The unchanging speed of light in a vacuum is a foundational fact of relativity. This constant speed has been tested to unprecedented accuracy, but there are some that argue that isn’t enough. In special relativity, it is assumed that the speed of light doesn’t depend upon what direction the light is traveling in, or where it is in space. Physical processes might affect the speed of light, but mere location and direction doesn’t. This is actually part of a broader metaphysical idea that the universe is homogeneous and isotropic. Basically, it’s the assumption that the laws of physics (whatever they might be) are the same everywhere in the universe. This is in contrast to ideas such as geocentrism, which assumes that Earth holds a special place in the cosmos. It’s been an assumption as far back as Newton, though it has been tested in several ways, and has held up so far. But what if the assumption about light is wrong? What if the speed of light is actually anisotropic?

The initial verification of an invariant speed of light comes from the Michelson-Morley experiment in 1887, which showed that the speed of light didn’t depend upon the motion of the Earth. This implied there wasn’t an absolute reference frame, or a luminiferous aether through which light propagated. Over the years the speed of light has been measured with ever greater precision, and it’s always appeared to be a physical constant. But most of these experiments rely upon light to make a round trip in two directions, so technically it’s a measure of the “two-way” speed of light. What hasn’t been done is a direct “one way” speed of light measurement. You might think that’s easy enough to do, simply measure the start and finish time of a photon, for example. But to do that you’d need to synchronize your clocks, which means you’d have to set them at the same time when they are side by side, then move one clock to the finish line. Of course, when you do that, the motion of the clock would affect its measure of time, and you can’t be sure they they are still in sync without assuming some model like special relativity.

Suppose then that the speed of depended upon its direction of motion? Suppose it travelled almost instantly when heading toward us, but at half the “speed of light” when traveling away from us. The round trip time would be the same as relativity predicts with a constant speed of light. Most physicists don’t worry about this kind of thing since relativity keeps passing all the tests, but philosophers love to explore these kinds of metaphysical weaknesses. So the “one-way light problem” appears every now and then in the literature.

So what if the speed of light isn’t the same when moving toward or away from us? Are there any observable consequences? Not to the limits of observation so far. We know, for example, that any one-way speed of light is independent of the motion of the light source to 2 parts in a billion. We know it has no effect on the color of the light emitted to a few parts in 10^{20}. Aspects such as polarization and interference are also indistinguishable from standard relativity. But that’s not surprising, because you don’t need to assume isotropy for relativity to work. In the 1970s, John Winnie and others showed that all the results of relativity could be modeled with anisotropic light so long as the two-way speed was a constant. The “extra” assumption that the speed of light is a uniform constant doesn’t change the physics, but it does make the mathematics much simpler. Since Einstein’s relativity is the simpler of two equivalent models, it’s the model we use. You could argue that it’s the right one citing Occam’s razor, or you could take Newton’s position that anything untestable isn’t worth arguing over.

Models such as anisotropic light are useful and interesting as a way of exploring the limits of what our scientific theories can tell us, but unfortunately they’re also used in a range of pseudoscientific models. In this case, the idea of a young Earth. One of the basic challenges for young Earth models is the starlight problem. If the universe is only a few thousand years old, how can we see light from the edge of our galaxy, much less other galaxies. One way to address this issue was to propose that the speed of light was much faster in the past, allowing distant starlight to reach us in a short time. But observations of line spectra from distant nebula shows that speed of light has changed no more than one part in a billion over the past 7 billion years. Then in 2010 Jason Lisle revived the idea of anisotropic light. If light moving toward us travelled at infinite speed, and away from us at half the traditional speed of light, then it would allow the most distant light in the young universe to reach us while still agreeing with relativity.

As crazy as that might sound, Lisle is right in claiming that such an effect would be indistinguishable from relativity, and this has made the work popular with young Earth supporters. However agreement with relativity isn’t enough. If light did actually reach us from distant galaxies instantly, we would expect galaxies at all distances (or more formally redshifts) to all look the same age. In fact, what we see is that more distant galaxies are younger than closer ones. If Lisle’s idea was correct, we wouldn’t see the magnification of distant galaxies due to cosmic expansion, nor fluctuations in a cosmic background, nor galaxy clustering in agreement with dark energy, nor a host of other observational results.

