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From: tjr@ihnet.UUCP (Tom Roberts)
Newsgroups: net.physics
Subject: Re: two questions for physicists
Message-ID: <180@ihnet.UUCP>
Date: Tue, 27-Nov-84 10:58:00 EST
Article-I.D.: ihnet.180
Posted: Tue Nov 27 10:58:00 1984
Date-Received: Wed, 28-Nov-84 03:18:44 EST
References: <6766@watdaisy.UUCP>
Organization: AT&T Bell Labs, Naperville, IL
Lines: 62

[Two Answers]

	(1) - Over the past little while there's been a lot
	of discussion on the net of "semi-cosmological" issues, my
	question is extremely simple. How can we talk about events
	which occurred when the universe was 10**-43 seconds old? Can
	such a concept as TIME exist when all the things we use to
	measure it are coalesced into one huge ball of superhot
	matter? For example, the best timepieces we have right now
	are atomic clocks, can we speak about a "time" before even
	atoms existed?

Time (and space) coordinate-systems are items of COGNITION, not
reality. They are used to THINK ABOUT and to DESCRIBE the real world,
but are not part of it (the things they describe, temporal and spatial
extent, are attributes of things in the real world).
Thus, a physicist does not require a timepiece in order to talk (or think)
about time. [There can be deep philosophical objections to this loose use
of language - I am avoiding mere technical jargon.] The subtleties involved
in discussing the first several seconds after the Big Bang are very
complicated, but do not invalidate the use of time in the discussion
(but you have to be VERY careful in specifying the coordinate-system).


	(2) - What justification is there for the
	omnipresent assumption that all sub-nuclear particles are
	the *same*. Are they indistinguishable because they are so
	small relative to any measuring device that any deviance
	cannot be detected? Or is it just an assumption about the
	sub-nuclear world that we've made and never thought about?

	watdaisy!gjerawlins

I know of no claims that ALL sub-nuclear particles are the same. Let us
rather consider whether all (sub-nuclear) particles of the same type
are the same. That is: "are all X-particles identical", where X belongs to
the set of known (or possibly unknown?) sub-nuclear particles {pi+, pi0, pi-,
neutron, proton, mu+, neutrino(mu), up-quark(?), .........}. [The experimental
differences between these types of particles can be dramatic, even though they
are so very much smaller than the measuring devices.]

At one level, their identicalness is what permits us to define a
particle-type (all pi+ mesons act similarly, in similar situations, thus
we call all particles that act that way pi+ mesons). This is no different
from why we call cats "cats". [All possible situations must be considered,
in principle.]

At a deeper level, the theories that seem to describe the properties and
actions of these particles are symmetric under the interchange of any
two identical particles (that's what makes them identical). While there
is no guarantee that these theories are "correct", they do have strong
experimental support, and are theoretically elegant. [Yes, there is really
no known all-encompassing theory of elementary particles, but the partial 
theories we do have exhibit this behavior, and there is reason to 
believe that this property will carry over into the "ultimate" theory 
(assuming there is one).] If there are no observable phenomena dependent
upon the interchange of two particles, they are identical (because that's
what the word means).


	Tom Roberts
	ihnp4!ihnet!tjr