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The One-way Speed of Light Controversy

Posted June 13, 2013 2:33 PM by Jorrie
Pathfinder Tags: relativity speed of light

The very title of this post may surprise and/or offend some relativistically minded readers. I will try to give a reason for this possible reaction, while at the same time try to narrow the gap between relativists and more practically minded engineers. To set the scene, I quote a piece from Einstein's 1905 paper on Special Relativity (1923 English translation), specifically on the definition of simultaneity.

"If at the point A of space there is a clock, an observer at A can determine the time values of events in the immediate proximity of A by finding the positions of the hands which are simultaneous with these events. If there is at the point B of space another clock in all respects resembling the one at A, it is possible for an observer at B to determine the time values of events in the immediate neighbourhood of B. But it is not possible without further assumption to compare, in respect of time, an event at A with an event at B. We have so far defined only an "A time" and a "B time." We have not defined a common "time" for A and B, for the latter cannot be defined at all unless we establish by definition that the "time" required by light to travel from A to B equals the "time" it requires to travel from B to A."

Einstein then described how the two clocks A and B can be set to a "common time", i.e. be synchronized in time readings. The method is simple in principle. Observers A and B are both stationary in the same inertial frame in free space and they measure the distance AB between them. They agree that A will send a light flash when her clock reads precisely time T1. Observer B sets and holds his clock on time T1 plus the time that the light pulse should take to reach him, i.e. AB/c. When he observes A's flash, he cancels the hold and let his clock run freely from T1+AB/c. They can now verify the process by B sending a light flash at a predefined time T2 and A simply checks that it arrives at time T2+AB/c. Their clocks are now "Einstein-synchronized".

By Einstein-synchronizing the two clocks we have forced the one-way speed of light to be measured as the same as its two-way speed, c. By demanding that every inertial frame synchronizes its clocks by this method, it is unremarkable that the one-way speed of light is observed to be c by all inertial observers and the same in every direction. Likewise, the fact that the two-way speed if light is c in all directions for all inertial frames is also unremarkable, provided that we forget about a medium for light propagation (the 'luminiferous aether'). Here we need only one clock to check the two-way speed in every direction possible - hence clock synchronization does not enter the picture. Better still, we can use an interferometer, just like Michelson-Morley did way back in the 1880s. It boils down to checking whether the round-trip time-of-flight for light over a given constant distance changes for different directions in space.

Even if the light signal was a stream of perfectly elastic balls, shot at identical speeds to bounce back from reflectors and their time-of-flight measured, we would not expect the direction to influence the round-trip time of the balls, irrespective of how the apparatus was moving in free space (provided that it was not nudged or accelerated, of course). As long as we leave the aether out of the picture, there is absolutely nothing to relativity. Granted, the whole picture is not quite as simple as that, but we do not need Lorentz-contracting arms of the apparatus (or time dilation) to explain the Michelson-Morley experiment.

Yes, if some observer who is rapidly moving relative to our setup was silly enough to attempt a measurement of the lengths of the arms of our interferometer, he will get shorter arms than what we reckon they are (in the direction of his movement). But that's his problem and we know why - he has the difficulty of reading his measuring tape simultaneously (on the fly), at both ends of the in-line interferometer arm; simultaneously according to his own set of synchronized clocks. We know that his clocks will not appear synchronized in our inertial frame, but this is perfectly predictable, using Einstein's relativity theory. There is obviously no physical contraction in any of our interferometer arms. They work just fine with the lengths that our workshop made them to have.

What about the tick rate of the clocks of this 'fly-by' observer? Will they tick slower than ours? Not in any absolute sense, but if we make careful observations, we may think that his clocks are losing time. The problem is that if he makes careful observations, he will think that our clocks are losing time. This is a scenario that I will handle later, but for now, forget about time dilation for purely inertially moving clocks. It is really a myth, just like the "luminiferous aether". They cannot both lose time relative to one other - it is just another observational issue. There are scenarios where the loss or gain of time is real, but this will have to stand over untill next time.

