[gaika]

It wouldn't break current theory, it would just mean that photons travel slower than "speed of light" and have non-zero rest mass. Constant c in relativity instead of speed of photons would just mean fastest speed possible.

[jxcole]

I have a comment about this. C, as is used in general relativity, is involved in a lot more than just the speed that light travels. It is also relevant to a lot of other equations, like time dilation in a gravitational field. Now, if this experiment resulted in changing our concept of c as the "speed of light" to the "speed of neutrinos", then your probably right. But I have to imagine that c has been verified experimentally in non-light related experiments.

For example, there is a certain speed where if you exceed it you are able to violate causal time relationships. I can't think of any experiments that would validate this. However, there is also the fact that theoretically, if you attempt to accelerate matter to the speed of light it's mass will increase infinitely. So if you accelerate it a little bit it's mass should increase a little bit, and you should be able to confirm the speed of light through an experiment where you measure infinitesimal increases in mass during large acceleration.

So my comment is that if he just broke the speed at which light travels, then everything is fine. But if he broke the speed at which you are able to violate causality, or the speed at which the mass of an object is infinite, then our entire understanding of physics is likely to be invalid.

Related reading - tachyon pistols

http://sheol.org/throopw/tachyon-pistols.html

[Jun8]

Not really. The reason the idea of fixing c as the speed limit is that the number arises naturally as the speed of EM waves from Maxwell's equations. Briefly put, these these equations are valid in all frames, the Galilean speed addition rule has to be wrong and c should be standard in all inertial frames.

So you can't just use c for "the highest speed any particle can have in vacuum".

[Florin_Andrei]

No.

Non-zero rest mass photons will break a lot of theories.

Plus, the "speed of light" is not merely an experimental result coming out of an interferometer. It's also a theoretical result, e.g. from Maxwell's equations - that's the one referred to by special relativity.

[hardtke]

An alternative is that neutrinos and photons travel a different distance because there are extra dimensions that affect the two types of particles differently. These are the so-called space-time foam models.

[dholowiski]

Don't forget... c is the speed of light 'in a vacuum'. Light travels slower through gas, water or glass. Light has even been slowed down to walking speed in a laboratory. The speed of light is not constant. The speed of light in a vacuum is. We assume.

[DougBTX]

> Light travels slower through gas, water or glass.

There is a good description of what is going on in this Stack Exchange post:

http://physics.stackexchange.com/questions/13738/propagation...

It explains why saying "c is the speed of light" makes sense, because when we say light is traveling more "slowly" through a material, we are including the time spent interacting with the material, being absorbed and re-emitted.

I'm bristling a little at your statement that "the speed of light is not constant". Imagine two men walking at the same speed from A to B. But one of them is walking in a straight line, while the other is zig zagging. It would be fair to say that the one walking in a straight line is travelling from A to B faster, even though they are both moving at the same speed. The speed of light is a constant, it is just that light travelling through a medium doesn't necessarily spend all of its time travelling in one direction.

[felipemnoa]

>>The speed of light is not constant

I think this is wrong. Regardless of the medium the speed of light is always constant. It seems to slow down because the photons are getting absorbed and re-transmited by atoms. But the speed of light is always the same regardless.

[jasonkolb]

Isn't the speed at which photons travel, by definition, the speed of light?

And as for the "c" in e=mc^2, doesn't this suddenly make "c" an unknown constant? Doesn't the fact that "c" changes suddenly change the values of the other variables in that equation as well? That seems pretty fundamental to me...

[zipdog]

As I understand it, Einstein's work rests on there being a fundamental maximum 'speed' and it seemed to him as though the speed of photons was that limit, so 'speed of light' became synonymous with this maximum. But it doesn't necessarily have to be so.

So if there's something faster, it changes our understanding of photons but not the existence of this fundamental maximum speed.

As you note, our efforts to measure c may have been off due to measuring the wrong thing, but I don't know the ramifications of a small % change in c.

(I'm not a physicist)

[kevinpet]

I think what he's getting at is that there's this value "c" that's really important to physics appearing in equations like e=mc^2 and determining the absolute upper bound on speed, and by the way, we used to assume that photons traveled at c, rather than their actual rate of 99.9975% of c.

I don't know whether changing c by this amount would break many experimental results. Adding a rest mass to photos sounds potentially revolutionary.

[iand]

Einstein based his theory on the maximum speed at which information can propogate. That's always been assumed to be the speed of light (photons). It may be possible that there is something else that can propogate information faster (e.g. neutrinos). There would still be an upper speed limit, but it wouldn't be the one we thought it was :)

[dfranke]

Isn't the speed at which photons travel, by definition, the speed of light?

Photons speed up and slow down routinely, depending on what medium they're traveling through. c, as it is used in the equations of relativity, is currently believed to be equal to the speed of light in a vacuum. But, with my limited knowledge of GR, my understanding is that gaika is correct and that the rest of the theory can still stand if this equality is broken.

[monochromatic]

It wouldn't break current theory for photons to have nonzero rest mass?