I had just been thinking the other day about how little one hears recently about the multiverse, with those previously involved in heavy promotion of the idea perhaps having thought better of it. Today however, Quanta has Physicists Study How Universes Might Bubble Up and Collide. This describes work of a sort that has become popular in recent years: study of various condensed matter systems, with a huge dollop of hype on top about quantum gravity based on some aspect of the condensed matter theory calculation having some vague relation to some calculation in some toy quantum gravity model or other.
I’ve written extensively here and elsewhere about the real problem with all claims by theorists to be studying the multiverse: they’re Theorists Without a Theory, lacking any sort of viable theory which could make the usual sort of scientific predictions. The main problem with the Quanta article is at the beginning:
What lies beyond all we can see? The question may seem unanswerable. Nevertheless, some cosmologists have a response: Our universe is a swelling bubble. Outside it, more bubble universes exist, all immersed in an eternally expanding and energized sea — the multiverse.
The idea is polarizing. Some physicists embrace the multiverse to explain why our bubble looks so special (only certain bubbles can host life), while others reject the theory for making no testable predictions (since it predicts all conceivable universes). But some researchers expect that they just haven’t been clever enough to work out the precise consequences of the theory yet.
Now, various teams are developing new ways to infer exactly how the multiverse bubbles and what happens when those bubble universes collide.
The big problem is with:
they just haven’t been clever enough to work out the precise consequences of the theory yet.
The reference to “precise consequences” is a common misleading rhetorical move, implying that there is no problem getting “imprecise consequences”, that the problem is just getting those extra digits of numerical precision. What’s really going on is that we know of no theoretical consequences of the multiverse, precise or imprecise, because there is no viable theory. The logic here is pretty much pure wishful thinking: if you look at colliding Bose-Einstein condensates and see a particular pattern, then if you saw a pattern like that in the CMB, you could try and infer something about your unknown multiverse theory. It’s not unusual for theorists to work on speculative ideas involving some degree of wishful thinking, but this is a case of taking that to an extreme.
Update: One of the very few theorists who has pushed back on the multiverse ideology is Paul Steinhardt. Howard Burton has posted here something from his interviews with Steinhardt, which includes this from Steinhardt:
“I’ve had this discussion where I’ll say, ‘Well, what do you think about the multiverse problem?’ and they reply, ‘I don’t think about it.’
“So I’ll say, ‘Well, how can you not think about it? You’re doing all these calculations and you’re saying there’s some prediction of an inflationary model, but your model produces a multiverse — so it doesn’t, in fact, produce the prediction you said: it actually produces that one, together with an infinite number of other possibilities, and you can’t tell me which one’s more probable.’
“And they’ll just reply, ‘Well, I don’t like to think about the multiverse. I don’t believe it’s true.’
“So I’ll say, ‘Well, what do you mean, exactly? Which part of it don’t you believe is true? Because the inputs, the calculations you’re using — those of general relativity, quantum mechanics and quantum field theory — are the very same things you’re using to get the part of the story you wanted, so you’re going to have to explain to me how, suddenly, other implications of that very same physics can be excluded. Are you changing general relativity? No. Are you changing quantum mechanics? No. Are you changing quantum field theory? No. So why do you have a right to say that you’d just exclude thinking about it?’
“But that’s what happens, unfortunately. There’s a real sense of denial going on.”
Update: Ethan Siegel has an excellent piece on the basic problem with string theory (to the extent it’s well-defined, it has too large a (super)symmetry group and too many dimensions, no explanation for how to recover 4 space-time dimensions and observed symmetry groups).
Here’s why the hope of String Theory, when you get right down to it, is nothing more than a broken box of dreams.
Update: If you’re looking for a detailed discussion of multiverse theories, of neither the usual promotional sort, nor the highly critical sort I specialize in, I can recommend Simon Friedrich’s new book Multiverse Theories: A Philosophical Perspective. Friedrich has a blog entry about the book here.
Why do writers at Quanta and other science magazines bother with this crud when there’s plenty of much more interesting actual new physics to write about? – you know, physics where people use theories to make predictions, test these predictions in the laboratory, and then refine their theories. Are condensed matter physicists just too busy making actual discoveries to chat to the journalists?
Quanta is more focused on the kind of actual new physics you suggest than just about any other publication, but I guess the multiverse is still irresistible for use as a hook to get attention.
