What Is Dark Matter Made Of with Dr. Surjeet Rajendran

What Is Dark Matter Made Of with Dr. Surjeet Rajendran

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Fraser Cain: Yep. All right, let's get started. Very good. All right. Speaking of quantum mechanics, we need some external observer to verify our existence. And then once that collapses the wavefunction, then we'll know we're actually live. But But until then, we're Schrodinger, his interview. And where do you where do you teach? You're in Dr. Surjeet Rajendran: Johns Hopkins?

Fraser Cain: Johns Hopkins. Okay. Perfect. Yeah, yeah. All right. And come on, somebody tells us that we exist. We do exist, which we know we exist. But, you know, I think require some kind of external validation to know that this thing is actually working. This technology, you know, I'm just going to take it on faith. I'm just going to assume that it

rikes Very dangerous, very dangerous way to do a scientific experiment. See, faith. Alright. Alright, let's get started. Well, hi, everyone. I'm Fraser Cain. I'm the publisher of Universe Today. I've been a space at astronomy journalist for over 20 years. And most of the time, I'm reporting news

stories, but I like to bring you behind the scenes to see how I interview some of the researchers that are doing the work. And today, I've got Dr. Surjeet, Rajendran, from Johns Hopkins University. And and he's here to answer all my burning questions about what comes next in the search for dark matter, and others suffer this. Well, Dr. Rajendra, thank you so much for for joining me today.

Dr. Surjeet Rajendran: Thank you so much for having me on. I'm sure it'd be a fun interview. Fraser Cain: Well, the question I always ask people is, is who are you in? What do you do? Dr. Surjeet Rajendran: Okay, so I am Surjeet Rajendran, I'm a professor of physics at Johns Hopkins. I'm a theoretical physicist. So you know, in principle, I'm supposed to

sitting out there on a very comfortable chair like this, and thinking of theories about the world. But I have sort of unusual interests in the sense that even though I like to be in like an in a very comfortable sort of lazy boy, I believe that the only way we will answer questions about the world is by doing experiments. And so I've been very actively involved in thinking about new ways in which one might perform experiments, what kinds of new technologies can want us to discover facts about the world. So I've been doing this now for about 15 years or so. And I've worked on a number of areas, for example,

thinking about different ways to detect gravitational waves, different ways to discover new forces of nature, as well as quite a lot of more recent work that has been aimed at trying to find new ways to detect dark matter, dark energy, stuff like that. So yeah, I mean, to me, that's very, very important. Because, you know, the way physics has been working, right, historically, it was a case that theory and experiments sort of right on top of each other, and we would like constantly have this very healthy mingling. But of late, that has been less true. And that's not good, right? Because at the end of the day, we want to figure out what's the nature by experiment? And that's always been my sort of main driving purpose. Fraser Cain: Yeah, it definitely feels that way that the theorists have gotten ahead of the experimenters. And and I

don't know whether this has somehow stalled the progress of science, but But definitely, you can see some theories like string theory of supersymmetry have have caught the fancy of the theorists community. And, and yet, it hasn't led to a lot of really practical ideas on how to test these ideas. And so they have to remain completely theoretical. I mean, there's

been some really interesting, I guess, side benefits from all this work in string theory, but not the advancements in the our fundamental understanding of, of the Universe, in the ways that we're hoping. So, how do you I mean, again, as a theorist, it's very, it's a very interesting perspective for you know, essentially the the experimenters are the heroes. You're the theorists cheering them on. How how do you think about this, then how do you? How do you think that as a theorist, you can bring this balance between the theorist and the experimenters more back in line? Dr. Surjeet Rajendran: Yeah. So you know, the way this sort of has operated for me practically right is basically the science is now live very, very big. So many people are out there,

they're doing their own thing. And they're extremely good at what they do. So for example, many of the things that I've been involved in involve, I would say, rather novel applications of extremely precise sensing technologies. So for example, there are these guys who call themselves you know, I mean, there's a whole field called amo, sort of atomic and molecular optics, right? And these guys have worked extremely hard to create very, very precise instruments that can measure Extremely small magnetic fields, very small accelerations, very small time changes, you know, you may have heard things like atomic clocks, extremely precise instruments. And they are the pros, you know, they want to come up with the best way to make that measurement. Right. But that's

pretty much what they are actually, experts on by themselves. They don't know how these technologies could potentially have applications outside of their specific domains of interest. And what I have been pretty successful, and I think had a lot of fun doing this sort of thinking about, well, how can we use a variety of these technologies to say discover facts about the world? So that's where I come in, because I think about a problem like dark matter. What do we know about dark matter? Very little, we know it exists. And we know that it's a new particle, it doesn't seem to be a modification of gravity. We roughly know a couple of things about its mass, you can ask how heavy could the dark matter be? And the bounds are pretty ridiculous in the sense that it says if it's heavier than about 10, to the 24 grams, okay, that's, that's, you know, if it's bigger than that, it's more like a size of a planet. And we would have seen that. So, you

know, it's not heavier than that. And on the lighter side, it's something like, you know, 10 to the negative 22 electron volts, which sort of roughly I don't know, in grams, well, that would even be more like kind of the negative 40 grams, something like that. So it's an enormous range of masses, but the dark matter exists, that's what all we know about the wall, right? So I've come to this perspective. And I say, look, the range of possibilities is so huge. How can I think about

systematically probing large parts of this parameter space, as opposed to saying, Hey, I liked this one theory. And I'm only going to go and test that one theory. Right. Some of that has been the attitude of the community. And to be fair, that

used to be the argue the community, people were like, they were very convinced, because of these mathematical reasons. They were like supersymmetry predicted a specific kind of dark matter particle, the wimp, okay, the weakly interacting massive particle. And people were extremely focused on finding that one particle and nothing else. So I come in and say, Well, why, you know, why are you

so convinced that I have something like 60 orders of magnitude mass that you think the answer is? That's right. To me, that sounds kind of crazy and confident? And I'm like, Look, we don't know anything. So we should be looking broadly. It almost Fraser Cain: sounds like it's a bit of a Hail Mary. That yeah, that they've they've because the math in the supersymmetry or string theory or whatever predicts a particle, you might as well try to see if that particle is there. And if you're done, then you get to roll up all of the excruciating work that it's actually going to take. But since that helped Hail

