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.


> Saibal Mitra wrote:
> > How would an observer know he is living in a universe in which
> > is lost? Information loss means that time evolution can map two
> > initial states to the same final state. The observer in the final 
> > thus
> > cannot know that information really has been lost.
> If the universe allows two different states to evolve into the same 
> state, the second law of thermodynamics wouldn't hold, and we would be
> to (in principle) contruct perpetual motion machines.
> I don't know why you say this can't be detected by an observer. In 
> all we have to do is prepare two systems in two different states, and 
> 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 
> very difficult to access.

Yes, in principle you could observe such a thing. But it may be that 
models exhibiting information loss look like model that don't have
information loss to internal observers. 't Hooft's deterministic models 
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 
yet) is as follows.

Consider first a model without information loss, like our own universe. 
is preventing us from converting heat into work with 100% efficiency is 
of information. If we had access to all the information that is present 
you could make an effective Maxwell's Daemon.

Lacking such information, Maxwell's Deamon has to make measurements, 
it has to act on. But eventually it has to clear it's memory, and that 
it ineffective.

To get rid of this problem Maxwell's Daemon would have to be able to 
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 
only work if the Daemon has control over the information loss process. 
information loss interferes with the actions of the Daemon, then it 
much use.

You could also think of the possiblity of some ''physical process'' 
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 
be overruled at will. The benefits come from that magical power.


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