[angryprofessor]

>Universes do not split like that. That would mean many universes were being created from nothing every instant. What actually happens is there are already infinitely many "universes" and they become different. And not one per outcome, but in proportion to the probability of outcomes.

It's not the writers of the article who have no clue. It's quantum physicists themselves.

In the Copenhagen interpretation of QM, a conscious observer induces wave collapse (a random process). In this interpretation, there are no other worlds. In Wave Collapse/Many Worlds, a parallel universe is created in which wave collapse occured but gave a different outcome.

There are different interpretations of QM (e.g. bohmian mechanics or GRW/stochastic collapse) which don't have this problem. If you are interested in QM, I strongly suggest looking these theories up. But Copenhagen QM (the theory in most textbooks) does have this problem.

The reporter did an adequate job of explaining the consensus view. The problem is really that the consensus view can allow for nonsense such as what was described in the article.

[pfedor]

"There are different interpretations of QM (e.g. bohmian mechanics or GRW/stochastic collapse) which don't have this problem. If you are interested in QM, I strongly suggest looking these theories up."

I respectfully disagree. You can spend years reading about interpretations and the gain in terms of your understanding of Quantum Mechanics will be zero. For anyone interested in Quantum Mechanics, I strongly suggest actually trying to learn it, instead of dabbling into confusing popular articles the authors of the majority of which have never studied theoretical physics themselves.

Please note I'm not saying that you don't know Quantum Mechanics. I just think your advice is bad.

So here's my advice to anyone out there who feels that they want to know more. See the Schroedinger equation. Learn how to calculate energy levels for the harmonic oscillator and for the hydrogen atom. Read first six chapter's of Paul Dirac's "The principles of quantum mechanics". The time spent reading about interpretations and philosophical implications of QM is wasted. People who do research in theoretical physics devote very little time to such considerations, or no at all.

[curi]

In Wave Collapse/Many Worlds, a parallel universe is created in which wave collapse occured but gave a different outcome.

That is incorrect. Many Worlds does not say universes are created all the time, it says they already exist and become differentiated.

BTW all the other interpretations are ridiculous attempts to avoid facing counter-intuitive facts. See, for example, David Deutsch's speech at the recent Everett conference http://www.philosophy.ox.ac.uk/misc/everett/Deutsch%20-%20Ap...

[hugh]

<i>That is incorrect. Many Worlds does not say universes are created all the time, it says they already exist and become differentiated.</i>

Actually there's no real consensus on that point, even among many-worlds theorists. Some folks suggest that universes "split", others suggest that they "become differentiated", and a third set of folks say that they do something which isn't really properly described by either of the above options. Deutsch has one of the more widely publicised many-worlds interpretations, but it's not the only one out there, and it's not necessarily the most widely accepted.

[curi]

I don't really care what views are popular, (though MWI is popular among top experts to whom it matters), I care which views are true. So what matters to me is which proponents say things that make sense and give good explanations and arguments. Deutsch does, including a powerful and concise argument in chapter 2 of his book. When someone gives a reasonable counter argument then I will reconsider, but that has not happened.

And, of course, Deutsch isn't the only example of the MWI side making more sense. Here is another source:

http://www.hedweb.com/manworld.htm

Edit: If you can give a non-ridiculous summary of any other interpretation, go ahead...

[angryprofessor]

Some of the many worlds variants make sense. I prefer bohmian mechanics (I can reason about it better), but I have no objection to MW. Ultimately I don't care one way or the other: the predictions for MY universe are the same, whether or not it is unique or one amoung many.

However, the point I was making is that most physicists believe in wave collapse. The writer of the article was simply providing a reasonably good summary of QM with wave collapse. According to Q1 of your article, this is still a mainstream belief except amoung string theorists, apparently. I agree, it's almost certainly wrong. But that's not yet a mainstream position.

As for other non-ridiculous interpretations, just look at bohmian mechanics or GRW. Bohmian mechanics postulates that particles exist and are guided by the wave function. The testable physical predictions are identical to many worlds. GRW makes different physical predictions (possibly testable in the near future), but the theory makes sense as well.

[curi]

There are two varieties of bohm interpretation:

one says that there are "hidden variables". what this means is all particles store information, from the start of existence, about all other particles they will ever have anything to do with. how they store this vast amount of information isn't explained. then, they act on the information. then somehow either this information is used by some physical process to follow the SE (Schrodinger Equation), or they don't bother with that and say the SE is sometimes false for some reason or other. notice all the parts where some explanation is left out. that makes it ridiculous to call this a reasonable alternative to MWI. MWI does flesh out details.

the other version -- the pilot wave theory -- says there are special particles that go along one groove of the wave function: our universe. this is a ridiculous attempt to make up some special status for our universe.

GRW is about the wave function randomly collapsing when there's enough information flow we might have to face the prospect of other universe-sized-things. why random? b/c that's better than saying "when we want it to so we can avoid MWI" and better than copenhagen's "when people are watching". how do they try to get away with this? well, mostly universes don't affect each other. so basically any time a universe isn't going to affect ours anymore, a "random" collapse occurs, and it disappears. this doesn't break things too too badly b/c ... it's a bit like saying anything so far away we can't see it, doesn't exist. frustrating to disprove. but still ridiculous.

[hugh]

My point is that there are other many-worlds interpretations apart from Deutsch's, which differ somewhat on philosophical issues like whether words really "split", as described above.

The main problem with them is that they're all a bit vague and handwavey -- the world is still waiting for somebody to come up with a truly rigorous treatment of many-worlds and its consequences, and how it works in non-toy systems (i.e. systems which don't just consist of a single observer observing a single observable).

The other problem is that most people writing on MWIs don't really acknowledge the existence of other MWIs, with the effect that everybody winds up talking past each other. Finally, it doesn't help that Everett's writings are so vague that we can't even figure out what he himself was saying.

Personally, I suspect that MWI is likely to be true, and that other interpretations have insurmountable flaws, but I'd be very careful about going around saying everything else is "ridiculous".

[curi]

You're right that I should be careful with what I call ridiculous. I am.

(Will now explain more details in response to the other guy's comment where he proposes bohm and grw theories as non-ridiculous)

[hugh]

Oh, and by the way, the MWI FAQ you posted actually contradicts you on this point, it seems:

"Q19 Do worlds differentiate or split?... Worlds do not exist in a quantum superposition independently of each other before they decohere or split. The splitting is a physical process, grounded in the dynamical evolution of the wave vector, not a matter of philosophical, linguistic or mental convenience"

Which proves my point about how there are a several different MWIs.

[curi]

Heh, you're right that it says that. That's a shame, it's largely good.