Absolute zero and Heisenberg uncertainty principle

Question

I got to read Feynman vol I and there was written that at absolute zero, molecular motion doesn't cease at all, because if so happens, we will be able to make precise determination of position and momentum of the atom. But we do know that Heisenberg uncertainty principle holds for microscopic particle IN MOTION....then what is wrong to consider that all molecular motion ceases at absolute zero...need some help!

Answer 1

Motion does not cease at absolute zero if the system you are looking at has a zero point energy.

In many systems, e.g. crystals, at low temperatures the atoms/molecules behave as harmonic oscillators, and the energy of a harmonic oscillator cannot be reduced to zero: there is always some minimum energy called the zero point energy. This means that at absolute zero the atoms in a crystal will not be stationary. There will be a small vibration corresponding to the zero point energy. This is most obvious for light atoms like Helium where the zero point energy is enough to keep the system liquid, so even at absolute zero Helium will not solidify unless it's put under pressure.

The situation is different for a free particle. In that case, at absolute zero the momentum is zero but then we have no knowledge about where the particle is (i.e. $\Delta x = \infty$). If we want to measure where the particle is we have to put some energy in, but then of course the system is no longer at absolute zero and the momentum is now non-zero.