I will try to focus on the things that are wrong and you should be aware of the fact that they were cherry-picked to a certain extent and you could cherry-pick some assertions which would make Sebens' essay look less bad. But such fundamental mistakes shouldn't be there at all, so his text is bad, anyway.
At the beginning, we learn how Sebens is wonderful:
... and like many philosophers of physics, knows the technical background behind relativity and quantum mechanics very well. ...Obviously, if he knew these matters "very well", he wouldn't be writing completely wrong – and virtually ignored – papers about the foundations of quantum mechanics. As a "philosopher", which is a euphemism for a "physics crackpot", he is receiving funding from the NSF, too. It can't shock you.
The first two sentences written by Sebens himself are:
In Newtonian physics objects always have definite locations. They are never in two places at once.It's being said as if these two sentences were equivalent. But they're not equivalent and they're problematic for tons of other reasons, too.
What is true about the first sentence is that classical physics allows us to assume that the locations may be completely well-known in principle. However, even in classical physics, they are often incompletely known, e.g. in statistical physics. And the incomplete knowledge is just being extended by quantum mechanics – with some new source of the uncertainty that can't ever be removed and that didn't exist in classical physics. But even classical physics admitted a description in terms of "what we know", not "what is out there", and it is this "what we know" description that has to be used if we switch to quantum mechanics.
Classical physics allows us to believe in the existence of a "meta-observer" who knows all the exact positions and velocities (or values of fields). This metaobserver may be called God. But in contrast with Sebens' claim, classical physics doesn't assert that God in this sense must exist.
The second sentence suggests that in quantum mechanics, a particle is in two places at once. But even though this language is often used in popular presentations and even physicists' sloppy formulations, it's not the case. When a particle is described by a wave function that is a superposition of "here" and "there", it doesn't mean that the particle is in two places at once. It means that the particle is "here OR there" (not "AND") and we can't know which is the right. But the locations "here" and "there" are only potential locations, not "objectively real locations" of anything, and they are mutually exclusive because the two position eigenstates are orthogonal to each other.
A quantum mechanical particle can't be in two places at once.
We are told that quantum theory requires us to revise this classical picture of the world, but what picture of reality is supposed to take its place is unclear.Quantum mechanics and its postulates and rules presented in the textbooks is what replaces classical physics. It is a "picture" of some sort. After all, we even use the phrases "Heisenberg picture" and "Schrödinger picture" for more particular choices in the quantum formalism.
This quantum mechanical picture or "these pictures" are not "pictures" in the sense of classical physics but there is no reason they should be.
At any rate, what replaced the classical picture is absolutely clear, and Sebens' sentence is just demonstrably false.
There is little consensus on many foundational questions: Is quantum randomness fundamental or a result of our ignorance? Do electrons have well-defined properties before measurement?Consensus is irrelevant for science. The fact that there is no consensus doesn't mean that science hasn't completely settled all these questions. The consensus doesn't exist – and will probably never exist – because many people who are completely incompetent or irrational or both (like Sebens and Carroll) are trying to offer their opinions and feelings as if they were a part of the scientific evidence. But they are not.
The quantum randomness is demonstrably fundamental – and it may also be interpreted as a result of our ignorance. But the point is that this ignorance is inevitable, due to the uncertainty principle. There is no way, not even in principle, to know in advance the state of all observables that may ultimately be measured. It's impossible because the observables that may ultimately be measured simply don't have and can't have well-defined values simultaneously, due to the nonzero commutators.
Sebens divides the pundits talking about the foundations of quantum mechanics to "doers" and "dreamers". Feynman is quoted as a "doer" and a quote in which Feynman says that the opposition to quantum mechanics is just due to the permanent self-brainwashing saying that "the world must be classical at the end" is the cause of all this nonsense.
Sebens' terminology, "doers" vs "dreamers", is of course extremely tendentious. It's virtually isomorphic to the terminology chosen by the notorious crank Lee Smolin who called people like himself "seers" while serious physicists were downgraded to "craftsmen". In both cases, the goal of the terminology is to pretend that the people doing the physics correctly are just some "superficial engineers" who don't study Nature deeply enough.
But that's of course a lie. The "doers" are better not only in doing real calculations and applying quantum mechanics – or particular quantum theories. They are also – and especially – vastly better in analyzing the truly profound conceptual questions about Nature and the character of things.
Dreamers, although they may often use the theory without worrying about its foundations, are unsatisfied with standard presentations of quantum mechanics. They want to know “how it can be like that” and have offered a variety of alternative ways of filling in the details.In science, you can propose numerous hypotheses but as long as it is science, they are being mercilessly compared against the empirical data (often with lots of calculations that have to be made as a part of the validation process) and perhaps also more abstract scientific evidence and principles extracted from the data in the past.
