# Has anyone measured the speed of light in one direction?

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• OscarCP
In summary, the video discusses the idea that the speed of light cannot be measured one-way, only it is possible to obtain a value for it in two-way measurements. It has implications for the whole of physics, and if the speed of light were different in different directions, many things in reality would be stipulated.
OscarCP
TL;DR Summary
Einstein in his 1905 famous paper "On the Electrodynamics of Moving Bodies" wrote that the idea that the speed of light is the same in all directions is a "stipulation". Meaning that is something conveniently taken to be true without experimental proof, because it fits known facts, leads to no contradictions, but is not based on direct one-way measurements, because these cannot be made.
I don't know if there are some here familiar with "Veritasium", a YouTube video channel dedicated to science and engineering. It was created and is hosted by Dr. Derek Muller. It has over 8 million subscribers and many of its programs have been watched millions of times.
Today I was watching an older one where the main subject was the idea that the speed of light cannot be measured one-way, only it is possible to obtain a value for it in two-way measurements, something that might have very deep implications for the whole of physics. These are measurements where basically light is sent on its way and simultaneously a very precise clock is started. The light crosses an accurately measured distance to a mirror where it is reflected and crosses the same distance back. Its arrival at a detector at the staring point stops the clock providing a time measurement of the round-trip flight. Dividing twice the measured distance by the time it took to go and come back along it is how the speed of light is precisely measured, according to Dr. Muller. As Einstein put it on his famous 1905 paper "On the Electrodynamics of Moving Bodies", the idea accepted as fact that light travels at the same speed in all directions is a "stipulation", meaning something that is accepted as true without proof in order to move on to more interesting things, as assuming it to be true is very useful and leads to no contradictions.
A consequence of this is the title of the video: "Why No One Has Measured The Speed of Light" Or, to be more precise, why no one has measured it ONE WAY. The consequence of this is that in some situations, the results would be the same if the light travelled at one speed one way and at infinite speed on the opposite way, as explained there. The argument in the video is quite compelling, but I think it is valid only within a certain limited description of the way things work: Special Relativity:

https://www.veritasium.com/videos/2020/10/31/why-no-one-has-measured-the-speed-of-light

This is my own take on what to me is a remarkable bit of news; I wonder what others here might have to say about this:

If the speed of light were different in different directions, I think that the truth of things such as the expansion of the universe, that go beyond what Special Relativity, within which the video's discussion is confined, can explain, would be also "stipulated". And consequently, also stipulated would be the notion of the Big Bang, when the universe began to expand from either a Plank-scale, or even a point-like fiery "egg" and time began. Just to mention two of the basic things that require, in my opinion, that light speed needs to be the same in all directions, at least in vacuum, for them to be more than assumptions or "stipulations" whose truth is ultimately as unknowable as the isotropic nature of the speed of light is said to be in the video. Among many other things that are accepted as experimentally verified facts. In other words: most and perhaps all of physics would be "stipulated" and not correctly assumed to be true.

Einstein wrote 1905 the paper where he stated that the idea that the speed of light is the same in all directions is an stipulation, that wether he meant it or not, has the implication that it and many other thing are also stipulations and not a demonstrated facts at a time when the expansion of the universe, for example, was not part of the scientific consensus and probably not even dreamed of. As that could only have happened after Einstein came up with General Relativity, that allows the existence of an expanding universe, and this and the observations of the astronomer Edwin Hubble were put together.
The constant one-way value of the speed of light towards us from distant stars is indirectly implied, I believe, by the red shift of the spectral frequency lines of elements that are supposed to be the same everywhere at every time, unless cosmology is wrong. This shift seems to exclude the possibility that light could have, for example, infinite velocity in one-way trips, because that starlight is coming one-way in all directions from the universe all around us.

Given all the consequences to physics and its relationship to reality of denying that the speed of light in vacuum is a constant in all directions that can be measured, if not directly, then indirectly to fit all known facts, this denial is the door to something either very profound, or very silly.

I imagine that some here might find it interesting as an intelectual exercise to explain their own positions on this intriguing idea.

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The speed of light is isotropic if you choose to use an orthogonal coordinate system and anisotropic if you choose to use a non-orthogonal one. There are no physical consequences to an individual's choice of coordinate system.

