Transitivity condition for Einstein synchronization and the one-way light speed

[elisir]

1) Consider that one way light speed is anisotropic.
2) Use the Einstein method to synchronize all the watches by the watch located at (x,y,z)=(0,0,0).

Now, all the watches are synchronized by the watch at (0,0,0) but they are not necessarily synchronized with each-other (Consider watches that are not in one line). This means that the transitivity condition is not met by the synchronization. So the synchronization is not an equivalence relation. If so, the one-way light-speed is an observable quantity regardless of how we synchronize watches. Perhaps you are not agree with me, but could you spare me what I am missing? Or do you also want to accept that one way light speed is a measurable quantity?

 

[JesseM]

Now, all the watches are synchronized by the watch at (0,0,0) but they are not necessarily synchronized with each-other (Consider watches that are not in one line).

How do you figure? If all the watches are at rest relative to one another, then if two pairs A,B and A,C meet Einstein's synchronization condition, then it's guaranteed that the pair B,C will meet it too.

 

[DrGreg]

1) Consider that one way light speed is anisotropic.
2) Use the Einstein method to synchronize all the watches by the watch located at (x,y,z)=(0,0,0).

These two conditions contradict each other. It's not possible for both to be true at the same time. If you use the Einstein method to sync all watches then you have forced the one-way speed of light to be isotropic. If you sync the watches in such a way that the one-way speed of light is anisotropic then you haven't used the Einstein method.

 

[pervect]

Consider a non-rotating inertial frame of reference. Then you can synchronize all clocks with a central clock via Einstein's method, and find that they are all synchronized with each other.

Now, however, suppose you are on a rotating frame of reference, a rotating disk. Then if you synchronize all clocks via Einstein's method to the central clock, you will find that the clocks at the edge are not synchronized via the Einstein's convention.

So, transivity of Einstein synchronization tells us something interesting about rotation, but I don't see how it tells us anything about the one-way speed of light.

 

[elisir]

Consider a non-rotating inertial frame of reference. Then you can synchronize all clocks with a central clock via Einstein's method, and find that they are all synchronized with each other.

Now, however, suppose you are on a rotating frame of reference, a rotating disk. Then if you synchronize all clocks via Einstein's method to the central clock, you will find that the clocks at the edge are not synchronized via the Einstein's convention.

So, transivity of Einstein synchronization tells us something interesting about rotation, but I don't see how it tells us anything about the one-way speed of light.

I totally agree with your points about rotation.

Suppose that we are in an inertial frame. Then consider the triangle ABC.
a) Synchronise A and B with the Einstein method.
b) Synchronise A and C with the Einstein method.
c) Measure the light speed that travels from B to C.
d) Measure the light speed that travels from C to B.
The Einstein synchronisation implemented in "a" and 'b" does not guarantee that the results of "c" and "d" coincide to each other. This means measuring the one-way speed of light in my eyes.

 

[elisir]

If you use the Einstein method to sync all watches then you have forced the one-way speed of light to be isotropic.

Use the Einstein method to sync all watches for the observer sitting at the origin. Then this observer can not measure the one-way speed of light. The one way-light speed is isotropic for this observer. But you have synchronised all the watches, so please ask other observers to measure the one-way speed of light.

 

[ghwellsjr]

It is not possible for anyone to ever measure the one-way speed of light. We can only measure the "average" round trip speed of light using a mirror at some measured distance away and it always calculates to the constant c. Einstein postulates that each half of the round trip measurement takes the same amount of time and this enables us to use his method to synchronize all the clocks that are stationary with respect to each other. If we synchronize clocks A and B and we also synchronize clocks A and C, then we know that clocks B and C will also be synchronized. Remember, the process of "synchronizing clocks" is nothing more than "defining" the two directions of light travel time between two points to be equal. If, after applying this definition, we attempt to measure the one-way speed of light, of course we will get c, but that is because we defined it that way.

 

[elisir]

It is not possible for anyone to ever measure the one-way speed of light. We can only measure the "average" round trip speed of light using a mirror at some measured distance away and it always calculates to the constant c. Einstein postulates that each half of the round trip measurement takes the same amount of time and this enables us to use his method to synchronize all the clocks that are stationary with respect to each other. If we synchronize clocks 1 and 2 and we also synchronize clocks 1 and 3, then we know that clocks 2 and 3 will also be synchronized.

I repeat myself. What is wrong with the algorithm below:
Suppose that we are in an inertial frame. Then consider the triangle ABC.
a) Synchronise A and B with the Einstein method.
b) Synchronise A and C with the Einstein method.
c) Measure the light speed that travels from B to C.
d) Measure the light speed that travels from C to B.
The Einstein synchronisation implemented in "a" and 'b" does not guarantee that the results of "c" and "d" coincide to each other. This means measuring the one-way speed of light in my eyes.

 

[ghwellsjr]

Opps, I edited my post while you added your last post, but I answered your question in my previous post.

You need to edit your post again to be consistent since you have added "d" where I think you mean "b".

 

[elisir]

You need to edit your post again to be consistent since you have added "d" where I think you mean "b".

