The Speed of Light is Not Constant

[Farsight]

He did. He used the word Geschwindigkeit. Go and google it. It means speed. 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”.

Note that rpenner attempted to persuade readers to ignore Einstein in this post and I rebuffed him in this post.

 

[krash661]

the propagation speed of the light with the place varies.

no, that's an incorrect translation,
which is only incorrect by your attempt to manipulate what he actually stated.
the actual translation is,
" the velocity of propagation of light varies with the place. "


think about this little statement

 

[rpenner]

You have emphasised relativity generally relinquishes the whole concept of a distant object having a well-defined speed. Understand this: we aren't talking about distant objects. We're talking about light in the room you're in.

Not co-located is not local for which the relevant positive description is "distant." This is clear from the differential language of general relativity, where an event's tangent space is flat but this tangent space is only a good approximation of curved space-time for a vicinity of the event which shrinks as acceleration of the event's world line or tidal forces increase and which also shrinks as one's interest in precision grows.

Your alternate hypothesis that there is a finite distance which is neither distant nor co-located requires inventing that Don Koks wrote about an absolute distance scale separating "distant" and "local" when he did not and that is at tension with the observation that lengths aren't even absolutely meaningful in special relativity. So the more parsimonious reading is that that which is not infinitesimally local is at a finite distance and therefore "distant" in the sense used by the article.

In summary, as reading the article starting at the heading "The Speed of Light as Measured by Non-Inertial Observers" (this is the section where the ceiling and floor observers are introduced) shows, the ceiling is a finite distance from the floor, therefore they are not local to each other therefore they are "distant" in the sense used by the article.

 

[PhysBang]

No, I'm banned from certain websites because some people like to tell everybody that they're the expert, and when somebody like me challenges their assertions with evidence and references, they permit trolls to make accusations and hurl abuse and derail the discussion and trash the thread. And if that doesn't work, they censor the discussion.

Well, you attempted to claim that you were and "expert" (your word) on the JREF forum and while you claim that you have produced evidence, that is a lie given your standard for evidence. You once wrote that the only form of evidence is observation, yet you have never once shown how your bizarre claims make predictions or in any way coincide with observations.

You claim that every scientist is wrong about dark matter in galaxies, yet you have never produced your own prediction of galaxy rotation curves. This seems to be to be nothing but trolling and hurling abuse.

The above is quite sufficient to counter both lpetrich's and your ad-hominem assertions.

On the contrary, it is demonstrating that our assertions are not ad hominems. The passage you cite shows that you are a liar and that our comments about your lies and cherry-picking are relevant to any discussion in which you arrive.

I'm not ignoring it, I'm quoting it. It's people like lpetrich and Physbang who are trying to get people to ignore it.

On the contrary, I have always asked for, and sometimes provided, specific details whilst you have carefully avoided giving details. lpetrich has always given the specific equations required for one to do the work of physics and produce results that can be compared to measurement. You have always failed to produce something that could be compared to measurement.

So spare me your the mendacious, evil and loathsome.

People have demanded many times that you quit your insults, yet you only seem to increase your insults. You never increase the details of your physics claims.

The "main thrust" of Baez paragraph is that the speed of light is NOT constant, and Einstein said it.

Sure, in a very specific way that you ignore because you do not want to learn the relevant mathematics.

I suspect that you are a scared man who knows that learning the mathematics will mean that you learn that you are grossly wrong.

Was there some part of Einstein means speed here that you didn't understand? You do recall that PhysBang said this on another forum?

"In a sense, this was done in 1905, when Einstein developed special relativity. This constancy of the speed of light is a postulate of the theory, so it is "shown" through the effectiveness and practicality of the theory. The same is true for general relativity, developed in 1915, which holds that the speed of light is constant at any infinitesimal region of a coordinate system".

Note that the speed of light is constant in an infinitesimal region? That means it isn't constant in the room you're in. And a curvature of rays of light can only occur when the speed of light varies with position.

That "the speed of light is constant at any infinitesimal region of a coordinate system" (a fact you deny, Farsight, since you claim that it changes as various points) does not mean that the speed of light "isn't constant in the room you're in". That requires additional information. It so happens that the general theory of relativity is quite specific on how speed changes for many definitions of speed. You have not yet given a definition of speed, which is not odd since it gives you a chance to weasel out of specific details.

You have emphasised relativity generally relinquishes the whole concept of a distant object having a well-defined speed. Understand this: we aren't talking about distant objects. We're talking about light in the room you're in.

Now that is just stupid: there are distances in a room, and you are clearly discussing things separated by a distance, i.e., the top of a room and the bottom of a room.

