# The Speed of Light is Not Constant

#### Farsight

I hope you agree that time travel is science fiction. It’s because clocks don’t literally measure the flow of time. What they really do is clock up some kind of regular cyclic motion and give a cumulative display that we call the time. That sounds like it’s no big deal. But it is. And it’s a whopper.

You’ll know about gravitational time dilation; clocks go slower when they’re lower. We allow for this in GPS, and it’s even detectable in a lab. See this interview with David Wineland of NIST: "if one clock in one lab is 30cm higher than the clock in the other lab, we can see the difference in the rates they run at". But what’s the big deal? Where's the whopper? The big deal is he’s talking about optical clocks, and when a clock goes slower it’s because the regular cyclic motion inside that clock is going slower. So what sort of regular cyclic motion might you find inside an optical clock? The motion of cogs? The motion of a quartz crystal? No. The motion of light. And the whopper is this: the speed of light is not constant.

That goes against the grain of what people say about relativity. People say things like "Einstein showed us that the speed of light is constant". He did in 1905, when he was doing special relativity. But check this out:

Einstein said:
1911: "If we call the velocity of light at the origin of coordinates c₀, then the velocity of light c at a place with the gravitation potential Φ will be given by the relation c = c₀(1 + Φ/c²)”.

1912: "On the other hand I am of the view that the principle of the constancy of the velocity of light can be maintained only insofar as one restricts oneself to spatio-temporal regions of constant gravitational potential".

1913: "I arrived at the result that the velocity of light is not to be regarded as independent of the gravitational potential. Thus the principle of the constancy of the velocity of light is incompatible with the equivalence hypothesis".

1915: "the writer of these lines is of the opinion that the theory of relativity is still in need of generalization, in the sense that the principle of the constancy of the velocity of light is to be abandoned".

1916: “In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position”.
That’s Einstein talking about the speed of light varying in a gravitational field. But if you ask around about all this, some will brush it off by pointing to the word velocity. They’ll say "It’s a vector quantity my boy. It’s speed and direction. The velocity changes because the direction changes". Guess what? That’s wrong. Go back to the original German, and what Einstein actually said was that a curvature of rays of light can only take place when die Ausbreitungsgeschwindigkeit des Lichtes mit dem Orte variiert. That translates to the propagation speed of the light with the place varies. The word “velocity” in the English translations was the common usage, as in “high velocity bullet”. This is crystal clear because Einstein referred to c which is the speed of light, and to "one of the two fundamental assumptions". That’s the special relativity postulate of the constant speed of light.

Have a read of Ned Wright’s Deflection and Delay of Light and note this: "In a very real sense, the delay experienced by light passing a massive object is responsible for the deflection of the light”. Light doesn’t curve because it curves, and it doesn’t curve because spacetime is curved. Einstein never said that. It curves because the speed of light varies with position. Like a car veers when it encounters mud at the side of the road. But those brush-off guys will dismiss this like they’re crazy. Do you want to see just how crazy? In relativity, we use an idealised optical clock called the parallel-mirror light-clock. It’s employed in the simple inference of time dilation due to relative velocity, which uses Pythagoras's theorem. OK, imagine we use two parallel-mirror light-clocks instead of the NIST optical clocks to demonstrate gravitational time dilation. Let’s exaggerate a little to make it more obvious. What you’d see is this:

Image credit: Brian McPherson

Yes it's exaggerated, and yes the mirrors should be tilted back a little, and yes the light pulses should curve a little. But those two light pulses aren’t going at the same speed. If they were, the clocks would stay synchronised. You’d have to be crazy to say the two light pulses were going at the same speed. And do you know what else is crazy? Check out the NIST caesium fountain clock. It’s used to define the second, like so:

"Since 1967, the second has been defined to be the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom".

See that mention of radiation? That’s light. Some will say there’s a peak frequency which is found and measured by the detector. But note that frequency is defined as cycles per second, and the second isn't defined yet. So what the detector effectively does is count incoming microwaves. When it gets to 9,192,631,770, that's a second. After that, the frequency is 9,192,631,770 Hertz by definition. Then we use the second along with light to define the metre, like so:

"Since 1983 the metre has been defined as the distance travelled by light in vacuum in 1⁄299,792,458th of a second".

We use the motion of light to define the second and the metre. And what do we use them for? To measure the motion of light. That’s why we always measure the speed of light to be 299,792,458 m/s. But it isn't constant. If it was, we wouldn't have a Shapiro delay. Those NIST optical clocks would run at the same rate. And they don't. Because the coordinate speed of light varies in a non-inertial reference frame. Like in a gravitational field. Like in the room you're in. And the coordinate speed of light is the speed of light. The measured speed of light isn't. Like Magueijo and Moffat say, that's just a tautology. And when you appreciate that, and look at the evidence and what Einstein said, it's as plain as day that the speed of light is not constant. As obvious as the nose in front of your face. As obvious as your blind spot. Once you know how to look.

