How to measure the one way speed of light.

[yuiop]

It is often stated that the one way speed of light cannot be measured and that the isotropic speed of light is just an assumption based on two way measurements and clock synchronisation conventions.

Here is a proposal for measuring the one way speed of light. It is not entirely original as it is based on the method used by Ole Romer to make this measurement by observing eclipses of Jupiter's moon Io, but is slightly simplified and idealised for easy analysis.

First we require a radial arm of length r. One end of the arm is anchored to an axis and the other end is free. A single clock is placed at A on the circumference of the arms arc. The arm is rotated to a high velocity v. A signalling flash device is placed at B on the opposite side of the circle. Each time the arm passes B the flash is triggered. The tangential velocity of the free end of the arm is computed as 2*pi*r/T, where T is the time to complete one full circle as measured by the clock at A. When the arm is rotating at a steady high velocity, some reading are taken. Let's say when the arm passes A the time is t1. The time t2 of the flash when the arm passes B, as measured by the clock at A when it receives the signal, is (pi*r/v)+(2*r/c) which includes the light travel time.

The one way speed of light is obtained by solving for c which is c=2*r/(t2-t1-pi*r/v) or 2*r/(t2-t1-T/2).

Now because the one way speed of light is often disputed, there may be some hidden tautologies in the above thought experiment. Can anyone say what they think they are?

 

[grav-universe]

Very intriguing. I can see no problems with it offhand. Even if the arm does not rotate rigidly, it should deform with a constant bend all the way around, and all we really need to know is where the end of the arm lies when it passes the clock and signalling device, which should also be constant, and can be measured easily all the way around while the arm is spinning to make sure it is following a circular path. So far it seems you have indeed found a method by which to measure the one way speed of light. Cool.

 

[grav-universe]

I will have to do some intense thinking about this, though, because according to our definitions of speed as you have applied them to the apparatus, it should be right, but that would mean there is only one speed photons or any other particles emitted in a particular way while traveling at a lesser speed can be measured, meaning there is only one way that frames can be synchronized while still following the same definitions of speed.

That is actually something I have been searching for the last couple of years, a way to synchronize frames according to some absolute principle. I thought it might have to do with something like bouncing a ball against a wall so that it is measured at the same speed in every direction to the wall as back at any speed, but that would be a principle that would have to be applied in a similar way as the first postulate about the laws being the same in every frame, which makes it an assumption. What you have shown would be applied absolutely, being more than just a principle, having everything to do with the definition of speed itself. Hopefully no problems will be found, because that would be absolutely amazing.

 

[grav-universe]

Okay yes, I think I see now. What you have done is found a much better and more natural way to synchronize frames than using light. If we have a clock at both A and B, then of course using light, we would set clock B to measure half the difference in the two way time for A when a pulse is emitted from A to B and reflected back to A. But by placing clocks A and B on opposite sides of the spinning arm, B would naturally have to be set such that it reads half the difference for a full rotation when the arm passes. I'm jealous, because that is what my bouncing ball thing was supposed to do, but not quite so neatly as what you have set up. The only potential problem I can see so far is verifying that the arm spins at a constant pace regardless of its frame, although there's no particular reason why it shouldn't.

 

[K^2]

First we require a radial arm of length r. One end of the arm is anchored to an axis and the other end is free. A single clock is placed at A on the circumference of the arms arc.

And this is where you end up having a problem. How do you synchronize clocks?

 

[Saw]

I wonder if there may be a problem with the time taken for the triggering mechanisms to do their respective jobs. When the arm passes by A, it has set to the clock in motion (a). When it passes by B, it has to release the flash (b). In both cases, that may be done through the arm moving a switch that sends and electric signal, which in turns activates a mechanism doing (a) or (b) as the case may be. If in (a) the electric signal travels, let us say, leftwards, then in (b) it must travel rightwards. Electricity does not travel at the speed of light, but it is subject to relativistic adjustments.

For example, in the display proposed by grav-universe where the arm is used for synchronizing clocks placed at A and B, another frame moving relative to the apparatus would argue that the clocks are not perfectly synchronized, precisely on the grounds that the time taken for the mechanisms to operate is different. In Galilean relativity, the velocity addition formula would guarantee that both signals would be simultaneous, since the forward one travels more distance but does it faster, while the backward one travels less distance but more slowly, one thing comepensating the other in exact terms, so that travel times are identical. But in SR the relativistic law for addition of velocities still provides for a difference of velocities as judged by a third frame, but one leading to a time difference between the two signals. (I know you know all that, but it write it to force myself to remember learned concepts.)