The main problem is with the physics community itself, which has over the years produced far too little pushback on multiverse pseudo-science. A rare exception is Paul Steinhardt. I just ran into Howard Burton’s interviews with him, will add a link to the posting. He quotes Steinhardt explaining that the physics community basically tries to sweep the multiverse problem under the rug instead of confronting it.
Steinhardt’s comments on the multiverse are misleading.
Some inflationary potentials give eternal inflation. Usually this occurs when they are extrapolated into a regime far from where the observed inhomogeneities are produced, and in a region where the calculations cannot be trusted any more.
Some inflationary potentials do not give eternal inflation.
The conflation of inflation, eternal inflation and multiverse (all distinct things) by Steinhardt et al is part of the multiverse hype problem, not a welcome respite.
I don’t want to start up the usual arguments over inflation, or get involved in arguments over different behavior of different inflationary potentials.
Steinhardt is one of the founding fathers of inflation, now argues against it. His main argument about this is that inflation doesn’t do one of the things it was supposed to do: explain why you get a smooth, flat universe without having to tune that into the initial conditions. As far as I can tell, he’s on solid ground when making that argument.
“What lies beyond all we can see?”
This answer is just as scientific as multiversology while having the added benefit of being much more delicious.
I should keep my mouth shut because of my ignorance, but it seems to me that mathematical work of Uhlenbeck and others on compactification of moduli spaces by adding `bubbles’ is important and interesting: and that if someone has something serious to say about `the multiverse’, they should be aware of that literature. As far as I can tell, the set of such persons is well-approximated by the empty set.
Not to change the subject, but as a chemist I think that Paul Steinhardt should have shared the 2011 Nobel Prize in Chemistry for quasicrystals. His contributions were deep and wide-ranging.
Check out particleclara on TikTok for songs and info about CERN and the LHC. Maybe not directly relevant to this post but it just went up today and it is certainly relevant to this general blog.
Is it possible that the writers of Quanta are simply not aware that there are other topics to write about in fundamental physics? Alternatives to string theory, multiverse and inflation, but still quite interesting, thought-provoking and equally captivating for the non-expert audience? Maybe they simply don’t know that such topics exist?
I saw a couple hopeful signs, with effort. Yeah, it’s “crud”. But unlike the majority of related content I’ve read, there was at least an attempt at moderation. Some work is characterized as “the [most] baby version of this problem that you can think of,”. If the reader doesn’t recognize how pointless it all is, they may still come away with a sense that the odds aren’t good: “you’ve taken a lot of things that are just very hard for physicists to deal with and mushed them all together and said, ‘Go ahead and figure out what’s going on,’”. There’s even a hint of an admission: “It’s a long shot”.
Not great, but not nothing. Multiverse Mania has set a low pop-sci bar, but I felt this article rose a smidgen above the norm.
I don’t think Steinhardt’s argument that inflation -> eternal inflation -> multiverse is on solid ground. Let me elaborate… (I won’t even go into the eternal inflation -> multiverse part.)
1) The eternal inflation regime only exists for potentials of certain shape. You can easily write a potential such that there is no eternal inflation.
2) Even for the potentials that have an eternal inflation regime, it is typically far separated from the observable regime.
For example, for the archetypical quadratic potential, observable modes are generated around 60 e-folds before the end of inflation, and eternal inflation regime is about 10^6 e-folds earlier. In terms of field value, this is a factor of 10^3. There is no reason to suppose that we can just extrapolate the potential there.
3) Even if the potential shape for large field values supports eternal inflation, there is no reason that the initial conditions have to be such that the field starts there. In fact, Steinhardt has criticised inflation with the argument that we don’t know the initial conditions – but his argument about eternal inflation requires specific initial conditions.
4) Eternal inflation (in slow-roll inflation) means that the power spectrum is of order unity, so perturbations are of the same order as the background and perturbation theory breaks down. So we cannot trust the calculation anymore.
This of course does not mean that eternal inflation is ruled out – simply that we do not know it to be an inevitable feature of inflation.
I find popular articles about this a bit depressing to follow, as they often focus on two sides who both think inflation->multiverse, divided only by whether they think this is good or not… Not really representative of the cosmology community.
The situation at Leicester again (somehow related to a previous post on the AI fad)
By the way, re the Quanta article, I’ve commented elsewhere how annoying I find it when such articles, often relating to some very outré, and lurid, notion, claim that the notion in question is being investigated by “physicists”, the very unspecific plural leaving the uninitiated with the impression that physicists as a class – en masse – are focused …
Anyway, “A physicist thinks there is a black hole in Newark, NJ”, in being singular, is less likely to grab eyeballs – and that, after all, is their business. Although I’d sure read that one.
