# Measuring One-Way Speed of Light

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1. Apr 22, 2014

### cooperm

Hey all, first I just want to say that I am by no means a physicist - just someone that is very interested in physics, and I have very little understanding of physics, but I am trying to learn.

I've been reading this article about anisotropic synchrony convention which mentions that we cannot measure the one-way speed of light because you would need 2 perfectly synchronized clocks, and to synchronize them perfectly you would already need to know the one-way speed of light first.

So this leads me to my 2-part question:

1. If you somehow made the light travel in a circle so that it travels back to you without going 2 ways, would you be able to calculate the one-way speed of light?

2. Is it possible to make light travel in a circle without affecting its speed, or at least affecting it in such a way where you would know exactly how much you affected it?

2. Apr 22, 2014

### ghwellsjr

The two-way measurement of the speed of light is a round-trip. Going in a circle is a round-trip. That does not get around the problem of measuring the one-way speed of light.

3. Apr 24, 2014

### Wes Tausend

George,

First let me say that the link that the OP (cooperm) posted in post #1 seems to have a pretty good explanation of why light can't be measured one way, a subject you so graciously explained to me in the recent past.

Second, it seems there may be a way to measure light one-way something on the order that cooperm queried, but it seems impractical. Since light can be completely absorbed by passing near a black hole, it seems there may be a narrow point of radius where light might not escape, but would not be absorbed either. Whatever this point is called (Schwarzschild radius?), it would be technically an orbit I think.

Hawking has stated something to the effect, in his Brief History of Time, that an orbit proceeds in a straight line because of curved space. So if one could practically (doubtful) measure the speed of one light orbit, the speed should constitute a one-way measurement using a single clock. The light should feel no acceleration. The single clock might run quite slow near such a field but it should display a reliably consistent rate, I think.

The flaw must be that the single clock would either have to also orbit at the speed of light to escape capture (an impossibility in which case the clock would feel no acceleration, nor slow) or perhaps less flawed, the single clock could pick up a specific light beam from afar, a beam that was known to have done a sling-shot around a black hole. Just short of such an orbit would be the ultimate observed gravitational lensing, I believe. Is such a sling-shot path ever possible in any manner, to arrive back at the coordinates where one started?

Before I get too involved, is the above-linked general "RationalWiki" website OK to reference here at physicsforums, or is it considered too controversial? I might like to reference the website in the future for the one-way light explanation, or other carefully chosen subjects.

Thanks,
Wes
...

4. Apr 24, 2014

### WannabeNewton

You can have a light beam freely falling in closed circular orbit at e.g. $r = 3M$ in Schwarzschild space-time (the so-called "photon radius") but a measurement of the average speed of the circulating light beam after a complete period, using a stationary (hence accelerating) clock at the same radius, would not constitute a measurement of the one-way speed of light as it involves a round-trip light circuit starting and ending at the same clock. A one-way speed of light "measurement" requires two spatially separated synchronized clocks and since clock synchronization is purely conventional, you cannot actually measure the one-way speed of light in the same way one measures the two-way speed of light. Rather the one-way speed of light is defined by the choice of synchronization convention.

5. Apr 24, 2014

### ghwellsjr

Wes,

I limit my discussions to Special Relativity. I don't know General Relativity. Maybe someone else can respond.

As far as the link goes, it is full of clutter, I would recommend the wikipedia article on the one-way speed of light.

EDIT: Looks like somebody already did. Thanks, WBN.

6. Apr 26, 2014

### Wes Tausend

Thanks for responding and the link advice, George. The Wikipedia link is pretty good without some of the apparent slants of the OP's link, and I never thought to specifically look for it. It helped me see WBN's point.

Sorry it took so long to get back and acknowledge this.

Thanks,
Wes
...

7. Apr 26, 2014

### Wes Tausend

I think I see your point WannabeNewton. The difference is partly accepted definition, and partly that the light would still suffer from the impossibilty of separating speed in one direction from that in the opposite (return) and knowing they are certainly identical.

