HDTenEightyP/NetNews huggingface dataset:
From: jbaez@riesz.mit.edu (John C. Baez)
Subject: Re: Computability of the universe
Organization: MIT Department of Mathematics, Cambridge, MA
Date: Tue, 15 Sep 92 22:16:29 GMT


Someone writes (I'm sorry not have the attribution, but you folks don't
know what contorted convolutions I have to do these days to post news
from here at UCR where nothing works):

>    The so-called "three body problem", computing the trajectory of
>.three (yes only 3) bodies under mutual gravitational (Newton-style)
>interaction, is not computable.

[in classical mechanics].  

Do you have a reference?  To be frank, I don't believe you; I think
you are mixing up uncomputability with the nonexistence of a
closed form solution.  It's an old result, due to Poincare I believe,
that this problem is not completely integrable.  But that has
practically nothing to do with uncomputability.  

Indeed, in a problem such as this a large amount of work goes into
stating precisely what one means by computability.  The point is that
there are collisions, so one could only hope for a global solution
on an open dense set of initial data at best.  If this is the case
one says one has "asymptotic completeness".  Asymptotic completeness
was an open problem for a long time in the classical n-body problem
with inverse square force law.  I forget if it has been solved.  If it
were, one would need to state precisely the sense in which this open
set was or was not computable, and in what sense the solution was or was
not computable for data in this open set.  

In a sense it doesn't matter since I showed the corresponding *quantum*
problem has computable unitary time evolution.  But it's interesting!
Newsgroups: sci.physics
From: daryl@oracorp.com (Daryl McCullough)
Subject: Re: Hidden variable theories, was: Uncertainty Princi
Organization: ORA Corporation
Date: Fri, 11 Sep 1992 15:18:00 GMT

In article <267@mtnmath.UUCP>, paul@mtnmath.UUCP (Paul Budnik) writes:

> The Shannon definition of information is that it allows one to select
> a limited number of alternatives from a wider range of possibilities.
> Encrypted data is information even though you cannot determine what
> the data is without a decryption key. In a test of Bell's inequality,
> we know from a local observation something about two things that
> happened at a distant site. These two things are the observation made
> at the distant site and the angle of a polarizer at a distant site. We
> do not know either of these, but we know something about the
> combination of the two of them. This is information and it is
> transmitted instantaneously if the instantaneous collapse postulate is
> true.

I don't think that Shannon's theory of information can be used to
support your claim that quantum mechanics involves instantaneous
transmission of information across large distances.

In order for there to be a transmission of information, there must be
a sender and a receiver of the information. Information is transmitted
to the extent that there is a correlation between the choices made by
the sender and the results observed by the receiver. There is no such
correlation in quantum mechanics. No matter what choice is made by the
sender in an EPR-like experiment, the probability distribution on
possible results for the receiver is unchanged.

The case with encryption is a false analogy. When a message is
encrypted, there is still a perfect correlation between messages sent
and messages received, so by Shannon, the information transmission
rate is the same as if the messages were unencrypted (actually
slightly less, since encrypted messages generally have more redundancy
than the unencrypted messages, depending on the encryption algorithm).
Encrypted messages *can* (and are) used to communicate.

Daryl McCullough
ORA Corp.
Ithaca, NY
Newsgroups: sci.physics
From: julian@fraser.sfu.ca (Julian Shillcock)
Subject: HELP WANTED
Summary: Help wanted retrieving codes from Allen & Tildesley book on liquids
Keywords: simulations, liquids, allen & tildesley
Organization: Simon Fraser University, Burnaby, B.C., Canada
Date: Tue, 15 Sep 1992 18:51:47 GMT


HI everyone,

I am using the book by Allen Aand Tildesley "Computer Simulation of Liquids".
In the back they advertise some codes for doing MD simulations that
should be available from : chem.tn.cornell.edu. They say to 
use the username statmech. But when I try this it gives a user-not-known error.
Has anyone tried to download these files? Succeeded?
I would appreciate it if anyone knows whether the username has been changed
or discontinued...?

Many thanks

julian shillcock

(julian@fraser.sfu.ca)
From: mcirvin@husc8.harvard.edu (Mcirvin)
Newsgroups: sci.physics
Subject: Re: Why do protons outnumber antiprotons?
Date: 4 Sep 92 21:41:04 GMT
Article-I.D.: husc8.mcirvin.715642864
Distribution: na

dwr2560@zeus.tamu.edu (RING, DAVID WAYNE) writes:

>I think the real problem with the hole theory is that bosons
>also have antiparticles, and the 'sea' can't fill up because there's no
>exclusion principle.

