Halloween is coming up, so let’s talk about the most prominent monster of the physics canon, the nightmare scenario.
Right now, thousands of physicists search for more information about particle physics beyond our current Standard Model. They look at data from the Large Hadron Collider to look for signs of new particles and unexpected behavior, they try to detect a wide range of possible dark matter particles, and they make very precise measurements to try to detect subtle deviations. And in the back of their minds, almost all of those physicists wonder if they’ll find anything at all.
It’s not that we think the Standard Model is right. We know it has problems, deep mathematical issues that make it give nonsense answers and an apparent big mismatch with what we observe about the motion of matter and light in the universe. (You’ve probably heard this mismatch called dark matter and dark energy.)
But none of those problems guarantee an answer soon. The Standard Model will eventually fail, but it may fail only for very difficult and expensive experiments, not a Large Hadron Collider but some sort of galactic-scale Large Earth Collider. It might be that none of the experiments or searches or theories those thousands of physicists are working on will tell them anything they didn’t already know. That’s the nightmare scenario.
I don’t know another field that has a nightmare scenario quite like this. In most fields, one experiment or another might fail, not just not giving the expected evidence but not teaching anything new. But most experiments teach us something new. We don’t have a theory, in almost any field, that has the potential to explain every observation up to the limits of our experiments, but which we still hope to disprove. Only the Standard Model is like that.
And while thousands of physicists are exposed to this nightmare scenario, the majority of physicists aren’t. Physics isn’t just the science of the reductionistic laws of the smallest constituents of matter. It’s also the study of physical systems, from the bubbling chaos of nuclear physics to the formation of planets and galaxies and black holes, to the properties of materials to the movement of bacteria on a petri dish and bees in a hive. It’s also the development of new methods, from better control of individual atoms and quantum states to powerful new tricks for calculation. For some, it can be the discovery, not of reductionistic laws of the smallest scales, but of general laws of the largest scales, of how systems with many different origins can show echoes of the same behavior.
Over time, more and more of those thousands of physicists break away from the nightmare scenario, “waking up” to new questions of these kinds. For some, motivated by puzzles and skill and the beauty of physics, the change is satisfying, a chance to work on ideas that are moving forward, connected with experiment or grounded in evolving mathematics. But if your motivation is really tied to those smallest scales, to that final reductionistic “why”, then such a shift won’t be satisfying, and this is a nightmare you won’t wake up from.
Me, I’m not sure. I’m a tool-builder, and I used to tell myself that tool-builders are always needed. But I find I do care, in the end, what my tools are used for. And as we approach the nightmare scenario, I’m not at all sure I know how to wake up.
4 gravitons 
I’m an amateur, but my take is that these are currently the five most immediately
promising areas for elucidating the standard model:-
1. Dark matter is a problem but, if you include axions, only some half of the likely parameter space has so far been explored?
2. I thought the most likely theory for dark energy is that it is the cosmological constant, and if so the challenge reduces as to how to derive its value from first principles?
3. As you explained in your previous post, you are fairly sure that neutrino experiments will provide at least some valuable answers.
4. Is spacetime quantum or classical? Apparently some experiments are being devised that may answer this question. (Is the possibility that spacetime is classical being too readily dismissed?)
5. Cosmologists are being deluged with data, are grappling with genuine problems, and should at some point be able to produce valuable insights. For example, apart from dark energy and dark matter, resolving the tension over the Hubble constant.
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So, some of these are indeed areas people find promising (and part of the point of the post was to lean towards the pessimistic side for Halloweenish reasons). But it’s also important to be aware that for most of this there are no real guarantees, and the “nightmare scenario” is still quite a real possibility.
Between axions and various axion-like particles there are a lot of things that haven’t been tested yet, of course, and there are various such particles that people are still optimistic about (though I get the impression that the original axion has fallen by the wayside a bit). But equally, dark matter could genuinely be none of these things, and undetectable in any of the experiments we’re doing, and we don’t really have a solid reason to insist that one possibility or the other is more likely.
