It is "her" complaint, not "his". This is Sabine Hossenfelder, a well known theorist (mostly for her outreach/popularization work).

And you are misrepresenting this"taboo" you are talking about. Thinking about what quantum mechanics "means" is how we got breakthroughs in quantum computing and theoretical computer science. Similarly, there is plenty of exciting deep theoretical work in particle physics beyond the smashing particles together type of experiments.

Oops, my mistake on the gender- Sabine is not a super familiar name. (Still dislike the tone, though; don't care how famous she is).

I also think the taboo has lifted a bit, as it's now possible for mainstream physicists like David Deutsch and Sean Carroll to build their careers on quantum foundations work. But I still think the physics community has a lot of baggage from the second half of the twentieth century to let go of.

It is not clear to me in what way the tone of your post differs from that of the article.

Here's a snippet:

> Another comment-not-a-question I constantly have to endure is that I supposedly only complain but don’t have any better advice for what physicists should do.

> First, it’s a stupid criticism that tells you more about the person criticizing than the person being criticized.

These feel closer in tone to "personal attack" on her critics to me than a discussion of their ideas.

There is also discussion of ideas in the article, which I have no problem with. But snippets like that feel like unnecessary salt that doesn't add anything. While her critics are (I agree, mostly) wrong, there is no reason to call them stupid.

i think its one of the classic logical fallacies - you can't disagree with me unless you have a better idea. it's a false dichotomy in the sense I can think you are wrong without further explanation. unless there's expt evidence but most of this is opinion-based so it's not open to that sort of progress.

'Physics is notoriously bad at having curiosity about strange, unconventional, or truly novel ideas.'

There is certainly no shortage of these!

What we are short of, are ideas that are:

- falsifiable

- testable by experiment, with current technology

Maybe we're short on those precisely because the strange ideas aren't given enough attention. Bell's Theorem was a landmark result that didn't come out of the Copenhagen interpretation, but because John Bell was fascinated by the de Broglie-Bohm interpretation.

The point is, unconventional perspectives give you new ways of looking at a problem that can yield new experiments.

That could be the case. I think the bigger issue is that many unsolved questions today are difficult to explore without expensive, highly-sophisticated equipment and larger teams of people.

>Physics is notoriously bad at having curiosity about strange, unconventional, or truly novel ideas.

Physics of today would be unrecognisable to a scientist at the start of the 20th century. Indeed the physics of 1935 would be unrecognisable to a scientist at the start of the 20th century. Our understanding progressed enormously AND it did so because people put forward radical theories in complete rupture with the established, 300-year old classical mechanics. There was no "traditionalist resistance" to it after it became apparent that classical mechanics failed to explain phenomena that quantum theory did explain.

>am astonished at how aggressively physicists resisted and suppressed "unconventional" interpretations of quantum mechanics (like the many worlds interpretation) in favor of the obviously-wrong Copenhagen interpretation [emphasis mine]

Ahahaha, you clearly have no idea what you're talking about. There is nothing "obviously wrong" about the standard Copenhagen interpretation (unless you have some new insight you would like to share), nor was there any suppression of ideas. Many debates have been waged in the past ~100 years, and many alternative interpretations have been put forward, like Bohmian theory, superdeterminism, or "shut-up-and-calculate".

>decided that it was meaningless to ask any deep questions about what QM actually meant

Physics, indeed all science, studies observable reality. Any "deep" questions about why or about things not measurable, quantifiable, or empirically observable, are by definition outside the scope of science. It is therefore as unreasonable as complaining about why don't geologists study epistemology. The answer is the same: it's outside the scope of their study.

I also don't like that your phrase seems to imply that what physicists study is not "deep" as opposed to your philosophical questions. There are many deep and beautiful ideas in physics.

> Physics, indeed all science, studies observable reality. Any "deep" questions about why or about things not measurable, quantifiable, or empirically observable, are by definition outside the scope of science.

This is wrong. Scientific theories provide predictive power, yes, but they should also provide explanatory power.

Which is to say, they should describe a model of the world because this is how we devise experiments, and experiments are clearly critical for science.

Two theories with equivalent predictive power but unequal explanatory power are not equally good theories.

