# The Speed of Light is Not Constant

To start, I don't have a position here re: the variable speed of light issue. But it has for a long time bothered me that it seems a theoretical leap to connect an atomic ground state optical frequency rate, with the speed of light...
It won't bother you once you understand it. See the OP. See the definition of the second? That concerns the NIST caesium clock. It's like their optical clock, but it employs microwaves rather than visible light. Look carefully at the definition of the second: "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". It's periods of radiation. Not frequency. It can't be frequency because frequency is cycles per second and we are defining the second isn't defined yet. So we're counting the microwaves coming past us, and when we get to 9,192,631,770 we say that's a second then we say the frequency of those microwaves is 9,192,631,770 Hertz by definition. Imagine we had a parallel-mirror light-clock and we counted 9,192,631,770 reflections then said that's a second/. Then the reflection frequency is 9,192,631,770 Hertz by definition.

...even where the emission of a photon is associated with electron transitions. Locally all light/photons have the same speed in vacuum regardless of their frequency, or the ground state frequency of the emitting atom.
Oh for God's sake,read the OP. We define the second and the metre using the local motion of light. Then we use them to measure the local motion of light. So we are guaranteed to always measure 299,792,458 m/s.

Might you reconsider your statement in the above quote with respect to the following?

Those NIST optical clocks are not based on or measuring the speed of light. They are based on ground state optical frequencies.
No. See above. Think of an ocean. Think of a wave traversing that ocean. Imagine you're above that ocean in a helicopter. Pacing that wave. It looks like a hump of water. A static hump of water. It doesn't have a frequency. It only appears to have a frequency to the guy in the boat. Because it passes him at some speed. And if it passes him slower, the frequency looks lower.

They do say something about how the optical frequencies of specific atoms are affected by location in a gravity well, but not so much about the speed of light... The implied application of a frequency change, to a change in the speed of light, seems to me to remain based on the underlying theory. May be true or may not, it does not seem to have been directly measured, relative to location in a gravity well, the velocity of light that is.
Shrug. What they say is wrong.

And aren't those optical clocks only used to set a standard for the second IOW to set other clocks? They don't actually keep time theirselves. Mostly they would just up the accuracy from cesium/microwave frequency clocks, as a standard to adjust or set something like a hydrogen maser...?
The NIST optical clocks will set the standard eventually.

Farsight, you do know that no one believes that (that you are a champion of Relativity), right?
Right. But you should.

I don't even think most of us believe you believe it. This is just a game you are playing.
I believe it. It's no game.

Still: be clear here: you do NOT accept SR's constancy of light speed postulate, right?
I accept it in that that was an SR postulate. But I reject it just as Einstein rejected it when he moved on to gravity. Look at the OP. Look at the Einstein quotes. And like I said to Declan, look at the dates. Then accept it, because I wouldn't lie to you.

Russ_Watters said:
Also, the Baez article: more out of context quotes (that is a key part of your game). Yes, space is curved. That quote means it isn't JUST space that is curved.
No Russ, space isn't curved in a gravitational field. It's inhomogeneous. And this is no game. This is a serious discussion thread, one where you are being told relativity the way it is, instead of the popscience version peddled by hacks and quacks.

The invariant speed of light is how he was able to even consider that spacetime might be dynamical in the first place.

I’ve always thought using only the term “invariant” for the speed of light, caused confusion. Of course it will always be measured in a vacuum by every direct observer to be 300,000 k/s (rounded). But the term doesn’t make it clear that 300,000 k/s is relative to the space-time the observer is in…

For example if I measure the distance to the moon with a laser near its average point from Earth I’ll get about 380,000 km. If suddenly the Earth took on a large increase in mass my clock (while still normal to me) is running slower relative to before, and the moon distance would now measure less than before too.

While I still measure light speed as 300,000 k/s, because my clock (time) is slower than before, and my space (moon distance) is smaller than before, I can conclude that 300,000 k/s in my earlier spacetime is not equal to 300,000 k/s in m new spacetime, I can only conclude 300,000 k/s is invariant to my particular spacetime. Clearly an invariant light speed is proportional to how I view time and space (distance), and they are both relative, dynamical terms. Perhaps it would cause less confusion to say light speed is invariant to any direct observer’s spacetime?