I mean "d"

If we synchronize clocks A and B and we also synchronize clocks A and C, then we know that clocks B and C will also be synchronized.

I disagree with this point. If one-way light speed is anisotropic, synchronisation of A and C, and synchronisation of B and C does not necessarily imply synchronisation of A and B.

As a simplified example: Please draw a triangle. Assume a clockwise speed and an anti-clockwise one for light on the edge of the triangle. The light pulse that travels clockwise shall go faster or slower than the one on the anti-clockwise direction. This difference is independent of how you synchronise your watches on this special case placed on the corner of the triangle. This difference is a physical quantity, while you are arguing it is not.

 

[JesseM]

I repeat myself. What is wrong with the algorithm below:
Suppose that we are in an inertial frame. Then consider the triangle ABC.
a) Synchronise A and B with the Einstein method.
b) Synchronise A and C with the Einstein method.
c) Measure the light speed that travels from B to C.
d) Measure the light speed that travels from C to B.
The Einstein synchronisation implemented in "a" and 'b" does not guarantee that the results of "c" and "d" coincide to each other. This means measuring the one-way speed of light in my eyes.

It does guarantee it, at least if the two-way speed is still c. Try coming up with an example and doing the math, you'll see (for example, suppose that in your chosen coordinate system, light has a speed of c+v in one direction and c-v in the opposite direction)

 

[ghwellsjr]

I mean "d"

Sorry, I didn't notice the difference in your lower-case nomenclature vs your upper-case.

I disagree with this point.

What specifically do you disagree with? That you cannot measure the one-way speed of light OR that you cannot define the two halves of the round-trip measured speed of light to take the same time OR just that synchronizing different pairs of clocks does not guarantee that other pairs will also be synchronized OR something else?

If one-way light speed is anisotropic, synchronisation of A and C, and synchronisation of B and C does not necessarily imply synchronisation of A and B.

Very true but Einstein defined light speed to be isotropic not anisotropic. If you want to define light speed to be anisotropic, then why do you care about Einstein's clock synchronization method?

As a simplified example: Please draw a triangle. Assume a clockwise speed and an anti-clockwise one for light on the edge of the triangle. The light pulse that travels clockwise shall go faster or slower than the one on the anti-clockwise direction. This difference is independent of how you synchronise your watches on this special case placed on the corner of the triangle. This difference is a physical quantity, while you are arguing it is not.

Are you suggesting a three-segment speed of light test in which we have two mirrors and one source/detector? First you aim the light at one mirror which reflects it over to the second mirror which then reflects it back to the source/detector and measure the time it takes for the pulse to get around back to the source/detector and then you repeat with the light aimed at the other mirror so that the light travels the same path in the reverse direction? Are you saying that if you perform this experiment you will get two different answers for the time it takes for the light to go around the triangle in the two different directions? If I understand your experiment correctly, why do you need to synchronize any watches when you can do the whole experiment with just one watch located at the source/detector of the light? Please clarify.

 

[JesseM]

Very true but Einstein defined light speed to be isotropic not anisotropic. If you want to define light speed to be anisotropic, then why do you care about Einstein's clock synchronization method?

Well, you can still analyze what will happen with clocks synchronized according to Einstein's method from the perspective of a coordinate system where the speed of light is anisotropic, you'll still get the same predictions about physical events like whether two clocks that have been previously synchronized with a third clock will be synchronized with each other (not 'synchronized' relative to this coordinate system, but 'synchronized' in the sense that if a signal is sent from one clock to the other and back, the midpoint of the readings when the signal left and returned to one clock is equal to the reading when the signal bounced off the other clock).

 

[elisir]

source/detector? First you aim the light at one mirror which reflects it over to the second mirror which then reflects it back to the source/detector and measure the time it takes for the pulse to get around back to the source/detector and then you repeat with the light aimed at the other mirror so that the light travels the same path in the reverse direction? Are you saying that if you perform this experiment you will get two different answers for the time it takes for the light to go around the triangle in the two different directions? If I understand your experiment correctly, why do you need to synchronize any watches when you can do the whole experiment with just one watch located at the source/detector of the light? Please clarify.

Thank you. We don't need synchronisation. Synchronisation is not needed in closed path. But there seems to be an unhappy/redundant marriage between one-way speed of light and synchronisation in mind of people.
Thank you. It is exactly the method that directly measures the one-way speed of light. This is the Trimmer experiment. But I have faced a large inertia which keeps repeating that the one-way light speed is not a measurable quantity. But the Trimmer method measures anisotropy in the one way light speed, in fact it has measured it about 40 years ago.

 

[JesseM]

Thank you. We don't need synchronisation. Synchronisation is not needed in closed path. But there seems to be an unhappy/redundant marriage between one-way speed of light and synchronisation in mind of people.
Thank you. It is exactly the method that directly measures the one-way speed of light. This is the Trimmer experiment. But I have faced a large inertia which keeps repeating that the one-way light speed is not a measurable quantity. But the Trimmer method measures anisotropy in the one way light speed, in fact it has measured it about 40 years ago.

