Re: MWI, Copenhage, Randomness (fwd)

2002-09-10 Thread Brent Meeker

On 09-Sep-02, Bruno Marchal wrote:


 Jesse Mazer wrote:



 Bruno Marchal wrote:




 Jesse Mazer wrote




 Ok, I think I see where my mistake was. I was thinking
 that decoherence just referred to interactions between a
 system and the external environment, but what you seem to
 be saying is that it can also refer to an internal effect
 where interactions among the components of a system with
 many degrees of freedom cause interference terms to become
 negligible. If that's correct, then when Wigner decided
 that interference would cause the wavefunction of the cat
 or Wigner's friend to collapse even before the box or
 the room was opened, then he was probably referring to
 this sort of internal effect, so my argument about using
 quantum computers to simulate truly impenetrable boxes
 would not make a difference.

 BM:
 decoherence refers to anything interacting with what you
 are, as observer, describing by a wave function, and which
 is not currently described by your wave function. (- need
 of a tensor product). IMO, it has been discovered by
 Everett and it explained why we don't feel the split or the
 differentiation. Decoherence is just entanglement with the
 the environment, it is the contagion of the superposition
 state, the linearity of the tensor product.



 JM:
 I probably need to read up on the actual mathematics behind
 decoherence before I can discuss it very intelligently.
 Brent Meeker seemed to say that even in the case of an
 isolated system whose wavefunction we know completely, if it
 has many degrees of freedom there will be an effect which
 approximates wavefunction collapse in which interference
 terms become neglible. Presumably this does not collapse
 the wavefunction onto any one particular classical state
 (dead cat vs. live cat), but by eliminating interference
 terms you get something similar to classical probabilities,
 where you're free to assume the cat is really in some
 state all along and your measurement just reveals that
 preexisting state (interference is the reason you get into
 trouble thinking that way about the quantum world, as is
 shown most clearly by the Bell inequality).

 I don't know whether this diagonalization effect in an
 isolated system would normally be called decoherence or if
 some other term would be used. I'd guess that they're two
 sides of the same coin, since if you knew the wavefunction
 for system + external environment it would itself have a
 large number of degrees of freedom, so the principle is
 probably the same. Also, I don't know whether Wigner was
 referring to an internal diagonalization effect or to
 entanglement with the outside environment when he argued
 that decoherence shows that the act of opening the box and
 observing the cat has no particular importance.






 BM: I don't see how the internal interaction could leads to
 decoherence, unless the information is not available to the
 observer. If a cat is in the (a + d) state in the box, and if
 we know the state of each air molecules in the box, we can
 in principle observe macro cat interferences. Obviously we
 cannot
 keep track of all those molecules and that's why in practice,
 even if the box completely isolates the cat and the air
 molecules we will not be able to see the
 interferences. So Brent is practically right, but the we
 loose the ability of witnessing interferences just if the cat
 interact with *any* particle we didn't
 keep track of, whether that particle was inside the box or
 not.

Right. Because we cannot construct an appratus that measures an
operator corresponding to determing the state of the cat and
all the particles the cat interacts with and which constitute
the cat, we cannot observe the interference between the very
complicated dead-subspace and live-subspace.  However while
this is a limitation in practice and not in the mathematics,
it is more than *merely* practical.  We, or anything
exhibiting intelligence, must have memory, i.e. irreversible
encoding of some past events/experience.  This implies that
we, and our instruments, must be macroscopic, quasi-classical
things.  So it is impossible that we, or other intelligent
beings, can experience the interference effects. I suspect
that this is a counter-argument to Deutsche's AI quantum
computer that experiences interference, but I haven't worked
it through.

...
 JM:
 Maybe since this is a computer simulation where we know
 the dynamical rules and initial state precisely, we would
 know just where to look for even the smallest interference
 effects, unlike in an ordinary macroscopic system where we
 don't have such detailed information. Also, we could run
 such a simulation over and over again from the same
 initial conditions, which would also help to detect small
 statistical deviations from classical predictions. I once
 read a comment by Deutch about decoherence where he said
 something like (paraphrasing) saying the interference
 terms are 'almost' zero is like saying someone is a 

Re: MWI, Copenhage, Randomness

2002-09-09 Thread Bruno Marchal



Jesse Mazer wrote:



Bruno Marchal wrote:




Jesse Mazer wrote




Ok, I think I see where my mistake was. I was thinking that 
decoherence just referred to interactions between a system and 
the external environment, but what you seem to be saying is that 
it can also refer to an internal effect where interactions among 
the components of a system with many degrees of freedom cause 
interference terms to become negligible. If that's correct, then 
when Wigner decided that interference would cause the wavefunction 
of the cat or Wigner's friend to collapse even before the box or 
the room was opened, then he was probably referring to this sort 
of internal effect, so my argument about using quantum computers 
to simulate truly impenetrable boxes would not make a difference.

