Adam Becker, a guy who wrote "What is Real? The Unfinished Quest for the Meaning of Quantum Physics" which was released yesterday, posted a blog post about Hugh Everett at Scientific American today:
In Fall 1955, Danish physicist Aage Petersen came to Princeton and debated two local grad students, Charles Misner and Hugh Everett.
At that moment, Hugh Everett was energized by ethanol. This compound could have caused Hugh Everett to see many worlds instead of one. (Everyone sees the Czech towers above twice because there's some mirror or glass on the wall.) Aage Petersen said some wrong things to provoke the other guys. In particular, he said that quantum mechanics didn't apply to macroscopic objects. Drunk Everett got upset and he decided to debunk this heresy in the most straightforward way. And because he saw multiple worlds at that moment, he chose many worlds to be the method to debunk Petersen's claim.
Now, Petersen's claim, as it stands, is utterly idiotic. Quantum mechanics obviously does apply to everything we can observe in our Universe, including macroscopic objects. If one divides the world to "things described by Hilbert spaces" and "things that aren't", one must be more subtle. This "Heisenberg cut" separating the worlds isn't a cut that forces macroscopic objects to be on the classical side.
Instead, it's a cut that forces microscopic objects to be on the quantum side; and a cut that allows large objects to be treated as a part of the observer or his measurement apparatus, and therefore treated classically. However, quantum mechanics always allows you to study how any system, including a macroscopic one, works inside. You may always include any object on the "observed side".
And this statement was no novelty in 1955. The whole condensed matter physics – as we understand the phrase today – is the application of quantum mechanics to macroscopic objects. Just look at this concise history of condensed matter physics.
The table discusses lots of events before 1925 – the Stone Age, Bronze Age, Iron Age, Democritus' atom, classification of crystal symmetries, laws for thermal and electric conductivity, Hall effect, electron in a classical metal, and a few others. However, the true modern condensed matter physics begins in 1925, when quantum mechanics is born, after 1926-1928 when the spin and the Fermi-Dirac statistics is understood (both Fermi-Dirac statistics and Bose-Einstein statistics are good tools that allow you to study macroscopic gases by quantum mechanics), and especially in 1928-1933 when people solve the electron in a solid using quantum mechanics. They discover the band structure and explain some forms of magnetism.
In 1947, true physicists discovered the transistor effect using quantum mechanics. All these insights were obtained by applying quantum mechanics to macroscopic objects, pieces of matter – systems of a macroscopic ensemble of electrons and nuclei. So the claim that quantum mechanics "breaks down" in any way when the systems are large is just bogus. The only correct statement is that for many questions about large systems, quantum mechanics is not needed in practice because classical physics is a good enough and simpler approximation.
Becker writes that Wheeler was a "political animal" – well, that may be fair – and consequently, he traveled to Copenhagen to inform Bohr about Everett's ideas for political reasons. Well, why would you politicize that? Wheeler simply thought that he had an interesting student who does things that directly compete with Bohr's philosophical, foundational comments about quantum mechanics. So he wanted to brag about that self-confident, provocative student, and/or be told that it wasn't any good. Wheeler obviously wasn't certain what the answer was.
Bohr told Wheeler that it was a pile of crap because it was a pile of crap. In particular, the "splitting of the worlds" made no sense. Even today, in 2018, it makes absolutely no sense and no fan of these Everett ideas can tell you anything whatsoever about the question whether the worlds split at all, when they split, why they split, how many branches there are. You may suggest several answers to each questions, none of them can be completed to a convincing let alone quantitative theory, and in fact, none of them has a significantly greater support among the Everett fans than others. They don't seem to care.