To conclude this post on a philosophical note, why are some scientists hooked on the "myths" of relativity - like the invariance of the one-way speed of light and Lorentz contraction, as if they are (sort-of) real physical realities? We have seen that they are both consequences of the convention that we use to synchronize clocks - the Einstein method. It was a brilliant masterstroke of the great man, because of all possible synchronization schemes, it makes the most sense - it makes physics "as simple as possible, but not simpler", as Einstein has once famously said.

Surprisingly, in years of studying relativity, I realized that physicists in the academic environment are not too dogmatic about these issues; a few of them actually convinced me that we cannot measure the one-way speed of light in any way - it is an assumption that is proven to be a good one by many experiments. Academics tend to look carefully at the origins of theories.

On the other hand, hands-on scientists using the one-way speed of light in their work (e.g. particle physicists at accelerators) can be more dogmatic. I think they are starting to forget the original assumptions made and just press on and use it to their advantage, almost as if it is the only way to view the world. It works, so no problem - why dwell on the origins of a successful theory.

This rigidity is perhaps also enhanced by the fact that the one-way speed of light is defined by international standards authorities to be a constant. Even the meter is defined in terms of the speed of light: "The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second."

-J

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#1

Re: The One-way Speed of Light Controversy

06/14/2013 12:21 AM

Will the speed of light change when passing through different garvitationl constant(g) or through strong electric/magnetic fields?.

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#2
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Re: The One-way Speed of Light Controversy

06/14/2013 12:43 AM

The speed of light never changes in vacuu, meaning if you go and set up a test in a gravitational field and measure it locally, you will get c. If you measure it non-locally, from a distance, you will get something lower than c, depending on the amount of spacetime curvature between you and the light path.

I do not know if electric/magnetic fields change the speed of light, but I guess they don't.

-J

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#3

Re: The One-way Speed of Light Controversy

06/14/2013 2:04 AM

If ever every engineer understands Relativity we probably could also have our houses build by Physicists!

Sorry had to conform to the fact that there of course is engineers that understand Relativity!

Will read later and respond to the actual information.

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Re: The One-way Speed of Light Controversy

06/14/2013 3:23 AM

If we consider that the speed of propagation of Light is fixed and constant and that the light wave does not necessarily travel in a straight line when going through space traversing different zones of different gravitational field or what we consider as different opacity, then measuring the time it took for the light wave to reach or cover a given distance does not give the actual speed of the light beam but might indicate the fact that that beam went via a longer or shorter path ... therefore, the different deviations and curvings have increased the actual distance covered while the speed was constant ... A different approach to the problem ???

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Re: The One-way Speed of Light Controversy

06/14/2013 5:59 AM

".. therefore, the different deviations and curvings have increased the actual distance covered while the speed was constant ... A different approach to the problem ???"

A different problem. The speed of light c is just locally defined, where it is a universal constant. Locally means measured over a short distance, ideally in free space where gravity is absent.

Over large distances in curved space there is no unique definition that we can call "the speed of light". What you will find in the literature is the 'Shapiro time delay of light", which is caused by curved spacetime.

-J

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#5

Re: The One-way Speed of Light Controversy

06/14/2013 4:49 AM

Hi Jorrie; While I find Einstein's theory on the speed of light between A & B logical, there are a few point that have crossed my mind, which you may have addressed, but? The speed of light, is it established at the point that it is initiated, the atom, or is it a function of its environment, a vacuum. I think that gravity refracts light not change its speed, possibly suggesting that light and gravity are initiated from the same source within the atomic structure? electron shell? But does a photon travelling through space for a few years and a photon travelling through space for a few thousand years have a constant speed, and does a photon travelling away from the sun travel at the same speed as a photon travelling towards the sun?

Regards JD.

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#7
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Re: The One-way Speed of Light Controversy

06/14/2013 6:10 AM

"...is it established at the point that it is initiated, the atom, or is it a function of its environment, a vacuum?"

