One way speed of light measurement

[The Baron]

TL;DR

I have an idea on how to measure the one-way speed of light.

Hi so to those of you who don't know the problem here is a video that explains it very well. -

I have an idea on how to measure it, can someone please tell me if, and why it is incorrect. Thank you!

Okay so we will have a timer, and a lot of mirrors, set up in a circle, a light beam is fired from the timer and goes in the path of the mirrors in all directions until it eventually returns to the timer and the speed is measured. But here is the catch, the distance between each mirror is different, so if there is a deviation the measurement will give us a speed greater or smaller than the speed of light. And we will either prove the speed of light is different in all directions or prove it's the same.

I am sure there is something wrong with my logic, can you please tell me what it is.

Thank you.

 

[Ibix]

You are measuring a round trip time, so that's a two way measurement. And assuming you use modern SI unit definitions, you're actually calibrating your rulers rather than measuring the speed of light, which is a defined constant these days.

 

[Dale]

We have had many many many threads on this topic. I would recommend reading over them.

The bottom line is that it is not a matter of clever experimental design. There is fundamentally no possible way to measure the one-way speed of light independently of the synchronization convention, because the one-way speed of light is defined based on the synchronization convention. Therefore, if you cleverly measure something that is independent of the synchronization convention then than measurement is not a measurement of the one-way speed of light, and if you cleverly measure the one-way speed of light then by definition it depends on the synchronization convention.

 

[The Baron]

You are measuring a round trip time, so that's a two way measurement. And assuming you use modern SI unit definitions, you're actually calibrating your rulers rather than measuring the speed of light, which is a defined constant these days.

It's a round trip because it returns to the same location, but the distance the light travels is different in each direction, so if the speed of light is different In a certain direction we will see it in the measured speed. So I don't really get what is the problem?

 

[Ibix]

but the distance the light travels is different in each direction,

No it isn't, or else the light can't return to where it started. For every metre it moves north it must move a metre south.

Dale's comment is much more fundamental. You cannot measure something that is a matter of definition without assuming your answer.

 

[.Scott]

It appears to me that you cannot even deduce that the one-way speed of light is always in the range of c/2 or greater. It could also be negative.

 

[Dale]

It's a round trip because it returns to the same location, but the distance the light travels is different in each direction, so if the speed of light is different In a certain direction we will see it in the measured speed. So I don't really get what is the problem?

The details of the setup are completely irrelevant, but @Ibix is right. What you describe is a round trip measurement. It is independent of any synchronization convention so by definition it is not the one way speed.

 

[The Baron]

The details of the setup are completely irrelevant, but @Ibix is right. What you describe is a round trip measurement. It is independent of any synchronization convention so by definition it is not the one way speed.

Yes, it is a round trip, but can't the round trip indicate about the one-way ?

 

[Dale]

Yes, it is a round trip, but can't the round trip indicate about the one-way ?

No.

 

[The Baron]

No.

why?

 

[Dale]

why?

I already explained why in post 3.

 

[The Baron]

I already explained why in post 3.

You explained that it is not a one way , to which I agree, where I disagree is that you say it is impossible to deduce information from round trip, to a one way.

 

[Dale]

You explained that it is not a one way , to which I agree, where I disagree is that you say it is impossible to deduce information from round trip, to a one way.

The round trip speed is independent of the synchronization. The one way speed depends on the synchronization. How do you propose to calculate a value which depends on the synchronization from one that does not?

 

[.Scott]

Okay so we will have a timer, and a lot of mirrors, set up in a circle, a light beam is fired from the timer and goes in the path of the mirrors in all directions until it eventually returns to the timer and the speed is measured. But here is the catch, the distance between each mirror is different, so if there is a deviation the measurement will give us a speed greater or smaller than the speed of light. And we will either prove the speed of light is different in all directions or prove it's the same.

If you break each segment of the light path into three perpendicular components, one where "c is not c" and then two others, then it may be easier to see why you haven't gained anything. We can call the "c is not c" component the z axis. So if you track the path along that z axis, it is clearly two-way and eventually returns to light source. Then just do the speed computations for each segment based on normal x and y and the non-normal z.

 

[The Baron]

The round trip speed is independent of the synchronization. The one way depends on the synchronization. How do you propose to calculate a value which depends on the synchronization from one that does not?

I didn't study this subject well enough so some of the things I am saying might be stupid

I am trying to suggest a way NOT to calculate the one way, but to deduce from the two way whether there is a thing to calculate. meaning if my experiment will work perfectly we will either know that the speed of light is constant in all directions, or we will no it is different in some directions.

 

[Ibix]

How do you propose to calculate a value which depends on the synchronization from one that does not?

This is the key point, @The Baron. It does not matter how cleverly you arrange your mirrors, what you are trying to do is like trying to detect the Greenwich Meridian. You can only do it if you smuggle your answer in somewhere.

 

[Ibix]

I am trying to suggest a way NOT to calculate the one way, but to deduce from the two way whether there is a thing to calculate.

And how can you do that? You have a round trip measure of $c$. By definition it's independent of the direction or orientation of your experiment, so it will always spit out the value 3×10⁸m/s. How will you use that unvarying number to deduce anything?