You may fool yourself by constantly calling your hypothesis "a dream" but science doesn't care about similar emotional labels and it may easily determine that everything written in the Sebens-Carroll papers are "wrong propositions". Fine, call them dreams but they are still stinky crap, OK?
The same comments apply to Sebens' negative propositions. He may repeat that he is not satisfied and "it cannot be like that" millions of times, and he surely will because he is a mindless parrot, but this won't change the fact that quantum mechanics is a theoretical framework – and the only known theoretical framework – that still agrees with all the observations ever made, and that is what matters in science, not "dreams", repeated slogans about "something that cannot be true", and scientifically indefensible "dissatisfaction".
Doers denigrate the dreamers for being unproductive, getting lost “down the drain.” Dreamers criticize the doers for giving up on one of the central goals of physics, understanding nature, to focus exclusively on another, controlling it.Right. A nice presentation of a confrontation. The only problem is an asymmetry: What the "doers" are saying is demonstrably true while what the "dreamers" are saying is demonstrably false. Quantum mechanics is primarily about the understanding of Nature, not about "controlling it". After all, quantum mechanics was discovered by pure scientists, theoretical physicists such as Heisenberg, Bohr, Dirac, and Pauli (and for purely scientific reasons), and not by any applied physicists or engineers. Quantum mechanics is the understanding of Nature at a much deeper level than classical physics and it's the "dreamers" who just don't want to accept the fact that deeper ideas than their superficial real-world experience has been needed to follow physics for almost 90 years. Quantum mechanics also led to very important practical applications but this fact shouldn't be used "against" quantum mechanics and because of causality, it doesn't imply that something is or was "excessively practical or superficial" about its foundations.
Sebens misinterprets another quote by Feynman as a call to consider then non-quantum alternatives to describe the quantum phenomena. But Feynman actually called for a physicist's remembering and considering many equivalent or dual perspectives – all of them correct, not mutually exclusive – on certain or all problems. He was talking about things like the different "pictures" in quantum mechanics, different bases of the Hilbert space, and so on. He was surely not saying that a physicist must permanently consider paradigms that don't agree with the evidence.
Sebens then "describes" (not really) the recent fad about many interacting worlds. He was not on the most publicized paper but wrote a related one. Instead of saying things that could at least remotely compare to my explanation of this idea, he focuses on bragging about the left-wing news servers that hyped this silliness. He also incoherently mixes these sentences with sentences about droplets as a realistic description of quantum mechanics. He also asks:
“I understand water waves and sound waves. These waves are made of particles. A sound wave is a compression wave that results from particles of air bunching up in certain regions and vacating other. Waves play a central role in quantum mechanics. Is it possible to understand these waves as being made of some things?”The probability amplitude waves may be proven not to follow from local (consistent with relativity) hidden variables, so in this sense, they are not "made of some things". However, if we wanted to envision a more general way how they could be "made of some things", we would run to the problem that "made of some things" isn't really well-defined in the most general imaginable theory. Is a D-brane made of fundamental strings? It depends what kinds of "making of" we allow. It is possible to determine whether a plastic object was made of LEGO but the building blocks of Nature aren't similar to the building blocks of LEGO so the ability to answer the LEGO question doesn't mean that a similar question may be answered in physics.
He shows a picture of a moving wave in the double slit experiment and says something that made me laugh out loud:
There’s a problem with thinking of the wave as made of something: the wave function assigns strange complex numbers to points in space instead of familiar real numbers.So the appearance of complex numbers is a "problem" for these "dreamers"! ;-) Cool. Complex numbers are more fundamental from an advanced algebra/calculus/geometry mathematical viewpoints, too. At any rate, there is no problem with them in physics, they are at least as consistent objects in mathematics as the real numbers, and they are badly needed at many places in physics, and especially in quantum mechanics where they become more vital than just some "tools to simplify certain calculations".
If you have problems with complex numbers, you will surely agree that the term "incompetent moron" is much more accurate than a "dreamer", won't you? Well, you should.
Sebens also immediately solves his "problem":
This can be resolved by focusing on \(|\Psi|^2\), the squared amplitude of the wave function, which is always a positive real number.Phases are erased, the problem is solved. Sorry, you haven't solved any problem, you have just tried to mask your stupidity. The (relative) phases of the wave function are exactly as important as the absolute values. They will decide about the measurements of all observables that don't commute with \(x\) – which are almost all observables. The momentum \(p\), for example, is encoded in the information how quickly the phase of \(\Psi\) is changing per unit distance.