That's also a problem, if you want to explain all particle non-
conservation in terms of holes.  The early attempts at QED were
actually in-between theories:  they described the only fundamental
boson in the theory, the photon, as the quantum of a quantized
electromagnetic field, but the matter particles were described
using the Dirac sea and particle-hole pairs.  So they didn't
encounter that particular problem, though, of course, they only
dealt with electromagnetism.
-- 
Matt McIrvin, Cambridge, Massachusetts, USA 
Newsgroups: sci.physics
From: jbaez@riesz.mit.edu (John C. Baez)
Subject: Re: Question of Theory of Everything (or Grand Unified theory)
Organization: MIT Department of Mathematics, Cambridge, MA
Date: Mon, 7 Sep 92 04:04:45 GMT

Don't ever believe anyone who claims that there WILL BE a shocking
scientific advance 10-20 years from now.  Scientific breakthroughs are
by definition unexpected until they happen.  The fans of superstrings
were very optimistic about 5 years ago and some of the more bold, or
if I may say so, arrogant, claimed that shortly we would have a fully
functioning "Theory of Everything".  The relative lack of hubbub about
string theory in the popular press lately is an indication of what has
happened in the meantime - lots of good mathematics but sort of a
quagmire when it comes to working out the physics.  In particular,
people had been excited for a while when it looked like there was an
almost unique sensible string theory, but now this no longer seems
true.  

I have my own prejudices about what are the best directions to look
for a theory of everything, and if you had been reading sci.physics
for the last year you would have seen me go on and on about them. 
Briefly, I favor the Ashtekar/Rovelli/Smolin loop variables approach
to quantum gravity, and its extensions to treat other forces.  Check
out the "Science and the Citizen" column of the latest Scientific
American (September 1992) for a nice introduction.  I am spending all
my time working on this stuff myself, however, so I am hardly an
unbiased party!  On good days I think, "Yeah, maybe in 10 or 20 years
we'll work out a theory of all the forces!"  But that's just my
natural optimism and good spirits.  We may never have a theory of
everything - indeed even if we get one we will never know for sure
that it is true - but all we can say for now is that physics is full, very
full, of mysterious puzzles!




Newsgroups: sci.physics
From: firth@sei.cmu.edu (Robert Firth)
Subject: Re: Computability of the universe
Organization: Software Engineering Institute
Date: Thu, 10 Sep 1992 18:36:03 GMT

In article <rwallace.716144999@unix1.tcd.ie> rwallace@unix1.tcd.ie (russell wallace) writes:

>Not so. A Turing machine can perform any feasible computation (the
>Church-Turing thesis).

Yes, I know the thesis.  The argument is circular, because it defines
"feasible" to mean "capable of being computed by a <machine equivalent
to> a Turing machine".  Unfortunately, a true random number is not
one of the things so computable, though it can of course be computed
using one of several flavours of the universal quantum computer.

The analogy with Greek geometry is exact.  They defined "constuctible"
to mean "constructible with only straight edge and compasses", whence
it follows trivially that straight edge and compasses can construct
all "constructible" figures.  Unfortunately, the trisector of an angle
is not one of those "constructible" figures, though it can of course
be constructed using the method of Archimedes.

The argument that a Turing machine can produce an arbitrarily close
approximation to a true random sequence is just as bogus as the
argument that successive bisections can produce an arbitrarily close
approximation to a trisection.  Yes, they can, but the issue is the
formal solvability of the problem, isn't it?
Newsgroups: sci.physics,sci.math
From: torkel@sics.se (Torkel Franzen)
Subject: Re: Report on Philosophies of Physicists
Organization: Swedish Institute of Computer Science, Kista
	<1992Sep13.050206.18067@CSD-NewsHost.Stanford.EDU>
	<TORKEL.92Sep13095337@bast.sics.se>
	<1992Sep13.174721.23818@CSD-NewsHost.Stanford.EDU>
Date: Sun, 13 Sep 1992 19:29:10 GMT

In article <1992Sep13.174721.23818@CSD-NewsHost.Stanford.EDU> pratt@Sunburn.
Stanford.EDU (Vaughan R. Pratt) writes:

  >The meaning of ZFC+# as a foundation for mathematics is no
  >different from say ZFC+V=L (all sets constructible), or any other
  >addition to ZFC.  It just means the same mathematics you've always
  >done, plus anything new you can get from this additional postulate.