Since you have no math, how do you even convince yourself of the things you say?

1. Two points in space, A and B.
2. There is distance between point A and B.
3. Light moves from point A to point B.
4. Light traveled a distance greater than zero.
5. If light traveled a distance greater than zero then a greater than zero time elapsed.
6. Period.

To summarise the above:

The speed of light in an extended space is not necessarily constant as measured by all observers.

And I say:

On the other hand, the speed of light locally (in a vacuum) is constant.

For example, in explaining the Shapiro delay, mentioned above, we note that if we assume a flat spacetime then the speed of light seems to slow as the light passes near the Sun. However, if we take the general relativistic description seriously, then the explanation for the apparent slowing of the speed of light is that there is actually more space near the Sun than there would be if the spacetime there was flat, because in fact the spacetime is curved. The light therefore has further to travel, which takes longer, even though its speed through the vacuum remains constant.

The speed of light in an extended space is not necessarily constant as measured by all observers.
It's more than that James. The speed of light in the room you're in is not constant as measured by you.

On the other hand, the speed of light locally (in a vacuum) is constant.
Only it isn't. If it was, those two NIST optical clocks, one 30cm above the other, would run at the same rate.

James R said:
For example, in explaining the Shapiro delay, mentioned above, we note that if we assume a flat spacetime then the speed of light seems to slow as the light passes near the Sun. However, if we take the general relativistic description seriously, then the explanation for the apparent slowing of the speed of light is that there is actually more space near the Sun than there would be if the spacetime there was flat, because in fact the spacetime is curved. The light therefore has further to travel, which takes longer, even though its speed through the vacuum remains constant.
See the quotes in the OP. Take Einstein seriously.

What a piece of work is a Farsight. In one single post, he has overturned all experimental data on relativity -they had failed to understand Einstein as he does!

Shame on everyone.

The General Theory of Relativity is more fun to discuss if you only ponder the speed of light, and not confuse the issue by adding in length contraction and the idea of geodesics. Anyone can be understand GR just as well as Einstein if you don't clutter it up with all gratuitous and unnecessary stuff like spaces and curvature and reference frames. (And it seems they are simplifying things even more by keeping their reasoning confined to moving in a circular world line.)

The speed of light is always the same...This is a postulate of relativity.....
It even remains the same when passing through denser mediums...It just has longer distances to travel after reflection/refraction.

Einstein said what he said, Declan. The principle of the constancy of the velocity speed of light is to be abandoned. Optical clocks run slower when they're lower. And there ain't no time flowing through 'em.

Einstein said what he said, Declan. The principle of the constancy of the velocity speed of light is to be abandoned. Optical clocks run slower when they're lower. And there ain't no time flowing through 'em.

Hmmm, this seems like an 'alternative theory' (alternative in that it is wrong based on all observations, experimentation and mathematics) to the accepted theory of relativity. I wonder why this particular thread (as well as others) is not in the alternative theory section? Perhaps it is thought that there could be some useful discussion on this subject. Well, I hope this 'useful discussion' does not confuse someone not schooled in science into thinking that farside's title is accurate, since we are in the physics section. The dismal state of scientific knowledge is bad enough in the US; they certainly do not need anything more to dumb them down further.

Hmmm, this seems like an 'alternative theory' (alternative in that it is wrong based on all observations, experimentation and mathematics) to the accepted theory of relativity. I wonder why this particular thread (as well as others) is not in the alternative theory section?
Because Einstein said the speed of light is not constant. And more importantly, the evidence says it too.

Perhaps it is thought that there could be some useful discussion on this subject. Well, I hope this 'useful discussion' does not confuse someone not schooled in science into thinking that farside's title is accurate, since we are in the physics section. The dismal state of scientific knowledge is bad enough in the US; they certainly do not need anything more to dumb them down further.
You've been dumbed down. I'm fixing it. Now try to read a bit more than the thread title.

Anyway, a neutral pion decays into two gamma rays $$\pi ^0\to \gamma +\gamma$$ ie light. In 1964 at CERN whether these pions were moving at 0.999 75c or stationary in the lab, the gamma rays that came from them were all measured to be the same speed.

Farsight is out to lunch like no other on this one. He may be challenging that light is not an electromagnetic wave that travels at a finite speed, or he wants to re-write Maxwell's equations.

Einstein said what he said, Declan. The principle of the constancy of the velocity speed of light is to be abandoned. Optical clocks run slower when they're lower. And there ain't no time flowing through 'em.

Of course he said what he said. I suppose you just overlooked what I said. Einstein said that in the period ca 1907 while he was stumped in formulation a theory that included gravity. At that time he still trying to work within Newton's notion of absolute space. After he pondered on a spacetime could be dynamical, he was able to progress in his work on GR. The invariant speed of light is how he was able to even consider that spacetime might be dynamical in the first place.

