Is there a physicist in the house?

From: Bill McDermith <bill_mcdermith_at_yahoo.com>
Date: Fri Oct 31 09:35:42 2003

Comments below...

Ron Hudson wrote:

> Heisenberg says we can't know the speed of <thing> and it's location
> at the same time.

Correct.

> What if I concentrate on location while timing <thing>

For <thing> small enough to show quantum effects, locating <thing> once
will change it's speed (momentum)...

> ie <thing> is at "5" and 1 second later it's at "35" is it not going
> "30" per second?
> and while it was going 30 per second didn't I see it at 5 and 35?

For quantum-sized <thing>, there would be no way to know the <thing> at 35
was the same <thing> at 5... Measuring <thing> at 5 disturbs <thing> --
see below

> (note to a real physicist this question is probably meaningless...)

Not meaningless... In fact, the basis of QM.

Usually stated, you can't know the momentum of a particle (p = mv, m = mass,
v = veloicty, p = momentum) and the position of a particle more
accurately than
the uncertainty introduced by Planck's constant h = 6.626068 ? 10-34 m2
kg / s
You usually talk about momentum because at relativistic speeds the mass
is not
a constant...

This is usually explained as uncertainty introduced by measuring one or
the other of
the two quantities momentum or position. A simple view is an electron --
if you shoot
it at a phosphor screen, it illuminates a spot, and you know exactly
where it is, but you
no longer know how fast it is travelling, as it gives up some of it's
energy to light
the phosphor. On the other hand, if you accelerate electrons between two
charged
plates, you can have a very good idea of the speed (therefore the momentum)
of the electrons (or think of them in a cyclotron) but you have no idea of
the actual position of them. The act of measuring one item affects the
other...

No one has come up with a way of measuring a particles position that doesn't
cloud the knowlege of the particles momentum, and the reverse.

Plank's constant is so small that these effects are only noticable on
the scale
of atomic particles, atoms, molecules, etc. and not on normal objects
that we
look at (the classical limit, where Newtonian mechanics is usually
used...), so
there is no problem with the fact that you can know the position and
momentum
of a billiard ball fairly accurately...

There is usually a good (math) expanation of this in textbooks with the
title
Modern Physics (2nd year college physics texts...) -- they usually also have
a good explanation of Relativity for someone who has had first year
college physics using Calculus. A good explanation for non-math
types is in a book called "Mr. Tompkins in Paperback"
by George Gamov -- has relatively clear explanations of both
QM and Relativity by writing a story where the speed of light
is low enough and Planck's constant large enough to make the
effects visible on "normal" objects...

Bill
Received on Fri Oct 31 2003 - 09:35:42 GMT