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Author Topic: Why there are no Multiple Universes
Kent
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I know this will ruin some sci-fi stories for you, but this article on the uncertainty principle is an explanation of quantum mechanics that I never got from my college professors.

quote:
This is why I say that the standard Many-Worlds language about "separate universes" is pernicious and misleading. What's going on here is not really a photon splitting into two photons in "separate universes," one taking each path, it's a photon wavefunction that is in a superposition state with random phases between the two pieces.

Why do we talk about decoherence as if it produced "separate universes?" It's really a matter of mathematical convenience. If you really wanted to be perverse, and keep track of absolutely everything, the proper description is a really huge wavefunction including that includes pieces for both photon paths, and also pieces for all of the possible outcomes of all of the possible interactions for each piece of the photon wavefunction as it travels along the path. You'd run out of ink and paper pretty quickly if you tried to write all of that down.

Since the end result is indistinguishable from a situation in which you have particles that took one of two definite paths, it's much easier to think of it that way. And since those two paths no longer seem to exert any influence on one another-- the probability is 50% for each detector, no matter what you do to the relative lengths-- it's as if those two possibilities exist in "separate universes," with no communication between them.

In reality, though, there are no separate universes. There's a single wavefunction, in a superposition of many states, with the number of states involved increasing exponentially all the time. The sheer complexity of it prevents us from seeing the clean and obvious interference effects that are the signature of quantum behavior, but that's really only a practical limitation.

Questions of the form "At what point does such-and-so situation cause the creation of a new universe?" are thus really asking "At what point does such-and-so situation stop leading to detectable interference between branches of the wavefunction?" The answer is, pretty much, "Whenever the random phase shifts between those branches build up to the point where they're large enough to obscure the interference." Which is both kind of circular and highly dependent on the specifics of the situation in question, but it's the best I can do.


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scifibum
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quote:
What's going on here is not really a photon splitting into two photons in "separate universes," one taking each path, it's a photon wavefunction that is in a superposition state with random phases between the two pieces.
It strikes me that the untrue interpretation, according to this guy, is much easier to understand (keeping acceptance in abeyance) than what he says is actually true.
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Lina Inverse
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If you may allow me to nit-pick for a moment, nothing in that article disproves the existence of multiple universes--it only says that the existence of multiple universes cannot be inferred by the wave-particle properties of light.

That is a really good article, though--I've pretty much given up on really understanding this stuff, but this article is very clear and comprehensible.

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threads
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quote:
In reality, though, there are no separate universes. There's a single wavefunction, in a superposition of many states, with the number of states involved increasing exponentially all the time. The sheer complexity of it prevents us from seeing the clean and obvious interference effects that are the signature of quantum behavior, but that's really only a practical limitation.
AFAIK, many-worlds still posits one wavefunction. "Universe" is a term used to refer to a branch of the wavefunction. It seems appropriate since the branches (or "universes") don't have a casual effect on each other.

Though I'm not any sort of authority on the matter.

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vulture
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The idea of Everett's "many worlds" interpretation of QM was to get around the problem of what happens when an observation is made. The Copenhagen interpretation ways that when an observation is made (the definition of which is vague) the wavefunction collapses from a wide range of possibilities to the single observed possibility. This posits something outside the scope of the mechanics of QM happening at that point - the evolution of the wavefunction is entirely deterministic until an observation is made, when something 'magic' happens.

Everett didn't like this, and wanted to keep the evolution of the wavefunction continuous even at the point of observation. To do this he posited that the wavefunction continued to evolve according the rules of QM, but an observer was limited to observing those portions of it consistent with his observations. The original version had one observer for each possible outcome, although other modifications have been proposed. Of course, all this is really doing is shunting the magic off somewhere else to keep the physical wavefunction sacrosanct. It's all a question of what you think is more fundamental I guess.

The only difference between the two interpretations is in the entirely unobservable effects on observers / wavefunctions when a (badly defined) observation is made.

