X-Message-Number: 17976
Date: Mon, 19 Nov 2001 21:24:05 -0700
From: Mike Perry <>
Subject: Re: MWI and waves

Robert Ettinger (#17962) made some comments on MWI and waves I'd like to 
respond to; Brian Wowk gave permission to quote from a private message of 
his. What follows is a sort of dialogue between the three of us.

Ettinger: I have not been able to get an answer from Deutsch [author of 
*The Fabric of reality* on the many-worlds hypothesis--MP] on the 
following--possibly because my question is just too stupid, who knows. 
Anyway: In "The Fabric of Reality" Deutsch seems to rely for proof of MWI 
mainly on single-photon interference. (You get different diffraction and 
interference patterns with different arrangements of slits, even if only a 
single photon at a time is involved.) He says that "shadow" photons (in 
other universes) interfere with "tangible" photons (in our universe). 
(Likewise with all objects or systems of any kind and any size.)

Perry: It's important to keep in mind, though, that really "shadow" is on 
an equal footing with "tangible"--other universes versus ours, with all of 
them equally "real."

Ettinger: Well, the first problem is that--as far as I can see--he offers 
no mechanism for this interference. Interference is classically 
understandable as a wave phenomenon, but he never mentions waves, just one 
photon interfering with another. This doesn't cut it. Secondly, there is no 
quantitative discussion. Perhaps this was because the book was intended for 
popular reading, but there should at least have been some reference to it.

Deutsch was being very colloquial (probably too colloquial) in discussing 
interference with photons in alternate universes. What's really going on is 
good old fashioned classical wave interference. The central tenet of MWI is 
that the entirety of reality consists of interfering quantum wavefunctions 
that never collapse. The real question is why, then, do we see discrete 
photons at all? This has actually been worked in quantitative detail. It 
turns out to be a decoherence process. See for example Zurek's Physics 
Today article
Roughly speaking, different parts of a wavefront impinging on different 
parts of a detector screen trigger wave processes within the screen that 
become decoherent with each other. The end result is that only wave 
processes corresponding to an infinitesimal portion of the original wave 
(i.e. a photon) retain coherence and the ability to be perceived by a 
single macroscopic observer. Of course this observer coexists with a 
plurality of decoherent parallel observers that have observed their photon 
landing on a different part of the screen. In other words, interfering 
particles is the wrong picture.

How many shadow photons interfere with each tangible photon? Do tangible 
photons also interfere with each other? If there are an infinite number of 
identical universes, as well as other infinities of different ones, as he 
appears to say, this quantitative question seems formidable, even though 
infinities can be ordered in the sense of measure

Perry: There is no reason to assume more than one identical 
universe--identity of indiscernibles.

Ettinger: One main argument of the MWI people is that a particle 
classically cannot interfere with itself, and yet this appears to happen. 
But we know there are so-called basic quantons (such as photons and 
electrons) and phenomenological quantons, such as phonons and several 
others. Phonons act like particles in certain ways, but result from wave 
phenomena in ordinary material media.

Perry: Even "ordinary" material is subject to quantum mechanics, like 
everything else as far as we know. Phonons in certain circumstances may 
behave enough like particles that a phonon quantum computer is possible. 
But this would not have to overturn MWI.

Ettinger: As far as quantum computation is concerned, if all quantons are 
phenomenological, then perhaps we are just dealing with another kind of 
analog computer, which can indeed work tremendously faster than an ordinary 
digital computer.

Perry: As I understand it, analog computation assumes, more or less, that 
quantities (voltages, for instance, which play an important role) are 
infinitely divisible, something at variance with quantum mechanics. An 
analog computer may be faster than a digital counterpart on certain 
problems. But there is a limit to this advantage, because you really don't 
have infinitely divisible quantities or perfect accuracy. The upshot is: 
there is no algorithm that would run in polynomial time on an analog 
computer that would not also run in polynomial time on a digital computer. 
This is not true of the quantum computer (we think); for example number 
factorization can be done in polynomial time (linear time actually) on a 
quantum computer but not as far as we know on a classical (digital) computer.

Ettinger: I remind readers that if MWI (or the Deutsch version of it) is 
correct, then apparently there always have been and always will be infinite 
numbers of identical copies of you,

Perry: Again, identity of indiscernibles makes it unnecessary to assume 
more than one *identical* copy of you exists at any given instant (though 
the full implications of this are rather profound).

Ettinger: as well as infinities of nearly identical copies, and further 
infinities of miserable, tortured ones, and other (smaller) infinities of 
blissful ones.

Perry: In other words, it seems that beings are more likely, perhaps 
infinitely so, to be miserable than blissful. Not at all the way I see it.

Ettinger: You are already immortal, very happy and also very unhappy, no 
matter what you do or don't do.

Perry: No, you are already immortal, yes, but what you are is tied to your 
perceptions--no more or less. In my book I argue that, while all can expect 
an ultimately happy outcome (for reasons discussed) the path to the happy 
outcome is *by no means* guaranteed smooth, short, or pleasant. What you do 
does make a difference. I make a case for the advantage of choosing 
cryonics over alternatives, in terms of the path you can expect to follow 
(ch. 13).

Mike Perry

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