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How the Many-Worlds Theory of Hugh Everett Split the Universe

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Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#11

Post by doubleunplussed »

lisper wrote: post #9
platz wrote: post #7

Earlier quoted context omitted.

That characterization of Woit does not appear quite accurate. Woit's claim may even be bolder, in that he claims that there is no functional difference between MWI and Copenhagen. > What strikes me when thinking about these two supposedly very different points of view on quantum mechanics is that I’m having trouble seeing why they are actually any different at all.

The main difference is one of rhetoric: Copenhagen insists that experiments have single outcomes. MWI insists that experiments have multiple outcomes in point of metaphysical fact. Another difference is that the MWI has to explain the Born rule (which is problematic), whereas Copenhagen simply takes it as an axiom. http://blog.rongarret.info/2019/07/the-trouble-with-many-wor...

It should be noted though that few interpretations have an explanation for the born rule.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#13

Post by lisper »

doubleunplussed wrote: post #11
lisper wrote: post #9

Earlier quoted context omitted.

The main difference is one of rhetoric: Copenhagen insists that experiments have single outcomes. MWI insists that experiments have multiple outcomes in point of metaphysical fact. Another difference is that the MWI has to explain the Born rule (which is problematic), whereas Copenhagen simply takes it as an axiom. http://blog.rongarret.info/2019/07/the-trouble-with-many-wor...

It should be noted though that few interpretations have an explanation for the born rule.

Right, most of them just take it as an axiom. But the MWI explicitly rejects the Born rule as an axiom, insisting that the Schrödinger equation is all the actual physics there is.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#14

Post by platz »

lisper wrote: post #9
platz wrote: post #7

Earlier quoted context omitted.

That characterization of Woit does not appear quite accurate. Woit's claim may even be bolder, in that he claims that there is no functional difference between MWI and Copenhagen. > What strikes me when thinking about these two supposedly very different points of view on quantum mechanics is that I’m having trouble seeing why they are actually any different at all.

The main difference is one of rhetoric: Copenhagen insists that experiments have single outcomes. MWI insists that experiments have multiple outcomes in point of metaphysical fact. Another difference is that the MWI has to explain the Born rule (which is problematic), whereas Copenhagen simply takes it as an axiom. http://blog.rongarret.info/2019/07/the-trouble-with-many-wor...

> The main difference is that the MWI has trouble explaining the Born rule.

To re-phrase, the claim in that article is that once MWI has done away with the Born rule, it has to deal with how the probabilities that we find in experiment are calculated under MWI.

.. Which is not obvious at first glance, because MWI seems to suggest everything happens with probability = 1.

Carroll acknowledges it is one of the challenges against MWI that needs to be answered.

Carroll offers "self-location uncertainty" as a way to recover the probabilities predicted by the Born Rule despite the determinism of the Schrodinger Equation

> But even if both people know the wave function of the universe, there is now something they don’t know: which branch of the wave function they are on. There will inevitably be a period of time after branching occurs but before the observers find out what outcome was obtained on their branch. They don’t know where they are in the wave function. That’s self-locating uncertainty, as first emphasized in the quantum context by the physicist Lev Vaidman. (https://news.ycombinator.com/item?id=20927410)

* edit - OP's post was edited but i'm leaving this post as-is.

> MWI insists that experiments have multiple outcomes in point of metaphysical fact.

That doesn't seem right. under MWI there is only one Wafefunction and one big quantum state. There are not additional 'worlds'

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#15

Post by lisper »

platz wrote: post #14
lisper wrote: post #9

Earlier quoted context omitted.

The main difference is one of rhetoric: Copenhagen insists that experiments have single outcomes. MWI insists that experiments have multiple outcomes in point of metaphysical fact. Another difference is that the MWI has to explain the Born rule (which is problematic), whereas Copenhagen simply takes it as an axiom. http://blog.rongarret.info/2019/07/the-trouble-with-many-wor...