Mary didn't pay off, now, you got to do it the hard way. And the hard way, I guess, is, is not confirming what you think, you know, it's disproving everything else until you're left with whatever remains that still fits within the we don't know what dark matter is, but it has to be within this range of mass. So what is the so if you take that kind of holistic view, what is the best way to kind of Hue away all the rock in this, or the marvel in this statue? In a more effective way? Dr. Surjeet Rajendran: It's an interesting kind of question, right? Where basically, you know, when I've advocated this point of view, people will go and tell me like sort of this was more true, but 10 years ago, where people were saying something like an old Well, this guy simply wants to go on a fishing expedition, right? Because that's, you know, that's what I'm doing. But I have the point of view. Yeah, I am going on a fishing expedition. Because we know there is some fish out there. That's the dark matter. We don't know what kind of fish

it is. Right? So it could be an extremely big thing like a blue whale, or to pay some a tiny little Finding Nemo starfish. So the job is to really figure out what kind of nets can I actually construct, which are able to go and find these varieties of fish, right? So if I use the kind of net that I need to catch a whale to try to catch Nemo, it won't work. So what I do really, is to think about it systematically, but I say, look, if I think dark matter is in this range of mass, okay, say this range of fish size, what are broad properties it could have? And once you think about it that way, you realize, well, look, what are we trying to do, right? We're saying there is some dark matter particle out there and detecting it means we want that particle to change some detector that we build out of atoms and light and stuff like that. So then you say, what

are some general ways in which various classes of particles could affect atoms and light? Turns out there aren't that many ways of doing them. So just to give you a simple example, if you Dark Matter has extremely low mass, right? So yeah, and actually like, here's an important point about it. We're talking about two very extreme ranges, right, like extremely light, dark matter, and extremely heavy dark matter. Now, here's one fact we know about the world, we know how much dark matter, energy density duras, the total amount of dark matter, energy we know. So if I think about the dark matter having a very, very small mass, because it's total energy density is fixed. There has to be a large number of dark matter

particles. Right? So when we think about trying to detect extremely light, dark matter, the way you want to view this is like a lot like how you view wind, right? So when you have wind wind, is the fact that a large number of air molecules that are moving past you, how do you detect when you could think about two ways of detecting when one of them will be efficient, the other will not be. So one way of detecting wind may have been for you to figure out if every single like trying to figure out the energy deposited by a single wind molecule, right? If we think about our detector that way. Now, of

course, we very hard because the wind molecule is so light that it doesn't deposit that much energy. So the way you detect when normally is that you build like a windmill, or a wind vane or whatever. But you're looking for the collective effects of all of these huge number of Bend molecules coming and hitting you. Right? Right. So in the other light, dark matter case,

that's kind of what you want to do, you don't want to look at the amount of energy that can be deposited by a single dark matter particle that is more appropriate for heavy dark matter. But for light, dark matter, because there's going to be a huge number of dark matter particles, you want to look for collective effects of these things. Fraser Cain: Right, and, and that collective effect could be seen at small scales or even large scales. Dr. Surjeet Rajendran: Yeah, there could be seen, in fact, a lot of work I've done is what is called small scale experiment, which basically is the idea that these collective effects can come to detectors that you have in your laboratory. And you can

see that effect. So I would say there are only about, you know, four or five of these collective effects. And I can list them all yeah, please work in the dark matter do right. So you have this wind of dark matter. And you're asking, how can the wind

affect particles in the world that I control? Things like neutrons, nuclear, yawns, electrons, photons, things of that kind. So what can this wind do, right, so this is when can come in, it can produce a photon, it can produce some light, what can go look for that this wind can come in, it can drive a current in a circuit and push on electrons. What can go look for that? This is when can come in. And if you have a spin of say, some electron or a proton or whatever, it can make that spin move back and forth. So you can look for the rotation of spins caused by this when it rolls directly pushed on body.

So if I have a mirror hanging somewhere, this wind can come and push on this mirror. Okay. And the thing is, we as humans have now gotten extremely good at measuring these effects. So for example, if the wind pushes on mirrors, what you're going to be doing is a lot like this experiment called LIGO, which looks for gravitational waves. So they're looking at like extremely small motions of these mirrors, right. So you could

pretty much use the same kind of technology to look for small motions of these murders, where the motion has been caused with the dark matter wind pushing on it. Humanity has also gotten very, very good at measuring small magnetic fields. So the dark matter came in, and let's say we created a small current in a circuit, that small current will also create a small magnetic field. And you can use that signal with a very powerful magnetometer to measure that the dark matter comes and make some spins wobble back and forth. As the spins wobble, they also change the magnetic field. And you can use a very, very precise

