How to measure the one way speed of light.

[harrylin]

After doing some rough sketches and calculations I have come to the conclusion (unfortunately) that any proposed anisotropy of light speed will be undetectable. It seems in the example above, that the arm going Northward going away from A arrives at B earlier than the isotropic case and gives time for the slow return light signal to arrive back at A at exactly the same time as the isotropic case.

If a long double arm is used, with a pivot at it centre so that it spans the full diameter of the circle and additional signalling devices are placed at the East edge (C) and the west edge (D), signals from C and D arrive back at A simultaneously with each other. This is despite the fact the arm bends due to differential speeds of the arm extremities on opposite sides of the circle. Additionally signals from C and D return to A at the same time as would be expected in the isotropic case. This is very unintuitive but seems to be the case on closer inspection.

It seems K^2 and MikeLizzi are correct in the concerns they raised. It also seems by implication that Ole Roemer only measured the two way speed of light in the eclipses of Io and not the one way speed of light as is sometimes suggested. Quite possibly, measuring the one way speed of light is tantamount to measuring absolute velocity, but that may be extrapolating too far.

Good analysis! And yes, truly measuring the one way speed of light is indeed tantamount to measuring absolute velocity.

 

[grav-universe]

Good analysis! And yes, truly measuring the one way speed of light is indeed tantamount to measuring absolute velocity.

Why do people keep saying this? If a frame truly measures the one way speed of light isotropically at c, then what is its absolute velocity? How would that be determined?

 

[Dale]

He obviously hadn't yet read about yuiop's setup.

There are quite a few ways we could tell an absolute frame of reference by measuring the one way speed of light using the spinning arm, but measuring an isotropic speed of c is not one of them. One way we could tell is if it doesn't follow the symmetry principle. For instance, once the arm is spinning, if the circumference is measured slightly wider from east to west than from north to south, besides telling us something unique about rigidity with stationary rulers as compared to spinning rulers, presumably the difference would become even greater with greater speed relative to the absolute frame and symmetrical only at rest to the absolute frame. We could also tell if the radius is different for different speeds in different frames or if different speeds require different synchronizations or something of the like, but there is no particular reason any of those should be the case.

Another way is if the one way speed is not measured isotropically at c when measuring using the spinning arm. In that case, only the absolute frame would measure an isotropic speed of light, but other frames would measure an offset, indicating the direction of travel to the absolute frame. We could also tell an absolute frame if different isotropic speeds were measured in different frames, although this last one would follow for the two way speed as well. But again, there is no reason why any of those should occur, although on the other hand, other than to achieve symmetry between frames as well as within them, there is otherwise no reason they couldn't occur as well, but can only be determined through experiment.

But whether or not the spinning arm measures the one way speed to be isotropic or anisotropic, or if we were to able to identify an absolute frame of reference, which would require just a small step from SR to LET anyway, SR being just one perfectly symmetrical condition of LET in that case, the spinning arm is still the most natural and a virtually absolute procedure for synchronizing frames, and once frames are synchronized in this way, or by any method that doesn't directly involve light for that matter but this method is special due to the absoluteness of rotation, then the one way speed of light can still definitely be measured, regardless of the results.

No, all it tells you about is the consequences of what you assumed. I.e. If you assume that the rod is straight then you measure c. If you assume the speed is not c then you measure that the rod is bent. And vice versa. It won't tell you anything about LET or any absolute reference frame because you can make it tell you whatever you want simply by choosing your simultaneity convention appropriately.

 

[ghwellsjr]

Good analysis! And yes, truly measuring the one way speed of light is indeed tantamount to measuring absolute velocity.

Why do people keep saying this? If a frame truly measures the one way speed of light isotropically at c, then what is its absolute velocity? How would that be determined?

As I said in post #29, if any inertial observer can measure anything that deviates from what he would measure if he were at rest in an absolute ether rest frame, he will have discovered that Special Relativity has been violated. So if you are at rest with respect to an absolute ether rest frame, you will measure the one-way speed of light to be c which means that both halves of the round trip speed of light measurement takes the same amount of time.

The point is that SR states that any inertial reference frame will have all the characteristics of an absolute ether rest frame and one of those characteristics is that the one-way speed of light is c for observers at rest in that frame of reference but not for observers moving with respect to that frame of reference. Therefore, the arbitrary choice of frame of reference determines whether any observer's measurement of the one-way speed of light is legitimately c or some other value.

So we cannot determine an absolute velocity just because we measure the one-way speed of light to be c in all directions.

 

[harrylin]

Why do people keep saying this? If a frame truly measures the one way speed of light isotropically at c, then what is its absolute velocity? How would that be determined?

OK this has already been answered but here's it in my words: I added "truly" to distinguish from "nominally" or "apparent". If a frame would truly measure the one-way speed of light, that would imply the use of absolute synchronization, so that a measured isotropy is also "absolute" or "true". True isotropy corresponds thus to rest in a stationary ether, or to zero absolute velocity.

