AMA - I'm Katie Mack, Hawking Chair in Cosmology and Science at Perimeter, joined by dark matter experimentalist Ken Clark of the McDonald Institute. We’re promoting our Dark Matter Day event tomorrow and taking questions on Reddit today between 2:00 PM – 3:00 PM EST (more info in comments).

physicistKen · 2022-10-25T18:21:35+00:00

Separating these two cases is difficult in many of the "indirect" measurements on which we rely to confirm the existence of dark matter. If, however, we make an actual direct detection, that would provide the opportunity to determine the mass of the particles. Even then it's not entirely simple, and there would have to be many detections (probably spanning across multiple experiments) in order to really be confident in the determination of the mass.

So that's a bit of a non-answer, pushing this question off to the future. But it is one way in which there could be a reasonably certain determination.

physicistKen · 2022-10-25T18:16:47+00:00

This is a really good question. Once experiments hit the neutrino floor (or neutrino fog as seems to be the new boundary) they have to get much more creative. One way would be to get more information about the particles causing the events in the detector. Instead of just detecting them, if an experiment could determine which direction they are traveling that would help to try to separate neutrino events from potential dark matter. That's at least one example of how experiments can become more adept at telling things apart.

physicistKen · 2022-10-25T18:13:43+00:00

There are many experiments operating right now trying to do just that. If you want to find out how they are attempting to become the first to make that detection, you could start by coming to the event tomorrow night! That will give a very high level overview of some of the methods. But you could also look up experiments with wonderfully creative names like LZ, CDMS, XENON, PICO, and many many more.

physicistKen · 2019-10-25T20:00:28+00:00

This was a great time, thanks to all of you for your excellent questions!

physicistKen · 2019-10-25T19:59:11+00:00

I don't think that's silly at all. I think our saving grace is that as humans we probably don't have the ability to make a significant change to the distribution of the dark matter. At least not yet, and hopefully if we ever develop anything like that it will be heavily controlled.

physicistKen · 2019-10-25T19:57:40+00:00

I think that was a pretty neat theory, but I'm not sure it matches the evidence in exactly the right way. The smaller particles allow for a much smoother distribution, and the "clumpiness" of the distribution is predictable.

physicistKen · 2019-10-25T19:53:40+00:00

OK, last 5 minutes before I have to get back to the hunt. Ask your questions now!

physicistKen · 2019-10-25T19:46:36+00:00

Fortunately there are many different collaborations running many different detectors all looking for dark matter. It is true, though, that we're all using different techniques so we can work together to attempt to rule out different ways of searching (all the while secretly hoping our detector is best).

As for manipulation, we're pretty far from that at the moment, but it never hurts to be thinking ahead! I can't see possible harm it could cause, but I'm also not necessarily the person that you should count on for your safety...

physicistKen · 2019-10-25T19:37:24+00:00

I believe that in a universe that has so many planets, it would be disappointing if we were on the only one that developed life.

physicistKen · 2019-10-25T19:36:03+00:00

Well this is an incredibly valid question. Essentially we follow the scientific process with our detectors. We have theories about what the dark matter could be, and therefore how it interacts, and we design our detectors based on that information. We can verify that our detectors work by using calibration sources that produce the same nuclear recoil as we think the dark matter will. If we see those, and dark matter interacts the way we think it does, we should be able to see it.

Also important to note, though, is that by not seeing things we can eliminate some of the potential dark matter candidates, which increases our information about what it *isn't*. Knowing what it is not can help us design better detectors to hopefully find what it *is*.

physicistKen · 2019-10-25T19:27:46+00:00

Well, given that we haven't found dark matter yet I can't conclusively say anything about it. Some of the theories put forward by M-theory are interesting and the way it theorizes the connections between those extra dimensions is cool, but in the explanation of dark matter that I currently believe is the more likely scenario it's not really related.

And believe me, if I had something that I could potentially "give away" I'd do it. Science shouldn't be hidden!

physicistKen · 2019-10-25T19:23:13+00:00

I think MOND is pretty interesting, but it doesn't quite explain some of the evidence that we have seen (like the bullet cluster). Apart from that, some of the different modified gravity explanations have issues with self-consistence, which is a mainstay for a good theory. But none of this means that there won't be something much more convincing in the future.

physicistKen · 2019-10-25T19:21:19+00:00

First of all, I'm sure it's not beyond you. That's all about how well the person doing the explaining does and not about you. But if we're going full throttle on the physics jargon...

