Re: Small probabilities

From: George Murphy <>
Date: Mon Nov 07 2005 - 09:45:44 EST

----- Original Message -----
From: "Glenn Morton" <>
To: "'George Murphy'" <>; "'Bill Hamilton'"
<>; "'Alexanian, Moorad'" <>
Cc: <>
Sent: Monday, November 07, 2005 8:23 AM
Subject: RE: Small probabilities

>> -----Original Message-----
>> From: George Murphy []
>> Sent: Monday, November 07, 2005 6:58 AM
>> On the more general question - to paraphrase our ex-Pres, it depends on
>> what
>> "observe" means. If the consequences of multiverse theories explain
>> observable phenomena better than other theories then they have to be
>> taken
>> seriously, even if the other universes aren't directly observable. & one
>> also has to ask whether they're unobservable _in principle_ or just due
>> to
>> our situation. E.g., in Linde's "bubble" version of inflation we could
>> (I
>> think) observe another bubble if we were close enough to the edge of
>> ours,
>> but the conditions that make life possible in our expanding bubble make
>> that
>> extremely unlikely.
> My understanding of Linde's version of inflation is untestable I quote
> Brian
> Greene
> "Instead, as Andrei Linde has proposed, there could have been mmany
> nuggets
> scattered here and there that underwent space-smoothing inflationary
> expansion. If that were so, our universe would be but one among many that
> sprouted--and perhaps continue to sprout--when chance fluctuations made
> the
> conditions right for an inflationary burst, as illustrated in Figure 11.2.
> As these and other universes would likely be forever separate from ours,
> it's hard to imagine how we would ever establish whether this 'multiverse'
> picture is true. However, as a conceptual framework, it's both rich and
> tantalizing." Brian Green, Fabric of the Cosmos, p. 320
> Lee Smolin adds the view that universes produce offspring with only
> slightly
> mutated physical constants via black holes. And in his view once again,
> no
> one can see past the black hole horizon to verify that Smolin is right.
> The
> multiverse idea is unscientific because it is untestable. Even the
> discovery of higher dimensions doesn't guarantee that other universes
> occupy
> those higher dimensions.
> I would also note that inflation actually explains a prediction of the big
> bang away. It explains why we don't see magnetic monopoles. Thus rather
> than
> explaining the existence of something, it is explaining the non-existence
> of
> something.

1) We don't know of a way to "observe" free quarks but there's a great deal
of observational data that's explained by quark theories, to the extent that
it would be high quixotic to claim that those theories are "untestable."
We're not at that point yet with multiverse theories but we might be some

2) I don't know why the inflationary result that the density of monopoles
should be extremely low amounts to "explaining away" any prediction of big
bang theory. It's misleading to regard inflationary cosmologies as
competitors with big bang ones. "Big bang" is a very loose term: the MWB &
light element abundances show that there was a "big bang" in the sense of an
extremely hot, dense state in the early universe. Saying that doesn't
commit us to the belief that the universe can be described arbitrarily
closely to "t = 0" by the simple cosmological models of GRT. It would be
more accurate to say that inflationary theories flesh out early (though
perhaps not the earliest) stages of big bang models, or that they are
covering theories of traditional big bang theories. & as part of that, they
explain (not explain away) the lack of monopoles in the present universe.

3) Explaining the non-existence of something can be very fruitful - cf. the
Michelson-Morley experiment & special relativity. Or the clue that Sherlock
Holmes found in "the curious incident of the dog in the night-time," the
curious incident being that "the dog did nothing in the night-time."

& with all that, I'm not a big fan of multiverse theories.

Received on Mon Nov 7 09:47:27 2005

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