On its own, relativity doesn’t require isotropy and homogeneity, even though we generally assume it to be true. But when we combine relativity with the confluence of evidence we have in astronomy, we find that assumption is not only justified, but valid to the limits of observation so far.

**Paper:** Md. Farid Ahmed, et al. *Results of a one-way experiment to test the isotropy of the speed of light*. arXiv:1310.1171 [gr-qc]

**Paper:** John Winnie. *Special relativity without one way velocity assumptions*. Philosophy of Science, Vol. 37, No. 2 (1970)

**Paper:** Jason P. Lisle. *Anisotropic Synchrony Convention—A Solution to the Distant Starlight Problem*. Answers Research Journal 3 191–207 (2010)

## Comments

Exceptionally well done.

Why do you continue to waste your/our time with such outrageously idiotic, religious-fanatic “young Earth” or “electric-universe” explanatory posts? When you justify legitimate science against insanity, no matter its quasi-scientific and/or mathematically possible foundations, you diminish our situation, if only a tad. It is EXPOSURE the fanatics (aka WACKOs) seek. Not acceptance or confirmation. Have you ever tuned to coast2coastam?

Please, leave bad science to the Bad Astronomer, and continue to explain our physical surroundings and entice explorations and pondering, as usual, with your insightful productions, but allow Skeptical Enquirers and others to debunk, as is their mission.

In the case of this post, I think it’s an interesting aspect of the limits of experimental data. I don’t look at fringe models all the time, but I think it’s good to talk about them occasionally, if only as a demonstration that we are open to alternatives, and that our rejection of them is not based upon ideology, but upon evidence.

I find it important to examine the “alternative theories” even the really cranky ones

not only to show that scientists are not closed minded but for us the amateurs

I am an astro photographer and is often asked about Electric,Holographic,Spiritual

or whatever Universe theory,or UFO ,Aliens, and the like. Brian clarifications and

refutations are a great help to explain, to whomever ask, why these ideas are not

relevant, useful or consistent with the evidence

I have spend thousands of hours in the desert under the stars, never saw UFO’s

nor met any aliens,although, I must admit, that sometimes when the laptop crashes

the battery goes dead, the scope act erratically I start to believe in “Trolls and genies !!!!

Thank you, Brian for at least attributing Metaphysics in your field. I’m a metaphysicist so your posts have inspired me as well. More radioactive powers to ya!

Hi Brian,

I have a new proposal how to measure one way speed of light. :

Let’s have set of points A, B and C . Let the distance AC=BC >>AB. (Theoretically point C should be at infinity). Let’s send a signal from A to B and from A to C simultaneously. When the signal reaches B, let’s reflect it toward the C. If the point C is at a large enough distance from A and B, signals from A to C and from B to C will be practically parallel. Distance AB divided by the time delay between AC and BC measured by the clock at C will be a one way speed of light from A to B.

For practical reasons the signal from C could be reflected back to A or B (or any other point D in close vicinity).

For example, if our distance AB=30km and point C is located on the Moon (if we could use one of the reflectors left there by NASA), cos(CAB)=cos (CBA)= 30km/385,000km=~1, the time for the light to travel to the Moon and back will be just over 2s. The time delay between signals from A and B will be the distance AB divided by one way speed of light regardless of the distance to C.

Regards,

Kris

Because it involves round-trip measurements, it’s still not one way. The key is that even though we can’t measure one-way light in the context of special relativity, other observations let us know that the speed is uniform.

I think we can synchronize distant clocks:

Let’s have a flashlight which rotates with angular velocity “w” while emitting the light. At a distant r from the flashlight let’s have two clocks A and B separated by distance d. Linear velocity of the beam at r from the flashlight will be v=wr. So rotating beam of light will activate the clock A and a certain time later will activate clock B. For our experiment to yield high enough accuracy we would need r and was as large as possible. So we would need fast spinning powerful flashlight which could shine the light visible from the big distance.

