Some News:
I’ll be in England later in the month, in Oxford much of the week of the 26th-30th. That week is the week of the 2022 Clay Research Conference and Workshops. The evening of Tuesday the 27th I’ll be giving a public talk on Unified Theories of Physics, sponsored by the Oxford Centre of the Institute of physics.
The 2022 HowTheLightGetsIn festival in London was supposed to be the weekend of September 17-18, but has been postponed two weeks because of the death of the Queen. It’s now scheduled instead for the weekend of October 1-2 and I’ll likely be there, participating in a couple of panel discussions.
More of the Same:
I’ve written too much here over the years about the problems with multiverse theories. For short versions, there’s also FAQ entries here and here, and a piece called Theorists Without a Theory I wrote for Inference. Seeing some recent things about this topic from people I generally agree with (e.g. here and here) leads to an uncontrollable urge to reiterate some of my arguments, so:
- You can’t argue against the concept of a multiverse in general, dismissing unobservable universes. If you had a very successful theory based on ideas that simultaneously implied successful predictions about what you can observe, as well as unobservable parallel universes, you could get indirect evidence for a multiverse. The strength of this evidence would depend on the details of the theory, but it’s logically possible that this could be strong evidence.
- Arguments pro or con about the “multiverse” that simultaneously engage with the many-worlds interpretation of quantum mechanics and inflationary or string theory landscape models are a waste of time. These are two completely different subjects, which raise completely different issues and have nothing to do with each other. For the rest of this I’ll stick to the second subject, ignore the first.
- If you want to have a serious discussion on this topic, it should be about a particular model or well-defined class of models. One popular class is inflationary models. Here people often write down a well-defined model, but the problem is that it’s a toy model (e.g. no SM fields, just gravity and a hypothetical inflaton field unrelated to any field for which we have a tested theory). Another popular class is the “string theory landscape”. Here the problem is that you don’t have a well-defined model. People who work on this work not with a well-defined theory but with a list of properties of a conjectured, currently non-existent, theory (e.g. “M-theory”). There’s nothing wrong with doing this to see if you get interesting predictions about the world, which would give you some confidence in the existence of the conjectural theory. There is something seriously wrong with doing this if after decades of work you find that the list of properties you have is vacuous in terms of explanatory power.
- It’s important to understand just how vacuous the “string theory landscape” class of models is. The problem is not just a measure problem on the space of possible universes, but much worse: one has no idea what this space is that you would like to put a measure on.
- “Pseudo-science” is an accurate description of “string theory landscape” research. People have complained to me that it is too harsh, should only be applied to activities of people who are abusing the good name of science for discreditable purposes. Doing something because you refuse to admit failure of a scientific idea you have a lot invested in seems to me a discreditable purpose.
Update: Joe Conlon is upset that I’ll be speaking in Oxford. He objects to my credentials, but perhaps my views on his field of string phenomenology (which are shared by a large fraction of the physics community) might have something to do with it. I’m wondering if Conlon also complained about this recent Oxford speaker (video here).
Peter wrote: “If you had a very successful theory based on ideas that simultaneously implied successful predictions about what you can observe, as well as unobservable parallel universes, you could get indirect evidence for a multiverse. The strength of this evidence would depend on the details of the theory, but it’s logically possible that this could be strong evidence.”
I do not know what it means for a theory to “imply” something that is unobservable, this is a meaningless phrase. Whatever you mean by it “implying” something, it is not the same as indirect evidence because there can be no indirect evidence for other universes.
Indirect evidence means that you have a mathematical structure in your theory that is *necessary* to describe what you observe even though it cannot itself be observed (eg, the wave-function, or gravitational waves). The entire problem with the other universe is that because they are not observable you do *not* need them to explain anything you in fact observe.
For this reason there is no evidence that you can gather for their existence, neither direct nor indirect.
The assumption “unobservable universes exist” is unnecessary to describe anything we observe. That’s it. You are claiming that this statement is wrong.
Peter wrote: “Arguments pro or con about the “multiverse” that simultaneously engage with the many-worlds interpretation of quantum mechanics and inflationary or string theory landscape models are a waste of time. These are two completely different subjects, which raise completely different issues and have nothing to do with each other. ”
In both cases physicists mistakenly believe that someone which is unobservable exists and fail to acknowledge that this existence is a superfluous assumption which is hence not scientific.
@Sabine Probably our host does not want us to discuss the many worlds intepretation.
However, it is easy to imagine a theory that has the same relationship to the multiverse as the theory of the quantum wave function has to MWI, and we can discuss this hypothetical theory instead.