magnetometer to measure that small change in the magnetic field. Right. So that's kind of how I think about systematically, Fraser Cain: right, and so then if you set up these experiments you created, you know, you set up for big experiments, side by side by side, one with a mirror one with the magnetometer one ticket, electric, current, etc. And you didn't detect any thing outside of I guess, you know, the cosmic rays coming through? Or do you know, all of the phenomena that you already know and can account for? Yes. What would that allow you to do in terms of your understanding of that, of that space, that search space for dark matter? Dr. Surjeet Rajendran: Yeah, this is a tough question, right? Because in a sense, this is one of the problems with a dark matter search business is that you don't know how far you need to go. Right? So basically, what we're learning is that if you don't see anything, you're basically saying, The Dark matter is not this kind of particle. What does the range of

interaction strength? Right? So that's what we're looking for. We're looking for a dark matter particle at a certain mass and a certain enter reactions strength and interaction strength is too weak, I cannot see it. So there could still be a dark matter particle out there it is that you require more and more sensitivity to get down there. And if we as human beings don't have that level of sensitivity, right now, we won't be able to find it. So it's a tough game, right? But basically, like, it's

a thing where there isn't a clear sign of saying, Stop. Now, this is a waste of time that that's just not true. You kind of just have to keep sort of digging. And so I would say it's a tough problem, because you need to go both in wide and mass as well as deep in coupling.

Fraser Cain: But aren't we I mean, aren't physicists already doing this dis process? I mean, when we talk about dark matter, we no longer like, we know that it's cold, dark matter. And so we know that it, you know, it probably is in hot neutrinos because of various constraints. So, and as you already mentioned, in the beginning, the mass constraints that dark matter could be, there is possible cross section sizes and so on. And then of course, there's a whole other field of MOND. How I guess, do you do you feel like, like, physicists kind of got halfway there by sort of figuring out that it's cold dark matter, and then have been continuing to search for the right particle, as opposed to just giving up to not knowing or giving into not knowing and then just working on constraining it before trying to identify what it is? Dr. Surjeet Rajendran: Well, I think it's very difficult to make a better constraints and what we have, right, so in some sense, the difficulty with dark matter is that so far, we've only known about his existence by gravity, right? That's how we know there's dark matter. And gravitation is not a

particularly powerful for force, even though it dominates the overall structure of the Universe. On like, you know, the constraints you get from gravitation are somewhat pretty weak, right? So that's why the parameter space is so large. And so this is a psychological thing about humans, right? So when they see the parameter space being so big, people have a tendency to psychologically say, Look, I am so convinced it must be at this point. Next, because the idea of searching No, this broad range of parameters scares people. Right? So these constraints, but these constraints are, in some sense, kind of artificial, because they're not actually based upon anything like truly real, right. So all you can ever do, in some

sense, is basically constrain a certain model, that's all you will never do, right? Until you find that thing. So you're gonna have been comfortable with that fact that the parameter space is so huge, that you want to look broad. Fraser Cain: But, but I think that process of, of constraining the search space would be helpful and appreciated, to know where to look, it's kind of like, you know, you drop your, you know, you drop your keys in a parking lot, and you don't know where to look. And, but if you can, somehow start to figure

out where it you know, where you know, where you walked. So once you start there, then you've constrained the search space. And that is very helpful. And I can totally, I can see this balance between constraining the search space, and then taking your shot and saying, Okay, this is it. And the experiments created. Yes, like when you think about the complexity and amazing sensitivity of something like Lego, right, which many people didn't even think was going to work? And when you understand the physics involved in that, in that experiment, it's mind bending? Absolutely. What kind of a leap of faith, I

guess, you know, would it be worth it to create an A machine that complicated and that sensitive, that's only job is to tell you where not to look? Dr. Surjeet Rajendran: Very good. So these are the two very important issues that he brought up. And they're both extremely central to this philosophy that I'm advocating in terms of how to do this, right. So first of all, you mentioned the thing about the parking lot that you dropped your keys. And, you know, you, obviously, if you knew where you walked, it makes sense for you to go and search where the, you know, on the route that you want, right, that makes sense. Now, here's the real story for dark matter, right? So it's something like the following, you know, that you drop your keys somewhere on planet Earth, that's all you know, right? That's huge space of parameters. Now, of course, it's very, very difficult to go

and search the entire planet Earth. So instead, what you do is that you go and talk to an astrologer who tells you hey, you know, do this weird turn of beliefs that I have? The dark matter must be in Maryland. Right? Of course. You know, it makes you even feel better about going and searching in Maryland because You're solid, you're told you that's what it's supposed to be. But it's not really based on anything, you know, actually factual, right? So the thing about physics is that we have to confront the problem at hand. The problem at

hand is that this is an immense effort. It's extremely hard to do. Right? So we can't make progress unless we're intellectually honest about what is known and what is not known.

The second thing you brought up is also a very important question, which is that would I advocate that people spend the amount of money and time that it took to find LIGO, to build LIGO to go after any specific kind of dark matter particle? Or Fraser Cain: no? I'm saying I'm saying just constrained the search space, its job is not to find particles, it's to tell you where not to look. Dr. Surjeet Rajendran: Yeah, but the thing? That's right, what I'm telling you is that this idea of knowing not we're not look is pretty hard, because it's so literal. And so all the time, right, we basically say, if there are particles of a certain kind, those particles would have been, you know, produced and stars and certain ways they would have shown up in our colliders in some way, right. So we do putting those constraints, it says that they are not that constraining. Right? It's like saying, for example, that you knew you drop your keys somewhere on planet earth, not the Solar System.