But I'm pretty sure the only way this experiment could fail to give equal times for the path in different directions would be if the two-way speed along any leg of the triangle (or any other straight-line path) was not equal to c. If that's correct, then I would say this experiment is not measuring anisotropy in the one way speed, rather it is just a particular type of test of the two-way speed.

 

[elisir]

But I'm pretty sure ..

I would highly appreciate it if you first would possibly put your "assurance" on a test on the simplified case I described above, repeated below:

Consider a triangle wherein the light speed in not the same for clockwise direction and anticlockwise one. In this example, the two-way speed of light is isotropic but it is the one-way speed which is not isotropic.

 

[JesseM]

I would highly appreciate it if you first would possibly put your "assurance" on a test on the simplified case I described above, repeated below:

Consider a triangle wherein the light speed in not the same for clockwise direction and anticlockwise one. In this example, the two-way speed of light is isotropic but it is the one-way speed which is not isotropic.

You're right, I was thinking of the type of anisotropic speed that results from doing a non-Lorentzian coordinate transformation on an inertial frame where the speed of light is isotropic (an example would be the Mansouri-Sexl transformation), but more generally it would be possible to have a theory where there is no frame in which the speed of light is isotropic.

An issue here is that if you have a theory with an anisotropic speed of light that isn't physically equivalent to SR (i.e. you can't do a coordinate transformation to get an SR inertial frame where the speed of light is isotropic), then even if you have one frame where observers at rest in that frame will measure the two-way speed to be c, I have my doubts that it'd be possible to have such a theory where observers moving at arbitrary constant velocities in this frame will also measure the two-way speed to be c regardless of which direction they aim the beam. And if there is only one preferred frame where the two-way speed is c, then experiments like the Michelson-Morley experiment should not be expected to show the two-way speed is constant when the experiment is performed at different points in the Earth's orbit.

 

[Dale]

Consider a triangle wherein the light speed in not the same for clockwise direction and anticlockwise one. In this example, the two-way speed of light is isotropic but it is the one-way speed which is not isotropic.

That sounds like a test of rotation, not anisotropy. I.e. the Sagnac effect. You are correct that the Einstein synchronization is not transitive in a rotating reference frame.

That said, there has been some research in this area:
http://ajp.aapt.org/resource/1/ajpias/v31/i7/p482_s1?isAuthorized=no

 

[elisir]

That sounds like a test of rotation, not anisotropy. I.e. the Sagnac effect. You are correct that the Einstein synchronization is not transitive in a rotating reference frame.

I agree that it can be used to test rotation too. But in this set-up, we do not need to keep something rotating. We just display the triangle a bit. Make it at rest with an inertial frame and perform the measurement. This is somehow similar to Sagnac effect, but it is not this effect.

 

[Dale]

Can you demonstrate mathematically what the difference is? It sounds exactly the same to me.

Also, the triangle cannot be at rest in an inertial reference frame since an inertial reference frame by definition is one in which the one-way speed of light is isotropic and equal to c. So you are by definition considering non-inertial reference frames. From your description it sounds like the specific non-inertial reference frame you are considering is a rotating reference frame (i.e. same metric as a rotating frame).

 

[elisir]

Can you demonstrate mathematically what the difference is? It sounds exactly the same to me.

The mathematical framework for the triangular system is that of Trimmer experiment, extended to resonators in section II of
http://arxiv.org/abs/1010.2057

Also, the triangle cannot be at rest in an inertial reference frame since an inertial reference frame by definition is one in which the one-way speed of light is isotropic and equal to c.

This is your assumption. If you assume that there exists a frame in which the Lightspeed is isotropic, you confine yourself to theories similar to that of Robertson-Mansuori-Sexl model. In these subset of theories, one-way light speed is isotropic.

So you are by definition considering non-inertial reference frames. From your description it sounds like the specific non-inertial reference frame you are considering is a rotating reference frame (i.e. same metric as a rotating frame).

If so, please assume that this sound of my writing is accidental. I did not mean it.

If we measure the one-way light speed and its anisotropy's profile coincides to that predicted by being in a rotating frame we than conclude that our frame is rotating. The one-way light speed anisotropy chosen by nature, may not necessarily coincide to that of any rotating frame.

 

[ghwellsjr]

When you described your triangular experiment in earlier posts, you didn't mention a piece of optical medium in one leg to slow down the speed of light which is what the Trimmer experiment had. Why? Do you consider it insignificant?

And from everything I could find about the Trimmer experiment, it yielded a null result, just like MMX. Why are you suggesting otherwise?

I looked at the paper that you linked to and it does not describe the results of an experiment but rather a proposed new experiment. Why did you reference it?

Finally, I cannot find enough information on the Trimmer experiment to understand what they actually did so could you please explain it?

 

[elisir]

When you described your triangular experiment in earlier posts, you didn't mention a piece of optical medium in one leg to slow down the speed of light which is what the Trimmer experiment had. Why? Do you consider it insignificant?

Imagine an arbitrary deviation from one-way anisotropy. The glass is there, to measure just one particular deviation. Measuring the rest of deviations, do not need a glass.

And from everything I could find about the Trimmer experiment, it yielded a null result, just like MMX. Why are you suggesting otherwise?