In contrast, if isotropy is merely by convention as in special relativity, then (nominal) light speed is simply made isotropic by convenient clock synchronization, and it is postulated that absolute rectilinear velocity cannot be detected: according to special relativity we will measure the same laws of physics with any inertial reference system and thus with any kind of experiment.

Note that absolute velocity could be inferred on other grounds. One may assume that the material universe will not move at high velocity relative to the ether, if it exists. Or, some people hypothesize that the CMBR is isotropic relative to the ether (or "space").

Harald

 

[Q-reeus]

Just wondering; supposing there really was some 'funny physics' going on whereby in some given inertial frame light really was faster one way than the opposite (always averaging to c in any 2-way experiment). Would it be a violation of SR/LET to be able to determine not the 'absolute' one-way c, but the difference |c⁺-c^-|, where the superscripts refer to propagation sense along a given axis?

 

[grav-universe]

OK this has already been answered but here's it in my words: I added "truly" to distinguish from "nominally" or "apparent". If a frame would truly measure the one-way speed of light, that would imply the use of absolute synchronization, so that a measured isotropy is also "absolute" or "true". True isotropy corresponds thus to rest in a stationary ether, or to zero absolute velocity.

In contrast, if isotropy is merely by convention as in special relativity, then (nominal) light speed is simply made isotropic by convenient clock synchronization, and it is postulated that absolute rectilinear velocity cannot be detected: according to special relativity we will measure the same laws of physics with any inertial reference system and thus with any kind of experiment.

Note that absolute velocity could be inferred on other grounds. One may assume that the material universe will not move at high velocity relative to the ether, if it exists. Or, some people hypothesize that the CMBR is isotropic relative to the ether (or "space").

Harald

After applying an absolute synchronization, frame A and frame B each measure the one way light speed isotropically at c. Which one is at rest with the ether at absolute zero velocity?

We must be defining absolute synchronization differently. It seems you are saying that it also infers an absolute frame. Absolute synchronization does not automatically imply asymmetry between frames, as an ether could exist now and we could apply LET rather than SR, but still could not determine its absolute frame because of the symmetry of the universe, making all observations and experiments relative between frames, so even an ether might not manifest itself as an absolute frame, but would be observed the same from any frame in that case.

Perhaps absolute is too strong a word. The most natural synchronization might be better, as it is applied within each frame, not between them. In any case, whether or not the synchronization is considered absolute doesn't matter. If an arbitrary synchronization is applied in the same way within all frames and it is symmetrical, we cannot tell the frame of an ether. If however, it is found to be asymmetrical in particular ways, then we can. If two different arbitrary synchronizations do not conform in the same way from frame to frame, then we can also tell. But otherwise, if all such synchronizations conform and are symmetrical, then we cannot.

 

[grav-universe]

Just wondering; supposing there really was some 'funny physics' going on whereby in some given inertial frame light really was faster one way than the opposite (always averaging to c in any 2-way experiment). Would it be a violation of SR/LET to be able to determine not the 'absolute' one-way c, but the difference |c⁺-c^-|, where the superscripts refer to propagation sense along a given axis?

It would be a violation of SR, which insists upon symmetry, although not LET, whereby we could tell an absolute frame since the two synchronization methods would not conform and we would no longer have symmetry between frames. Presumably the absolute frame in that case would be the one and only frame where light would still be measured isotropically, while all others produce an offset.

 

[Q-reeus]

It would be a violation of SR, which insists upon symmetry, although not LET, whereby we could tell an absolute frame since the two synchronization methods would not conform and we would no longer have symmetry between frames. Presumably the absolute frame in that case would be the one and only frame where light would still be measured isotropically, while all others produce an offset.

grav-universe - my understanding is SR and LET are indistinguishable in any operational sense. The talk has been though that a differential in c is allowed in principle in both theories, but since one-way measurements relying on some clock sync scheme are unobtainable, it could never be found. What I am toying with is an idea for determining difference in one-way speed that uses no clock sync. Not fully worked out, but as time permits may post on it.

 

[harrylin]

Just wondering; supposing there really was some 'funny physics' going on whereby in some given inertial frame light really was faster one way than the opposite (always averaging to c in any 2-way experiment). Would it be a violation of SR/LET to be able to determine not the 'absolute' one-way c, but the difference |c⁺-c^-|, where the superscripts refer to propagation sense along a given axis?

The first is not "funny physics": according to SRT it is possible to adjust the clocks of any inertial frame such that the one-way speed becomes equal to the two-way speed and equal to c. Obviously that applies to your "given inertial frame" in which "light really is faster one way than the opposite (always averaging to c in any 2-way experiment)".

