Re: [asa] (planck limits) Thermodynamics & Eternal Universe - A Question

From: <>
Date: Thu Oct 02 2008 - 17:48:39 EDT

As I pointed out, combining general relativity with quantum theory is what shows that there are limits on the measurability of lengths & times. The equivalence principle of general relativity is an essential part of the argument. (It does not, however, depend on Einstein's field equations.)

However, the limit on mesurability of lengths & times only shows that the metric concept can't be extended below the Planck limit. It doesn't mean that space-time couldn't be a continuum without any metric structure below that limit. That's one reason why I wish Schroedinger's affine field approach in the 40s had gotten more attention. Unfortunately most people saw it just as a variant of Einstein's non-symmetric extension of general relativity, which is wrong.


---- Don Winterstein <> wrote:
> ..Presentations of loop & string quantum gravity theories sometimes give the impression that the Planck limit is just an ad hoc assumption.
> Speaking of loop quantum gravity: Theorist Martin Bojowald has a short (but featured) article in the October '08 Scientific American in which he treats space as made up of "atoms" with diameters equal to the Planck length. These atoms have physical reality, which is predicted from "applying quantum principles to the loops." He faults General Relativity for assuming space is a continuum and concludes that, because it isn't, you can't get a density singularity and hence no Big Bang. What really happened, he says, is a Big Bounce, which followed collapse of a previously existing universe. And properties of the space atoms are what provided the driving force behind the inflationary expansion of the early universe.
> Interesting ideas, but ready to be swallowed?
> Don
> ----- Original Message -----
> From:<>
> To:<>
> Sent: Thursday, October 02, 2008 7:50 AM
> Subject: RE: [asa] (planck limits) Thermodynamics & Eternal Universe - A Question
> To flesh this out a bit more, you can't measure space or time intervals smaller than the Planck values. That is because of the joint effects of the uncerratinty principle & the influence of gravitational fields on rods & clocks. If you try to measure smaller & smaller time intervals with precision the uncertainty principle requires greater & greater uncertainty in the mass (=energy) of your clock, & that uncertainty in energy means a large uncertainty in the gravitational field & thus in the clock's measurement.
> This is worth emphasizing because presentations of loop & string quantum gravity theories sometimes give the impression that the Planck limit is just an ad hoc assumption.
> Shalom,
> George
> ---- "Dehler wrote:
> > Dividing numbers can go on ad infinitum, because numbers are figments of our imagination. Dividing spacetime, according to accepted theory, has limits. Of course, if you conceptually isolate one of those smallest possible elements of spacetime, you can conceptually mark off subdivisions; but such subdivisions would have no relevance to any physical process.
> >
> > In response to "Coope": My understanding is that physicists believe the 2nd Law holds everywhere in our expanding universe, but I guess it would not hold everywhere in a contracting universe, because ultimately that would be like having the air go spontaneously back into the balloon--in obvious violation of the 2nd Law.
> >
> > Don
> .

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Received on Thu Oct 2 17:49:14 2008

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