The title of the book emphasizes their misgivings about the probabilistic interpretation of quantum mechanics, and describes how Schrödinger’s cat arose out of discussions with Einstein. More of the book though is actually about their efforts to generalize GR and find a unified geometrical theory of gravity and electromagnetism. This began almost as soon as the field equations for GR were in place (1915). The book’s stories of media hype for bad ideas, involving physicists given rock-star academic positions at institutes set up for them make clear that some contemporary problems go back much further than I’d ever realized.
Einstein’s later work is, for good reason, dismissed as misguided, since it ignored quantum theory. He and Schrödinger did however have good reasons for skepticism about the Copenhagen interpretation of quantum mechanics. The measurement problem has turned out to be a very subtle one, with the cat experiment a very good way of making clear the problem. Their enthusiasm for ideas about unification that weren’t working was also way ahead of their time…
]]>Horgan: In 1995 you told me that it’s a “terrible time for particle physics.” Are you feeling any better about your field now? Are there any particular advances that give you hope?
Weinberg: I’m not much more cheerful.
…
Horgan: Do you still believe in the attainability of a “final theory” of physics, one that ends what you called “the ancient search for those principles that cannot be explained in terms of deeper principles”?Weinberg: I still expect there to be a final theory, but I’m less confident that humans will discover it in this century.
When asked about when string theory should be abandoned as a dead end he ignores that part of the question:
Horgan: In your new book, To Explain the World, you write that “scientific theories cannot be deduced by purely mathematical reasoning.” Doesn’t that principle apply to string theory? At what point, if ever, should string theory be abandoned as a dead end?
Weinberg: String theory may be inspired by mathematical reasoning, but not deduced, and certainly not confirmed.
He defends the multiverse with
Further, if we find some future theory that does make successful predictions about a lot of things, which turn out to be true rather than false, and if that theory also predicts the existence of a multiverse, then we should take that prediction seriously even though it can’t be tested directly.
which is true enough, but doesn’t address the fact that there is no such theory. The string theory landscape “prediction” of a multiverse is exactly the opposite sort of thing, not a corollary of successful predictions, but something being invoked as an excuse for failure to make predictions about anything at all.
For something more substantive, I recommend Alessandro Strumia’s theory summary for Moriond 2015. It has a lot of interesting commentary about a range of phenomenological topics. On the multiverse and anthropics he takes a quite different point of view than Weinberg:
Nobody talked about anthropics at Moriond 2015. This has an anthropic interpretation: Moriond is not in California. Clearly, social factors are playing a role, as always when experiments cannot set the issue. On one side, `having discovered the multiverse’ is physically indistinguishable from `having pursued a failed unification program’, but sounds much better. On the other side, future physicists could consider us as crazy for not having immediately accepted anthropic arguments.
He discusses “naturalness” extensively, including explaining why anthropics is no solution, since it doesn’t explain an unnaturally small Higgs mass.
]]>These days, things have changed. If you’re at Perimeter, prominent activities include:
On the other hand, it you’re in Santa Barbara these days, you might be participating in a KITP conference on Quantum Gravity Foundations. This is featuring very little about the technical issues in superstring theory being discussed at Perimeter, but a lot of discussion of deep conceptual issues in quantum gravity. There’s also a lot of willingness to throw out standard axioms of physics, maybe even quantum mechanics. They’re even letting Carlo Rovelli talk.
The sort of speculation going on at the KITP is featured on the cover of this month’s Scientific American, and this week Quanta magazine will be publishing a series of pieces on something related, the “ER=EPR” conjecture. There’s debate whether anyone really understands this and whether it is consistent with standard quantum mechanics. It also features a diagram that people call the “octopus” diagram. Back in the day it was Lee Smolin who was getting grief for an “octopus” diagram (see here), yet another way in which things have changed.
For a more balanced view of quantum gravity issues, you might want to spend your time in France, where the IHES recently hosted an interesting series of surveys of the subject (see here), and the Quantum Gravity in Paris conference featured more specialized talks. In the category of quantum gravity topics I wish I had more time to learn about, Kirill Krasnov’s talk was presumably related to this recent work, which looks intriguing.
]]>If you’re not listening to Villani tonight, you could be watching a PBS Nova program on mathematics, The Great Math Mystery. Among the mathematicians interviewed will be my colleague Dusa McDuff. As for the question on the PBS site:
Is math a human invention or the discovery of the language of the universe?
the answer is the latter.
What some mathematicians might consider the “Great Math Mystery” is whether Mochizuki really has a proof of the abc conjecture. There finally will be the topic of a workshop involving experts in the field, to be held this December in Oxford. Still no paper from Go Yamashita about this, but here you can find some photographs of the boards from his talks in Kyoto last month. Mochizuki himself has a new paper, inspired by conversations with Fesenko.