And it is also no problem to measure a difference of two (apparent) one-way speeds - that is actually done with GPS: in GPS the one-way speed of light is made isotropic wrt the ECI frame, which makes is anisotropic relative to the surface of the Earth. But of course that isn't "absolute" at all, it's as measured with (or "with respect to") the ECI frame.

Harald

 

[harrylin]

After applying an absolute synchronization, frame A and frame B each measure the one way light speed isotropically at c. Which one is at rest with the ether at absolute zero velocity?

We must be defining absolute synchronization differently. It seems you are saying that it also infers an absolute frame. Absolute synchronization does not automatically imply asymmetry between frames, as an ether could exist now and we could apply LET rather than SR, but still could not determine its absolute frame because of the symmetry of the universe, making all observations and experiments relative between frames, so even an ether might not manifest itself as an absolute frame, but would be observed the same from any frame in that case.

Perhaps absolute is too strong a word. The most natural synchronization might be better, as it is applied within each frame, not between them. In any case, whether or not the synchronization is considered absolute doesn't matter. If an arbitrary synchronization is applied in the same way within all frames and it is symmetrical, we cannot tell the frame of an ether. If however, it is found to be asymmetrical in particular ways, then we can. If two different arbitrary synchronizations do not conform in the same way from frame to frame, then we can also tell. But otherwise, if all such synchronizations conform and are symmetrical, then we cannot.

I don't know of another meaning of absolute synchronization than the Newtonian one, which assumes to establish "absolute time" - the antithesis of what special relativity postulates to be possible. Therefore relativity claims that synchronization is "relative" in every sense of the word, even if we choose to make it asymmetrical.

 

[grav-universe]

grav-universe - my understanding is SR and LET are indistinguishable in any operational sense.

They are if all frames are completely symmetrical, which so far they appear to be. LET covers a broader scope than SR, however. SR only works if all frames are symmetrical, while LET does not necessarily require it.

The talk has been though that a differential in c is allowed in principle in both theories, but since one-way measurements relying on some clock sync scheme are unobtainable, it could never be found. What I am toying with is an idea for determining difference in one-way speed that uses no clock sync. Not fully worked out, but as time permits may post on it.

That would be interesting, but you would still need something physical to base the synchronization upon by which to measure the one way speed, whether it requires one clock or two clocks or no clocks. In that case you are really just comparing two physical systems to see if they conform. That is basically what yuiop's setup does, comparing the natural synchronization of a spinning arm to a synchronization using light.

 

[grav-universe]

I don't know of another meaning of absolute synchronization than the Newtonian one, which assumes to establish "absolute time" - the antithesis of what special relativity postulates to be possible. Therefore relativity claims that synchronization is "relative" in every sense of the word, even if we choose to make it asymmetrical.

Well, by asymmetrical I mean, like with the spinning arm for instance, once the arm is spinning, we measure it to be slightly oval-shaped. Presumably then, only one frame would measure a perfect circle. Otherwise we are not really discussing an absolute time per say, as might be applied between frames, but only within frames according to some natural synchronization, and that same synchronization would apply equally and symmetrically to every frame. But you are right, synchronization itself is relative, figuratively speaking, so you have convinced me to drop the word "absolute" as applied to the synchronization method, especially since its meaning can apparently be taken different ways. Although in that respect, even if an ether exists, measurements can still be taken "relative" to the ether, depending upon how one thinks about it, so all can be considered relative with or without an ether, just adding one more universal entity to be relative to, similar to how one might measure relative to the CMB, even if it were to alter our measurements in some way. However, more than just being relative, two such "natural" synchronizations, as with light and rotation, must really be precisely the same as clock settings would be determined from either, otherwise they do not conform to the frame in the same way for both methods and we could determine the one frame where they do. Not that it would matter much if we did, though, since there is nothing in physics that would ultimately prevent it except a postulated principle of symmetry within and between frames, but otherwise there is no reason it couldn't occur.

 

[Q-reeus]

The first is not "funny physics": according to SRT it is possible to adjust the clocks of any inertial frame such that the one-way speed becomes equal to the two-way speed and equal to c. Obviously that applies to your "given inertial frame" in which "light really is faster one way than the opposite (always averaging to c in any 2-way experiment)".

Sorry but cannot follow your logic here. Clocks can be made to read anything we wish, but that's beside my point entirely.

And it is also no problem to measure a difference of two (apparent) one-way speeds - that is actually done with GPS: in GPS the one-way speed of light is made isotropic wrt the ECI frame, which makes is anisotropic relative to the surface of the Earth. But of course that isn't "absolute" at all, it's as measured with (or "with respect to") the ECI frame.

Harald - Not sure if you have worked as a consultant or whatever in GPS, but chucking in 'ECI frame' without explanation had me scrambling for one. Got lucky first Googling attempt: http://en.wikipedia.org/wiki/Earth-centered_inertial. Now could you please explain just how the GPS situation involving orbiting satellites has a bearing on an effectively inertial lab frame where we are working with some fixed (unspecified for now) apparatus, apart from occasionally reorienting the apparatus direction?

