Speaker 1: Hey, Daniel, who do you think would win in a
Speaker 1: fight theoretical or an experimental physicist?
Speaker 2: That depends are we're talking arm wrestling or like integration
Speaker 2: competitions into what mathematical race?
Speaker 1: Then I think I would put my money into theoretical physicist.
Speaker 1: I mean, no offense.
Speaker 2: Maybe we have to do the experiment, or maybe you.
Speaker 1: Should keep this theoretical. I don't know if you want
Speaker 1: to pick a fight.
Speaker 2: Well, maybe the two sides of the field just compliment
Speaker 2: each other beautifully.
Speaker 1: Is that all it takes us? Some compliments and you
Speaker 1: guys are back as friends.
Speaker 2: Theories are cheap, right, They don't need money for experiments,
Speaker 2: They just need compliments in theory.
Speaker 1: In my experience, hi am Warham, a cartoonist and the
Speaker 1: creator of PhD comics.
Speaker 2: Hi, I'm Daniel. I'm a particle physicist and a professor
Speaker 2: at UC Irvine, And back in the day, I did
Speaker 2: want to be a theorist.
Speaker 1: Back in the day, How old were you.
Speaker 2: When I started grad school? I wasn't sure if I
Speaker 2: wanted to do experimental or theoretical physics, So I guess
Speaker 2: I was in my early twenties, which by now is
Speaker 2: pretty far back in the day.
Speaker 1: Did you actually get a choice, like they offer you
Speaker 1: an option of which way to go, or do you
Speaker 1: have to, like, I don't know, test into it.
Speaker 2: You definitely have to kind of try out and work
Speaker 2: with the theorists if you want to be a theorist.
Speaker 2: But you have all the options when you start grad school.
Speaker 2: You could end up being an experimental particle physicist or
Speaker 2: a theoretical cosmologist or whatever. All those paths are available.
Speaker 2: You just gotta like it enough and be good at it.
Speaker 1: So what happened? Why didn't you pick the theory?
Speaker 2: I discovered I just didn't like writing down equations as
Speaker 2: much as a theorist. They would sit there and like
Speaker 2: develop several different mathematical fonts to write their equations in,
Speaker 2: and I was like, wow, I'm just not loving this
Speaker 2: as much as they're loving this.
Speaker 1: It sounds like you were against the idea of it
Speaker 1: in theory.
Speaker 2: My experience was the experiments were more fun.
Speaker 1: But anyways, Welcome to our podcast Daniel and Jorge Explain
Speaker 1: the Universe, a production of iHeartRadio.
Speaker 2: In which we try to blur the line between theory
Speaker 2: and experiment. We want to talk about all the concepts
Speaker 2: in theoretical physics that try to explain what's going on
Speaker 2: in our world, but we also try to touch back
Speaker 2: on the ground and understand what experiments are telling us
Speaker 2: about the nature of reality, what is Nature actually saying
Speaker 2: to us as she spins the story of the universe,
Speaker 2: And then we try to explain all of it to you.
Speaker 1: That's right, because it is a pretty storied universe, full
Speaker 1: of amazing little details and facts and things to discover
Speaker 1: out there that we are still puzzling over and which
Speaker 1: require all kinds of scientists to figure out, theorists and experimentalists.
Speaker 2: And in the history of physics we have made progress
Speaker 2: in lots of different ways. Sometimes the theorists have come
Speaker 2: up with a clever idea, a suspicion about how the
Speaker 2: universe might work, with lots of cool directions for experimentalists.
Speaker 2: Go out and check this thing, Measure how light bends
Speaker 2: around the sun, see if you can find the Higgs boson.
Speaker 2: Those can be wonderful directions to help unravel the mysteries
Speaker 2: of the universe. But sometimes the experimentalists lead the way,
Speaker 2: turning on particle smashers and discovering gobs and gobs of
Speaker 2: new particles that nobody expected.
Speaker 1: I guess my question, Daniel is why can't you be both?
Speaker 1: Why can't you be a theoretical and an experimental physicist.
Speaker 2: I'm doing my best. Actually i'm doing my best. But
Speaker 2: the reality of academia these days is to get one
Speaker 2: of these jobs, you have to be the world's expert
Speaker 2: in some subfield. And that makes it really hard to
Speaker 2: sort of live between two fields because you have to
Speaker 2: be like the top person in that field that year.
Speaker 2: And so if the theorists aren't sure, if you're a
Speaker 2: theorist and the experimentalist aren't sure if you're an experimentalist,
Speaker 2: nobody's going to give you that job. So you've got
Speaker 2: to sort of get the job in one category and
Speaker 2: then inch your way over to the other one if
Speaker 2: you're interested. That's kind of cliquish, it's definitely very cliquiche. Absolutely,
Speaker 2: these fields form and then they protect themselves and it
Speaker 2: can be hard for new kinds of subfields to emerge,
Speaker 2: Like right now we have the emergence of physicists who
Speaker 2: are experts in machine learning, and people aren't sure is
Speaker 2: that theoretical is it experimental? Because you're running a bunch
Speaker 2: of calculations nobody's really sure. Everybody knows that it's valuable,
Speaker 2: but we aren't quite sure where to put them.
Speaker 1: That's because they're robots? Are they in disguise?
Speaker 2: We're all just biological robots?
Speaker 3: Man?
Speaker 1: Oh, there you go.
Speaker 2: Aren't you the expert in squishy robots?
Speaker 1: I am, Yeah, Well I used to be, at least
Speaker 1: a lifetime ago. We're a couple of lifetimes ago.
Speaker 2: Now back in the day. Is there such a thing
Speaker 2: as a theoretical roboticist?
Speaker 1: Uh? Yeah, there's a lot of theory in robotics as well.
Speaker 1: But no, as we I guess we're not as clique
Speaker 1: as you're just a roboticist. If you're into robots, you're
Speaker 1: just a roboticist.
Speaker 2: New York because you just build your own friends. You're like, hey, look,
Speaker 2: I don't need people's friends. I can build my own.
Speaker 1: Yeah. But as you said, I guess you need both
Speaker 1: kinds of endeavors or search. You need experimental research and
Speaker 1: you need theoretical research in order to figure out how
Speaker 1: things work in the universe. Because I guess you need
Speaker 1: to come up with a theory so that you can
Speaker 1: prove it with an experiment, and you need an experiment
Speaker 1: to prove the theories. Otherwise there's no science.
Speaker 2: That's sort of a theoretical way of thinking about it,
Speaker 2: that we come up with the theories and improve them
Speaker 2: with experiment. Remember that sometimes experiments don't just prove theories.
Speaker 2: They blow up theories and tell us that the universe
Speaker 2: is different from the way we understand it and operates
Speaker 2: in some other way we don't yet understand. Like the
Speaker 2: photoelectric effect was a demonstration that boy, we really don't
Speaker 2: understand at light and how it works, and it took
Speaker 2: a few years before the theorist came up with any sort
Speaker 2: of explanation for it.
Speaker 1: Yeah, but I guess experimenter's lunches kind of experiment blindly, right.
Speaker 1: You usually have some sort of theory at hand when
Speaker 1: you design your experiments, when you go out there and
Speaker 1: turn stuff on.