> There is nothing "obviously wrong" about the standard Copenhagen interpretation

Can you tell me how to calculate whether a system of particles will cause another system of particles to collapse or not?

Can you tell me under what circumstances a system of particles will evolve unitarily or not?

Can you shade the region of the spacetime diagram of EPR where the wavefunction is collapsed? How about in a delayed choice quantum eraser experiment?

If you tell me "you're not allowed to ask those questions" (or "hm, I never thought of that!"), then you're directly illustrating the complaint here about physics!

The common narrative is that Copenhagen, many worlds, and the other interpretations of QM are all equivalent, but they are not. Copenhagen adds an extra physical event-- collapse-- where the wavefunction is suddenly nonunitary. The burden is on Copenhagen to tell me how and when this happens, and in fact to prove that it happens at all. Many worlds, on the other hand, predicts the same phenomenon-- the apparent collapse of a wavefunction to an eigenstate-- without adding unitarity violation, or any other phenomena at all beyond the normal, extremely well-verified mechanics of multi-particle system scattering; it merely treats the environment is a multi-particle quantum system.

Many worlds is the null hypothesis (no, the extra worlds are not extra suppositions, they are predictions of known physical laws), and the burden is on Copenhagen to show that unitarity violation exists, and the burden is extremely high (possibly insurmountable?) for EPR and eraser experiments.

When Copenhagen was devised, most believed that there was some "underlying state" in QM, and that measurement told you something about what the underlying state was. Bell's theorem should have sent a shockwave through the community which forced everyone to reevaluate fundamental assumptions and self-correct wrong ideas. But by accident of history, John Bell was too shy to publish in a major journal, and no one even read his result for four years. The implications of Bell's theorem were slow to diffuse through the community, and so the disruptive moment of reckoning that should have happened never came. By the time it was well-accepted, the narrative in physics had become "you're not allowed to ask about what quantum mechanics means, just shut up and calculate", and so the cognitive dissonance was cast aside. Annealing this attitude and this misstep away is taking excruciating decades.

EDIT: I also don't mean to imply that there aren't deep and beautiful things in modern physics. There are! But physics is concertedly avoiding asking deep questions in areas where (I strongly believe) it is most important. The claim that "the meaning of QM is outside the scope of science" is exemplary, I think, of that attitude.

>Can you tell me how to calculate whether a system of particles will cause another system of particles to collapse or not?

You appear to be somewhat mistaken. Copenhagen interpretation does not postulate any specific explanation or mechanism for wave-function collapse, merely that "upon measurement, the wave-function collapses into an eigenstate of the observable being measured". This, of course, is a physically verified phenomenon. Now Copenhagen purposefully leaves the precise meaning of "measurement" undefined, since in my view there is no convincing empirical evidence that supports a specific mechanism for this phenomenon. Other interpretations posit mechanisms (decoherence (doesn't fully account), von Neumann "consiousness" (not empirical), etc.) for this collapse.

My biggest complaint about many-worlds interpretation is how it is in its essence a non-scientific theory, as it makes assertions about an unobservable reality. It postulates other parallel realities that by definition do not communicate. Again, this makes it intrinsically a non-scientific theory.

>Can you tell me under what circumstances a system of particles will evolve unitarily or not?

Everywhere except on measurement, in which the state collapses.

>How about in a delayed choice quantum eraser experiment?

I'm not familiar with this experiment.

>If you tell me "you're not allowed to ask those questions"

By all means, ask as many questions as you want. That's after all the essence of scientific endeavour. But it's not very nice to misrepresent other positions, nor is it to claim everyone else is deluded (without strong evidence on your side at least).

>Bell's theorem should have sent a shockwave through the community which forced everyone to reevaluate fundamental assumptions and self-correct wrong ideas.

Bell's inequalities force no re-evaluation. They simply prove that the search for a local hidden-variable theory is impossible. It certainly raised important ideas for research, but it does not do anything to discredit established QM.

>The claim that "the meaning of QM is outside the scope of science" is exemplary, I think, of that attitude.

If a physical argument can be made about this problem, then it's in the scope of physics. Otherwise, it is not. As simple as that.

> This, of course, is a physically verified phenomenon

No it's not. There are interpretations in which collapse does not exist, so these experiments are not measuring what you think they're measuring unless you already assume the conclusion.