Suppose we had a theory which precisely defined a space X, with points of the space corresponding to different effective field theories or different values of the physical constants, and gave an evolution rule for a function f on the space X which can be interpreted as the density of universes with those laws of physics (for example a form of “cosmological natural selection”). Furthermore, one could calculate the likely behavior of the function f, and find that it concentrated on physical theories matching our own observations. (Or to avoid retrodiction, one could even imagine that the fundamental constants of physics in our universe were observed to change slightly over time, and this theory was capable of predicting that change via a flow on the space X.)
Supposing we had such a theory, it would seem reasonable to me to refer to the points of the space X as distinct universes, which the theory describes the dynamics of.
I’m with Dr. Hossenfelder on this, at least to the extent my grasp of the philosophy allows. It’s one thing to say your equation has, e.g., two solutions, one of which fits an empirical observation. It’s quite another to say the other possibility is “real”. One possibility that I’ll never observe is myself writing something about this without feeling the need to use scare quotes. Any kind of multiverse which must deny contact with experiment for any other part of the ensemble seems simply to fall on the other side of the line of demarcation. Why are we forced to acknowledge the other solutions “exist”?
That’s my biggest beef with the Everettians. Why is it so intellectually dishonest or lazy to simply dismiss my “other selves” with a shrug, or view them as artifacts with no scientific relevance. I suppose it forces me to insist that science is ultimately an empirical discipline, but that’s a hill I’m willing to die on.
Will,
Yes, please don’t try and start a discussion of MWI. To my mind at least, the issues there are quite different (QM is a well-defined theory, for one thing, and MWI enthusiasts often say their claim is just that all you need is QM).
However the scenario you describe is closely related not to MWI, but to the string landscape. My main problem with this scenario is just that it’s a hypothetical that has nothing to do with the current state of theory (no one has anything like the kind of theory you postulate). I see no point in arguing about a failed theoretical idea by starting with “assume the kind of theory desired exists and is completely successful”.
Sabine can answer you if she wants. I’ll write another comment related to yours.
I took “imply” to be mathematical/logical imply =>. My understanding is that GR mathematically describes what goes on just inside the event horizon of a black hole. I would say GR implies what is going on there, and it is not observeable to us. It would be fair to me to say all the evidence supporting GR is indirect evidence of what is going on just within the event horizon.
Sabine (and LMMI),
Different people think about this differently, here’s my take:
A fundamental physical theory is a model (expressed of necessity in the language of mathematics) which comes with a way to relate some of its elements to experimental observations. This last feature allows the model to simultaneously explain what we observe and be tested (if it says we should observe X and we observe not X, it is wrong).
A good theory is one that is simple, and based on few assumptions, from which flow many non-trivial predictions about what we will observe, predictions that can be and have been checked. The ratio between information out and information in is very high.
A bad theory is typically one that starts out simple, but makes wrong predictions. You can fix these wrong predictions by making the theory more complicated, but what you end up with is something useless: information out is just reprocessed information in, you are learning nothing new from such a theory.
Applied to the multiverse issue, I see no argument of principle that you can’t find a very good theory with elements that correspond to our observations and have been successfully checked, but also other elements that don’t. If a model is simple and highly successful, and non-observable elements are closely linked in its structure to tested, observable elements, it would be reasonable to say that one has evidence that the non-observable elements exist.
You can argue that elements of a model that cannot be tested by observation should just be cut out of it (characterized perhaps as “mathematics, not physics”, or “unscientific”). But I can also take the attitude that what’s important is how good the model is, and a good model can consistently contain observable and unobservable elements, it just has to have a convincing explanation of what you observe and what you don’t.
What’s really important to make clear is that the deadly problem the string theorists have is a very conventional one: they have a bad, failed model. They’re trying to wriggle out of this inconvenient situation by invoking non-existent successful models and getting people wrapped up in arguments of principle over those. Best to not do this since it’s besides the point, a waste of time, and accomplishes what those with failed ideas want: obfuscating the fact of their failure.
Peter: from the history of physics, do you have any example in minds of such non-observable predictions that would comes from the mathematical structure ?
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For instance in GR the mathematical structure (space-time is a curved manifold) yields only observable predictions that, indeed, made the theory successful (binding of the light, redshift etc). Even the effect that at the time seemed unobservable (gravitational waves, black holes) are nowadays observed.
I would say the same holds true for gauge degrees of freedom. They may well be classically unobservable, but lead to some quantum effect.
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How can one claim that a mathematical structure needed for the coherence/beauty/simplicity of the theory is forever unobservable ? Negative numbers (as I read, don’t remember where) where invented just for the convenience of calculation, but they were denied of any meaning: what sense would it make to give meaning to something which is less than nothing ? Well, today negative numbers are very concrete, to anybody who has ever experienced a deficit on one’s bank account 🙂
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But if something that is now unobservable might become observable one day, then I find it misleading to call it “multiverse”. It is just a part of our universe that at the moment is not accessible to our experiment.
martibal,
I don’t think issues of mathematics have anything to do with this, other than the fact that mathematics is the language you use to write the theory down. There are all sorts of issues about how this language describes elements and relations of the theory, and how you get from elements of the theory to experimental observations (the LHC detectors are not measuring the values of quantum fields…). But they’re besides the point I’m trying to make.
