# When a star explodes, does the shock wave

1. Dec 21, 2007

### trendal

When a star explodes, does the shock wave move faster than the local speed of light? If it does, what effects would this have?

A little explaining is in order...

While the actual speed of light does not change (c in a vacuum) the overall progression of light is greatly slowed down due to the conditions inside a star - I think it takes somewhere in the thousands of years for a photon originating in the core to make its way to the outside.

When a star explodes (supernova) it does so much faster than a thousand years - I think I've heard something on the order of a hundred minutes or so. So while the shock wave from the explosion moves at a much slower speed than c in a vacuum, it does move much faster than c in the star.

What effects would this have?

mods, please move this thread if it is in the wrong area!

2. Dec 21, 2007

### DaveC426913

While this is true, I do not beleive that is equivalent to saying that "the speed of light inside the star is on the order of .5million km per thousand years".

3. Dec 21, 2007

### trendal

What do you mean? Why not?

4. Dec 21, 2007

### trendal

Recently we have slowed the speed of light down to something like 20m/s with Bose-Einstein condensates, and it is common to say that "the speed is 20m/s"...so I don't see why you wouldn't say "the speed of light in the sun is 0.5Mkm/1000 years".

In either case, "light in a vacuum" retains the same speed...its overall progress is just slowed down through interacting with matter.

Last edited: Dec 21, 2007
5. Dec 21, 2007

### kudoushinichi88

The speed of light in vacuum is constant. Photons cannot travel faster than this speed, but it can definitely travel slower than this speed. And NOTHING in space can travel greater than the speed of light!!

Why is it so difficult to understand that c is unchanging?

Of course, c only appears to be a constant as far as experiments tell up till now...

Last edited: Dec 21, 2007
6. Dec 21, 2007

### D H

Staff Emeritus
The energy embodied in the photons that originate in the core of a star does eventually leave the star, and it does take a long time for that energy to travel from the core to the surface. However, the photons that originate in the core of a star never make it to the surface of the star.

Photons created by fusion travel but a short distance before being absorbed by ions. A bit later, an ion that absorbed some photon will emit one or more photons, but of a different frequency and in a random direction compared to the captured photon. This is a diffusion process.

7. Dec 21, 2007

### trendal

I worded that poorly...

Call it the "group velocity" of light in a star, if you want. It is slow. Or call it energy.

And for the love of god, stop trying to explain to me that "c is unchanging"...I know that. That's in a vacuum...and a star is about as far from a vacuum as you can get. Yes, I know that the space in between particles is a vacuum...but again in a star there is far less of that space for light to travel.

8. Dec 21, 2007

### D H

Staff Emeritus
You still worded that poorly.

Suppose you encapsulate a strong, steady source of gammas in a thick lead ball. The gammas will only penetrate a little bit into the ball before being absorbed. The absorbed gammas will heat the lead ball. This heat will diffuse to the surface of the ball and leave the surface as black-body radiation. The ball is indeed absorbing photons (gammas) at the center and emitting photons (infrared, for example) at the surface, but it is incorrect to think of this diffusion process as representing a light transmission process.

9. Dec 21, 2007

### kudoushinichi88

I think I understand that you mean the shockwaves inside the star when it explodes... And you are referring to the speed that light travels inside the star.

Then I have no idea what's the answer.

10. Dec 21, 2007

### trendal

Whether it is a transmission process or not isn't the point of the argument I was getting at...regardless of what is happening to the energy the fact remains that the shock wave produced by a supernova travels faster than the group velocity of light in a star.

Think of Cerenkov radiation...that is essentially what I'm getting at.

Last edited: Dec 21, 2007
11. Dec 21, 2007

### D H

Staff Emeritus
Wrong. The speed of light in a star is orders of magnitude higher than the mean speed diffusion computed as the ratio of the star's radius to the mean diffusion time. A diffusion process is akin to a random walk. "Two steps forward, one step back."