The latter. Wikipedia: "The classical behaviour of the electromagnetic field is described by Maxwell's equations, which predict that the speed c with which electromagnetic waves (such as light) propagate through the vacuum is related to the electric constant ε0 and the magnetic constant μ0 by the equation c = 1/√ε0μ0."

A photon always travels at c, as measured locally in free space, even close to a black hole. If you sit far from the b/h and could somehow watch a photon going radially inwards towards the b/h, you would find that it is delayed, but as I wrote to the prior respondent, it has nothing to do with the speed of light, which stays c all the way down...

-J

PS to everyone: lets try to keep this thread for the special relativistic speed of light in flat space. There are apparently enough issues around this limited scenario that needs to be cleared up (judging from that long, abortive thread on Doppler and the one-way speed of light). We can discuss the "general relativistic speed of light" in a different thread if required.

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#8

Re: The One-way Speed of Light Controversy

06/14/2013 1:46 PM

Good thread as usual, Jorrie. The only thing I don't understand is where the title came from. In the controversial thread that spawned this, there were many controversies, so it was difficult to tell what the main one was, but you were the only one to mention the one-way speed of light.

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Re: The One-way Speed of Light Controversy

06/14/2013 3:44 PM

Hi S,

The 'controversy' is actually evident in that not all scientists agree on the 'conventional nature' of the one-way speed of light. I participate on two other forums where the opinions are split between them.

-J

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#10
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Re: The One-way Speed of Light Controversy

06/15/2013 2:20 AM

Hi again S; for everyone's understanding, maybe I should have said something on the relationship between the one-way speed of light and ralfcis' "light-barrier" of the other thread.

The 'light-barrier' is understood to mean that we cannot accelerate any particle to exceed the speed of light as measured in our own inertial frame. However, the particle just keeps on piling up more and more momentum as we attempt to push it closer and closer to this 'barrier'. And its redshift (or blue-shift, depending on the direction) keeps on growing. Note that the old way of stating this might have been "the particle keeps on piling on more and more mass", but this is no longer used because we know that the rest mass of the particle remains constant in this scenario.

Now, if the one-way speed of light is fixed by a clock synchronization convention, then the 'light-barrier' must be fixed by that same convention, not so? More correctly, one should rather say that the 'light-barrier' is an observer dependent concept. This 'private barrier' is due to the fact that we have to use clocks and light or other electromagnetic waves to observe speed. Or, perhaps it is just convenient for us...

Advanced civilizations at other places in the universe might have taken another view of speed. They could perhaps have decided to use 'rapidity' to express the property that we call 'speed' (the rapidity φ corresponding to velocity v is φ = tanh−1(v/c). For low speeds, φ is approximately v/c.) The rapidity of light is infinite, so the light-barrier would have been more logical (infinite). One can do all your relativity using rapidity, but the equations look somewhat more intimidating, I guess.

On the other hand, using rapidity means we can just add them together like we are used to for low speed calculations, so that φ = φ1 + φ2, not v = (v1+v2)/(1+v1v2), like we have with relativistic speeds.

-J

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Re: The One-way Speed of Light Controversy

06/15/2013 2:38 PM

Hi Jorrie,

I think this is helpful. I like the term momentum much more than mass for a relativistic particle. Mass is often equated with "quantity of matter", and so mass "from my point of view" didn't settle well, but momentum is always from my point of view because I accept that all motion is relative. The rapidity concept is interesting. I like it.

-S

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#12

Re: The One-way Speed of Light Controversy

06/17/2013 1:54 PM

May I use your thread to think out loud? Our discourse in the other thread with our Canadian friend and your post here renewed my interest in this matter.

I want to bring forward a little thought experiment.

The experiment acknowledges the existence of 4 dimensions which are: the three dimensions in space and as a fourth the time.

Lets assume that 3 dimensions are variable and one is constant.

I think I do just describe the known space time relation, if we assume the time is the dimension that stays constant.