I am not suggesting otherwise. The Trimmer experiment returned null results for one-way light speed. The MMX returns null results for anisotropy of two-way light speed.

I looked at the paper that you linked to and it does not describe the results of an experiment but rather a proposed new experiment. Why did you reference it?

You asked a mathematical framework for Sagnot and Trimmer experiment. I pinpoint to the mathematical framework for the Trimmer experiment. I hoped that this would helped you realize the distinction between Trimmer and Sagnot experiment.

Finally, I cannot find enough information on the Trimmer experiment to understand what they actually did so could you please explain it?

They considered a triangular interferometer. They measured how the phase difference between light moving clockwise and anticlockwise on the perimeter of the triangle, as function of the configuration of the triangle to the constant starts. They find no deviation for anisotropy of one way light speed with the precision of cm/s.

The whole aim of this post, is to rise the question and debate why now, today, some of us say and keep repeating that the one-way light speed can not be measured.

 

[ghwellsjr]

When I keep repeating that the one-way speed of light cannot be measured, I am merely pointing out what Einstein said in his 1905 paper. He described the experimental method of measuring the round-trip speed of light using a source/detector and a mirror a measured distance away and timing the round trip and calculating the speed. He then explained that this does not say anything about the one-way speed of light and then made the bold statement that in any inertial reference frame, we are free to define the two halves of the round-trip measurement to take the same amount of time. This is the essence of SR.

Many people don't realize that this is what he is saying because they think the statement that the speed of light is constant in all inertial reference frames is referring to something that has been measured and confirmed (which is true for the round-trip but not the one-way). If we could measure the one-way speed of light, we could identify an absolute rest frame and relativity would be out the window.

 

[elisir]

When I keep repeating that the one-way speed of light cannot be measured, I am merely pointing out what Einstein said in his 1905 paper.

Einstein wrote his paper correctly. In page 3, of the english translation of his paper, here
www.fourmilab.ch/etexts/einstein/specrel/specrel.pdf
After defining synchronisation he writes that

We assume that this definition of synchronism is free from contradictions,
and possible for any number of points; and that the following relations are
universally valid:
1. If the clock at B synchronizes with the clock at A, the clock at A syn-
chronizes with the clock at B.
2. If the clock at A synchronizes with the clock at B and also with the clock
at C, the clocks at B and C also synchronize with each other.​

A general anisotropy of the one-way light speed contradicts these assumptions (provided that you use the Einstein method to synchronise all watches with the one at the origin, as this method is assumed in the paper). Nonetheless, the Einstein's work that introduces SR is right by itself.

 

[ghwellsjr]

Prior to Einstein, everyone believe that there was an absolute aether rest frame and only in that one frame was light isotropic, in all other frames, light is anisotropic. They attempted to build a science on that concept.

Einstein said, you can define light to be isotropic in anyone of those other frames and it will be exactly the same as if it were the absolute aether rest frame.

So the issue of whether light is "really" anisotropic or isotropic in any inertial frame is moot in SR.

Einstein agreed that a general anisotropy of the one-way light speed apparently contradicts these assumptions but that it was only apparent and not a real contradiction.

 

[Dale]

The mathematical framework for the triangular system is that of Trimmer experiment, extended to resonators in section II of
http://arxiv.org/abs/1010.2057

Thanks for the link. This is exactly what I thought you meant and it is what is known as a ring-laser. This type of device is well understood and uses the Sagnac effect to measure rotation. Note the discussion in the ArXiv about the beat frequency, which is the way a ring-laser measures rotation. You are, in fact, describing a rotating reference frame and you are, in fact, correct that Einstein synchronization is not transitive in rotating reference frames.

Your proposed experiment generates a shift (beat frequency) which is related to the rotation of the device in an inertial frame and not anyone way anisotropy. There is simply not enough information available from this experiment to measure anything more.

This is your assumption. ...
If we measure the one-way light speed and its anisotropy's profile coincides to that predicted by being in a rotating frame we than conclude that our frame is rotating. The one-way light speed anisotropy chosen by nature, may not necessarily coincide to that of any rotating frame.

No, it is not my assumption, it is the DEFINITION of an inertial frame that the speed of light is isotropic and equal to c.

I don't think that you understand the situation. There is NO WAY to measure the one-way speed of light, and there is no such thing as the one way speed of light "chosen by nature". The one way speed of light is something that a human being DEFINES by virtue of CHOOSING an arbitrary synchronization convention.

Imagine an arbitrary deviation from one-way anisotropy. The glass is there, to measure just one particular deviation.

That would introduce a deviation in the two-way anisotropy of the speed of light.

 

[elisir]

Your proposed experiment generates a shift (beat frequency) which is related to the rotation of the device in an inertial frame and not anyone way anisotropy. There is simply not enough information available from this experiment to measure anything more..

Do not keep the system rotating. Rotate the triangle. stop rotating. Now every thing is in inertial system. Measure the beat frequency. Does the beat frequency depend on the configuration of the triangle? if yes, you have detected the one-way anisotropy of light.

