Is there a physicist in the house?

From: der Mouse <mouse_at_Rodents.Montreal.QC.CA>
Date: Wed Nov 5 17:59:01 2003

> Fundamentally you can't disprove Heisenberg with Newtonian laws, so
> if you think you can, you are likely wrong (to very high degree of
> liklihood).

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.)

> People have already mentioned how acceleration and smallness can mess
> up the measurement, but it's probably more than that. Indirectly
> related might be the Bose-Einstein concentrate. Scientists slowed
> down subatomic particles to a near stop, so they knew the position
> and the speed right? Apparently the particles by theory and by the
> experiment just "deres" or become big fuzzballs that have no definite
> position.

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.

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Received on Wed Nov 05 2003 - 17:59:01 GMT