Luckily the Creator left such a flashlight for us. It is called a pulsar. As we know pulsars can spin extremely fast (even more than 700 rotations/s) and are very stable, rivaling accuracy of atomic clocks.

Let’s make a rough calculations: if r=1100ly(=~10^19m, w=10hz our linear speed will be 6.28x10x10^19m/s=6.28×10^20m/s.

If our distance d from A to B is 6.28×10^6m (less than 0.1s for the light to travel from A to B and back; this is important that we are able to exchange information between clocks A and B within the time shorter than one full rotation of the pulsar), the signal from the pulsar will arrive at A 10^-14s earlier than at clock B. I think that if we are not very fussy, we could say the signals arrive at A and B simultaneously.

Now to synchronize the clocks, at the beginning of the pulse from the pulsar detected at A we send the information from A to B that at the beginning of the next pulse from the pulsar clock at B should be started. When the next pulse from the pulsar arrives, we start both clocks A and B which will now be (almost perfectly) synchronized.

Having two clocks synchronized, we can easily measure one way speed of light.

Kind regards,

Kris

Dear Brian,

You suggest in your article that one-way light speed measurement is not possible since synchronized clocks are not available. However the very successful GPS has accurate synchronized clocks that can be used to make a one-way light speed measurement. The result is light speed c-v for light travelling east and c+v for light travelling west where v is the speed of the Earth’s surface at the particular latitude. In other words while two-way light speed is isotropic, one-way light speed is anisotropic contrary to the accepted view.

You also suggest that the equations of special relativity continue to hold even with one-way light speed anisotropy providing there is two-way light speed isotropy. However the Lorentz transformations of special relativity predict one-way light speed constancy which is inconsistent with one-way light speed anisotropy.

I would be interested to read your comments on these two points.

GPS clocks are still subject to relativity. For a true one-way speed of light you’d need two synchronized clocks that stay synchronized when they are relocated, which relativity says isn’t possible. So while one-way experiments have been done, they still don’t disprove models that claim a universal two-way speed of light. In the post itself I linked to a paper that derives the equivalence between experimental results and a two-way speed of light. So you can formulate a model in which two-way light speed is constant, one-way light speed isn’t, and get exactly the same results as standard relativity.

Keep in mind this post is not arguing that relativity is somehow wrong, or that the speed of light isn’t isotropic. The point is that experiments have limitations, and its useful to know what are assumptions of a model and what are confirmed results. Just in terms of special relativity, we don’t need to assume isotropic light speed, though it is useful to do so. There is also no experimental or theoretical reason to presume light isn’t isotropic.

Dear Brian,

I have done a fair amount of work with the GPS and am very familiar with its operation. Contrary to your claim, GPS clocks stay synchronized even as the GPS satellites move around the Earth. The necessary clock adjustments are programmed into the computer software and the resulting clock synchronization ensures that the system works as well as it does. Therefore two ground-based synchronized GPS clocks can be used to measure one-way light speed. This is a very straightforward concept. The problem for physicists is that the result is one-way light speed anisotropy which is inconsistent with the light speed invariance postulate of special relativity.

Regarding my second point on special relativity, you state, “we don’t need to assume isotropic light speed.” I am contending that it is necessary to assume isotropic light speed since the Lorentz transformations predict light speed isotropy (this is easily shown by differentiation) and therefore would be inconsistent.if light speed were anisotropic. Therefore any finding of one-way light speed anisotropy would be problematic for special relativity.

I would welcome your further comments.

Ah, now I understand. You aren’t asking for clarification, you want me to debate your particular alternative model. In that case you can contact me directly to discuss payment for vetting your work.

No I do not want you to vet my work! You wrote an article making certain assertions and invited comment. I was simply commenting by challenging some of your assertions. Your initial response was quite unconvincing. Based on your latest response, we can end this exchange.