No, it is not my assumption, it is the DEFINITION of an inertial frame that the speed of light is isotropic and equal to c..

Could you please spare me why you expect to find what you have defined as intertial frame in nature?

I don't think that you understand the situation.

I totally disagree. Should I start to insult back? I understand you, I expect you reflect and try to understand me.

There is NO WAY to measure the one-way speed of light.

You are wrong. There exist some ways. Your assumptions or pre-assumptions are not consistent.

There is no such thing as the one way speed of light "chosen by nature". The one way speed of light is something that a human being DEFINES by virtue of CHOOSING an arbitrary synchronization convention.

If you refer to Einstein synchronisation, kindly refer to page three of the English translation of his original paper. He had been extremely careful to state his assumptions. Please take cautious, under the circumstances that his assumptions are violated you can not use the output of his assumptions.

That would introduce a deviation in the two-way anisotropy of the speed of light.

This is hypocrisy, I was answering how a piece of glass is needed in that particular method to measure a particular profile of one-way light anisotropy. Your reply indicates that you are saying that for a piece of glass, light speeded changes in both directions inside it. We are both right. Please don't employ hypocrisy.

 

[matheinste]

1) Consider that one way light speed is anisotropic.
2) Use the Einstein method to synchronize all the watches by the watch located at (x,y,z)=(0,0,0).

Now, all the watches are synchronized by the watch at (0,0,0) but they are not necessarily synchronized with each-other (Consider watches that are not in one line). This means that the transitivity condition is not met by the synchronization. So the synchronization is not an equivalence relation. If so, the one-way light-speed is an observable quantity regardless of how we synchronize watches. Perhaps you are not agree with me, but could you spare me what I am missing? Or do you also want to accept that one way light speed is a measurable quantity?

My problem is, like many others, that although I realize that the speed of light may not be isotropic, the one way speed cannot even in principle be measured.

Irrespective of whether or not it can be done, it certainly has not been done. I am not familiar with modern methods of measuring c, but I would imagine that, if you can measure the average two way speed, then the technical ability to measure the one way speed is available, should it be possible in principle.

My knowledge of simple physics is not large but as far as I know, to measure speed you must know distance and time. Distance presumably is no problem, but time?

How do you break the circle of time and synchronization?

Please indulge the uninitiated amongst us, and provide an answer to this basic question, or tell me why it is not a problem or why the measurement has not been done.

Matheinste.

 

[elisir]

My knowledge of simple physics is not large but as far as I know, to measure speed you must know distance and time. Distance presumably is no problem, but time?
How do you break the circle of time and synchronization?

Imagine that streets have an upper speed limit and this speed limit is the light speed.
Imagine that you have a car that could instantly reach the speed of light.
Imagine that you don't want to get any speed ticket, but you will derive as fast as possible.
Imagine that streets are not symmetric. They do not have the same upper speed limit in opposite direction.

Now the one-million dollar question is that, can anyone measure the one-way speed limit by just looking at where and when you start deriving and when and where you stop?

Now look at this picture:

This is the situation I described by words above.

If you derive in one direction and come back on the same path, reach to the point you started deriving from and tell us how much time your journey takes, we can not deduce the one-way upper speed in your path.

If you derive in a triangle, and tell us whether you rotated clockwise or anticlockwise, we can measure the one-way upper speed limit. Look at the picture, rotating clockwise takes less time or anticlockwise direction? the answer is clear.

Only in the case that you don't come back to the point that you started deriving from, we need clock synchronisation. So, please derive back to the point you started deriving from. This is the point that you break what you call the circle of time and synchronisation. Once you reach to the point you started from, we do not need synchronisation.

Please indulge the uninitiated amongst us, and provide an answer to this basic question, or tell me why it is not a problem or why the measurement has not been done.
Matheinste.

This experiment has been done by Trimmer in 1973. But Trimmer et al. reported two results at one paper, one of them is wrong. The results on the one-way anisotropy is correct. Placing both the right and wrong results in one place might be a reason that people have not noticed the Trimmer's work
ref. : Phys.Rev.D8:3321-3326,1973, Erratum-ibid.D9:2489-2489,1974

 

[matheinste]

Hello elisir,

How does this differ essentially from having a light source next to a clock and a mirror at a known distance. You don't synch clocks but all you measure is the average speed of light. Your more complex path can be broken down in many ways into components which have equal path lengths in opposite directions, the same as the mirror scenario.

I am a little short on time and so will take a closer look later but it looks a non starter to me.

Matheinste.

 

[elisir]

How does this differ essentially from having a light source next to a clock and a mirror at a known distance. You don't synch clocks but all you measure is the average speed of light. Your more complex path can be broken down in many ways into components which have equal path lengths in opposite directions, the same as the mirror scenario.

I am a little short on time and so will take a closer look later but it looks a non starter to me.

The geometry you describe, in the closed path you propose, the light speed in both direction exist. In triangular geometry, once you choose clockwise or anti-clockwise direction only one direction of the light speed exists. You can not break down let's say the clockwise direction on the triangular path into superposition of two-way round trip on straight lines.