Speaker 2: It's a bit of a raging debate right now in
Speaker 2: experimental physics whether we should be focused on searching for
Speaker 2: the ideas that theoretical physicists are suggesting, or whether we
Speaker 2: should be developing strategies that are more just exploratory that
Speaker 2: leave us open to surprises. Like when you turn on
Speaker 2: the Hubble Space telescope and look out into space. Sure,
Speaker 2: you want to see the things that you had in
Speaker 2: mind to look at, but you're also open to like
Speaker 2: seeing aliens waving at you, or seeing new kinds of
Speaker 2: stuff you didn't even expect to see.
Speaker 1: But I guess also at the same time, we're getting
Speaker 1: to a spot where you know, things are so complex
Speaker 1: and so subtle and so hidden that you kind of
Speaker 1: need to know what you're looking for in a way, right,
Speaker 1: it's kind of hard to just like look for everything.
Speaker 2: It is really hard to look for everything you really
Speaker 2: put your finger on it, especially when your data is
Speaker 2: very statistical. If you do like a single experiment and
Speaker 2: you get some weird result, you might be able to say, hey, look,
Speaker 2: there's definitely something new here. But if the data are subtle,
Speaker 2: if the new things appear as like trends in your data,
Speaker 2: then you're right, it can be hard to know how
Speaker 2: to find them. So then you have to play some
Speaker 2: clever statistical arguments and say, well, you're the kinds of
Speaker 2: things that we could see, and here are the ways
Speaker 2: that we could search for them. So you have to
Speaker 2: do a little bit more work to define the kinds
Speaker 2: of things you might be able to see. Even if
Speaker 2: you aren't sure which specifically might pop up in your data.
Speaker 1: Well, sometimes there are cases where both the theories and
Speaker 1: the experimental lists are stumped. And that is the case
Speaker 1: where non physics. There's kind of a big hole in
Speaker 1: physics in terms of our knowledge of how things work
Speaker 1: in the universe.
Speaker 2: That's right, at the most fundamental level, we still don't
Speaker 2: really understand the basic rules of physics. We have two
Speaker 2: pillars of modern physics relativity that tells us about space,
Speaker 2: time and gravity, and quantum mechanics that tells us about
Speaker 2: particles and forces, and we just don't know how to
Speaker 2: bring them together. And it's important because it has to
Speaker 2: do with one of the most basic questions in physics,
Speaker 2: which is what is the universe made out of? What
Speaker 2: is the fundamental fabric of reality? After all?
Speaker 1: Yeah? And is it soft and comfortable? Is what I
Speaker 1: want to know.
Speaker 2: It seems to have a little bit of spandex in it.
Speaker 1: Here, you guys, long is a stretchy that can accommodate
Speaker 1: all sizes.
Speaker 2: Because my waste is not the size it was back
Speaker 2: in the day.
Speaker 1: You want the universe to kind of expand with you,
Speaker 1: your mind and your waste. But yeah, there's kind of
Speaker 1: a big hole in our understanding of the universe, and
Speaker 1: it has to do with gravity. We're not quite sure
Speaker 1: where gravity falls, whether it falls or it fits with
Speaker 1: quantum mechanic skill theory, or whether it works the way
Speaker 1: that Einstein envisioned in special relativity.
Speaker 2: Right, that's right. Einstein's special relativity tells us about light
Speaker 2: and how it propagates. His theory of general relativity tells
Speaker 2: us about space time and how it bends. And these
Speaker 2: two theories are in conflict and tell us very different
Speaker 2: stories about the nature of the universe. But so far
Speaker 2: we haven't been able to figure out a way to
Speaker 2: test them without building a solar system sized particle collider
Speaker 2: or peering inside a black hole. So experimental physicists have
Speaker 2: not really been able to contribute to this conversation until now.
Speaker 1: So the deal in the podcast, we'll be asking the question,
Speaker 1: can we test quantum gravity in a tabletop experiment? And
Speaker 1: right here the word tabletop I think of board games.
Speaker 1: Is this what we're talking about? Like a little cardboard
Speaker 1: unfolding thing with pieces, And then you test quantum gravity exactly.
Speaker 2: You can download the schematics from the internet and print
Speaker 2: out your own Nobel Prize winning experiment. Do you go?
Speaker 1: Is it called settlers of quarks or quantum ton.
Speaker 2: I'll leave you to do the branding of it. But
Speaker 2: when we say tabletop experiment in physics, we basically mean
Speaker 2: something not like the large Hadron collider or something that
Speaker 2: doesn't require a ten billion dollar facility staffed by thousands
Speaker 2: of people. We mean the kind of thing a single
Speaker 2: physicist could do in their laboratory, in the basement of
Speaker 2: your nearby university.
Speaker 1: I see you're talking about a million dollar tabletop, not
Speaker 1: a billion dollar tabletop.
Speaker 2: Exactly, just like everybody has a million dollar table in
Speaker 2: their kitchen. No, it's really like a single physicist experiment,
Speaker 2: something you could do in a reasonable physics lab, not
Speaker 2: something people are going to be doing on their kits table.
Speaker 1: Well, as usually, we were wondering how many people out
Speaker 1: there had thought about this question or perhaps have any
Speaker 1: ideas about how to do it.
Speaker 2: So thanks very much to everybody who participates in this
Speaker 2: segment of the podcast. If you've been listening for years
Speaker 2: and would like to hear your voice speculating about the
Speaker 2: topic of the day, please write to us two questions
Speaker 2: at Danielandjorge dot com everybody's welcome.
Speaker 1: So think about it for a second. Do you think
Speaker 1: we can test quantum gravity on somebody's table? Here's what
Speaker 1: people had to say.
Speaker 4: Well, since quantum gravity is, you know, with the gravity
Speaker 4: of the really small, I don't see why the experiments
Speaker 4: with it couldn't be done on a tabletop. I just
Speaker 4: have no idea what those experiments would even begin to
Speaker 4: look like.
Speaker 3: Though.
Speaker 2: If yes, then it will come to our table soon.
Speaker 1: But till then, I don't think it is possible at all.
Speaker 2: Uh, yeah, you probably could, but probably not today.
Speaker 3: I do not feel like we could test quantum gravity
Speaker 3: in a tabletop experiment because you.
Speaker 1: Need a lot of gravity for it to work, and
Speaker 1: I don't think the Earth has that kind of gravity.
Speaker 1: All right, not a lot of optimism, I like the
Speaker 1: person who said the tabletop, I don't think so. But
Speaker 1: maybe a desktop or on the floor, or on a shelf,
Speaker 1: maybe mountain top maybe, yeah, tabletop on a mountain. There
Speaker 1: you go, lower gravity.
Speaker 2: Well, we talked recently about how to measure big g
Speaker 2: and that experiment was definitely done on a mountain side
Speaker 2: swinging pendulums next to a big mountain in Scotland. So yeah,
Speaker 2: you can do funny gravity experiments on tops of mountains.
Speaker 1: And I like the person who said probably, but not today,
Speaker 1: Like is today a bad day for that? Were they
Speaker 1: busy that day? How about next week? Next week work?
Speaker 2: Please fill out this doodle pole for when we will
Speaker 2: win a Nobel prize.