You may propose the splitting of worlds whenever some state evolves into a superposition. But "a superposition" is an ill-defined phrase because only "superposition of some particular vectors" is well-defined. You must always say "superposition of what". So every state in the Hilbert space is a superposition of something. Does it mean that the splitting takes place at every moment? What are the pieces that the wave function splits into in that case? If it only takes place at some special moments when the terms in the superposition are sufficiently separated from each other, how would you measure the separation? And which separation would be "enough" to split the worlds – a binary question? Or should the splitting of the worlds only take place when an observer decides to measure something or something else? In one world, one kind of a measurement is made, in another world, another measurement is made.
There's clearly no way to determine the preferred moments. No way to pick any preferred basis to define "the superpositions". No way to define "the distance" between the terms in the superposition for a general quantum system. None of these ideas can possibly work. Advocates of the many worlds philosophy claim that they have the "interpretation" that adds nothing to mathematics of quantum mechanics. But if their "interpretation" made any sense, they would have to add lots of these things about "when, how, and into what the splitting occurs". They don't have any answers which is why they're not adding anything to the mathematics of quantum mechanics – but it also means that their theory is guaranteed to remain incomplete wishful thinking. One of these problems would be enough to kill the idea – but the idea has about 10 such lethal problems.
On top of that, even if you solved these problems in some way, the many worlds theory will have nothing to do with science – with predictions. All predictions of quantum mechanics have the form of probabilities, continuous numbers assigned to possible results of experiments, or their functions or functionals. And no Everett's fan has an idea how these probabilities could be written into the many worlds, or extracted from the many worlds. It's just not possible. If this many world theory predicts something, it's the wrong prediction that all probabilities should be rational – the number of worlds would be the denominator because if several worlds obviously exist, they should be "equally likely". Well, the actual outcomes in quantum mechanics are not. It just doesn't make the slightest sense. And all predictions in quantum mechanics are functions of these continuous probabilities. Because the many worlds philosophy can't be reconciled with the continuous probabilities at all (or it seems to predict wrong probabilities), it can't be reconciled with the predictions as such – it cannot possibly have anything to do with science within the quantum mechanical framework.
So the thesis was reduced to some straightforward comments about entangled states – including apparatuses etc. (those things were largely copied from von Neumann who wrote about similar things in the 1930s) – and some vague and ambitious (but less ambitious) additions that make no sense. Some people say that Everett didn't really want "many worlds" (and those were only added later by DeWitt) but he did. The "many worlds" themselves and the splitting was just removed from his initial draft because it was way too stupid to go through the PhD defense at Princeton – let alone one that Niels Bohr himself was aware of.
While the "splitting of the worlds" was too stupid for Princeton, it was smart enough relatively to people like David Deutsch, and lots of the readers of his and similar people's popular articles and books. So they talk about splitting of the worlds even in 2018. And the more the education of the people will deteriorate, i.e. the closer to Idiocracy we will get, the higher percentage of people claiming to know "something about quantum physics" will mention "splitting of the worlds" even though it makes absolutely no sense.
In 1955, things were still alright and Everett couldn't possibly get a postdoc job. He didn't care – he didn't even want one (or a faculty job, for that matter). He wanted the money, Becker writes, and lots of fine food, beverages, and coituses that may be purchased for the money. So he got those things through his job in the Pentagon and happily day after a heavy heart attack at age of 51.
This particular cult has two aspects: the ideas and the men. The ideas suck and are unusable in physics. But one can show that lots of this movement is about the humans as well. I simply can't understand why some people would choose Hugh Everett, a totally superficial, materially oriented, alcohol and other sins enjoying chap, as their role model. I think it's no coincidence – a big dose of populism or "revolutionary spirit at any cost" helps to power this unscientific cult.
At any rate, this cult is everywhere these days. So people are increasingly guaranteed to hear conspiracy theories about the evil dinosaur Niels Bohr, the opportunist or spineless "homo politicus" John Wheeler, and the fresh genius Hugh Everett who was discriminated against. In reality, Bohr was a really deep thinker, Everett wasn't discriminated against but he was a guy who needed and got a higher salary than the actual physicists, and John Wheeler was simply a nice chap who was confused enough so that he needed to hear an appraisal by Bohr.