Also in New York this week, Bjorn Poonen will be speaking on Thursday. His topic is a heuristic argument that there is a finite bound on the rank of elliptic curves. For notes from a talk of his about this last year, see here.
Update: The Villani IAS talk is available here.
Update: At David Mumford’s blog he has a long and very interesting posting about the state of mathematical research publishing.
Update: One more piece of math news. Dan Rockmore has set up a public version of his Concinnitas Project, which lets people post, with explanation, a picture of their choice of a “most beautiful mathematical expression”. See here for details.
]]>The BBC has gotten very excited about this whole LHC thing.
Update: Now it’s two beams at 6.5 TeV. They just need to be careful to avoid beam collisions until the press event is organized…
Update: Maybe they weren’t careful enough. The Monday morning beam commissioning reports “Possibly first collisions”. No confirmation of this from the experiments, or officially from CERN.
]]>To get some idea of the scale of this funding, note that the entire NSF budget for theoretical HEP is about $12 million (the DOE spends about $50-60 million, but that supports groups at the labs, as well as computational hardware, and is decreasing). The Templeton Foundation has an endowment of over $3 billion (growing rapidly), and pays out over $100 million in grants/year (also growing rapidly). I don’t think my skills as a fantasist are good enough to imagine what this means for ten years from now in the future.
In other multiverse news, the Literary Review of Canada has published a review by David Orrell of the recent Unger/Smolin book, and an exchange of letters between him and Matthew Kleban. I wrote something about the book here, and I’m in many ways not very sympathetic to the point of view of Orrell and Unger/Smolin, especially about the role of mathematics in physics.
I’m more on Kleban’s side about mathematics, but the way he paints multiverse studies as the latest scientific descendant of the mathematics-driven successes of physics of the past is highly problematic. While this is a point of view favored at Stanford and at Templeton (Kleban has a $175,000 grant from them), I don’t think it’s a defensible one. Kleban’s arguments are
More to the point is the string landscape, a relatively concrete structure believed to follow from the mathematics of string theory.
Here “relatively” is a weasel word (relative to what?), masking the fact that we don’t at all know what the structure of the string landscape is.
contrary to Unger and Smolin’s assertions, recent work indicates that current or near-future cosmological observations – specifically, the detection of positive spatial curvature – would falsify the landscape (if it is false).
The situation with the measurement of spatial curvature is that recent Planck results give |Omega_K| less than 0.005 and the expectation is that it is zero to a much higher accuracy than that, way beyond anything measurable (this is considered one of the main arguments for inflation). This “prediction” isn’t “recent”. Susskind’s book on the multiverse ten years ago gave this one bit of sign information as the only prediction of the multiverse (see here). Shortly thereafter some authors were arguing that you could get positive curvature from the string landscape (see here). I have no idea if they’re right, but in a recent paper Kleban himself writes about this:
Positive curvature would probably not completely end discussion about a multiverse but it would be very bad news for the eternal inflation/CDL bubble nucleation framework.
and I think Orrell has it right that
I would be interested to see if the detection of positive spatial curvature actually falsified the theory – wouldn’t it just adapt?
Furthermore, the theory can be used to predict the signatures of cosmic bubble collisions: violent events where two previously separate “universes” collide.
There’s no evidence at all for such “signatures”, and I don’t think there’s any plausible argument for why they’ll appear in new data given that they haven’t been seen yet (I wrote here about Kleban’s Columbia talk about this). Final data from Planck on polarization are expected soon, but this is so implausible that I’m not sure Planck will even bother to look.
The problem with this kind of “testable prediction” is that it’s much like my claiming that my theory that the universe is controlled by a giant turtle is testable and predictive, since if you saw a big picture of a turtle in the CMB, that would be strong evidence for my theory. There was a reason Popper went on about falsifiability…
the standard model of particle physics combined with Einstein’s theory of general relativity – two of the most well-established theories in physics – predict a large landscape quite similar to that of string theory.
This one brings back the “string wars” era, since I haven’t heard anyone trying to use it (based on this) since 2007. Whenever people make a “string theory is just like the standard model” argument I’m never sure what to respond. How do you argue with someone trying to claim that the most successful physical theory ever, by far, is “quite similar” to a theory that has had zero success? It’s kind of like trying to argue with someone who wants to tell you that black is white, because they’re both kinds of grey. Surely they’re not serious?
In this case, sure, if you put the standard model on a complicated space-time background, added lots of fluxes, etc. to the background, maybe you could turn it into as useless a theory as string theory. This doesn’t mean it’s “quite similar”.