What I’m talking about is the possibility of a model having elements that are a fundamental part of a model’s structure (independent of how you write this down), but not accessible to experiment. I can’t think of a good historical example, maybe there aren’t any. But again, arguing over this abstractly is besides the point, so I’m not interested in trying to come up with examples and do so. The string theorists point to a logically possible class of models with this characteristic. I don’t think they’re wrong that these are logically possible, the problem is that all attempts to construct a model in this class which is consistent (mathematically and with observation) have failed utterly.
Martibal
You are asking Peter: “from the history of physics, do you have any example in minds of such non-observable predictions that would comes from the mathematical structure ?” Then you mention ” for instance in GR the mathematical structure …… yields only observable predictions……”
A good answer for you will be the Godel solutions to GR equations. Essentially, they are mathematically feasible, but they show “time looping” that was never observed!
Your question look trivial, but in fact it is at the core of any sustainable theory: how nature select in the general mathematical landscape this equation and not any other?
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@Peter Woit,
Well, if people disagree with your claim that “If you had a very successful theory based on ideas that simultaneously implied successful predictions about what you can observe, as well as unobservable parallel universes, you could get indirect evidence for a multiverse” by saying it’s impossible that a theory could imply unobservable parallel universes, I don’t think it’s possible to respond without sketching in more detail what a theory that implies unobservable parallel universes to look like.
I did indeed intend it to be similar to a potential version of the string multiverse theory, although it’s also a bit inspired by some other things.
@LMMI The issue is not just that an equation may have two solutions but that there is some dynamics involving multiple universes that (indirectly) makes predictions. You can’t just drop some of the universes in this scenario any more than you can drop some elements of any other dynamical system and get an equivalent system.
Of course no one is forced to believe anything and you can always deny the existence of whatever you please, but if theories of this type ever make solid predictions and do better than competing theories, any kind of consistent principle that denies the other universes will also have to imply that stars stop existing when they get far enough away from us and silly things like that.
I’m not advocating for “dropping” something so much as acknowledging a certain state of ignorance about its “existence”. Do I conclude “it’s as if there was another universe” or “there is another universe”? However unavoidable the question, I’ll never be able to settle the matter empirically. Hence my bias is to say that’s not a scientific question.
I mean, like, I accept the theory of object permanence — the theory that the stuff in back of my head continues to exist, that it doesn’t disappear and then reappear when I turn around — even in situations where I can’t use mirrors or cameras or any other mechanism directly to confirm the stuff’s continued existence. I accept this theory purely on the basis of its simplicity, even though it postulates the existence of entities that (by definition) are unobservable in principle to me at the relevant times.
Or, worse, simply call radically different things ‘the multiverse’, one of which is literally our universe past the cosmological horizon, and one of which is the strongest possible Platonism, ascribing physical reality to every logically consistent mathematical structure.
[I rechecked the WP article to see what it said about various people’s multiverse classifications, and ideas like “Given infinite space, there would, in fact, be an infinite number of Hubble volumes identical to ours in the universe.” really just trigger me, as a mathematician. Thus stuff needs citations to outright mathematical criticism.]
@Marvin: Goedel solution is usually put apart because, besides the equation of general relativity, one takes into account in the description of nature some causality requirements, that seem incompatible with the existence close timelike curves. This is not as if there were a landscape of solutions to Einstein equations, all equally physically relevant, and for some unknown reason nature selects one or the other.
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Then one may discuss if these causality requirements are legitimate (for instance, putting causality aside, is it possible for an extended body to have all its point following close timelike trajectories ? If not, then is this really a causal problem if an elementary particles follows a close time like curve ? etc). It might even happen than in the future, one finds some situation in which Goedel solution makes sense. But this does not mean that in our universe Goedel solution is excluded, and it exists in some “multiverse”.
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It is a bit like Dirac equation: energy was not supposed to be negative but if, in a precise mathematical sense, one takes solution of Dirac equation of negative energy, then one discovers antimatter. This does not mean that matter lives in a universe, and antimatter lives in another one.
@Peter: I agree with your view on string landscape (it is inconsistent both with experiment and from the mathematical point of view). I am just a bit puzzled by your disagreement with Sabine.
Maybe this is just a matter of vocabulary, whether one intends by “multiverse” the elements of a theory that cannot be directly observed (for the moment, but with no certainty that they may not become observable in the future) or another “reality” which, by definition, has not any possible interaction with our universe.
