From: John.C..Baez@mailbox.bison.mb.ca (John C. Baez)
Newsgroups: sci.physics
Subject: Re: Defining local, causal etc., was: Hidden variable theories
Date: 25 Sep 92 07:50:15 GMT

Organization: MIT Department of Mathematics, Cambridge, MA

In article <5755@osc.COM> Joe Keane <jgk@osc.com> writes:
>In article <1992Sep20.070040.26815@galois.mit.edu> jbaez@riesz.mit.edu (John
>C. Baez) writes:
>>I am simply saying that if you put an electron into a spin-up state along the
>>z axis and then measure its spin along the x axis, you will get a measurement
>>that's either spin up or spin down.  The *model* only tells you that these
>>two possible outcomes are equally likely.  *Reality* - that is, actually
>>doing the experiment - will give one or the other answer.
>
>I don't agree with this last sentence.  It's just collapsist propaganda.  If
>you want to defend this statement, please describe an experiment which would
>show that you are correct.

Do the experiment I described.  You will either get 

1) spin up

or 

3) spin down

The theory does not predict which answer you will get.  It only
predicts that if you repeat the experiment lots of times, the
frequency of each outcome should approach 50%.

That is all I am trying to say.

>
>>All I am saying, in a convoluted way perhaps, is that QM is an essentially
>>probabilistic theory: that knowing what pure state a system is in does not
>>enable us to predict with certainty the outcomes of all experiments.
>
>It's well known that you can't predict the future with certainty.  A hundred
>years ago people believed this was a practical limitation, and that you could,
>in principle, simulate the universe determinstically.  Now we don't think this
>is true.

That is my point.  I am afraid we agree.

>
>>What I'm trying to note here is that this means that we cannot say "forget
>>reality, I've got my isomorphic model programmed into the computer here and I
>>can simulate reality with the model and see what happens."
>
>If the model is correct, there is no difference.  I could run a thousand tests
>at CERN and fake a thousand on my computer and no one would be able to tell
>which is which. 

True.  My point was simply that you would get different answers!  E.g.,
the first "real" decay might shoot out some pions, while the first
simulated decay might shoot out a kaon.  This is obvious and not very
bad, so please don't think I'm claiming this is some wonderful discovery
on my part.

>Me too.  John is normally pretty sensible, but he seems to be saying a lot of
>collapsist stuff, but then he says he doesn't like collapse.

I hope you take my normally good track record into account and try to 
find some way whereby what I am saying is actually consistent.

Dale (who you quoted) eventually understood me, so it can be done!  :-)

>So reality is probabilistic and the model is probabilistic.  They seem pretty
>similar in that respect.  The only thing we need to check is that they give
>the *same* probabilities for various situations.

Reality gives a definite outcome for any given single experiment.  This
cannot be predicted by the model, which only computes a probability.
This is the difference between reality and the model.
I am repeating myself over and over again....

>See, now you're sounding sensible again.  So where did you get all the stuff
>about ``what WILL happen''?

Do the experiment I describe at the beginning of this post, once.
Do it!  Something will happen!

 * Origin: The Electronic Mailbox (1:348/7.0)