Also, your view on other interpretations of QM are outdated. Arguably one of the more famous results on quantum mechanics, Bell's theorem, would have never existed if not for Bohmian mechanics.

Don't discount the value of explanatory power. The fact that other interpretations provide far more explanatory power than Copenhagen makes them far more valuable. Many important results in quantum foundations would have never happened if everyone were a Copenhagenist.

You really want to look into the delayed choice eraser... it's mind blowing.

And it is still explained by the Copenhagen interpretation and there’s no problems with it. It’s like all those SR paradoxes that sound interesting but don’t disprove the theory — just the fact you don’t fully understand it.

The major interpretations of QM are all similar in the fact that they are not testable and don’t really affect the resulta. It’s metaphysics.

Also, all the non-MW interpretations say "and then choose a random outcome" without any clue as to how randomness might occur. MWI can propose an actual mechanism for randomness: sampling a space of alternatives, aka the Sleeping Beauty approach. To me this is pretty close to a knockdown argument in favour of MWI.

Demystifying the Delayed Choice Experiments: https://arxiv.org/abs/1007.3977

I am not sure that paper knows it, but it is essentially showing that many worlds is correct-- that the wavefunction never collapses:

> the implications on conditional probabilities hold for other measurements throughout the entire spacetime, present and past. [emphasis theirs]

And the math in the paper which supports that statement works by keeping all the superpositions around and allowing us to project to them at any time. This is the picture of many worlds! Copenhagen says the opposite: When you make a measurement, the unmeasured superpositions go away. The paper confirms, that's nonsense! If you shade any region of the spacetime diagram where the superposition is gone ("collapsed"), you'll be wrong!

The author says: "Note: in this work, we will not make any reference to why this (apparent) collapse occurs. Not only is this a much harder issue, it is simply not relevant to discussion we will present."

Also: "It is important to note that arriving at our conclusions did not require introducing new physics. We only relied on elementary quantum mechanics: not on novel ‘backwards time’ concepts, nor on any particular interpretation: we only used the Born rule ‘as is’. [...] With the remarks and intuition presented here, there really is no mystery whatsoever in any of the discussed experiments."

This experiment is not more mysterious than the rest of QM, but of course if you find the whole theory unsatisfactory you won't be satistified with this explanation either.

That language is why I think the authors aren't aware that they are advocating many worlds.

Also, this is a fantastic example of the "don't ask questions" attitude I think is so shameful. If they had bothered to take the small step of asking, they'd have come to a clear, evidence-based refutation of Copenhagen! There are regions of spacetime after the measurement, where the wavefunction is not collapsed, which the authors explicitly point to. That's in direct contradiction to the premise of Copenhagen.

The point is that the order of the measurements is irrelevant: QM predicts the same probability distribution of outcomes. So the problem can be solved using either order and the response is the same. I am not sure what premise of Copenhagen is being contradicted.

Edit: solving the problem in the "natural" order would give the same probabilities but is much more difficult to handle. You need to get a probabilistic outcome for the position of each single photon on the screen, which "collapses" the state and determines the wave function of the other photon. At this point, the first photon has been detected somewhere but it can only be labeled as "interference" or "not interference" later after the detection of the second photon. The probability of being labeled as "interference" or "not interference" does depend on the position (because the collapsed wave function depends on what the outcome of the previous measurement was). When everything is said and done, looking at the subset of events labeled "interference" there is an interference pattern and looking at the subset of events labeled "not interference" there is not an interference pattern. There is no mystery.

> I am not sure what premise of Copenhagen is being contradicted.

Before measurement, the state of the particle system is Σ α_i|i>. Copenhagen says, "after you measure the system, all but one of the α_i go to zero". The authors don't do that, and in fact say that you can't. Unless I am misunderstanding something, they keep all the α_i around at all times and project the measurement for each particle separately, regardless of whether it has been (or will be) measured somewhere else. There is nothing philosophically or physically wrong with this, as they point out, but it is different than what Copenhagen says you should do when you measure something.

And if you look at what it means (which they refuse to do), you'll see that after Alice makes her measurement, Bob's quantum state is still explainable in terms of a superposition. When is the measurement (joint or individual) "finished"? Answer, per the authors: Never. The superposition permeates spacetime. That's how we escape the need for a causal connection between Alice and Bob's measurements, and, naturally, that's how many worlds does it.

