From: Rob Harris Cen-IT (Rob.Harris@bournemouth.gov.uk)
Date: Fri Jul 16 1999 - 09:01:03 MDT
> QM says if I measure the position x, there is a range of
> values that it can take, and there is no way for me to predict the
> outcome of the experiment beforehand.
>
OK - after all, that's why we're taking the measurement.....
> Let's say I measure the
> position and come up with a value of 2. *Now*, if I measure the
> position again right away, QM says that I am guaranteed to get the
> same value, 2.
>
Uhh.....in the same time position ? Surely if you take another reading at a
later time - no matter how small the timescale, we can expect a slight
change in the reading due to system progression...?
Alternatively - are you talking REALLY small time scales? I mean, viewing
the progression of time as a quantized sequential transition from one time
"position" to another, rather than an analogue, constant progression? If so,
and you are saying that the second reading is taken within the same time
chunk, then I follow you.
> Thus, for example,
> to measure the position of an electron, you have to bounce a
> photon off of it.
>
With our current technology.
> But these are just
> manifestations of the underlying theory, which says,
> unequivocally, that you cannot simultaneously know both
> observables.
>
What is the basis for this premise?
> The theoretical limit of this uncertainty is
> defined by Plank's constant, hbar.
>
What do you (everyone) think about this? It seems to me that the
obvious way to avoid the uncertainty principle is to use non-intrusive
methods of measurement. For instance, determining the composition of stars
by detecting photons from it. You can get all the photons you want without
actually affecting the star's composition - you could have a legion of
photon-munchers eating the photons as soon as they leave the star's surface,
and you wouldn't affect the star's composition in any way.
Also, I don't buy the double measurement commutability thing. Surely
the critical factor involved in commutability in this case is whether the
first measurement affects the system. Well, whether you're measuring
position with a photon (which will alter the system by 1 photon of whatever
energy and velocity), THEN another position measurement, or position THEN
momentum, the disruption has occurred at the point of the original
measurement, making the nature of the intended secondary measurement
irrelevant - The system was already disrupted by the first measurement.
I could be off track here due to my lack of understanding of wave
functions, and their properties and implications. What do you all think?
Rob.
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