Re: Measurement in QM FAQ question
From: Edward Green (spamspamspam3_at_netzero.com)
Date: 07/20/04
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Date: 20 Jul 2004 14:22:09 -0700
spambin@tiscali.co.uk (DDEckerslyke) wrote in message news:<b6a4c181.0407200141.5e2bd4aa@posting.google.com>...
> OK i've read the FAQ here
> http://www.mtnmath.com/faq/meas-qm.html
> wherein lies the link to Schrodinger's Cat for a 6th grader
> http://www.mtnmath.com/cat.html
> from which this is taken
>
> "To understand Schrödinger's cat one must understand what a strange
> theory quantum mechanics is. In all other scientific theories we have
> models of how we think things work. For example we know that distance
> traveled equals speed times velocity. If you travel for two hours at
> 50 miles an hour you will go one hundred miles. We can measure the
> time with a clock and the distance with the odometer on your car.
> Quantum mechanics is not like that. What we measure in experiments is
> not described by quantum mechanics. Instead quantum mechanics gives
> the probability that we will make a given measurement.
>
> "Probabilities occur all the time in science because we almost never
> know everything we need to make a completely accurate prediction. For
> example if you really want to make a trip of a hundred miles you can
> not know ahead of time exactly how long it will take. You might run
> into a traffic jam. You can only give an estimated time. In quantum
> mechanics probabilities are different. They are not considered to
> result from our limited understanding of the universe but to be
> fundamental. Of course Einstein thought this was mistaken but most
> physicists do not agree with him."
>
> Continuiung the analogy of the car journey: Observing whether you have
> arrived at your destination does not determine your arrival it just
> confirms it, or otherwise, in the mind of the observer. Similarly with
> Schrodinger's Cat. It's either alive or dead but the observer doesn't
> know till they look. Or am I missing the point? Did Scrodinger use the
> example of the cat to highlight the apparent absurdities of the
> participant observer?
>
> ISTM making the observation does not determine/decide the state of the
> system it just means the observer now knows what the state of the
> system is. Now AIUI this is exactly the cherished notion of an
> objective universe 'out there' that QM undermines but I'm not clear
> how exactly it is undermined. Help?
In order to give empirical meaning to "is there an objective universe
out there", we would have to adopt a model which gives this meaning:
this is the standard way in physics. At this level physics tends off
into philosophy, but I am of the minority opinion that there is no
necessary logical gap -- whatever the question, the investigatory
mechanism remains the same.
I'm not about to try to formulate such a model here, but I will
suggest that purely classical measurment can share the property of
"things not existing until we observe them", whereas the quantum
ansatz allows preparation of systems with definite properties, which
have effectively been "measured" with arbitrarily high precision and
left in the measured state. So the alleged dichotomy between
classical and quantum measurement is over-rated: it depends what we
are measuring.
It is because we have no model at the level where we denigrate the
investigation as "metaphysics" that we can't even seem to formulate
the possibilities clearly -- definitely a pre-paradigm effect.
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