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Yet, I do not find innocent that string’s hype use the word “multiverse”. It is again a way to oversell the theory using some “sexy” terminology: the “tiny little vibrating strings that describe everything” are now replaced by “a landscape of possible universes”. This is far more appealing to the public than twistors or noncommutative algebras. Pointing this out, as Sabine does, does not seem to me so useless.
martibal,
Just to make clear my problem with the string landscape: it’s not that it’s inconsistent with observation or mathematics, it’s that there is no theory. People insist on talking about this subject without noticing that the “theory” they are talking about is nothing more than some hopes and dreams adopted by a group of people as a tactic to avoid admitting that a specific research program has failed utterly. My main disagreement with Sabine and others is that they argue as if there is a theory, engaging in arguments about what is “real”, what is “unscientific”, what is “ascientific” etc., etc. which are completely besides the point. If you argue about this kind of thing, there’s no right answer and it completely obfuscates what is going on, and lets the people doing this get away with it (and “it” is convincing people unification is hopeless, and salting the ground so that no one will ever try again).
For more along the same lines of what I’m disagreeing with, this morning there’s a tweet from Will Kinney
https://twitter.com/WKCosmo/status/1569647758604124163
“This gets multiverses exactly right: they arise naturally out of the math, but there is no way — even in principle — to know whether or not other universes exist.”
He’s referring to certain specific models of inflation, which have nothing to do with the string theory landscape models where you get different physics in different universes. People really need to make substantive arguments about specific models, not empty statements covering all sorts of completely different sorts of theories.
Also, not a big fan of bringing “math” into these empty arguments. The sort of math used in these models is very basic college-level stuff. Referring to “math” this way is a bit like if whenever I discuss anything with anyone, I make a big point of going on about how they are using “English”, as if that were relevant to their argument.
Consider the EW phase transition in the standard model. The SM is sufficiently well established that nobody would seriously argue with the statement that this transition took place in the early universe (people might argue about the details). However, it is unlikely that any super-advanced civilization will ever observe the transition in the laboratory (the energy density is 10^8 times higher than the critical energy density of the QCD transition, and the energy density created in pp or heavy ion collisions only grows very slowly with the beam energy in a collider, roughly like the inelastic cross section log(E)^2).
We could imagine that the SM predicts (it does not — but it could have) that the transition produces domains in which the strength of the weak interaction differs. I would be prepared to consider this as a multiverse scenario, one that does rely on well established physics.
Thomas Schaefer,
Thanks, that’s a good example of how one might have a well-tested theory that implies unobservable regions with different physics.
This inspires a different example of the point I’m trying to make. One can in principle imagine “brane-world” scenarios in which at LHC energies one could excite brane degrees of freedom that would decouple from our brane as a separate stable brane. Suppose one started doing these experiments and saw exactly what the theory predicted, including phenomena corresponding to exactly what one would expect to see in such a decoupling event. Should one say that such decoupled branes don’t “exist” and any reference to them or their properties is “unscientific”?
I’m inclined to think there’s an important distinction between assuming something empirically manifest continues to exist when I’m not looking at it, and assuming something I never observed and can never be observed, ever, does. I don’t find conflating such things to be persuasive enough, but I do admit those who do have a point.
Kaku has become a crank (the one good thing about him writing books about quantum computing is that he is not writing them about string theory). I wasn’t aware of his talk at the Oxford Union; the Oxford Union is an undergraduate student debating society (albeit one with a long and notable history) with a reputation of inviting controversial speakers, mostly on politics, and no one looks to the Oxford Union for guidance on the state of theoretical physics.
You are not a crank and I enjoy reading your blog. I do (obviously) think you wrong on the scientific substance, and, yes, do not think the UK’s professional society for physicists should be presenting you to the public as a scientific authority on unification (and the level of research activity does indeed feed into this view). (But, also, as I said elsewhere, I am strongly against rescinding invitations once made.)
But there is nothing personal to this; your views are well known and while I think them wrong and misguided, they are always expressed in reasonable, if direct, language.
I’d be quite happy to meet for coffee or something similar when you are in Oxford, let me know if interested.
Joseph Conlon,
Thanks for the comment! I’ll contact you privately to see if we can find time to meet up when I’m there, I would enjoy doing that.
I’m in the middle of preparing that talk, it will mostly be things that much if not most of the theoretical physics community would agree with (even the skepticism about string theory unification). At the end there will be something about my own recent ideas about unification, with warning labels these are speculation not now vetted by the community. Some of this will be about a fundamental role for twistors, which should be less controversial in Oxford than anywhere else in the world.