I don't now how you can think that you will be taken seriously if you think that there is no distance between the floor and ceiling of any room.

 

[lpetrich]

Farsight's most recent post fits my description so well. Book-thumping, the terrible wrong of denying Einstein, you name it.

More seriously, I've attempted to find the coordinate speed of light to first order in curvature. Find a null geodesic from point A to point B. Then find a spacelike one from A to point X and a timelike one from X to B. Select X so that the spacelike and timelike geodesics have perpendicular directions there. Find the ratio of distance (AX) and time (XB). That will give the coordinate speed of light. I've attempted to find it, and I get a rather complicated expression.

$$ \Delta v = - \frac{1}{6s^2} R_{ijkl} x_1^i x_2^j x_1^k x_2^l ,\ (x_1 \cdot x_1) = s^2 ,\ (x_2 \cdot x_2) = - s^2 ,\ (x_1.x_2) = 0 $$

AX = x1, XB = x2, R = the Riemann curvature tensor, Delta v = (AX distance) / (XB time) - 1

So one does get a change in the observed speed due to curvature, but to lowest order, it's (curvature) * (measurement distance/time)[sup]2[/sup], and it depends on what point one is using as a distance and time reference. This is contrary to what Farsight seems to be claiming, that the speed of light changes in some globally well-defined way in GR.

 

[Farsight]

no, that's an incorrect translation

It's correct. Einstein referred to the SR postulate, which is the constant speed of light. And his statement doesn't make sense if you insist on vector-quantity velocity. It would be tautological nonsense wherein "light curves because it curves".

Not co-located is not local for which the relevant positive description is "distant."

Rpenner, light in the room you're in is not "distant". The NIST optical clocks are 30cm apart. That's not "distant".

This is clear from the differential language of general relativity, where an event's tangent space is flat but this tangent space is only a good approximation of curved space-time for a vicinity of the event...

You still don't understand GR at all. Your pencil doesn't fall because spacetime is curved. Spacetime curvature relates to the tidal force. It isn't detectable in the room you're in. But your falling pencil is. It falls down because what you'd call the tangent space is tilted. But the space in the room you're in isn't tilted, and nor is it curved. Instead it's inhomogeneous. Einstein referred to this in his 1920 Leyden Address:

"...recognition of the fact that 'empty space' in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gmn), has, I think, finally disposed of the view that space is physically empty."

Your alternate hypothesis...

It isn't my alternate hypothesis. Don Koks said what he said, so did PhysBang, and so did Einstein. The speed of light varies with position. It isn't constant in the room you're in, because the space is inhomogeneous in the room you're in. Because the energy tied up as the matter of the Earth conditions the surrounding space, and the effect of this diminishes with distance.

that there is a finite distance which is neither distant nor co-located requires inventing that Don Koks wrote about an absolute distance scale separating "distant" and "local" when he did not and that is at tension with the observation that lengths aren't even absolutely meaningful in special relativity. So the more parsimonious reading is that that which is not infinitesimally local is at a finite distance and therefore "distant" in the sense used by the article.

You are hoist by your own petard, because light moves from A to B when you measure its speed, and you call that a local measurement. Even when A and B are 3 metres apart. And yet if those NIST optical clocks were even more accurate they would lose synchronisation when one was 3cm above the other. Or 3mm.

In summary, as reading the article starting at the heading "The Speed of Light as Measured by Non-Inertial Observers" (this is the section where the ceiling and floor observers are introduced) shows, the ceiling is a finite distance from the floor, therefore they are not local to each other therefore they are "distant" in the sense used by the article.

You're left in the ridiculous situation wherein you have to claim that anything that isn't infinitesimal is "distant" rather than local, and yet in the same breath you have to claim that something that isn't infinitesimal is local rather than distant. Reductio ad adsurdem, rpenner. So start paying attention to what Einstein said instead of dismissing him with specious claims that relativity has moved on. And start paying attention to the evidence too.

 

[Farsight]

Smoke and Mirror Alert:

More seriously, I've attempted to find the coordinate speed of light to first order in curvature. Find a null geodesic from point A to point B. Then find a spacelike one from A to point X and a timelike one from X to B. Select X so that the spacelike and timelike geodesics have perpendicular directions there. Find the ratio of distance (AX) and time (XB). That will give the coordinate speed of light. I've attempted to find it, and I get a rather complicated expression.

$$ \Delta v = - \frac{1}{6s^2} R_{ijkl} x_1^i x_2^j x_1^k x_2^l ,\ (x_1 \cdot x_1) = s^2 ,\ (x_2 \cdot x_2) = - s^2 ,\ (x_1.x_2) = 0 $$

AX = x1, XB = x2, R = the Riemann curvature tensor, Delta v = (AX distance) / (XB time) - 1

So one does get a change in the observed speed due to curvature...