Such an obtuse method might allow a single clock to be used, but the logical conclusion would be the same as using a mirror and a single clock. In thinking it over, one might be able to someday do such a measurement by passing light in such an orbit as to pass close to the black hole in perihelion and measuring it at a less stressful point of aphelion. But there is no reason to wish to do so at this time because of reasons you have stated.

As a bit of trivia, I did see such a sling-shot set-up mentioned before. A similar point came up once before on some forum, or another, where it was supposed that the traveling twin of Einstein's Twin Paradox would not undergo a discernable acceleration by carefully sling-shooting around a star in a u-turn, and could thereby return to earth without "feeling a return" acceleration and affecting his aging. (If I recall that scenario correctly.)

One other way-out experimental method might have occurred in a once-imagined closed universe where a beam of light could be supposed to leave the universe in a single direction and reappear from behind still traveling the same direction. This could then be repeated in the opposite direction and times compared as of a single clock. I believe the first flaw would be, that we are quite certain that we do not presently live in a closed universe. That, and secondly, the light might take forever to return.

Wes
...

8. Apr 26, 2014

### Staff: Mentor

There's some history at work in the terminology here.

The fundamental issue is actually the difference between two-clock and one-clock measurements of the speed of light; the former require a synchronization convention and the latter do not. When this issue first came up, general relativity and the possibility of curved light paths were decades in the future; only straight-line light paths in flat space-time were considered so a one-clock measurement necessarily involved a mirror or equivalent, and therefore was a two-way measurement. In principle GR suggests that there are other ways of making one-clock measurements, ones that don't have an obvious "out" and "back" to make the phrase "two-way" feel natural. But that's just a problem with the terminology, not the physics.

After a century of established practice, I don't expect the terminology to change. Everyone knows what I mean by measuring the "two-way" speed of light, but I'd get blank looks if I started talking about "one-clock" measurements.

9. Apr 26, 2014

### DrStupid

In theory a one-way measurement of the speed of light would be possible with two parallel slits rotating on the same axis. But in praxis it would be difficult to get the required accuracy.

10. Apr 26, 2014

### ghwellsjr

I don't understand your measurement but I can assure you that it's not an issue of accuracy. There is always some assumption that is made in these kinds of proposals that is tantamount to saying that the light takes the same amount of time to go one way as it takes to go the other way.

11. Apr 26, 2014

### DrGreg

It's worth pointing out that if you use a non-standard synchronisation method, it doesn't just result in an anisotropic speed of light, it affects the coordinate speeds of everything else as well. For example, a rotating disk that appears rigid with standard synchronisation would not appear rigid with a non-standard synchronisation; points on the circumference wouldn't all move with the same, constant coordinate speed. So an assumption of rigidity is tantamount to an assumption of standard synchronisation.

12. Apr 26, 2014

### DrStupid

I mean something like this:

The distance between the slits can be measured by use of any suitable reference.
The orientation of the slits would be adjusted in rest. After starting rotation any oscillations need to fade out before the measurements starts. Than the angular velocity can be measured locally at any point of the axis.

The maximum sensor signal is reached after n full rotations of the slits. The speed of light results from the corresponding angular velocity and the distance between the slits.

As it is a one-way measurement there is no other way. Therefore no corresponding assumptions are required.

13. Apr 26, 2014

### ghwellsjr

Yes there is. DrGreg already addressed it. You have an assumption of rigidity.

Look at it this way, you either have to drive the shaft at one location and assume that when the oscillations fade away, the slits are synchronized or you have to drive the shaft at more than one location and use some other method to assure that they are synchronized.

14. Apr 26, 2014

### DrStupid

There is no rigidity required. Elasticity is sufficient.

15. Apr 26, 2014

### Wes Tausend

...
DrStupid,

It seems the above claim of a single clock, from rotating slits on the same axle, stems from the belief of an equivalent of a solid bar locking the hands of two distant clocks together so that the clocks must logically run concurrent. Or, if one must insist it is one long single clock, the speed of light is measured within the contained volume of the one clock. Interesting take.