(And if we take it further and ask, when Bob makes his measurement, "What is the state of Alice's particles?" we'll see that she is in a superposition of being entangled with each of the superpositions of the particle, which remains in superposition before, throughout, and after our ~measurement of~ entanglement with it).

Please see what I added to my previous comment. You could in principle do it in the "right" order and you would get the same result. They routinely solve quantum optics problems using standard QM and the experimental results match the predictions.

What he shows in that paper is that the order of the measurments is irrelevant [1]. So he does solve the problem in the reverse order where it can be done easily writing a few quantum states [2].

Note that QM is not about causal connections, is about correlation. Once a pair is correlated, the correlation may appear when measurements are done. But it's not that observing one outcome here and now "causes" a particular outcome there and later (or before). One doesn't need to keep superpositions to think it "correlation" terms (instead of "causal" terms).

[1] Using the projection postulate in the derivation: "Say we have indeed measured on B and got OB = bJ . The state then collapses onto ..." He concludes: "None of this looks very surprising, but we want to stress that the total probability to find OA = aI and OB = bJ does not depend on the place or time at which the measurements occur."

[2] He also uses the projection postulate here in the usual way: "So the experimental outcome (encoded in the combined measurement outcomes) is bound to be the same even if we would measure the idler photon earlier, i.e. before the signal photon by shortening the optical path length of the downwards configuration. Then, if the idler is detected at D4 for example, the above state ‘collapses’ onto ..."

Yep, I'm with you that we don't need to bring causality into it.

And yes, you don't need to ask "when does the wavefunction collapse" to manipulate joint probabilities of measurements that happen at disparate locations and times. In fact, that's my objection: If you do ask, you find that there is no consistent answer! (And I suggest it's because wavefunction collapse is not a thing the universe does).

Re: "interference tagging"-- do you have a link to some material? (I'd love to understand something specific before commenting).

EDIT: Also, I didn't see Appendix B at first-- The authors do understand and even advocate the Everettian view! Though I still don't quite understand/agree with their earlier timidity about finding and interpreting conflicts with Copenhagen.

> If you do ask, you find that there is no consistent answer!

I'm not sure what's the problem. Why do have to "ask" if the answer doesn't really matter? What answer more consistent than "it doesn't really matter" would you like? Anyway (standard) QM is a non-relativistic theory, QFT may be more satisfactory from that point of view.

Re: "interference tagging" - what I mean is that first you detect the photons and later check if they "did happen" to go through two slits (interference appears) or one slit (no interference). But the interference pattern is not visible for a single photon and at that point the individual events are still a superposition of both possibilities (so for the events at a certain position part of them will be in the end identified as coming trough one slit and some of them from both). Only after the second measurement is done you know how to group the previoulsy recorded events to see the interference. It's not that the later measurement causes interference to appear. Or at least it doesn't affect at all where the photons were detected, it just lets you know how to group the existing events to make it apparent (selecting only those where, once the full mesurement on the pair has been done, the path taken remains uncertain).

Edit: maybe this picture helps https://upload.wikimedia.org/wikipedia/commons/thumb/c/c8/De...

If all the events are taken together there is no interference pattern. But when they are grouped according to where the second photon is detected in two cases there is still no interference but in the other cases complementary interference patterns appear.

> Why do have to "ask" if the answer doesn't really matter?

You tell me; Copenhagen is the one that says collapse exists. It sounds like maybe we are on the same page that collapse isn't necessary to explain quantum mechanical observations? In that case, we are both Everettians :).

To explain quantum mechanical observations you need "collapse" (i.e. the projection postulate: immediately after a measurement the state of the quantum system is the projection on the corresponding eigenspace of the operator). I don't know what do you gain by saying that it's not "real" and that it's just "as if".

Because if we say that some of those alpha_i physically go to zero at any point (e.g., "after"), our predictions are wrong, in agreement with the paper. We have to account for the fact that those alpha_i|i> are still nonzero and "existing", and that our projection onto them is only zero for the time being. A different choice in EPR or a quantum eraser experiment may bring our projection onto those states back out of orthogonality-- or maybe not, if we never make those choices. But if we believe we have the physical freedom to manipulate our experiments, we can't get away with saying those extra "universes" (basis states) physically disappear.