Whether you think of dark energy as potentially just a cosmological constant depends a bit on whether you think inflation is needed, because with inflation you really need it to be a field that changes over the history of the universe. (From a string theory perspective you also don’t expect there to be any constants like that just sitting around without a physical justification.) But yeah, “nightmare scenario” may well be that it’s indistinguishable from a cosmological constant as far as we can probe.
Neutrinos indeed imply some real guarantees for genuine BSM physics. The nightmare scenario there is that we never really go beyond “parameter-fixing”: we could get to a point where we’ve fixed most of the masses and mixing angles, and still have no idea how they get their mass (Majorana or Dirac) or how the other implications work out (the inevitable extra heavier neutrinos). The experiments we have can rule out a couple of options depending on how they turn out, but can’t fix the model completely, even to the level we have for other particles getting mass from the Higgs.
Spacetime being classical would have very weird implications, mostly because singling out any specific thing as classical has very weird implications, because it forces you to decide which interpretation of quantum mechanics is true, something that quantum mechanics seems very good at avoiding. The upcoming experiments can rule out certain specific proposals, though mostly pretty weird ones (for example, Penrose has argued that some of the experiments trying to set up gravitational superpositions won’t work). But precisely because this is a matter tied to interpretations of quantum mechanics, there’s pretty much infinite wiggle room for someone to come up with an interpretation of “spacetime is classical” that avoids any given feasible experiment.
Cosmologists have a lot of data, but real limits on what they can do with it. (Statistically, because our universe is in some sense “only one sample”, there are real hard limits to how much we can learn.) It’s also filtered through astrophysics, which means that some mysteries and tensions may just turn out to be a confusion about how stars and galaxies form rather than fundamental physics. Over time we’ll learn more, but unless we get lucky we may just learn enough to rule out some models.
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Most so-called “tensions” between the standard model and observational evidence, such as dark matter and dark energy, are really tensions between the standard model + general relativity + Lambda CDM and observational evidence, and it could easily be that there is something wrong with general relativity or Lambda CDM rather than the standard model. All our evidence for the phenomena attributed to dark matter and dark energy comes from astrophysical and cosmological sources, and absolutely none of it from particle physics experiments, and it could equally be explained by alternatives to general relativity and alternatives to Lambda CDM instead of some particle physics mechanism.
I would say it is much more likely that something is wrong with general relativity or Lambda CDM, because there are a significant number of other tensions in astrophysics and cosmology, ranging from CMB dipoles and other cosmic dipoles, to the Hubble and S8 tensions, to wide binaries behaving like MOND, to the impossibly early galaxies discovered at JWST, and a whole host of other tensions sitting around 2-sigma to 5-sigma.
This might be a real nightmare scenario for particle physics – there’s a revolution in fundamental physics but it deals entirely with the non-quantum sector – gravity and cosmology. Particle physics stays completely the same because there is still nothing in contradiction with the standard model except for neutrino oscillations and still no way of distinguishing between the multiple different theories for neutrino oscillations. All the funding and grants and attention amongst young experimentalists and theorists gets redirected to astrophysics and cosmology for the exciting new revolution taking place, while particle physics is left all by itself as a underfunded, ignored, and stagnant field.
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There are a couple of different distinctions one can draw here. There was one I had in mind, and there are a few different ones implied in different parts of your post.
What I had in mind was not really a distinction between particle physics and cosmology, but between fundamental physics and non-fundamental physics. I think it’s really possible at this point that we don’t have anything more to learn in my lifetime about fundamental physics, even including modifications of GR and LamdaCDM. It’s quite possible that all of the tensions people have seen are the result of some combination of bad statistics and systematics of galaxy and star and black hole formation, and that going anywhere beyond our current knowledge of the fundamentals (i.e. “dark matter exists”) is not something we’ll be able to do. We can still learn more about what kinds of fundamental theories are possible/consistent/have particular consequences…some people have a lot of fun with that, and that does have some appeal to me. But the question “what is the Lagrangian that best describes our universe” may well be one where we just don’t have that much more to learn for a long time. That, in my mind, is the “real” nightmare scenario.