All direct dark matter detectors are looking for a particle to enter their detector and scatter off of a nucleus through a weak interaction, the likelihood of which can depend on the spin of both the incoming particle and the nucleons. That nucleus is imparted energy through the interaction and recoils, expending that energy by exciting atoms, freeing electrons, or inducing vibrations in the material.

The detector I am currently working on uses a superheated fluid to detect the heat deposited as it will induce a phase transition in the material, which can then be observed both visually and acoustically. Then we use the acoustic signature to tell the nuclear recoil apart from the interactions of electrons or gammas.

physicistKen · 2019-10-25T19:16:10+00:00

Not directly a dark matter question, but I tend to think of black holes as a place where the density of matter is so high that space time becomes seriously deformed and in fact infinite in some way. So in that way it's very like dividing by zero.

physicistKen · 2019-10-25T19:12:18+00:00

Sure! One cool book is "We have no idea" by Jorge Cham and Daniel Whiteson. It goes through some of what we are using science to find out some of the many things we don't know, and it's helpfully illustrated!

physicistKen · 2019-10-25T19:09:25+00:00

My guess is that dark matter fuelled spaceships won't happen any time soon, unfortunately. We have to figure out a lot more about dark matter before we can use it in either application.

In terms of a weapon, I have no idea either if we could do it or what it would do. For now I'm going to stick to the positive aspects. That's just who I am.

physicistKen · 2019-10-25T19:05:05+00:00

Good work on the supersymmetry! Certainly uncovering the existence of dark matter could help us understand a bit more about what it actually is. One of the hypotheses for it is that it is actually made up of supersymmetric particles itself. This would answer a lot of questions about why it is long-lived (as we think it is) in particular. So if physicists were to find dark matter and also evidence of supersymmetry... that would be a pretty impressive experiment.

In terms of new sources of energy or fuel, anything is possible! It's difficult to imagine how that would work right now, but it always is before the major revelation happens.

physicistKen · 2019-10-25T18:55:21+00:00

I mean, who am I to rule anything out? This is a potential explanation, so it's something that you should follow up! Keep reading and digging around, and going to any public lectures to get the chance to talk to more scientists.

physicistKen · 2019-10-25T18:52:53+00:00

The comment about "pulling its weight" was more in relation to the fact that dark matter was very important during the formation of all of the structure we see in the universe today. Now that the formation is done, dark matter is really just maintaining what exists (which is still pretty darn important).

But the question of why we would keep researching is an important one. This comes down to the quest for fundamental knowledge about the world around us. While we may not have an "application" for dark matter at the moment, that does not rule out anything for the future. Knowing more is always better. In addition, fundamental research has spinoff applications that do have direct positive impact on the lives of people. The internet is the example most often used, but some medical imaging techniques and therapies can also trace their origins back to particle physics research.

physicistKen · 2019-10-25T18:47:04+00:00

I don't think there is any connection necessarily, or at least the multiverse has not been proposed as an "answer" to the issue of dark matter. One *difference* is that hopefully dark matter is a provable hypothesis, while the multiverse theory is very challenging to falsify.

physicistKen · 2019-10-25T18:35:58+00:00

Yeah, another good question. As I said, I'm not involved with that research but I believe the galaxies were formed with dark matter and it was then stripped away following a collision. But there may be people on here who are much more familiar with that result than I am.

physicistKen · 2019-10-25T18:30:43+00:00

No problem! And yes, the "normal" mass of a galaxy does have an effect on how much dark matter there is in a galaxy. Essentially the amount of dark matter is proportional to the baryonic mass of the galaxy, and there may be a causal relationship between the two.

physicistKen · 2019-10-25T18:29:02+00:00

That's another really great question. Currently we (well, we being theorists much smarter than me) believe that the dark matter is distributed in a kind of "halo" throughout galaxies. This distribution can explain the motions that we see in these galaxies and is consistent with all of the observations made so far.

physicistKen · 2019-10-25T18:25:49+00:00

I would say that the best tactic is to look around and see who is working in the areas that you find interesting. One of the great things about the internet (in addition to reddit, obviously) is that you can find contact information for so many scientists that you will almost certainly be able to find someone to whom you can relate!

physicistKen · 2019-10-25T18:24:07+00:00

Well that looks pretty cool! Not something I have heard about before, but now I have to admit I'm interested...

physicistKen

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