Right. So there's certainly, you know, when when you lost your keys, yeah, you don't have to search the entire universe or the entire galaxy, but it's still planet Earth. So that's what I'm telling you that the constraints are loose enough, right, that you're kind of in a situation where you still need this rather broad program. But the point is this right that, let's say, theoretically, the scrubbing possible, suppose every singles Dark Matter mass that I want to search for, if it required several billion dollars of money, and you know, 30 years of people's efforts? Would I be advocating that is searched for all these dark matter particles the same time, but even if I had a candidate, it's not possible, right? There's only so many physicists in the world, the physics budget is whatever it is. So you can't be searching for all of these things at the same time. But that's not true. What's true is that each of

these experiments typically cost about a few million dollars. And it requires a few groups, you know, maybe like 10 to 20 people working for maybe 10 years, to kind of substantially begin. Right? So it's a it's a, I kind of view this balance in the following way. You know, if you were going to spend billions of dollars on money, and 30 years of people's lives, you better be god damn sure that the thing that you're going to search for is actually there, Fraser Cain: like the Large Hadron Collider, like, like, Dr. Surjeet Rajendran: it might be like to be, like, definitely want to be there. It was suddenly, what are you spending

30 years and billions of dollars going out to the heads? Same as Lego? Lego made sense? Because you knew the world is gravitational waves at some level? Right? Okay, they had some slop about maybe a couple of orders of magnitude. They don't exactly know what that would be. But you are guaranteed that if you could make the technique work, which of course was unclear in the beginning, but you knew that if you could spend that billion dollars and get it down there, you'll see something that's not true. But dark matter, right? There is no dark matter particle where anyone is convinced that all you need to do is spend $1,000,000,000.30 years of effort and you are going to find it. If there was a dark matter

candidate, I would be all for it. But that's just not true. Fraser Cain: Now, you know, you've started originally talking about how we might search for light dark matter. Yeah. Do you have sort of a similar set of thinking about much more heavy dark matter? Dr. Surjeet Rajendran: Yeah, I do, actually. So this is more of

been my more recent interests. So I spent the bulk of my career thinking about the Ultralight, Dark Matter sector. But more recently, I've been thinking about extremely heavy dark matter, and how one could actually go and find it. So there are again, different classes of these guys. So the challenge with heavy dark matter, typically is the fact that the number density is so low now, right, because again, the total mass, the total energy density is fixed. As he said, If

the mass of the Dark Matter gets very, very heavy, the number of dark matter particles is extremely small. So you run into the following problem that I as a human being can build some detector on my ground. You know, it's typically a few meters in size, if I get very aggressive and I can operate it for a year.

So all I can ever do in my experiment that I build on the ground is wait for a dark matter particle to go through it. But the dark matter is so heavy, right? It will never go through an experiment that I build in one year. Right? Because it's just so rare that I'm it's so hard for me to find it. The question is, what do you do in that case? What kind of strategic thinking can you actually have? So I've been exploring sort of two possibilities. One is that it does something dramatic to an astrophysical body like a star. Okay, so one of the works that I was involved in, it was kind of a Good idea that basically, it turns out that certain kinds of dark matter particles like primordial black holes, you know, these are extremely heavy objects. These things can go to

like white dwarf stars, you know, like the stars that we have around us. And they can actually make them blow up into supernova. Wow. Okay. And it's a crazy thing, because it turns out that a white dwarf is essentially like a nuclear bomb waiting to explode. It's got all these carbon nuclei all sitting

next to each other, and the carbon wants to fuse. But what's happening is that in a white dwarf, the temperature by itself was so small, that the carbon does not undergo fusion because it has to go through this, you know, fusion barrier, right fusions very hard. But what can happen is that when there's very, very heavy dark matter particles go through, they can heat up a tiny part of the star to very high temperatures. And that then makes the carbon over there undergo fusion. And then all the energy released in that process can then go nearby to heating up other carbon atoms, making the whole thing explore like a ball, right? Fraser Cain: And I guess it's similar. So the way like a type

one, a supernova goes off exactly, because I've got a buildup of material on the surface, that chain reactions across it just blows the whole thing apart. But it has to be a process. Right, right. Okay, that's really interesting, huh? So. So that's one way, are there some other places that you might

be able to see them? Dr. Surjeet Rajendran: Yeah, so I've kind of recently been involved in a very fun project with a group of guys at Harvard, University of Maryland College Park. Basically, the idea is that, you know, one of the issues we ran into was that look, if I build an some detector for that's a meter squared in size, and I operate for a year, then only, you know, certain mass of dark matter particles can go through, how can I change that, but one thing I can actually do is I can leverage the fact that the Earth is very old. So let me go and look at some very, very ancient rock. Okay, so that means this rock has been around for about a billion years or so. And what

you can look for is basically, as a dark matter went through, it could have left unusual tracks in the rock. Okay, so the idea is basically that that basically, let me go and look at a meter squared of rock in, actually the jack hills of Australia, that's our place where we think this is the best place to do it. So we have all these ancient quartz in that area. And if you are very, very heavy, dark matter going through, it will basically create this very, very long line of damage the rock. And people these days have very fancy ways of imaging rock, you know, with all kinds of fancy technology.

