Do interactions slow down a particle?

Question

I wonder whether particles, especially such as photons and neutrinos, do slow down when they interact in any way? E.g. how is it going on for the Compton effect but how is it in general like in QED or so?

Answer 1

This is true in certain cases (not true in the case of a photon) . Many fundamental particles acquire their mass by interacting with the Higgs field. And we know that massless particles travel at the speed of light. This means that interacting with the Higgs field indeed slows down the particles.

Is this true for interactions other than the Higgs? Not really. According to Einstein's relation $m=\frac{E}{c^2}$, potential energy due to interactions may cause a system's inertia to increase or decrease. However, this does not mean that their velocity slows down.

Furthermore, a quantum mechanical particle does not have a well defined momentum. But the expected value of the momentum evolves according to the Ehrenfest's theorem:

$$\frac{d\langle P\rangle}{dt}=\left\langle-\frac{dV}{dX}\right\rangle$$

You can see that a quantum mechanical particle may accelerate or decelerate due to interactions similar to in classical mechanics.