# Absolute zero and Heisenberg uncertainty principle

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!

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 I've read your question two times. I don't get what you're asking. Can you try to elaborate a bit further? I especially don't get your last sentence. – sebastian Mar 7 at 8:58 I mean to say that the heisenberg rule holds only 4 microscopic particle in motion...but at absolute zero motion is assumed to be absent...hope this helps... – A4KASH Mar 8 at 20:08

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.