The many worlds interpretation, if taken at all literally (rather than being taken as an instrumental interpretation strictly of the mathematics involved), strikes me as completely ridiculous.

Of course, I have pretty much a logical positivist bent on such things.

The consensus from my fairly recent read of Max Tegmark's Multiverse book among physicists is that the MWI is the correct approach — Question

Not only not the consensus, but apparently not even the majority view.

I find it hard to concieve the MWI due to the rather infinite amount of realities there may be; but, so do many mathematicians have qualms with dealing with real infinities. — Question

Well, the idea that the universe is spatially infinite was commonplace throughout the history of thought, and among today's cosmologists this is probably much closer to a consensus. And that doesn't even require the acceptance of any particular interpretation of quantum mechanics. So I don't think the infinitude of the world - or worlds - is all that controversial.

A quasi deterministic universe always seems more appealing; but, why can't we have determinism within a many worlds interpretation. — Question

MWI is a deterministic theory (in a way).

An infinity here, an infinity there, and pretty soon you're talking Big Science. Quantum mechanics are formulated in infinite Hilbert spaces or universes, but the mathematics also display no preference for the arrow of time. The implication, as far as I'm concerned, is that we are observing a universal recursion in the law of identity and there is no humanly discernible explanation. We are using nature to study nature, while the void laughs in our faces, yet, like ants climbing the Empire State building we cling to our belief that we can grasp the reality of our situation by merely climbing higher. Only God can see the back of their own head without using a mirror and when we no longer make distinctions between who we are and what we are doing we embody the truth. That makes life and the laws of physics metaphorical rather than metaphysical and there should be times when we perceive both nonlinear spatial and temporal effects that resemble universes and different times merging, thus, conflating the identities of space and time in every way imaginable.

I don't. Mostly because I don't see what it adds to Copenhagen interpretation, and Copenhagen interpretation is what we focused on several years back in the class where we learned about such things.

But I've been out of the loop on that for a long time, too.

I went to a live talk by a bloke called Marcus du Sautoy only 24 hours ago, in which he argued among other interesting things that the Many Worlds Interpretation was to him a good argument against intelligent design. Of all the gin joints in all the worlds, every equation and constant necessary for life is present in this one gin joint world we're in, while there are zillions in which the math doesn't add up. I'm thinking about it :)

I'm interested in getting a straight answer to the question, "if 'many worlds' is the solution, what's the problem?' I asked that on Physics Forum, which produced various convoluted responses, before the thread was locked.

For those who haven't seen it, an excellent article on Hugh Everett III in Scientific American.

A report on a straw poll of physicists from Sean Carroll, 'The Most Embarraing Graph in Quantum Physics', showing that the Copenhagen Interpretation is still most popular. (I'm currently reading Manjit Kumar's excellent history of QM and am just up to the section where Heisenberg and Bohr are not speaking on account their differences over interpretation.)

I posed an interesting question some time ago to those interested about whether QM obeys causality.

https://www.physicsforums.com/threads/does-quantum-mechanics-obey-causality.881156/

The opinions were interesting.

Never knew the CI was still alive and well.

The problem is more of a question -- while we can predict various phenomena using QM, what do the postulates and predictions of QM indicate about the nature of nature/reality?

Initially the equations developed in QM didn't predict anything as much as they resolved certain paradoxes. The structure of the atom was the question.

But the solution presented seemed to contradict a number of beliefs that one would draw from classical physics and thermodynamics. And, furthermore, seemed to border on the incoherent -- and certainly contradicted leading theories of the atom at the time.

I find the idea of decoherence too at odds with the MWI to take the MWI seriously.

Mind you, under the MWI, there is no decoherence. Every reality is essentially a decoherence from the original state (big bang) to the present.

I canno't grasp of a universe without decoherence given how macroscopic events are deterministic and at odds with the randomness and indeterminacy of QM.

That's not really a straight answer, though! I'll take the plunge: I think the factor which motivated Everett was this:

[The Copenhagen] approach privileges the external observer, placing that observer in a classical realm that is distinct from the quantum realm of the object observed.

(From the Scientific American profile.)

Now, I think it was intolerable for there to be a suggestion that 'the mind of the observer' has a role in the outcome. After all, that torpedoes the whole principle of objectivity. But the only way to get rid of it, was to propose that the Universe actually splits when the observation is made!