 

[Q-reeus]

They are if all frames are completely symmetrical, which so far they appear to be. LET covers a broader scope than SR, however. SR only works if all frames are symmetrical, while LET does not necessarily require it.

Not sure if you are referring to an 'aether wind' or such, but LET still gives the same results as SR even here.

That would be interesting, but you would still need something physical to base the synchronization upon by which to measure the one way speed, whether it requires one clock or two clocks or no clocks. In that case you are really just comparing two physical systems to see if they conform. That is basically what yuiop's setup does, comparing the natural synchronization of a spinning arm to a synchronization using light.

Will have to go back and check what yuiop was actually proposing (never got much interested till now). But obviously some kind of physical comparison has to be made - no 'mind powers' stuff here!

 

[grav-universe]

Not sure if you are referring to an 'aether wind' or such, but LET still gives the same results as SR even here.

Considering ourselves stationary and we are in a ship while another ship is traveling away from us along the x-axis at v, if the ether physically contracts the ship along the line of motion and slows the ship's clocks and natural processes, each by precisely by a factor of sqrt(1 - (v/c)^2), then the ship observers will also measure the same isotropic speed of light, the same relative speed for us, and the same length contraction and time dilation when measuring the length of our ship and our clocks when using their own contracted rulers and slowed clocks. Since the measurements are precisely the same from either frame, we would not know if we were the ones that are stationary to the ether and the other ship is moving through it or the other way around. The mathematics is consistent where each frame measures precisely the same things of the other. However, that is just one condition in LET that allows symmetry within and between frames, so that we cannot identify the frame of the ether, the same as with SR. The length contraction and time dilation are two factors that can be made unknown variables in LET, though, so complete symmetry is not the only solution.

Will have to go back and check what yuiop was actually proposing (never got much interested till now). But obviously some kind of physical comparison has to be made - no 'mind powers' stuff here!

 

[Q-reeus]

Considering ourselves stationary and we are in a ship while another ship is traveling away from us along the x-axis at v, if the ether physically contracts the ship along the line of motion and slows the ship's clocks and natural processes, each by precisely by a factor of sqrt(1 - (v/c)^2), then the ship observers will also measure the same isotropic speed of light, the same relative speed for us, and the same length contraction and time dilation when measuring the length of our ship and our clocks when using their own contracted rulers and slowed clocks. Since the measurements are precisely the same from either frame, we would not know if we were the ones that are stationary to the ether and the other ship is moving through it or the other way around. The mathematics is consistent where each frame measures precisely the same things of the other. However, that is just one condition in LET that allows symmetry within and between frames, so that we cannot identify the frame of the ether, the same as with SR.

We're in perfect harmony here.

The length contraction and time dilation are two factors that can be made unknown variables in LET, though, so complete symmetry is not the only solution.

Meaning I take it length and time wrt the 'absolute' or 'locally absolute' aether frame, as we have just agreed that between any two inertial frames the transformation relations are 'fixed'.

 

[chinglu1998]

The first is not "funny physics": according to SRT it is possible to adjust the clocks of any inertial frame such that the one-way speed becomes equal to the two-way speed and equal to c. Obviously that applies to your "given inertial frame" in which "light really is faster one way than the opposite (always averaging to c in any 2-way experiment)".

And it is also no problem to measure a difference of two (apparent) one-way speeds - that is actually done with GPS: in GPS the one-way speed of light is made isotropic wrt the ECI frame, which makes is anisotropic relative to the surface of the Earth. But of course that isn't "absolute" at all, it's as measured with (or "with respect to") the ECI frame.

Harald

What do you means is made?

Light is isotropic in ECI otherwise, you could not synch clocks in it. This is a well know fact in the mainstream.

 

[grav-universe]

Meaning I take it length and time wrt the 'absolute' or 'locally absolute' aether frame, as we have just agreed that between any two inertial frames the transformation relations are 'fixed'.

The relations for the transformations are fundamentally fixed in variable form regardless of the kinematic theory for how the universe operates, whether it be SR or LET or anything else. Let's say we are stationary in a ship within our own frame and an identical ship passes us in the opposite direction along the x axis, identical in the respect that they measure the same proper length d of their ship as we do of ours. Both ships synchronize the clocks at the front of their ships to T=0 upon passing, when the fronts coincide, which is the origin of each frame. According to our frame, the other ship's clocks are ticking slower by a factor of z, their rulers and ship are length contracted by a factor of L, and they have synchronized so that to us, the clock at the rear of their ship reads a greater time than the front clock by tl, so the other frame is synchronized with an additional time of tl per length L d. An event then occurs according to our frame at coordinates t, x. According to the other frame, the event would occur at x - v t, but their rulers are also contracted, so they measure

x' = (x - v t) / L

The event occurs at time t to us, so when the clock at the front of the other ship reads z t, but the simultaneity difference adds a time tl per length L d in the negative direction also, so the time upon a clock that coincides with the event in the other frame will read

t' = z t - (tl / (L d)) (x - v t)

