# I How can a Photon apparently transfer Momentum without any Mass?

1. Aug 4, 2017

### KBon

Due to Compton - Scattering it seems that a Photon changes the Momentum of the electron, although it doesn't have any mass. How should I imagine this?

2. Aug 4, 2017

### vanhees71

A photon is massless but has energy and momentum. It's the mode of an electrodmagnetic field, and you have
$$E=|\vec{q}|c,$$
where $\vec{q}$ is the momentum of the photon.

In relativistic physics you don't need a mass to have a momentum!

3. Aug 4, 2017

### Staff: Mentor

Relativistically, the general relationship between energy, momentum, and mass is $E^2 = (pc)^2 + (mc^2)^2$ where p is the magnitude of the momentum. For a photon, m = 0 which gives vanhees71's formula, in different notation.

4. Aug 4, 2017