BM:
decoherence refers to anything interacting with what you are, as
observer, describing by a wave function, and which is not currently described
by your wave function. (- need of a tensor product).
IMO, it has been discovered by Everett and it explained why we don't feel the
split or the differentiation. Decoherence is just entanglement with the
the environment, it is the contagion of the superposition state, the
linearity of the tensor product.



JM:
I probably need to read up on the actual mathematics behind 
decoherence before I can discuss it very intelligently. Brent Meeker 
seemed to say that even in the case of an isolated system whose 
wavefunction we know completely, if it has many degrees of freedom 
there will be an effect which approximates wavefunction collapse 
in which interference terms become neglible. Presumably this does 
not collapse the wavefunction onto any one particular classical 
state (dead cat vs. live cat), but by eliminating interference terms 
you get something similar to classical probabilities, where you're 
free to assume the cat is really in some state all along and your 
measurement just reveals that preexisting state (interference is the 
reason you get into trouble thinking that way about the quantum 
world, as is shown most clearly by the Bell inequality).

I don't know whether this diagonalization effect in an isolated 
system would normally be called decoherence or if some other term 
would be used. I'd guess that they're two sides of the same coin, 
since if you knew the wavefunction for system + external 
environment it would itself have a large number of degrees of 
freedom, so the principle is probably the same. Also, I don't know 
whether Wigner was referring to an internal diagonalization effect 
or to entanglement with the outside environment when he argued that 
decoherence shows that the act of opening the box and observing the 
cat has no particular importance.






BM: I don't see how the internal interaction could leads to decoherence, unless
the information is not available to the observer. If a cat is in the
(a + d) state in the box, and if we know the state of each air molecules in
the box, we can in principle observe macro cat interferences. 
Obviously we cannot
keep track of all those molecules and that's why in practice, even if the box
completely isolates the cat and the air molecules we will not be able 
to see the
interferences. So Brent is practically right, but the we loose the ability
of witnessing interferences just if the cat interact with *any* 
particle we didn't
keep track of, whether that particle was inside the box or not.



JM:



But this makes me wonder about the thought-experiment by David 
Deutsch which Hal Finney brought up, in which interference shows 
that an isolated A.I. was splitting into multiple versions which 
experienced different outcomes. Presumably a simulation of an 
intelligence would have a lot of degrees of freedom too, so why 
wouldn't decoherence ruin things?

Ok, but then SWE is wrong at some point. Where do you think?


What does SWE stand for?



Sorry. It is Schroedinger Wave Equation.




JM:
Maybe since this is a computer simulation where we know the 
dynamical rules and initial state precisely, we would know just 
where to look for even the smallest interference effects, unlike 
in an ordinary macroscopic system where we don't have such 
detailed information. Also, we could run such a simulation over 
and over again from the same initial conditions, which would also 
help to detect small statistical deviations from classical 
predictions. I once read a comment by Deutch about decoherence 
where he said something like (paraphrasing) saying the 
interference terms are 'almost' zero is like saying someone is a 
little bit pregnant. His argument would probably be that although 
decoherence may explain why the world looks approximately 
classical in the many-worlds framework, it doesn't remove to 
postulate those other worlds in the first place.


BM: I don't understand your last sentence.