Wrong! The change in the observed speed isn't due to curvature. That confuses cause and effect. Light curves because the speed of light varies with position, in line with the gradient in gravitational potential. The derivative of potential. Reimann curvature is associated with tidal force which is the second derivative of potential. See the picture on the right of the Wikipedia Riemann curvature tensor article. You plot the observed speed using optical clocks, and you plot out a gradient. Only when you keep on plotting you see that this gradient is gradually changing. Your plot is curved. And it's a plot of the speed of light.

This is contrary to what Farsight seems to be claiming, that the speed of light changes in some globally well-defined way in GR.

I'm referring you to Einstein and the Baez website. And they both agree with me.

 

[PhysBang]

Rpenner, light in the room you're in is not "distant". The NIST optical clocks are 30cm apart. That's not "distant".

You have a serious mental problem if you think that 30cm is not a distance.

Please read the physics that you cherry-pick from.

You still don't understand GR at all. Your pencil doesn't fall because spacetime is curved.

Writes the person who cannot even describe the falling of a pencil in classical mechanics.

You have been asked repeatedly to show how to use your physics to describe a physics application. You can't. The best you could do is produce an equation from Einstein that used a constant speed of light without any comment on how to use it.

Please learn the physics you insult.

It isn't my alternate hypothesis. Don Koks said what he said, so did PhysBang, and so did Einstein.

We all said things you selectively quote out of context, ignoring our larger body of statements. You also ignore the fact that Einstein never used inhomogeneous space in his work and that you have been unable to show us how to do any physics problem with inhomogeneous space. Your one reference, a specific paper where one relativistic result with light is reproduced using a prism effect, is not relevant as there is no source that uses inhomogeneous space to describe a falling object. This being the case, it is your hypothesis, a so far failing hypothesis.

The speed of light varies with position.

A misreading of everything you have quoted.

You are hoist by your own petard, because light moves from A to B when you measure its speed, and you call that a local measurement. Even when A and B are 3 metres apart.

No. Please read.

And yet if those NIST optical clocks were even more accurate they would lose synchronisation when one was 3cm above the other. Or 3mm.

Exactly. When things are at a distance, they are at a distance.

You're left in the ridiculous situation wherein you have to claim that anything that isn't infinitesimal is "distant" rather than local,

By definition.

and yet in the same breath you have to claim that something that isn't infinitesimal is local rather than distant.

Only you are making the claim that there is a class of distances that are not infinitesimal yet are not a finite distance.

And start paying attention to the evidence too.

When you have the ability to show us how to calculate the trajectory of a falling object, then you might have evidence.

 

[PhysBang]

Wrong! The change in the observed speed isn't due to curvature. That confuses cause and effect. Light curves because the speed of light varies with position, in line with the gradient in gravitational potential.

Then, please, show us your equations.

I'm referring you to Einstein and the Baez website. And they both agree with me.

No, so far they were very careful to draw a distinction between local and distant that you deny. That makes you a liar when you appeal to them and a fraud in general.

 

[Farsight]

When you have the ability to show us how to calculate the trajectory of a falling object, then you might have evidence...

Then, please, show us your equations...

Hiding behind mathematics won't help you, PhysBang. Nor will calling me a liar. Because you're on record as saying this:

"In a sense, this was done in 1905, when Einstein developed special relativity. This constancy of the speed of light is a postulate of the theory, so it is "shown" through the effectiveness and practicality of the theory. The same is true for general relativity, developed in 1915, which holds that the speed of light is constant at any infinitesimal region of a coordinate system".

The speed of light is only constant in an infinitesimal region. So it isn't constant in the room you're in. Now is it?

 

[Farsight]

...You're left in the ridiculous situation wherein you have to claim that anything that isn't infinitesimal is "distant" rather than local, and yet in the same breath you have to claim that something that isn't infinitesimal is local rather than distant. Reductio ad adsurdem, rpenner. So start paying attention to what Einstein said instead of dismissing him with specious claims that relativity has moved on. And start paying attention to the evidence too.

Cat got your tongue, rpenner?

 

[lpetrich]

Rpenner, light in the room you're in is not "distant". The NIST optical clocks are 30cm apart. That's not "distant".

By ordinary standards, yes, but that does not make any difference.

You still don't understand GR at all. Your pencil doesn't fall because spacetime is curved.

Except that it does. I studied GR as a graduate student, so I can claim at least a little bit of expertise on it.