Speaker 1: There you go. Yeah, I guess people didn't feel like
Speaker 1: it could work. But let's find out, Daniel step us
Speaker 1: through this. What is quantum gravity?
Speaker 2: So when we say quantum gravity, what we mean is
Speaker 2: a theory that explains both the quantum mechanical behavior super
Speaker 2: tiny particles, the way like electrons and photons do things
Speaker 2: that baseballs and basketballs and mountaintops don't do. You know,
Speaker 2: they don't move in smooth paths. They have weird quantized
Speaker 2: energy levels. They can be in a superposition of different states,
Speaker 2: like maybe they're here, maybe they're there. They can interact
Speaker 2: with each other and interfere in all sorts of complicated
Speaker 2: ways described by their wave function. And we want a
Speaker 2: theory that explains gravity as we know it. That things
Speaker 2: seem to move in these inertial pass through curved space time,
Speaker 2: and that mass in space tends to bend the path
Speaker 2: which affects the motion of other mass. So we have
Speaker 2: these two very different theories of the universe, and so
Speaker 2: far we can't bring them together. So quantum gravity would
Speaker 2: be a theory that explains both these things somehow harmoniously.
Speaker 2: But it's not a theory that we have today.
Speaker 1: Well, I guess maybe step us through a little bit
Speaker 1: of what we haven't been able to bring these two
Speaker 1: things together as far as I understand it. It's kind
Speaker 1: of due to two things, right, Like, one is that
Speaker 1: we haven't measure the gravitational force at the level of
Speaker 1: quantum particles, right, that's one thing. And also we don't
Speaker 1: know what happens to general relativity when you get down
Speaker 1: to that small level.
Speaker 2: Too exactly, I think you put your finger on it. Really,
Speaker 2: we don't know what the gravity is for little particles.
Speaker 2: The gravity for a baseball or for a moon. We
Speaker 2: think we understand and we've been able to test that.
Speaker 3: Right.
Speaker 2: We see moon's orbiting planets, we see planets orbiting suns.
Speaker 2: We see how gravity works. But that's all really really
Speaker 2: big stuff. What we don't know is what happens when
Speaker 2: you have gravity for particles, because particles are super duper tiny,
Speaker 2: which makes it really complicated. For two reasons. One is
Speaker 2: that they have almost no gravity. Remember that gravity is
Speaker 2: like the weakest force in the universe, and so the
Speaker 2: other forces overwhelm it. You try to do experiments with electrons,
Speaker 2: then their charge is much more powerful than their mass. Right,
Speaker 2: the electromagnetic force is much more powerful than the gravitational
Speaker 2: force on an electron. So it's basically possible to measure
Speaker 2: the gravitational force on an electron.
Speaker 1: Can I ask why that is?
Speaker 3: Though?
Speaker 1: Like, couldn't I shoot an electron from here to London
Speaker 1: and see if it curves with the curvature of the Earth.
Speaker 2: You could try that, absolutely, I think you probably shouldn't
Speaker 2: shoot beams across the surface of the Earth without getting
Speaker 2: signatures from everybody who might live in between. But say
Speaker 2: you did that, the electron would be effected by all
Speaker 2: sorts of charged particles between here and London, right, There'd
Speaker 2: be lots of other effects on the electron which would
Speaker 2: swamp out any gravitational effects.
Speaker 1: But I guess maybe, like from a satellite, I'm thinking,
Speaker 1: you know, I just shoot a whole bunch of them,
Speaker 1: and wouldn't the effects from other things kind of even
Speaker 1: out If you shoot a bunch of them out, Like,
Speaker 1: don't we have like quantum drives or like electron cannons.
Speaker 1: What happens if I just shoot them out there in space?
Speaker 1: Do they keep going straight or do they bend?
Speaker 2: Yeah, you could build an electron gun and put it
Speaker 2: in space and shoot them out, but still it would
Speaker 2: be dominated by the effects of other particles. Remember space
Speaker 2: is not totally empty. There's cosmic microwave background photons there,
Speaker 2: there's other charge particles from the Sun, and all of
Speaker 2: these would dominate the fate of that electron. Really, the
Speaker 2: problem is that the charge is more powerful than the mass.
Speaker 2: We talked about this once, and this is either because
Speaker 2: gravity itself is just weaker than the other forces for
Speaker 2: reasons we don't understand, or because electrons are just packed
Speaker 2: with a lot of charge compared to how much mass
Speaker 2: they have. You can think about it sort of either way.
Speaker 2: But that just means that the effect of gravity is
Speaker 2: tiny compared to the effect of electromagnetism. So to do
Speaker 2: that experiment, you'd need to isolate those particles from any
Speaker 2: sort of effect. And today we'll talk about an experiment
Speaker 2: that's going to try to do that, all.
Speaker 1: Right, So then that's where quantum gravity comes in. It's
Speaker 1: kind of a is it a theory or an idea
Speaker 1: that tries to bring these two big ideas together.
Speaker 2: It's not a theory. It's like a category of theories.
Speaker 2: It's like a dreamt of theory. What we want is
Speaker 2: a theory that bring these two things together. We don't
Speaker 2: have one. We don't know what the theory of quantum
Speaker 2: gravity is. You know, sometimes you have like ten different
Speaker 2: theories that describe the universe. The experiment has to go
Speaker 2: off and tell you which one is correct. Right now,
Speaker 2: we have zero. We have zero theories that explain quantum
Speaker 2: mechanics and gravity at the same time. So we sort
Speaker 2: of need an experimental result to be like, hey, this
Speaker 2: is the right direction though where hey, here's something to
Speaker 2: grab on to, here's a clue. But there's the second
Speaker 2: reason why these experiments are difficult that we didn't get
Speaker 2: to yet. One is just that gravity is so weak,
Speaker 2: and the other is that these particles do things that
Speaker 2: we don't know how to explain with gravity, Like particles
Speaker 2: don't have smooth paths. It's not like the electron is
Speaker 2: always somewhere, has some velocity. You know, you want to
Speaker 2: calculate the gravity of an electron, you have to know
Speaker 2: where it is, so you know how far away it is,
Speaker 2: you can calculate it's gravity. But electrons don't have specific
Speaker 2: locations that have probabilities, so we don't know. For example,
Speaker 2: if an electron, when it has probabilities to be in
Speaker 2: multiple places, does it mean it has like multiple different
Speaker 2: possible gravities. We just don't know how to do gravity
Speaker 2: for things that have uncertainties in their locations.
Speaker 1: You mean, we don't know how to do that if
Speaker 1: gravity was not a quantum force, right, Like you're sort
Speaker 1: of assuming that. I guess you want gravity to be
Speaker 1: like a quantum force like the other forces that we
Speaker 1: know about, right.
Speaker 2: Yeah, that's sort of one of the basic questions when
Speaker 2: you want to build the theory of quantum gravity, like
Speaker 2: is it a quantum force? If so, then two electrons
Speaker 2: interacting gravitationally wouldn't like collapse each other's wave functions. Some
Speaker 2: bits of one wave function would interact with some bits
Speaker 2: of another wave function, and they could do all sorts
Speaker 2: of weird quantum interactions. But if gravity is actually a
Speaker 2: classical force and not a quantum force, then it would
Speaker 2: collapse the wave function sort of like when you use
Speaker 2: a detector in a double slit experiment, it forces the
Speaker 2: particle to pick one of the options instead of the
Speaker 2: other one. So we just don't know, like is gravity classical,
Speaker 2: is it quantum mechanical. We just don't even know where
Speaker 2: to begin.