Why was Ethan Siegel’s piece published in Forbes, of all places? Who is the audience?
It is well written and accurate, but the nut graf is the third from last, topic sentence: “If String Theory is correct, then somehow — and nobody knows how — this ultra-symmetric state broke, and it broke incredibly badly.”
That word “incredibly” is doing (as they say) a lot of heavy lifting there. Siegel’s use of language is otherwise careful. Is he really making an argument from intersubjective plausibility? And if he is, isn’t that just another manifestation of the same lgical fallacy as the arguments from “naturalness”? To whom, exactly, and why, exactly, is this symmetry breaking “incredible”? There is such a thing as scientific imagination; whose imagination fails here, and whose does not?
Siegel has a regular column on the Forbes site, often dealing with fundamental physics, similar to this one. He does a good job of trying to explain serious physics to a general audience.
The point he is making I think is the main one that needs to be made, and that rarely gets explained. It’s been the problem with string theory (and GUTs and supersymmetry) since the beginning. They posit huge new symmetry groups (in the case of string theory, 10d super-Poincare as well as things like two copies of E8) which imply physics that looks nothing at all like what we observe. You then have to come up with symmetry-breaking mechanisms which explain why and how the symmetry is broken to the much smaller (4d Poincare + SU(3)xSU(2)xU(1)) we observe.
Calling this symmetry breaking “incredible” is perfectly reasonable. That this happens is an extraordinary claim that should require extraordinary evidence. Instead there’s none at all. The original hope that an exotic compactification scheme using Calabi-Yaus would solve the problem was always highly speculative and hard to believe (it could properly be described as “incredible”). That doesn’t mean people shouldn’t have looked into it, but for a long time it has now been clear this doesn’t work.
I think Ethan Siegel’s account of the dream of string theory: “… that we can take this theory, like some enormous unbroken box, and stick the right key in it and watch it crumble away, leaving only a tiny piece left that perfectly describes our Universe…” is cogent and realistic; but then IICC, the Planck mass is something like twenty orders of magnitude greater than the Higgs mass, which sounds like plenty of room for new physics. It reminds me of people like Lyell who came to understand that the age of the world was much greater than they had imagined. An old academic cartoon shows two bearded guys at a blackboard, one says to the other, `It’s not that they want you to promise results; it’s that they want you to promise results IN THEIR LIFETIME!’ Apparently Archimedes, based on work of Aristarchus, estimated the size of the universe as roughly two light-years; today we might say he was off by ten orders of magnitude. It is not so clear to me that string theory should be abandoned just because it has a lot of unexplored room in it.
PS the previous comment escaped before I had the chance to say that all this hype nevertheless creeps me out.
If he had used the word “improbably” rather than “incredibly”, I would have had no point to make. Perhaps I had none, even so.
Simon Friedrich’s blog post is well-written and diplomatic. Though I think none of the hard critics of the multiverse claim that might not live in a multiverse.
” these large groups are enormous, like a block of uncut marble, and we want to get just a tiny, perfect statuette (our Standard Model, and nothing else) out of it.”
“String Theory truly is: a large, unbroken box that must somehow crumble in this particular, intricate fashion, to recover the Universe we observe”
He is mixing his metaphors — and very badly.
I concur with Bernard, and I’m in disagreement with Friedrich on other matters. I don’t understand how a concept that may be as malleable as “God”, and may require god-like powers to probe, can be described as “far from unscientific”. It’s far from helpful, at best. Of course there are mysteries grounded in good science that may be beyond humans to explain or test, but that’s hardly a new or especially deep idea. That the obvious happens to take a on a most abstruse manifestation in the speculative realm of untestable hypotheses doesn’t strike me as terribly “serious”. There’s a mountain of negative data to stack up against the notion of new symmetries where they should have been found in the most “natural” places, tons of non-evidence of collisions with bubble universes and so forth. It’s not as if these idea haven’t been quite thoroughly vetted, and we’re butting up against real physical limits to testing them any further, quite aside from the fact that they’re no longer promising. Where’s practical “science” in any of these multiverse scenarios to hide any longer? Aren’t such concepts already so seriously constrained by evidence as to stretch credibility to the breaking point? I don’t think there’s much room for diplomacy, anymore, however congenial one wishes to be. Hasn’t “the multiverse” already conclusively failed?
As a group theorist, I can confirm that these large groups really are enormous. However, they bear no resemblance to a block of uncut marble. Nor do they bear any resemblance to the universe we observe.