Re: causing trouble

[Tim Gwinn <***>]

JohnM said:
> Question about Q-measurement (I really know very little about Q-science):
> Isn't it changing the system only because what we call
> 'measuring" is indeed
> the destruction (extraction?) of a substantial component in the
> model-system
> we visualize? In which case the word is used wrongly.

John,

I would say that it is because all "measurement" is, at root, the physical
interaction of (at least) two systems: the system under study and the meter
(i.e., measuring device). In the case of quantum measurements of
non-commuting variables, like position and momentum, we do not (currently)
have a way of physically interacting with the quantum system in a way which
ascertains the value of one of these variables without the physical
interaction causing some change in the other variable that is greater than
some particular degree of precision.

Another way to say it is that a meter works by having a dynamic induced on
it by the object system in the physical interaction of the two systems. In
most macroscopic measurements, the meter is designed to NOT induce a
detectable dynamic on at least some variable(s) of the object system. In the
case of quantum measurements, we cannot design a meter which does not induce
a detectable dynamic on some variable(s) of the object system. To me, it
remains debatable whether this is truly a fundamental physical limitation or
whether future knowledge will allow us to design meters that avoid this
problem.

Regards,
Tim