# Why photons can't afford more than light speed ?

1. Jun 29, 2010

### oneat

Why photons can't afford more than light speed?
They are not limited by E=mc^2.

2. Jun 29, 2010

### JDługosz

It is the nature of space-time. Read up about Special Relativity, then come back to the question.

3. Jun 29, 2010

### Gerenuk

No, all particles obey this equation. Even photons. Obviously you cannot use the usual gamme factor.

4. Jul 1, 2010

### Gear.0

But that equation is not even what shows that normal matter can't go at light speed.

It is shown with:
$$p = \gamma m v$$
But, this is the equation for bodies with mass only and it shows they cannot reach c.

For massless particles (photons) the above equation does not apply, you must use this instead:
$$p = \frac{E}{c}$$
which doesn't have the same problem. Furthermore, we know that for waves the wavelength * frequency = velocity of the wave, but for light the wavelength and frequency are inversely related by the constant c, so when you find wavelength*frequency you get that they equal "c", which should be the volocity, so v=c.

I'm just sort of beating around the bush though, there are much better explanations. I know there is a way to derive it from maxwell's equations.
Sorry I can't give a better explanation but that is why I didn't respond to this thread right away, I waited a day.

5. Jul 1, 2010

### HallsofIvy

The fact that light always has speed c, relative to any observer, is a result of experimental evidence. It is not proven from other properties, other properties are derived from that observation.

6. Jul 1, 2010

### cragar

you can show that light travels at c with maxwells equations
c= 1/(sqrt(eq))
where e= permittivity constant
q= magnetic constant
and when maxwell came across this result , he realized that this was always constant no matter how fast you were going , and this was always in the back of his mind ,
And Einstein used this fact to start creating relativity ,