So the idea basically, is for us to get a very old piece of rock from these hills, and image them and see that these very unusual tracks in the rock, Fraser Cain: that's really neat, you know, would be even better, would be a chunk of the Moon. I've, I've heard, I saw paper, someone talking about how the Moon is like the perfect place to get a historical record of essentially, cosmological events in the Universe that you will have written in the regolith, every supernova, every gamma ray burst, everything that's happened relatively nearby over the course of billions of years, you just take a great big core sample, bring it home, and then slowly pull it apart bit by bit. And you would and you'd be able to sort of roll back but I can imagine taking a cubic metre of the Moon and bring that home. And and then you'd have

something that that wasn't affected by by various Earth, creatures, and so on. That's really, that's really neat. What about sort of stuff that's in the middle? So you've like on the one side, you're you're detecting on mass, this wind? And on the other hand, you're detecting giant particle events, primordial black holes, passing through your planet, and so on? Is there What about the stuff that's in the middle? Dr. Surjeet Rajendran: Yeah, the stuff in the middle is also very interesting. And you know, I've kind of thought a lot about. So it's an interesting problem. So you know, the traditional community, the people who do what is called weakly interacting massive particles, dark matter, or dark matter, these guys are there in the middle in many ways, and they have actually done a lot of very important work. And so what is

now true is that it's sort of conventional. Okay, I would even call it conventional. I mean, you know, because they've been pushing the envelope on it continuously. It is now possible through a lot of very hard work by the community to detect something like the dark matter comes and hit something, let's say or get absorbed, and deposits about an electron volts of energy. Okay? Now one electron volt is something that we in particle physics is, you know, well we use it all the time as always a very large number, but it's not it's like an incredibly small amount of energy sector, the negative 19 joules are very, very small amount of energy. So they're actually so good now at measuring these very, very small one electron volt amounts of energy deposition. And the

reason why they can do that is because the dark matter deposits that one electron water energy, it can typically ionized particles, okay? And the way that works is that, you know, how do you see a small amount of energy, fundamentally, you want to create what I would call as an amplifier, right, you have something that's the positive small amount of energy. And that's very hard to see. So you need to increase his effects. So one way in which these guys do this is basically they will apply a large electric field, let us say, and something comes in, it hits something deposits some heat, and that causes something to get ionized. Okay, sometimes some electron gets ionized. Once you have the electron getting ionized to put

a large electric field, that ion now gets pulled up at very high energy. And they can then see that so that fundamentally amplified the effect of the dark matter. The difficulty with this approach is that you have to be able to deposit enough energy to ionize objects. And in our world, there is a minimum amount of energy you need to ionize they're about, you know, point five, Evie, something like that, like not that much below and electron volts. So to go below that you require newer ideas.

And what I've been involved in, it's sort of a interesting field the called single molecule bandits. And here's kind of the idea, okay, so as a cartoon, here's how it works. Imagine I take a bunch of spins, you know, like a magnet. And I apply a magnetic field in the opposite direction. So the spins are all

aligned in one direction, the magnetic fields in the opposite direction. What that means is basically that the spins are not in their ground state, they're not the lowest energy, right? They're excited, they're on the wrong direction, they're anti aligned. So they'll flip, they will actually be able to release energy. But you can create materials where the Spirit is,

is anti aligned, right? So so there's a stored energy in that system. What I was thinking of was basically that, well, maybe something can come and hit those spins, deposit some heat. And we deposit that heat, the the spins that absorb that heat there, they jump to the ground state, they flip. And when they flip,

they release energy that they've stored in them, because of the the excitation the magnetic field, and that energy can then go and cause the nearby spins to also fly. Fraser Cain: Right? I see I see. But but in in general, though, you're essentially taking again, some volume of shielded volume, hopefully below, you know, below the surface of the Earth or whatever, and you are measuring the entire energy that is in that area, and you're looking for anything that's out of the ordinary, that is essentially some kind of interaction from a medium mass particle, that's, that's coming out. And that's fine. And the challenge is that our ability to multiply the the detections has reached is a certain minimum. And but if we

could develop more fundamental ways of multiplying, the whatever, whatever interactions are happening, then that allows you to search a lot deeper and detect far more sensitive events. Dr. Surjeet Rajendran: That's correct. Yeah, so the challenge is really in this game in the middle is about trying to detect smaller than smaller amounts of energy, right, so you need better and better amplifiers. And what I've been involved in

is sort of ways in which the conventional amplifiers are based upon using electric fields and ionized material. And I've been trying to do that on the magnetic side, simply because the amount of energy it takes to cause these magnetic spin flips is a lot lower than what it takes to ionize an atom. Fraser Cain: So then let's put it all together. So at this point, you know, you've got sort of some ideas for the for the lightest particles some ideas for the middle particle, somebody's for the heaviest particles, you know, right now science is facing, do we invest in what comes after the Large Hadron Collider, they, you know, the superconducting supercollider some and and it's, you know, the goal is to essentially just to smash particles at random and see what happens and hope that something is in there, which feels like that's still part of what you're describing just in a very expensive way. How would you apportion funds this point, to

try to make some meaningful progress in the search for dark matter? Dr. Surjeet Rajendran: Well, so yeah, you know, the Dark Matter game is a lot cheaper, of course than building a collider. Right? So as you said, these are a few million dollar experiments. And they only require maybe 10s of people working for maybe 10 years, right. So there's a lot lot cheaper than what is being done for the next collider, the collider is basically 10s of billions. Decades of work. Right. So, as far as a dark matter game is concerned, I think, you know, I, the community has, by and large, I think, embraces approach, that maybe a decade or something ago when I was beginning to do this, the community is extremely focused on a small number of experiments. But today, in fact, there are a number of people

around the world who are actually coming up with their own ways of detecting dark matter, and I'm not the only guy in this game. There are plenty of people trying to do this. And many of those experimental efforts are also funded simply because as I say, right, this is one of those things where it is not that enter that resource intensive for any one person to do. Plus there is a feature that sort of sociologically, right, the Allied see thinks that these are like large experiments, you know, they are. And when you have a large experiment, there's a lot of management involved. And you know, if people feel constrained, I think sociological in terms of what they can do what they can play with. Here, you're kind of in a situation where people who are more independently motivated, are able to get into the game and do meaningful things, right.

