I'm not a physicist, so I'm asking a question. How much of this "we
have no information loss in this universe" prinicple are we simply
assuming at the outset? I know that a lot of it is unverified theory,
like in the case of Stephen Hawking's black hole vs. no black hole from
infinity argument, etc. For instance, are we simply assuming, by the
sacred laws of thermodynamics, that in the quantum background there is
always an antiparticle for each particle in order to annihilate each
other? Or could it be that particles and antiparticles appear and
disappear asymmetrically on their own, under our observational radar,
even though that wouldn't be elegant? Perhaps all these undetectable
asymmetries add up to cancel out any observable asymmetries. Weirder
things have happened in quantum physics. Are we assuming by elegance
that there is no information loss? You can just tell me to go back to
my math if you want.
Tom
>
> Saibal Mitra wrote:
> > How would an observer know he is living in a universe in which
information
> > is lost? Information loss means that time evolution can map two
different
> > initial states to the same final state. The observer in the final
state
> > thus
> > cannot know that information really has been lost.
>
> If the universe allows two different states to evolve into the same
final
> state, the second law of thermodynamics wouldn't hold, and we would be
able
> to (in principle) contruct perpetual motion machines.
>
> I don't know why you say this can't be detected by an observer. In
theory
> all we have to do is prepare two systems in two different states, and
then
> observe that they have evolved into the same final state. Of course in
> practice the problem is "which two different states?" And as I suggest
> earlier, it may be that for anthropic reasons one or both of these
states
is
> very difficult to access.
>
Yes, in principle you could observe such a thing. But it may be that
generic
models exhibiting information loss look like model that don't have
information loss to internal observers. 't Hooft's deterministic models
are
an example of this.
I'm also skeptical about observers being able to make more efficient
machines. The problem with that, as I see it (I haven't read Lloyd's
book
yet) is as follows.
Consider first a model without information loss, like our own universe.
What
is preventing us from converting heat into work with 100% efficiency is
lack
of information. If we had access to all the information that is present
then
you could make an effective Maxwell's Daemon.
Lacking such information, Maxwell's Deamon has to make measurements,
which
it has to act on. But eventually it has to clear it's memory, and that
makes
it ineffective.
To get rid of this problem Maxwell's Daemon would have to be able to
reset
its memory without changing the state of the rest of the universe. This
could possibly be done in an universe with information loss, but that
could
only work if the Daemon has control over the information loss process.
If
information loss interferes with the actions of the Daemon, then it
isn't
much use.
You could also think of the possiblity of some ''physical process''
which
would be sort of a ''passive Maxwell's Deamon'' that could reduce the
entropy in such universe. Using that you could create a temperature
difference between two objects. To extract work you now need to let heat
flow between the two objects. So, at that stage you need an entropy to
increase again.
So, to me this doesn't seem to be a generic world in which you have
information loss, rather a world in which it is preserved but where it
can
be overruled at will. The benefits come from that magical power.
Saibal
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Received on Tue Apr 11 2006 - 14:02:02 PDT