It does agree with what I once imagined time to fundamentally be. In the most simple system, time can be measured only by a uniform rotation as a periodic event comparison ratio to any other event to be timed. In more complicated systems, other forms of time measurements might be attempted, but all distill to any sine wave or frequency, and they are still only a distended rotation. Therefore time is always a periodic rotation to me, and the direction of rotation does not affect the Arrow of Time being one direction. I am always looking for the root cause, or definition, and I could be wrong here.

Thanks,
Wes
...

16. Apr 26, 2014

### WannabeNewton

That doesn't matter. The "experiment" still overtly requires the use of clock synchronization so you're back to square one. If you have a journal reference attempting to bypass the use of clock synchronization in any and all "measurements" of the one-way speed of light then please link it.

17. Apr 26, 2014

### ghwellsjr

If your shaft is elastic, then from where does you confidence come that the two slits pass the light at the same time?

18. Apr 26, 2014

### Staff: Mentor

How do you measure $n$?

19. Apr 26, 2014

### DrStupid

The slits can not pass the light at the same time because the light needs some time to travel from one slit to the other.

20. Apr 26, 2014

### DrStupid

The experiment results in a sensor signal as a function of the corresponding angular momentum. This function is periodic and n is "measured" by counting the maximums: n=0 for the first maximum for the non rotating slits (obviously not very useful), n=1 for the second maximum and so on.

21. Apr 26, 2014

### ghwellsjr

Your picture shows that light is passing through both slits at the same time. Of course it's not the same photons passing through both slits at the same time. I thought the idea was that a photon passing through the first slit while it was on the top would pass through the second slit exactly one rotation later while it was on top and you vary the speed of rotation until you see light come through. Isn't that what is supposed to happen? And unless you assume that both slits are on top at the same time, in other words, that the shaft is rigid, then how can you determine when a given photon got from the first slit to the second slit?

22. Apr 26, 2014

### Staff: Mentor

You mean angular velocity, correct? That's what you're measuring, according to your description. I'll assume that's what you mean below.

Ok, so the experimental results will be a curve of sensor signal vs. angular velocity. But the angular velocity that appears in the experimental results is measured at some particular point on the axis; there is no guarantee, without an assumption of rigidity, that the angular velocity at the point of measurement equals the angular velocity of the slits, because the whole assembly is elastic.

Even if you let all oscillations damp out as best you can, there is no guarantee, again without an assumption of rigidity, that the relative orientation of the slits under steady rotation is the same as it was in the original rest state. And if you try to measure angular velocity at multiple points, or try to measure the actual relative orientation of the slits while in motion, or try to drive the shaft at multiple points to control the slits' relative orientation (as ghwellsjr pointed out), you have the usual clock synchronization issues.

Finally, you have an obvious clock synchronization issue between the sensor and the measurement point for angular velocity; these two measurements are at spatially separated points, so you can't just assume the two signals are referenced to the same time, which means you can't even generate the experimental result you are describing without making some assumption about clock synchronization.

23. Apr 26, 2014

### DrStupid

Yes, you are right.

The angular velocity is not the problem. It could be measured by Doppler effect. The problem is the same orientation of the slits in their co-rotating rest frame. This is guaranteed because the assembly is elastic. Elasticity implies that the device returns to its position of rest after the accelerating angular momentum is removed. It might start oscillating first but after thermalization of the mechanic energy the slits will be in their original relative position again.

If the construction is sufficiently balanced I do not see why there should be a torsion without angular momentum. However, if such a torsion exists and leads to a wrong result it should be detectable because it would hardly give the same wrong result for any configuration.

These measurements neither needs to be performed at spatially separated points nor at the same time. Thus I do not see any clock synchronization issue.

24. Apr 26, 2014

### DrStupid

As mentioned above the shaft does not need to be rigid. Elasticity is sufficient to guarantee that the slits are in their original relative orientation as soon as the rest position is reached.

25. Apr 26, 2014

### ghwellsjr

Do you accept without question or exception all the tenets of Special Relativity, including the Lorentz Transformation?