A Is the Many Worlds Interpretation Widely Accepted Among Physicists?

[stevendaryl]

MWI, as I have heard the term used, suggests many independent worlds that continuously separate from each other and never recombine.

No. In the original development of MWI by Everett, he never assumed anything other than unitary evolution according to Schrodinger's equation. The thing about worlds "splitting off from one another and never recombining" is not an additional assumption; it follows from decoherence. If you have a superposition of two states, \alpha |\phi_1\rangle + \beta |\phi_2 \rangle, the only way to "observe" such a superposition is through interference. To have interference effects, you need to have a "final state" |\psi_{final}\rangle such that both

|\phi_1\rangle \Rightarrow |\psi_{final}\rangle

and

|\phi_2\rangle \Rightarrow |\psi_{final}\rangle

have non-negligible probabilities. But if |\psi_1\rangle and |\psi_2\rangle are macroscopically distinguishable, then there is no such final state |\psi_{final}\rangle reachable from both. If you make a macroscopic change, such as a cat dying, then from then on, the world will be different because that cat died. So effectively, macroscopically distinguishable states never recombine. So you might as well treat them as separate possible worlds. But there is no need for any special, irreversible "splitting" process.

I think an easy contrast between MWI vs. non-MWI is the Schrodinger Cat. An MWI proponent would say that there are independent instances of both a dead cat and live one. I would say that the experiment is impossible. You can't separate the cat or the radioactive decay particle from the rest of the universe.

That's exactly what MWI says. Rather than a superposition of a live cat and a dead cat (which is so unstable as to be practically impossible), the situation evolves into an either/or of (1) a world in which the cat is alive, and (2) a world in which the cat is dead.

A decay decision is imposed on the particle based on its participation in the universe and the result is a cat that is definitely dead or alive - but not both. The interaction between the cat and the particle itself affects the timing of the particle decay. The reason I say this is that the alternative is to say that the result seen when the box is open is the result of a decision made with information that did not exist before the experiment started - that a parameter was added to the universe. In MWI, this parameter is what would differentiate between one world and another. In non-MWI, superpositioning never creates a state with information other than that it started with.

I'm not sure I understand the distinction you are making. The idea behind MWI is just ordinary quantum mechanics, but where you move the boundary of what you consider "the system" to include more and more of the universe. You can either view

• the atom of uranium as "the system", and the cyanide canister is measurement device, or
• the atom + cyanide canister is the system, and the cat is an observer, or
• the atom + cyanide canister + cat is the system, and the person opening the box is the observer, or
• the atom + cynanide canister + cat + person is the system, and a second person is the observer, or
• ...
• the entire universe is the system

However, if you want to describe the system using a pure state (wave function) as opposed to a mixed state (density matrix), then that is only possible if the system is (at least temporarily) isolated. Only at the very small (a single atom) and the very large (the whole universe) can the system be considered isolated, so the use of pure states only applies at those two levels.

I don't know what creates apparent "collapse", but my thought is that some inconsistencies occur as the wave function involves more and more mass. The problem with this "inconsistency" notion, is that it presumes that ultimately only one path will prove out to be free of inconsistencies - but I can't imagine anything that would drive that number to 1 rather than 0 or some large number. Still, something drives events towards apparent collapse. What we observe and remember are fully collapsed events - not a blurred unresolved super-positioned history seen with a blurred unresolved super-positioned mind.

Well, MWI definitely doesn't predict that anyone sees blurred superpositions.

 

[Demystifier]

Planetology is usually applied classical (non-quantum) physics, though.

So is cosmology (except at very early times).

 

[Demystifier]

Well, I suppose the evolution of a galaxy would fit into the Copenhagen Interpretation if you viewed the first 14 billion years as a very time-consuming experiment, and assume that there is no definite state of the galaxy until astronomers evolve to study it.

Exactly!

 

[secur]

People figure if that's the best real scientists can do, they can do better with no knowledge at all - and they're right.

They are not right - it is only that the (perhaps) best real scientists do to turn science into vey simplified but animated popular science that dispenses with formulas. They are usually told by the scientists that there is a BIG difference between real science (whose language is math) and popular science (whose language is plain English) - but they ignore this important extra piece of information.

I am convinced that pop science makes much more good than harm. Yes, it creates a few crackpots, but they are an exception rather than a rule. Most people who read popular science do not become crackpots. Instead, they become people with some rough understanding of science aware of the fact that their understanding is only rough. Besides, some of them eventually become true scientists themselves, precisely because they initially got interested in science by pop-science books.

Certainly a lot of popular science books are good. For instance "The Milky Way" by Bart Bok; "Road to Reality" by Roger Penrose. Even the bad ones are, perhaps, 2/3 good. But the errors (mostly in philosophy not physics) are terrible. For instance David Deutsch in "Beginning of Infinity" actually tries to "prove" that infinity exists in the real world! Clearly he's never even heard of Aristotle, Kant, and others who long ago dissected his elementary errors. Richard Feynman in "Lectures" said (paraphrasing, don't have the book handy) that QED "explains why the cream in your coffee swirls, it even explains why you like cream in your coffee." Then there's "Quantum Suicide" - 'nuff said.

You both feel that modern physics is basically in great shape and little peccadilloes like these can be shrugged off as harmless. Whereas I see a big problem: hubristic rejection of real-world experimental data in favor of sheer fantasy.

Such books (and attitudes) do, indeed, help attract the next generation of scientists. But they repel the cream of the crop.

 

[atyy]

Does MWI assume classical spacetime?

 

[Demystifier]

Certainly a lot of popular science books are good. For instance "The Milky Way" by Bart Bok; "Road to Reality" by Roger Penrose.

The "Road to Reality" is really good, but I wouldn't classify it as popular. It requires certain mathematical literacy. I would call it semi-popular.

 

[Demystifier]

Does MWI assume classical spacetime?

It depends. You can apply MWI to any quantum theory, from non-relativistic QM to quantum gravity.

 

[stevendaryl]

You both feel that modern physics is basically in great shape and little peccadilloes like these can be shrugged off as harmless. Whereas I see a big problem: hubristic rejection of real-world experimental data in favor of sheer fantasy.

I'm probably going to be sorry that I asked, but what's an example of real-world experimental data that is being rejected by physicists out of hubris?

 

[stevendaryl]

Does MWI assume classical spacetime?

MWI really amounts to an attempt to apply quantum mechanics to the entire universe, instead of splitting the universe into observer + system. I don't think that it depends on any particular assumptions about what the universe is like, other than that it can be described by quantum mechanics.

However, there is a huge problem with MWI, which is reconciling its picture of amplitudes evolving unitarily with the way the world appears to us. It seems possible to me that we have to assume something special about the universe in order for MWI to reproduce a world like ours (with macroscopic objects having approximately definite positions and momenta, and with microscopic objects obeying the Born rule for probabilities).