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