Re: Particle Position

[John Denker <jsd@MONMOUTH.COM>]

At 09:32 AM 5/10/99 -0700, William Beaty wrote:
> How "wrong" is this statement?  :
>
>   When it's interacting with an atom, it's a particle.  When its flying
>   through space it's a wave.

Welllll.......  That statement has some strengths and some weaknesses:

1) To my way of thinking, there is no such thing as a particle and no such
thing as a wave --- there is only stuff.  The terms "wave" and "particle"
are inherited from 19th-century physics.  They apply, if at all, to certain
asymptotic situations, in which case we can perhaps say "this stuff is
acting like a particle, approximately" or "this stuff is acting like a
wave, approximately".

2) The quoted statement correctly calls attention to the fact that how
stuff behaves depends on what it is interacting with.

However, the situation of interacting with an atom is by no means
sufficient to guarantee that stuff is acting like a particle.

a) It is a fact that we can build voltmeters.  These measure (to a good
approximation) the voltage operator, (a dagger plus a), which is linear in
the field operators.  Stuff interacting with a voltmeter looks relatively
wave-like.

b) It is also a fact that we can build photon-counters.  These measure (to
a good approximation) the number operator, (a dagger a), which is not
linear in the field operators.  Stuff interacting with a counter looks
relatively particle-like.

c) We can build all sorts of other apparatus (quantum-crypto apparatus, EPR
apparatus, ...) where we cannot analyze the situation using 19th-century
concepts.

Cheers --- jsd