From: hal@finney.org
Date: Sat May 19 2001 - 23:56:49 MDT
Lee writes:
> But here was my idea from many years ago: suppose (to make
> the paradox simpler) we have a Contrarian. This person will
> always say "Yes" if he is supposed to say "No" and vice-verse.
> The Contrarian receives information from the future that he
> is about to say "Yes", so naturally he says "No". Well, then
> after he writes down his answer somewhere, and some time travel
> apparatus gets hold of it, it's now the "No" answer that flies
> backwards in time. This time, the past Contrarian is presented
> with the "He said No" information, and so says "Yes". Then,
> later, when **his** information is written down and then used
> by the time travel apparatus, the cycle repeats.
The Extropy article by Hans Moravec can be found at
http://cart.frc.ri.cmu.edu/users/hpm/project.archive/general.articles/1991/TempComp.html.
He analyzes electronic circuits suitable for computers which contain a
negative time delay element. This sends signals into the past and adds
extraordinary power to computers if properly exploited.
He considers an electronic analog of the Contrarian, a time-delay
circuit which has an inverter. If the present value of the circuit is
1, it goes to 0 in the past, and vice versa. Moravec claims that the
inverter circuit would actually reach an intermediate voltage level,
balanced between 0 and 1. Although digital circuits are of course
designed to avoid this state, the peculiar circumstances make this the
only stable and consistent state.
Applying this to the human Contrarian, I suppose one possible result
is that he receives an unintelligible or ambiguous signal. Although
the system would be designed to avoid this, it is the only stable
state and so this is what will happen.
I find this more convincing than Deutsch's interpretation because I
believe it can be rigorously analyzed quantum mechanically in some
simple cases. A similar example is the famous Novikov billiards shot
in which balls come out of wormholes before they go in, yet attempts
to set up "Contrarians" produce consistent but surprising paths.
(This was used dramatically in Robert Forward's novel Timemaster.)
He analyzed this in full QM detail to show how it would work.
I don't believe Deutsch's explanation are similarly backed up. The idea
of identifying parallel worlds with time travel endpoints is just hand
waving. The Deutsch parallel worlds are QM many-worlds branches and so
they have a mathematical relationship with each other. Deutsch never
shows that a time travel machine would have specific quantum mechanical
effects that would correspond to ending up in a different Everett branch.
(I may be wrong about this, maybe he has a rigorous analysis that I am
not aware of.)
Hal
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