What I meant was although in practice decoherence might seem to 
solve the measurement problem and remove the need for other worlds, 
in 

Re: MWI, Copenhage, Randomness

2002-09-06 Thread Bruno Marchal





Re: MWI, Copenhage, Randomness

2002-09-06 Thread Bruno Marchal



Jesse Mazer wrote





Ok, I think I see where my mistake was. I was thinking that 
decoherence just referred to interactions between a system and the 
external environment, but what you seem to be saying is that it can 
also refer to an internal effect where interactions among the 
components of a system with many degrees of freedom cause 
interference terms to become negligible. If that's correct, then 
when Wigner decided that interference would cause the wavefunction 
of the cat or Wigner's friend to collapse even before the box or 
the room was opened, then he was probably referring to this sort of 
internal effect, so my argument about using quantum computers to 
simulate truly impenetrable boxes would not make a difference.


decoherence refers to anything interacting with what you are, as
observer, describing by a wave function, and which is not currently described
by your wave function. (- need of a tensor product).
IMO, it has been discovered by Everett and it explained why we don't feel the
split or the differentiation. Decoherence is just entanglement with the
the environment, it is the contagion of the superposition state, the
linearity of the tensor product.




But this makes me wonder about the thought-experiment by David 
Deutsch which Hal Finney brought up, in which interference shows 
that an isolated A.I. was splitting into multiple versions which 
experienced different outcomes. Presumably a simulation of an 
intelligence would have a lot of degrees of freedom too, so why 
wouldn't decoherence ruin things?

Ok, but then SWE is wrong at some point. Where do you think?



Maybe since this is a computer simulation where we know the 
dynamical rules and initial state precisely, we would know just 
where to look for even the smallest interference effects, unlike in 
an ordinary macroscopic system where we don't have such detailed 
information. Also, we could run such a simulation over and over 
again from the same initial conditions, which would also help to 
detect small statistical deviations from classical predictions. I 
once read a comment by Deutch about decoherence where he said 
something like (paraphrasing) saying the interference terms are 
'almost' zero is like saying someone is a little bit pregnant. His 
argument would probably be that although decoherence may explain why 
the world looks approximately classical in the many-worlds 
framework, it doesn't remove to postulate those other worlds in the 
first place.


I don't understand your last sentence.


Bruno





Re: MWI, Copenhage, Randomness

2002-09-06 Thread Jesse Mazer

Bruno Marchal wrote:




Jesse Mazer wrote





Ok, I think I see where my mistake was. I was thinking that decoherence 
just referred to interactions between a system and the external 
environment, but what you seem to be saying is that it can also refer to 
an internal effect where interactions among the components of a system 
with many degrees of freedom cause interference terms to become 
negligible. If that's correct, then when Wigner decided that interference 
would cause the wavefunction of the cat or Wigner's friend to collapse 
even before the box or the room was opened, then he was probably referring 
to this sort of internal effect, so my argument about using quantum 
computers to simulate truly impenetrable boxes would not make a 
difference.


decoherence refers to anything interacting with what you are, as
observer, describing by a wave function, and which is not currently 
described
by your wave function. (- need of a tensor product).
IMO, it has been discovered by Everett and it explained why we don't feel 
the
split or the differentiation. Decoherence is just entanglement with the
the environment, it is the contagion of the superposition state, the
linearity of the tensor product.

I probably need to read up on the actual mathematics behind decoherence 
before I can discuss it very intelligently. Brent Meeker seemed to say that 
even in the case of an isolated system whose wavefunction we know 
completely, if it has many degrees of freedom there will be an effect which 
approximates wavefunction collapse in which interference terms become 
neglible. Presumably this does not collapse the wavefunction onto any one 
particular classical state (dead cat vs. live cat), but by eliminating 
interference terms you get something similar to classical probabilities, 
where you're free to assume the cat is really in some state all along and 
your measurement just reveals that preexisting state (interference is the 
reason you get into trouble thinking that way about the quantum world, as is 
shown most clearly by the Bell inequality).

I don't know whether this diagonalization effect in an isolated system would 
normally be called decoherence or if some other term would be used. I'd 
guess that they're two sides of the same coin, since if you knew the 
wavefunction for system + external environment it would itself have a 
large number of degrees of freedom, so the principle is probably the same. 
Also, I don't know whether Wigner was referring to an internal 
diagonalization effect or to entanglement with the outside environment when 
he argued that decoherence shows that the act of opening the box and 
observing the cat has no particular importance.


But this makes me wonder about the thought-experiment by David Deutsch 
which Hal Finney brought up, in which interference shows that an isolated 
A.I. was splitting into multiple versions which experienced different 
outcomes. Presumably a simulation of an intelligence would have a lot of 
degrees of freedom too, so why wouldn't decoherence ruin things?