(thumping of Einstein's Leyden address snipped...)

All he was saying in it was that the metric of space-time can vary more-or-less arbitrarily, and that space-time is thus a dynamical entity.

(about my calculation of observed-velocity deviation due to space-time curvature ...)

Smoke and Mirror Alert:

Wrong! The change in the observed speed isn't due to curvature. That confuses cause and effect.

Except that that's what I found with my calculation. I did it in the small-size limit so one can easily estimate how large an effect that will be. However, Farsight did not point out anything wrong with my calculation, and instead reiterated his interpretations of his chosen quotes.

 

[brucep]

Farsight's most recent post fits my description so well. Book-thumping, the terrible wrong of denying Einstein, you name it.

More seriously, I've attempted to find the coordinate speed of light to first order in curvature. Find a null geodesic from point A to point B. Then find a spacelike one from A to point X and a timelike one from X to B. Select X so that the spacelike and timelike geodesics have perpendicular directions there. Find the ratio of distance (AX) and time (XB). That will give the coordinate speed of light. I've attempted to find it, and I get a rather complicated expression.

$$ \Delta v = - \frac{1}{6s^2} R_{ijkl} x_1^i x_2^j x_1^k x_2^l ,\ (x_1 \cdot x_1) = s^2 ,\ (x_2 \cdot x_2) = - s^2 ,\ (x_1.x_2) = 0 $$

AX = x1, XB = x2, R = the Riemann curvature tensor, Delta v = (AX distance) / (XB time) - 1

So one does get a change in the observed speed due to curvature, but to lowest order, it's (curvature) * (measurement distance/time)[sup]2[/sup], and it depends on what point one is using as a distance and time reference. This is contrary to what Farsight seems to be claiming, that the speed of light changes in some globally well-defined way in GR.

I'm pretty sure you understand how it's derived from the metric.

dr/dt = 1-2M/r radial remote coordinate speed of light. What Farsight doesn't understand is this is coordinate dependent. He refuses to even acknowledge the physics. This is what the 1 and 2M/r represent in the derivation. 1 is the invariant we measure in all local frames which can be derived from the minkowski metric for flat spacetime and 2M/r is the metric curvature component and represents a specific position on the spacetime manifold. That makes the remote observation/reckoning/measirement coordinate dependent. All local measurements are made in the [flat] tangent space of the spacetime manifold and are invariant over the entire spacetime manifold. Everywhere in the universe. The remote coordinate speed of light is specific to the coordinates 2M/r and varies with r [choosing units r=nM]. Farsight chooses the remote coordinates as preferred when he concludes the speed of light varies in the gravitational field. The same goofball mistake he makes when he says black holes can't exist. He's had years to learn something about how the theoretical model GR works. Never happened.

 

[nimbus]

For those who are missing farsight...
John Duffield

I feel I do understand something about the pseudo-Riemannian geometry. Let me give you a taste of it by stepping down a dimension:

HERE

 

[Farsight]

That was over a year ago, nimbus. You can always find me somewhere on the internet if you want to talk to me. Here's a nice article on physicsworld: electrons in magnetic field reveal surprises.

"...Farsight chooses the remote coordinates as preferred when he concludes the speed of light varies in the gravitational field. The same goofball mistake..."

It isn't a goofball mistake. They aren't remote coordinates in the room you're in. Remember this from the Baez website:

"Given this situation, in the presence of more complicated frames and/or gravity, relativity generally relinquishes the whole concept of a distant object having a well-defined speed. As a result, it's often said in relativity that light always has speed c, because only when light is right next to an observer can he measure its speed—— which will then be c. When light is far away, its speed becomes ill-defined. But it's not a great idea to say that in this situation "light everywhere has speed c", because that phrase can give the impression that we can always make measurements of distant speeds, with those measurements yielding a value of c. But no, we generally can't make those measurements. And the stronger gravity is, the more ill-defined a continuum of observers becomes, and so the more ill-defined it becomes to have any good definition of speed. Still, we can say that light in the presence of gravity does have a position-dependent "pseudo speed". In that sense, we could say that the "ceiling" speed of light in the presence of gravity is higher than the "floor" speed of light.

Einstein talked about the speed of light changing in his new theory. In his 1920 book "Relativity: the special and general theory" he wrote: "... 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 [...] cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity [Einstein means speed here] of propagation of light varies with position." This difference in speeds is precisely that referred to above by ceiling and floor observers."