Speaker 1: And when you say classical, you mean like basically not
Speaker 1: quantum mechanical, like not fuzzy, not uncertain.
Speaker 2: Yeah, exactly, we mean not quantum is sort of an
Speaker 2: overused word. Some people say classical to me, not relativistic,
Speaker 2: like Newtonian, but today we mean not quantum mechanical. So
Speaker 2: we don't know if gravity, like really is just classical
Speaker 2: the way Einstein described it, thinking about space as smooth
Speaker 2: and continuous and everything having passed, or if it is
Speaker 2: a quantum effect, in which case it could either be
Speaker 2: a force like you suggested, mediated by weird gravitons, or
Speaker 2: maybe like space itself is quantum mechanical and uncertain. If
Speaker 2: gravity is the curvature of space, maybe space itself can
Speaker 2: be like maybe bent here and maybe bent there in
Speaker 2: some weird quantum mechanical way. There's so many possible directions
Speaker 2: for quantum gravity, nobody really knows which one is going
Speaker 2: to build a viable theory that even can do calculations.
Speaker 1: All right, well, let's get a little bit deeper into
Speaker 1: quantum gravity and whether or not we can test it
Speaker 1: and test it for under a billion dollars, because I
Speaker 1: guess the cheaper the better. We'll dig into that, but first,
Speaker 1: let's take a quick break. All right, we're talking about
Speaker 1: quantum gravity and whether or not that is a thing
Speaker 1: at all, whether it will bring together quantum mechanics in
Speaker 1: general relativity to give us one theory of the universe,
Speaker 1: and whether or not we can even design experiments to
Speaker 1: test such a theory.
Speaker 2: I like your threshold of a billion dollars.
Speaker 1: Yeah, well though these days with inflation, maybe that's more
Speaker 1: like ten billion.
Speaker 3: Though.
Speaker 2: You know, if we could spend a billion dollars and
Speaker 2: get the answer to quantum gravity, I'm pretty sure we
Speaker 2: would do it. The truth is, the experiments might cost
Speaker 2: a lot more than one billion dollars.
Speaker 1: All right, well, let's dig into the cost of these experiments.
Speaker 1: How can we test quantum gravity and figure out whether
Speaker 1: or not it's a real thing or not.
Speaker 2: Well, you had sort of the right idea, which is like,
Speaker 2: let's just zoom in on a quantum particle and look
Speaker 2: at its gravity somehow. But remember the scale of things
Speaker 2: we're talking about here, Like these particles are super duper tiny,
Speaker 2: and the effects we're talking about what happened on really
Speaker 2: really short distance scales. Like gravity gets more powerful when
Speaker 2: things get closer together. In order for gravity to be
Speaker 2: powerful enough for us to really test it, you need
Speaker 2: to get things together to like the Plank scale distances
Speaker 2: we're talking about, like ten to the minus thirty five meters.
Speaker 2: So until recently it seemed like, well, the only way
Speaker 2: to test quantum gravity is to have like a microscope
Speaker 2: that can see effects at the scale of ten to
Speaker 2: the minus thirty five meters, which felt almost impossible.
Speaker 1: Now, I guess, pain me a picture here of what
Speaker 1: it is that you would be trying to do. Like,
Speaker 1: for example, what if I just take a bunch of
Speaker 1: hydrogen atoms. Like a hydrogen atom is just an electron
Speaker 1: and a proton, so it's perfectly balanced in terms of charge.
Speaker 1: And I know that if I stick up bundle I'm
Speaker 1: in a container, they'll sort of tend to fall down
Speaker 1: because of gravity, right, They'll sort of accumulate the pressure
Speaker 1: of the hydrogen tank will be higher at the bottom
Speaker 1: than at the top. That means gravity is working on
Speaker 1: them and it is pulling them down. Can I build
Speaker 1: some sort of model or theory that kind of models
Speaker 1: or tells me how it's working at the quantum level.
Speaker 2: Well, there's the theoretical difficulty, and then there's the experimental difficulty.
Speaker 2: On the theoretical side. Like we've tried to build those theories,
Speaker 2: they just don't work. Gravity is complicated because everything is
Speaker 2: affected by it. It's not like electromagnetism where you can
Speaker 2: like shoot out photons and those photons themselves don't feel electromagnetism, right,
Speaker 2: Photons don't interact with other photons. Gravity interacts with everything
Speaker 2: with energy. So when you try to build a quantum
Speaker 2: theory of gravity, like including the exchange of gravitons, those
Speaker 2: gravitons amid other gravitons which feel those gravitons, and it
Speaker 2: gets very hairy, very quickly. We talked once about the
Speaker 2: strong nuclear force, which has a similar property that it's
Speaker 2: gluons amid other gluons which affect other gluons, and it's
Speaker 2: a nightmare to do any calculations. Gravity is even more
Speaker 2: complex than that, and that's sort of one of the
Speaker 2: reasons why it's been so difficult to build a theory.
Speaker 2: So anytime people build a theory of quantum gravity, it
Speaker 2: just sort of predicts nonsense. We just can't mathematically make
Speaker 2: it work. And then there's the experimental challenge. And what
Speaker 2: you're talking about is like trying to build a setup
Speaker 2: where you can see the gravitational effects on particles. But
Speaker 2: the experiment that you describe like a bunch of hydrogen,
Speaker 2: you know, those are classical effects. The fact that those
Speaker 2: hydrogen atoms are quantum particles is irrelevant to the fact
Speaker 2: that they have more pressure on the bottom of the
Speaker 2: tank than the top of the tank. Oh, I see.
Speaker 1: You're trying to kind of like see what happens to
Speaker 1: gravity at the quantum distance level. Right, that's kind of
Speaker 1: the problem, right, Like you might be able to design
Speaker 1: a hydrogen gun something that shoots hydrogen atoms and you
Speaker 1: can track how the gravity affects its path, maybe, but
Speaker 1: that doesn't necessarily tell you whether or not there's like
Speaker 1: uncertainty or whether the there's fuzziness at the you know,
Speaker 1: really small distance.
Speaker 2: Exactly in the same way that like every time you
Speaker 2: toss a baseball, and principle you're tossing quantum objects right
Speaker 2: at a baseball. That just a bunch of quantum objects, and
Speaker 2: definitely they're feeling gravity. We're not asking like, do electrons
Speaker 2: and protons feel gravity? We're pretty sure they do. We're
Speaker 2: asking is how does their quantum mechanicalness interact with their
Speaker 2: gravitational attraction, you know, when they're doing their weird quantum stuff.
Speaker 2: How does gravity play a role with that? You know,
Speaker 2: if you have a particle that like has a possibility
Speaker 2: to be here and they're simultaneously, what is its gravity?
