A while back, when the MiniBoone experiment announced evidence for a sterile neutrino, I was excited. It’s still not clear whether they really found something, here’s an article laying out the current status. If they did, it would be a new particle beyond those predicted by the Standard Model, something like the neutrinos but which doesn’t interact with any of the fundamental forces except gravity.
At the time, someone asked me why this was so exciting. Does it solve the mystery of dark matter, or any other long-standing problems?
The sterile neutrino MiniBoone is suggesting isn’t, as far as I’m aware, a plausible candidate for dark matter. It doesn’t solve any long-standing problems (for example, it doesn’t explain why the other neutrinos are so much lighter than other particles). It would even introduce new problems of its own!
It still matters, though. One reason, which I’ve talked about before, is that each new type of particle implies a new law of nature, a basic truth about the universe that we didn’t know before. But there’s another reason why a new particle matters.
There’s a malaise in particle physics. For most of the twentieth century, theory and experiment were tightly linked. Unexpected experimental results would demand new theory, which would in turn suggest new experiments, driving knowledge forward. That mostly stopped with the Standard Model. There are a few lingering anomalies, like the phenomena we attribute to dark matter, that show the Standard Model can’t be the full story. But as long as every other experiment fits the Standard Model, we have no useful hints about where to go next. We’re just speculating, and too much of that warps the field.
Critics of the physics mainstream pick up on this, but I’m not optimistic about what I’ve seen of their solutions. Peter Woit has suggested that physics should emulate the culture of mathematics, caring more about rigor and being more careful to confirm things before speaking. The title of Sabine Hossenfelder’s “Lost in Math” might suggest the opposite, but I get the impression she’s arguing for something similar: that particle physicists have been using sloppy arguments and should clean up their act, taking foundational problems seriously and talking to philosophers to help clarify their ideas.
Rigor and clarity are worthwhile, but the problems they’ll solve aren’t the ones causing the malaise. If there are problems we can expect to solve just by thinking better, they’re problems that we found by thinking in the first place: quantum gravity theories that stop making sense at very high energies, paradoxical thought experiments with black holes. There, rigor and clarity can matter: to some extent they’re already there, but I can appreciate the argument that it’s not yet nearly enough.
What rigor and clarity won’t do is make physics feel (and function) like it did in the twentieth century. For that, we need new evidence: experiments that disobey the Standard Model, and do it in a clear enough way that we can’t just chalk it up to predictable errors. We need a new particle, or something like it. Without that, our theories are most likely underdetermined by the data, and anything we propose is going to be subjective. Our subjective judgements may get better, we may get rid of the worst-justified biases, but at the end of the day we still won’t have enough information to actually make durable progress.
That’s not a popular message, in part, because it’s not something we can control. There’s a degree of helplessness in realizing that if nature doesn’t throw us a bone then we’ll probably just keep going in circles forever. It’s not the kind of thing that lends itself to a pithy blog post.
If there’s something we can do, it’s to keep our eyes as open as possible, to make sure we don’t miss nature’s next hint. It’s why people are getting excited about low-energy experiments, about precision calculations, about LIGO. Even this seemingly clickbaity proposal that dark matter killed the dinosaurs is motivated by the same sort of logic: if the only evidence for dark matter we have is gravitational, what can gravitational evidence tell us about what it’s made of? In each case, we’re trying to widen our net, to see new phenomena we might have missed.
I suspect that’s why this reviewer was disappointed that Hossenfelder’s book lacked a vision for the future. It’s not that the book lacked any proposals whatsoever. But it lacked this kind of proposal, of a new place to look, where new evidence, and maybe a new particle, might be found. Without that we can still improve things, we can still make progress on deep fundamental mathematical questions, we can kill off the stupidest of the stupid arguments. But the malaise won’t lift, we won’t get back to the health of twentieth century physics. For that, we need to see something new.
4 gravitons 
Add to your analysis that people just love to hear bad news, they tend to see problems and catastrophes everywhere focusing on the negative side of things.
Journalists often say “good news doesn’t sell newspapers”.
That’s why for example crackpot scenarios for the “end of the world” are so popular.
So if you want to sell you must create problems even if there is none.
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Well those books can be easily subsumed by saying: “Failed physicists want to see physics fail”, A lot of effort went into this over the years, instead of spending it in actual research.
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I don’t think that’s quite fair. Popularization is popularization, even if it’s negative popularization, and in my experience it isn’t any more of an obstruction to research than teaching or textbook-writing is. It doesn’t seem to have made Woit a worse mathematician, or Hossenfelder a worse physicist.
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“There are a few lingering anomalies, like the phenomena we attribute to dark matter, that show the Standard Model can’t be the full story.”
Your careful wording is appreciated.
For what it’s worth, dark matter phenomena, is not just the most stark empirical evidence that we absolutely must go beyond the existing SM + GR paradigm somehow or other.
It is also, thankfully, just about the only area of fundamental physics where new experimental data on multiple fronts, that can shed light on what exactly it is, keeps surging in almost every week from astronomy observations. These observations are made by a great many independent teams of investigators, keeping the field at least partially grounded in reality, and also making the field less vulnerable to group think as HEP where experiments at a single facility in a single town, the LHC, dominate the field (even though it isn’t quite the only game in town).
Not only can we measure reality, we have multiple kinds of qualitatively different kinds of data to look at, many of which have large sample sizes.
We can look a gravitational waves and related visible light observations of neutron stars crashing into black holes. We can look at the CMB. We can look at multiple colliding galactic clusters. We can measure the dynamics of wide binary stars. We can observe the dynamics of stars with all sorts of relationships to the Milky Way. We can look at ultra-diffuse galaxies. We can look at the 21cm observations. We can look at the dynamics of thousands of galaxies and make inferences from that. We can make ultra-precise solar system observations. We can run N-body simulations. We can make analytical calculations with simplifying assumptions whose impact is relatively easy to quantify. We can compare expected and observed Big Bang Nucleosynthesis data under various theories. We can look a lensing data with large sample sizes. We can compare one way of looking at a gravitational system against a different kind of observations of the same system to see if the observations are consistent.
And, while the phenomena and data are “complicated” and require sifting through a lot of detail, the dark matter particle or modified gravity theories that have to explain them need to be pretty simple to be close approximation of the default GR with no dark matter null hypothesis in the circumstances where that null hypothesis is sufficient to perfectly describe what we see to the limits of observational precision.
The rules involved in any given hypothesis may be harder to implement, but the actual number of rules involved is on the same order of magnitude as chess, or even checkers, and the rules that are being proposed are not all that much profoundly harder to work with than many of the rules we’ve accepted into the SM and GR. Like QCD, doing calculations with some of the possible rules may only be possible to work with at a practical level via approximation and some new amplitude tricks.
So, get back to work and be inspired about it, darn it.
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Interesting… Find it stimulating that information, so far, on the dark matter phenomena challenges so many, with so little evidence, to suggest so much.
But then again, I’m just a goober from Alabama with quirky reading habits…
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I’ve got a vision of the future, 4gravitons. But I don’t think you’ll like it. You don’t need a new particle. You need to understand the particles you’ve got. Starting with the photon, then the electron. When you do, I’m afraid to say you will come to appreciate that there’s a lot wrong with the Standard Model. A lot.
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Stau sleptons?
Click to access 1809.09615.pdf
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Maybe! We’ll see if it holds up. (The impression I have is that the specific proposal of stau sleptons is just one possibility, not specifically evidenced for aside from people having suggested it in the past.)
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