Just my opinion. "Fred".

I do. All but one of your quotes was prior to 1916, when Einstein published his general theory. So, we see him groping towards it gradually, up until the last statement you quoted. Of course, in 1916, there was still a lot that Einstein didn't understand about the implications of his own theory. We've had almost 100 years since then to get a handle on it all.

Well put, and all the implications since then have gone through peer review and practical application in some areas.
And lo and behold, it is only in relatively recent times we have had the Internet and forums such as this.

I accept it in that that was an SR postulate. But I reject it just as Einstein rejected it when he moved on to gravity.
Ahh, so you DO recognize that Einstein's own understanding changed over time. Interesting. So why do you continue to insist on using Einstein quotes from before he published GR to support your admittedly non-mainstream views?

No Russ, space isn't curved in a gravitational field. It's inhomogeneous. And this is no game. This is a serious discussion thread, one where you are being told relativity the way it is, instead of the popscience version peddled by hacks and quacks.
Hacks and quacks because their understanding of the universe has evolved since Einstein? So tell me: is Einstein also a hack and a quack for having inconsistent/evolving views?

Worse, you criticize the mainstream for evolving views while the mainstream still accepts SR as-is, but you don't. So aren't you being hypocritical there?

-You say you are with Einstein, but you aren't.
-You say you are against an evolving view, but you support one.

Ahh, so you DO recognize that Einstein's own understanding changed over time. Interesting.
Yes of course I do. I'm the one who does. I'm the one who knows he postulated his constant speed of light in 1905 but by 1911 was starting to say it didn't apply to gravity.

Russ_Watters said:
So why do you continue to insist on using Einstein quotes from before he published GR to support your admittedly non-mainstream views?
I don't. The quotes are from the period when he developed GR, and from after he published it. Look at the dates in the OP quotes. I'm not lying about them.

Russ_Watters said:
Hacks and quacks because their understanding of the universe has evolved since Einstein?
No. Because they don't understand it. Because they peddle a cargo-cult travesty of Einstein's relativity that nobody understands. And yet the original is easy to understand.

Russ_Watters said:
So tell me: is Einstein also a hack and a quack for having inconsistent views?
No. He developed relativity. It didn't all happen overnight.

I think I have a way to settle the argument. Farsight is saying that time itself is not variable, only that physical clocks run at different rates in different gravities. But we can force some clocks to run at the same rates. So here is an experiment that is quite practical IMO. One clock will be a stable crystal oscillator and the other a phase-locked VCO that is locked to the crystal oscillator. We are forcing the two clocks to run at the same rate with a phase-lock. Position the crystal clock on the floor and the phase-locked VCO near the ceiling. If time is not variable then the clocks should be locked in phase if measured at the height of the phase detector (ceiling) in the PLL or measured at the crystal on the floor. But if time is really variable then the measurement made at the height of the crystal oscillator (floor) then the phase should drift. I don't know what the phase drift rate would be but you could run the experiment for days or even years with a frequency counter in A/B mode.

It seems so simple. Am I missing something?

Right. But you should.

I believe it. It's no game.

I accept it in that that was an SR postulate. But I reject it just as Einstein rejected it when he moved on to gravity. Look at the OP. Look at the Einstein quotes. And like I said to Declan, look at the dates. Then accept it, because I wouldn't lie to you.

No Russ, space isn't curved in a gravitational field. It's inhomogeneous. And this is no game. This is a serious discussion thread, one where you are being told relativity the way it is, instead of the popscience version peddled by hacks and quacks.

Please explain the difference between a locally curved field and a local inhomogeneous field. If a field is affected in any way by the presence of a massive body is that not defined as an inhomogeneity in that field. If that field is a gravitational field, why would it not qualify as an inhomogeneity?

