# Matter @ speed of light and e=mc^2

1. Mar 4, 2009

### Pjpic

I seem to have read that matter can't be accellerated to the speed of a photon because it would take an infinite amount of energy. But it seems that mc^2 = E means that matter can be transformed to E (and doesn't E move at the speed of light?)

2. Mar 4, 2009

### ZapperZ

Staff Emeritus
But when that matter has been transformed into "energy", it is no longer identical to that matter and it can do whatever an "energy" can do.

When you tranform $10 into a sandwhich, you can now eat that sandwich. Does this mean that your$10 bill can also be eaten like that sandwich? How tasty is that? Probably if you drown it in a lot of ketchup.

Zz.

3. Mar 4, 2009

### Pjpic

When you tranform $10 into a sandwhich, But, I thought the rule was that you'd have to have an infiinte amount of money to catch the sandwich. Maybe, you could hand over the$10 at the exact same time as you ate.

4. Mar 4, 2009

### Bob S

In the transformation of mass to energy, you have to conserve both energy and momentum (and quantum numbers). Consider an electron in a piece of aluminum. Stop a positron in it. It will find an electron and both masses will be converted to pure energy; two 511 keV photons. So here both energy and momentum are conserved.

5. Mar 4, 2009

### ZapperZ

Staff Emeritus
You are not interesting in figuring out on what is wrong with your "logic", are you?

Zz.

6. Mar 4, 2009

### Bob S

When a light beam passes through a clear glass disk with index of refraction n, the velocity of the photons in the glass are reduced to a value v = c/n, or about 2/3 the velocity of light. When a proton from the Fermilab Tevatron, with an energy equal to about 1000 proton rest masses and speed nearly the speed of light, passes through the glass, its velocity would be nearly 1.5 times the velocity of the photons. So matter can travel faster than the speed of light, but not in a vacuum.

Last edited: Mar 4, 2009
7. Mar 5, 2009

### lightarrow

No, the total mass doesn't change. See related threads.

8. Mar 5, 2009

### Nabeshin

Pretty sure he means rest mass...

9. Mar 5, 2009

### lightarrow

...me too.
I'm not joking, see related threads. For example, post #45 of this thread: