I am a dark matter physicist ask me anything!

Can you explain what you do in a very dumbed down form?

Sure! Basically we have proof that there is something existing in the galaxy, but have no idea what it is. So what I do is try to find some way to have that dark matter give us evidence that it actually exists. So far all the evidence is just based on our observations of the influence on other things, but we want to see it on its own directly.

Why do you have to look for dark matter underground? Is it hiding from us there or something?

This is a fantastic question. What we know about the dark matter is that it is very unlikely to interact in our detectors. So if we were to run our experiments on the surface, we would see all of the many things (like background radiation) going on around us all the time instead of the dark matter we want to see. Going underground removes most of those background events and lets us focus in on just the dark matter (hopefully...).

Can you 'feel' when you're 2km underground by changes in pressure, temperature, etc.? Or does it just feel like being indoors on the surface?

..Does the huge long lift ride ever make you feel like you're actually loading a new area, in a computer game..?

What particle interactions might allow us to observe dark matter? Something neutrino-ey (hence being underground)?

What proportion of dark matter theory is empirically testable? If any stuff isn't testable, is that just because we haven't built the right sort of detectors (or whatever!) yet, or is it untestable even in principle?

You can absolutely feel the difference. The pressure 2km underground is about 30% higher than on the surface, so on the way down your ears pop a lot and you can feel the difference. It's also pretty hot since the walls of the mine are about 40C. Fortunately we keep our labs air conditioned to a comfortable 20C (mostly to save the experiments, not the physicists). And the lift (called a "cage" here) is not that long. It can take about 3.5 minutes to get all the way down if you don't stop.

This is very neutrino-y in it's interaction since we look for the dark matter to interact with a nucleus and it's actually the movement of that nucleus that we are hoping to see.

The current theories that we are looking for are certainly testable, but there are many different theories out there that may not be. I'll just keep hoping that it's something that can be seen in my detector (or one like it I guess).

OK, so the lab is apparently kept at 16C, and the unexposed rock is 45C. Science is all about corrections.

What do you think about that dark matter free galaxy that was recently discovered? Think it's real?

This is a great question. I think it is probably real (I wasn't involved with that research, but good scientists are) and I think it's pretty cool. In fact, I think of it more as the "exception that proves the rule" since there are so few galaxies out there without dark matter and they are so anomalous that we can pick them out of the huge surveys of astronomical objects.

Are there theories on how these dark matter free galaxies came to exist? I mean, did they form that way, or did they lose their dark matter somehow like in a collision where the normal matter got left behind?

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.

You say you hunt for dark matter. Can you tell us what dark matter tastes like once you've caught it, and are there any special licenses or permits you need to become a dark matter hunter?

Edit: Jokes aside, you have a very cool job. I'm curious, if dark matter particles exist would they be able to interact with other particles? Would forces like gravity and electromagnetism influence them at all?

My personal hope is that dark matter is some new flavour that will blow everything else away. So far there's no license required, but maybe eventually it will be endangered and we'll have to set up limits.

But seriously, we do expect it to interact with other particles, just very rarely. That's how our detectors work (mostly... we hope). In terms of forces, we know it has to interact with gravity since that's how we got most of our evidence for its existence. But we know for sure that it doesn't interact electromagnetically since light shoots right through. We currently think it might interact with the weak force (and we use that to try to see it) but the strong force is definitely out.

It’s my understanding all the prevailing theories being tested are yet undiscovered particles.

Are we coming to the point where we need theorists to come up with new testable hypothesis’s that don’t require a particle?

Could this be a phenomenon of gravity itself?

Do you have any fear that the phenomenon of Dark Matter is “untestable”? Something like multiverse influence that would be very difficult to prove.

Yeah, there's a lot of good stuff in this question. The most popular idea right now is that the dark matter is a particle that we haven't found yet. So theorists are coming up with lots of ways to explain what that is, and therefore how we can find it. There have been many attempts made to explain our evidence for dark matter using modifications to gravity, but so far none of them are able to explain other observations properly. Maybe some time in the future they could, but so far it doesn't really work. So we're treating the new particle as the best theory and working from there!

Sometimes the "untestable" idea does keep me up at night thinking I am wasting my life. But we still have lots of areas to search in, so I'm not giving up just yet.

ELI5: how does a dark matter experiment work? If we don't know what it looks like or what it does, or anything - how can we tell if we found it or not?

Awesome. I love this question.

Basically we are looking for the dark matter to come into our experiment and hit something. When it hits the thing in our detector, it makes light or heat or charge. The experiments are designed to record the change because of that hit. The biggest problem is that lots of things can hit our detectors, so we do things like going underground to hide from them. We also work to tell dark matter hits from other hits.

What are the chances "dark matter" is actually just nothing, and doesnt exist at all?

Well hopefully the chances are not good. We have quite a bit of evidence using many different methods and observational sources that say that *something* exists. Now we just want to detect it directly so that we can learn more about what it is.

Are we sure that dark matter even exists, if so how do we know, and how didn't we know about until recently? Also, what can we do with dark matter? And how does it relate to dark energy?

Even though a lot of the action has been recent in the searches, we've had evidence for the existence of dark matter for a while. You can go back to the 30s to find people already talking about galaxy rotation curves being incorrect (and naming it "dark matter"). There was even more evidence in the 70s that we are missing some mass, so that has provided motivation for all of these search experiments.

Dark matter doesn't really relate to dark energy except in the name. It's a bit unfortunate that they both got the "dark" part.

What can a 16 year old sitting at home do to get more hands on experience about your field?

There are lots of things you can do! If you're close to a science centre or university, there are often exhibits or information that you can get. But for real hands-on experience, there are summer schools offered by different labs and universities that provide a great way to get to play with some of this science. Less hands-on options are things like the social media presence of labs or experiments that you can research. Also, send a scientist an email! Scientists love emails. They are usually (I hope) happy to answer questions about what they do. Most of the time the trouble is to get them to STOP talking about what they do...

Wow sounds amazing! Would you mind sending me a few contacts in my PMs so that I get to know more people and get in touch?

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!

Do you believe dark matter can eventually be used as a energy source similar to the way we use solar radiation?

Wouldn't that be nice? I can't rule that out, since there are lots of smart people that may be able to some day make some cool energy source out of it. Unfortunately we just don't know enough about it yet to say either way. Check back once we have proof of existence and we can go from there!

fair enough. To that effect, do you have a recommendation for a good book or documentary laying out the working theories?

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!

Are you skipping any meetings today for this?

As a matter of fact, @mjbressler, there are important dark matter meetings going on right now. But I jumped at the chance to answer all of these great questions!

Hi Dr. Clark, a couple of questions. ELI5

1. will google's recent quantum supremacy claim help us in about investigating dark matter?
2. what's next if do discover some evidence of WIMPS

1. It could. Advances in quantum computing will expand our ability to model the universe, and therefore could tell us a lot about how dark matter works.
2. If we do find evidence of WIMPs we are going to want to know a lot more about the particles themselves. At that point all experiments will be focusing on trying to get all the information we can about how exactly the dark matter interacts.

thanks for getting back. for a chemist who is interested in the field of particle physics, what's the best way to keep ourselves up to date that doesn't require rigorous research or investing too much time

Particle physics is very fortunate in that it seems to be interesting to lots of people, which is encouraging for people like me to see. But that means that there are many references out there that can give you a good understanding of the field without becoming too bogged down in the details.

How do you see the WIMP search field reacting to another 10 years of null results? There's always more DM models to explore, but it's not clear to me that any will pick up significantly more motivation in the next 10 years (from indirect searches or other cosmology).

Yeah, this is another really good question. We (the community of dark matter hunters) have been pursuing this for quite a while without success. I think there is a faction of the field that thinks we had better find something soon, but there will always be more theories to explore... well, until we find it. In terms of the motivation, something else could come along to give us extra ideas, but I tend to agree with you that we have sufficient evidence that we should keep looking.

What do you think are the odds that dark matter and dark energy (as concepts) are a result of us not fully understanding how gravity works? Einstein improved on Newton’s theories, so what’s the likelihood of someone coming along and improving (extending, really, since they’ve been proven) on Einstein’s theories?

Of course, it’s unknowable, but do you suspect we have a lot more to learn about gravity?

I think the gravity explanation is an interesting one, but any successful modification to the theory is going to have to be able to explain all of the evidence we see (such as the bullet cluster, which is both evidence and a neat astronomical observation). If theorists can come up with something that does that, and it's not impossible that they can, I would have to rethink.

Heard about the recent observation of Xe 124 half life decay in a dark matter detection facility. So, are there any other interesting observation/phenomenon seen in a dark matter facility?

There have been a few other observations that come along with dark matter experiments, but we're really trying to keep our eyes on the prize.

What are your recommendations for a student in college for astrophysics? Also, I've seen many theories on what dark matter may actually be, what do you think it is?

My primary recommendation for a college student is to follow your interests. There are so many cool things going on right now that it's difficult to go wrong in choosing a field. But if you're not enthusiastic about what you're doing, the work is going to be much harder.

As for what I think it is, I'm hoping it's a particle, and preferably a weakly interacting one. After all, I've put lots of my time into building detectors to find that kind.

Can you tell me how can new techniques in Computer Science such as Machine/Deep Learning or Computer Vision help you/other researchers in this area? Or maybe in particle physics? One day I dream of working at CERN as an intern or even as a data scientist (if it is there). Thank you!