Definition of INHOMOGENEITY

1: the condition or an instance of not being homogeneous

2: a part that is not homogeneous with the larger uniform mass in which it occurs; especially : a localized collection of matter in the universe

I’ve always thought using only the term “invariant” for the speed of light, caused confusion. Of course it will always be measured in a vacuum by every direct observer to be 300,000 k/s (rounded). But the term doesn’t make it clear that 300,000 k/s is relative to the space-time the observer is in…

For example if I measure the distance to the moon with a laser near its average point from Earth I’ll get about 380,000 km. If suddenly the Earth took on a large increase in mass my clock (while still normal to me) is running slower relative to before, and the moon distance would now measure less than before too.

While I still measure light speed as 300,000 k/s, because my clock (time) is slower than before, and my space (moon distance) is smaller than before, I can conclude that 300,000 k/s in my earlier spacetime is not equal to 300,000 k/s in m new spacetime, I can only conclude 300,000 k/s is invariant to my particular spacetime. Clearly an invariant light speed is proportional to how I view time and space (distance), and they are both relative, dynamical terms. Perhaps it would cause less confusion to say light speed is invariant to any direct observer’s spacetime?

Just my opinion. "Fred".

I think you are trying to over think a rather simple property of SR & GR. Stated as simply as possible: All observers will find that the speed of light is the same, regardless of whether they are looking at light in their own reference frame or someone else's reference frame. Everyone will measure it speed as 3 x 10^8 m/s. Any light they measure from whatever source will be just that. What differences they will observe which ARE frame dependent are time dilation and space contraction. But light they will all agree on. It is invariant because it is same to all observers and doesn't need any adjustment to relate one frame to another.

Distance/time = speed. Distance(contracted)/time(dilated) = speed(unchanged.)

It won't bother you once you understand it. See the OP. See the definition of the second? That concerns the NIST caesium clock. It's like their optical clock, but it employs microwaves rather than visible light. Look carefully at the definition of the second: "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". It's periods of radiation. Not frequency. It can't be frequency because frequency is cycles per second and we are defining the second isn't defined yet. So we're counting the microwaves coming past us, and when we get to 9,192,631,770 we say that's a second then we say the frequency of those microwaves is 9,192,631,770 Hertz by definition. Imagine we had a parallel-mirror light-clock and we counted 9,192,631,770 reflections then said that's a second/. Then the reflection frequency is 9,192,631,770 Hertz by definition.

Oh for God's sake,read the OP. We define the second and the metre using the local motion of light. Then we use them to measure the local motion of light. So we are guaranteed to always measure 299,792,458 m/s.

No. See above. Think of an ocean. Think of a wave traversing that ocean. Imagine you're above that ocean in a helicopter. Pacing that wave. It looks like a hump of water. A static hump of water. It doesn't have a frequency. It only appears to have a frequency to the guy in the boat. Because it passes him at some speed. And if it passes him slower, the frequency looks lower.

Shrug. What they say is wrong.

The NIST optical clocks will set the standard eventually.

The point was and is that optical clocks are not measuring the speed of light, so they are providing no direct evidence that the speed of light is either constant or variable.

You keep using this example attached to a pong like graphic and claim it proves something about the speed of light. Re-read your own last response and point to direct evidence of any value for the speed of light.

Or any experiment using optical or even cesium clocks to measure the speed of light, where the experiments are sufficiently separated in a gravitational field that they measure the speed of light to be anything other than constant.

I suspect any real experimental confirmation will have to wait until someday the speed of light in vacuum is measured somewhere far from the surface of a gravitationally significant mass.., using an appropriately accurate clock which can also be shown to be gravitationally time dilated.

Huh? The gif is a parallel-mirror light-clock idealised exaggeration of the NIST optical clocks, which run at noticeably different rates when separated by a vertical distance of 30cm.

So the top clock is rotating with the earth at the same rotational velocity as the bottom clock? In other words, if the two clocks both rotate exactly one earth rotation, then the top clock traveled a further distance than the bottom clock in one rotation of the earth, right?

Farsight, the universe is way weirder than you think.