 

[ghwellsjr]

Your pictures are the same ones from the link you provided in which you said that you only provided the link to answer DaleSpam's request for a mathematical demonstration of the Trimmer experiment. I didn't think you were promoting this new proposed experiment.

When I looked at the paper and saw the diagrams with the cars on the streets and read the explanation, it made absolutely no sense to me but I didn't ask for an explanation because you didn't indicate you were putting any confidence in this proposed new experiment but now it appears that you are. But, your explanation makes no sense. I have questions:

Why are there two lanes going one way but only one lane going the other way?
Why is there only one car shown in the top graphic but two cars shown in the bottom graphic?
Is there any significance to the fact that the two-lane direction is outside the one-lane direction in the triangle and therefore a longer distance?
Are these graphics applicable to the Trimmer experiment or only to the new proposed experiment?
Isn't it the case that in the Trimmer experiment and the new proposed experiment, the light traverses exactly the same path in the two directions?
Why doesn't the graphic show something equivalent to the piece of glass or other optical device that is present in the real experiments?

But to answer your million dollar question: of course, we super-beings looking down on this graphic could measure the one way speed of the cars because we have super clocks and super rulers that allow us to determine the real graphic distances and times. But graphic-beings can't "see" ahead to know when the car reaches the other end of the track.

That is our problem when we try to measure the one-way speed of light. Once the light leaves our source, we have no idea where it "really" is until something (a detector or mirror) a known distance away communicates back to us that it has arrived at that point and then it takes time to communicate that information back to us so unless we make some assumptions about how long it takes for the communication of that information back to us, we cannot determine absolutely when the light reached our detector or mirror.

How does your graphic (or the Trimmer experiment or the new proposed experiment) solve this problem?

 

[matheinste]

The geometry you describe, in the closed path you propose, the light speed in both direction exist. In triangular geometry, once you choose clockwise or anti-clockwise direction only one direction of the light speed exists. You can not break down let's say the clockwise direction on the triangular path into superposition of two-way round trip on straight lines.

My basic geometry may be a bit rusty but is it not the case that in a journey, starting from and returning to the same point, the total integrated displacement by all paths is zero, or better to say in this case, that in all directions, the components, referred to a set of coordinates, of a journey by any arbitrarily chosen path to any point, and the components of an arbitrarily chosen return path, referred to the same coordinate axes, sum to zero. They are all in principle analogous to a straighline path out and back. Of course such an "integration " of paths may not be true in a non conservative field or spacetime, but we are dealing with such advanced scenarios here, are we?.

Matheinste

 

[DrGreg]

It seems to me that describing this as a measurement of the one-way speed of light is misleading (and why everyone else in this thread has objected). It would be more reasonable to describe it as a measurement of the "three-way" speed of light (round a triangle in each direction).

Special relativity asserts that the three-way speed of light should be the same either way round an inertial triangle, but not for a rotating triangle. So this experiment is really to confirm that the Sagnac effect is zero when the angular velocity is zero. It says nothing at all about the one-way speed of light which is determined purely by your choice of coordinates (or synchronization convention) and can't be determined experimentally.

 

[elisir]

...

I discard your teasing questions.

But to answer your million dollar question: of course, we super-beings looking down on this graphic could measure the one way speed of the cars because we have super clocks and super rulers that allow us to determine the real graphic distances and times. But graphic-beings can't "see" ahead to know when the car reaches the other end of the track.

Perhaps this is another teasing. I have better method to tease. Look :P .. please discard the possibility that you are a super being and answer.

That is our problem when we try to measure the one-way speed of light. Once the light leaves our source, we have no idea where it "really" is until something (a detector or mirror) a known distance away communicates back to us that it has arrived at that point and then it takes time to communicate that information back to us so unless we make some assumptions about how long it takes for the communication of that information back to us, we cannot determine absolutely when the light reached our detector or mirror.

Have you ever heard of the detectors? Measure the time. Send a pulse to a mirror, have that pulse reflected back to another mirror, and from the second mirror to your detector near the place that you have emitted it from. As soon as you detect the pulse, measure the time. Reflect before you write.

How does your graphic (or the Trimmer experiment or the new proposed experiment) solve this problem?

Read the papers for detailed mathematical answer to your questions.

 

[elisir]

Special relativity asserts that the three-way speed of light should be the same either way round an inertial triangle, but not for a rotating triangle. So this experiment is really to confirm that the Sagnac effect is zero when the angular velocity is zero. It says nothing at all about the one-way speed of light which is determined purely by your choice of coordinates (or synchronization convention) and can't be determined experimentally.

First thank you. If we go to theories that include a generalisation of special relativity that allows for anisotropy of one-way light speed, such as SME, then the triangular geometry will measure the anisotropy of the one-way speed of light. This is similar to Sagnac effect but it is not that effect. It measures the one way light speed.