You can make coordinate choices that lead to infinite speed of light in one direction in cosmological spacetimes as well. Nobody does because it makes the maths harder and disguises interesting features of those spacetimes with no benefit, but you can do it. The detected frequencies of light, on the other hand, are physical facts and they don't go away whatever choices you make. Changing your coordinate system simply leads you to use different concepts in your description.

In short, all of this is stuff I would file under "silly". There's no physics here, just the freedom to make your maths harder.

PeroK, vanhees71, Dr.AbeNikIanEdL and 2 others
Ibix said:
In short, all of this is stuff I would file under "silly". There's no physics here, just the freedom to make your maths harder.
Great! The score so far: Silly 1, Profound 0.
Anyone for "Profound"?

OscarCP said:
Great! The score so far: Silly 1, Profound 0.
Anyone for "Profound"?
Probably. The actually useful question is: is there anyone who even half understands the maths who would go for profound?

Nugatory and Motore
Another real and relevant question: Has anyone measured the speed of light one way?

OscarCP said:
Another real and relevant question: Has anyone measured the speed of light one way?
Sure. Ole Rømer did it in 1676. It's not impossible to do, it's just impossible to do without assuming a relationship between your answer and the two-way speed (edit: or disguising a two-way measure somehow) so you're actually measuring the two-way speed and assuming a ratio to get the one-way speed. Rømer assumed the speeds were equal, although being more than two centuries in advance of Einstein he naturally didn't think of it that way.

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cianfa72 and vanhees71
OscarCP said:
TL;DR Summary: Einstein in his 1905 famous paper "On the Electrodynamics of Moving Bodies" wrote that the idea that the speed of light is the same in all directions is a "stipulation". Meaning that is something conveniently taken to be true without experimental proof, because it fits known facts, leads to no contradictions, but is not based on direct one-way measurements, because these cannot be made.

this denial is the door to something either very profound, or very silly
I think that categorization is rather silly. That there are only two possibilities, silly or profound.

The one way speed of light is a convention that we are free to choose. So let’s look at other conventions:

SI units. Silly or profound?
The dimensionality of units. Silly or profound?
The right hand rule. Silly or profound?
The origin of your coordinates. Silly or profound?
The direction of your axes. Silly or profound?
The reference node in a circuit. Silly or profound?
The sign of an electron’s charge. Silly or profound?
Your generalized coordinates. Silly or profound?
The location of 0 potential. Silly or profound?

I don’t think that conventions are generally either dichotomously characterized as silly or profound. It is important to understand when something is a convention and not get trapped into mistakenly treating it as part of nature.

Choosing good axes, for example, can make a homework problem easier. So I don’t think that is silly. But does it follow then that it is profound? I don’t think so either.

The choice of the one way speed of light is a convention. It is not forced on us by any observation. That includes the cosmological observations that you mistakenly believe require it.

Recognizing the fact that it is a convention allows us freedom. We will almost always choose Einstein’s convention, but sometimes his convention makes things more difficult. In those cases we are free to choose a different convention that is more useful for that problem. Is that silly or profound? Or is it just a convention?

Motore, topsquark, vanhees71 and 3 others
Dale said:
I think that categorization is rather silly. That there are only two possibilities, silly or profound.

The one way speed of light is a convention that we are free to choose. So let’s look at other conventions:

SI units. Silly or profound?
The dimensionality of units. Silly or profound?
The right hand rule. Silly or profound?
The origin of your coordinates. Silly or profound?
The direction of your axes. Silly or profound?
The reference node in a circuit. Silly or profound?
The sign of an electron’s charge. Silly or profound?
Your generalized coordinates. Silly or profound?
The location of 0 potential. Silly or profound?

I don’t think that conventions are generally either dichotomously characterized as silly or profound. It is important to understand when something is a convention and not get trapped into mistakenly treating it as part of nature.

Choosing good axes, for example, can make a homework problem easier. So I don’t think that is silly. But does it follow then that it is profound? I don’t think so either.

The choice of the one way speed of light is a convention. It is not forced on us by any observation. That includes the cosmological observations that you mistakenly believe require it.