I think the distinction between whether something is a problem with the Standard Model, or GR, or LamdaCDM is a false one. You can put new physics (new terms in the Lagrangian) into whichever bucket you like, without some extra purpose in mind that’s just an aesthetic choice.
You also draw a distinction between quantum and classical, but I don’t think that’s right either. Neutrino masses were discovered through a quantum process (oscillation), but the masses themselves are likely to be classical. Supersymmetric particles, if they were discovered at the LHC, would have been seen because of a quantum process, but their existence itself would be classical, supersymmetry would be a classical fact about the laws of physics. And on the other hand various approaches to dark matter and to modifying GR rely on one level or another on quantum mechanisms in order to avoid breaking something important.
A more important distinction, which you gesture at, is one of methods. Particle physicists, especially particle experimentalists, can do certain kinds of things: build colliders, build big tanks of liquid to catch particles, etc. It may well be that there is something more to learn about fundamental physics (so not the full nightmare scenario above), but that those methods can’t say anything about it. And yeah, that’s its own kind of more localized nightmare scenario, with exactly the kind of consequences you’re imagining.
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The actual real nightmare scenario that no particle physicist wants to consider is that the global economy completely collapses similar to the Great Depression 90 years ago, and a lot of firms go out of business and a lot of governments default on their debt. Universities, research institutes, and governments have to cut particle physics research programs so that they could survive in the harsh new economic reality, and progress in particle physics is halted for many decades. Even worse, wars in America and Europe begin and CERN, Fermilab, and other particle accelerators and laboratories get irrecoverably damaged in the resulting conflicts, setting particle physics back decades.
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It seems like this nightmare might bleed over into quantum cosmology also, yes?
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Absolutely, yeah, see my response to Madeleine above. If you want to find out something like “what is the potential of inflation”, not only might there be no available evidence for a while, there’s a point at which there may simply not be enough available evidence period, since we have only one universe of statistics to use.
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Would a “realist” interpretation of quantum mechanics do anything to change this nightmare?
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I mean, you can pick any interpretation you want, there’s still no guarantee you can get any evidence that can tell you whether you’re right!
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“I’m a tool-builder, and I used to tell myself that tool-builders are always needed. But I find I do care, in the end, what my tools are used for. And as we approach the nightmare scenario, I’m not at all sure I know how to wake up.”
I’m finding this a bit hard to translate. Are you saying you don’t know what use amplitudeology will be, if it doesn’t get applied to beyond-standard-model physics?
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I can see some things amplitudeology will get used for (in particular, gravitational waves). But I’m not sure that motivates me in the same way as the prospect of new fundamental physics.
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Suppose I were to tell you that the standard model is the correct effective field theory up to the Planck scale, that everything in the dark sector comes from quantum corrections to Einstein gravity, that ultimately there’s a specific string vacuum which explains all this as well as the values of the yukawas, and that there’s a revolutionary amplitudeological perspective on this final theory? 🙂
That would be new fundamental physics, but not in the sense of giving us new particles to detect.
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Heh, ok, yes, one option is for there to be “purely theoretical progress”, someone coming up with a theory that’s so good that we don’t worry that we can’t find any new evidence for it.
I don’t think there’s much reason to think that’s likely, but it would certainly be cool!
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I have been wanting to add here, that I owe much of this vision of what could be, to Marni Sheppeard (1967-2021), whose PhD thesis was apparently 20 years ahead of its time.
http://dx.doi.org/10.26021/6326
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Wow, I hadn’t heard of Marni Sheppeard before, she had quite a story!
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