From: brahm@cco.caltech.edu (David E. Brahm)
Newsgroups: sci.physics
Subject: Re: NEWS: Galileo Cleared of Heresy Charges by Vatican
Date: 8 Nov 1992 02:57:06 GMT
Organization: California Institute of Technology, Pasadena
NNTP-Posting-Host: punisher.caltech.edu
Summary: More Quantum Cosmology

[I discussed the coordinate singularity at t=0 and Quantum Cosmology...]

crb7q@kelvin.seas.Virginia.EDU (the ever-vigilant Cameron Randale Bass) says:
> Simply saying that it is an inappropriate question of coordinates
> seems to beg the question of 'beingness', since you clearly seem
> to be saying that there is an initiating 'event'.

To understand how quantum cosmology "explains" the universe, you must stop
thinking in terms of sequential causation, i.e. that something (in t<0)
"caused" the Big Bang (t=0), which subsequently evolved into the universe.
There was no t<0.  Nevertheless, we can ask about (relative) quantum
probabilities for universes, and show that ours is "likely."

> Um, like everyone else I enjoy getting 'something' out of 'nothing'.  So
> explain to me how one does this, in even the most general terms...

Feynman's way of understanding why a baseball goes from the pitcher to the
catcher along a parabolic path is that it "actually" goes along all
possible paths, with each path weighted by a phase (called the "action"),
and that every physical measurement actually probes the sum of these
weighted paths (the "Feynman path integral").  For the most part these
phases cancel each other out, except for a small subset of paths where the
phase is stationary, so effectively the path which extremizes the action
dominates the sum.  The classical approximation is to consider only this
path, and the extremization of the action is where classical equations of
motion come from.

For the universe, one can consider a sum of "histories," which are 4D
manifolds with metrics (modulo diffeomorphisms).  Hartle and Hawking
proposed very simple boundary conditions, restrictions on the histories
analogous to restricting the baseball to start at the pitcher and ending at
the catcher: "This class consisted of curved spaces, without singularities,
and which were of finite size, but which did not have boundaries or edges."
This is called the "No Boundary" proposal.

Once again, the sum over all histories is dominated by a small subset, and
the task of quantum cosmology is to show that this dominant path is in fact
the 4D macroscopic universe we call home.

Thus the way we get "something out of nothing" is that we start with
everything possible, all realizable universes, and show that our own
dominates the path integral.

I'll close with a quote from Hawking ("The Origin of the Universe") germane
to our discussion:
     "...So God would not have the freedom to choose the initial
  conditions.  Of course, God would still be free to choose the laws
  that the universe obeyed.  However, this may not be much of a choice.
  There may only be a small number of laws, which are self-consistent,
  and which lead to complicated beings, like ourselves, who can ask the
  question: What is the nature of God.  Even if there is only one,
  unique set of possible laws, it is only a set of equations.  What is
  it that breathes fire into the equations, and makes a universe for
  them to govern?  Is the ultimate unified theory so compelling that it
  brings about its own existence?  Although Science may solve the
  problem of how the universe began, it can not answer the question: why
  does the universe bother to exist?  Maybe only God can answer that."

--
Staccato signals of constant information,  |  David Brahm, physicist
A loose affiliation of millionaires and    |  (brahm@cco.caltech.edu)
  billionaires and Baby ...                |---- Carpe Post Meridiem! --
These are the days of miracle and wonder,  | Disclaimer:  I only speak
And don't cry, Baby, don't cry, don't cry. | for the sensible folks.
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