 

[elisir]

My basic geometry may be a bit rusty but is it not the case that in a journey, starting from and returning to the same point, the total integrated displacement by all paths is zero, or better to say in this case, that in all directions, the components, referred to a set of coordinates, of a journey by any arbitrarily chosen path to any point, and the components of an arbitrarily chosen return path, referred to the same coordinate axes, sum to zero. They are all in principle analogous to a straighline path out and back. Of course such an "integration " of paths may not be true in a non conservative field or spacetime, but we are dealing with such advanced scenarios here, are we?.
Matheinste

Dear Matheinste,

You are right that any-closed path can be broken to pieces. We then can integrate over the pieces. But on every piece of line, we have two bits of information: the light speed in both directions.

When you consider a straight line, you break the line, on every piece you have moved twice. You sum over the directions too. You sum over extra bits of information. At the end, you have only integral.

When you apply your breaking procedure on a triangle, you sum over pieces. But you do not sum over the extra bits of information that exists on each piece. You can define two integral over the pieces, one that includes the bit of clockwise direction and one that includes the bit of anti-clockwise direction.

 

[ghwellsjr]

I have better method to tease.

Yes, you do, goodbye.

 

[elisir]

Yes, you do, goodbye.

It was the thing that I did not know. Have good times.

 

[Dale]

Do not keep the system rotating. Rotate the triangle. stop rotating. Now every thing is in inertial system. Measure the beat frequency. Does the beat frequency depend on the configuration of the triangle? if yes, you have detected the one-way anisotropy of light.

Don't you see the problem in this? The way you determine whether or not the frame is inertial is by this measurement. When the beat frequency is non-zero the frame is non-inertial, therefore you can NEVER get a non-zero reading in an inertial frame by DEFINITION. It does not depend on the configuration of the triangle.

Could you please spare me why you expect to find what you have defined as intertial frame in nature?

Because more than 100 years of scientific evidence shows it: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

I totally disagree. ... I understand you, I expect you reflect and try to understand me.

I not only understand your idea, but I immediately recognized your proposed experiment as ring interferometer of the kind commonly used in inertial navigation systems to measure the Sagnac effect. The fact that you did not see this relationship to the Sagnac effect shows that you, in fact, do not fully understand the topic. That is not an insult, that is the whole purpose of this site: to educate about mainstream physics. You are in the right spot to learn.

You are wrong. There exist some ways.

Then please provide an example of some way to measure the one-way speed of light. A ring interferometer is not such a device.

Should I start to insult back? ... This is hypocrisy, ... Please don't employ hypocrisy.

elisir, you need to take a breath and calm down. If you feel that stating that you don't understand something is an insult then you probably don't have the temperament for this site. I find it interesting that you consider my assertion that you didn't understand the topic (a minor deficit in education) to be an inappropriate insult, but you consider your assertion that I am a hypocrite (a major deficit in character) to be appropriate. It doesn't bother me either way; I have been called much worse. But I find the juxtaposition rather amusing. You need to develop a little bit of a "thick skin" and not be so sensitive in this medium.

 

[elisir]

Don't you see the problem in this? The way you determine whether or not the frame is inertial is by this measurement. When the beat frequency is non-zero the frame is non-inertial, therefore you can NEVER get a non-zero reading in an inertial frame by DEFINITION. It does not depend on the configuration of the triangle.

A rotating frame leads to a specific beat frequency. That is the beat frequency is a functional of the parameters determining the configuration of the triangle and the circular velocity of the inertial frame. Measure the beat frequency, if that frequency profile coincides to the profile you expected you will conclude that you are in a rotating frame. Your measured quantity may deviate from this profile.

Because more than 100 years of scientific evidence shows it: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

Even I started from the original paper of Einstein. Special relativity is moot to the possibility of anisotropy of the one-way light speed. Please note that anisotropy in the one-way light speed contradicts the assumption of Einstein that his synchronisation is an equivalence relation. Once the light speed is anisotropic, his assumption on page three of the English translation of his paper breaks. Please do not use the outcome of an argument when the assumption is broken.

I not only understand your idea, but I immediately recognized your proposed experiment as ring interferometer of the kind commonly used in inertial navigation systems to measure the Sagnac effect. The fact that you did not see this relationship to the Sagnac effect shows that you, in fact, do not fully understand the topic.

I do agree that ring experiment is not capable of measuring one-way light speed. Consider an inertial ring or a rectangle. In this path, for each direction present in the path, light speed in both direction exists. Let me illustrate it more:
Consider a rectangle ABCD:
Consider a pulse of light moving on the rectangle from A to B, B to C and C to D, and finally back to A.
The light speed in direction of AB, and BA; and BC to CB equally contribute to the time for the travel of pulse. Only two-ways of light speed contributes to ring or rectangle.

This is the reason that I disagree with you. You know Sagnac experiment and you are saying the Sagnac experiment does not measure one-way light speed. I agree with you on this fact.

However, once you consider a triangle path, you see that you can measure the anisotropy of the one-way light speed.

That is not an insult, that is the whole purpose of this site: to educate about mainstream physics. You are in the right spot to learn.

Thanks. I try to correct myself.

Then please provide an example of some way to measure the one-way speed of light. A ring interferometer is not such a device.