  What this seems to say is that a "foundation for mathematics" is any
formal theory extending ZFC, possibly subject to some adequacy
conditions which are unclear from your formulation. Why do you call
this a "foundation for mathematics"? It seems to pertain, rather, to a highly
formal kind of mathematics, of limited interest.

   >You might object that # has no mathematical content.

  Why should I do that? # is an ordinary (false) arithmetical statement.

   >It is probably wishful thinking to hope that one of not-# or # might
   >turn out to imply one of CH or not-CH (continuum hypothesis).

  I believe it is well known that no arithmetical statement
undecidable in ZFC can imply (in ZFC) either CH or its negation.
From: matmcinn@nuscc.nus.sg (Mcinnes B T (Dr))
Newsgroups: sci.physics
Subject: Re: Question of Theory of Everything (or Grand Unified theory)
Date: 8 Sep 92 02:30:27 GMT
Organization: National University of Singapore

Please excuse me if you have already debated the hell out of this
one......but I would like to make the following observation. It strikes
me that [eg] superstring theory is not easy to understand. Suppose that
in the 80's it had turned out that string theory is right. Then we would
have to conclude that the universe is incomprehensible to all of
humanity except for a tiny coterie of people. Now: is it not truly
astounding that the complexity of the universe is so precisely
calibrated to the intelligence of homo sapiens? So that it is just
barely within our comprehension, to the extent that a few hundred of us
can understand it? Aren't we lucky? Would one not expect a priori that
the universe should be either easily comprehensible to all, or vastly
beyond the intellect of even a Fields medallist?  :)
Newsgroups: sci.physics
From: pratt@Sunburn.Stanford.EDU (Vaughan R. Pratt)
Subject: Re: Color of electric charges (silly question)
Organization: Computer Science Department,  Stanford University.
Distribution: na
Date: Sun, 13 Sep 1992 22:23:30 GMT

In article <1992Sep13.210328.24283@galois.mit.edu> jbaez@riesz.mit.edu (John C. Baez) writes:
>When doing mathematics I am constantly visualizing things,

Me too!  It makes algebra *so* much easier.  And teaching is a lot more
fun when you can draw pictures of things instead of writing out strings
of symbols.

The trouble is that a lot of people (maybe more so in computer
science?) are symbol oriented.  They get very disoriented by your
pictures and accuse you of being too intuitive.  Then you have to
translate for them, which is not only a pain but meanwhile you aren't
talking to the visual people, so you end up explaining everything
twice, or ignoring the symbol people if there isn't time.  But the
worst is program committees, which don't give you enough room to
explain everything twice.

>but never, it seems, in color!

Me too!  Even when I'm thinking about red-and-black trees the nodes are
white and black.  (Come to think of it, maybe that's because they're
colored that way in Cormen, Leiserson and Rivest.)

So what's the cause?  Do we think monochromatically because the great
majority of textbooks are monochrome, or are textbooks monochrome
because their authors have colorless ideas (when not sleeping
furiously)?

When I run out of bandwidth on the blackboard I revert to colored
chalk.  Axiom: there's never enough colored chalk, you have to bring
your own.

-- 
======================================================| God found the positive
Vaughan Pratt   pratt@cs.Stanford.EDU   415-494-2545  | integers, zero was
======================================================| there when He arrived.
Newsgroups: sci.physics
From: jbaez@riesz.mit.edu (John C. Baez)
Subject: Re: Hidden variable theories, was: Uncertainty Princi
Organization: MIT Department of Mathematics, Cambridge, MA
Date: Fri, 11 Sep 92 19:27:13 GMT

I hate to be such a wet blanket, but it might improve the clarity of
the discussion if some of the folks who keep bandying about the word
"local" would take the time to define what they mean.  I can think of
3 distinct definitions off hand.

1.  A linear operator A is local if Apsi in an open set only depends
on psi in some open set.  Here psi is a function on some topological
space, e.g., a wavefunction on R^3.  In this sense the Hamiltonian for
Schroedinger's equation is local, since it's a Laplacian plus
potential.  One could be moved by this to say that Schroedinger's
equation is local.

2.  A wave equation on spacetime is causal if initial data on some 
open set U of a spacelike surface determine the solution at points x such 
that every timelike geodesic hits U.  Such points are said to be in
the "domain of influence" or "causal shadow" of U.  Schroedinger's
equation is not causal but the Klein-Gordon and Dirac equations are. 
Some of you seem to be using the word "local" for this, but "causal"
is traditional.   It is an axiom in most formulations of quantum field
theory that it be causal.