Actually, you stated misconceptions (not understanding relativistic synchronization, for example) and invited me to pick them apart. What you didn’t state is that your misconceptions are part a model you developed which claims the invalidity of relativity and an anisotropic speed of light. You didn’t even bother to look at the references in the post itself.

Technically you didn’t want me to vet your work, you wanted to play the game of having me explain science to you while you continually shift the goalpost. We can play that game, but I have an hourly rate.

I have another proposal to (at least theoretically) synchronize the distant clocks:

Let’s have two light sources at points A and B separated by distance d and sending constantly (perpendicular to AB) signals to clocks at A’ and B’

Let’s have an opaque rigid rod of the length d traveling with constant speed v(non relativistic) parallel (and very close to) the line AB from B towards A . Initially the light from B to B’ will be blocked and the light from A to A’ will be allowed to be transmitted . When front end of the rod will start cutting off the light from A to A’ the light from B will start to be transmitted to B’ . At this moment we will have both clock at A’ and B’ synchronized.

Hi Brian, I enjoyed this article!

Having always assumed that the speed of light is isotropic, I have just discovered and begun to learn about the anisotropic synchrony convention; thus I found your article and others. It is stated repeatedly that the measurement of one-directional light is impossible due to relativity, but my question is this: is it possible show with some confidence that light never travels at infinite speed?

As an example: at the end of Cassini’s mission, NASA listed the one-way signal time as 85 minutes. I am assuming this meant 170 minutes were required for a full round-trip communication (send/acknowledge) sequence to complete.

It would seem that if the speed of light is infinite in one direction (to the “observer”), then the Cassini probe would have observed the signal from Earth instantly (from its perspective), and immediately responded with a return signal. Then on Earth, being the “observer” of the signal originating from Cassini, we would have instantly seen the response (i.e. no time delay from Send to Acknowledge on Earth).

However, this was not the case, as 170+ minutes delay was imposed after sending a message for a response to be received from the probe.

Could this time delay be used to confirm that neither light traveling toward or away from an observer is traveling at infinite speed? (It would not confirm a uniform speed, but would rule out infinite speed?) Or does relativity still render this type of measurement impossible? And in such an example, does it make any difference that the reply is NOT reflected light, but rather a generated response from the probe itself? (Bouncing light off a mirror would be reflected; however, sending a radio signal from an antenna would be generated.)

I’m sure I’m misunderstanding or missing several points, but I’d really appreciate your thoughts on this to help me understand more clearly.

Thank you!

No, in terms of the physics generating a new signal would be the same as reflecting a signal. The speed in a particular direction would depend upon the nature of spacetime.

Thank you so much for your reply. To clarify, you are saying light’s speed may be different when traveling through different paths in spacetime, and the speed differences are due to spacetime itself, not the observer’s position? thus light’s speed through any given path would be arbitrary?

My other question is about light which has no return path. For example, photons from a star which hit a detector on Earth (your eye) would not be reflected back, thus they only traveled in one direction. Isn’t it necessary at that point to assume this light’s speed to have been c, since there’s no return path and no possibility to average the forward and return speeds to arrive at the required c?

Or is this again an application of the anisotropic principle and the light could travel at different speeds at various points along the path? But even if it travels at different speeds through different parts of spacetime in between the source and destination, wouldn’t the average speed in one direction still need to equal c?

Again thanks for taking the time to field my silly questions.

Best,

Ryan

The way 1-way speed of light problems are posed is that there is an inherent difference of speed in particular physical directions. In contrast, the isotropic assumption is that the speed is the same in all directions. The most common use of the 1-way speed argument is that Earth is “special,” so the speed toward Earth is ultra-fast, but in other directions is regular c. There’s other ways make the argument, but they all come down to some “special” frame of reference. Relativity argues that there isn’t a special reference frame, and that idea agrees with observation.

The speed of light is constant relative to the earth’s centered non-rotating reference frame. It is experimentally verified that the speed of light is not constant relative to earth’s surface (Michelson-Gale, GPS).