It is not so clear to me, however, how this applies to the original display by yuiop. Do you see yourself a problem?

 

[Q-reeus]

yuiop - taking the lazy way out here:
http://www.amnh.org/education/resources/rfl/web/essaybooks/cosmic/p_roemer.html"
http://arxiv.org/abs/1011.1318"
http://arxiv.org/pdf/1003.4964v1"
Hope these help!

 

[yuiop]

And this is where you end up having a problem. How do you synchronize clocks?

The beauty of the thought experiment is that there is only one single clock that makes all the timing measurements, so there is no need to synchronise any clocks.

 

[K^2]

Oh, I see. So instead of using light to carry information back, you are using a massive object. Id est, the rotating arm.

Well, then your problem is exactly the same. You have to assume that the arm takes the same amount of time to travel one way as the other. If you aren't ready to make that assumption for light, why should you make it for massive object?

 

[MikeLizzi]

Oh, I see. So instead of using light to carry information back, you are using a massive object. Id est, the rotating arm.

Well, then your problem is exactly the same. You have to assume that the arm takes the same amount of time to travel one way as the other. If you aren't ready to make that assumption for light, why should you make it for massive object?

Now that took me a few minutes to think about. But yes, I agree. I wonder if the same problem exists with the original roemer experiment? Can we know the diameter of Earth's orbit to the degree of accuracy necessary for that version of the experiment?

 

[yuiop]

Oh, I see. So instead of using light to carry information back, you are using a massive object. Id est, the rotating arm.

Well, then your problem is exactly the same. You have to assume that the arm takes the same amount of time to travel one way as the other. If you aren't ready to make that assumption for light, why should you make it for massive object?

This is a valid concern. One way to evaluate the significance of this is to assume that the one way speed of light is something other than the average of the two way speed of light and see if the experiment would detect this anomaly. Of course it is reasonable to require that physical objects (e.g. the arm) are constrained to move relative to the anisotropic speed of light in the direction they are moving. (We can't have massive objects overtaking photons.) For example the arm tip going Northwards at 0.8c is not the same as the arm tip going Southwards at 0.8c in the "big picture".

I propose this test case. Master clock A is at the South end of the circle and signalling device B is at the North end. We conjecture that the anisotropic speed of light is 1.5C going North and 0.75C going South and at C going East or West. (C is the average two way speed of light). What would the apparatus measure? Will the anisotropy of the speed of light be undetectable? I have reasons to think not. I will come back to this after more analysis.

 

[grav-universe]

Oh, I see. So instead of using light to carry information back, you are using a massive object. Id est, the rotating arm.

Well, then your problem is exactly the same. You have to assume that the arm takes the same amount of time to travel one way as the other. If you aren't ready to make that assumption for light, why should you make it for massive object?

Oh, wait. That's the way I compared it too in regards to how the one way light speed is normally defined, but actually there's something a little more absolute about it, having to do with the symmetrical rigidity of the spinning apparatus. That is, with the arm, we can actually set both clocks at the same time, without regard to the rotational speed. Before spinning, the arm extends directly between points A and B. After spinning, whenever each end of the arm passes A and B, each clock ticks off one interval of time, in the same way as a light clock but with a single extended rigid object, and according to the symmetry principle of the frame, there should be no reason to assume that the ends of the arm do not reach each clock simultaneously, although we are also assuming the symmetry principle in the first place, so that frame considers clocks A and B to be synchronized, and of course other frames would claim that the apparatus is not rigid in the same way as they would not with the M-M apparatus, but viewing it while the arm is in motion.

 

[K^2]

You cannot use symmetry here. If it holds, then there is no need to measure one-way speed of light. If it doesn't, then matter will not really move the same way either, and what we think of as a straight line might not actually be a straight line.

 

[grav-universe]

You cannot use symmetry here. If it holds, then there is no need to measure one-way speed of light. If it doesn't, then matter will not really move the same way either, and what we think of as a straight line might not actually be a straight line.

We are not assuming the one way speed of light to conform to the symmetry principle. If it does, then we have the same synchronization by either method. If it doesn't, then we have to decide whether to synchronize using light or rotation, but again, not that there would be any reason both methods wouldn't conform, as well they should, but we can't assume that beforehand.