In some cases it is a safe approximation to ignore those extra states for the remainder of our experiment/calculations, but with a small change to the experiment we can make that a bad approximation.

I am afraid that you have not understood the paper.

a) You do the measurement first on the "screen" side (and project the quantum state of the pair of photons according to the measurement, the "extra universes" disappear). You do then the measurements on the "idler" side (and project again the quantum state according to standard QM).

b) You modify slighly the setup to reverse the order of the measurements. You do the measurement first on the "idler" side (and project the quantum state of the pair of photons according to the measurement, the "extra universes" disappear). You do then the measurements on the "screen" side (and project again the quantum state according to standard QM).

QM predicts that the outcomes in the original experiment (a) and the "reversed" experiment (b) are the same. And those predictions are verified empirically.

> But by accident of history, John Bell was too shy to publish in a major journal, and no one even read his result for four years.

Why do you say that his 1964 paper (I assume) was not read by anyone for four years?

He published in a tiny journal that was only in print for a handful of years, which was read by very few people. Bell himself said that no one contacted him or even mentioned it to him until four years after its publish date, and that researcher only came across it by pure happenstance. The whole story is detailed in Becker's book; it's fascinating.

Thanks for the reference.

As if it's known that meaning itself is not a physical quantity

> There is nothing "obviously wrong" about the standard Copenhagen interpretation.

Copenhagen interpretation showed up when they were faced with a choice between preserving locality or determinancy.

They chose wrong — we know now QM is non-local, and that the underlying justification for the Copenhagen model is an extraneous philosophical proposition.

But much like an extra dependency in a software project, no one wants to remove it now that it’s used everywhere, and there’s a lot of “good enough” stuff using it.

That said, it seems to be one of the major impediments to a unified theory: by dropping the extraneous assumption, we have fewer things to reconcile with GR, and can start looking for GR geometries that have quantized non-local behaviors.

ie, dropping Copenhagen and giving geons another look is probably worth it. (And is basically what loop quantum gravity people are doing, as far as I can tell.)

> we know now QM is non-local, and that the underlying justification for the Copenhagen model is an extraneous philosophical proposition.

What underlying justification are you talking about and what do you understand by "the Copenhagen model" precisely? At least in the Einstein vs Bohr debates the one denying that QM could be a complete theory because of its non-locality was Einstein, I think.

From Wiki:

> Einstein's refusal to accept the revolution as complete reflected his desire to see developed a model for the underlying causes from which these apparent random statistical methods resulted. He did not reject the idea that positions in space-time could never be completely known but did not want to allow the uncertainty principle to necessitate a seemingly random, non-deterministic mechanism by which the laws of physics operated.

The underlying assumption for Copenhagen was to try to preserve locality by assuming non-determinism. However, there’s no saving locality — and non-locality is enough to leave determinism — so there’s no reason for the non-deterministic axiom.

I think I also meant “definite” instead of “deterministic”, but it works out the same.

Copenhagen does not require nonlocality

No, but if you have non-locality, then you don’t need the non-definiteness of Copenhagen, and could do with something like a Bohm or MW interpretation.

Or you could do with Copenhagen...

I found this quote from David Lindley in a review of Adam Becker's book referenced in this thead (http://www.math.columbia.edu/~woit/wordpress/?p=10147):

"The problem with Copenhagen is that it leaves measurement unexplained; how does a measurement select one outcome from many? Everett’s proposal keeps all outcomes alive, but this simply substitutes one problem for another: how does a measurement split apart parallel outcomes that were previously in intimate contact? In neither case is the physical mechanism of measurement accounted for; both employ sleight of hand at the crucial moment."

I don't think that criticism really understands what Everett says-- the evolution of the universal wavefunction is unitary. There is no "splitting". There is only uncertainty about what part of phase space you are in (and any measurement that tightens your certainty in one axis of phase space will broaden it along some other axis).

quote from previous post that is potentially relevant to your dislike of the "superior, disdainful tone":

> later someone sent me a glorious photoshopped screenshot (see above) which shows me with a painted-on mustache and informs us that Sabine Hossenfelder is known for “a horrible blog on which she makes fun of other people’s theories.”