Good to see a cordial exchange here between JC and PW. However I’d like to suggest, with all due respect, that JC is on rather shakier ground relative to PW than he seems to suppose. If fundamental physics has been stuck for a generation, who contributes more: one whose wheels spin at high speed (=copious “research output”) or one who tries to show that the spinning is idle? This is science, not history, so we can’t say definitely and for all time that the spinning *is* idle. But we also can’t say – and given recent history really shouldn’t say – that the spinning *isn’t* idle. This is so despite what is surely a very strong tendency among wheel-spinners (if that’s what they are) to regard what they do as progress. This suggests two very different kinds of interest: that of “active researchers” in a specific research program, and that of practitioners more generally who believe that the research program is a failure. Even if PW weren’t proposing to share speculative ideas in fundamental physics at Oxford but only a critique of what he takes to be a failed research program, those who think of themselves as contributors to the program would do well to receive him not just cordially but modestly.
“If fundamental physics has been stuck for a generation, who contributes more: one whose wheels spin at high speed (=copious “research output”) or one who tries to show that the spinning is idle?”
`If” is having carry quite a lot in this sentence: certainly, *if* fundamental physics (or anything else!) is headed down a dead end, there is no credit for continuing to drive in that direction. But, generally, my view of the history of theoretical physics is that everyone should be rather humble about their ability to judge good ideas and dead ends on timescales much less than a couple of decades, and in many cases much longer than that.
And I never really get why people think string theory is an example of failed program (as I describe in my book, it has delivered too much to many different areas). To me, the highway which really is littered with the corpses of dead and never-to-be-resurrected ideas is that of BSM model building.
Peter Woit,
Have you ever written down your thoughts on MWI? I would be curious to know.
Pedro,
I don’t remember exactly where I wrote about this. If you look through the blog postings in this category
https://www.math.columbia.edu/~woit/wordpress/?cat=21
you’ll find various places where I wrote something.
I don’t want to start a discussion here, but a quick thing to point out is that MWI proponents don’t have any new equations or new theory, they say just use the standard Schrodinger equation. They argue that talking about a multiverse solves interpretational problems, but to me this just is verbiage that doesn’t address the difficult underlying questions, which really are about how to understand the physics of how classical behavior and our picture of a classical world emerges from the quantum theory.
Peter writes
“… and lets the people doing this get away with it (and “it” is convincing people unification is hopeless, and salting the ground so that no one will ever try again).”
That may be one possible angle. But lately I have been wondering that, while perhaps not hopeless on scientific grounds (although that’s open to debate), it seems hopeless to me in terms of how the community reacts, the particle physics community in particular. They seem to have very rigid preconceptions on how unification should work and be done. If your theory doesn’t meet that, then they will ignore you. A case study for me regarding this has been Connes’ spectral standard model. It’s a purely geometrical unification of all forces and even the Higgs field into a single framework and action functional. The early versions had many problems, but the current incarnation is quite good. My point is not to do propaganda for this model, but it’s quite remarkable how little attention it receives from that community. My feeling is that this is because the unification is classical (and then unification doesn’t seem to solve the non renormalizability of gravity) and that no new, beyond SM particles/fields are predicted at higher energies (maybe only one more at most). These two things go against core traditional beliefs in the community on this topic since many decades.
So, it seems to me that, even if you are Connes, Fields medal etc., unification is hopeless. What’s left for us common mortals, then? Unification seems hopeless because the particle physics community is making it so. It’s not even the science what is counting here. Maybe that’s one of the reasons nobody will ever try again, why bother? Nobody will listen to you. I’m very pessimistic, to be honest.
>>People insist on talking about this subject without noticing that the “theory” they are talking about is nothing more than some hopes and dreams adopted by a group of people as a tactic to avoid admitting that a specific research program has failed utterly.
Seriously, Peter? We *do* actually have a nonperturbative string theory—not a set of “hopes and dreams”, but a complete theory. AdS/CFT tells us that string theory on anti-de-Sitter space is dual to N=4 Super-Yang-Mills. I see holographic dualities every single day in my research, and that’s actually how I *define* non-perturbative superstring theory on that background—the dual to the superconformal field theory. It’s clear that you’re still living in the 1990s, unwilling to accept, or even unaware, of the enormous progress in understanding the nonperturbative behavior of string theories enabled by holographic dualities. We have a theory now, Peter.
And as somebody who is so enthusiastic about the Standard Model and QCD, I’d imagine you’d be ecstatic about the enormous progress in understanding the nonperturbative behavior of QCD that has been enabled by string theory and AdS/CFT and similar dualities. Surely that progress in understanding QCD is more than enough to justify this research project. But I guess you’d rather stick your head in the sand than admit defeat.
The Angry M-theorist,
A major failed hope and dream is that AdS/CFT has anything to do with the real world (it says nothing about 4d string vacua that would give the SM + gravity).
