From: cal@otter.hpl.hp.com (Colin Low)
Newsgroups: alt.magick
Subject: Re: Magical Power(book)
Date: 13 Aug 92 12:48:18 GMT
Organization: Hewlett-Packard Laboratories, Bristol, UK.

Some questions open up a can of worms; sometimes the discussion
winds down into a nit-picking and opinionated stalemate, and sometimes
the worms just keep on coming - it's worms all the way, to paraphrase
a famous comment about Indian cosmology. The discussion about `magic'
and `doing stuff' has opened a large can of worms, and I thought life
might be simpler if I talked about causality, because I thought I understood
what the word meant, but Le Grand was right to point out that a wiseass
Greek had already muddied up the water, and promptly steered us towards
the causal equivalent of solipsism.
	Somebody mentioned Bell's Theorem. You can't trust philosphers, but
perhaps physicists have some clues about causality. Perhaps we can find
some arena of discourse where the question gets clearer instead of muddier.
I started thinking about this, and I thought all of you who thought
simple questions ought to have simple answers might be amused by the result;
what follows has no explicit connection with magic, but anyone with skill
at curing headaches might find their abilities in demand.

There is an advertisment on British television at the moment which shows
the interior of a pub filmed in reverse - the beer leaps out of the glass
into the pump, the dart leaps out of the board into the player's hand, the
publican hands money to the customers. We recognise instantly that the
`arrow of time' is going the wrong way, and it is part of our normal
experience that there is an arrow of time, and it does run one way only.
This is formalised as the second law of thermodynamics - in a closed system,
entropy will always increase. Entropy is something we understand intuitively -
a broken glass has more entropy than a whole glass, liquid in a bottle
has less entropy than liquid spilt on the floor, a dog in a park has
more entropy than a dog in the dog-house. We can measure entropy. Given
two events in a closed, macroscopic system we observe an increase in
entropy and tag one event as `before' and one as `after'. Once we can talk
about `before' and `after' we can talk about `cause' and `effect'. Given
a set of events, we can make lots of little flags labelled 'cause' and
'effect' and go around labelling the events. Event A is the cause of
event B; event B is the effect of event A. This is something we are
very familiar with in our lives, this notion of causal relationship, and
we maintain a large list of these relationships because it is useful
to know about these things - I press the switch and the light goes on, I
press the throttle and the car goes faster, I drop the box and my foot hurts.
	At a microscopic level none of this seems to work. When you are
looking at electrons, or electromagnetic radiation, the idea that you
can tell the arrow of time by running the film backwards doesn't work. Run
the film backwords and you see one of the things you could see when you
run the film forwards. The technical name for this is `time symmetry'; the
laws of physics are time symmetric. Because you can't tell which way
the film is running, you can't tag events with `before' and `after'. It doesn't
seem to make much sense to talk about causality.
	As an example, one of the solutions of the time symmetric Maxwell's
equations (which cover a *lot* of physics) is a spherically expanding
shell of light which expands out to infinity - you switch on the torch,
and the light comes out. This is called the `retarded' solution. There is
also a completely symmetric 'advanced' solution where a wavefront converges
from the future onto the filament of your bulb and makes it glow white
hot. Why do we see the retarded solution and not the advanced solution?
Feynman looked at this problem and came up with something fascinating called
"The absorber theory of radiation". The equations of physics, such as Maxwell's
equations, or Schrodinger's equation, are time symmetric differential equations
with multiple solutions; this is normal for differential equations, and
the correct solution is normally selected by selecting a set of `boundary
conditions'. The boundary condition on a violin is your finger; your
finger selects a particular solution for the vibration of the string. Feynman
argued that there must be a boundary condition which selects between
advanced and retarded solutions, and that is why we only see retarded
solutions. The boundary condition for electromagnetic waves is the
universe - the entire universe acts as a boundary condition, and if you
try to model the propagation of advanced and retarded waves in an
isotropic, four dimensional, spherically expanding universe, the solutions
do indeed come out differently.
	The point I am trying to make is that far from physics throwing
light on causality, we find some really hard problems. At a microscopic
level, with one very esoteric exception, we find complete time symmetry
(or more technically, PCT symmetry). This isn't "new physics", something
really weird and wonderful. We are going back at least one hundred years
or more. It is hard to talk about causality because all you have to do
is run the film backwards and all the little flags have to be swapped over.
There is a macroscopic "arrow of time", and a great deal has been written
on trying to tie together macroscopic irreversability and microscopic
time symmetry, but as far as I know this is still one of those open-ended
areas where physics looks more than a little untidy.

Colin