Ok, but then SWE is wrong at some point. Where do you think?


What does SWE stand for?


Maybe since this is a computer simulation where we know the dynamical 
rules and initial state precisely, we would know just where to look for 
even the smallest interference effects, unlike in an ordinary macroscopic 
system where we don't have such detailed information. Also, we could run 
such a simulation over and over again from the same initial conditions, 
which would also help to detect small statistical deviations from 
classical predictions. I once read a comment by Deutch about decoherence 
where he said something like (paraphrasing) saying the interference terms 
are 'almost' zero is like saying someone is a little bit pregnant. His 
argument would probably be that although decoherence may explain why the 
world looks approximately classical in the many-worlds framework, it 
doesn't remove to postulate those other worlds in the first place.


I don't understand your last sentence.


What I meant was although in practice decoherence might seem to solve the 
measurement problem and remove the need for other worlds, in principle even 
tiny interference effects are just as much in need of an explanation as 
large ones, and decoherence will not make interference disappear completely 
(as I argued above, we should be able to detect tiny interference effects in 
simulations of macroscopic systems on a quantum computer, unlike in ordinary 
macroscopic systems where we don't have enough information to know where to 
look for such tiny effects). If you view the universe as a giant 
computation, the only way to duplicate interference effects precisely is to 
compute all those other histories--I think this is the point you were making 
about Bohm and his rejection of COMP, since computing the behavior of the 
pilot wave would probably be equivalent to computing all possible 
histories of the system you are 

Re: MWI, Copenhage, Randomness

2002-09-05 Thread Brent Meeker

On 04-Sep-02, Tim May wrote:

 On Wednesday, September 4, 2002, at 02:44 PM, Hal Finney
 wrote:

 Tim May wrote:

 In weaker forms of the MWI, where it's the early state
 of the Big Bang (for example) which are splitting off
 into N universes, De Witt and others have speculated (as
 early as around 1970) that we may _possibly_
 see some evidence consistent with the EWG interpretation
 but NOT consistent with other interpretations.

 I'm not familiar with the details of this. But I know
 that much of
 the impetus for increased acceptance of MWI models comes
 from the
 cosmologists.

 It was in DeWitt's article, Quantum mechanics and
 reality, Physics Today, September 1970, reprinted in the
 collection The Many-Worlds Interpretation of Quantum
 Mechanics, edited by Bryc DeWitt and Neill Graham, 1973.

 Moreover a decision between the two interpretations may
 ultimately be made on grounds other than direct laboratory
 experimentation. For example, in the very early moments of
 the universe, during the cosmological Big Bang, the
 universal wave function may have possessed an overall
 coherence as yet unimpaired by condensation into
 non-interfering branches. Such initial coherence may have
 testable implications for cosmology. (p. 165 of the
 reprint volume).

But to maintain strictly unitary evolution the branches are
only non-intefering when measured by macroscopic operators.
 For some (impossible to realize operators) there will
still be cross terms.


 By the way, issues of observers and measurements are
 obviously fraught with Chinese boxes types of problems.
 In the Schrodinger's Cat pedantic example, if the cat
 alive or cat dead measurement is made at the end of one
 hour by opening the sealed box, what if a video camera had
 been also sealed inside the box, and had seen the cat
 breathe in the cyanide gas at 10 minutes into the
 experiment? Does this imply the wave function collapsed
 at the time of the measurement by the human observers, at
 the one hour point, or at the time the video camera
 unambiguously recorded the cat's death?

Alive and dead are very macroscopic operators (average of
lots of micro-states) and so the cats interaction with it's
environment (the box) will very quickly diagonalize the
Alive X Dead density matrix.  To introduce observers as
having a special effect seems to introduce an aphysical
mysticism.

 One could arrange a thought experiment involving literally
 a series of boxes within boxes, each being opened at, say,
 one minute intervals after the cyanide was released or not
 released. One set of observers sees the cat either alive
 or dead at the end of the canonical one hour period. But
 they are sealed inside a box. After one minute, their box
 is opened, and the observers in the next-larger box then
 see the collapse of the wave function at the 61-minute
 point. After another minute, their box is opened and a
 new set of observer sees the collapse of the wave
 function at the 62-minute point.

 And so on. (I don't know if I'm just reinventing a thought
 experiment someone developed many decades ago...it seems
 like a natural idea.)