A straight line isn't a straight line anyway with SR. For instance, if the arm remains more or less rigid according to the frame of the apparatus, a frame moving relative to the apparatus, synchronized to light speed, will observe something close to the diagram below with the four colored lines at various alignments for the arm, although not drawn precisely.

 

[yuiop]

I propose this test case. Master clock A is at the South end of the circle and signalling device B is at the North end. We conjecture that the anisotropic speed of light is 1.5C going North and 0.75C going South and at C going East or West. (C is the average two way speed of light). What would the apparatus measure? Will the anisotropy of the speed of light be undetectable? I have reasons to think not. I will come back to this after more analysis.

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.

 

[grav-universe]

I'm not sure what you mean with your last post. The spinning arm gives a different means by which to synchronize the clocks around the circumference, something other than using light signals. For instance, if the clocks are equally spaced around the circumference with the ends of separate arms originally directed at each one, then when the arms are spun at a steady pace and each reaches the next clock on the circumference, each of the clocks are set to T=0, at which point they are considered synchronized within the frame. Then the difference in times for light pulses to travel one way between any of the clocks can be measured using this synchronization of the clocks for the frame.

 

[ghwellsjr]

After doing some rough sketches and calculations I have come to the conclusion (unfortunately) that any proposed anisotropy of light speed will be undetectable.

I'm glad that you have seen the light.

See this thread for another in depth discussion on this topic:

https://www.physicsforums.com/showthread.php?t=448493&highlight=trimmer

 

[grav-universe]

I'm glad that you have seen the light.

See this thread for another in depth discussion on this topic:

https://www.physicsforums.com/showthread.php?t=448493&highlight=trimmer

I don't quite understand your post either. If we were to synchronize clocks using light to begin with, then of course we couldn't determine the one way speed of light, sure, since if we were to use the Einstein synchronization method, for instance, then the one way speed would be as defined by the synchronization procedure itself. But we are not referring to light at all for the synchronization of clocks with the spinning arm. We can outline the points that the end of the arm passes to be sure it follows a perfect circle, as it should if we are not to determine any absolute motion, and it should do so at a steady pace through all points around the circle for the same reason. Since we know the length of the path the end of the arm follows and the time to complete a full rotation, we know its rotation speed, and all clocks around the circumference can be synchronized accordingly, and their synchronization will remain the same regardless of how the rotation speed is changed, so we have a natural almost absolute type of synchronization procedure. From this, between two of the clocks that are now synchronized in this manner or even with a single clock with the experiment yuiop set up, we can directly measure the one way speed of light. Most likely it would still be c, yes, but there is otherwise no connection between synchronization procedures that would tell us this beforehand, not without actually performing the experiment.

 

[harrylin]

yuiop - taking the lazy way out here:
http://www.amnh.org/education/resources/rfl/web/essaybooks/cosmic/p_roemer.html"
http://arxiv.org/abs/1011.1318"
http://arxiv.org/pdf/1003.4964v1"
Hope these help!

Nice collection, especially the last one says it all. :-)

Regards,
Harald

 

[grav-universe]

Nice collection, especially the last one says it all. :-)

Regards,
Harald

The last paper deals only with measuring the straight line path of light reflected off of a mirror. The synchronization of clocks along a straight line, however, is arbitrary. We can synchronize them to measure half the two way speed of light using the Einstein simultaneity convention or any number of a possible infinite number of synchronizations, as long as the clocks are set linearly. That is, we can add some amount of time to each clock per distance in a linear fashion and measure an anisotropic speed of light if we want to, it is completely arbitrary. The only rule governing this is that the clocks must be set such that if an object is traveling inertially, at a steady pace without any forces acting upon it, its speed should be measured to remain constant, so the difference in times measured for the object between clocks should be made proportional to the distance traveled by the object. Without this, our definition of speed becomes virtually meaningless, if we were to allow clocks to be set arbitrarily between themselves within a frame. Of course, likewise, they must run at the same rate as each other as well, which can be tested using inertial objects also, so that all must conform to these rules of speed to be useful.

Not just light, but massive objects traveling at less than c can have arbitrarily measured speeds along a straight line path in the same way, so no experiments that can be performed with bouncing light off of mirrors or massive objects off of walls will determine any differently. I know because I have spent the last couple of years trying, but the best I could come up with is relationships that include no absolute notions of motion or synchronization methods. Rotation, however, is absolute. I thought about using a rotating disk to find relations, but then I thought there would be additional complications with the clocks on the disk that might involve GR, so didn't get into it, but the experiment yuiop set up requires no clocks on the disk itself, only stationary clocks around the circumference.