I tend to think the implications for causality are the most "offensive" aspects of CI -- well, at least they *were*; not any longer. It was that not just the complexity of a system giving rise to uncertainty, but even the most simple system, down at the smallest, is not deterministic, ala CI, but stochastic, which ran against a number of assumptions of physicists at the time.

So that's another way of saying the same, but I was trying to generalize to allow not just what's on the table, but even new ways of interpreting the postulates. It's good to be aware of that history, but no need to pin oneself down either. I'm not really overly committed to CI, it's just what I'm most familiar with, and makes sense of the postulates.


I could see your point on what motivated Everett, though.

Itend to think the implications for causality are the most "offensive" aspects of CI

That is what also really annoyed Schrodinger and Einstein. Heisenberg was quite at home with the 'quantum jump' whereas Schrodinger said he 'hated the whole thing'.

Why do you say 'not any longer'? What has changed?

Last year a mathematical study indicated that, assuming quantum Indeterminacy rules the universe, then the vanishingly tiny effects of gravitational time dilation may very well explain most of the weirdness we see in the behavior of quanta. The tools to test the idea might require three years to develop, but the implication is that simply adding what we consider small amounts of mass when they form atoms is enough to explain the collapse of the wave-function. Which actually means there may be no collapse of the wave function because what we are viewing is merely juxtapositions. A shadow, for example, can be said to behave in a similar manner to quanta which can be described as yin-yang dynamics and the Monstrous Moonshine Conjecture being confirmed means the toolbox of physicists to explore quantum mechanics in the everyday world is now beginning to the cover the basics.

Quantum simulators and topological insulators are the hot thing because quanta being analog it means you don't need a full fledged quantum computer to do a wide variety of calculations. Analog is the more duh!, kick the damned thing approach that, nevertheless, can be incredibly fast, efficient, and even creative. It also means that just understand the analog language of nature could provide a wide variety of cheap and easy to use tools. With the first publication of the 500 states of matter it means the dream of the alchemists should be accomplished within the next century.

A good example of analog quantum mechanics is physicists recently discovered a combination of materials that self-organize to produce what they call quantum logic gates which can then be mixed and matched in a variety of ways to form a full fledged quantum computer. Nature herself, can provide the simulators with a little encouragement because she's a bit vein.

Well, the idea that the universe is spatially infinite was commonplace throughout the history of thought, and among today's cosmologists this is probably much closer to a consensus. — SophistiCat

So they believe in a real, physical infinity, as opposed to a mathematical infinity? I thought infinities in physics meant there was a problem with the theory requiring revision. Maybe it's just a personal preference, but infinity seems like zero or imaginary numbers to me. A useful concept that has no real embodiment. For example, there is such a thing as one rock (as in a single, countable, physical object), but there isn't actually zero rocks, anymore than there are physically zero unicorns, that's just a useful conceptual tool.

Of all the gin joints in all the worlds, every equation and constant necessary for life is present in this one gin joint world we're in, while there are zillions in which the math doesn't add up. I'm thinking about it — mcdoodle

So this isn't a violation of Occam's razor?

Of course, I have pretty much a logical positivist bent on such things. — Terrapin Station

There needs to be empirical evidence backing it up at some point, or else it will always remain an interpretation. If no empirical evidence can ever be given, then it's not scientific, but it's rather metaphysics, akin to saying we're living inside a simulation.

Why do you say 'not any longer'? What has changed? — Wayfarer

I say "no longer" because it's institutionalized now. While there are those who disagree with an interp, and it's understood that the question of interpretation is not settled (and sometimes posited that it could not be settled), it's not offensive in the sense that it was before. I mean, as I noted, that's what I was taught. So it's not exactly a scientific controversy when it's textbook (even if it is acknowledged that the question is not settled)

I can think of two reasons why that might be the case.

One, scientific thought changes not just with experiments, but with the deaths of those who postulate scientific truths. Many a scientist has gone to their grave against the consensus when their "opponents" won the general agreement of scientists. So the proponents of CI, MWI, Bohm, etc. are dead, and therefore the arguments aren't carried with the same sort of conviction. And, in the meantime, none of them really won out. CI has enjoyed the most renown probably because it was first, more than anything.

Two, the cultural milieu of this particular scientific thread has changed. QM was developed on the continent, where philosophy enjoyed a higher degree of respect within academic institutions. A lot of the questions that drove QM were part of a philosophical concern (not strictly, but partially). They were interested in the nature of reality and the nature of, well, nature. But Americans aren't as patient with these sorts of questions. They tend to enjoy the results of technological progress more than questions about what a scientific theory might mean about the nature of the world. Where these were a part of the scientific tradition, the victors of the two world wars fractured that tradition and had it reborn elsewhere, with different cultural values and educational goals.