 Seen this way, the collapse of the wave function in the
 Schrodinger's Cat thought experiment is seen as a problem
 of knowledge, not something quasi-mystical about an
 instantaneous collapse of some psi-squared function.

 (More interesting are the delayed choice experiments.)

In Bohm's QM the universal wave function determines
everything and the apparent randomness we see it just like
the randomness of statistical mechanics, it comes from our
inability to take into account all the non-local effects of
all the rest of the universe.  It solve the collapse of
the wave function problem as you suggest by making it a
collapse of our uncertainity and it's perfectly
understandable then that different observers collapse it
at different times.  The problem is that BQM isn't Lorentz
invariant - but people are working on that.

...

Brent Meeker
The concept of 'measurment' becomes so fuzzy on reflection
that
it is quite suprising to have it appear in physical theory
at the
most fundamenatal level.
  -- J. S. Bell




Re: MWI, Copenhage, Randomness

2002-09-05 Thread Jesse Mazer

Brent Meeker wrote:

On 04-Sep-02, Tim May wrote:

  By the way, issues of observers and measurements are
  obviously fraught with Chinese boxes types of problems.
  In the Schrodinger's Cat pedantic example, if the cat
  alive or cat dead measurement is made at the end of one
  hour by opening the sealed box, what if a video camera had
  been also sealed inside the box, and had seen the cat
  breathe in the cyanide gas at 10 minutes into the
  experiment? Does this imply the wave function collapsed
  at the time of the measurement by the human observers, at
  the one hour point, or at the time the video camera
  unambiguously recorded the cat's death?

Alive and dead are very macroscopic operators (average of
lots of micro-states) and so the cats interaction with it's
environment (the box) will very quickly diagonalize the
Alive X Dead density matrix.  To introduce observers as
having a special effect seems to introduce an aphysical
mysticism.

But even if one understands that conscious observers are not necessary to 
collapse the wave function, Tim's questions do not go away. One could 
always imagine that the box in the Schroedinger's cat experiment was made of 
some super-material that blocked interaction between the inside and the 
outside so effectively that decoherence was completely eliminated, so from 
the outside the cat would have to be treated as being in a macroscopic 
superposition until the box was opened, even though the cat (or a video 
camera inside the box) would remember having been in a single definite state 
all along.

  One could arrange a thought experiment involving literally
  a series of boxes within boxes, each being opened at, say,
  one minute intervals after the cyanide was released or not
  released. One set of observers sees the cat either alive
  or dead at the end of the canonical one hour period. But
  they are sealed inside a box. After one minute, their box
  is opened, and the observers in the next-larger box then
  see the collapse of the wave function at the 61-minute
  point. After another minute, their box is opened and a
  new set of observer sees the collapse of the wave
  function at the 62-minute point.

  And so on. (I don't know if I'm just reinventing a thought
  experiment someone developed many decades ago...it seems
  like a natural idea.)

Yes, this is similar to the Wigner's friend thought-experiment. The 
physics dictionary entry on Schrodinger's cat at 
http://physics.about.com/library/dict/bldefschrdingerscat.htm describes it 
briefly:

Wigner's friend is a variation of the Schrödinger's cat paradox in which a 
friend of the physicist Eugene Wigner is the first to look inside the 
vessel. The friend will find a live or dead cat. However, if Professor 
Wigner has both the vessel with the cat and the friend in the closed room, 
the state of the mind of the friend (happy if there is a live cat but sad if 
there is a dead cat) cannot be determined in Bohr's interpretation of 
quantum mechanics until the professor has looked into the room although the 
friend has already looked at the cat. These paradoxes indicate the absurdity 
of the overstated roles of measurement and observation in Bohr's 
interpretation of quantum mechanics.

By the way, anyone interested in the measurement problem and interpretations 
of QM might want to take a look at the book Foundations and Interpretations 
of Quantum Mechanics by Gennaro Auletta...I haven't read it, just browsed 
it (it's a bit over my head for now), but it looks like a very comprehensive 
phone-book sized reference on these issues, with a lot of discussion of new 
interesting experiments probing quantum weirdness. There's a brief 
description and a table of contents at 
http://www.wspc.com/books/physics/4194.html

--Jesse

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Re: MWI, Copenhage, Randomness

2002-09-05 Thread scerir


 Yes, this is similar to the Wigner's friend thought-experiment. 