> The main difference is that the MWI has trouble explaining the Born rule. To re-phrase, the claim in that article is that once MWI has done away with the Born rule, it has to deal with how the probabilities that we find in experiment are calculated under MWI. .. Which is not obvious at first glance, because MWI seems to suggest everything happens with probability = 1. Carroll acknowledges it is one of the challenge…

> OP's post was edited

For the record, the original just said (as quoted in the above response) "The main difference is that the MWI has trouble explaining the Born rule." Upon reflection I decided that wasn't really the main difference, that the main difference is the difference in ontology. But the fact that the MWI has to recover the Born rule from the linear dynamics is a major consequence of this difference in ontology.

The jury is still out about whether or not self-location uncertainty actually allows the Born rule to be recovered without begging the question. If it works, it would be a major breakthrough. Personally, I'll give long odds against.

[UPDATE]:

> There are not additional 'worlds'

Um, what do you think that the "MW" in "MWI" stands for?

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#16

Post by platz »

lisper wrote: post #15
platz wrote: post #14

Earlier quoted context omitted.

> The main difference is that the MWI has trouble explaining the Born rule. To re-phrase, the claim in that article is that once MWI has done away with the Born rule, it has to deal with how the probabilities that we find in experiment are calculated under MWI. .. Which is not obvious at first glance, because MWI seems to suggest everything happens with probability = 1. Carroll acknowledges it is one of the challenge…

> OP's post was edited For the record, the original just said (as quoted in the above response) "The main difference is that the MWI has trouble explaining the Born rule." Upon reflection I decided that wasn't really the main difference, that the main difference is the difference in ontology. But the fact that the MWI has to recover the Born rule from the linear dynamics is a major consequence of this difference in o…

> the main difference is the difference in ontology

Sure, I'm not sure how much Science has to say about ontology but I'm interested in the outcome.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#17

Post by lmm »

lisper wrote: post #9
platz wrote: post #7

Earlier quoted context omitted.

That characterization of Woit does not appear quite accurate. Woit's claim may even be bolder, in that he claims that there is no functional difference between MWI and Copenhagen. > What strikes me when thinking about these two supposedly very different points of view on quantum mechanics is that I’m having trouble seeing why they are actually any different at all.

The main difference is one of rhetoric: Copenhagen insists that experiments have single outcomes. MWI insists that experiments have multiple outcomes in point of metaphysical fact. Another difference is that the MWI has to explain the Born rule (which is problematic), whereas Copenhagen simply takes it as an axiom. http://blog.rongarret.info/2019/07/the-trouble-with-many-wor...

Conversely, Copenhagen-like explanations need to explain (or postulate) a decoherence/collapse operation that violates all the usual rules of QM: it would be non-time-symmetric, non-unitary, likely non-local (propagating faster than light), ...

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#19

Post by smackay »

fallingfrog wrote: post #8

You can do a really simple thought experiment that provides a persuasive argument in favor of the many worlds theory. Or at least, it shows that wave function collapse is relative to an observer, not universal everywhere. Start with Schroedingers cat: the cat is in a box, it’s in a state of quantum superposition. It’s 50/50 alive or dead. Now a researcher opens the box: the cat is found to be either alive, or dead, a…

But surely the first researcher is also in a state of quantum superposition and they are 50/50 opening/not opening the box.

So how does time come into the equation? And does that mean that any observation is localised and there's no need for many worlds except in unobserved areas of the universe?

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#20

Post by lmm »

lisper wrote: post #13
doubleunplussed wrote: post #11

Earlier quoted context omitted.

It should be noted though that few interpretations have an explanation for the born rule.

Right, most of them just take it as an axiom. But the MWI explicitly rejects the Born rule as an axiom, insisting that the Schrödinger equation is all the actual physics there is.

MWI is not incompatible with postulating the Born rule if you want to. It just doesn't elevate it to an axiom of the theory. It's like doing constructive mathematics: you're not denying the law of excluded middle, you can postulate it and it will have the usual consequences, but the core theory doesn't rely on it, and it's possible to hope for a more elegant alternative.
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