🚀 go-pugleaf

[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.