Mehran- OK, another of your pointed, thoughtful questions.
The thing is that, so far as we now undrstand things, electrons
simply don't have exact positions. In, for example, the ground state of
a hydrogen atom the electron state really is spread out and NOTHING IS
CHANGING, not position or momentum or anything else. Now when something
else (maybe say a high-energy photon) comes along things do change, and
some sort of 'measurement' process may occur. The outcome can be an
electron whose position or momentum (but not both) are at least for a
while more sharply defined than in the initial ground state.
If the electron starts out in some higher-energy state, it will
ultimately radiate away the extra energy into electromagnetic waves, so
something will change. An electron in the ground state cannot radiade
energy because there is no lower-lying state it can fall into; that's
what's meant by the ground state. It's a non-classical feature of
quantum mechanics that ground states exist at all. If you try to use a
picture of electrons moving about in some sort of trajectory to
calculate how fast an excited atom radiates energy, you won't get the
right answer except in some special cases.
In other words, we have no reason to think that the electron
'seems' to be a cloud. It's far easier to make a self-consistent
description in which the electron IS a cloud, not a point darting
p.s. Here's another little fact that may help bring out these
mysteries. The photons emitted are electromagnetic waves, so you might
be tempted to think that they have particular values of electric and
magnetic fields, just as you are tempted to think that electrons have
position and momentum. However, for a single photon (or for ANY exact
number of photons) the expected value of the fields is zero. On a small
scale, things just don't have the properties we might naively expect
them to have using what we know from classical physics.
(republished on 07/21/06)