Speaker 2: So you've got to get something to be showing as
Speaker 2: quantum effects, which means really small distances and revealing its
Speaker 2: gravitational interactions, which requires really really large masses, which is
Speaker 2: why some people are excited to see inside black holes,
Speaker 2: because that's where you have like, really really really big
Speaker 2: masses squeeze down to quantum distances, and so what's going
Speaker 2: on inside a black hole would really tell us about
Speaker 2: the nature of quantum gravity and therefore the deepest nature
Speaker 2: of space time itself. Of course, we can't see inside
Speaker 2: black holes, so those secrets are hidden from us.
Speaker 1: Yeah, you might want to let that one go. It
Speaker 1: seems like we're never going to find out what's inside
Speaker 1: of a black hole.
Speaker 2: There's even a theory called cosmic censorship that suggests will
Speaker 2: never be able to answer this because the answers are
Speaker 2: always going to be hidden behind some weird horizon. It's
Speaker 2: sort of a pessimistic approach.
Speaker 1: WHOA, I didn't know there was a ratings board for
Speaker 1: the universe.
Speaker 2: And there are even some theorists that suggests this whole
Speaker 2: enterprise is a waste of time. Like Freeman Dyson, the
Speaker 2: guy who thought of Dyson's fears. He likes to think
Speaker 2: that we live in a dualistic universe, that quantum mechanics
Speaker 2: and gravity just sort of like rule in different regimes
Speaker 2: and they never actually overlap at any place where they
Speaker 2: come into contact is hidden from us by these event horizons.
Speaker 1: Like maybe gravity is classical and it's not quantum. But
Speaker 1: you're saying, or he's saying that at the quantum level,
Speaker 1: there's things that are happening that you will never find out.
Speaker 2: Yeah, exactly that maybe you don't have a single theory
Speaker 2: at the universe. You like two theories and each have
Speaker 2: their own regime and they never overlap anywhere we could
Speaker 2: test them. But a lot of people don't really like
Speaker 2: that theory. I really don't like that theory because I
Speaker 2: want there to be one theory of the universe, one
Speaker 2: thing that explains everything, and I'd love to see these
Speaker 2: two different concepts battle it out. I want to force
Speaker 2: the universe to show us what the answer is.
Speaker 1: But I wonder couldn't they Couldn't he be right though, Like,
Speaker 1: couldn't it just be the gravity, you know, bend space
Speaker 1: and quantum fields and quantum particles exist in that bent space.
Speaker 2: Yeah, he could be right. But if we can come
Speaker 2: up with some experiments that force quantum mechanics and gravity
Speaker 2: to speak at the same moment, to say, like, all right,
Speaker 2: here's what happens when you have a particle that has
Speaker 2: two possibilities and it has some gravity, then we'll know.
Speaker 2: And maybe he's right. Maybe gravity really is classical. And
Speaker 2: what happens when particles interact gravitationally is that their wave
Speaker 2: functions collapse, because that's what happens when classical objects interact
Speaker 2: with quantum objects. But it sure would be nice to know.
Speaker 1: Yeah, gravity is pretty classic. So talk to us a
Speaker 1: little bit about how we've been trying to study this
Speaker 1: or get answers to this question.
Speaker 2: So, other than wishing we could see inside a black hole,
Speaker 2: the other typical tool in our toolkit is a particle collider.
Speaker 2: You build a big particle smasher, you pour a lot
Speaker 2: of energy into one tiny little spot. Then you can
Speaker 2: like break open bonds. You can see how the pieces interact.
Speaker 2: But in order to see gravitational effects, you would need
Speaker 2: so much energy. You'd basically need like the Plank energy.
Speaker 2: It would require building a collider that's like the size
Speaker 2: of the galaxy in order to get enough energy into it.
Speaker 2: Or some calculations suggest if you build a particle collider
Speaker 2: that big, it would collapse into a black hole.
Speaker 1: Wait, why do you need so much energy?
Speaker 2: Because gravity is really really weak, which means it operates
Speaker 2: on really small distance scales. In order to get to
Speaker 2: small distance scales, you need a lot of energy. It's
Speaker 2: sort of like the de Burgly wavelength, right, Like the
Speaker 2: wavelength of a particle is inversely proportional to its momentum,
Speaker 2: and so the more momentum an object has, the smaller
Speaker 2: its wavelength. And you want to see like really really
Speaker 2: short distance effects, you need really really high energy probes.
Speaker 2: So you need like super duper high energy particle collisions
Speaker 2: in order to see things happening at really short distance scales.
Speaker 1: Why because I guess the more energy two particles have
Speaker 1: when they smash into each other somehow, that gives you
Speaker 1: more resolution in space.
Speaker 2: Yeah, exactly, The more momentum the particle has, the smaller
Speaker 2: the wavelength of their wave function. You can think of
Speaker 2: the motion of every particle is described by a little
Speaker 2: wave function that determines what happens to it, the same
Speaker 2: way you can think of like light as a wave, right,
Speaker 2: it's wiggling around, And if you're using light to see things,
Speaker 2: you can only really see things that are the wavelength
Speaker 2: of that light or larger. Anything smaller than that wavelength
Speaker 2: the photon sort of can't interact with it. And so
Speaker 2: you want to see really really small effects, you need
Speaker 2: really really high energy photons or in our case, we
Speaker 2: need really really high energy particle beams to see very
Speaker 2: very short distance interactions.
Speaker 1: Right, Because I guess you need things with mass, right
Speaker 1: to test the quantum gravity or gravity at the quantum level.
Speaker 1: And so that's also true for things with mass, Like
Speaker 1: the faster they're going, the smaller they are, is.
Speaker 2: That what you're saying effectively, the smaller their wavelength. Is.
Speaker 2: Another way to think about it is that you need
Speaker 2: enough energy in those collisions to make gravity stronger, Like
Speaker 2: you want to overcome the electromagnetic force and the strong
Speaker 2: force and make gravity as powerful as those other forces
Speaker 2: so that you can see its effects. You need to
Speaker 2: pour a lot of energy into those collisions, because the
Speaker 2: power of gravity is linked to the mass and to
Speaker 2: the energy of these things. So you pour enough energy
Speaker 2: into one little location, you'll get a very strong gravitational interaction.
Speaker 2: So if we want to see the gravitational effects on
Speaker 2: quantum particles, you need to pour a lot of energy
Speaker 2: into one little spot.
Speaker 1: And is that the only way to do it through
Speaker 1: particle colliders? Isn't there some like I don't know, like
Speaker 1: aim your beams better approach or make smaller wavelength particles.
Speaker 1: I don't know, Like can we do this without making
Speaker 1: a black hole in our solar system?
Speaker 2: The short answer is no. I mean, we do our
Speaker 2: best with particle beam aiming already. But really the limitation
Speaker 2: is the energy of the particles, and we have them
Speaker 2: going as fast as we can, and the wavelength of
Speaker 2: the particle is determined by its energy. So really sort
Speaker 2: of at the limit there. We talked recently about other
Speaker 2: strategies for accelerating particles that might make it smaller, faster, cheaper.
Speaker 2: So there might be a breakthrough in accelerator technology which
Speaker 2: could leap us up like a factor of ten or
Speaker 2: one hundred. But we are like a factor of a
Speaker 2: trillion away from being able to test quantum gravity in
Speaker 2: particle colliders. So really, nowhere in the near future will
Speaker 2: particle colliders be able to answer this question?
