Is there a physicist in the house?

And if you have such an argument and it _is_ right, all you've shown is that Newtonian mechanics and Heisenberg are incompatible, that they cannot both be right. Which, if either, _is_ right is a question for experiment, not theorizing. (My money would be on Heisenberg in such a case.) That's part of it; the Pauli exclusion principle is also related. The exclusion principle says that particles with non-integer spin (fermions, such as electrons with their spin ??) cannot be identical: two such particles must differ in location, or spin, or some such. This is why atoms have electron shells that can fill up - for example, the first shell is spatially symmetric, and thus can tolerate only two electrons, one with spin +? and one with spin -?; the next shells are lobed, with three such available (one for each spatial dimension), holding two electrons each. Fermions display Fermi-Dirac statistics, I think the term is. Particles with integer spin (bosons, such as photons with their zero spin) can be as identical as they care to; they are said to display Bose-Einstein statistics. As you say, when particles get very cold, their momentum becomes very small, to a high degree of precision, which means their position becomes correspondingly _un_certain: they fuzz out, to put it loosely. But if you try this with fermions, they resist overlapping because of the exclusion principle[%]; you have to do it with bosons to get anything really interesting. (This is why helium-4 becomes superfluid but helium-3 doesn't: that extra neutron makes the difference between the nuclei being fermions and their being bosons.) With fermions, all you get is zero-point motion (particles still jiggling around some regardless of how close to absolute zero you get; they refuse to sit still because that would give them very precise position _and_ velocity, and fuzzing out into a B-E condensate would make them overlap with other fermions.) [%] Actually, "because of" is a very imprecise way of putting it. We do not know _why_ the world works this way; we do not know even that there _is_ a "why". All we know is _that_ it does, and the exclusion principle is just a description of one aspect of the way the world works. To say that the exclusion principle is _why_ something happens is, strictly speaking, mistaking description for causality. /~\ The ASCII der Mouse \ / Ribbon Campaign X Against HTML mouse(a)rodents.montreal.qc.ca / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B