So as the last paper claims, we cannot determine an absolute synchronization by which to set clocks along a straight line, so can't measure the one way speed of light along a straight either with clocks set arbitrarily along the same path, but it does not mention rotation. While we can add so much time to each clock per distance along a straight line indefinitely, we cannot do so around the circumference of a circle. When starting at clock A and working our way around the circumference, adding some time to each clock per arc, we would eventually reach A again, so there would be a discrepency there. All we could do is to set all the clocks ahead or behind by the same amount, but the overall synchronization would still be maintained. So beyond reforming our definition of speed upon which physics is based, and which can't really be redefined with without making a mess of things, rotation does seem to be an absolute method by which to synchronize frames. The first article where the one way speed of light is measured from Jupiter and Io by Roemer may be an extension of this, where the same basic principle is applied to orbits.

 

[Dale]

A straight line isn't a straight line anyway with SR. For instance, if the arm remains more or less rigid according to the frame of the apparatus, a frame moving relative to the apparatus, synchronized to light speed, will observe something close to the diagram below with the four colored lines at various alignments for the arm, although not drawn precisely.

I think this is the key point. In the non-inertial reference frame the synchronization convention is defined such that the arm is bent. This bent arm means that the flash is triggered either earlier or later such that you get the value you would expect from a straight arm in an isotropic-c frame.

Thanks for this very interesting problem, yuiop!

 

[harrylin]

The last paper deals only with measuring the straight line path of light reflected off of a mirror. [..]

So as the last paper claims, we cannot determine an absolute synchronization by which to set clocks along a straight line, so can't measure the one way speed of light along a straight either with clocks set arbitrarily along the same path, but it does not mention rotation. [..] The first article where the one way speed of light is measured from Jupiter and Io by Roemer may be an extension of this, where the same basic principle is applied to orbits.

The last paper not only mentions but even discusses rotation, in the form of an orbit! I copy-paste here the intro of that section: the "Romer Experiment, which measured the speed of light via changes in light transmission time from Jupiter and its moons [...] This effectively entails a time difference measured via a single clock on the Earth as this clock moves to different positions in the Earth’s orbit."

Cheers,
Harald

 

[grav-universe]

In the non-inertial reference frame the synchronization convention is defined such that the arm is bent.

Did you mean the frame moving relative to the spinning arm?

 

[grav-universe]

The last paper not only mentions but even discusses rotation, in the form of an orbit! I copy-paste here the intro of that section: the "Romer Experiment, which measured the speed of light via changes in light transmission time from Jupiter and its moons [...] This effectively entails a time difference measured via a single clock on the Earth as this clock moves to different positions in the Earth’s orbit."

Cheers,
Harald

Okay yes, in appendix ll, right. It doesn't really discuss it, though, just simply stating it involves a single clock, and then goes on to say there is an on-going debate about slowly transporting clocks and about whether an absolute synchronization exists, the present consensus being that it does not. With yuiop's experiment, however, we can see that a single clock is all that is really necessary with rotation as well as apparently with orbits.

 

[Dale]

Did you mean the frame moving relative to the spinning arm?

No, I just meant that any frame where the one-way speed of light is not c is a non-inertial frame in the sense that the laws of physics don't take their "textbook" forms.

 

[yuiop]

Thanks for this very interesting problem, yuiop!

Cheers

If you or anyone else is interested, I have devised this formula for an isotropic speed of light that satisfies the requirement that the two way speed of light is always c.

$$C_{\textrm{one-way}} = c(1 + 0.5^{-\cos(\theta)})/2$$

If $\theta=0$ is North then the speed of light in the North direction is 1.5c and 0.75c in the South direction and c in the East and West directions. Note this quantity is just a magnitude and the direction is given by $\theta$ and that velocities of all massive particles must also be scaled by this factor. In the OP thought experiment, the arms must move with scaled velocity too (to prevent exceeding the speed of light) and when this is done, any proposed anisotropic speed of light is undetectable.

 

[grav-universe]

Cheers

If you or anyone else is interested, I have devised this formula for an isotropic speed of light that satisfies the requirement that the two way speed of light is always c.

$$C_{\textrm{one-way}} = c(1 + 0.5^{-\cos(\theta)})/2$$

If $\theta=0$ is North then the speed of light in the North direction is 1.5c and 0.75c in the South direction and c in the East and West directions. Note this quantity is just a magnitude and the direction is given by $\theta$ and that velocities of all massive particles must also be scaled by this factor. In the OP thought experiment, the arms must move with scaled velocity too (to prevent exceeding the speed of light) and when this is done, any proposed anisotropic speed of light is undetectable.

