# Photon and its momentum

1. Mar 16, 2005

### scilover89

Momentum can be categorised under electromagnet force, and electromagnetic force carrier is photon. But photon have momentum. Isn't this paradoxical?

2. Mar 16, 2005

### Antiphon

The classical electromagnetic field has momentum. The photon is the
quanum mechanical minimum unit of this field, and it also carries this
momentum.

3. Mar 16, 2005

### Sterj

Mhh, how can it carry momentum if it has mass of zero? Momentum is p=m*v. Or is here the meaning of the quantum mechanical operator?

4. Mar 16, 2005

### Antiphon

According to E=mc^2, energy and mass are eqivalent in this case.

Moving energy must carry momentum. It does not require the
presence of a classical mass.

A compressed spring is a little heavier than a loose one. If they move at
the same speed, the compressed one has more momentum.

5. Mar 16, 2005

### Staff: Mentor

No, in relativity theory it's possible for a particle to have momentum and energy even though it has zero mass. The general relationship between mass, energy and momentum is

$$E^2 = (pc)^2 + (mc^2)^2$$

Set $m = 0$ and you have $E = pc$ which in fact has been verified for electromagnetic radiation.

6. Mar 16, 2005

### Sterj

So for photons this equation can be written as:
E^2=(pc)^2

7. Mar 16, 2005

### Antiphon

Yes. E=hf, p=f/h where h is Planck's constant, when you are talking about
photons. For classical fields, momentum/m^2=ExH/c^2, energy/m^2=ExH.

8. Apr 27, 2010

### Strafespar

Yeah, you are thinking in terms of classical physics.

9. Apr 28, 2010