Now back from a trip to the West Coast, here are some accumulated things that may be of interest:

- One thing I didn’t do while there was attend the 2020 Breakthrough Prize symposium. For videos of three talks about supergravity, see here. At the time of the award I wrote here about why a \$3 million prize for a failed idea about particle theory was a bad idea. Listening to the talks, I think an even worse idea is telling the public that this is a great example of why they should trust science.
- For another dubious idea from the West Coast, in January the KITP is bringing high school teachers to Santa Barbara to teach them about Spacetime, Holography, and Entanglement. Most of the programs the KITP has run for teachers (see here) have been devoted to explaining important, solid science. Back in 2001 when they promoted string theory I thought that was a bad idea, this latest one isn’t much better. Again, when the credibility of science is under attack, why go to the public (or, in this case high school teachers) to promote a highly speculative research program? Is it really a good idea for high school teachers to be exposed to this kind of hype, presumably with the hope that they’ll somehow transmit it to their students?
- On the evergreen topic of bad multiverse science, Scott Alexander here defends multiverse speculation, responding to Jim Baggott’s article. He and the authors of the more than four hundred comments debate at length a red-herring issue. Alexander writes:

My understanding of the multiverse debate is that it works the same way [as respectable paleontology]. Scientists observe the behavior of particles, and find that a multiverse explains that behavior more simply and elegantly than not-a-multiverse.

Yes, if theorists had a simple, elegant multiverse theory with lots of explanatory power, you could get into interesting arguments about its testability and whether the idea was solid science or not. The problem is that no such multiverse theory exists. If you want to talk about the MWI multiverse, your problem is that solving the measurement theory problem by just saying “the multiverse did it” may be “simple” and “elegant”, but it’s also completely empty. If instead you want to talk about the cosmological multiverse, the problem is that you don’t have a theory at all (and the actual fragments of a theory you do have are complicated and ugly). For more about this, see my posting and article on Theorists Without a Theory.

For something more positive, while traveling I noticed two quite interesting articles which explain in a detailed technical way approaches to two of the great unsolved problems of our time, while carefully discussing why the approaches have not (yet?) worked, leaving the great problems unsolved.

- For mathematics and the Riemann Hypothesis, see Alain Connes and Caterina Consani’s article The Scaling Hamiltonian, about the attempt to understand the zeros of the Riemann zeta function in terms of the properties of a specific Hamiltonian operator, which in some sense is a generator of a group of scaling transformations.
- For physics and quantum gravity, see Donoghue’s A Critique of the Asymptotic Safety Program, which has a detailed discussion of the problems with making sense of both quadratic gravity Lagrangians and the idea of a non-trivial fixed point gravity theory theory. I was interested to see that he has a lot to say about the Lorentzian vs. Euclidean signature issue, something often ignored.

Finally, I recommend reading Elizabeth Landau’s interview at Quanta with astronomer Virginia Trimble and Trimble’s excellent advice to us all:

Pay attention. Someday, you’ll be the last one who remembers.

**Update:** Two more math-related items well worth a look:

- Dan Rockmore at the New Yorker on Where do ideas come from?
- A profile of Terry Tao at the Princeton Alumni Weekly.

**Update**: Yet another blog entry from Scott Alexander about the multiverse, with more hundreds of comments. What is it with the fascination for this empty argument?

At BBC Science Focus, more multiverse promotion from Sean Carroll. He does end though by getting to the real point (note that when theoretical physicists say a question is “hard to answer”, it means they have no idea how to answer it):

Many-Worlds is a lean and mean theory, but it’s possibly too lean and mean; there is very little structure to rely on, so questions like “Why do probabilities behave the way they do?” and “Why is classical mechanics such a good approximation to the world we see?” are hard to answer.

This is exactly the problem that those arguing over this at Slate Star Codex and elsewhere don’t seem to understand: saying “all is the Schrodinger equation” doesn’t tell you how to connect the theory to the world we observe. Adding in an ontology of multiple universes does nothing at all to solve this problem.

The interview with Trimble is also interesting for the way it contrasts her mindset with that of the woke young interviewer. Trimble isn’t about to be patronized or felt sorry for. For details see — https://luysii.wordpress.com/2019/11/12/tough-60s-chick-disappoints-woke-young-interviewer/

I’d rather try and moderate a discussion of everyone’s favorite ideas about fundamental physics than one about Trimble’s views on sexual harassment. Have left the luysii comment so those who want to discuss this can do so elsewhere.

“Is it really a good idea for high school teachers to be exposed to this kind of hype, presumably with the hope that they’ll somehow transmit it to their students?”

From some experience I say this is really bad since neither the exposed students nor the teachers have any idea how speculative these ideas are. I give “pro bono” talks about fundamental physics to young kids and they keep asking about the multiverse (which is close to science fiction). If anyone is to expose untrained audiences to supergravity, one has to be super honest about how not backed up by data this is (followed by a discussion on the importance of following the data and not abandoning the scientific method).