Can I ever tell you that! In fact, the PICO experiment (that I work on) recently had a student use machine learning techniques to try to tell the signal that we want from the background. It's a very cool way to possibly get more out of the data than we could previously. Data science is definitely having a huge impact on many different fields, so continuing on that path is a good way to go.

What's your best guess to what dark matter is? Or can you give us some examples of its properties?

The properties that we know about dark matter are that it interacts with gravity but doesn't interact electromagnetically. Other than that we are trying to nail down the rest. What we would like to know is how likely it is to interact with matter (the cross-section) and the mass of the particles.

Is there a supermassive black hole in the dark matter less galaxy that we found?
Where is the dark matter allocated inside a galaxy?Inside the galaxy or just in its halo?

I'm not involved with the dark matter free galaxy, so I'm afraid I don't know if that one does have a supermassive black hole at the centre. But I can say something about the distribution! We expect that the dark matter in a galaxy is distributed all around the galaxy itself in kind of a halo shape, as you say. That doesn't mean that there isn't any throughout the rest of the galaxy, just that the majority is found there.

Thanks a lot Ken!And one last question if you may! Does the mass of baryonic matter of a galaxy plays a role in how much dark matter mass is in a galaxy?

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.

Is there any evidence to suggest that dark matter may be related to the multiverse theory?

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.

Interesting. As a layman, I just wondered if it could somehow be some strange type of shadow of actual matter from another universe.

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.

What kind of effects does dark matter exert on matter that we can observe? How do we know the dark matter is there?

This is a great question! We actually know that dark matter is there because of the effect it has on galaxies around it. Basically the gravitational interaction of dark matter and galaxies produces a measurable effect that lets us know it exists. We have other proofs like the signature of dark matter in the Cosmic Microwave Background, but the first evidence came through gravity and observations.

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

If you were to find dark matter, would it be possible to contain somehow?

Yeah, that would be pretty neat. Unfortunately it interacts so rarely with everything around it that it would be really difficult to trap it. I wouldn't rule out anything that could happen in the future though!

do you get free mcdonalds for life?

Would that be a good thing? How long do you think a person could eat McDonalds before getting pretty tired of it?

But the name of the institute is after Art McDonald who won the 2015 Nobel prize with the SNO project. I've never thought to ask him if HE gets free food...

When will we have dark matter fueled spaceships?

Could we build a dark matter weapon? What would it do?

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.

Why does the smell of a fart fade away?

Not *exactly* dark matter, but basically everything naturally diffuses from an area of high concentration to an area of low concentration. So the fading away is just gases being gases.

Will uncovering the behavior and existence of dark matter allow us to find its supersymetrical counterpart? Potentially leading to new sources of fuel and energy?

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.

Yes of course, correct me if I'm mistaken but is that the essence of Quantum Chromo Dynamics? Is it theorized that potentially dark matter and it's supersymetrical makeup exists within one of the 11 theorized dimensions of M-theory? Kinda like entangled with it?

I've personally done extensive research on the Higgs Boson, Superstring theory, and what the LHC is doing to experiment for uncovering a well defined theory of Quantum Gravity , and all of this is too fascinating not to ask. Forgive me if I sound arrogant, I just would really like to understand what you can say without giving it all away hahaha.

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!

Uses of dark matter in our lives?

Well, so far dark matter has helped us by allowing our galaxy to form, which eventually led to us being able to ask the question. (Simulations say that without dark matter we probably wouldn't have any structures forming at all and therefore we wouldn't exist.) But in our day-to-day lives it's probably not pulling its weight any more. It's all around us all the time, but it interacts so rarely that it doesn't have a large impact. But just like any research-based project, we can't predict all of the wonderful things that will come out of it. Right now the methods being developed for these experiments are being used in things like medicine to improve our lives in general. An example would be the discovery of the electron, which has then gone on to give us things like reddit!

How warm is dark matter?

What are the optimal temperatures theorized for finding dark matter?

What are the temperatures inside the machines to detect DM, before and during experiments?

What's the lowest temperature that has been experienced during experiments and are there were significant changes related to low temperatures?

Well, dark matter in the way that we look for it is often called "cold" since it is moving way slower than the speed of light. But it doesn't necessarily have a temperature as such. The optimal temperature for finding it varies from experiment to experiment. For example, the PICO experiment (that I work on) operates around 16C, but the CDMS experiment (that I used to work on) ran at around 12mK, or roughly -270C. They use this very low temperature as a way to detect the heat deposited by the dark matter interaction to try to detect it.

what's most important thing(s) to know about this topic that would help us in our life?

I personally believe that one of the things we can achieve in our lives is to advance knowledge about the world around us. Right now there is something out there (dark matter) which we don't really know much about. So I think it's worth doing what we can to see how much we can learn! Curiosity can definitely be a worthwhile motivation.