So you can be your own person trying to do your own kind of thing. And so there is an element of it attracting, I would say, the more small scale entrepreneurial side of physics, that people want to play with their own technology, you know, are able to get into these things. Right. So, so that way the investments can be, I would say, fairly broad. And it's not

too intensive for any one person. Fraser Cain: It's, I mean, I totally agree with you, you know, when I think about our reporting, you know, we're talking about various interesting tabletop experiments being done in the search for dark matter, or relatively small, something set up in an old salt mine, or which, as you say, you know, is is sub million dollars or a few million dollars expense, a few people have one researcher and a few grad students are, are often not reporting, you know, they're all saying we didn't find anything, but but still are helping to constrain that, that search space, and so does it. I mean, does it feel like the one I sort of think about, like you could, you could set up your mirror or whatever, and you could set up to a certain level of sensitivity, and then you could run that experiment for a year, and you would find no result. But if you had made a bigger mirror and a more sensitive, or maybe you could find a result, next time around, and so it's sort of iterative, yes. In that, and it feels to me like one of

the things that scientists don't like to do is to report a negative result. Does that. And you know, it's like you want to, you want to say that you found something you don't want to say that you've you spent the last year and you didn't find anything, even though finding nothing is is incredibly important to constraining this parameter space? Do you think that limits? The way scientists work at all? Dr. Surjeet Rajendran: No, not really, because here's the thing that I learned, right, which is that when it comes to experimental as, like, one of them was very blunt with me, he basically said, look so deep, all you are doing is giving me excuses to play with my toys. So these guys love to build these

really, really sensitive instruments. Yeah. And I'm just giving them an excuse to get in there. And like, you know, play with all the tools that they have to make more and more precise experiments. So it's, you know, I agree, like people would have, I mean, everybody wants a discovery, right? That's, that's what we're, that's really what we're trying to do. But it's not like, in this game, especially, again, as I said, it sort of attracts a certain kind of person who's willing to go in there and play with these things. Right. Like, that's kind of what they're really excited about. And so for

them, sociologically, I have not really found it too difficult to even motivate people to do this, because most of them do. No, I mean, pretty much everybody does know that most of the time, they're going to be reporting results. Right. So a meaningful result, as you say, it constrains something it tells us what we don't know, I mean, you know, that, that we know that the dark matter is not particle X, right? We know that. So that's, of course meaningful. But ultimately, I think what motivates them is just the fact that they love the technology, right? They just love playing with it.

Fraser Cain: So what is it a Nobel Prize for no results? Dr. Surjeet Rajendran: Right? That will be a very easy Nobel Prize to get Fraser Cain: that but a spectacularly no results, right, like somebody who went above and beyond the call of duty to, to not find anything. Dr. Surjeet Rajendran: And actually, it was an example of such a prize that was given to is not the Nobel Prize people, but his breakthrough foundation. So they gave this Breakthrough Prize, which is like $3 million to this group at the the University of Washington. So these guys were basically trying to figure out if the force of gravity is exactly like what Newton had predicted down to like a scale of like a distance of a micron. Okay? So you know, we know gravity works very well

and big distances, you know, that's gonna be measured. It's possible there are deviations from the law of gravity at shorter and shorter distances, of course, then find anything, but it was really heroic effort to actually get down there and show that gravity does work, like how you think it's supposed to work. Right? That heroic effort was justifiably recognized by this very nice prize that they got. So there are several such adventures that I think people do, for example, the for a long time you They have been engaged in a game of trying to see if electron has had a dent in it. So you know, this is called the electric dipole moment of the electron.

And for 50 years, humanity has been trying to find this little dent in the electron, and we haven't found it yet. And experiments have gotten more and more and more heroic, they have required more and more and more technology. And these guys have really been, you know, pushing the envelope on this regularly.

So I think, you know, yeah, it is heroic effort. And in my opinion, it should be it is certainly worthy of all the all the recognition that I think I think that requires, Fraser Cain: yeah, yeah. And it because it really feels to me like, like, it is as important as the people making the discoveries, you know, if you have, if you've removed every negative possibility, right, what remains, you know, could be interesting. So then, so then it sounds like, partly this is a technology issue. So is so from a sort of, like an engineering

standpoint, or a funding standpoint, how could we get these people better tools? Dr. Surjeet Rajendran: Right, so in terms of like, I would say, funding, that's an interesting sort of thing that you land in there. So here's a fact about the world. Right? So a lot of our money for how research gets done gets a portion by government grants. Yeah, you know. And so the technologies that we talked about, right things like basically, measuring very precise magnetic fields, measuring very small accelerations, very small time changes, things of this kind. Traditionally, the funding for this comes from organizations like NIST, right, the National Institute of Standards technology, because they're interested in creating very, very precise measurements. So they will give you money to