I don’t think the MWI multiverse is completely useless. It says that everything is quantum — there is no split between a macroscopic quantum world of observers and a microscopic quantum world of particles, the way that some people interpret (or at least used to interpret) the Copenhagen interpretation.

Of course, you

canbelieve that everything is fundamentally quantum without subscribing to the MWI, but my impression is that there are a lot of physicists who don’t truly believe this (for example, anybody who believes there is a sharp distinction between virtual particles and real particles is classifying particles into two classes, one purely quantum and the other more real).Peter Shor,

The problem with just saying “everything is quantum” (or with invoking splitting universes) is that it’s an empty statement. You’re saying nothing about the real problem (how does the classical world of our observations emerge from the underlying quantum formalism?). Yes, “everything is quantum” is simple and elegant, and I even believe it’s most likely true, but it’s not a non-trivial answer to the deep problem.

The criticism of an empty non-answer to a problem as being not testable/falsifiable is a way of pointing to the emptiness: if an idea about physics is non-empty, not just a bunch of words, you should be able to do something non-trivial with it that you can’t do without it. If this new, different thing has some connection to the real world, however indirect, you expect it to imply some in principle observable difference.

Off topic, but was wondering whether you’ve yet had any chance of reading the new bio of Jim Simons by Zuckerman, and whether you’re planning to write a review of it at some point?

theoreticalminimum,

Just finished reading it late last night, will write about it here very soon.

Peter Woit:

If you don’t say “everything is quantum,” there’s no problem. There’s a classical world and a quantum world, and you don’t

needan explanation of how the classical world emerges from the quantum world.You have to admit that “everything is quantum” before you can start to develop the theory of decoherence.

Certainly, some people were wondering why the classical world wasn’t quantum before MWI — we have Schrödinger’s cat and Wigner’s friend — but my impression is that most physicists didn’t even think about this problem. This seems to be confirmed by the historical development. Considering the following timeline (based on quotes from Wikipedia):

and

I agree with you that the statement “everything is quantum” is not an answer. But I think it’s part of some very important questions.

To continue my previous comment, while you can’t test the MWI experimentally, you can indeed test the statement “everything is quantum” by doing experiments that look for interference between larger and larger objects. And so far, they haven’t found a limit.

Peter Shor,

I don’t think we really disagree. Yes, you need to first pose the problem. My point is just that invoking MWI at the level of “everything obeys the Schrodinger equation, and our observed universe is one of many” sounds simple and elegant, but it does zero to solve the problem. And you can see this by noting that there is no testable distinction with the “textbook” or “Copenhagen” interpretation.

The theory of decoherence is very much non-empty, and, as a result, is very much testable.

It’s reasonable to complain about some many worlds promoters, but the part that’s reasonable to complain about is not any flaw in MWI, but just the fact that they are presenting the existence of parallel universes as a big, exciting scientific discovery. As you note it’s not even a scientific theory, just the avoidance of some bad theories and philosophical positions (objective collapse, hidden variables, denial of objective reality). It shouldn’t even be that surprising. After hundreds of years of every scientific theory that implies that I, personally, am in some way special or unique, getting knocked down, are we really supposed to keep falling for it?

You are of course correct that MWI does not in itself contain any mathematical model of how, in fact, quantum states decohere. But the argument that we should take the interpretational point of view of whoever does the best mathematical (or experimental) work in this area seems shaky. Given that any correct quantum mathematics is compatible with any mainstream quantum interpretation, why should we not take the best mathematics and combine it with what we separately understand to be the best interpretation?

I read the beginning of one of the papers in the mathematical theory of decoherence that you linked on this blog, and noticed that it relied on a system of axioms about what observers observe and how that relates to the wave function. I don’t think it explained what an observer is or how to tell which (if any) observers exist. If you add to this theoretical system any plausible definition of how to tell which observers exist from the wave function, you can conclude that there exists an observer which observes one outcome of the experiment and there also exists an observer which observes the other outcome, i.e. many worlds. I don’t think this reasoning is illegitimate because I haven’t written a paper on the mathematical side of decoherence.

There were historical predecessors to decoherence theorizing before Zeh, going back to Rosenfeld and company in the 1950s and even Heisenberg in the 1930s. Why the subject took so long to develop is an interesting question of psychology and perhaps even sociology, but telling why an idea got overlooked is always tricky. For example, Wigner nearly got the no-cloning theorem in 1961 but narrowly missed the point.