Now the idea is that for light the dimension that is constant is not the time but instead it is one of the spatial dimensions.

I have been wondering if this could explain the behaviour of light!

In order to have a "constant" speed (unchangeable by its relative movement) it needs to either defeat space or time. Let's make this space instead of time. I hope it presents itself that we can keep the speed of light constant if we vary the time instead of "distance".

So if for light it is time that is out of whack, how would we perceive this? Maybe as a One Way Speed, as a dependency to one spatial direction. We don't know which spatial dimension is affected. It might not be necessary to know but we still have to consider the other two spatial dimensions. What of them? I was wondering if we could describe a wave a three dimensional construct, which is made up of 2 spatial dimensions and time. Come to think of it I believe that this is exactly how we can describe a wave. Lets call it ""Area" happen over time". Would this be correct?

Last assumption in this little experiment is, that light is a particle, let's say a "Photon". This photon has no weight. Why? Because it has only two dependent spatial dimensions! Bluntly said we cannot calculate a mass without the volume! I always wondered why we cannot really grasp the photons, but just now I think we have not found the two dimensional handle!

We can see light, but then if above is somehow true, we see something that is there but not really there! How so? Well we certainly do perceive and we also can measure the wave behaviour of light. Blue red yellow all just wave lengths!

According to our experiment light still fluctuates in 2 of our 3 space dimensions and in time. The way it interacts with our dimensional system would be as a wave or in wave form! In a long stretch it could explain that during the lights interaction with our dimensions it is affected by our worldly things which there is a particle speed change when going through materials, reflections and so on. But it also could explain the duality of light as particle and wave.

Let's end the experiment here. There is still lots of thoughts but very much undeveloped. Maybe you or anyone else has an interest in thinking in this direction. I am not even sure where this all leaves us? Has it been brought up before? Is it just a rewrite of simultaneity? Is this relativity?

Would be interesting to hear what you have to say!

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#13
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Re: The One-way Speed of Light Controversy

06/17/2013 4:58 PM

I think you intended Jorrie to answer this, But I'll take a shot.

Lets assume that 3 dimensions are variable and one is constant. I think I do [did?] just describe the known space time relation, if we assume the time is the dimension that stays constant.

I don't think any scientist believes that time is constant. Many books talk about the one-way direction of time. I either read or "heard" somebody say that we are moving through time at the speed of light. This doesn't explain why light speed is always the same no matter the speed of the object producing the light IMHO.

photon has no weight. Why? Because it has only two dependent spatial dimensions! Bluntly said we cannot calculate a mass without the volume!

An interesting thought. String theory has "objects" with one, two, and three dimentions. I don't care for it, but that's just me. I can't get my head around the rest of your post.

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Re: The One-way Speed of Light Controversy

06/17/2013 9:28 PM

Ok the idea with my first comment was that time is constant. But after I posted I realised this is not what it could be. Initially I was thinking about that the space is constant and time is running. But if we look at it from the prospective of ourselves we can move within space but time is constantly moving for us! Hence I named time the constant and space the variable. However you turn it there is always one dimension that is locked in while the others are changeable.

I cant really make sense out of it yet other than to think that there is a relativity to it.

I probably need to read something more about string theory. Thanks for the hint

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#15
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Re: The One-way Speed of Light Controversy

06/18/2013 1:21 AM

"However you turn it there is always one dimension that is locked in while the others are changeable."

In Einstein's General relativity, no part of the four dimensional spacetime (4-space) is "locked", because it can be curved and hence changed by energy. It is only when we examine a tiny piece of it in our own neighborhood ("locally) that it seems "fixed and flat."

The easiest way to think about 4-space is that everything moves at a constant 4-velocity = c through it*. This means that more velocity component in any spatial direction has to mean less component in the time dimension. This concept works for both flat and curved 4-space. Massless particles like photons that move at c in 3-space hence do not move in the time dimension at all.

* http://en.wikipedia.org/wiki/Four-velocity

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