build a really good clock, for example, a really good magnetometer. But they don't, per se care about trying to find dark matter, that is not part of their government remit. Right? Right, finding the dark matter is supposed to the job of the Department of Energy, or the NSF or people like this. So traditionally, it has been the case that any you know, and it's an interesting challenge, right? Because first of all, these guys have to be able to demonstrate that they can actually build a really good clock, I mean, without a good clock, you can't do any of these things anyway. But once you've got a really good clock, there's an extra amount of effort, you need to really try to use it for dark matter, right, because there a lot of extra technologies that are required to make that very specific scientific application, which does require specific extra funding. And oftentimes, it used to be the case that there were cracks over there where someone will find you to do X. But these guys don't think that is their job to find dark

matter, they'll go and tell you go get money from somewhere else to get, you know, money for this. So what has been helpful in this game is that a number of private foundations have stepped in, like the Simons foundation, the housing Simons Foundation, the Moore Foundation, all these guys who basically are willing to sort of get into that area, but the look for the fact that yeah, there is actually pretty good reasons to think that if you spend money here, I might actually be able to use this technology to find some fundamental physics parameter. So they have come in very helpfully to initiate some of these applications. And once that has actually happened, that

has actually helped stimulate the government also, to move into this area. And, you know, sort of like start funding these things. So then today, it is, in fact, the case that due in part to, you know, the like, there's a number of activity going on in the government about building quantum technologies overall.

And that has overall led into an pretty substantial infusion of funds everywhere. And some of that has also been directed now into this kind of dark matter research. Fraser Cain: In the field of astronomy, and cosmology, there's a bit of a renaissance going on, mostly around the the Hubble constant and this idea of the crisis in cosmology that that the measurements of the expansion of the Universe more recently, and the measurement as it is in the cosmic microwave background, differ, and yet, the error bars have reduced to the point that they don't overlap. And so there was something wrong. There's something with the standard model of the

Universe as we understand it, that is, is insufficient to explain it. And this is exciting. You talk to any cosmology talk to any astronomer and they are they are stoked that they have finally they have a crack that they can now wedge in and start to try to understand more about the fundamental nature of the Universe. Particle Physics seems stuck, that the Large Hadron Collider has been grinding away, testing these individual theories, as we talked about. Do you think that particle the particle physics community is ready for a revolution and where do you think that's, that starts Dr. Surjeet Rajendran: It is a great question. So Here's how I to view this problem, right? So yes, you are correct, the particle physics seems stuck. Now there are two issues with this right? One is that if you think about particle physics, right, all the problems that we were inspired by, like, what is the nature of dark matter? What is dark energy? Why is the Higgs word it is? Why is the Universe expanding at the rate at which I which is expanding? What solves the black hole information problem? None of these problems have been solved, right? Like, yes, it is true that people, you know, had certain ideas on how to approach them. And those ideas, unfortunately, didn't pan

out. But the problem is haven't gone away. Right. So this is now the problems that our generation has the ability to go and tackle. So I look at this the following way, you know, as a young person in the field, right? Let's think about this following very interesting issue. What is it that's difficult about a young person making progress? The difficulty is that the old people know a lot, they have already been in the field well before you so they have all the technologies in the world to go and attack a problem that you do not yet know how to tackle. But now notice the interesting fact that in this particular area, right now, you are in a very interesting situation, where the old guys don't necessarily know anything better than you do, right? Because they were trying to find certain things and those things didn't pan out. Great. So you

are now at the same level as they are, and you are not sociologically depressed because your ideas didn't pan out. So you are an enthusiastic young person who can play this game just as well as any of the perfect time to be young person in the field, and sort of do really innovative work. And the you know, to me, like, I'm always motivated by these things on how to do this. The second question that you asked us

about, like, how do you make progress in particle physics? Right, like, what is it that really is necessary? And this is because physics, I would say it's very like a two dimensional plot, right? So in a sense, we know that is dark matter, we know those dark energy, we just don't know what they are. So think about this following issue, right? When I say there is a new particle in the world, there are two parameters that are of interest to me. First is the mass, the energy that I need to go in order to look at the stuff. And the second is

basically the coupling how strong this particle is in talking to us. Now, particle colliders have done a great job in the last 100 years of probing a variety of physics. But what particle colliders are doing is basically they're going in one dimension is access, they're going to more and more energy, higher and higher mass. But the range of couplings strengths to

the particle collider can probe is very narrow, because they only have so many particles that can collide per second, which basically means their ability to produce particles that have extremely small couplings, is very limited. So they have only in some sense, probed physics in a one dimensional plot, going more and more in energy, but not much in coupling. So sorry, Fraser Cain: so I just want to understand this. Sorry, I apologize. Um, so you're saying like, if you would say run your particle accelerator at lower energies, but maybe more collisions, you could back to that wind analogy that you could detect more of a collective wind of a whole bunch of smaller, less energetic particles that would give you information, sort of a new space. Is that my understanding that correctly?

Dr. Surjeet Rajendran: Well, so actually, it's a little somewhat different from that. So I would say in a particle collider, right, you're sort of limited by the number of particles per second the machine can produce, so you only have so many of them. And if you want to produce a very weakly coupled particle,

a very light particle, yes, what you require is an enormous number of collisions, you don't care so much about the energy at which you're colliding, right? But you need an enormous number. Okay. But it's very hard for them to produce an enormous number of these particles that's just not technologically feasible right now. So what part of the colliders do is that they get a fixed number of these particles coming out? And then they can make ways to make them collide at higher and higher energy? Fraser Cain: Yeah, yeah. And so and so that you could with if

the particles are much lighter, it's by having a larger number of the collisions, as opposed to cranking the energy, the energy up to produce more amounts of particles? Yeah, yeah. And so I was sort of saying it sort of goes back to your original idea of right for right for dark matter. Right. And that we've been focused on the the big bullets as opposed to the wind.