Weyl understood the mathematics of entanglement as early as 1931 — he called it a kind of

Gestalt,since Schrödinger hadn’t coined the modern word yet. In principle, he or anybody else after that could have said, “Hey wait, a generic unitary on a joint system will have no reason to respect the tensor-product structure, so it will rotate a tensor-product state into one that isGestalt-y, whose marginals will be mixed, I wonder what that implies?”I think that an underrated factor in the history of philosophizing about quantum mechanics is that, for much longer than we intuitively think nowadays, not everybody

trustedquantum theory in their bones yet. Weyl himself expected it to be “subsumed” into a better theory. As late as 1938, Kramers was saying that “everyone knew that the quantum theory was provisional” and that there was no guarantee that “after some years, Schrödinger’s equation would still be the root of everything.” (See the proceedings for the 1938 Warsaw conference, which also report that Heisenberg was speculating about some kind of breakdown of quantum mechanics at higher energies.)Will,

I think the essential problem here is that, while many if not most of us believe quantum mechanics should also in principle describe observers, we don’t know how to do this. There quite possibly are many different ways of thinking about what an “observer” is. It’s perfectly plausible that in some definitions of “observer”, time evolution take states with one “observer” into states with multiple, effectively disconnected, “observers”. You can postulate such a definition with such behavior if you want, but this doesn’t really buy you anything. If anyone asks you to say anything about the real world, all you can do is go back to what Copenhagen/textbook tells you to do.

What I see people arguing about MWI doing is starting with a sensible starting point (all is quantum), then ignoring the difficult problem this poses of how to describe observers, measurements, etc. and calculate something about them, instead starting in on empty discussions about conjectural behavior of undefined concepts.

Blake Stacey,

I’m sure you’re right that part of the explanation for the history of this subject is that its founders though of it as provisional. They had just been through a period of huge revolutions in physics, and expected another one soon.

Another part of the explanation may be the very limited experimental tools they had available, with lots of kinds of quantum behavior experimentally inaccessible. The context of the “Copenhagen” argument that you have to describe your experimental apparatus classically was one of much more limited kinds of equipment. If they had, say, a scanning tunneling microscope, they might have argued very differently.

If I may venture one Trimble comment not on the foregoing subject, oh, man do I miss those ApXX reviews. The typescripts were a thing to behold – just riddled with typos (

manyof which the U. of C. Press failed to catch; c’mon, you call yourself an editor and don’t know when a zero should be anOand vice versa?) and general zaniness. They rivaled Olin Eggen, whose secretary also prepared his submissions.I just wish they would have let me have one rather than tossing them to the nearest warm body.

I like the formulation:

> It’s perfectly plausible that in some definitions of “observer”, time evolution take states with one “observer” into states with multiple, effectively disconnected, “observers”.

But the key point has to do with a certain stronger, but equally true statement:

> It’s clear that in every plausible definition of “observer”, time evolution takes states with one “observer” into states with multiple, effectively disconnected, “observers”.

You’re completely right that this doesn’t make any progress on the difficult problem of defining observers and related concepts (which are not entirely problems in physics but also moral philosophy, psychology, possibly biology (of course if we use measuring devices and not humans as our observers these should merely be questions of physics and engineering)). But how much progress can we make on these questions if we can’t even get this one basic fact right without 60+ years of bitter war?

Peter Shor,

”anybody who believes there is a sharp distinction between virtual particles and real particles is classifying particles into two classes, one purely quantum and the other more real”

No. The more typical belief is that real particles are quantum, while virtual particles are purely illustrative tools for discussing high-dimensional integrals. See

https://www.physicsforums.com/insights/physics-virtual-particles/

“Of course, you can believe that everything is fundamentally quantum without subscribing to the MWI”

I do believe that, based on the much more rational thermal interpretation featured in my new book

https://www.degruyter.com/view/product/537801

Scott Alexander believes in the multiverse because Eliezer Yudkowsky believes in it – he comes from the LessWrong “rationalist” subculture.

Arnold Neumeier:

I didn’t say that the “more real” particles weren’t quantum in some respects.

But in actual fact, there isn’t a sharp distinction between “real” electrons and “virtual” electrons. It’s a continuum, even though dividing electrons into “virtual” and “real” can be useful for intuitive understanding.

This idea that real electrons exist and virtual electrons don’t actually exist (and are just a tool for doing calculations) seems to me to be a misconception carried over from the Copenhagen interpretation. To treat physics consistently, you either have to say that “real electrons” are just an approximation, or say that “virtual electrons” actually exist.

Peter Shor,

”there isn’t a sharp distinction between “real” electrons and “virtual” electrons. It’s a continuum”

This is not correct; you seem to mistake virtual electrons for short-living effective electrons.

As explained in detail in the Insight article linked to in my previous comment, there is a continuum between stable and unstable particles, but not between real particles (defined by having a state) and virtual particles (defined as internal lines in Feynman diagrams). It is impossible to write down the state of a virtual particle.

” “real electrons” are just an approximation”

Yes, since real electrons in QED are defined only asymptotically, at times $\pm \infty$.

langlands has some new (september) comments on the geometric langlands controversy. don’t think i’ve seen this on your blog yet so thought you / your readers might like to be aware. https://publications.ias.edu/node/2700