This kind of anonymous comment is word for word the kind of thing I got a lot of nearly 20 years ago. Back then I wasted too much of my time trying to have serious discussions with such commenters. Now I’m a lot older, and somewhat wiser…
Okay, Peter. Let’s talk about the landscape. It’s extremely frustrating to me that you, and the other folks in this thread (Sabine, etc.) keep harping on this point that the landscape of string vacua furnishes no predictive power. The opposite is actually true. Statistical surveys of string vacua have made predictions about particle masses borne out by experiment. We aren’t at the point yet where we can predict precise ranges of physical constants from statistical arguments about string vacua, but we HAVE made predictions confirmed by experiment. This is not my research area, but I invite you to read this excellent review article
https://arxiv.org/pdf/2202.11578.pdf
Examining vacua compatible with KKLT moduli stabilization, the authors construct a distribution of SUSY masses and discover that sparticle masses are likely beyond the reach of the LHC. Thus, the fact that the LHC has not yet found SUSY is, in fact, resounding experimental confirmation of the KKLT landscape of string vacua. It’s hilarious to me that you keep yapping here on your blog about the LHC “not finding SUSY.” while that’s actually the biggest piece of experimental evidence for string theory and the KKLT landscape we have.
The Angry M-Theorist,
This really does take me back. The problem with anonymous comments like yours is that it’s impossible to tell whether the commenter is a troll or an idiot. So, at some point I just started deleting all these things. Will revert to that policy now.
@Alex (16 September)
I find the remarks, especially those regarding Connes’ approach, inspiring and deserving some further discussion, so please forgive me if I am going a bit out of topic here.
First of all, I would like to point out a nice interview of Alain Connes in chapter 6 of the relatively recent book “Conversations on Quantum Gravity” by Jay Armas.
There (at the end of page 139) in the discussion on the discovery of the mathematics underpinning the renormalization process, we find the following sad and disillusioned statement:
“These breakthroughs were enough for me to come to terms with my discontent with the method used by physicists. The sociological fact that they have not been understood by physicists nor are they part of their general knowledge is totally unimportant to me.”
One might of course imagine that all of non-commutative geometry has been receiving the same cold reception by the community of theoretical physicists for very similar reasons (lack of ability to appreciate the mathematical details) … for sure, the initial failed prediction of the Higgs mass (coupled with the usual “Popper nonsense” about crucial experiments ditching entire theories) has been easily exploited by the “mathematically lazy” … but there might be a few other factors involved here.
While, in one way or another, string theory or loop quantum gravity have been claiming from the very start to propose fundamental theories of physics solving both (ST), or at least one (LQG), of the unification and the quantization of gravity problems, non-commutative geometry (apart from rare exceptions, for example Jesper Grimstrup’s attempt) has been less assertive, more humble in goals and has almost never been considered, even by Alain Connes himself (private conversation in Cargese 1997), as an approach dealing with foundations of quantum gravity: the spectral triples of Connes’ standard model are supposed to be a description of geometry only at the (Euclidean) “classical level” (see page 152 of the interview), to be further subject to “second quantization” and “renormalization” treatments, in exactly the same way and with exactly the same issues as any other classical field theory undergoing quantization.
Hence it is not surprising that talks and contributions in non-commutative geometry often appear in the “phenomenology” section of physics conferences, where people consider “approximate models”.
My (wrong?) feeling is that the possible fundamental nature/meaning of “(non-commutative) geometry in a fully covariant” field theory context is left unexplored also because of some current deep ideological impasse: when all of the quantum gravity communities are in perfect agreement on the “emergent nature of space-time geometry” (and the lack of meaning of geometry in a fundamental theory) and somehow even Alain Connes himself does not dare to question such dogma
[quoting for example from page 140 and the mid/end of page 149 of the previously mentioned interview:
“I find it quite an ill-conceived idea that one could quantise gravity as one quantises all other fields because I don’t believe that geometry makes any sense, say, above the Planck energy. This belief is shared by many people. I think that in the same way that the Brout-Englert-Higgs particle emerges from symmetry breaking [16, 17], the actual geometry of spacetime also emerges from symmetry breaking and does not make sense beyond a certain energy level. One of my leitmotifs for developing non-commutative geometry has been to try to understand how geometry can actually emerge from purely Hilbert space considerations.”
“In particular, we have to imagine a more fundamental theory in which the geometry doesn’t make any sense beyond the Planck scale but appears out of spontaneous symmetry breaking when the temperature is lowered.”
“So my belief is that there is an enormous symmetry which is the symplectic unitary group; this symmetry is spontaneously broken and only then the geometry emerges. This is one possible scenario and if it holds it shows that it is vain to try and quantise gravity in a given geometrical background in which case one is quickly confronted with the lack of renormalisability, since at sufficiently high energies the geometry itself will have totally disappeared.”]
… there seems to be little space left to propose variations to the common narrative in which non-commutative geometry is just taking the place of classical geometry in some mid-low energy emergent scenario adding “corrections” to usual treatments, rather than conceptually changing the panorama.
