RE: [asa] (Planck limits) Thermodynamics & Eternal Universe - A Question

From: Alexanian, Moorad <>
Date: Fri Oct 03 2008 - 10:03:26 EDT

In the middle 70’s I wrote a paper on the very early universe and had a gas of black holes filling up the whole space, sort of a hard sphere gas at close packing. At the time I thought of that being the makeup of some sort of granular space—not space-time as John Wheeler speculated later on—since I considered time to be a continuous. I thought that was too drastic so this assembly of black holes became massive hadrons at later times. This transition from black holes to massive hadrons may have happened at the Planck length but I do not recall right now.



From: [] On Behalf Of Don Winterstein
Sent: Thursday, October 02, 2008 6:13 PM
Subject: Re: [asa] (planck limits) Thermodynamics & Eternal Universe - A Question


...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?




        ----- Original Message -----



        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.
        ---- "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 Fri Oct 3 10:50:36 2008

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