From: "Dan Minette"
> From: Robert Shaw >
> >
> > From: "Dan Minette" <[EMAIL PROTECTED]>
> > >
> > >
> > Wavefunction collapse is an arrtifact of the Copenhagen interpretation.
> > It works when doing calculations but, as Schrodinger's cat shows, it
> > is not really sound.
> >
>
> Well, you can call it what you will. First the wave function is is
> superposition of a number eigenstates, then it exists in an eigenstate or
a
> smaller number of eigenstates.
That's only true if you ignore the quantum behaviour of the measuring
system, and the rest of the universe.
> Schrodinger's cat didn't necessarily show that this wasn't sound. It
> certainly is at odds with common sense metaphysics, but that doesn't make
it
> unsound. Fairly well respected people, like Wigner, took that very
> seriously. I had the chance to briefly talk with Wigner, BTW, about this
> stuff before he died.
It certainly works, for the purposes of calculation, but what counts
as a measurement? It doesn't explain why quantum theory doesn't
appear to apply to the measuring device, but if it did apply the
that would appear inconsistant with wavefunction collapse.
>
> > Decoherence is an approach that suggets the wavefunction doesn't
> > actually collapse, it's just that for classical systems all trace of
> > interfeerence is washed out by interactions with the rest
> > of the universe.
>
> I understand why the off diagnal terms go away, that makes sense. The
> remaining question is why there is only one term that is on the diagonal
> that is selected.
There isn't. If the off diagonal terms are zero then the diagonal terms
don't
interact with each other. This means all the diagonal terms can be present,
rather than the wavefunction collapsing. It just looks like a collapse from
our limited perspective.
> >
> > You are not asking a meaningful question.
>
> I'm a plumber. I ask questions that can be answered experimentally. Let
me
> restate it clearly:
> If we have instruments that can measure the position and momentum of the
> balls to within theoretical limits at time t, and then again at time t+dt,
> what is the maximum value of dt such that we can predict the measured
> position at time t+dt to better than 1 cm? That's a pretty simple
question.
Are you talking about a classical system, or a quantum mechanical one?
With a classical system dt can be arbitarily large.
For a quantum system, you have to do the entire calculation on a QM
basis but, unlike the classical case, you haven't given enough information
to fully specify the initial state.
To use the QM uncertainties to do a classical calculation mixes two
incompatable theories. That might work sometimes, but the results
are not automatically meaningfull.
> >
> > Either way events are theoretically predictable to arbitary accuracy.
>
> No, they are not. Events are things that happen Predictable means you will
> know what will happen. Think about what answer you will get for a totally
> quantum description of the positions of the balls after 1 minute: it
will
> be very close to uniform probability of them being anywhere on the table.
> That tells me that we cannot predict the position of the ball on the
table.
But, in QM, the position of the ball on the table is not a meaningful
question.
You can no more talk about the position of the ball than about the
coordinates
of an electron in an atom.
If you stick to questions that are meaningful in QM, then you can predict
their answers to arbitary accuracy, if you know the initial state to
sufficient
accuracy.
