On Sat, May 1, 2010 at 11:07 PM, Rex Allen <rexallen...@gmail.com> wrote:

> 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.

"Physicalism" is much too vague--we can imagine a wide variety of *possible*
laws of physics, including ones where the universe would be predicted to
maintain a consistent low entropy (I think the old Steady State cosmology
would be an example, with new low-entropy matter and energy being
continually created in empty space). Carroll's reductio ad absurdum relied
on more specific features of the laws of physics in *our* universe, like
time-symmetry. But if we're talking about the laws of physics in our
universe, it's not as if we already know that these laws would naturally
lead us to predict Boltzmann brains would be more common than ordinary
observers and so we have to tack on the "Past Hypothesis" as an extra
assumption. The point is that since we don't yet know the ultimate laws of
physics, we can't yet calculate a probability distribution for histories of
the universe/multiverse that would allow us to decide whether ordinary
observers are likely to be more common than Boltzmann brains or vice versa.
The "Past Hypothesis" basically amounts to the idea that if we *did* know
these ultimate laws, they would indeed imply that in an "average" history of
the universe/multiverse, ordinary observers will be more common than
Boltzmann brains. And Carroll proposes a specific way the laws of physics
might work that could make this plausible.

> 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.
> Oops.
> 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"
> hypothesis.

He doesn't specifically address the hallucination possibility, but I think
one could naturally extend his argument to deal with it. His initial
argument assumes that a Boltzmann brain would have a properly-functioning
sensory system, and that it would be overwhelmingly likely that such a brain
would perceive high-entropy surroundings rather than low-entropy
surroundings...so, the fact that we *do* see low-entropy surroundings can
itself be seen as a falsification of the
Boltzmann-brain-with-functional-sensory-system hypothesis. It is true that a
Boltzmann brain in high-entropy surroundings might not have a functional
sensory system and might instead hallucinate a low-entropy world. But I'd
argue that if we consider all possible arrangements of a set of particles
that would be sufficiently brainlike to be conscious, we should expect the
average observer produced by a random fluctuation would have an arrangement
with the *bare minimum* of order needed for consciousness, which I think
would be likely to involve much more chaotic perceptions than we actually
have (for example, I can still be quite conscious while hearing and seeing
only white noise). So I would say there's a good qualitative argument for
thinking the Boltzmann brain hypothesis should predict that our perceptions
would be hightly chaotic, and this prediction is not borne out by my

> 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.

And again, if we knew the ultimate laws of physics, then with enough
computing power and mathematical savvy, the question of which are more
common could be *settled* one way or another without any extra tacked-on
assumptions. And Carroll gives a qualitative argument about how known
physics could be extended in a way that could in fact naturally lead to the
prediction that ordinary observers would be more common. So unless you can
show that his speculations about future physics are likely to be false, you
can't treat the Boltzmann brain possibility as a reductio ad absurdum
against physicalism.

> >> 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?
In the abstract, without considering anything about myself beyond the fact
that I'm a conscious observer, I would say it's quite possible either is
true (though I would say that the argument about the typical Boltzmann
brain's experience being much more chaotic than my own should cause me to
strongly favor the former possibility as more likely). The question could
really only be settled if we knew the ultimate laws of physics and could
compute the likelihood of various possible histories. You seemed to be
saying that somehow we *already* knew enough about physics to conclude that
entropy should have been higher in the past unless an extra logically
independent "Past Hypothesis" saying the opposite was tacked on, so that's
what I was disagreeing with.


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