On Sat, May 1, 2010 at 8:40 PM, Jesse Mazer <laserma...@gmail.com> wrote:
> On Sat, May 1, 2010 at 1:43 PM, Rex Allen <rexallen...@gmail.com> wrote:
>> On Thu, Apr 29, 2010 at 10:58 PM, Jesse Mazer <laserma...@gmail.com>
>> wrote:
>> >
>> > I think you've got the argument wrong.
>> I think you're wrong about my getting the argument wrong.  :)
> I suppose it depends what you mean by "the argument". It is possible you
> could find *some* mainstream scientist who seriously considers the
> possibility that all our historical records of a low-entropy past are wrong
> or that we are actually Boltzmann brains with false memories, but for any of
> the physicists I have read who have brought up these ideas, like Sean
> Carroll and Brian Greene, it is completely clear to me that they only
> consider these to be reductio ad absurdum arguments, not that they actually
> think these are likely to be true. If you disagree, I suggest you haven't
> actually read these authors very carefully, or haven't really understood
> what you read.

Well, I think the passage I quoted pretty much stands on it's own.
Without the extra assumption of the Past Hypothesis, the data we have
available leads to a conclusion that isn't cognitively stable when
combined with the assumption of physicalism.

I would take this as a reductio ad absurdum argument against physicalism.

The eternal recurrence problem is a related, but not identical,
problem than the issue introduced by the principle of indifference.
Here Sean invokes probabilistic reasoning on infinite sets, which
Brent and I are still discussing.  Though I just noticed that we
accidently wandered off the main list into a private email exchange.

Anyway.  Onwards:

> Then on p. 223 he explains in more detail why we can be confident we aren't
> Boltzmann brains: because the level of order we experience is far greater
> than what the vast majority of possible Boltzmann brains should be predicted
> to experience (though he does bring up the possibility that our experience
> of an orderly environment could just be a hallucination).

This was one of the points of my "The 'no miracles' argument against
scientific realism" thread...which died an untimely death.

So how does he rule out this hallucination possibility?  Or the
Boltzmann brain simulator possibility?  What facts do we have about
the nature of reality that rules it out?

Another extra assumption.  The "we can trust our observations, even
though our observations imply that we can't trust our observations"

Quoting the book, page 363:

"This version of the multiverse will feature both isolated Boltzmann
brains lurking in the empty de Sitter regions, and ordinary observers
found in the aftermath of the low-entropy beginnings of the baby
universes.  Indeed, there will be an infinite number of both types.
So which infinity wins?  The kinds of fluctuations that create freak
observers in an equilibrium background are certainly rare, but the
kinds of fluctuations that create baby universes are also very rare.
Ultimately it's not enough to daw fun pictures of universes branching
off in both directions of time; we need to understand things at a
quantitative level well enough to make reliable predictions.  The
state of the art, I have to admit, isn't up to that task just yet.
But it's certainly plausible that a lot more observers arise as the
baby universes grow and cool toward equilibrium than come about
through random fluctuations in empty space."

SO.  I think it's significant that *even with* all of his auxiliary
hypothesis, he still judges it likely that Boltzmann brains do exist.
And in such numbers that it's not clear whether they are more or less
common than "normal" observers.

>> BUT these things all inevitably lead to more questions.  There seem to
>> be only two possible "final" answers:
>> 1)  Everything exists.
>> 2)  Reality is essentially arbitrary.  There is no reason why
>> existence is this way as opposed to some other way.  It just is.
> Even if "everything exists", there is still the possibility of some definite
> probability distribution on this "everything"--either a probability
> distribution on all possible universes/computations/mathematical structures,
> or a probability distribution on all possible observer-moments. It's quite
> possible that the probability distribution would be such that observers who
> had *true* memories of a low-entropy past would be much more common than
> random Boltzmann brains with no memories or false memories.

Isn't it also quite possible that the opposite is true?

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