about it. The OP is living on borrowed time!
And why would that be? Did I violate a forum rule not to write anything that disagrees with Einstein's postulates?

Magatron said:
Did I violate a forum rule not to write anything that disagrees with Einstein's postulates?

No, you are not listening to what others are saying, and it seems that you are not here to learn. Just to force one us your view of things, which of course is wrong.

weirdoguy said:
Sigh... Again - can you give us your definition of reference frame? It's wrong, but it can be a basis for further discussion.
Okay, a frame in which a light beam can be seen to exist when all frames in relative motion to it are removed. Anything else?

weirdoguy
weirdoguy said:
No, you are not listening to what others are saying, and it seems that you are not here to learn. Just to force one us your view of things, which of course is wrong.
Oh, is this a school? I mistook it for a physics forum. Do threads get closed if nobody can refute the author's conclusion using logic and reason rather than somebody's "postulates"?

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weirdoguy
This thread is currently under review.

Magatron said:
Okay, a frame in which a light beam can be seen to exist when all frames in relative motion to it are removed. Anything else?

Again - general definition of a reference frame. What you said can't be treated as that.

Magatron said:
Or IS it? That was just Einstein's postulate, he merely postulated it. Definition of "postulate": suggest or assume the existence, fact, or truth of (something) as a basis for reasoning, discussion, or belief.
It has been experimentally confirmed many times since then. As Robertson showed about 60 years ago, we no longer need the postulates. We can simply use experimental results.

PeterDonis and Motore
Magatron said:
Light was not generated in all frames, the laser was only in the rocket's frame.
This is not correct. A reference frame is just a coordinate system (in one common usage). The laser was generated. You can identify the event of the generation of the laser in all coordinate systems. So indeed the laser was generated in all frames.

cianfa72
Magatron said:
Do threads get closed if nobody can refute the author's conclusion using logic and reason

I'm sorry but Universe does not care about your, or mine, "logic". Especially when you don't even know the very basics of theory you are refuting.

Dale
Magatron said:
And why would that be? Did I violate a forum rule not to write anything that disagrees with Einstein's postulates?
Actually, yes, you did. We are here to teach mainstream physics as it is practiced by professional scientists. All posts must be consistent with the professional scientific literature, which includes Einstein’s postulates.

As such, this thread is now closed. You are welcome to post another thread in a similar vein provided you keep it more focused on standard physics. Disputing well-validated science is off topic everywhere on PF.

Dale said:
All posts must be consistent with the professional scientific literature, which includes Einstein’s postulates.
But even more than that, as you pointed out in an earlier post, we have known for decades experimentally that the speed of light is observed to be the same in all reference frames. So it's not a postulate any more, it's an experimental fact. And all posts here must be consistent with known experimental facts.

Dale

## 1. How is light velocity measured?

Light velocity is measured by using the equation c = λν, where c is the speed of light, λ is the wavelength, and ν is the frequency. This equation is derived from the fundamental relationship between the speed of light and the properties of electromagnetic waves.

## 2. What is a thought experiment?

A thought experiment is a mental exercise that explores a hypothetical situation or scenario in order to test a theory or concept. It involves using logic and reasoning to analyze and understand a particular concept or phenomenon.

## 3. Why is it important to measure light velocity correctly?

Measuring light velocity correctly is important because it is a fundamental constant in physics and plays a crucial role in understanding the behavior of light and other electromagnetic waves. It also has practical applications in fields such as astronomy, telecommunications, and navigation.

## 4. Can light velocity be measured in a vacuum?

Yes, light velocity can be measured in a vacuum. In fact, the speed of light is defined as the speed at which light travels in a vacuum. This is because a vacuum has no particles or matter to slow down the speed of light.

## 5. How does the thought experiment for measuring light velocity work?

The thought experiment for measuring light velocity involves a hypothetical scenario in which a person is standing on a moving train and shining a flashlight towards a mirror at the other end of the train. By measuring the time it takes for the light to travel to the mirror and back, and taking into account the speed of the train, one can calculate the speed of light. This thought experiment helps to illustrate the concept of relative velocity and how it affects the measurement of light velocity.

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