dfsiemensjr@juno.com wrote:
>
> On Sun, 23 Jul 2000 20:57:08 -0400 George Murphy <gmurphy@raex.com>
> writes:
> > dfsiemensjr@juno.com wrote:
> > .....................
> > > However, I will say dogmatically
> > > that all scientific explanation will be in terms of natural
> > events, just
> > > as the scientific explanation of the Big Bang can only work back
> > as far
> > > as 10^-43 sec after the event. Calling the Big Bang "creation" is
> > outside
> > > of the scientific explanation.
> >
> > To the 2d sentence & 1st 1/2 of the 1st, yes. "Creation" in
> > the strict sense
> > is a theological term, & natural science deals with natural
> > phenomena. But limitation
> > of scientific explanation to t > 10^-43 sec is a statement about the
> > current state of
> > physics & may be made obsolete by an adequate quantum theory of
> > gravity. Of course that
> > will still not mean that physics can answer all our questions - such
> > as why the
> > pattern described by that theory is instantiated.
> > Shalom,
> > George
> >
> I stand corrected for not anticipating the unified field theory. But a
> question remains in my mind. I understood that the Big Bang is a
> singularity and that science cannot explain singularities. Does that mean
> that we cannot get back to t=0? Or have I misunderstood? Or, to take a
> different tack, if the eventual quantum gravity supports the "bubble" or
> "manu universe" approach, would that mean that the Big Bang was not a
> singularity?
Dave -
It occurred to me after posting this that a brief article on this topic might be
of value because a lot of writers on science-theology (e.g., Drees' _Beyond the Big
Bang_ & Worthing's __God, Creation, and Contemporary Physics_) talk about the Planck
time as a limit without giving any explanation of either why or how it is a limit, &
thus contribute to an air of mystery about it. For now a few comments as a downpayment:
1) The Planck time Tp = (hG/2*pi*c^5)^(1/2) ~ 10^-43 sec as a limit can be
derived in several ways by I think the clearest is as follows. Both quantum theory &
general relativity speak about effects on measurements of time intervals, the first
because of the energy-time uncertainty relation & the second because the gravitational
field of a mass will affect the rate of a clock. Each of those effects by itself can be
minimized, the first by allowing an unlimited uncertainty in energy of the system & the
second by eliminating masses which might affect the rate. But both can't be done
together. Energy is mass, and a big energy uncertainty means a big mass uncertainty
which means a big uncertainty in gravitational effects on a clock, while if you minimize
the mass you make the energy uncertainty very small & the time uncertainty big. When
this is worked out quantitatively you find that for times comparable with Tp or less the
uncertainty in a measured time interval would be comparable with the interval itself.
2) This means that our present theories of quantum mechanics and general
relativity can't consistently be applied together to phenomena on time scales of Tp or
smaller, or length scales of Tp/c or smaller. That's true for _any_ phenomena, not just
those of the early universe or the end of gravitational collapse. In most situations
we're working at much larger scales & don't have to worry about that - even though we
know our theories are really incomplete. But in the earliest stages of the universe (if
"early" is even meaningful) there _is_ no larger scale. Physics at the Planck scale is
all there is.
3) What is needed to overcome this is a correct theory of quantum gravity,
which we don't have yet. Such a theory would not necessarily be a "theory of
everything", even though some people hope for quantum gravity to come out of a
successful TOE, such a superstrings. But it's possible that a correct quantization of
gravitation would still leave gravity separate from the other interactions.
4) The fact that we don't have such a correct theory doesn't mean we don't know
anything at all about what such a theory will be. The argument under 1) is an example
of a foray into this area, & some others can be made. (E.g., we can be pretty sure that
gravitation _does_ have to be quantized and that there are gravitons which have spin 2.)
5) The classical space-times usually taken as models of a universe with a big
bang are singular. This means that they are incomplete: It isn't really that there's
an event t = 0 where funny things happen, but that there is no event t = 0: It's as if
a point were ripped out of the manifold. It may be that in a correct quantum theory of
gravity there will be appropriate nonsingular space-times but this is not certain. (A
lot hinges on my weasel word "appropriate.")
Shalom,
George
George L. Murphy
gmurphy@raex.com
http://web.raex.com/~gmurphy/
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