# Why light travels

1. Jul 12, 2005

### ludi_srbin

How does light travel? What gives the energy to the photons to travel at c? And if in order for something to travel at c you need an infintie amount of energy, how come photon travels at c?

2. Jul 12, 2005

no, you don't. photons are massless. $$E=hf$$

3. Jul 12, 2005

### εllipse

To elaborate, it only takes an infinite amount of energy for something to travel the speed of light if it has a rest mass (a mass in its own frame of reference) because relativistic mass increases as speed increases. But for something that doesn't have a rest mass (such as a photon), it has to move at the speed of light in order to even exist at all. (Or atleast, that's what the equations seem to suggest to me. I posted my reasoning behind this here: https://www.physicsforums.com/showthread.php?t=79637 and nobody told me I was wrong so I assume it was correct.)

Last edited: Jul 12, 2005
4. Jul 12, 2005

### Chronos

Here is a link to get you started:

Special Relativity as a Physical Theory
http://www.arxiv.org/abs/physics/0410124

And here:
http://physics.nyu.edu/hogg/sr/

5. Jul 12, 2005

### Igor_S

I think modern interpretation is that mass doesn't change (it's the same in all inertial systems), but energy does according to formula:

$$E = \frac{mc^2}{\sqrt{1-(v/c)^2}}$$.

So mass is the same in the eyes of every intertial observer (the term "invariant" is commonly used).