Is the speed of light constant?

[rbj]

sorry, this might sound like a stupid question but what does 'gtr' stand for? general something relativity?

General Theory of Relativity. i always refer to it as "GR" if i need an acronym.

say Doc: i wanted to check out that creationist site (for giggles, if anything else) and you deleted the reference before i got a chance to. and, there is no "history" to the page that i know of.

grrr.

how else am i going to be entertained today?

 

[Doc Al]

say Doc: i wanted to check out that creationist site (for giggles, if anything else) and you deleted the reference before i got a chance to. and, there is no "history" to the page that i know of.

I'll PM you.

 

[pervect]

As I pointed out above, there ARE astronomical measurements that seem to suggest that the speed of light HAS changed, not by much but by a measurable amount.

There are some astronomical measurements that suggest the speed of light (more precisely, the fine structure constant) may have changed, and other astronomical measurements that suggest it hasn't changed.

For one popular account of one experiment that has failed to find a variation in c, see for instance http://www.newscientist.com/article.ns?id=dn4844

While the experimental results do not agree with each other at this point, all the positive findings are tiny, typically talking about variations less than a part per million.

 

[turbo]

Turbo, I hope that in future you will be more careful to avoid the appearance of pushing a fringe viewpoint by presenting a misleading description of some mythical "Einstein's view" (To mention just one objection, while by an arguably perverse shift in viewpoint one can model weak-field lensing in terms of a kind of "refraction", you seem to have forgotten about strong field lensing, which is much more complicated, yet described in fully nonlinear gtr exactly the same way--- see Chandrasekhar, Mathematical Theory of Black Holes.)

I have referred to Einstein's writings because they represent his attitudes at the time that his theory of GR was fresh and current. If you want to refute these ideas, which he forcefully presented in a number of venues, you should link to to refutations that are contemporary, well-documented, and well-motivated. You will not be able to do so. Einstein claimed that gravitational lensing is due to classical optics, saying that

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. Now we might think that as a consequence of this, the special theory of relativity and with it the whole theory of relativity would be laid in the dust. But in reality this is not the case. We can only conclude that the special theory of relativity cannot claim an unlimited domain of validity; its result hold only so long as we are able to disregard the influences of gravitational fields on the phenomena (e.g. of light).

Einstein's contemporaries were not happy with his progression toward a vacuum that was an active player in refraction, but they were not able to nay-say him. They were unhappy with the establishment of a non-classical ether, though Einstein pursued this for a long time. You may be smarter than Einstein, but you've got to prove it before I'll believe it.

 

[Chris Hillman]

Turbo, IMO it is an insult to the leaders of the Golden Age to imply that GTR has not been "fresh" since 1915. I repeat, IMO your historical comments are misleading and even incorrect, but my basic point is that arguing over how to understand Einstein's views on topic T c. 1915 (as far as we can judge from surviving documentation) is not particularly helpful for understanding GTR. I repeat: IMO, your comment which I quoted was seriously misleading to Kristy and appears to be pushing a point of view which would have impeded her progress toward understanding gtr.

I have liberally sprinkled this post with IMOs and suggest that at this point we should simply agree to disagree, since it is clear that you have no intention of abiding by my advice to stress our best current understanding of GTR when trying to give newcomers to relativistic physics some useful pointers in PF discussions such as this thread.

I don't want this thread to be derailed by some argument between us. I sense that you may have more to say about why you think that understanding the history of physics is important or even essential for understanding modern physics. If so, I request that you start a new thread on that topic, and I will try to drop into explain my more or less opposing view (unless of course your essay is so convincing that I decide to agree with you!). Fair enough?

 

[DaveC426913]

You're gonig to ask us to start counting off the crackpot gambits one-by-one?

it hasnt been accepted by the physics community because it suggests the universe is so young that evolution would not have had enough time to get to where it is today.

1] "the scientists are conspiring to hide the truth"

this is the kind of huge piece of evidence against the slowing light speed theory i was talking about in an earlier post saying how the idea 'holds water' until someone states a reason why it wouldn't work.

2] "I've made a nice tidy model. If you can't tell me why it won't work, it must be true."



The Lockness Monster is HUNGRY!

 

[pervect]

I'm confused. If the speed of light is changing, does that mean that a vacuum's refractive index is also changing? What effect would the speeding up / slowing down of lights speed have on various models etc...

Also, I read somewhere about light being refracted by gravity...but how does this work since it has no mass. My original thoughts were about general relativity and how gravity affects time, so if light is near a massive object, time is slower and therefor it will go slower...? Can anyone tell me how this works?

Let me give a simpler response than the one you've been given.

What the value of 'c' is depends on whose clocks and whose rulers you use. Due to effects such as "gravitational time dilation", clocks and rulers don't necessarily agree. So, the value you will measure for 'c' depends on your choice of clocks and rulers.

(This is a very slight oversimplification, but it will point you in the right direction).

With one particular choice of clocks and rulers, the speed of light is always equal to 'c'. Thus the "modern interpretation" of GR is to use this choice by default, and say that the speed of light is always equal to 'c'. This corresponds to using "local" clocks and rulers. One way of putting it - the speed of light may appear to be different from 'c' from a distant location, but if you actually go there and measure it, you find that it hasn't changed.

You might want to look at the sci.physics.faq

in particular:

Einstein went on to discover a more general theory of relativity which explained gravity in terms of curved spacetime, and he talked about the speed of light changing in this new theory. In the 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 of propagation of light varies with position. Since Einstein talks of velocity (a vector quantity: speed with direction) rather than speed alone, it is not clear that he meant the speed will change, but the reference to special relativity suggests that he did mean so. This interpretation is perfectly valid and makes good physical sense, but a more modern interpretation is that the speed of light is constant in general relativity.

The problem here comes from the fact that speed is a coordinate-dependent quantity, and is therefore somewhat ambiguous. To determine speed (distance moved/time taken) you must first choose some standards of distance and time, and different choices can give different answers. This is already true in special relativity: if you measure the speed of light in an accelerating reference frame, the answer will, in general, differ from c.

In special relativity, the speed of light is constant when measured in any inertial frame. In general relativity, the appropriate generalisation is that the speed of light is constant in any freely falling reference frame (in a region small enough that tidal effects can be neglected). In this passage, Einstein is not talking about a freely falling frame, but rather about a frame at rest relative to a source of gravity. In such a frame, the speed of light can differ from c, basically because of the effect of gravity (spacetime curvature) on clocks and rulers.

If general relativity is correct, then the constancy of the speed of light in inertial frames is a tautology from the geometry of spacetime.

 

[turbo]

I don't want this thread to be derailed by some argument between us. I sense that you may have more to say about why you think that understanding the history of physics is important or even essential for understanding modern physics. If so, I request that you start a new thread on that topic, and I will try to drop into explain my more or less opposing view (unless of course your essay is so convincing that I decide to agree with you!). Fair enough?

Fair enough, Chris. I will take some time in the next day or so to pull in some relevant references. One in particular ("On the Ether", 1924) is available only in hard-copy, so I'll have to transcribe relevant portions of that.