# Speed of light

1. Oct 31, 2007

### Foxcroft

Hello, back again.

I have another question that I have been pondering about.

The question I don't get is: light has mass yeah?? even if it's a really small amount. But then how does light go as fast as the speed of light?? Because you would need infinite energy to get at that speed, carrying that mass a long time/distance at that speed surely must use a lot of energy.

But then I thought if E=mc2 then entergy is mass x speed of light2 so doesn't that mean, that if it's traveling at the speed of light, it creates it's own energy? That's the only explanation I could come up with.

What flaws do I have, as I'm sure I've got some.

Thanks Jack.

2. Oct 31, 2007

### malty

Energy has mass, but it's only when it's travelling close to the speed of light that this mass becomes significant.

Think of this way, if no object can reach c then how does it slow down in a vacuum if it experienced a constant acceleration, the only way to decrease the acceleration is to increase the mass.

In the case of light itself, well it's light nothing can travel faster than it, and it contains a tremendous amount of energy (afterall it IS a form of energy); light from the stars travels millions of light years yet it still can do work on the electrons within the human eye.

Last edited: Oct 31, 2007
3. Oct 31, 2007

### stewartcs

When you say "mass" do you mean "relativistic mass" (mr) or "invariant mass" (m0)? These two meanings are not the same. The invariant mass of a particle is independent of its speed v, whereas relativistic mass increases with speed and tends to infinity as the speed approaches that of light, c.

This can be stated mathematically as,

mr = E/c^2

m0 = sqrt(E^2/c^4 - p^2/c^2)

4. Oct 31, 2007

### ZapperZ

Staff Emeritus

Zz.

5. Oct 31, 2007

### pixel01

I think every object must have two parts of energy : the rest (mc^2) and the moving which is kinetic energy. Because mass can be converted to energy so light is just a special case when mass is converted to energy.

6. Oct 31, 2007

### stewartcs

Cool, didn't know that was there.