Those are the fundamental forms of the transformations which would be used regardless of the kinematic theory, so variable with LET. Of course with SR, we have the symmetry principle, whereby z = L = sqrt(1 - (v/c)^2) = 1 / y and tl = z L d v / (c^2 - v^2) = d v / c^2, which gives

x' = y (x - v t)

t' = t / y - (y v / c^2) (x - v t)

= y (t / y^2 - (v / c^2) (x - v t))

= y (t (1 - (v/c)^2) - v x / c^2 + (v/c)^2 t)

= y (t - v x / c^2)

 

[harrylin]

Sorry but cannot follow your logic here. Clocks can be made to read anything we wish, but that's beside my point entirely.

Harald - Not sure if you have worked as a consultant or whatever in GPS, but chucking in 'ECI frame' without explanation had me scrambling for one. Got lucky first Googling attempt: http://en.wikipedia.org/wiki/Earth-centered_inertial. Now could you please explain just how the GPS situation involving orbiting satellites has a bearing on an effectively inertial lab frame where we are working with some fixed (unspecified for now) apparatus, apart from occasionally reorienting the apparatus direction?

It's an essential point of SRT that because clocks can be synchronized according to our wish, for our convenience, this directly affects apparent isotropy of light speed. For GPS it was most convenient to choose the ECI "frame" as "rest frame". Consequently clock synchronization makes radio waves appear to propagate isotropically relative to the ECI frame, wrt which the Earth rotates. The relative speed (also called "closing speed") of radio waves and a GPS receiver at rest on the Earth is c-v. This is merely an example that a nominal difference of one-way speeds can be measured; it has no direct bearing on the example of the OP but the same can be done for two one-way light speeds. I gave that example because it was unclear to me what you had in mind and hoped to achieve, but I suspected that it was based on a misunderstanding related to these matters.

 

[harrylin]

What do you means is made?

Light is isotropic in ECI otherwise, you could not synch clocks in it. This is a well know fact in the mainstream.

That is presenting the facts upside-down! At the risk of SRT being deleted by "mainstream", here's the "principle of the constancy of the velocity of light" as reiterated by Einstein in 1907:

"We [...] assume that the clocks can be adjusted in such a way that
the propagation velocity of every light ray in vacuum - measured by
means of these clocks - becomes everywhere equal to a universal
constant c, provided that the coordinate system is not accelerated.

- http://www.soso.ch/wissen/hist/SRT/E-1907.pdf

Harald

 

[Q-reeus]

The relations for the transformations are fundamentally fixed in variable form regardless of the kinematic theory for how the universe operates, whether it be SR or LET or anything else. Let's say we are stationary in a ship within our own frame and an identical ship passes us in the opposite direction along the x axis, identical in the respect that they measure the same proper length d of their ship as we do of ours. Both ships synchronize the clocks at the front of their ships to T=0 upon passing, when the fronts coincide, which is the origin of each frame. According to our frame, the other ship's clocks are ticking slower by a factor of z, their rulers and ship are length contracted by a factor of L, and they have synchronized so that to us, the clock at the rear of their ship reads a greater time than the front clock by tl, so the other frame is synchronized with an additional time of tl per length L d. An event then occurs according to our frame at coordinates t, x. According to the other frame, the event would occur at x - v t, but their rulers are also contracted, so they measure

x' = (x - v t) / L

The event occurs at time t to us, so when the clock at the front of the other ship reads z t, but the simultaneity difference adds a time tl per length L d in the negative direction also, so the time upon a clock that coincides with the event in the other frame will read

t' = z t - (tl / (L d)) (x - v t)

Those are the fundamental forms of the transformations which would be used regardless of the kinematic theory, so variable with LET. Of course with SR, we have the symmetry principle, whereby z = L = sqrt(1 - (v/c)^2) = 1 / y and tl = z L d v / (c^2 - v^2) = d v / c^2, which gives

x' = y (x - v t)

t' = t / y - (y v / c^2) (x - v t)

= y (t / y^2 - (v / c^2) (x - v t))

= y (t (1 - (v/c)^2) - v x / c^2 + (v/c)^2 t)

= y (t - v x / c^2)

Understand what you're getting at I think - it's only the inter-frame measure product x't' that's 'invariant' for a given relative frame velocity, and either factor x' or t' could vary arbitrarily within that, so L can vary arbitrarily. Problem I see here is that SR Doppler shift is known to high precision - that fixes the t' part (the t'= y (t - v x / c^2)), thus forcing....