The ceiling speed of light in the presence of gravity is higher than the floor speed of light. A curvature of rays of light can only take place when the velocity [Einstein means speed here] of propagation of light varies with position. Could it be any simpler? All you have to do is appreciate that time travel is a fantasy because clocks clock up motion, not the literal flow of time. Then you know that when a light clock goes slower, it's because light goes slower.

 

[origin]

because clocks clock up motion, not the literal flow of time. Then you know that when a light clock goes slower, it's because light goes slower.

Don't you realize that you are denying what Enstein said? I thought that was some sort of great sin for you. You know farsight if you are really all that interested in relativity, why in the name of all that is holy don't you take classes at a community college to get the math you need. As soon as I retire I am going to go back and take courses. At most schools they have programs that make it really cheap for someone over 55 to take classes. I think it is like $50 per course at our CC.

 

[lpetrich]

That was over a year ago, nimbus. You can always find me somewhere on the internet if you want to talk to me. Here's a nice article on physicsworld: electrons in magnetic field reveal surprises.

Yet more book-thumping. Why not read Imaging the dynamics of free-electron Landau states : Nature Communications : Nature Publishing Group and try to understand its math?

They aren't remote coordinates in the room you're in.

It's remote in a relative sense.

Back to the nominal subject of this thread, in a post shortly before this one, I'd calculated how much a measured speed of light can vary as a result of curvature effects when curvature is relatively small. I avoided coordinate dependence by using geodesics. It turns out to be a function of the location of the hinge point, as it may be called, between the spacelike segment and the timelike segment whose lengths go into the speed-of-light measurement.

Let's put the issue in another way. If you wanted to find how fast you walk or how fast your car can go, you don't use the great-circle distance between your reference start and end points. You measure the distance along the path, even though it may be noticeably greater than the great-circle distance.

 

[przyk]

They aren't remote coordinates in the room you're in.

For the purpose of what's discussed on that FAQ -- the ambiguity in defining speeds and other quantities in remote locations -- yes they are. That ambiguity isn't normally noticeable over short distances in a weak gravitational field (like across your room), but there isn't any cutoff distance below which it disappears completely in GR.

 

[PhysBang]

It isn't a goofball mistake.

This is correct, it is a straight up lie from Farsight.

Farsight knows that Baez is speaking about remote coordinates across the room but he lies about this.

Look what quotes from Baez and look at what he leaves out.

Carlip, Gibbs and Kok write, "Each observer is going to measure the speed of light to be c in his vicinity, but I can't accurately talk about the speed of a distant light ray (or anything else), because I can't enlist anyone to make measurements for me in such a way that we all agree on what space and time standards we're using."

They then write, "Given this situation, in the presence of more complicated frames and/or gravity, relativity generally relinquishes the whole concept of a distant object having a well-defined speed. As a result, it's often said in relativity that light always has speed c, because only when light is right next to an observer can he measure its speed—— which will then be c. When light is far away, its speed becomes ill-defined. "

So when they talk about an "ill-defined" speed, they are talking about speeds when considered at a distant.

And the paragraph that Farsight never quotes, right before the ones that he does, reads:

"It's easy to build a continuum of observers in flat spacetime with everyone inertial, who each measure events only in their vicinity. It's possible but much harder to do the same for a uniformly accelerated frame. For more complicated frames and also for real gravity, we find that I simply can't populate space with a continuum of observers who all agree with me on distances and simultaneity. We just won't have a common standard of rulers and clocks. Each observer is going to measure the speed of light to be c in his vicinity, but I can't accurately talk about the speed of a distant light ray (or anything else), because I can't enlist anyone to make measurements for me in such a way that we all agree on what space and time standards we're using."

The passage that Farsight likes begins with "Given this situation" because the situation in question is the speed of light at different distance in a room.

Now you might be tempted to say that Farsight is just too stupid to read that essay all the way through and that he just missed the relevant parts of the essay. That would be a mistake. Many people have pointed out to Farsight the selective way that he is presenting this passage. Farsight knows that the source doesn't match his Farsight-Relativity theory, but he pretends it does anyway. That's lying.

Heck, the source straight up denies Farsight-Relativity. They authors write, "In such a frame, the not-quite-well-defined "speed" of light can differ from c, basically because of the effect of gravity (spacetime curvature) on clocks and rulers." In Farsight-Relativity, it supposedly works the other way around, though since Farsight can't show you exact predictions or equations for Farsight-Relativity, we'll just have to take his word for it.

They aren't remote coordinates in the room you're in.

And there is the lie.

 

[chinglu]

He did. He used the word Geschwindigkeit. Go and google it. It means speed. 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”.

Note that rpenner attempted to persuade readers to ignore Einstein in this post and I rebuffed him in this post.

You did not prove RPenner was wrong.