Speaker 1: All right, well, I think part of what we're going
Speaker 1: to be talking about here today are experiments that have
Speaker 1: kind of ideas about how to test this without destroying
Speaker 1: the Solar system. And they involve diamonds and lasers and
Speaker 1: space lasers. No tabletop lasers in space. No, no tabletop
Speaker 1: lasers in Pasadena. Oh that take up space in Pasadena.
Speaker 2: A table near you?
Speaker 1: All right, Well, to dig into it, Daniel, What is
Speaker 1: the first of these experiments?
Speaker 2: So the first of the experiments involves diamonds, and the
Speaker 2: goal here essentially is to create a situation where a
Speaker 2: particle has the probability of being in two places at once,
Speaker 2: and then you test its gravity. You see, if it's
Speaker 2: gravity really is sort of like split between its two
Speaker 2: possible locations, or if when you probe it with gravity,
Speaker 2: it somehow collapses into just having one possible location. Because
Speaker 2: remember this, quantum particles can do this weird thing. They
Speaker 2: can be in a superposition like if there's two possibilities
Speaker 2: for an electron, it doesn't have to choose A or B.
Speaker 2: It can maintain both possibilities until something interacts with it classically,
Speaker 2: which forces it to choose. That's the weird thing about
Speaker 2: quantum mechanics and something we don't understand. So this is
Speaker 2: very hard, of course, because particles are very small and
Speaker 2: they're very delicate. But they've come up with a clever
Speaker 2: way that they think might be possible, and it involves
Speaker 2: electrons embedded in falling diamonds.
Speaker 1: Sounds like a rap video where there's like money and
Speaker 1: diamonds falling from the sky. Break it down for us.
Speaker 1: How does his experiment work?
Speaker 2: Is you take a very tiny diamond and has a
Speaker 2: nitrogen added inside of it, like embedded inside the diamond.
Speaker 2: And this has a cool property, which is that if
Speaker 2: you zap it with a laser, the electrons have a
Speaker 2: probability to absorb that photon, which case they flip their
Speaker 2: spin to be up, or to ignore that photon and
Speaker 2: flip their spin to be down. So you shoot a
Speaker 2: laser at this diamond, and now it's in a quantum
Speaker 2: superposition of two possibilities. Maybe the electron inside there on
Speaker 2: the nitrogen is spin up, and maybe it's spin down.
Speaker 2: So you have your particle now in a quantum superposition.
Speaker 2: But what you need is for it to be in
Speaker 2: a quantum superposition of two locations rather than two spins.
Speaker 2: So then you pass it to a little magnetic field.
Speaker 2: The magnetic field will push it left or right based
Speaker 2: on the spin. So you have this falling diamond which
Speaker 2: passes through a magnetic field and it either moves left
Speaker 2: or it moves right. Now if it's in a quantum superposition,
Speaker 2: and then it has both possibilities to move left and
Speaker 2: to move right. So now it's location depends on this quantumness.
Speaker 2: It's the same thing for another diamond nearby. Now you
Speaker 2: have this pair of falling diamonds, each of which has
Speaker 2: the possibility to be in two slightly different locations, and
Speaker 2: you see how they interact. Do the possibilities for one
Speaker 2: diamond interact with the possibilities for the other diamond, or
Speaker 2: do the two diamonds like collapse each other's wave functions.
Speaker 1: I see, So you embed a little nitrogen atom into
Speaker 1: the diamond Ezaly with a laser, and now the nirogen
Speaker 1: atom has quantumn certainty, which kind of extends to the
Speaker 1: whole diamond. Is basically what you're saying, right like if
Speaker 1: I don't know, there's quantum certainty about the electron into nitrogen,
Speaker 1: and that means there's quantum certainty about the whole diamond
Speaker 1: because it could be swinging right or left. And now
Speaker 1: if you put two of them together really close, they
Speaker 1: should interact with gravity. And so now you have the
Speaker 1: system where you have two quantum objects interacting with gravity exactly.
Speaker 2: And they have some really clever mathematical way to tell
Speaker 2: if the two diamonds interacted in a quantum way or
Speaker 2: if the two diamonds interacted in a classical way, Like
Speaker 2: if they interacted in a quanti when you measure the
Speaker 2: spins of those electrons after they fall far enough in
Speaker 2: your experiment, they'll have some cool correlation to them, and
Speaker 2: if they interact it in a classical way, then they'll
Speaker 2: be uncorrelated, like whether they're spin up or down will
Speaker 2: just be random, and so because of the weird rules
Speaker 2: of quantum mechanics, you can tell whether quantum mechanics has
Speaker 2: been at play in the gravitational interaction, Like did gravity
Speaker 2: cancel out the quantum mechanic effects because it's really just
Speaker 2: a classical force, or did it allow the quantum uncertainty
Speaker 2: to be maintained, meaning that gravity would be a quantum
Speaker 2: mechanical effect not a classical effect.
Speaker 1: Well, I guess quantum mechanics aside. Can you measure gravity
Speaker 1: the force of gravity by just dropping two diamonds together
Speaker 1: and seeing if they attract each other? Is that like
Speaker 1: a real thing you can do.
Speaker 2: It's a real thing you can try to do. That's very,
Speaker 2: very difficult because little diamonds have very very gentle gravity,
Speaker 2: and so this is not something we think we can
Speaker 2: do today. There's a group in the UK that thinks
Speaker 2: that they can figure out how to do this, and
Speaker 2: there's lots of complicated steps involved and they're hoping to
Speaker 2: maybe pull this off sometime in the next ten years.
Speaker 2: There's a lot of really complicated moving parts involvement getting
Speaker 2: the nitrogen inside the diamond, flipping its spin with a laser,
Speaker 2: beam getting two pairs of diamonds to fall simultaneously close
Speaker 2: enough each other that maybe they have a gravitational interaction.
Speaker 2: Now you don't actually have to see any sort of
Speaker 2: like gravitational pull. You're not measuring like how far did
Speaker 2: the diamond move because of gravity. You're just bringing them
Speaker 2: close enough together that you think gravity is at play.
Speaker 2: The gravity like wakes up and says, ooh, there's something
Speaker 2: going on here. You don't have to measure the gravity.
Speaker 2: You just have to see if gravity messes up the
Speaker 2: quantum state. I see.
Speaker 1: But to measure the quantum state at the end, wouldn't
Speaker 1: you be doing something like measuring that whether or not
Speaker 1: the two diamonds were attracted to each other gravitationally or not.
Speaker 2: No, all you need to do is measure the spins
Speaker 2: of those electrons embedded inside the nitrogen atoms in the diamonds.
Speaker 2: You don't have to see the gravitational effects directly. It's
Speaker 2: sort of like in the double slit experiment when you
Speaker 2: add a detector and that ruins the interface spearance. We're
Speaker 2: adding gravity to a quantum interaction and seeing if it
Speaker 2: ruins the interference or not.
Speaker 1: M But would that necessarily tell you anything about quantum
Speaker 1: gravity or gravitons.