> The truly horrible thing about this blog, however, is that I’m not making fun. String theorists are happily studying universes that don’t exist, particle physicists are busy inventing particles that no one ever measures, and theorists mass-produce “solutions” to the black hole information loss problem that no one will ever be able to test. All these people get paid well for their remarkable contributions to human knowledge. If that makes you laugh, it’s the absurdity of the situation, not my blog, that’s funny.

http://backreaction.blogspot.com/2018/10/dear-dr-b-what-do-y...

> The modern astrophysical world view, which began with Galileo, and its challenge to the adequacy of the senses to reveal reality, have left us a universe of whose qualities we know no more than the way they affect our measuring instruments, and — in the words of Eddington — "the former have as much resemblance to the latter as a telephone number has to a subscriber." Instead of objective qualities, in other words, we find instruments, and instead of nature or the universe — in the words of Heisenberg — man encounters only himself.

-- Hannah Arendt, "Vita Activa"

Heisenberg and others knew that, I'm sure many current scientists do too, but going by output, the situation is dire, the intertubes are full of people talking about being "unbiased" or "objective" even about human matters, when not even the hard sciences are truly objective! It's like people simply agreed that because they're not religious or whatever, they're now correct. If they are wrong about anything, since they supposedly are on the side of science, they can be sure that will be corrected, so they can consider themselves correct even today. Before you know it, you have superstitious people who think their superstition constitutes the absence, the opposite of superstition.

We went from Heisenberg to a chemistry teacher cooking crack while his assistant gets all bug-eyed about the power of "science, bitch!". Or this "critique" of the game Soma I saw recently, where the Youtuber mentions that "people who are smarter than you and I are saying the universe might be a simulation". What I see is people booby-trapping everything with stupidity.. making things so stupid, and then glorifying that, that injecting even a lick of seriousness into anything would cause massive offense, hurt a lot of egos. To me it's the analog of religious fundamentalists going to church 20 times a day to get hyped and primed with caricatures of those not of their flock, and fantasies of how great it's all going to be when those are gone.

But I'm not sure I can blame physicists for that. I don't know enough about what they do, I see what "everyone" else is doing, and that's horrid by itself. It's not the job of physicists to ask deep questions about what it all means, it's the job of everybody.

> suppressed "unconventional" interpretations of quantum mechanics (like the many worlds interpretation) in favor of the obviously-wrong Copenhagen interpretation.

They’re just interpretations and they all work equally well and produce the same predictions. It makes no sense to call any of them obviously wrong. It’s personal preference.

Is it possible though, that Copenhagen is wrong in a similar manner to epicycles that give the correct numerical answer?

That's the impression I get as a non-physicist, that yes it's kind of an aesthetic preference, but on the other hand aesthetics are important and sometimes are pivotal in finding new insights.

Ugly may not be completely objective, but it isn't completely arbitrary either.

I should correct an important misconception. Epicycles do not produce correct predictions.

Epicycles can produce correct point location of planets, but do not produce correct phase. Galileo observed heliocentrism-compatible phase of Venus with his telescope. That was the crucial experiment, not the simplicity.

I don't think it's important to my point. The point remains that multiple theories can be equivalent in some restricted sense, yet the more aesthetic one might have greater potential.

But it matters. All the major interpretations give you the same results (otherwise we’d just test it and rule out the wrong ones).

Copenhagen is wrong to the degree that any theory can be called wrong. It posits specific physical events (collapse) which are not only unsupported by evidence, and not only contradicted by evidence (delayed choice quantum eraser experiments), and not only logically self contradictory (when it is not too vague to make concrete statements about the phenomena it is explaining, it is circular-- our theory of electrons cannot include scientists (or the macroscopic world in general) as a fundamental element, when scientists themselves are explained in terms of electrons), but ontologically inferior to many worlds, which explains all the same phenomena without adding any new suppositions (and no, the extra worlds are not extra suppositions, they are natural predictions that arise by simply following well verified quantum laws to their natural conclusion, by treating the environment as a quantum mechanical system of particles).

MW might be simpler in the way it arises but while it produces the same results it doesn’t really matter. And it definitely has enough supporters to ruin the “stubborn physicists” hypothesis.

Sabine Hossenfelder is a woman.

His?