Speed of light times 1.4

gravywavy
gravywavy
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RE: I too am adding my

Message 15889 in response to message 15885

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I too am adding my 2cents worth. I am currently running EINSTEIN, ROSETTA, and CLIMATE PREDICTION(since 2004) to my satisfaction.
...
I am giving all of my accounts equal time (33.33% each, at present) and would like to add SETI to the mix (ultimately 25% each).

If you have successgully attached to SETI, then as soon as work becomes available you will automatically drop from 3 lots of 33% to four lots of 25%, at least if you are using the client version 5.2.x.

At present Rosetta sometimes declines to give out work, but usually this lasts for only an hour or two and you'd need to look through tthe messages tab to spot this.

R~~

~~gravywavy

Solomon
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RE: RE: It would seem, to

Message 15890 in response to message 15888

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Quote:
It would seem, to me, there is no such thing as 'constant'; Why? When you add variables such as temperature, pressure, and media these ALWAYS changes the parameters of the problem in question.

Light travelling through a medium ( non-vacuum ) is a modelled as a process of absorbtion followed by re-radiation, primarily by interaction with the electrons in ordinary matter. There is vacuum propagation between each interaction, matter being mostly 'empty' space. This predicts well for the states of matter found hereabouts anyway. The 'c' constant appears as a factor in the mathematical 'propagator' of quantum electrodynamics, where the overall calculation involves Feynman's sum over histories or path integral to get the amplitudes, phases and then probabilies which yield the final prediction of behaviour. In the absence of matter this method predicts the classical straight line movement with classical speed c.
So yes, the distance that light travels, divided by the time it takes to do it is going to vary depending on what it passes through. I think the merit in claiming 'c' as a constant of nature is to separate out light's intrinsic behaviour ( when left alone, so to speak ) from it's behaviour during interactions by passing through matter.
One thing I've wondered about is whether relativistics needs to take into account indices of refraction etc. After all Einstein grounded alot of his conclusions by considering light signals between separated points in space. Now if the light is slowed because it has to 'stop and talk to the electrons' on the way, then that is going to have an effect. I can't imagine a GPS system not having to account for the timing change because the signal from the satellites is passing through the atmosphere, to achieve the required accuracy.
Also the propagation of other forces are based on 'c' too. The weak and strong nuclear forces are too short range to really worry about over everyday distances, but gravity isn't. I'd assumed that matter would affect gravitational radiation via it's mass/energy content, however small, but that we could ignore electric charge etc. Maybe this is the area which is not yet unified, but I'm not sure.

The importance of the speed of light in relativity is that any observer in an inertial frame of reference will see light travelling at the same speed, as long as it's propagating through a vacuum. However, this will not hold if the light is travelling through a medium. This means that a material's index of refraction will depend on its speed. However, this can be understood in terms of the contraction of its length in the direction of motion.

As for GPS, I'm not sure whether the medium needs to be taken into account, as the speed of light in air is extremely close to that in vacuum. It may be that other sources of error in the system are significanly larger than this. I'm not really sure.

Michael Karlinsky
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RE: As for GPS, I'm not

Message 15891 in response to message 15890

Quote:

As for GPS, I'm not sure whether the medium needs to be taken into account, as the speed of light in air is extremely close to that in vacuum. It may be that other sources of error in the system are significanly larger than this. I'm not really sure.

There is a short Wikipedia paragraph about GPS and relativity,
which in turn points to an excellent document on this topic.

Michael

Mike Hewson
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[quoteThere is a short

Message 15892 in response to message 15891

[quoteThere is a short Wikipedia paragraph about GPS and relativity, which in turn points to an excellent document on this topic.
Michael

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

gravywavy
gravywavy
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RE: One thing I've wondered

Message 15893 in response to message 15888

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One thing I've wondered about is whether relativistics needs to take into account indices of refraction etc.

This is a very astute point.

For calculating the standard results in special and general relativity light is always deemed to be in vacuo, travelling at c.

However, many of the experiments to detect the aether depended on the properties of material media. The reason we can't detect ourt motion throught the aether was thought for a while to be because a medium like air or water dragged the aether along with it.

Experiments were done that successfully measured the aether's drag in both water and air, and these pre-date special relativity so SR had to give a good alternative account of what was going on in those experiments. This explanation was that the light in the water was not travelling at 'c', so of course there could be a measurable difference in its speed depending whether it moved with or against the flow of the water.

The carefully calibrated aether drag coefficients of various substances turn out to be just a number that depends on the refractive index of that substance and on nothing else.

So far, so good.

However, people are now asking if there is a difference according to the orientation of the flow of the water to the motion of the Earth through space when doing these Aether drag experiments. Both SR and General Relativity say there should not be; some alternative theories predict a very small residual effect. If such an effect exists then SR and GR are 'only' very good approximations to reality (like we might say Newton gave us a good apporximation to reality in certain situations).

River~~

~~gravywavy

Ben Owen
Ben Owen
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Oops, looks like my first

Oops, looks like my first answer missed something implicit in the original question:

It's the speed of light in vacuum that's constant, and it's that constant that appears in all the equations of relativity. The speed of light in a medium does indeed change with fluctuations in temperature, density, etc. But what appears in the geometry of spacetime, etc is always the speed of light in vacuum.

By the way, be careful with Wikipedia. Their page is right that in-medium effects (ionospheric delay etc) are important, but that remark about the constancy of c not working in a rotating frame is wrong. As of right now, that wrong remark seems to have been stricken from the page, but there are others.

Oh, yuck, I just looked at their page on neutron stars and it's awful....

Wikipedia seems to be best at dealing with non-technical things because it's written by an army of volunteers. For technical subjects you really need an expert to sit down and do it, and there aren't that many experts.

And: In principle gravitational waves would indeed slow down a little bit from c as they go through matter. But it's way too small to be measured because the interaction with matter is very weak. I vaguely remember doing this calculation as a homework problem in grad school and the answer was that if you had a high enough density of matter to significantly affect the waves, the matter was about to collapse and form a black hole.

Hope this helps,
Ben

Ed Otto
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Well, some Italians have

Well, some Italians have recently experimentally verified that the speed of light is NOT a constant. The papers are available on the phys paper servers.

MarkF
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Ed, your citation is not

Ed, your citation is not particularly helpful. Can you provide any additional details such as the author(s) names or the title of the paper?

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