3.  A physical theory is local if any pure state of the system is
determined by its restriction to any family of open sets U_i which
cover space.  Roughly put, you can figure out everything about what's
going on by doing experiments in local regions and piecing together
the information without worrying about correlations.  In this sense
classical field theories are local but quantum field theories are not.

I know that 9/10's of the fun about arguing is not knowing what the
other guy means, so feel free to ignore this attempt at making things
precise.
From: jbaez@riesz.mit.edu (John C. Baez)
Newsgroups: sci.physics
Subject: Re: Did electric/magnetic symmetry "break"?
Date: 5 Sep 92 17:08:53 GMT
Organization: MIT Department of Mathematics, Cambridge, MA

In article <1992Sep5.073528.16705@asl.dl.nec.com> terry@aslws01.asl.dl.nec.com (Terry Bollinger) writes:
>Premises:

> o  If you ignore interactions with electrically chaged matter, electric
>    and magnetic fields behave very similarly.

Quite true.  Mathematicians call this symmetry duality.  The vacuum
Maxwell's equations are dF = 0, d*F = 0, which are obviously symmetric
under the map F -> *F, since **F = -F.  This corresponds to E -> B, 
B -> -E.  

> o  However, they are highly assymmetrical in their relationship towards
>    matter.

Quite true.  The non-vacuum equations become dF = 0, d*F = J.  That's
because there are electric charges and currents but no magnetic.

>     E.g., the electric field has associated with it two particles
>    (among others) called the electron and positron, with opposite charges
>    of (say) 1 unit each.  The equivalent particles for the magnetic field,
>    if they exist at all, would have to have charges with magnitudes about
>    63 times as great.

Huh?  63??  I never heard about that - where'd you get that number from?
What units are using to compare electric and magnetic charges?  Frankly,
I'm very dubious.
Newsgroups: sci.physics
From: marty@amaterasu.physics.uiuc.edu (Marty Gelfand)
Subject: Re: Where do physics graduates look for work?
Organization: Dept. of Physics, Univ. of Illinois at Urbana-Champaign
Date: Fri, 11 Sep 1992 01:38:23 GMT

In article <mxr.716170528@neptune> mxr@neptune.dsc.com (Max "Jellied Griddle Suicide" Rible) writes:
>abbasi@star.enet.dec.com  writes:
>>Ok, you worked hard to get your physics degree, now what do you do?
>>where do you look for work? when I mean work, I mean physics/science
>>related, not flipping burgers.
>
>>are there actually physics/science work out there?
>
>Every person I know personally who has a PhD in physics (any branch)
>is either working in academia or the computer industry.  [...]

And don't forget Wall Street.  Sometimes they can use people who know
some math (..."rocket scientists" they call them).  (Plus you can
calculate how long it takes to hit the ground after jumping from the
18th floor on Black Monday :_) 

One of my PhD classmates (in physics) is currently on the Street,
and friends from college who went straight into finance have hired
PhD mathematicians.  So if New York City isn't your idea of hell on
earth, think about it.

Marty Gelfand
Newsgroups: sci.physics


>"rotate the Heaven
>of fixed stars, and see what happens to the water in the bucket".
>
>What many people now believe is that, could we indeed do the experiment,
>the water would still climb the walls of the bucket, and that this would
>be caused by the rotation about it of the stars.  And if there were no
>stars, there would be no effect on the bucket no matter what.

I think this is wrong. Say there was only one star, which was just a
flashlight (not much mass compared to the Earth). Now if the rest of
the empty universe, containing this flashlight, spun around the Earth,
why would you expect to see the water move in the bucket? This flashlight
has very little mass and is far from the Earth; it's really just a marker.

In fact if you spun it very fast, it's weak gravitational field wouldn't
move the water at all, as it's forces would average out due to the time
constant associated with the Earth's gravitational field, the water's mass,
and the water's viscosity. But if you spun the Earth very fast, this
would not be the case and the water would eventually spill out of the
bucket.

There is a very weak coupling of a spinning mass to induce spinning on
another mass. But the spinning of the flashlight and universe would
not produce the same effect (imperceptible really) as the Earth
spinning at the same rate.