Wigner later (1983) changed opinion and wrote 
that decoherence forbids superposition of states like  
 c1 |s 1 |friend 1 + c2 |s 2 |friend 2
After that in QM the conscious being  - i.e. the friend 
who tells that  he already knows whether the outcome is 
|s 1 or |s 2 - plays no role. Von Neumann, London,
Bauer, Wigner thought (and many more still think) that 
consciousness was able to collapse a superposition 
(E.P.W., Collected Papers, VI, pages 225-244)
s.










Re: MWI, Copenhage, Randomness

2002-09-05 Thread Jesse Mazer

scerir wrote:

Wigner later (1983) changed opinion and wrote
that decoherence forbids superposition of states like
  c1 |s 1 |friend 1 + c2 |s 2 |friend 2
After that in QM the conscious being  - i.e. the friend
who tells that  he already knows whether the outcome is
|s 1 or |s 2 - plays no role.

But can decoherence really forbid macroscopic superpositions in principle, 
or only in practice? To build quantum computers, people have to figure out 
clever tricks to keep fairly large systems in quantum coherence, even though 
under normal circumstances decoherence would prevent this. Is there a limit 
to how far we could take this? If quantum computing follows something like 
Moore's law, it seems concievable that we could eventually simulate 
something as complex as a cat-in-a-box, which we'd have to treat as being in 
a superposition of states as long as coherence was maintained among the 
elements of the computer itself. And we don't need something so complex to 
get these sorts of paradoxes--at the bare minimum, we just need a system 
complex enough to keep internal records of its own state (analogous to the 
cat's brain or the videocamera in the box) so that we must treat the system 
as being in a superposition as long as coherence is maintained, but once 
coherence breaks down we can look at the internal records and see that the 
system *appears* to have been in a definite state all along (Is there any 
well-defined notion of what it means for a system to keep records of its own 
history, BTW? Maybe this is related to Maxwell's demon and the 
thermodynamics of computation?) Then we could then extend this to a 
nested-box scenario where one subsystem is in coherence relative to a 
larger system, which is itself in coherence relative to the outside world, 
and then decoherence occurs between the system and the subsytem while the 
system as a whole remains in coherence from an outside point of view 
(which would be like Wigner's friend looking in the box while both the box 
and the friend were still sealed in a room).

Von Neumann, London,
Bauer, Wigner thought (and many more still think) that
consciousness was able to collapse a superposition
(E.P.W., Collected Papers, VI, pages 225-244)

Did Wigner only believe this until his change of opinion in 1983, or did he 
continue to think this way afterwards?

--Jesse


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Re: MWI, Copenhage, Randomness

2002-09-05 Thread Hal Finney

Jesse Mazer wrote:
 But can decoherence really forbid macroscopic superpositions in principle, 
 or only in practice? To build quantum computers, people have to figure out 
 clever tricks to keep fairly large systems in quantum coherence, even though 
 under normal circumstances decoherence would prevent this. Is there a limit 
 to how far we could take this? If quantum computing follows something like 
 Moore's law, it seems concievable that we could eventually simulate 
 something as complex as a cat-in-a-box, which we'd have to treat as being in 
 a superposition of states as long as coherence was maintained among the 
 elements of the computer itself.

David Deutsch took this idea farther, I think in his 1985 paper.  He proposed
the following experiment:

1. Create a large quantum computer
2. Create a conscious AI program
3. Run the AI on the QC
4. Let the AI make a two-way quantum observation
5. Keep the AI in a superposition of the two states a la Schrodinger's cat
6. The AI announces that it has made a definite, precise observation
7. Recombine the two states and reveal interference

This shows that the conscious mind of the AI was in two states, and both
were equally real.  Hence one of the following must be true:

A. Consciousness doesn't collapse the wave function
B. AI's aren't conscious
C. QM is wrong

and more generally we get a choice between:

A. MWI is wrong
B. QM is wrong

People like Penrose believe that QM is wrong, that there is a yet-unknown
law called objective reduction which makes state function collapse an
objectively real phenomenon and determines when and how it happens.  But
if you're not willing to go that far, it is hard to deny the force of
Deutsch's thought experiment.