 

[Dale]

I didn't study this subject well enough so some of the things I am saying might be stupid

I am trying to suggest a way NOT to calculate the one way, but to deduce from the two way whether there is a thing to calculate. meaning if my experiment will work perfectly we will either know that the speed of light is constant in all directions, or we will no it is different in some directions.

And how exactly are you proposing to “deduce whether there is a thing to calculate”? Please use Reichenbach’s synchronization convention and derive how your measurement can “deduce whether there is a thing to calculate” regarding the synchronization vector that controls the one way speed of light.

 

[Ibix]

Reichenbach’s synchronization convention

I've heard you mention this a few times, but I don't know what it is. Do you have a good reference? A quick Google turns up quite a lot of ResearchGate and academia.edu stuff, but I'm not sure how reliable that is.

 

[Dale]

I've heard you mention this a few times, but I don't know what it is. Do you have a good reference? A quick Google turns up quite a lot of ResearchGate and academia.edu stuff, but I'm not sure how reliable that is.

Here is Reichenbach’s original 1924 book: https://books.google.com/books/about/Axiomatization_of_the_Theory_of_Relativi.html?id=OztALUF8EMoC but I don’t actually use it.

This paper is a much broader overview: https://www.sciencedirect.com/science/article/abs/pii/S0370157397000513?via=ihub

It describes many different approaches that are equivalent but I just use Reichenbach’s name since he has priority. Note, this is paywalled but there are non paywalled versions on the web that I won’t link to directly.

Also, the Wikipedia page is decent and has good references: https://en.wikipedia.org/wiki/One-w...ansformations_with_anisotropic_one-way_speeds

 

[Sagittarius A-Star]

Also, the Wikipedia page is decent and has good references: https://en.wikipedia.org/wiki/One-w...ansformations_with_anisotropic_one-way_speeds

In the OPs video, at time 11:18, they show the same formulas for one-way speed of light and for generalized Lorentz time transformation as in this Wikipedia link.

I found two other links:

Reichenbach made an extension of Einstein’s synchronization procedure in which the one-way speed of light was undefined and it was just imposed a causality condition corresponding to taking tA’ > tB > tA. This led to a definition of coordinate time in terms of the expression tB = tA + ϵ(tA’ – tA), where 0 < ϵ < 1

Source (page 182):
https://www.redalyc.org/pdf/3397/339751454003.pdf

... and:
https://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/significance_conv_sim/index.html#epsilon

 

[Algr]

if my experiment will work perfectly we will either know that the speed of light is constant in all directions, or we will no it is different in some directions.

The problem is that due to the nature of how space/time works, both of those interpretations are equally valid descriptions of what is happening. Thus no experiment will ever disprove either possibility.

 

[The Baron]

The problem is that due to the nature of how space/time works, both of those interpretations are equally valid descriptions of what is happening. Thus no experiment will ever disprove either possibility.

Hmmm so you are saying we can't prove or disprove it based on...

 

[The Baron]

And how exactly are you proposing to “deduce whether there is a thing to calculate”? Please use Reichenbach’s synchronization convention and derive how your measurement can “deduce whether there is a thing to calculate” regarding the synchronization vector that controls the one way speed of light.

As i have said before, i suggest we deduce it from the deviation in the calculated speed.

 

[Dale]

As i have said before, i suggest we deduce it from the deviation in the calculated speed.

Show your math please, using Reichenbach’s synchronization convention or another similar convention. Mathematically, how can your measurement provide any information about $\epsilon$?

I have told you that it cannot work and why it cannot work. You want to ignore that, so back up your claim rigorously.

 

[Ibix]

As i have said before, i suggest we deduce it from the deviation in the calculated speed.

But you haven't shown any mechanism for calculating a speed, except for looking at the round-trip speed which would always be $c$. As you've described it there is no way to use your experiment to derive anything - so you need to do as Dale says and provide details of what measurements you would take and what calculations you would do with them. Otherwise, you are just repeating "I am sure it's possible to do the impossible" with no justification.

 

[The Baron]

Show your math please, using Reichenbach’s synchronization convention or another similar convention. Mathematically, how can your measurement provide any information about $\epsilon$?

I have told you that it cannot work and why it cannot work. You want to ignore that, so back up your claim rigorously.

i don't know what is Reichenbach’s synchronization convention

But here is a "calculation" because I really don't know how to do this

 

[Ibix]

I really don't know how to do this

Then why do you think it will work? I'm not being snarky - I'm asking you to reflect on your thinking process. We've told you several times why no experiment can do what you want, and you admit that you don't really understand what you are doing. Why are you pursuing something you don't understand instead of asking questions about our answers (answers from people who do understand what we're doing), which might advance your understanding?

I don't have anything on this phone that can read docx files. It would have been helpful to use the LaTeX feature of this forum. There is a howto linked just below the reply box.

 

[Vanadium 50]

It would have been helpful to use the LaTeX feature of this forum.

It would not. There's no calculation there other than distance over time.

 

[Vanadium 50]

As i have said before

Repeating a wrong (and unsubstantiated) statement does not make it right.