Is there a physicist in the house?

From: der Mouse <mouse_at_Rodents.Montreal.QC.CA>
Date: Wed Nov 5 15:15:00 2003

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

Not quite.

The uncertainty principle says that certain pairs of measurements
cannot be made to a greater combined precision than a particular
constant.

One of those pairs is location and momentum: when a particle's position
and momentum are measured, the product of the uncertainties in the
measurements cannot be less than Planck's constant (loosely put; it may
be Planck's constant over two pi or some such). Indeed, I've seen it
said that a particle _does not have_ position and momentum to greater
combined precision than permitted by the Uncertainty Principle - not
just that there are "true" values we just cannot measure with
certainty, but that the supposed true values do not exist.

Yes, it's grossly counterintuitive. Quantum mechanics is like that.

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

> ie <thing> is at "5" and 1 second later it's at "35" is it not going
> "30" per second?

There are some problems here.

First, if you aren't constantly observing <thing>, you can't tell
whether it's the same <thing>.

Second, your magnitudes are _way_ too high. For objects of macroscopic
size - the sort of thing your physical intuition leads you to picture -
the uncertanties are, quite literally, lost in the noise. For example,
if you try to measure the location of a mustard seed, the uncertainty
demanded by the Uncertainty Principle is so small that the molecules
constantly evaporating from and being re-absorbed by the seed's surface
provide plenty of uncertainty.

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Received on Wed Nov 05 2003 - 15:15:00 GMT