Recognizing the fact that it is a convention allows us freedom. We will almost always choose Einstein’s convention, but sometimes his convention makes things more difficult. In those cases we are free to choose a different convention that is more useful for that problem. Is that silly or profound? Or is it just a convention?
I entirely agree. I wrote the "profound or silly" line to get people moved to reply and get the thread going. I stand by my statement on cosmological observations, because they do not need to be explained by assuming different speeds of light when the light from stars and galaxies, etc. travels one-way towards us coming in all directions from all over the sky. One speed suffices to explain their redshifts..

As to Rømer measuring the one-way speed of light?.

Rømer actually did not measure the one-way speed of light, for example according to this Wikipedia article, and many, Einstein included, must have been well aware of this. The reason he did not measure it one-way is subtle but also undeniable:

https://en.wikipedia.org/wiki/One-way_speed_of_light

Relevant excerpts:

Rømer's measurement

(*) "The first experimental determination of the speed of light was made by Ole Christensen Rømer. It may seem that this experiment measures the time for light to traverse part of the Earth's orbit and thus determines its one-way speed, however, this experiment was carefully re-analysed by Zhang, who showed that the measurement does not measure the speed independently of a clock synchronization scheme but actually used the Jupiter system as a slowly-transported clock to measure the light transit times."

(**) "The Australian physicist Karlov also showed that Rømer actually measured the speed of light by implicitly making the assumption of the equality of the speeds of light away and towards its source." (**)(*) To measure the time between the sending from A and receiving at B a light signal, two clocks, one at A and the other at B cannot be perfectly synchronized by putting them first together but then moving them apart, according to Special Relativity. But they can be as little de-synchronized as one wished by moving one of the clocks from A to B as slowly as necessary.

(**) Because Rømer assumed that the speed of light when the Earth was moving away from Jupiter was the same as when it was moving towards Jupiter. See here, in particular, the diagram he draw illustrating the observations around first and second quadrature, between the reappearances and disappearances of Io from Jupiter's shadow and accompanying explanation:

https://en.wikipedia.org/wiki/Rømer's_determination_of_the_speed_of_light

The idea of different referent frames, Cartesian and no Cartesian giving different speeds s ingenious, but does it yield different speeds in opposite directions, which is what is of interest here?

Things can be very curious when things remain within the domain where Special Relativity is sufficient to explain them.
I encourage people to watch the Veritasium video to see for themselves the very interesting discussion there, that makes perfect sense, in my opinion, when kept within such a domain.

After watching the video, we may discuss this further.

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OscarCP said:
Rømer actually did not measure the one-way speed of light, for example according to this Wikipedia article, and many, Einstein included, must have been well aware of this.
Indeed. As I said, it's not possible to measure the one way speed of light without assuming a relationship between the one-way and two-way speeds. But something like what he did is the only thing you can call a one-way measure.

OscarCP
OscarCP said:
(*) To measure the time between the sending from A and receiving at B a light signal, two clocks, one at A and the other at B cannot be perfectly synchronized by putting them first together but then moving them apart, according to Special Relativity. But they can be as little de-synchronized as one wished by moving one of the clocks from A to B as slowly as necessary.

Slow clock transport is equivalent to Einstein synchronization, which is not the only possible definition of clock synchronization, as the video shows.
Wikipedia said:
Those experiments directly establish that synchronization with slow clock-transport is equivalent to Einstein synchronization, which is an important feature of special relativity. However, those experiments cannot directly establish the isotropy of the one-way speed of light since it has been shown that slow clock-transport, the laws of motion, and the way inertial reference frames are defined already involve the assumption of isotropic one-way speeds and thus, are equally conventional.
Source:
https://en.wikipedia.org/wiki/One-way_speed_of_light

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vanhees71 and OscarCP
This was intended as an answer to Ibix, but it could be also in part to Sagittarius, if one looks at it at the level of General Relativity and considers the cosmological red shift:

True enough, except this is not what can be called a one-way measurement, as it is explained in the video, so it is a convention, as Einstein implied, not a conclusion from the result of an actual one-way measurement.
Which, right or wrong, I think it makes most of everything in physics also a convention for the reasons I have already explained in the OP. Which, if correct, would also be "profound", would it not?

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Dale said:
Silly or profound?
In the words or the great Nigel Tufnel, "There's a fine line between stupid and clever".

vanhees71
Good quotation!
But looking at, for example, the Wikipedia article or the video, it does not look like this is considered to be on the "stupid" side of this line. Which makes it, in my opinion, interesting and worthy of being discussed in "Physics Forums." So here we are.

