# Why light travels

#### 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?

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

#### 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.)

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#### Igor_S

εllipse said:
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.)
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).