# Mass and energy

1. Jan 21, 2005

### pnazari

Ok I have some general questions about mass and energy. I was wondering if photons have any energy themselves? And if they do, why is it that energy can travel at the speed of light but mass can not (ie, if they are interchangeable why cant mass travel at light speed). Thanks for your time.

2. Jan 21, 2005

### dextercioby

Photons can have virtually any energy...But photons do not have rest mass,that's why they always travel at "c" in vacuum.The relativistic mass of the photon (given by Einstein's equation) has nothing to do with its rest mass and hence with its speed...

What do you mean,"energy can travel of the spped of light"...??PHOTON'S ENERGY TRAVELS AT THE SPEED OF LIGHT.It's also valid for any other massless particles...

Daniel.

3. Jan 21, 2005

### pnazari

What I mean is that if photon has energy and it is travelling at c then it's energy is also travelling at c...i dont even know if this is a valid statement...

4. Jan 21, 2005

### dextercioby

Yes,in the case of the photon,its energy travels at "c",simply because the photon IS ENERGY.Quantum of energy for the electromagnetic field...

Daniel.

5. Jan 21, 2005

### pnazari

So then if the photon is just energy travelling at c, and since energy and matter are interchangeable, then why is it that matter cant travel at c? That was my original question.

If I understand what you said previously, it is because the photon has zero rest mass?

6. Jan 21, 2005

### dextercioby

Matter which has nonzero rest mass cannot travel at "c".Period.

Yes.

Daniel.

7. Jan 21, 2005

### pmb_phy

Yes. Photons have energy E = hf. A photon has zero proper mass m0 (aka "rest mass") but a non-zero inertial mass m = p/v = p/c = E/c2. If you'd like to then you can think of light as carrying mass with it.

Pete