Speaker 2: It wouldn't tell you that much, but it would be
Speaker 2: a powerful clue. It would tell you if gravity is
Speaker 2: classical or quantum mechanical. Like, if gravity is classical, it'll
Speaker 2: act like a detector and it'll collapse those wave functions
Speaker 2: and destroy this interference. If gravity is quantum mechanical, it won't,
Speaker 2: and everything quantum mechanical will stay quantum mechanical, and you
Speaker 2: get all sorts of weird interference. So that just tells
Speaker 2: you if gravity is classical or quantum mechanical. It doesn't
Speaker 2: tell you like, oh, space is quantized, or oh there
Speaker 2: are gravitons. It doesn't tell you which theory of quantum gravity,
Speaker 2: but it is a powerful clue. It would mean, for example,
Speaker 2: if we know gravity is quantum mechanical, the Freeman Dyson
Speaker 2: is wrong about classical gravity and quantum mechanics being able
Speaker 2: to play together.
Speaker 1: Yeah, he could be wrong, in which case he might
Speaker 1: need to stick to them making vacuum tees. All right, Well,
Speaker 1: that's one experiment, and I guess it's in progress. I
Speaker 1: guess they're designing it or making it. Where are they
Speaker 1: with that?
Speaker 2: This physicist at University College London who's leading a team
Speaker 2: of researchers who are trying to make this work. And
Speaker 2: there's folks in Santa Barbara as well, and they're trying
Speaker 2: to work on this. But you know, there's a lot
Speaker 2: of complicated steps and making this thing do its stance
Speaker 2: and being sure you know, what they're doing is a
Speaker 2: lot of pieces involved, lots of complicated experimental cleverness really
Speaker 2: required just to be able to do this test. So
Speaker 2: they're hoping sometime in the next ten years to be
Speaker 2: able to pull this off.
Speaker 1: All right, Well, let's get to the second of these
Speaker 1: potential experiments to measure quantum gravity. We'll dig into that,
Speaker 1: but first let's take another quick break. All right, we're
Speaker 1: talking about quantum gravity and whether or not it's a thing,
Speaker 1: whether gravity is quantum mechanical or is it pretty and
Speaker 1: classical and doesn't care about quantum mechanics and this weirdness
Speaker 1: of things being uncertain, And so we talked about one
Speaker 1: possible experiment that it might look at that using falling diamonds.
Speaker 1: And there's another interesting potential experiments happening also, right.
Speaker 2: That's right, And this one is being developed and built
Speaker 2: in your.
Speaker 1: Backyard, like literally my backyard.
Speaker 2: Look at your window.
Speaker 3: Man.
Speaker 2: You ever wonder what those people are told you?
Speaker 1: What?
Speaker 2: No, it's at cal Tech. Both the theorists and the
Speaker 2: experimental list are at Caltech, and it's a really cool idea.
Speaker 2: And what they're trying to do in this experiment's completely
Speaker 2: different from the other one is try to see if
Speaker 2: space itself is quantum mechanical. Like, if gravity is quant
Speaker 2: mechanical and there are gravitons, then that would mean that
Speaker 2: graviton should be like popping out of the vacuum all
Speaker 2: the time, the same way that other quantum particles are.
Speaker 2: Like if you go out in the middle of empty space,
Speaker 2: there's nothing there, there's still always a little bit of
Speaker 2: energy in the quantum fields, which means that like those
Speaker 2: fields can turn into particles briefly and then back into
Speaker 2: potential energy. So if space itself is quantum mechanical, if
Speaker 2: gravity is quantum mechanical, then gravitons should also be popping
Speaker 2: out of the vacuum. There should be like effectively tiny
Speaker 2: little ripples in space making quantum size gravitational waves.
Speaker 1: WHOA wait, I think you just confused me a little bit.
Speaker 1: So I thought there were two possibilities. Either gravity is
Speaker 1: quantum mechanical or space is quantized. Which one are you
Speaker 1: talking about here?
Speaker 2: Here, we're talking about gravity being quantum mechanical, that there
Speaker 2: exist gravitons which mediate the force of gravity, which in
Speaker 2: this theory would be a quantum force like the other
Speaker 2: forces in the universe.
Speaker 1: Okay, so we're not talking about quantizing space itself.
Speaker 2: That's right. We're not talking about quantizing space and like
Speaker 2: a space foam. But we're talking about space being filled
Speaker 2: with a quantum force of gravity, which would have fluctuations
Speaker 2: in it, right, And those fluctuations would be like quantum
Speaker 2: gravitons popping in and out of the quantum gravitational field.
Speaker 1: I see. So anything that is quantum or has a
Speaker 1: quantum field, by its nature, by its kind of statistical
Speaker 1: random nature, has these particles popping out of nothingness. But
Speaker 1: doesn't it need some sort of like energy to it.
Speaker 2: It does, But quantum fields always have energy to them.
Speaker 2: They can never relax down to zero because the uncertainty principle,
Speaker 2: the minimum energy level of a quantum field is always
Speaker 2: above zero energy, which is why there's always energy in
Speaker 2: quantum fields, which is why there's energy in all of
Speaker 2: space because of its quantum nature.
Speaker 1: Well, that's kind of an odd idea, Like what happens
Speaker 1: if a graviton appears out of nothingness, well.
Speaker 2: Mostly almost nothing, because gravitons would be super duper tiny,
Speaker 2: gravity is super duper weak, and so it would be
Speaker 2: basically impossible to see these things what have effects on
Speaker 2: super tiny distance scales we typically can't probe. But a
Speaker 2: theorist that Caltech, Catherine Zurich, came up with this idea
Speaker 2: that maybe gravitons can all work together. Instead of just
Speaker 2: looking for one graviton, maybe you can look for like
Speaker 2: larger effects of graviton sort of working together to make
Speaker 2: something else emerge from this quantum craziness. And she designed
Speaker 2: an experiment to maybe see that.
Speaker 1: Hmmm, interesting, Well, we actually have an interview of Daniel
Speaker 1: talking with professor Catherine Zurich from cal Tech about her
Speaker 1: idea for this experiment.
Speaker 2: That's right. Kathy and I have known each other since
Speaker 2: we were POSTOCX, and so I called her up and
Speaker 2: asked her about her crazy idea to not build a
Speaker 2: black hole in Pasadena.
Speaker 1: I feel like it's a little says, you have to
Speaker 1: throw that disclaimer in there, it's like, what are you
Speaker 1: guys doing? I am not destroying your town if that's
Speaker 1: what you're asking. First of all, let's get that clear.
Speaker 2: That didn't make you feel any better.
Speaker 1: Nobody asked I wasn't something I was concerned about before.
Speaker 3: All right.
Speaker 2: In that case, I'm also not testing any nuclear weapons
Speaker 2: in Pasadena.
Speaker 1: Oh good, thank you. What else are you not doing
Speaker 1: in Pasadena? Let's go down the list. All right. Well,
Speaker 1: here is Daniel's interview with Professor Catherine Zurich from cal Tech.
Speaker 2: All right, so it's my pleasure to welcome Professor Katherin Zurchod.
Speaker 2: Thank you very much for chatting with us.
Speaker 3: It's my pleasure to join you.
