From: Iain Strachan <igd.strachan@gmail.com>

Date: Fri Aug 17 2007 - 09:29:15 EDT

Date: Fri Aug 17 2007 - 09:29:15 EDT

I'm probably a bit out of my depth here as regards the physics, but will

attempt a couple of responses ...

I respectfully disagree with your disagreement.

*>
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*> First of all, it makes little sense to me to talk about the physically
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*> possible values of the physical constants. For us they seem to be free
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*> parameters. What is the range of physical values for G? How would you pick
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*> that range to perform a Bayesian analysis? It seems to me you can't.
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*>
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I think possibly you can, for example by placing a weak constraint, which we

know must be true, which yields a range of values, from which the strong

constraint (habitability) requires a much smaller range. For example, we

might require the universe not to collapse back on itself within, say a year

of the Big Bang. This will place an upper limit on G. Presumably the lower

limit would be 0. But the stronger constraint (stars, galaxies etc) would

presumably select a very small fraction of the range we initially got from

the weak constraint. Presumably one could envisage even stronger

constraints that would narrow the range even further - for example stars

nucleosynthesising elements higher than helium. (I'm guessing this isn't to

do with G, but hopefully you get the idea - I'm guessing that a universe of

stars that only undergo D-D fusions is far less finely tuned than one in

which Carbon and Oxygen are synthesised for fusion).

But also (and this is just at the perimeter of my knowledge), it is still

possible to do a Bayesian analysis using an "Improper Prior", where the

prior probabilities do not integrate to unity, though the posterior

probabilities do.

http://en.wikipedia.org/wiki/Prior_probability#Improper_priors (The

limitations of my knowledge are that I'd come across the term in my studies,

and hence was able to know the term in order to search for it in Wikipedia!)

All we can approximate is the range that life requires. Is that range a

*> modest fraction of the measured value? If so, we have fine tuning.
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*>
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Yes, ... but how do you define what you mean by "modest"? Without some

appeal to probabilities, how do you know that, e.g. 10^(-6) is a "modest

fraction"? My guess is that intuitively you feel that this is "modest"

because you think that if the range to pick from were 0 to that value, then

you have a 1 in a million shot at hitting it. But then, of course, you've

implicitly assumed a prior distribution.

*>
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*> Scenario Two: We develop a fundamental theory that predicts the value of
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*> the cc. Now we are surprised, not that the cc was chosen from such a large
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*> set to have a value in the habitable range, but by just the opposite: that
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*> the fundamental theory spit the value out—habitability was built into the
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*> theory. Fine tuning survives. (And here we have effectively ruled out the
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*> multiverse as an explanation for fine tuning.)
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*>
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I think Dawkins talks about this in The God Delusion, as the alternative to

the multiverse - that the physical constants might be the only values they

could be, just like pi is the ratio of the circumference to the diameter of

a circle. In this case there is no fine tuning - more like a mathematical

proof that life must exist. Dawkins certainly doesn't think this is fine

tuning.

Iain

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Received on Fri Aug 17 09:29:32 2007

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