# What's so special about light?

1. Aug 5, 2013

### copernicus1

Hi, I confused about the relationship between light and $c$ on the one hand and massless particles and $c$ on the other. Does light travel at c because it's light and that's all there is to it, or does it travel at c because it just happens to be another massless particle?

I know that classically $c=1/\sqrt{\epsilon_0\mu_0}$, which seems to indicate that the speed of light has everything to do with electromagnetism, but then why does the speed of light apply so much more broadly?

2. Aug 5, 2013

### phinds

What you call "the speed of light" is really "the universal speed limit". That is, it isn't what it is because it's the speed that light travels at, it is a more fundamental thing than that and light travels at it because light is a massless particle. Any massless particle will travel at the universal speed liimit. Also, gravity waves are believed to travel at that speed

3. Aug 5, 2013

### copernicus1

Okay, thanks, that's sort of what I thought. Are there other ways of defining c besides in terms of the permittivity and permeability constants? This seems to tie c to the electromagnetic interaction in some special way.

4. Aug 5, 2013

### Bill_K

c is the fundamental constant in Maxwell's Equations. It's a mistake to think in terms of two fundamental constants ε0 and μ0, which are presented that way to satisfy the engineers. Recall that μ0 is nothing more than a defined number, 4π x 10-7.

5. Aug 6, 2013

### harrylin

It is well known that c is tied to electromagnetic interactions as light is electromagnetic, but c is defined as the speed of propagation in vacuum (for sound, c is the speed of sound and for light it's the speed of light). In other words, the fact that electromagnetic properties allowed to predict the propagation speed of electromagnetic waves does not define that speed - but those constants are definitely tied together. And since SR we know that c is also the limit speed of everything material; that was the mathematical outcome from the observations that the speed of light is not only a constant c, independent of the source, but also invariant. That doesn't make light special, but it does make its vacuum propagation speed c special.

Last edited: Aug 6, 2013
6. Aug 6, 2013

### nitsuj

I agree, I can't help but feel c is ALL about geometry & sequencing (ordering). From there physics happens.

7. Aug 6, 2013

Uh ... huh ?