OscarCP said:
I think it makes most of everything in physics also a convention for the reasons I have already explained in the OP.
No. Everything in physics must be expressible in terms of invariants, and anything that is just a convention is not observable but rather is a choice of interpretation of measurements. The one way speed of light is such an interpretation, but the two way speed is not.

OscarCP said:
I wrote the "profound or silly" line to get people moved to reply and get the thread going.
So clickbait. It is not very appreciated here. It makes you seem less reasonable and it wastes time and effort as it has here. Now, instead of discussing the substance of your argument we are discussing the clickbait. Please refrain from that in the future.

OscarCP said:
I stand by my statement on cosmological observations, because they do not need to be explained by assuming different speeds of light when the light from stars and galaxies, etc. travels one-way towards us coming in all directions from all over the sky. One speed suffices to explain their redshifts.
You can stand by it all you like, but it is missing the point. Yes, one speed suffices, but the observations are also consistent with an anisotropic one way speed. The whole point of the conventionality of the one way speed is not that anisotropy is required, but that it is in fact a valid arbitrary choice.

vanhees71, Motore, Doc Al and 3 others
Ibix said:
Indeed. As I said, it's not possible to measure the one way speed of light without assuming a relationship between the one-way and two-way speeds. But something like what he did is the only thing you can call a one-way measure.
Ibix said:
No. Everything in physics must be expressible in terms of invariants, and anything that is just a convention is not observable but rather is a choice of interpretation of measurements. The one way speed of light is such an interpretation, but the two way speed is not.
And that I think is both correct and what I would call a concise observation on a profound aspect of meaning in physics I have not seen discussed before. Glad to have brought this to "Physics Forums." However, as shown in the video, when it comes to choosing an interpretation, if one chooses the interpretation that in one direction, from A to B "c" equals its textbook value and in another, from B to A, it is infinite, one gets a self-consistent picture of a strange kind of world.

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OscarCP said:
And that I think is both correct and what I would call a concise observation on a profound aspect of meaning in physics I have not seen discussed before.
I wouldn't have thought "physics can't depend on a human perception of it" to be a particularly profound observation, but if you say so.
OscarCP said:
However, as shown in the video, when it comes to choosing an interpretation, if one chooses the interpretation that in one direction, from A to B "c" equals its textbook value and in another, from B to A, it is infinite, one gets a self-consistent picture of a strange kind of world.
Hopefully you mean c/2 one way and infinite the other way. Actually, you get a non-standard description of the same spacetime everyone else recognises, and the key point is that it's the description and the relationship between terms used and reality that has changed, not the world.

vanhees71, Motore and malawi_glenn
Ibix said:
I wouldn't have thought "physics can't depend on a human perception of it" to be a particularly profound observation, but if you say so.

Hopefully you mean c/2 one way and infinite the other way. Actually, you get a non-standard description of the same spacetime everyone else recognises, and the key point is that it's the description and the relationship between terms used and reality that has changed, not the world.
Good point about c/2.I missed that. One thing that got my attention in the video is the amusing conversation over a radio link between the Flight Center CAPCOM and the astronaut on Mars trying to synchronize their watches so both show the same time. It is a simplification of what the real situation would be, as they are stationary relative to each other.

OscarCP said:
it is a convention, as Einstein implied, not a conclusion from the result of an actual one-way measurement.
Yes.

OscarCP said:
Which, right or wrong, I think it makes most of everything in physics also a convention for the reasons I have already explained in the OP
No. See below.

OscarCP said:
If the speed of light were different in different directions, I think that the truth of things such as the expansion of the universe, that go beyond what Special Relativity, within which the video's discussion is confined, can explain, would be also "stipulated". And consequently, also stipulated would be the notion of the Big Bang
These claims are wrong. The expansion of the universe and the Big Bang are invariants and do not depend on any human choice of convention.

OscarCP said:
The constant one-way value of the speed of light towards us from distant stars is indirectly implied, I believe, by the red shift of the spectral frequency lines of elements that are supposed to be the same everywhere at every time
Wrong. Our spacetime model of the universe is not derived solely from red shift observations and does not require any particular convention for coordinates or the one way speed of light or anything else. Cosmologists use a particular coordinate chart for models of the universe out of convenience, not necessity; you could make a different coordinate choice and have a different one-way speed of light in your coordinate description of the model and still have all of the same invariants, such as red shifts. The real physical content of the model is the relationships between different invariants and different observables: for example, the history of expansion of the universe that we have in our best current model is based on, among other things, looking at the detailed relationship between redshifts, luminosities, and angular sizes for all of the objects we can observe.