--------------------------------------------------------------------------
Ric Peregrino     c/o Hewlett Packard Co.        I represent only myself
ric@spd.hp.com    1501 Page Mill Rd. Bldg. 5M    I may be wrong, maybe not
415-857-7526      Palo Alto, CA 94304            Do DC photons exist?
--------------------------------------------------------------------------
Newsgroups: sci.physics
From: cannon@scs.unt.edu (Dustin Cannon (Lab 3460))
Subject: Grad School
Organization: School of Community Service, UNT
Date: Sat, 12 Sep 1992 20:18:35 GMT


	I was wondering if anyone could suggest a good grad school for me.  
My emphasis is theoretical physics, I would like a school strong on 
that.  I'm a senior at the University of North Texas, with a physics major 
and math minor.  I have a 4.0 in my major and a 3.7 cumulative.  I have not 
taken my GRE yet, so I can't give you those scores.
	I would prefer a place where I'm not simply given a project by a 
professor and he says "See ya finals week."
	As far as money goes, I'm going to need TAships, TFships, grants, or 
scholarships of some sort.
	Well, those are the facts.  Any suggestions? Thanks in advance for 
any advice.
_________________________________________________________________
Dustin W. Cannon      "Just because some of us can read and write
University of          and do a little math, that doesn't mean we
North Texas            deserve to conquer the universe."
                                                -Kurt Vonnegut
Newsgroups: sci.physics,sci.math,sci.logic
From: jbaez@riesz.mit.edu (John C. Baez)
Newsgroups: sci.physics
Subject: Re: It's Down to the Last Blank Check
Date: 10 Sep 92 17:57:05 GMT
Distribution: na
Organization: MIT Department of Mathematics, Cambridge, MA

In article <1992Sep10.012507.23803@nuscc.nus.sg> matmcinn@nuscc.nus.sg (Mcinnes B T (Dr)) writes:
>I think GE Brown must be chairman for the House Committee for the
>propagation of the New Age.  It's terrifying that an idiot of this order
>can occupy a position of such importance. The worst thing is that he
>jumbles up matters of genuine scientific interest [eg how to achieve
>zero population growth] with stupid nonsense like how to persuade people
>not to regard material possessions as a measure of wealth.

The simplest way to do the latter might be to deprive them of all
material posssessions, which seems to be the current strategy.  :-)
Newsgroups: sci.physics
From: johan@blade.stack.urc.tue.nl (Johan Wevers)
Newsgroups: sci.physics
Subject: Re: Hidden variable theories, was: Uncertainty Princi
Date: 9 Sep 92 12:52:41 GMT

campo@sunthpi3.difi.unipi.it writes:

>Again, instantaneous transmission of information happens in an
>underlying hidden variables theory. In the quantum theory, since it
>cannot be used to send a signal, this transmission of information is
>absent by definition. Also only the hidden variables theory has to
>give up causality; e.g. quantum electrodynamics strictly respects
>causality.

I thought that QED was also a theory which gave it's answers as
probabilities. Furthermore, I used to see QED as an underlying
theory for QM which does not contradict with QM on matters as
uncertancy.

It also gives a model for the double slit experiment: an isolated
electron does not exist. Because of higher order Feyman diagrams,
`an electron' is a whole bunch of electrons+photons+other particles.
Because electrons are indistingluisable, we cant point at `the original
electron', but each of the virtual electrions has a contribution to the
transport from `the electron' through a slit. So each electron has wave
behaviour, also if they pass the slits one by one. The position
and momentum for each virtual foton/electron/other particle exists
objective, but because we have to integrate over a lot of probabilities
we get uncertancy.

Does anyone here know wether this view can be right?

******************************************************************** 
* J.C.A. Wevers                *         u\  _\       /e+        2 * 
* johan@blade.stack.urc.tue.nl *     1 |   \ u \  X  /         |   * 
******************************** P = - |    >   >---<     + ...|   * 
*     The only nature of       *     N |   /   /     \         |   * 
*     reality is physics       *         u/   /d     u\            *
********************************************************************
Newsgroups: sci.physics
From: pratt@Sunburn.Stanford.EDU (Vaughan R. Pratt)
Subject: Re: Report on Philosophies of Physicists
Organization: Computer Science Department,  Stanford University.
Date: Tue, 15 Sep 1992 01:37:01 GMT

In article <1992Sep14.191207.21572@guinness.idbsu.edu> holmes@opal.idbsu.edu (Randall Holmes) writes:
>>This was proved by showing that it
>>was equivalent to (a slight strengthening of) not-# for PA
>
>This is flat wrong.  # is false in the standard model of arithmetic
>(in ZFC, if you like); the Paris-Harrington Theorem is true there.
>Thus the PH theorem cannot be a strengthening of #.

I didn't say it was.  It strengthens not-#.

-- 
======================================================| God found the positive
Vaughan Pratt   pratt@cs.Stanford.EDU   415-494-2545  | integers, zero was
======================================================| there when He arrived.
Newsgroups: sci.physics