Someone recently promised me a million dollars if the universe turns out to be conscious. Do you think the universe is likely to be conscious? (I already know I'm not likely to get my money even if it turns out to be true.)

Also, do you think dark matter will unleash the dark side of the force and cause an imbalance in the universe if we were to use it?

How does one prove that the universe is conscious? I'm afraid you will probably never see that million.

I guess the second part depends on how we "use" it. Dark matter is all around us all the time, passing through us constantly. I also think that humans probably can't significantly impact the universe as a whole, but let's leave that for future generations.

No idea how that could be validated or falsified but this quote says that, so that was why I asked. https://www.nbcnews.com/mach/science/universe-conscious-ncna772956

The notion of a conscious universe sounds more like the stuff of late night TV than academic journals. Called by its formal academic name, though, “panpsychism” turns out to have prominent supporters in a variety of fields. New York University philosopher and cognitive scientist David Chalmers is a proponent. So too, in different ways, are neuroscientist Christof Koch of the Allen Institute for Brain Science, and British physicist Sir Roger Penrose, renowned for his work on gravity and black holes. The bottom line, Matloff argues, is that panpsychism is too important to ignore.

“It’s all very speculative, but it’s something we can check and either validate or falsify,” he says.

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

How do I get a job doing this? I'm certain that I can spend years looking for something only to never find it.

Experience: Searching for love for the last 30 years...

All you really need is a PhD in physics (or some related field) and the drive to keep going with the search. All of these experiments also involve students starting at the undergrad level.

How could dark matter affect the cosmic fabric? Is it in somehow like the effect of the black hole?!..and if it is...then how could the (DM and Black Holes) work together in a galaxy system or structure? I mean for example, in one galaxy having one black hole at the centre, that responsible for holding the galaxy stars orbiting around it by its massive gravity.. and then, of course, there is a dark matter distribution that responsible for stars motion... so how could they be linked together to hold up the galaxy!!?

We definitely know that the dark matter in a galaxy can influence the travel of stars, and the distribution can make a difference. So both dark matter and black holes do have an effect on everything around them, through gravity.

What is the worst part about being physics faculty?

There are lots of fantastic things about being physics faculty. Having the opportunity to work on cutting edge science experiments with great collaborators is tough to beat. I guess the worst part the ongoing process of having to secure funding to continue progress on our experiments. Lots of great physicists have great ideas, but there is a limited pool of money available.

How are we so sure that dark matter is matter and not something like primordial eddies/puckers/dimples/tangles in the fabric of spacetime? We only know it's there because of how it warps spacetime, so what if it's just a topological feature of spacetime. <kook> And what if these tangled puckers are occasionally popping free, releasing their potential energy, and what if that's the source of dark energy? </kook>

It may be possible that the dark matter is not a particle at all, but some kind of dimensional artifact as you describe. That is going to be more difficult to detect, and nothing that I work on will be able to find it in this case. But in general we still believe that the best bet is the particle-based explanation based on the evidence that we've collected in the past. The scientific community is searching for particles, at least for now.

What do you believe, but cannot yet prove in your field?

Well the primary thing I believe but cannot yet prove is the existence of dark matter. There are other things, like how neutrinos get accelerated to incredible energies, that I have opinions about, but I wouldn't go so far as to say I believe them. At least not yet.

Is there anything known about the distribution of dark matter in space? Is it at predictable places in galaxies? Near stars? Around planets?

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.

Why do you think that dark matter still needs to be researched? As you answered already, it's not "pulling it's weight anymore"

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.

Can you explain what you do in a non-dumbed down form? I want a sense of how intense your work is and how beyond me it might be. I never see physicists talk without toning themselves down.

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.

Haha well thanks but I'm sure I'd be able to understand what you said a lot faster had I payed attention in my science classes in high school, a bit of a mistake seeing how I'm becoming interested in science now.

Either way from what I understood I want to know how you know this machine can detect dark matter? Dark matter is both something that it completely new to us and we're unsure even exists. How do you know it will even work?

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*.

That must mean you go through many different detectors huh? Do you guys have many theories about what dark matter is capable of if we are able to find and manipulate it? Could manipulating it at all cause possible harm?

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...

Haha I'm not worried about my safety. But I read what you said about how scientists believe that it may have pulled together the universe as we know it. I suppose I wonder if messing with it in the wrong ways could be like messing with cornerstone of a building. I might sound very silly but I wonder if it could make reality itself fall apart if it did something so significant.

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.

What do you think about the alternative explantions for the rotation of galaxies, i.e. MOND?

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.

And what do you think about the "the mysterious mass is just a gigantic bunch of brown dwarfes" theory? And thanks for your answeres!

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.

Do you believe in aliens?

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.

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

How do black holes work? Is it something to do with dividing by zero?

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.