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John Carlos Baez

Maybe you've heard the big news about LUX-ZEPLIN, the dark matter detector. They put 10 tonnes of liquid xenon in a tank, 1.5 kilometers down in a mine. The idea is if that if dark matter is made of weakly interacting massive particles, these might hit a xenon nucleus and make it emit some light.

They've been waiting for 280 days and they haven't seen anything interesting. That's the big news. 😕 They claim this rules out the possibility that dark matter is made of weakly interacting massive particles that are more than 9 times as heavy as a proton.

It must be somewhat discouraging doing this work. To paraphrase one of the team members: "we are the world leaders in not detecting dark matter". But it's good that people are trying. While you may have already decided physicists will never find dark matter, they have to actually look. In particle physics, you never discover a new particle until you do.

Alas, it's completely possible that dark matter is made of stuff that 𝑜𝑛𝑙𝑦 interacts gravitationally with other matter, so that we'll never find it with detectors like LUX-ZEPLIN, yet it still exists.

It's also completely possible that something other than dark matter - something we don't understand yet - is making galaxies spin faster than they should given the matter we see, and form earlier than they should given the matter we see, and so on. But while this is easy to say, it's incredibly hard to make up a theory that explains all these phenomena without dark matter! People are trying to make up new theories of gravity that do the job, but these theories have a lot of problems.

So, the mystery endures.

newscenter.lbl.gov/2024/08/26/

19 comments
nick

@johncarlosbaez for sale / 7 tons of liquid xenon / never interacted with.

John Carlos Baez

@nickzoic - It's a great deal. You just have to go 1.5 kilometers down a mine shaft and bring up a 7-ton tank of liquid that must be kept at -100 Celsius.

wired.com/story/how-dark-matte

AstroMike

@johncarlosbaez Doesn’t it limit the cross-section more so than the mass?

John Carlos Baez

@AstroMikeHudson - I don't know why their press release phrases it in terms of the mass: there must be extra assumptions put in, to get that conclusion. Maybe it's simply because no journalist could understand "cross section". Here's a graph they released which more accurately conveys what they actually found. The horizontal axis is mass in GeV/𝑐², and the proton mass is about 1 GeV/𝑐².

From here:

indico.uchicago.edu/event/427/

Abdullah Khalid

@johncarlosbaez @AstroMikeHudson What I would like is a graph of uncertainity in the cross-section as a function of the number of days the experiment has run. That would be fun.

John Carlos Baez

@abdullahkhalids - yes. They are driving the cross-section down to zero. The experiment has run for 220 days, and they plan to go on for 1000 days.

(I said 280 days in my post, but that includes 60 days of an earlier run.)

@AstroMikeHudson

The Right Irreverend

@johncarlosbaez Sometime back in the Tao of Physics days, I remember reading somewhere that when a theoretical physicist, through their math, postulates the existence of a new, not yet seen particle, that this event suddenly creates the reality and the actual particle is discovered! Or… maybe the theory just gives us the tools to know where to look and what to look for, so that’s why we find the actual particle.

John Carlos Baez

@dadakopf - Antimatter is an interesting case.

Dirac's theory of the electron predicted that it had a positively charged antiparticle of the same mass. He chickened out and suggested this was the proton. The mathematician Weyl said essentially "come on, Dirac, that doesn't have the same mass as the electron!" Then Dirac got brave enough to predict the electron had a positively charged antiparticle of the same mass. Then the experimentalists saw it!

But the funny part is, the experimentalists had already been seeing this antiparticle in photographic plates, spiraling the opposite way from the electron in the Earth's magnetic field. But they didn't believe that was possible, so they though someone had accidentally flipped those plates over.

@dadakopf - Antimatter is an interesting case.

Dirac's theory of the electron predicted that it had a positively charged antiparticle of the same mass. He chickened out and suggested this was the proton. The mathematician Weyl said essentially "come on, Dirac, that doesn't have the same mass as the electron!" Then Dirac got brave enough to predict the electron had a positively charged antiparticle of the same mass. Then the experimentalists saw it!

The Right Irreverend

@johncarlosbaez I have this vague memory that somewhere in Wittgenstein, he says that if you were walking down the street and there was something there utterly foreign, or unknown to you, that you wouldn’t see it. Maybe there has to be some sort of psychological twist to make the unseen seen or something. Dark matter is dark because we don’t know how to see it and thus we don’t know what it looks like. Can math function as a corrective lens to focus our eyes on what’s out there?

Simon Lucy

@dadakopf @johncarlosbaez

If there is invention preceding discovery then it's definitely in maths.

The principle of adding stuff to a model to make it 'work' is as old as brains.

David Quintero

@johncarlosbaez the possibility that dark matter could only interact gravitationally, being its true nature locked from us, forever, is something I heard on Sean Carroll's podcast some time ago. It caused a great impact on me. Somehow, I want to rebel against that possibility, against that the universe will never allow us to know something so important about it.

John Carlos Baez

@davidsuculum - I sympathize. If you were a mathematician pondering all the dozens of fascinating and vital unsolved problems, not knowing which might be *forever* unsolvable due to Goedelian incompleteness, you might feel even worse. But I doubt the universe feels any obligation to share its secrets. So we should probably just relax and enjoy the mysteries, even as we try to figure things out.

The good news is that we are getting so much purely gravitational information about dark matter - through gravitational lensing, and its effects on galaxies and galaxy formation - that it's becoming extremely tricky to fit all the data we have with a model that's not quite elaborate.

@davidsuculum - I sympathize. If you were a mathematician pondering all the dozens of fascinating and vital unsolved problems, not knowing which might be *forever* unsolvable due to Goedelian incompleteness, you might feel even worse. But I doubt the universe feels any obligation to share its secrets. So we should probably just relax and enjoy the mysteries, even as we try to figure things out.

The Blue Wizard

@johncarlosbaez @davidsuculum Yes, tricky. Yet we still do need more data. I have said that before.

It is a bit like the classification of various stars in late 19th/early 20th century, with Hertzsprung-Russell diagram, etc. and we finally figured out what those are and where they came from. Reading the history of astronomy is interesting and instructive in how science is done!

Michael Richardson

@johncarlosbaez There is a very interesting storey about lead used to line the tanks of such experiments: youtube.com/watch?v=o0A9M5wHBA

Ciggy Bringer of Smoke

@johncarlosbaez

Wanna help me with a prank where I crawl out of the tank at exactly 1 year

Condalmo.

@johncarlosbaez These results must have them dazed and confused

Serge Droz

@johncarlosbaez the fate of dark matter experiments. I once saw a cartoon by Sidney Harris of mounteners walking down a hole, saying: " Because it's not there"

ma𝕏pool

@johncarlosbaez If I'm reading this chart from WP right, 9GeV/c2 mass cuts out WIMPs as explanation. Would it also detect composite/macroscopic DM, non-topological solitons?

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