I am over-repeating myself. Triangular path enables us to measure one-way light speed. While ring or rectangular does not make us enable to measure the anisotropy of the one-way light. Please refer to
Phys.Rev.D8:3321-3326,1973, Erratum-ibid.D9:2489-2489,1974
and read this paper carefully. This is an established example. After reading this paper, you may wonder why the community has forgotten this method. I wonder for this too.

elisir, .. You need to develop a little bit of a "thick skin" and not be so sensitive in this medium.

Thanks for your advice, I shall try my best to consider it.

 

[Dale]

Measure the beat frequency, if that frequency profile coincides to the profile you expected you will conclude that you are in a rotating frame. Your measured quantity may deviate from this profile.

It doesn't matter what frequency you measure. If you measure one beat frequency you will determine that you are rotating at some specific angular velocity. If you measure a beat frequency 2x greater then you determine that you are rotating at twice the angular velocity. There simply is no data left over to additionally measure any anisotropy. Any frequency can be accounted for by rotation.

Please do not use the outcome of an argument when the assumption is broken.

The point is that if the assumptions were broken then the resulting theory would not accurately predict the results of experiments. Since the results of experiments agree to very high precision with the predictions of the theory we can conclude that any violations of the assumptions are very small.

I do agree that ring experiment is not capable of measuring one-way light speed. Consider an inertial ring or a rectangle. In this path, for each direction present in the path, light speed in both direction exists. Let me illustrate it more:
Consider a rectangle ABCD:
Consider a pulse of light moving on the rectangle from A to B, B to C and C to D, and finally back to A.
The light speed in direction of AB, and BA; and BC to CB equally contribute to the time for the travel of pulse. Only two-ways of light speed contributes to ring or rectangle.

This is the reason that I disagree with you. You know Sagnac experiment and you are saying the Sagnac experiment does not measure one-way light speed. I agree with you on this fact.

However, once you consider a triangle path, you see that you can measure the anisotropy of the one-way light speed.

The shape of the path does not matter for the Sagnac effect, it depends only on the area of the loop and the angular velocity. The triangular ring interferometers are well understood and behave the same as a square or circular interferometer of the same area. In fact, many commercially available laser ring gyroscopes (interferometers) are triangular shaped:
http://www.airpower.au.af.mil/airchronicles/aureview/1985/may-jun/siuru.html
http://www.google.com/images?q=lase...itle&resnum=3&ved=0CEEQsAQwAg&biw=996&bih=970

With this many devices on the market if triangular shaped gyros were sensitive to some additional effect that could degrade their performance in detecting rotation then they would universally be avoided due to competitive pressures.

 

[elisir]

The shape of the path does not matter for the Sagnac effect, it depends only on the area of the loop and the angular velocity.

There exists no angular velocity in Trimmer method. The triangle is not rotating, the angular velocity is vanishing. So what you measure is not Sagnac effect. The Trimmer paper is about five pages:
http://prd.aps.org/abstract/PRD/v8/i10/p3321_1
Please take a glimpse on this paper.

 

[Dale]

There exists no angular velocity in Trimmer method. The triangle is not rotating.

How do you know that it is not rotating?

 

[elisir]

How do you know that it is not rotating?

A quick reply:
I do not rotate it myself. My laboratory might be rotating. I measure the phase difference of lights moving clock-wise and anti-clockwise on the perimeter of various triangles. If the phase difference is a functional of the area of the triangle (as you suggest that the Sagnac effect is), then it is Sagnac effect. I find that my laboratory is rotating. I find the angular velocity.

If the phase difference is a functional of the perimeter of the triangle, than rotation can not describe what I see. It is due to anisotropy of the one-way light speed.

 

[Dale]

That is fair enough, now you have two measurements so you could in principle tease out two effects.

There are different-sized triangular laser ring gyros available commercially. To my knowledge none of the designers have reported any effect of the perimeter, which would easily have been uncovered during routine testing.

 

[elisir]

There are different-sized triangular laser ring gyros available commercially. To my knowledge none of the designers have reported any effect of the perimeter, which would easily have been uncovered during routine testing.

True. This null result, however, should lead to a bound on the anisotropy of the one-way light speed. The one-way light speed is isotropic with the precision of these commercial gyros. I am quite sure that this null result has not been translated to the anisotropy of one-way light speed. Such a translation sounds very nice to me.

 

[Dale]

True. This null result, however, should lead to a bound on the anisotropy of the one-way light speed. The one-way light speed is isotropic with the precision of these commercial gyros. I am quite sure that this null result has not been translated to the anisotropy of one-way light speed. Such a translation sounds very nice to me.

So go ahead and do so. You consider yourself to be highly motivated and knowledgeable on the subject, do you not?

 

[elisir]

So go ahead and do so. You consider yourself to be highly motivated and knowledgeable on the subject, do you not?

In so doing, one uses the commercial equipments to extracts scientific data from. I gauss that this can not reach the precision of 0.1 nm/s on the anisotropy of the one-way light speed in mSME inferred from the absence of Cerenkov radiation for the ultra-high-energy-cosmic radiation, or 1 nm/s of the proposed cavity resonators. It would reach the precision of about or around cm/s, which is the precision of the Trimmer experiment in 1973, or about of the Tobar et al. experiment in 2009. It would be nice, but its outcome shall not improve the best current bounds.