Thanks, M, very insightful and quite true. You might find this article, Quantum Mysticism: Gone but Not Forgotten, of interest. It notes:

Schrödinger’s lectures mark the last of a generation that lived with the mysticism controversy. As Marin explains, quantum mechanics up to World War II existed in a predominantly German context, and this culture helped to form the mystical zeitgeist of the time. The controversy died in the second half of the century, when the physics culture switched to Anglo-American. Most contemporary physicists are, like Einstein, realists, and do not believe that consciousness has a role in quantum theory. The dominant modern view is that an observation does not cause an atom to exist in the observed position, but that the observer finds the location of that atom.

Schrodinger, Pauli, Heisenberg, and Eugene Wigner were philosophically inclined and educated. Schrodinger learned Greek and Latin (had some spare time in senior school!) and was influenced by Schopenhauer. In later life, Wigner read on Vedanta, and Heisenberg published well-regarded books on physics and philosophy (from a generaly Platonist perspective). I don't think the contemporary commentators (with exceptions like D'Espagnat, Penrose, Henry Stapp) have any kind philosophical depth; they simply resolve all of the philosophical conundrums by invoking parallel or multiple universes. (You could say that having swept all the philosophical problems under the rug, they need a bigger rug!)

They tend to enjoy the results of technological progress more than questions about what a scientific theory might mean about the nature of the world — Moliere

Most likely because they're on the corporate or military-industrial payroll, and they're being paid to shut up and calculate (although again with noble exceptions).

A quasi deterministic universe always seems more appealing; but, why can't we have determinism within a many worlds interpretation. — Question

??????!

I don't know how you failed to notice, but Many Worlds is deterministic. In fact, it is the entire point of it!

Many Worlds, is not just deterministic, it is unitary and local. All dynamics is unitary; the Schrödinger Equation is obeyed by all things at all times.

It was in fact Schrödinger who first discovered the other Worlds, but he was reticent to talk about them, because he knew other people would think he was crazy. It was left to Everett to summon the courage to develop the idea, motivated as he was by the desire to unify QM and general relativity. Everett paid the ultimate scientific price for his discovery.

There has been some progress since Everett. The Born Rule is now dropped as an axiom of QM, Decoherence has been discovered, and the quantum computer has been discovered, all as a result of Everett's idea.

If we go a bit further back in time to 1935, the Bohr-Einstein debate was essentially about the nature of science. Einstein was a realist - he thought scientific theories were about what exists in reality; Bohr was an anti-realist. Out of this debate came Einstein's discovery of Entanglement.

I'm going to chalk-up Entanglement to Everett's side of the argument, because it still is an argument between realists and anti-realists.

There needs to be empirical evidence backing it up at some point, or else it will always remain an interpretation. If no empirical evidence can ever be given, then it's not scientific, but it's rather metaphysics, akin to saying we're living inside a simulation. — Marchesk

All quantum interference experiments are evidence of Many Worlds.

A particularly fun experiment is the Elitzur-Vaidman bomb tester. All interaction free measurements are evidence of Many Worlds.

The famous Before-Before experiment is evidence of Many Worlds, as are all experiments on entanglement.

The quantum computer was invented to test Many Worlds.

Quantum Cosmology can't be done outside Many Worlds.

Many who work on quantum foundations will disagree with you that MW is an interpretation, as it has fewer axioms than standard QM.

I find the idea of decoherence too at odds with the MWI to take the MWI seriously. — Question

?????!

Decoherence was discovered and developed under Everettian* quantum mechanics!

*While H. D. Zeh - the discoverer of decoherence - was an Everettian, he developed a flavour of Many Worlds known as "Many Minds".

The many worlds interpretation, if taken at all literally (rather than being taken as an instrumental interpretation strictly of the mathematics involved), strikes me as completely ridiculous. — Terrapin Station

The old argument from personal incredulity!

Don't take MW seriously, just take the Schrödinger Equation seriously!

All quantum interference experiments are evidence of Many Worlds. — Tom

@Tom - could I put the question to you: what problem is the 'many worlds' interpretation a solution for? Why is it necessary to invoke 'many worlds'?

Many Worlds does not invoke anything, let alone many worlds.

Your question is like - "What problem does 'elliptical planetary orbits' solve? Why does Newton invoke ellipses?"