Update: Just noticed another recent essay about the multiverse, Marcelo Gleiser’s examination of whether Fairies live in the multiverse.
]]>With the agreement of Grothendieck’s family, the work of the Circle to bring Grothendieck’s unique story and writings to the public has resumed.
I do very much like one thing in the new version, the section about possible unification of ideas of quantization and of symmetry, where he speculates:
Quantization and fundamental symmetry will not appear as separate principles, but as two aspects of a deeper unity.
That’s pretty much one of the main motivations of the book I’m writing (see here).
Update: There are rumors going around tonight that there’s been a hoax perpetrated on the arXiv, something like the Sokal hoax. This has to do with an hep-th posting entitled Riding Gravity Away from Doomsday, which has appeared under the name of a very prominent string theorist, Ashoke Sen, winner of the $3 million Milner Fundamental Physics Prize. What I’m hearing is that no one can believe that Sen could possibly have seriously written something this silly, so it must be some sort of hoax. Speculation is that the hoax could have been carried out to make the hep-th moderators look bad, by showing that they’ll agree to anything, no matter how absurd, if it invokes the Landscape and the multiverse. Some think that Sen’s account must have been hacked and then used to post the nonsense paper, others think that Sen himself is behind the hoax, having had enough of the Landscape business. I’ll update this as more information becomes available.
Update: At least some papers on the arXiv still are serious.
Update: Beams are back in the LHC, successfully circulated at 450 GeV on Easter Sunday (live blog here). Next step, ramp up to 6.5 TeV.
]]>I am not a proponent of the idea that our Big Bang universe is just part of a larger multiverse.
About string theory, the LHC and SUSY, the exchange went:
If the LHC finds no evidence for supersymmetry, what happens to string theory?
Damned if I know!
Weinberg went on to respond to the issue of the testability of string theory by discussing the possible measurement of primordial B-modes, without mentioning that string theory makes no predictions at all about this.
Update: Sabine Hossenfelder performs the public service of reading the “PARALLEL UNIVERSES” paper and explaining what is going on here.
Update: This year’s Abel Prize went to John Nash and Louis Nirenberg. Nature News has a story here. The award to Nash was for his work on PDEs and the Nash embedding theorem. He already has an Economics Nobel, for his work on game theory. This surely makes him the first person to win not-quite-Nobels in two completely different fields.
Update: Also at Nature, news about the LHC problem.
]]>There’s also a review of a book from a few years ago about Einstein’s search for a unified theory, Einstein’s Unification by Jeroen van Dongen. The review addresses something I mentioned in my recent essay about mathematics and physics, that the development of GR provides a good example of a successful theory coming out of not just experiment and “physical intuition”, but motivated also by the serious use of deep mathematical ideas. According to the review:
Einstein employed two strategies in this search [for the GR field equations]: either starting from a mathematically attractive candidate and then checking the physics or starting from a physically sensible candidate and then checking the mathematics. Although Einstein scholars disagree about which of these two strategies brought the decisive breakthrough of November 1915, they all acknowledge that both played an essential role in the work leading up to it. In hindsight, however, Einstein maintained that his success with general relativity had been due solely to the mathematical strategy. It is no coincidence that this is the approach he adopted in his search for a unified field theory.
Besides the fact that Einstein said so, other evidence for the primacy of the mathematical strategy in this case is the simultaneously successful work by mathematician David Hilbert, who was definitely pursuing the mathematical strategy.
While I think there’s an excellent argument that a mathematical approach was crucial in Einstein’s discovery of the field equations, the later history this book deals with also shows the dangers this can lead to. Einstein spent much of the rest of his life on a fruitless attempt to get a unified theory by pursuing the same mathematics he had so much success with in the case of GR. It’s a good idea to keep in mind both examples. On the one hand, trying out some new deep mathematical ideas can lead to success, on a time scale of a few years. On the other, if you’ve spent 30 years pursuing a mathematical framework that has gone nowhere, maybe you should do something else. A lesson that Einstein’s successors at the IAS might want to keep in mind…
The story about new tests of the equivalence principle contains the usual nonsense about testing “string theory predictions”:
Using beryllium and titanium, they found gravitational and inertial mass equal to one part in 10 trillion, as they reported in Physical Review Letters in 2008. That’s not quite precise enough to test string theory predictions.
That “string theory predicts violations of the equivalence principle” is what used to be called a “factoid”, something not true repeated so often that it becomes a fact. It seems though that usage has changed, with “factoid” now often being used to refer to something true. A new word is needed.
Update: See here for an article by Michel Janssen and Jurgen Renn discussing in detail the question of the “mathematical” versus “physical” strategies in Einstein’s discovery of the GR field equations.
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