From: burns@latcs1.lat.oz.au (Jonathan Burns)
Newsgroups: rec.arts.sf.science
Subject: Re: Parallel Universes
Date: 17 Apr 92 05:02:21 GMT
Organization: Comp Sci, La Trobe Uni, Australia
Originator: burns@latcs2.lat.oz.au


In article <1992Apr15.215637.8148@ugle.unit.no> stig@Lise.Unit.NO
(Stig Hemmer) writes:

> This question isn't just directed to Jonathan but to anyone who think
> they know the answer.

Well I'll _try_...


> If I have understood things correctly the MWI people state that
> nothing really significant happens when you do a measurement.

Yes, pretty much. Other interpretations have to include, or get
around including, a completely new mechanism, "The Collapse of
the Wave Function". What we all agree on is that a measurement
gives us information we didn't know before. In one-world 
interpretations, the information has to be _generated_ at
measurement time (collapse theories), or else amplified from
information already there (hidden-variable theories).

That implies something horrid, in certain cases where one quantum
event gives the possibility of two outcomes at a measuring instrument
here, and also the possibility of two outcomes at another m.i.
over there. When the two m.i.'s compare notes later, they find their
new informations correlated: if I measure a value 'north', say, then
your measurement says 'south'; or vice-versa. And this happens _every
time_.

But how can this be? If our theory was that the instruments decided
the measurement values (maybe by collapsing the wave function somehow),
how could they agree to decide opposite values? They were too far
apart for a light-signal to have got from one to the other in time.

That's the EPR paradox in a nutshell. It has been attacked by supposing
that both m.i.'s were recording information already present (local
hidden variable theory); or that potential information was passing
back in time from the instruments to the original quantum event; and
there's supposed to be some other kind of attack that denies "the
objectivity of the universe", whatever that means.

Experimental work so far (the Alain Aspect experiment) says that
quantum mechanics gives the right predictions in a situation where
communication between the m.i.'s would have to be faster than light.

> If this is so, how does they explain the recent experiments where
> measurement one place are affected by whether or not a measurement is
> made elsewhere?

As you say; except that one would prefer to say that the measurements
are correlated. Whether one _affects_ the other is the Big Question.

> (What happens is that a wave-like multi-hump interference pattern
> changes into a particle-like single-hump (non)interference pattern)


Not quite. Just going on the facts, we can't state that there _is_
an interference pattern. There are only three measurements, one early,
two later. The wave pattern is a catalogue of possible outcomes, and
may be only our mathematics. We can verify the catalogue by doing lots
of measurements. What QM predicts is that the catalogue can be sorted
into exclusive sets of outcomes, in various different ways (north-south,
east-west, etc), and gives these outcomes certain complex-number weights.

Now. As Nick Haines says,

> The result of both measurements is determined by the `choice' of
> universe made at an earlier time (ie by which universe was selected
> when the two particles in the EPR experiment were interacting).

In other words, the original event begets two possibilities: 

	here	there

	north	south
and
	south	north

In single-world theories, we are faced with the previous unappealing
choices: a hidden variable which has already selected one of the two,
or else FTL collusion between the instruments.

In the MWI, the wave-function is really a catalogue of universes, of
which two sets became distinguishable at the time of the original
event. In each universe there is a pair of instruments, and it's
only a matter of which instrument records the distinction first. 
There _is_ a hidden variable, and it tells us which flavour of
universe we're in. (Sort of a process id, if you're into Unix :-))

I don't think I want to go into the business of non-commuting variables
(e.g. that the multiverse is divided east-west as well as north-south).
The bottom line is that the MWI makes sense, but doesn't make any
difference to our catalogue of potential outcomes.

But with all respect to Janis, this is not just a topic that physicists
bull on about over coffee. That may be all it's _fit_ for, but they
do actually write papers about it, and the day may come when we either
spot something practical in all this, or change our terms of reference
because of it. After all, we lately had Roger Penrose speculating that
wave-function collapse is real, and we're only getting into the realm
of QM experiments with quantum-scale instruments.

A happy Easter to all.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jonathan Burns        | His wit never failed him, and he laughed
burns@latcs1.lat.oz.au| himself to death over a book of the dying
Computer Science Dept | words of famous men.
La Trobe University   |                  Disch, _Camp Concentration_
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Newsgroups: rec.arts.sf.science