You keep going on about your clocks running slower as if that means light is traveling slower. That is a false premise. Because not only is time slower, but the length contraction makes the distance shorter, so light's speed is the constant in all frames. You can not present time dilation without length contraction, they are inextricably connected. Considering only one is a false premise at the start.

It's starting to appear that you are intentionally trying to leave out length contraction so you can argue endlessly ad infinitum that light speed is variable. It's not a hard concept, so is it intentional? Unless you have a completely new argument or line of reasoning, I'll leave you to the game with someone else.

Einstein said what he said, Declan. The principle of the constancy of the velocity speed of light is to be abandoned. Optical clocks run slower when they're lower. And there ain't no time flowing through 'em.

Einstein said no such thing....
You are handling the truth rather lightly and taking statements out of context.

SR/GR are as is.
Your continued denying such facts here are just a local concern that genuine lovers of science here have to wear.
You are making no difference, nor will you ever make any difference to the knowledge contained in the halls of science, and the accepted mainstream view.
In more basic language, you are pushing shit uphill.

Of course he said what he said. I suppose you just overlooked what I said. Einstein said that in the period ca 1907 while he was stumped in formulation a theory that included gravity. At that time he still trying to work within Newton's notion of absolute space. After he pondered on a spacetime could be dynamical, he was able to progress in his work on GR. The invariant speed of light is how he was able to even consider that spacetime might be dynamical in the first place.
Declan, look at the dates on the Einstein quotes. He was still saying the SR postulate had to be ditched in 1916. The whole point of what I'm saying is that what you've been taught is not in accord with Einstein.

You keep going on about your clocks running slower as if that means light is traveling slower. That is a false premise. Because not only is time slower, but the length contraction makes the distance shorter, so light's speed is the constant in all frames. You can not present time dilation without length contraction, they are inextricably connected. Considering only one is a false premise at the start.
It's no false premise. Read the OP in time travel is science fiction. There is no time flowing within a clock. A clock "clocks up" some kind of regular cyclical motion and shows you a cumulative result that you call the time. When a clock goes slower it's because that motion goes slower.

It's starting to appear that you are intentionally trying to leave out length contraction so you can argue endlessly ad infinitum that light speed is variable. It's not a hard concept, so is it intentional?
I'm not missing it out. Think it through. Since 1983 the metre has been defined as the distance travelled by light in vacuum in 1⁄299,792,458th of a second. When your light goes slower your second is bigger. Then your slower light and your bigger second cancel each other out, and the metre is unchanged.

Unless you have a completely new argument or line of reasoning, I'll leave you to the game with someone else.
It isn't new. Einstein said the speed of light wasn't constant in a gravitational field.

Anyway, a neutral pion decays into two gamma rays $$\pi ^0\to \gamma +\gamma$$ ie light. In 1964 at CERN whether these pions were moving at 0.999 75c or stationary in the lab, the gamma rays that came from them were all measured to be the same speed.

Based on which clock. And compared to what? These little details are important and should be clearly identified and made allowances for when making adamant claims for a 'same speed of light' in all circumstances. Don't you think?

One thing I do know...
The speed of light is constant...it never changes...not even through a denser medium.

Could you explain what you mean by that? Because as it's written it's most definitely wrong.

Last edited:
Could you explain what you mean by that? Because as it's written it's most definitely wrong.

If you are referring through the medium bit, all I'm referring to there is that photons/light actually have a longer distance to travel, after reflection/refraction and such.
So it will appear to "slow down"

If you are referring through the medium bit, all I'm referring to there is that photons/light actually have a longer distance to travel, after reflection/refraction and such.
So it will appear to "slow down"

That is wrong. That is why "c" is specified as "in a vacuum". Light does slow down as it passes through a medium. That is the basis of the entire field of optics. Various mediums slow it by different amount. That is why lenses work. That is how a prism works. It is just one of those 1st Principles that everyone should know as second nature. There is nothing controversial about it all. It's experimentally demonstrated in all introductory physics labs.

Google up refraction and dispersal. There are tons of materials available.

Here’s the idea: a medium, whatever it is, is made up of molecules. When a photon (light particle) hits a molecule it is sometimes absorbed. Its energy is turned into raised electron-energy-levels, or vibrations and flexing, or movement. In short order (very short order) the photon is spit out of the other side, none the worse for wear.

In between molecules light still travels at light speed. It’s just that, with all those molecules around, it’s always darting ahead, getting absorbed, pausing for a moment, then being re-emitted. On the scale we’re used too, this happens so much and so fast that you don’t notice the starting-and-stopping. Instead you notice an average slowing of the light.

That is, if light always takes about 33% longer to travel through water than air (and it does) due to absorption and re-emission, you’d say “ah, light travels slower through water!”. The fact that that isn’t quite the case is rarely important.