To PeterDonis, Thanks for your detailed reply, but I must clarify here that did not say that the model of spacetime has been derived from red shifts, but that the fact that the speed of light is the same in all direction can be shown to be true using those shifts -- as much as anything can be "shown to be true" in physics, that is.

And this is directed to all to those interested in this conversation: Please, watch the video (its URL is in my comment at the start of this thread), that is a not a very long one and is well and seriously presented by someone that I believe understands physics, and then come back and tell me on what you agree and on what, if anything, you disagree with it. Thanks.

OscarCP said:
the fact that the speed of light is the same in all direction can be shown to be true using those shifts
It can't. Isotropy of the speed of light is a coordinate choice, a convention. It's not an invariant that can be derived from some other invariant (like observed redshifts).

Motore and Dale
PeterDonis said:
It can't. Isotropy of the speed of light is a coordinate choice, a convention. It's not an invariant that can be derived from some other invariant (like observed redshifts).
Then, if I stipulated (using Einstein terminology in his 1905 paper) the speed of light to be 0,0001 m/s, what would happen to my interpretation of those measured red shifts?

And I repeat my request for thoe interested in this topic to watch the video and come back adn opine on it.

OscarCP said:
if I stipulated (using Einstein terminology in his 1905 paper) the speed of light to be 0,0001 m/s
You can't stipulate this in all directions. You can only stipulate it in one direction. Then you would have to figure out the implications for other directions (for example, in the opposite direction to the one you stipulated, the one-way speed of light would be many orders of magnitude faster than the standard value, instead of slower).

OscarCP said:
what would happen to my interpretation of those measured red shifts?
I don't know what you mean by "interpretation". If you constructed your model correctly, all of the invariants in it would be exactly the same as in the standard model that cosmologists use. That would include all observed red shifts. You would assign very different coordinates to the points of emission of the light that had those redshifts, but coordinates don't have any physical meaning; they are just labels for events. The physical meaning of any model is in its invariants, and red shifts are invariants, so they must be the same in any valid model, no matter what coordinates it uses.

OscarCP said:
I repeat my request for thoe interested in this topic to watch the video and come back adn opine on it.
The author of the video is not the one posting here. You are. You are making claims, and you should expect people to respond to those claims.

PeroK, Motore, Vanadium 50 and 1 other person
Interesting. Then, I'm curious to know, if I stipulated (using Einstein terminology in his 1905 paper) the speed of light to be textbook c in one direction and 0 m/s in all the others, what would happen to the interpretation of those measured red shifts in light coming in all directions from stars and galaxies across the sky?

And I repeat my request for those interested in this topic to watch the video and come back and opine on it. I really want to hear what they make of it. Is it OK? Is it well-presented garbage? Thanks.

OscarCP said:
the fact that the speed of light is the same in all direction can be shown to be true using those shifts
No, it cannot. First, the speed of light being the same in all directions is not a fact. As we have discussed it is a convention. Second, the observed shifts are consistent with any choice of the convention. So our observations don’t show anything one way or the other about the topic.

OscarCP said:
And I repeat my request for thoe interested in this topic to watch the video and come back adn opine on it.
I think most of us have seen it already, it is not new. My opinion of the video is that it is a decidedly low point for Veritasium. It should just have been a brief mention of the conventionality of the one way speed of light. The “wonderment” in his voice makes it seem much more important than it merits. Also, he seemed unaware of any of the actual professional scientific discourse on the topic

OscarCP said:
if I stipulated (using Einstein terminology in his 1905 paper) the speed of light to be textbook c in one direction and 0 m/s in all the others
You can't stipulate that either. You can only stipulate the one-way speed of light in one particular direction. Then the known fact that the two-way speed of light is ##c## in all directions is sufficient to dictate the one-way speed of light in all other directions besides the one you stipulated.

OscarCP said:
what would happen to the interpretation of those measured red shifts in light coming in all directions from stars and galaxies across the sky?
You keep using the word "interpretation" Red shifts aren't an interpretation. They are a direct observable. You don't "interpret" them. You just measure them.