You keep saying that, that any proposed anisotropic speed of light is undetectable. What does that mean exactly? From the looks of it, all you are really doing with this and your previous post is changing the synchronization of the frame and testing to see if light pulses that arrived at A simultaneously before will still arrive simultaneously and if the varying measure of rotational speed around the circumference remains consistent with the anisotropic speeds of the light pulses, but of course it will be, since changing the settings upon the clocks does not actually change the events as they occur according to the original synchronization, so I don't see how randomly re-synchronizing is useful or what it is supposed to demonstrate. I thought the purpose was to measure one way light speed using some form of absolute synchronization method, which you have found.

 

[Dale]

Cheers

If you or anyone else is interested, I have devised this formula for an isotropic speed of light that satisfies the requirement that the two way speed of light is always c.

$$C_{\textrm{one-way}} = c(1 + 0.5^{-\cos(\theta)})/2$$

If $\theta=0$ is North then the speed of light in the North direction is 1.5c and 0.75c in the South direction and c in the East and West directions. Note this quantity is just a magnitude and the direction is given by $\theta$ and that velocities of all massive particles must also be scaled by this factor. In the OP thought experiment, the arms must move with scaled velocity too (to prevent exceeding the speed of light) and when this is done, any proposed anisotropic speed of light is undetectable.

This works, but since the one-way speed of light is entirely dependent on the synchronization convention it is usually easier to specify in terms of the transformation of the time coordinate. In your case:
T = t - y/(3c)

This formula produces the anisotropic speed of light you mention above and includes the directional dependence, etc.

 

[ghwellsjr]

There are several reasons that I keep saying that we cannot measure the one-way speed of light, not the least of which, it's what Einstein said in his 1905 paper and what he repeated in several explanations in the years following.

But beyond that, the one-way speed of light is the characteristic of an absolute ether rest frame. By that, I mean, only when you are at rest in the ether will the two halves of the measureable round-trip (from light source/detector to mirror and back to light source/detector) light time be equal. If you are moving with respect to the ether, then it will take longer for the light to get to the mirror than it will for the light to get back to the source/detector (or the other way around). Being able to identify the difference in these two times is identical to being able to identify the absolute ether rest frame.

When you attempt to make the measurement, you get a null result, just like MMX. What if the first time Michelson did his experiment and could not measure any ether wind, he stopped right there and wrote his paper declaring that he had discovered the absolute ether rest frame? What a co-incidence!

The same thing happens when you attempt to measure the one-way speed of light, you will measure that it is identical to c. Now you can claim that you have discovered the absolute ether rest frame. But you better not do it again after you have accelerated with respect to your first measurement or you will discover that nature had fooled you just like it would have fooled Michelson if he had done his experiment just one time.

There is another way to consider this problem. Suppose in your rest frame you measure the one way speed of light and get c. In other words, both halves of the round-trip speed of light take the same time. Then you observe someone else traveling at a high rate of speed. In your rest frame, that other person will measure the round trip speed of light to be c (keeping in mind time dilation and length contraction and relativity of simultaneity) but you will conclude that each half of the round trip will not take an equal time. However, as you observe him measuring the time for the two halves, you see that he also concludes that they are equal (again because of time dilation, length contraction and relativity of simultaneity). Keep in mind, I'm only considering one frame of reference here.

Now why does Einstein say that the assignment of equal times to both halves of the round trip speed of light measurement is an arbitrary definition, as opposed to anything that can be measured? It's because the alternative theory popular at the time, Lorentz Ether Theory, maintained that there was a single preferred absolute ether rest frame and that we on the Earth are also traveling with respect to this rest frame. We are like the other observer in my above example. We could use a single agreed upon rest frame as the absolute rest frame as our single definition from which to make all measurements. Then when we use that definition to assign the times of the two halves of the round trip light speed experiment, we will say that they are not equal, just like we will say that our lengths are contracted in the direction of the ether wind and our clocks are running slow and we have simultaneity issues but they are all consistently resolved by referring back to our arbitrarily agreed upon absolute rest frame.

 

[grav-universe]

There are several reasons that I keep saying that we cannot measure the one-way speed of light, not the least of which, it's what Einstein said in his 1905 paper and what he repeated in several explanations in the years following.

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.

 

[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