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- Path: sparky!uunet!elroy.jpl.nasa.gov!nntp-server.caltech.edu!brahm
- From: brahm@cco.caltech.edu (David E. Brahm)
- Newsgroups: sci.physics
- Subject: Re: NEWS: Galileo Cleared of Heresy Charges by Vatican
- Date: 9 Nov 1992 05:55:43 GMT
- Organization: California Institute of Technology, Pasadena
- Lines: 65
- Message-ID: <1dkugvINNgpn@gap.caltech.edu>
- References: <1992Nov7.163126.20553@murdoch.acc.Virginia.EDU> <1dhvm2INN6e5@gap.caltech.edu> <1992Nov8.044014.26633@murdoch.acc.Virginia.EDU>
- NNTP-Posting-Host: punisher.caltech.edu
- Summary: Yet more on quantum cosmology
-
- [My description of quantum cosmology as a Feynman sum over histories...]
- > There was no t<0. Nevertheless, we can ask about (relative) quantum
- > probabilities for universes, and show that ours is "likely."
-
- Proving that even aero engineers waste their weekends on UseNet,
- crb7q@kelvin.seas.Virginia.EDU (Cameron Randale Bass) wrote,
- > OK. Tell me how to think of it.
-
- Well, Dale, that's what my last post was supposed to do. Of all the
- possible 4D manifolds (satisfying some constraints), ours is the one that
- dominates the path integral.
-
- > I am fine with there being no t<0, but in what manner was
- > the initiating 'event' set up?
-
- 'Set up' sounds to me like you're still asking about 'beforehand'. I am
- probably guilty of using misleading terminology when I first spoke of our
- universe as an "event" in a "spacetime-less background"; I should have said
- a "history" in a "Feynman sum", but at the time I hadn't defined those
- terms. By "event" I did not mean the Big Bang event, i.e. (t=x=0).
-
- > Talk of relative quantum probabilities of universes is all fine and
- > good, but it seems to be giving me no information.
-
- Hawking claims that the 'no boundary' proposal makes some predictions, e.g.
- that there are no true singularities; and that density fluctuations all
- started in the ground state and then were inflated, leading to a Zel'dovich
- spectrum which is consistent with COBE results.
-
- > ...'Without singularities' seems to be an odd a priori restriction
- > coming from Hawking. It also seems to be odd in that you mention
- > the 'coordinate singularity at t=0' earlier.
-
- At the moment I think this is an ad-hoc assumption. Without it I don't
- think anyone knows how to make any predictions. Please note that the
- coordinate singularity at t=0 is not a (physical) singularity, though.
-
- > See the trouble one gets into? There will always be an assumption
- > in such any theory that cannot be explained, and must be defined.
-
- A sweeping statement I would like to see proven. The fact that quantum
- cosmology is not (yet) a complete and convincing theory does not show that
- such a theory is impossible; at least some people have the guts to try.
-
- matt@physics.berkeley.edu (Matt Austern) complained,
- > It doesn't seem at all clear to me that there is any sensible measure for
- > an integral over the space of all possible four-dimensional manifolds.
-
- True, and I believe the state of the art at the moment is to consider a
- very tiny subset of histories (i.e. a few free parameters) which is easier
- to work with.
-
- So, yeah, the field is still in its infancy, and may all turn out to be
- garbage; that's how science goes. I find it interesting mostly in that it
- attempts to bring science to bear on those "deep" questions of existence
- once thought to be purely the realm of religion. Whether it will do so
- successfully remains to be seen. And now you know everything I know about
- quantum cosmology, and more.
-
- --
- 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.
-