 

[Q-reeus]

It's an essential point of SRT that because clocks can be synchronized according to our wish, for our convenience, this directly affects apparent isotropy of light speed. For GPS it was most convenient to choose the ECI "frame" as "rest frame". Consequently clock synchronization makes radio waves appear to propagate isotropically relative to the ECI frame, wrt which the Earth rotates. The relative speed (also called "closing speed") of radio waves and a GPS receiver at rest on the Earth is c-v. This is merely an example that a nominal difference of one-way speeds can be measured; it has no direct bearing on the example of the OP but the same can be done for two one-way light speeds. I gave that example because it was unclear to me what you had in mind and hoped to achieve, but I suspected that it was based on a misunderstanding related to these matters.

Thanks for your response. I appreciate a worldwide navigation system needs a standardized reference frame to work from - ECI in this case. Knowing next to nothing about the actual workings of the GPS network, am thinking when you refer to a relative speed of c-v, this translates as a Doppler shift measurement, inferring relative velocity between that satellite emitter and terrestrial GPS receiver (or extended by computation to some other physical reference object). If I recall right there is normally four such satellites always in view to accurately 'quadrangulate' position and speed. That of itself doesn't allow one-way light speed measurements. As you put in #51, we know SR postulates that (one-way) c is an invariant in any frame, whether light is being emitted within that frame, or received from a source in that or any other frame.
EDIT: Just came across an article "One-Way Light Speed Determination Using the Range Measurement Equation of the GPS"; Stephan J. G. Gift. Looks like I misinterpreted what you have been arguing - sorry. After just a brief skim, I have the impression what the author is claiming is one-way c is actually Sagnac. The consensus view is one-way can't be done:http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#one-way_tests" But then goes on to quote upper bounds!

Because I'm increasingly of the view there is little if any physical sense to a genuine one-way variation in c, might as well give a brief sketch of what was planned, but now canned. For all I know it's old hat but here goes anyway. Basically, two long, closely spaced parallel lengths of say optical fiber are fed by a splitter from a common laser oscillator, and each match terminated at the far end, so there are no reflected waves. The fibres are very slightly different in phase constant, and thus a kind of 'potential interference pattern' exists between adjacent sections of fibers owing to the different guide wavelengths. Connect a sampling probe between the two at or near a node. If there is a one-way c, the location of such a node should be pushed variously towards or away from the laser source depending on relative orientation between fibers and presumably the direction of 'aether flow' (what else could be postulated as a variable c cause?). Moving the arrangement slowly through various angles, while maintaining a null at the probe by means of a variable phase section in one arm of the feed splitter, it should be possible to work out both the magnitude and sense of 'delta c'. Probably not sensitive enough in that form at least, but in principle it should work. No clocks! Most likely just hunting for a ghost, so have no interest in pursuing it further. Is there an obvious fatal design flaw?

 

[harrylin]

Thanks for your response. I appreciate a worldwide navigation system needs a standardized reference frame to work from - ECI in this case. Knowing next to nothing about the actual workings of the GPS network, am thinking when you refer to a relative speed of c-v, this translates as a Doppler shift measurement, inferring relative velocity between that satellite emitter and terrestrial GPS receiver (or extended by computation to some other physical reference object). If I recall right there is normally four such satellites always in view to accurately 'quadrangulate' position and speed. That of itself doesn't allow one-way light speed measurements. As you put in #51, we know SR postulates that (one-way) c is an invariant in any frame, whether light is being emitted within that frame, or received from a source in that or any other frame.
EDIT: Just came across an article "One-Way Light Speed Determination Using the Range Measurement Equation of the GPS"; Stephan J. G. Gift. Looks like I misinterpreted what you have been arguing - sorry. After just a brief skim, I have the impression what the author is claiming is one-way c is actually Sagnac. The consensus view is one-way can't be done:http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#one-way_tests" But then goes on to quote upper bounds!

I found that paper but have reason to suspect that it's not peer reviewed and it's certainly not free of errors - already its main message is wrong. However, the "Sagnac correction" of GPS is effectively the same as the one-way speed of light relative to a moving object (in modern jargon, their "closing speed" c-v).

Because I'm increasingly of the view there is little if any physical sense to a genuine one-way variation in c, might as well give a brief sketch of what was planned, but now canned. For all I know it's old hat but here goes anyway. Basically, two long, closely spaced parallel lengths of say optical fiber are fed by a splitter from a common laser oscillator, and each match terminated at the far end, so there are no reflected waves. The fibres are very slightly different in phase constant, and thus a kind of 'potential interference pattern' exists between adjacent sections of fibers owing to the different guide wavelengths. Connect a sampling probe between the two at or near a node. If there is a one-way c, the location of such a node should be pushed variously towards or away from the laser source depending on relative orientation between fibers and presumably the direction of 'aether flow' (what else could be postulated as a variable c cause?). Moving the arrangement slowly through various angles, while maintaining a null at the probe by means of a variable phase section in one arm of the feed splitter, it should be possible to work out both the magnitude and sense of 'delta c'. Probably not sensitive enough in that form at least, but in principle it should work. No clocks! Most likely just hunting for a ghost, so have no interest in pursuing it further. Is there an obvious fatal design flaw?