Of course, the “emergence of geometry from Hilbert space” has also a much deeper meaning, where “emergence” stands for a new “spectral approach” to geometry (but then a full spectral theory of non-commutative algebras is still missing and taking the usual “dual” point of view, shifting focus on algebras, … seems to be not enough to give a clear meaning to geometry in quantum gravity).
Best Regards (sorry for the long post) 😉
Paolo
“Arguments pro or con about the “multiverse” that simultaneously engage with the many-worlds interpretation of quantum mechanics and inflationary or string theory landscape models are a waste of time. These are two completely different subjects, which raise completely different issues and have nothing to do with each other.”
What about this paper, Peter, which suggests otherwise (i.e., that the MWI and eternal inflation are somehow related?)
https://arxiv.org/pdf/1105.3796.pdf
Does this offer you any great insight into how classical behavior emerges from quantum? Or into inflation?
Guy in CS,
Waste of time applies. I’ve never heard of anyone who thinks that makes any sense.
“I’ve never heard of anyone who thinks that makes any sense.”
What about Leonard Susskind and Raphael Bousso? Presumably they think the idea makes sense? And presumably you’ve heard of them?
If you’ve identified flaws in their reasoning, perhaps you could bring that up with them, rather than categorically rejecting their work as “nonsense.”
By the way, Bousso has over 8,000 citations, and Susskind over 39,000. So I find it hard to believe that, working together, they would write something that makes no sense to anybody.
Anonymous Theorist,
The “Multiverse Interpretation” paper was eleven years ago. If you can find a recent article by anyone (including Bousso-Susskind) that explains this in a comprehensible manner and makes a compelling case that it is not nonsense, please provide that rather than citation counts.
I recall a comment from Steven Weinberg somewhere that he could make no sense of this. Weinberg was more polite, less polite was Lubos Motl, who wrote “they’re on crack”, see
https://www.math.columbia.edu/~woit/wordpress/?p=3723
Peter,
A couple points.
1. Please think about the way your attitude here would affect a vulnerable young graduate student or a postdoc who’s trying to make it in the competitive environment of high-energy theory. When they see a professor like yourself trashing others’ work, calling it “nonsense”—the work of physicists with tens of thousands of citations under their belts—won’t they be afraid that the established professors would trash their own unorthodox ideas even more? Graduate students and postdocs need to feel like their potentially unorthodox theoretical contributions will be welcomed, rather than trashed as “nonsense.” You are really doing a disservice to the community with this kind of language. Again, point out a specific flaw in Susskind’s reasoning rather than categorically condemning his idea.
2. If there’s one thing I can’t stand, it’s hypocrisy. You claim that string theory is “not a true theory.” There’s a complete theory for perturbative superstring scattering amplitudes on flat ten-dimensional spacetime, but no complete “theory” that would tell us about the moduli space of four-dimensional string vacua. It’s true, there is only the conjectured “M-Theory,” and we don’t yet have a handle on what the degrees of freedom are for that theory (although we have some ideas). But if that’s your central criticism of the string research project, I could EQUALLY WELL say that about your “unification theory,” Peter! Your so called twistor unification of gravity and the standard model is a joke. It’s manifestly background dependent. In fact it’s all on flat spacetime. The twistor geometry here only tells you about flat Minkowski spacetime. The Penrose-Ward transform here only tells you about conformally invariant wave equations on Minkowski spacetime. You’re examining quantum fields on flat four-diemsnional space. So when you claim that you have a theory of “quantum gravity” and the geometry is organized around flat Minkowski spacetime with the fixed background, it makes me laugh. How can you attack string theory when your own unification theory only works on a flat Minkowski background?
Anonymous theorist,
I think you’re doing much greater damage to young theorists by insisting that the way to evaluate theoretical ideas is by counting up the citation numbers of the authors, and that authors with high citation numbers cannot possibly be writing nonsense. If you think the Bousso-Susskind multiverse interpretation should be taken seriously, explain why or point to such an explanation. It’s not my job to write out detailed explanations of why some particular piece of nonsense that no one is paying attention to is nonsense (despite the author’s citation numbers).
About string theory, the problem is not that it’s an incomplete theory, it’s that everything that is known about the theory points to it looking nothing like the real world. You can only get the real world in a very complex and poorly understood way, and it looks like if that works you could get anything you want the same way.
About my own ideas, they’re definitely much more incomplete than string theory. I’m enthusiastic, continuing to work on them and in the middle of writing up more. And no, what I’m trying to write down in more detail is not a theory of flat space-time. Gravity is included in a chiral version, the sort of thing Ashtekar and then the LQG people have worked with, with some differences (including working in Euclidean signature). But, this is really off-topic here, and given that you evaluate ideas based on citation counts, I’m clearly not going to get anywhere with you.