There is certainly no requirement that many worlds is true, no matter how much sci-fi writers may like it.

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Ron Lambert
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I think that the primary arguments against the multiple-universe idea are that:

(1) It is wasteful (creating a whole additional universe of matter and energy every time you flip a coin, etc.)

(2) There can be no moral right and wrong, because even if you choose to do good, there will always be an alternate universe where you chose to do evil, so there really is no free will, and so no moral accountability.

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vulture
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quote:
Originally posted by Ron Lambert:
I think that the primary arguments against the multiple-universe idea are that:

(1) It is wasteful (creating a whole additional universe of matter and energy every time you flip a coin, etc.)



Rather extravagantly wasteful. It doesn't so much violate Occam's razor as batter it with a brick

quote:

(2) There can be no moral right and wrong, because even if you choose to do good, there will always be an alternate universe where you chose to do evil, so there really is no free will, and so no moral accountability.

I'm not sure that that counts as an argument as what is physically possible though [Smile]

Actually, I disagree with the premise of the argument anyway. In order for there to be universes where I have done some specific evil, it has to be possible for me to do that specific evil. But is it possible? The mere fact that we are people with remarkably consistent personalities suggests that the range of possible actions that I am realistically choosing between very often don't include me (or Ron) doing evil. I have never killed anyone, despite potentially millions of moments where I might have chosen to do so (in circumstances where other people have chosen to commit murder). I am consistently someone who has made the same choice every time. So, if the many worlds interpretation is true (which I personally think is total bunk), then can there be a universe (or a great many universes) in which I have committed murder? Doesn't seem that likely - the chance in any given moment is at best (worst) miniscule, and could well be non-existent, simply because I am not the kind of person who would chose to do that.

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threads
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quote:
Originally posted by vulture:
quote:
Originally posted by Ron Lambert:
I think that the primary arguments against the multiple-universe idea are that:

(1) It is wasteful (creating a whole additional universe of matter and energy every time you flip a coin, etc.)



Rather extravagantly wasteful. It doesn't so much violate Occam's razor as batter it with a brick
From Wikipedia:
quote:
MWI, being a decoherent formulation, is axiomatically more streamlined than the Copenhagen and other collapse interpretations; and thus favoured under certain interpretations of Ockham's razor. Of course there are other decoherent interpretations that also possess this advantage with respect to the collapse interpretations.
The Copenhagan interpretation requires an additional premise that the wavefunction actually collapses.
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vulture
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quote:
Originally posted by threads:
From Wikipedia:
quote:
MWI, being a decoherent formulation, is axiomatically more streamlined than the Copenhagen and other collapse interpretations; and thus favoured under certain interpretations of Ockham's razor. Of course there are other decoherent interpretations that also possess this advantage with respect to the collapse interpretations.
The Copenhagan interpretation requires an additional premise that the wavefunction actually collapses.
The Copenhagen interpretation is bunk also IMHO [Smile] . And the use of Occam's razor in favour of the MWI is a bit rich - it's kind of like Pascal's wager: it only looks convincing if you accept it's basic premise. Namely, if you have a problem with the idea of the collapse of the wavefunction, and find the MWI attractive for that reason, then yes, the fact that it avoids the collapse of the wavefunction might look like a simplification of something unnecessarily complex in the Copenhagen interpretation.

Conversely, if you find the idea of multiple universes fundamentally silly, then it is much 'simpler' in Ockham's sense to work on the assumption they don't exist.

FWIW I've never met a scientist yet who cared much one way or the other on the subject. Most who actually use QM just accept that it is a recipe for making accurate numerical predictions, and live with the fact that none of the proposed interpretations of QM is either satisfactory or complete. And that talking about it is so much unverifiable hot air that will all come crashing down when someone finally comes up with a self-consistent theory of mechanics that underlies QM, and actually shows what is going on at the QM level (glossing over the whole issue of the relationships between mathematical models and the real world...)

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Kent
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I just accept it on faith too vulture. [Smile]
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