- yes it sounds to me a bit like an experiment with cables that was done some years ago in Belgium and from which a positive effect was claimed... let's see the FAQ:

http://www.phys.ncku.edu.tw/mirrors/physicsfaq/Relativity/SR/experiments.html#Experiments_not_consistent_with_SR

["Amateurs look for patterns, professionals look at error bars" - hmm, that's way oversimplified - but that's another topic!]

Ah I found it, something like that was done once by de Witte (regretfully the experiment isn't directly discussed there). Anyway, in theory such experiments will only eventually find effects from the non-inertial motion of the Earth. The Silvertooth experiment there also looked for effects on phases, and was also discredited on theoretical grounds (the alternative theory could not explain the claimed results either). If I correctly recall (for I have now no time to redo the analysis), waves (from one side as well as from two sides) interfere the same when in motion as in rest - perhaps someone else can eventually correct me and elaborate.

 

[Q-reeus]

I found that paper but have reason to suspect that it's not peer reviewed and it's certainly not free of errors - already its main message is wrong. However, the "Sagnac correction" of GPS is effectively the same as the one-way speed of light relative to a moving object (in modern jargon, their "closing speed" c-v).

As things stand you've got me confused, hopefully just a case of unspoken qualifications. In #31 "And yes, truly measuring the one way speed of light is indeed tantamount to measuring absolute velocity.", which I took as saying it was an impossibility. but then in #50 "The relative speed (also called "closing speed") of radio waves and a GPS receiver at rest on the Earth is c-v. This is merely an example that a nominal difference of one-way speeds can be measured..." - Gift's paper was I thought arguing just that. Let's bypass all the technical intricacies of how GPS works and consider physics in some nominally inertial lab frame. A basic consequence of a real nonreciprocal one-way c : from the fundamental relation c = lambda*f, if f is fixed any nonreciprocal c automatically means a nonreciprocal lambda in that frame. This regardless of whether the light source is within that or another frame. Do you agree with this? And contrary to the consensus viewpoint, lambda variation is at least in principle readily measurable (see below).

- yes it sounds to me a bit like an experiment with cables that was done some years ago in Belgium and from which a positive effect was claimed... let's see the FAQ:
http://www.phys.ncku.edu.tw/mirrors/physicsfaq/Relativity/SR/experiments.html#Experiments_not_consistent_with_SR
["Amateurs look for patterns, professionals look at error bars" - hmm, that's way oversimplified - but that's another topic!]

Thanks for that link - more details on one-way tests than the Baez site.

Ah I found it, something like that was done once by de Witte (regretfully the experiment isn't directly discussed there). Anyway, in theory such experiments will only eventually find effects from the non-inertial motion of the Earth.

Not so - see below.

The Silvertooth experiment there also looked for effects on phases, and was also discredited on theoretical grounds (the alternative theory could not explain the claimed results either). If I correctly recall (for I have now no time to redo the analysis), waves (from one side as well as from two sides) interfere the same when in motion as in rest - perhaps someone else can eventually correct me and elaborate.

Found the 1986 paper by Silvertooth - seems OK in principle. Interested though in any actual detailed critique you may know of. A very similar style of differential phase type test was performed by C Navia et al, referenced in the above link. Interesting that the only criticism leveled there was one of sloppy technique - but nothing about fundamental theoretical inadequacy.

Originally Posted by Q-reeus:
grav-universe - my understanding is SR and LET are indistinguishable in any operational sense.

"They are if all frames are completely symmetrical, which so far they appear to be. LET covers a broader scope than SR, however. SR only works if all frames are symmetrical, while LET does not necessarily require it."

That was from #42. I would now have to agree with grav-universe on that point: SR by the 2nd postulate forbids one-way c variation, whereas LET implicitly requires it. My assumption was LT's automatically compensate in both theories. That's only true for two-way measurements and 'normal physics' which depends on invariant two-way c. Hence one-way tests are crucial to distinguish between SR and LET - and differential phase type measurements are perfectly capable in principle of doing that - as per c = lambda*f discussed above. How so? Well my proposed twin fibers setup for one. A nonreciprocal lambda equally stretches/compresses lambda along both lines - and the associated node locations that for sure can be measured; just a question of required sensitivity. Another even simpler example, not very sensitive but 'for sure' in principle - a single match terminated line containing one or more sections of a very low Q almost '1/4 lambda' transmission cavity. Won't go into the details here, but not hard to show that any non-reciprocity in c and thus lambda will result in a corresponding reflection phase and amplitude variation. Once we see past the false dilemma of 'clock sync' and realize phase methods completely bypass this issue, 'impossibility of one-way c measurement' now looks pretty stupid imho. I've done a 180 degree flip on outlook here. Desperately resisting being sucked into a SR/LET theoretic dispute BH!

