# Considering the diffusion of a single particle

Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero

As I quoted through the Wikipedia,

The diffusion should be affect of the thermal motion. As on it my first feeling was what is the fundermental force acts on it with the main 4 fundermental forces. I found much details from Force causing diffusion but arise a question considering a single particle.

As this ,when we consider a set of particles, each can collide with the other particle, the particles are moving from the more concentrated place to the lower one.

If we take a vacuum tube and keep a particle there, in the temperature of absolute zero, and then slowly increasing the temperature, the particle starts to move with the thermal motion.The is no way for any collision or way to vary it's momentum which force affects for this motion by the main 4 fundamental forces?

• I'm not sure what the question is here?
– Gert
Commented Sep 10, 2019 at 15:34

If the single particle is in a vacuum tube, then there really isn't a way to define the temperature. Temperature is a concept that only applies to collections of a very large number of particles, since temperature is somewhat a measure of the average kinetic energy of a system.$$^*$$ But for your single particle it has a well defined kinetic energy already. We don't (can't?) apply the ideas of statistical mechanics to it.
$$^*$$ I say somewhat because other degrees of freedom not related to the kinetic energy can contribute to the temperature of a system, but for a system of non-interacting, single particles (like an ideal gas) then it is fine.