Newton never invokes ellipses, they are a consequence of his theory.

MW is notable for its lack of invocations - it does not invoke the Born Rule - it derives it, and it does not invoke wavefunction collapse, or state-vector reduction.

Yet another thing that MW does not invoke is Classical Mechanics, which is required under the Copenhagen Interpretation. Neither does it invoke consciousness to get around "Wigner's Friend" type experiments.

So if 'many worlds' doesn't invoke 'many worlds', why is it called by that name? ('Invoke' meaning 'to cite or appeal to (someone or something) as an authority for an action or in support of an argument.)

Is the following description accurate?

The many-worlds interpretation is an interpretation of quantum mechanics that asserts the objective reality of the universal wavefunction and denies the actuality of wavefunction collapse. Many-worlds implies that all possible alternate histories and futures are real, each representing an actual "world" (or "universe"). In layman's terms, the hypothesis states there is a very large — perhaps infinite — number of universes, and everything that could possibly have happened in our past, but did not, has occurred in the past of some other universe or universes. The theory is also referred to as MWI, the relative state formulation, the Everett interpretation, the theory of the universal wavefunction, many-universes interpretation, or just many-worlds.

Newton never invokes ellipses

I had the idea that it was Kepler who discovered the elliptical orbit of the planets.

So if 'many worlds' doesn't invoke 'many worlds', why is it called by that name? ('Invoke' meaning 'to cite or appeal to (someone or something) as an authority for an action or in support of an argument.) — Wayfarer

It was called "The Relative State Formulation" by its originator. "Many Worlds" was a catchy name coined by DeWitt. Those working in foundations, seem to prefer "Everett Interpretation", though recently its started to be called Unitary Quantum Mechanics or even simply Quantum Mechanics

As I said, Many Worlds doesn't invoke anything - it is simply quantum mechanics taken as a universal theory.

The quote you provide rightly states that many worlds are an implication of Unitary Quantum Mechanics. It then wrongly states that they are a hypothesis, though that seems more like sloppy use of language.

I had the idea that it was Kepler who discovered the elliptical movement of the planets. — Wayfarer

Kepler invoked ellipses, Newton did not.

Very slippery answer. According to the Relative State Formulation, there are many worlds, y/n.

I'm sorry; I should have stated that the other way.

What I meant to say in my non-educated understanding is wave function collapse. I don't believe the wavefunction does not collapse in MWI and decoherence is simply the wavefunction striving towards the mean.

I never bought into the idea that you can stand in front of an automatic machine gun and have realities in which it does not fire indefinitely/sporadically/once/none at all.

t was in fact Schrödinger who first discovered the other Worlds, but he was reticent to talk about them, because he knew other people would think he was crazy. — Tom

So how do you know he talked about them?

It was left to Everett to summon the courage to develop the idea, motivated as he was by the desire to unify QM and general relativity. Everett paid the ultimate scientific price for his discovery. — Tom

A martyr to boot. (Although, as a consolation prize, he made a fortune plotting the re-entry paths for ICBM warheads.)

There has been some progress since Everett. The Born Rule is now dropped as an axiom of QM, decoherence has been discovered, and the quantum computer has been discovered, all as a result of Everett's idea.

I don't believe that it is possible to fully explain decoherence in English, although as I understand it, the problem it solves is only that it shows that Schrodinger's cat would really be either dead, or alive, but not both, because on macro scales, the effect of the uncertainty principle is cancelled out by the interaction of so many states:

Every real system, whether quantum or 'classical' (such as a life-sized cat), is in contact with an external environment -- a messy, noisy collection of atoms whose state can never be perfectly known. This coupling between a quantum system in a superposition and the environment in which it is embedded leads the system to 'collapse' or decay over time into one state or another. This process is known as decoherence.

The rate of decoherence depends on the size of the quantum system. Physicists can now create and maintain quantum particles such as atoms or single photons of light in superpositions for significant periods of time, if the coupling to the environment is weak. For a system as big as a cat, however, comprised of billions upon billions of atoms, decoherence happens almost instantaneously, so that the cat can never be both alive and dead for any measurable instant.

Nature.

Decoherence, however, doesn't solve the 'observer problem' with respect to sub-atomic particles.

So how do you know he talked about them? — Wayfarer

"The Interpretation of Quantum Mechanics: Dublin Seminars (1949-1955) and Other Unpublished Essays" Schrödinger 1995. p 19