 

[ghwellsjr]

I would now have to agree with grav-universe on that point: SR by the 2nd postulate forbids one-way c variation, whereas LET implicitly requires it. My assumption was LT's automatically compensate in both theories. That's only true for two-way measurements and 'normal physics' which depends on invariant two-way c. Hence one-way tests are crucial to distinguish between SR and LET - and differential phase type measurements are perfectly capable in principle of doing that - as per c = lambda*f discussed above. How so? Well my proposed twin fibers setup for one. A nonreciprocal lambda equally stretches/compresses lambda along both lines - and the associated node locations that for sure can be measured; just a question of required sensitivity. Another even simpler example, not very sensitive but 'for sure' in principle - a single match terminated line containing one or more sections of a very low Q almost '1/4 lambda' transmission cavity. Won't go into the details here, but not hard to show that any non-reciprocity in c and thus lambda will result in a corresponding reflection phase and amplitude variation. Once we see past the false dilemma of 'clock sync' and realize phase methods completely bypass this issue, 'impossibility of one-way c measurement' now looks pretty stupid imho. I've done a 180 degree flip on outlook here. Desperately resisting being sucked into a SR/LET theoretic dispute BH!

The only difference between LET and SR is a philosophical one; LET claims that nature operates on a single unidentifiable absolute ether rest frame in which we are almost always moving with respect to, while SR claims that every inertial observer can consider himself to be at rest in, what amounts to, the absolute ether rest frame. There can be no test that can distinguish between the two.

 

[Q-reeus]

The only difference between LET and SR is a philosophical one; LET claims that nature operates on a single unidentifiable absolute ether rest frame in which we are almost always moving with respect to, while SR claims that every inertial observer can consider himself to be at rest in, what amounts to, the absolute ether rest frame. There can be no test that can distinguish between the two.

That's what I've thought till now. As for LET reference frame, 'absolute' need not mean that as such - I'm rather partial to an increasing view that CMBR provides a reference gauge for a 'locally absolute' rest frame. Think of the analogous 2D balloon model - any point locally stationary on the skin can serve as a local rest frame, though no globally 'absolute rest frame' is possible. But really I think it gets down to whether non-reciprocal c is in principle detectable - currently I would say yes - via properly designed one-way differential phase measurements. If you know of a reference that can shoot that idea down, please post it!

 

[harrylin]

As things stand you've got me confused, hopefully just a case of unspoken qualifications. In #31 "And yes, truly measuring the one way speed of light is indeed tantamount to measuring absolute velocity.", which I took as saying it was an impossibility. but then in #50 "The relative speed (also called "closing speed") of radio waves and a GPS receiver at rest on the Earth is c-v. This is merely an example that a nominal difference of one-way speeds can be measured..." - Gift's paper was I thought arguing just that. Let's bypass all the technical intricacies of how GPS works and consider physics in some nominally inertial lab frame. A basic consequence of a real nonreciprocal one-way c : from the fundamental relation c = lambda*f, if f is fixed any nonreciprocal c automatically means a nonreciprocal lambda in that frame. This regardless of whether the light source is within that or another frame. Do you agree with this? And contrary to the consensus viewpoint, lambda variation is at least in principle readily measurable (see below).

Thanks for that link - more details on one-way tests than the Baez site.

Not so - see below.

Found the 1986 paper by Silvertooth - seems OK in principle. Interested though in any actual detailed critique you may know of. A very similar style of differential phase type test was performed by C Navia et al, referenced in the above link. Interesting that the only criticism leveled there was one of sloppy technique - but nothing about fundamental theoretical inadequacy.

Aargh - I spent one hour on replying (basically it was all "no" + long explanations) but lost it all because I didn't make a backup and this site threw me out... too bad, won't have time anymore and should be sleeping now. If you still have any question one or two days from now, please ask again!

 

[ZapperZ]

Er.. not sure if anyone brought up this link already (I didn't read through all 4 pages of this discussion), but in case it hasn't, one might want to look at this review:

http://arxiv.org/abs/1011.1318

Zz.

 

[Q-reeus]

Er.. not sure if anyone brought up this link already (I didn't read through all 4 pages of this discussion), but in case it hasn't, one might want to look at this review:

http://arxiv.org/abs/1011.1318

Zz.

Yes it was listed early on in the thread, but I never got to read all of it myself. They claim clock synch is no barrier to making one-way tests, and quote Clifford Will to that effect. On the other hand these reviewers deny it can truly be done; ie. 'one-way' is always de facto 'two-way': http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#one-way_tests"