So the top clock is rotating with the earth at the same rotational velocity as the bottom clock? In other words, if the two clocks both rotate exactly one earth rotation, then the top clock traveled a further distance than the bottom clock in one rotation of the earth, right?

Farsight, the universe is way weirder than you think.

Is it possible to postulate two clocks having the "same rotational speed" but at different distances from the center, without a marked difference either in "length traveled" or "arriving at different times"?

Seems to me, in order to keep synchronicity, one clock would have to travel faster (or slower) than the other, or tick faster (or slower) than the other.

So the top clock is rotating with the earth at the same rotational velocity as the bottom clock? In other words, if the two clocks both rotate exactly one earth rotation, then the top clock traveled a further distance than the bottom clock in one rotation of the earth, right?

Farsight, the universe is way weirder than you think.

Careful, mate. A non-rotating planet would result in time rate difference due to gravity, as illustrated by EMPIRICALLY every day. The Earth's rotation respective SR velocity-difference effects on said two clocks only centimeters above each other is practically insignificant compared to the gravitational effects, especially in strong gravity wells near the surface of NEUTRON STARS etc.

Careful, mate. A non-rotating planet would result in time rate difference. The Earth's rotation affect of clocks only centimeters above each other is practically insignificant compared to the gravitational effects, especially in strong gravity wells near the surface of NEUTRON STARS etc.

naah, keep it simple. Extraordinary examples only prove they are extraordinnary. If I have a central point around which two synchronized clocks rotate at the same speed but at different orbital distances from the central point, synchronicity is lost immediately.

Careful, mate. A non-rotating planet would result in time rate difference due to gravity, as illustrated by EMPIRICALLY every day. The Earth's rotation respective SR velocity-difference effects on said two clocks only centimeters above each other is practically insignificant compared to the gravitational effects, especially in strong gravity wells near the surface of NEUTRON STARS etc.

There is no such animal as a non rotating planet. Planets rotate because power=work/time. There is no such thing as a rotation that is not power in this universe. This is something so bizarre you have no idea.

This diagram is wrong, I think.
Shouldn't the lower bats be closer together if they are in a stronger gravity field?

That is still a hypothetical assumptions/interpretations from mainstream 'theory', not actually observed as 'the mechanism involved in reality'.

Just like the other 'usual cop-out' from mainstream 'theory' which offers that other naif hypothetical interpretation/assumption that the "path length traveled by the photon is longer because the stronger gravity 'increases' the spacetime path".

Note that both naif cop-outs CONTRADICT each other: The former would have the path between mirrors "shorter"; while the latter would have the path "longer" between mirrors.

Talk about naif 'explanations' wanting it BOTH WAYS, hey?

Let's drop all the naif abstract 'explanations' and just follow the reality as it empirically presents itself. That is the only way we will all ever get out of the mire of abstraction upon abstraction which leads further and further away to naif so-called 'explanations' that are nothing of the kind, especially if they CONTRADICT themselves with every newly ad hoc introduced abstract 'fix'. Good luck to us all.

Is it possible to postulate two clocks having the "same rotational speed" but at different distances from the center, without a marked difference either in "length traveled" or "arriving at different times"?

Seems to me, in order to keep synchronicity, one clock would have to travel faster (or slower) than the other, or tick faster (or slower) than the other.

Faster or slower? How would you measure that, when fast or slow is the measure of time and distance.

The reality here is of course that the mainstream understanding of GR works very well.

CERN, NASA, ESA, JPL and all other centers of learning are in total agreement.

What I find curious is the fact that the "existence of time" as an independent dimension is fact. As far as I know there is not a shred of proof of that claim.

We use the term "time" only in relation to an action in a geometric "field" or "space". But like the claim of a god, no one has actually proven that time does exist independent of space and without measurable action time cannot emerge as a property of measurement.

I have never heard anyone able to disprove that time is a "result" and emerges along with the action of the field or object. I am not arguing against the accepted relationship between space and time. I am questioning the assumption of the passage of time in the absence of space or action.