If you mean, what kind of spacetime model would you infer from the redshifts and other observable data, of course it would be the same one we infer with the standard choice of coordinates that cosmologists use, because all of the invariants are the same, and, as I have already said several times now, the physical content of the model is in its invariants. Same invariants, same model. How you choose to put coordinates on it is irrelevant.

cianfa72
Motore, Dale and OscarCP
OscarCP said:
I repeat my request for those interested in this topic to watch the video and come back and opine on it. I really want to hear what they make of it. Is it OK? Is it well-presented garbage?
The more you keep making this request, the more it seems like you are trying to dodge the responses to your own claims. As I said before, the author of the video is not the one posting here, You are. If you make claims, you need to own them, and acknowledge responses to them. You should not be trying to deflect attention to someone else's video. If all you wanted was people's opinions on the video, then you should have just asked for that, and stopped there. But you didn't.

PeterDonis said:
You can't stipulate that either. You can only stipulate the one-way speed of light in one particular direction. Then the known fact that the two-way speed of light is ##c## in all directions is sufficient to dictate the one-way speed of light in all other directions besides the one you stipulated.You keep using the word "interpretation" Red shifts aren't an interpretation. They are a direct observable. You don't "interpret" them. You just measure them.

If you mean, what kind of spacetime model would you infer from the redshifts and other observable data, of course it would be the same one we infer with the standard choice of coordinates that cosmologists use, because all of the invariants are the same, and, as I have already said several times now, the physical content of the model is in its invariants. Same invariants, same model. How you choose to put coordinates on it is irrelevant.
You don't mean to say that the distance to stars and galaxies and the age of the universe are irrelevant, do you? Becase if the light sources' positions change with their coordinates and their distances to us and between themselves change, all of it so the redshifts stay the same if light is stipulated to be different in different directions, those things, distances and age of the universe, it seems to me, won't stay the same.

In other words: rather than a repeated blanket statement about "invariants", I would prefer an explanation or a reference that actually explains things beyond technicisms.
And to hear what people here make of the video.

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OscarCP said:
Becase if the light sources' positions change with their coordinates and their distances to us and between themselves change, all of it so the redshifts stay the same if light is stipulated to be different in different directions, those things, distances and age of the universe, it seems to me, won't stay the same.
Very hard to read that sentence.

We do use SR (and GR) in our GPS satellites, and they seem to work fine... there is no problem in "just being able to measure the two-way speed of light".

OscarCP said:
In other words: rather than a repeated blanket statement about "invariants", I would prefer an explanation or a reference that actually explains things beyond technicisms.
Spacetime is what it is. The paths of objects through spacetime are what they are. The interaction between a photon and your detector is what it is, so the wavelength you measure is what it is. Why the photon has the frequency you measure has an invariant explanation in terms of those invariant things. They do not always match up nicely to familiar terms like "position" or "age".

If you want to split spacetime up into space and time so that you can talk in 3 dimensional terms about where the photon was emitted and where it was received, and separately talk about when it was emitted and when it was received you need to make a decision about how you want to do that splitting. How you do it defines what you mean by space and times, so it defines what you mean by when something was emitted and where, and it defines what you mean by what happens to the light as it travels. None of this makes a blind bit of difference to the universe, it just changes how you are describing it.

malawi_glenn
malawi_glenn said:
Very hard to read that sentence.

We do use SR (and GR) in our GPS satellites, and they seem to work fine... there is no problem in "just being able to measure the two-way speed of light".
Yes, both Special and General relativity work very well when corrections based on them are applied to the signals of the GPS satellites. (NB: the measurements made using their signals are one way.) I happen to have worked for almost thirty years, among several other things, developing techniques to position objects to better than a couple of inches, some of those fixed to the Earth, as in geodetic stations, and others while moving (from Antarctic glaciers, cars, ships, jet planes, to artificial satellites) using GPS and its more recent comparable system GALILEO from the European Union.
And the best answer so far on this topic, I think, has been this one about the ability to measure the two-way speed of light in all directions. I'll go with that. Thanks.

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OscarCP said:
Dale said:
I think most of us have seen it already, it is not new. My opinion of the video is that it is a decidedly low point for Veritasium.

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