Oh, and also, I’m currently on vacation in one of the most spectacularly beautiful places I’ve ever seen, in the Lake District of England. So, shouldn’t be spending time on this anyway…
Speaking of hypocrisy:
Anonymous Theorist, point 1: don’t trash other people’s unorthodox ideas, as it might discourage others
Anonymous Theorist, point 2: Peter Woit’s unorthodox idea is ‘a joke’ and ‘makes me laugh’
“I could EQUALLY WELL say that about your “unification theory,” Peter!”
Some red flags from Anonymous Theorist tells me to leave well alone:
1) Presenting arguments in ALL CAPS.
2) Using “quotation marks” to undermine others work.
3) Attacking others work instead of defending your own position.
Enjoy the Lake District. It is a beautiful place 🙂
Felix/Sam,
The problem with anonymous comments like that one is that I honestly can’t tell whether they’re written seriously, or whether they’re written by a troll trying to discredit string theory (and waste my time…).
@anonymus theorist: String theory has been involving thousands of people, for many years, it has spread into common culture thanks to lies (“the idea is very simple: the world is made of tiny little vibrating strings”), trying to impose itself by intimidation (“the only game in town”, “no other approaches to quantum gravity is viable”) and claiming to absorb other ideas in a strongly dishonest way (“non commutative geometry is just the geometry of string theory” “Connes model are the low energy limit of string theory”). So please keep a low profile regarding hypocrisy and any other moral qualities.
Regarding Peter, your attitude makes me think of those heterosexuals that complain about the gay pride and want an heterosexual pride: as far as I know, no kids have been kicked out from home because straight. As far as I know, Peter’s idea are not studied in the most reknown institute, receiving large quantity of money (how many breakthourghprize to twistors ?).
And please, don’t pretend to take care of young people’s future. If you were really concerned with this, then you would urge young people to have a critical view on strings and not to listen to propaganda.
Some years ago I have heard Vaffa at ICTP in Trieste, followed by Connes. Vaffa’s propaganda in front of students that, most likely, could not know the real state of the art of the field was a shame.
But at least he put his face.
Peter,
For what it’s worth, I’d put money on “anonymous theorist” being an actual theorist and not a troll. They clearly have some technical knowledge about string/high-energy theory. If they are a troll, they’re probably somebody in the hep-th community and not a random layman. It seems to me that a genuine internet troll would be more interested in stirring up culture war nonsense than, say, wasting the time of a moderately well-known physics blogger.
Anyway, the arguments here are almost nostalgically reminiscent of the 2000s era string wars. A blast from the past, as it were.
Peter,
A troll or not a troll, it is actually these kinds of heated arguments that make your blog more interesting than others. Please keep replying wisely to these kinds of attacks from arrogant people.
Btw, I find the citation count for judging the value of others’ works an extremely interesting topic (Susskind himself used it in one of his arguments against Smolin). Why don’t you devote a whole entry in your blog just for this citation count issue? It is at the root of the socialogy that led to the present crisis in high energy physics.
@Anonymous Non-Theorist:
It seems to me that citation count is close to useless when you have a faddish area (high-energy physics) some of whose leading practitioners are producing vacuous papers which it seems everybody feels compelled to cite anyway, because otherwise they might upset these practitioners, something which they fear would negatively impact their careers.
Mathematics is much better off — when Michael Atiyah announced his proof of the Riemann hypothesis, most mathematicians paid no attention to it.
Thank you Peter Shor!
Your point raises another interesting topic which is: why socialogy works in high-energy physics, but does not in mathematics. That’s why I asked Peter to open a new thread in his blog for this citation issue because it’s a rich (but off) topic. I remember Peter promised once in this blog to dedicate a post to the difference between math people and physics people and how can one be both. So maybe he could include in that topic this citation issue as well 😉
Re Peter’s early comment beginning: “Sabine (and LMMI), Different people think about this differently, here’s my take: …”
(First-time reader here.) This is one of the best short statements I’ve come across of how theories-models-observations relate, and what counts as evidence, in any scientific inquiry. So, thanks a lot Peter!
Do you (or others who are sympathetic) have any recommendations for longer no-nonsense elaborations of this particular “take” on these epistemological issues. (I know they have been discussed endlessly over centuries, just asking if there is any article, book, or blog post that someone thinks is especially good/worth reading). Thanks!
Scott Aaronson, the point is that when you accept such a theory you are doing philosophy (however reasonable and “obvious” you might think that philosophy is!) rather than science.
Adam Treat: not quite. It follows from accepted physical theories such as Newtonian physics that the back of my armchair continues to exist even when I do not lean against it, or look at it. You may try to cook up a physical theory in which the back of the armchair disappears, but it’s simply not the theory that’s in physics textbooks.
Asking what are the consequences of a given physical theory should still be called science (armchair science?) rather than philosophy, in my opinion.