Hal Finney




Re: MWI, Copenhage, Randomness

2002-09-05 Thread scerir

J. Mazer [about Wigner and consciousness]

 Did Wigner only believe this until his change of opinion in 1983, or did he 
 continue to think this way afterwards?

Wigner wrote (Nov. 18, 1978) ...

 ... as far as living organism of any complexity are concerned, the same
initial state hardly can be realized several times. There are no two
identical people and if we repeat the same experiment on the same
individual the initial conditions are no longer the same - the individual
will remember at the second experiment the event of the first one -
his mental outlook will have changed thereby. This means that the
relevant statements of the theory encompassing life will be terribly
different from those of the present natural sciences.

and also ...

I do not believe there are two entities: body and soul. I believe that life
and consciousness are phenomena which have a varying effect on the
event around us - just as light pressure does. Under many circumstances,
those with which present-day physics is concerned, the phenomenon
of life has an entirely negligible influence. There is then a continuous 
transition to phenomena, such as our own activities, in which this
phenomenon has a decisive influence. Probably, the behaviour of
viruses and bacteria could be described with a high accuracy with
present theories. Those of insects could be described with a moderate 
approximation, those of mammals and men are decisively influenced
by their minds. For these, present physical theory would give a false
picture even as far as their physical behavior is concerned.

E. P. Wigner, Philosophical Reflections and Syntheses,
Springer, 1995, page 272








Re: MWI, Copenhage, Randomness

2002-09-05 Thread scerir

J. Mazer:
 But can decoherence really forbid macroscopic superpositions in principle,
 or only in practice?

Well, experiments have been done many times, showing
the effect of decoherence on (macroscopic) quantum superpositions
http://physicsweb.org/article/world/13/8/3/1
http://physicsweb.org/article/news/4/7/2/1
http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v406/n6791/abs/4
06043a0_fs.html





Re: MWI, Copenhage, Randomness

2002-09-05 Thread Jesse Mazer

Brent Meeker wrote:

OK, consider a single excited hydrogen atom in a perfectly
reflecting box. Has it emitted a photon or not?  QM will
predict a superposition of photon+H and H-excited in which
the amplitude for H-excited decays exponentially with time.
  But the exponential decay is only approximate it actually
decays, not to zero, but to a small value corresponding the
an equilibrium state in which the probability of the photon
being emitted is balanced by the probability of it being
reabsorbed.  So is what is in the box - photon+H or
H-excited?  The answer is, in effect, both. What's in the
box is the wave-function that describes the superposition.

You can't get the paradox back by supposing there's a video
camera watching the state, because the video camera is a
macroscopic object with so many degrees of freedom that
when it detects the photon the system (camera + H + photon)
will go into a superposition of states which, when
projected onto (emitted, not-emitted) will be essentially
diagonal, i.e. the wave-function will be collapsed.

Ok, I think I see where my mistake was. I was thinking that decoherence 
just referred to interactions between a system and the external environment, 
but what you seem to be saying is that it can also refer to an internal 
effect where interactions among the components of a system with many degrees 
of freedom cause interference terms to become negligible. If that's correct, 
then when Wigner decided that interference would cause the wavefunction of 
the cat or Wigner's friend to collapse even before the box or the room was 
opened, then he was probably referring to this sort of internal effect, so 
my argument about using quantum computers to simulate truly impenetrable 
boxes would not make a difference.

But this makes me wonder about the thought-experiment by David Deutsch which 
Hal Finney brought up, in which interference shows that an isolated A.I. was 
splitting into multiple versions which experienced different outcomes. 
Presumably a simulation of an intelligence would have a lot of degrees of 
freedom too, so why wouldn't decoherence ruin things? Maybe since this is a 
computer simulation where we know the dynamical rules and initial state 
precisely, we would know just where to look for even the smallest 
interference effects, unlike in an ordinary macroscopic system where we 
don't have such detailed information. Also, we could run such a simulation 
over and over again from the same initial conditions, which would also help 
to detect small statistical deviations from classical predictions. I once 
read a comment by Deutch about decoherence where he said something like 
(paraphrasing) saying the interference terms are 'almost' zero is like 
saying someone is a little bit pregnant. His argument would probably be 
that although decoherence may explain why the world looks approximately 
classical in the many-worlds framework, it doesn't remove to postulate those 
other worlds in the first place.

--Jesse

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