Dr. Surjeet Rajendran: Exactly, exactly. Yeah. Yeah, for sure. I would say like the issue really, is that you know, and because it's very, very hard for a collider to increase the number of particles is colliding, you just require newer ways of finding these very, like big clickable particles. What you require are basically more and more precise technologies. Right? Because these weakly coupled particles have an extremely small effect. So you want more and more sensitive

instruments, as opposed to just the raw power that a collider provides. So I would record I would suggest that really you should be thinking about how do I go deeper and deeper and coupling smaller and smaller coupling. And that is kind of where this the wind is blowing now is the sense that we are now the ability to produce extremely precise quantum sensing technologies, which we so far haven't had. Right? So sort of thinking about historically, let's ask this question, how come in the last 100 years, we built incredibly amazing colliders? Why did that happen in the last 100 years? Why did that not happen? 200 years ago, right? It's because of the fact that towards the end of the 1800s, and the beginning of the 1900s, we as human beings, we mastered the electromagnetic force, right? 20 years ago, nobody knew anything about it.

And the last 100 years, we built the engineering to be able to crank an enormous amount of power into radio waves. That's what these colliders are really about. And that technology, God has a long way, right. So as long as you gave me a fixed number of particles, I could really push them to very, very high energies, we were able to do that. Right now, the time we

live in humanity, this is the time when we are really getting to that level of mastery over quantum techniques that are able to measure things very accurate. Right, we're sort of beginning our exploration of extreme precision instruments. So we are, I would say, at the level of quantum technology, the same way where we were in terms of electromagnetic technology in the early 1930s. Right, when people are just thinking about, well, how can I use this RF technology to do new physics.

That's what you know, the great people of that era, like Lorenz and Chamberlain, or what those guys were thinking about. And today, I will say, we are at that level where Oh, I have really good quantum engineering, the quantum engineering is able to do very good clocks. But I have now not used it so far to go really after physics, just like how Chamberlain and you know, Lorenz, and all these guys managed to use RF technology to really go after physics.

Fraser Cain: And so maybe the next version of the Large Hadron Collider allows maybe doesn't produce more energy than the LHC. But gives you more particles to collide, to expand the search space into those into those two, two into those two directions. Dr. Surjeet Rajendran: Yeah, so what do they like to see per se?

Because I see the wrong kind of machine to do, and it just has to be like a very quantum inspired style. But yes, that's the kind of idea that I'm thinking about it. Yeah, Fraser Cain: that's really interesting. So place your bets, when you know, when do you think that we will have some kind of answer? For what dark matter is? Dr. Surjeet Rajendran: Oh, I really don't know. It's a physics is an interesting thing, right? Like, it's basically this this thing, especially like in our kind of physics, right? Like, you know, it goes with this name of fundamental physics, you know, whatever, right. So, for us, things are very binary, right? Like, you either discover something, you know, very discovered very rare. But when you discover it, it's a

screw. It's just forever, right? So like, it's a very binary kind of output in terms of what you're looking for. And you just don't know. I mean, like, it's very hard to make a bet on whether or not in my lifetime, I will find what the dark matter is, right? Like it's, it's a, it takes a certain kind of, I think, psychology to be okay with that. Yeah, right, that I

could spend my entire life looking for it. And that's fine. If I don't find it, it's okay. Fraser Cain: Yeah. But it does feel I mean, I, you know, you mentioned a little earlier about the, like, the string theorists who spent the last 30 years working on their math, nothing's panned out. And I'm sure they're having this kind of existential crisis about their lives at this point. You're like, Why? Why did I do this? And, and maybe the lesson that can be learned is a fundamentally different way to conduct the search. So that you

don't get you don't rabbit hole so hard, and instead, stay flexible. Keep your mind open flip from experiment to experiment. And, and when those no Nobel Prizes, as you as you go until collectively, humanity knows where it's car keys are I just mentioned every single analogy into one year, but yeah, Dr. Surjeet Rajendran: yeah, exactly. Yeah. And

unfortunately, there is no better way of doing it for dark matter, right. And I know that there's a difference between the Dark Matter thing and string theory in the sense that at the end of the Dark Matter game, you know, these guys look for weekend recognized particles for 30 years, they actually know that there is no particle called the WIMP that they look for. Right? That's a true result about nature, right? They actually know for sure, there isn't a particle over there. The

string theorists cannot even tell you that right? They can actually even tell you what they don't know. are what they do know? Right? That's a big difference, because you're what we know. It's the truth. It is knowledge. Fraser Cain: Yeah, yeah. So if you, you know, you mentioned sort of this next generation, the young people, the people coming in to physics and astronomy, particle physics, fundamental physics, what advice would you give to them as they start on their journeys? Dr. Surjeet Rajendran: Yeah, this is exactly what I was advocating a little bit earlier, which is that they're entering this field in a very unique time where the old people have no advantage over the young people, right? It's a beautiful time to get into the community at this time when you want to just as good as anybody else. And that just means that like, essentially, you know, and for me, right, it's like vulnerable, I look at particle physics, I see unsolved problems, I see blue sky, things that nobody's tried before. And so it's a very

funny thing where people sort of get in, and there's sort of a little bit depressed by the fact that the older guys are depressed, like, who cares? Lots of stuff for you to do. So it's, again, right, it's a little bit about the personality that person comes in, where you have to be willing to be like, Yeah, this is a great time to be in, simply because the problems are so huge. And the territory is so wide open, that

2022-04-19 10:05

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