# At absolute zero, all particle motion does what?

Oct 3, 2016

Stops (at least, for atoms... not for molecules). This can be related back to the third law of thermodynamics:

There exists an arbitrary condition called absolute zero in which the entropy of a perfect crystal is chosen to be zero.

That is, at absolute zero, there is perfect order in the system, and it is defined that only one microstate is supposedly available: stillness.

Each ensemble of microstates must be consistent with a given macrostate, but with only one microstate, there is only one macrostate. So we'd be able to observe (in the macrostate) as well that there is no motion (theoretically).

We could have also realized that temperature is defined as the average kinetic energy (or at least directly proportional to it, which is good enough), and at absolute zero, temperature is $\text{0 K}$.

Thus, the average kinetic energy is $0$ at $\text{0 K}$ and it is said that there is no motion in the system, unless it is a molecule. Then it can vibrate with a zero-point energy of about $\frac{1}{2} h \nu$.