Re: PhD-thesis on Observational Selection Effects

From: Brent Meeker <>
Date: Tue, 04 Jul 2000 18:08:20 -0700

Hello Jacques

On 03-Jul-00, Jacques Mallah wrote:
>> From: Brent Meeker <>
>>> Of course, if we already had a theory of quantum gravity, and if the
>> math calculation already yielded the prediction that p=x, then p(o|1) (if
>> you like, p(x|1)) would be 1 - based on that math, not on the fact that we
>> do see x.
>> That's exactly what I took Candidate #1 to be - we have a theory that
>> mathematically constrains all parameters either to particular values or to
>> ranges which are consistent with SAS's. If that was not what you meant by
>> Candidate #1 (and apparently it wasn't), why didn't you consider that as
>> Candidate #4; after all it's what most physicist hope and work for.
> The whole point is that right now, we don't have such a theory! And
> what my analysis seems to point to is that there isn't one.

What analysis is that? The hypothesis that every mathematically consistent
universe exists may admit infinitely many in which there are no free
parameters and in which there are sentient beings.

> If, on the other hand, we had a theory that exactly specified (up to
> many decimal places) each physical constant, and it predicted the values we
> see, then that theory would have a huge probability advantage over #2 (which
> only constrains the values to a range, albeit a narrow one).

I assume you say this because, although p(o|2)=p(o|4)=1, you assign a lower
prior probability to 4 so that p(4)>>p(2).

> The advantage will always
> belong to *some* type of ensemble theory, erasing the anthropic
> "coincidence".
> Ensemble #2 easily beats non-ensemble #1, but "apparent ensemble #2 with
> Tegmark ensemble" is not necessarily any more likely than such an "apparent
> non-ensemble #1 with Tegmark ensemble". Tricky! (And similar to the effect
> of the MWI on "god's coin toss".)
> But important to point it out now, lest physicists should come up with a
> unique consistent set of physical constants, and then try to use that as
> evidence against the MWI way of thinking about physical laws! It wouldn't
> be.)

It would not disprove an ensemble theory, but it would make it irrelevant.
Imagine Ptolemy saying to Newton, "That all possible epicycles exist is the simplest
hypothesis. Your elliptic orbits and theory of gravity are not evidence
against it."

> You still don't seem to understand what the MWI says. In both 1) and 2)
> you seem to assume branching is some special physical process beyond normal
> Shrodinger evolution. You probably have seen a lot of books characterize
> the MWI that way.

Apparently I don't. I was relying on David Deutsch's description of an
experiment to distinguish Everett's MWI from the other interpretaitons [Quantum
Concepts in Space and Time, Penrose & Isham, pp 219]. "...At this point an
outside observer could measure O(T) but the result would be to transfer the
split to himself..." Deutsch seems to think a split has occured in the quantum
process, which is a parallel computation, and if it is observed the observer
will split. Is it your interpretation that these splits don't occur or that
they are not physical processes or they are just part of the unitary evolution?

> As I said, in both my version of the MWI and Everett's, "branching" is
> just a manner of speaking about the typical behavior in Shrodinger evolution
> where decoherence occurs.

I understand decoherence is just part of the unitary evolution and hence to get
from the *almost* diagonal density matrix to a definite classical value still
takes a step.

A theory of mind is needed by *any* theory to
> generate *any* prediction about observations. I attempt to formulate one in
> my CWIA.

In Deutsch's paper [ibid] he requires a 'mind' for his crucial experiment; but
the mind seems a very simple one. The hard part about the experiment is
keeping all the interactions reversible.

> Finally, it's obvious to me that a stochastic theory, which says that
> the Kolmogorov complexity of the physical world increases with every event,
> continuously violates Occam's razor.

It certainly leads to an increase in information - but not number of
hypotheses. In any case Occam's razor is a rule-of-thumb; you're trying to
promote it to a law of physics.

vr, Brent
Received on Tue Jul 04 2000 - 19:21:44 PDT

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