On Sat, Dec 17, 2011 at 11:59:07AM +0100, Bruno Marchal wrote:

... snip ...
> >In a multiverse, the
> >counterfactuals are realised, but in different branches.
> Not necessarily. If the computation is classical, it is the same in
> the normal continuations. The classical counterfactuals are not
> realized in a quantum multiverse. You have to put the Klara in some
> superposition state to do that. You need a quantum Olympia.

They will automatically be in superposition, being just a classical
device replicated across the branches. But they are not quantum
devices (in the sense of processing qubits).

> >Hence those
> >"inert" parts are no longer inert in all branches.
> I don't see this.

Consider an instruction that monitors the circular polarisation of a
photon. If the photon is left polarised, then the program branches, if
right, the program continues to the next instruction.

In Maudlin's initial run, suppose the program didn't branch (we
now eliminate from consideration the MW branch in which the program
did branch prior to the construction of Olympia).

Now Maudlin construct an Olympia with without branching at the
point. Or we do your step of replacing the branch instruction by a no
operation, as the result will be the same. The we run Olympia, with a
Klara attached at that step.

In the Multiverse branches where the photon is right polarised,
Olympia continues on, and the Klara remains inert. In the branch where
a left polarised photon is observed, the Klara springs to life, and
implements the missing branch instruction.

This is what I mean. At all stages, Klara and Olympia are classical
computing devices, embedded in a Multiverse.

> >If they were, they
> >could be removed from the computer altogether, without affecting the
> >computation.
> But that is the case for the computation under consideration.

No. See above.

> >
> >>
> >>So you are introducing a different kind of physical multiverse,
> >>which would handle the counterfactuals. But this will not work.
> >>Either this physical multiverse, which plays the role of the
> >>generalized brain, is Turing emulable, in which case I can emulate
> >>it in a single Turing machine, for which the MGA will apply again.
> >>Or it is not Turing emulable, but then the need of it will
> >>contradict the comp assumption.
> >>
> >
> >This step, as I understand it, is a form of dovetailing. Nobody really
> >thinks of the dovetailer algorithm as instantiating consciousness, so
> >the move is ultimately invalid, I would think.
> The problem is there. With comp + the physical supervenience thesis,
> the dovetailing algorithm does instantiate consciousness.

The dovetailer instantiates consciousness in exactly the same way that
a random block of marbles instantiates the statue of David.

I think that for most people call a shapeless block of marble 'David'
is a bit perverse.

> >It is not a question of the parallel realities playing a role in the
> >computation, but in the supervenience. Maudlin's argument says If
> >COMP, then supervenience on single universe is contradictory. But it
> >doesn't say anything about supervenience on multiple parallel
> >realities.
> Those are relevant for the relative measure on continuations.

Quite possibly. But that is an independent question to Maudlin's
argument. I'm trying to stay focussed here.

> >>>>
> >>>>>If you then fold the multiverse back into a
> >>>>>single universe by dovetailing, one can then reapply the Maudlin
> >>>>>move.
> >>>>
> >>>>Indeed. That is the key point.
> >>>>
> >>>>
> >>>>
> >>>>>But then, in that case, one can embed that result into a
> >>>>>Multiverse, and the cycle repeats.
> >>>>
> >
> >I think I'm coming around to the view that neither of the above steps
> >are valid - but one could equally say they are as valid as each other.
> Not sure I see which steps you are talking about. The MGA is a
> reductio ad absurdum from comp + physical supervenience.

The step of folding a Multiverse into single universe by
dovetailing, followed by reembedding the single universe back into
another multiverse. It is what we've been discussing, but is _not_
part of either MGA nor Maudlin. 


> >
> >>This is not
> >>different than the comp or quantum immortality argument. The fact
> >>remains: the physical activity in one normal branch missing the
> >>register is the same as the physical activity in some branch not
> >>missing it, for the same particular computation.
> >
> >In all branches, or just special ones? If all branches, then the
> >register is totally unnecessary.
> In this case the same computations, with the same inputs are done in
> all branches.

But then, this is not a Multiverse. By definition, a multiverse's
branches will be distunguished by the inputs.

> >If just a special pair of branches,
> >then Maudlin's argument shows that supervenience must occur across
> >more branches than those two.
> There is no more branches. We are now simulating all the branches in
> a single reality. If that is not possible, then comp is already
> false.

We're talking past each other here...


> OK, but then to make your argument you have to shift toward a multi-
> multiverse, given that we have come back to a single universe. And
> the argument can continue: I will just simulate that
> multi-multiverse in a single branch on a single classical universal
> machine. If that is not possible, then comp is false. If that is
> possible, then MGA will apply again.

As I said above, in simulating the multiple branches of the Multiverse
by dovetailing, we are no longer instantiating consciousness. (Just
like all blocks of marble are not David.) This step is basically
invalid. It does not imply COMP is false, though.

Of course, if you can think of another way of simulating a multiverse
within a single universe, I wil naturally reconsider...

> >
> >But this doesn't answer the why question. I could imagine that you
> >might feel that Multiverses are otiose, so would prefer a derivation
> >of their existence from something "simpler" - eg arithmetic of the
> >whole numbers.
> Not at all. It is the idea that there is primary matter which is
> otiose, or epistemologically contradictory. It is physicalism which
> is show wrong. The multiverse is shown to be emergent from a numbers
> multi-dream. Physics becomes a branch of machine's theology.
> >
> >That's fine and dandy - but the Multiverse is not otiose - it is far
> >less of an impost than a single reality.
> Yes. We agree on this. It is the main theme of this list. The
> question is not about their existence, but their primitivity.

Actually, I have no problem with this. I am quite persuaded by Kant's
concept of the unknowable noumenon to apply Laplace's "Sire, je n'ai
besoin de cet hypothese" to the whole issue of primitive reality. I
don't need the MGA to conclude that. But others seem to require a
primitive "something" to exist, and are most upset at
immateriality. I'm happy to work out what is the limit of our
knowledge and move on from there. If any universal system can
reproduce phenomena, then that suffices. It is a meaningless question
to ask "which universal system" - it could be all of them, or even nothing
at all.

Coming back to the MGA - it would be interesting to know whether your
movie graph construction escapes the multiverse embdding move. Assume
your filmed graph has had most of its nodes removed, but by
coincidence, a supernova sends a blast of photons that exactly
reproduce the graph in one of the MW branches. In the vast majority of
branches, the filmed graph is a dead as a doornail.

Does this render the graph conscious in that single lucky branch?

I think this question bears on the issue of Boltzmann brains too,
something we haven't discussed enough here.



Prof Russell Standish                  Phone 0425 253119 (mobile)
Principal, High Performance Coders
Visiting Professor of Mathematics      hpco...@hpcoders.com.au
University of New South Wales          http://www.hpcoders.com.au

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