He tries to say that \(|\Psi|^2\) – with the phase indefensibly ignored – doesn't determine the probability but a density. Except that it demonstrably doesn't. One particle is seen on the photographic plate so operationally, the squared wave function surely does determine probability and their distributions, not densities of real things. Suddenly, he gets distracted by the fact that for many particles, the wave function has to be a function of \(3N\) variables. That's important for entanglement, he realizes, but it's very clear that this "complication" is just another "problem" for him, like complex numbers.
This presentation of him is a weird sequence of several paragraphs in which he is simultaneously struggling with some complete basic introductory issues of quantum mechanics; and he wants to pretend that he has some better theory "beyond QM" at the same moment.
To make the story short, he says that \(|\Psi(x_1,y_1,z_1,x_2,y_2,z_2)|^2\) etc. doesn't describe the probability but the density of parallel universes. Well, even in the context of classical physics, any probability may be visualized as a relative fraction of repetitions of an experiment – which may be called "parallel universes" if you imagine that the other histories are somewhere "out there". So this "insight" – a way to make the notion of the probability more tangible – is not new at all. That's how people are normally explaining probability to start with. The other possible outcomes exist in the "multiverse of possibilities". The other outcomes are "not really real for us" but you may view them as "real from some meta-viewpoint".
However, what's new – and completely wrong – is that this visualization of the probability density is used in quantum mechanics. In quantum mechanics, you actually can no longer visualize the probability density in this way because there are many mutually "incompatible" probability densities. The quantities like \(x\) are not the only ones that can be measured. The system must be ready for a measurement of \(p\), too. The probability density in the momentum representation may be calculated from the wave function, too. But it cannot be calculated from \(|\Psi(x)|^2\) because in this probability density, as I have already said, the information about the phases is completely missing and this information is completely essential – actually more essential than the absolute values – for the calculation of the probability distributions \(\rho(p)\) of different momenta.
If you imagine that the parallel worlds are already objectively distributed in the \(x\) variable in a particular way, then you have picked a preferred observable, \(x\), where the measurements may be OK. But all other observables will fail to be represented correctly in this "dot-based" visualization. The uncertainty principle simply prevents you from replacing the wave function with some classical information about a particular observable such as \(x\).
In my October 2014 essay, I discussed the lethal flaw of the Many Interacting Worlds from a slightly different, but ultimately equivalent angle: I treated that proposal as a variation of the Bohmian mechanics.
I ignore one paragraph by Sebens where he just says quantum-mechanics-resembling things that may sound OK to you if you're not careful enough but they are completely wrong. But let me end up with his last paragraph:
There is of course much more to the story than what’s been said here. One particularly intriguing consequence of the new approach is that the three sentence characterization of Newtonian physics with which this post began is met. In that sense, this theory makes quantum mechanics look like classical physics. For this reason, in my paper I gave the theory the name “Newtonian Quantum Mechanics.”LOL. That's hilarious. Just to be sure, the theory doesn't just make the world "look" like nonrelativistic Newtonian classical physics. It claims that the world really is described by such a theory. And Sebens thinks it's a great victory to brag about.
So "dreamers" not only want to "undo" the quantum revolution. They pretty much want to undo the transition of physics to "field theory" as well and claim that the world is described by a classical deterministic Newtonian mechanics.
For these "dreamers", everything that has been found in physics since the time of Newton – field theory, partial differential equations (in hydrodynamics, electrodynamics, and elsewhere), the statistical description of thermodynamics, special relativity, general relativity, quantum mechanics, quantum field theory, and of course string theory – was just a sequence of mistakes, increases in the complexity of our theories that were not necessary anyway, and all the evidence that led to these developments in physics has been just a conglomerate of illusions and tricks.
Now, "dreamers", how stupid you have to be to believe a conspiracy theory like that, the opinion that 300 years of the scientific progress – that has led to a dramatically deeper understanding of the Cosmos at the theoretical level as well as an unprecedented technological progress – is just an illusion, a sequence of paradigm shifts that shouldn't have taken place? How blinded and arrogant you have to be not to see that your vision that the world must be controlled by classical Newtonian mechanics should be imposed on physics regardless of any detailed evidence because you consider all the new insights of physics of the last 300 years – as well as complex numbers – to be "problems" for you?
The only real problem, Mr Sebens, is the combination of complete stupidity and arrogance, values that you reconcile so smoothly and naturally.
I am totally infuriated by this junk because complete idiots who are, scientifically speaking, at least 300 years behind the times claim – and use similar idiots in the media to claim – that they are close to cutting-edge science. You can only "dream" about being in the average. Real cutting-edge researchers have not only mastered insights that changed physics 90 years ago, like quantum mechanics. In the most high-brow disciplines, one must really know most of the discoveries made 20, 10, and sometimes even 2 years ago, otherwise he's out. A straight reversal of 300 years of the scientific progress is "somewhat" (insanely) unlikely and all the evidence points against this possibility.