Speaker 2: So help me understand, first of all, how it's possible
Speaker 2: at all to see effects of quantum gravity. We understood
Speaker 2: for a long time that these things were just on
Speaker 2: the plank scale. How do they sort of work together
Speaker 2: to emerge to some signal we can see experimentally.
Speaker 3: So it's just like smoke. So if you ask yourself
Speaker 3: the question how to smoke spread? So there are interactions
Speaker 3: of molecules on very short distance scales, much shorter than
Speaker 3: what we can observe and yet you can see the
Speaker 3: effects of those short distance interactions simply by waiting a
Speaker 3: while for the effects of those short range interactions to
Speaker 3: accumulate over time. So that's a physical analogy for what
Speaker 3: we're interested in doing. So we have these quantum fluctuations
Speaker 3: on very short distance scales. So in this case it's
Speaker 3: the plank length, which is about to into the minus
Speaker 3: thirty five meters. And the idea is that if those
Speaker 3: quantum fluctuations accumulate over long times, then we can observe them.
Speaker 3: They're still very small, but we can observe them then
Speaker 3: with sufficiently precise instruments.
Speaker 2: So what makes quantum fluctuations add up to make a
Speaker 2: microscopic effect and what makes them not because sometimes they don't. Right,
Speaker 2: you have like a bunch of electrons in a baseball,
Speaker 2: they have all such fluctuations those average out to nothing.
Speaker 2: You can see what makes these guys add up over
Speaker 2: longer distance scales.
Speaker 3: So it's really the fact that you're losing information. Any
Speaker 3: measurement that you make is over a finite time. So
Speaker 3: you know, I turn on my instrument, let's say it's
Speaker 3: in an intraferometer, and the light goes out. It comes
Speaker 3: back and I make a measurement of it. And so
Speaker 3: what it does, what an instrument does, is it defines
Speaker 3: what we call a horizon. So there's a region of
Speaker 3: the space time that I measure and there's a region
Speaker 3: of the space time that I don't, and so that
Speaker 3: leads to a quantum mismeasurement effect that accumulates over time.
Speaker 3: So you're absolutely right that, you know, normal systems, where
Speaker 3: we can confine all of our information to a particular region,
Speaker 3: there's no information that's going to accumulate over time. But
Speaker 3: in this case, we can't actually confine quantum fluctuations. There's
Speaker 3: just part of the space time that we can't measure,
Speaker 3: and so what we're doing now is quantifying how much
Speaker 3: of that information is lost over the period of time
Speaker 3: that I make that measurement.
Speaker 2: Very cool, and so what kind of models of quantum
Speaker 2: space time is as sensitive to generally any kind of
Speaker 2: model where space time has quantum fluctuations or only specific
Speaker 2: sort of kinds of ideas.
Speaker 3: So what we're trying to show is that this effect
Speaker 3: occurs very generally across the space of theoretical ideas that
Speaker 3: people explore, you know, commonly, So what do I mean
Speaker 3: by that exactly, So we're still trying to understand a
Speaker 3: precisely what are the minimal sets of requirements that you need.
Speaker 3: At minimum, we need quantum fluctuations at the plank scale,
Speaker 3: so that has to be there, and those quantum fluctuations
Speaker 3: have to accumulate into the infrared. And there are various
Speaker 3: ways that we can see that. We can see it
Speaker 3: actually coming out in a quite broad range of theories
Speaker 3: where we can just write down some general properties of
Speaker 3: the theory and then crank through it and we see
Speaker 3: this effect come out. So we think it's pretty generic.
Speaker 2: Wonderful, And so why can't existing interchometers like LEGO, which
Speaker 2: is already very very precise, why can't it's signatures of
Speaker 2: this quantum fluctuation.
Speaker 3: So we actually think that LEGO is not very far
Speaker 3: from being able to see it. But one of the
Speaker 3: reasons why LEGO is not optimally sensitive to this signal
Speaker 3: is because they recycle their light by which I mean
Speaker 3: the light beam goes out and it comes back, and
Speaker 3: they don't make a measurement of it. After one round trick,
Speaker 3: it actually goes out and comes back many times before
Speaker 3: they make a measurement of it. And so for the
Speaker 3: signals that they're interested in, which come from you know,
Speaker 3: let's say black holes merging, that's fine because there's a
Speaker 3: classical source that generates a wave at some frequency. In
Speaker 3: this case, we're also interested in gravitational waves, but they're
Speaker 3: gravitational waves that come from the vacuum fluctuating, and they're uncorrelated.
Speaker 3: If I measured the system over time scales that are
Speaker 3: long in comparison to the light crossing time, So the
Speaker 3: fact that ligo weights and beam goes out and comes
Speaker 3: back many times before they measure it means that they're
Speaker 3: actually averaging down their signal, and so they have a
Speaker 3: reduced sensitivity to it in comparison to if they had
Speaker 3: this same that they could measure the same space time fluctuation.
Speaker 3: But they did it after the light just went out
Speaker 3: and came back. Then we claim that you can actually
Speaker 3: see this signal.
Speaker 2: Do these space time fluctuations look different from a gravitational
Speaker 2: way you could get from black hole collisions, for example.
Speaker 3: Yeah, they do. So one thing that's different about this
Speaker 3: signal in comparison to what you would get from let's
Speaker 3: say black hole mergers is in that case, the signal
Speaker 3: is the signal. It doesn't depend on my measuring apparatus.
Speaker 3: If I have a gravitational way of coming in at
Speaker 3: some frequency. It's like your radio station is broadcasting something
Speaker 3: and it has a frequency, and that's just you know,
Speaker 3: you tune it to some station, and that's what it is.
Speaker 3: In this case, what you measure actually depends on your apparatus,
Speaker 3: like your interferometer. So if I have a smaller apparatus,
Speaker 3: my signal is going to be coming in at a
Speaker 3: higher number radio station. Then if I have a bigger apparatus,
Speaker 3: then it comes in at a lower frequency station. The
Speaker 3: reason for that is because it's the quantum mismeasurement. And
Speaker 3: of course how much you're mismeasuring the space time depends
Speaker 3: on how big you know, the volume of space time.
Speaker 2: You're measuring affects your horizon.
Speaker 3: Yeah, it depends on the size of your horizon. That's
Speaker 3: another way of saying it. It depends on the size
Speaker 3: of your horizon, depends on how many quantum degrees of
Speaker 3: freedom are fluctuating inside your volume, which depends on how
Speaker 3: big your horizon is. And so as a result, you know,
Speaker 3: you would really know about this signal. First of all,
Speaker 3: it would have a very particular shape, but it would
Speaker 3: depend on your measuring apparatus, So you could compare between
Speaker 3: different instruments and then start to tell what the source
Speaker 3: of it would be.
Speaker 2: And can you also see things unexpected, like if there's
Speaker 2: a general enough detector that you might see things that
Speaker 2: aren't these quantum fluctuations, then aren't gravitational away some black
Speaker 2: holes but something else, you know, surprise?
Speaker 3: Yeah. Sure, So these instruments that were interested in building,
Speaker 3: they can be sensitive to anything that's generating gravitational waves
Speaker 3: in that same frequency range. So the signal definitely has
Speaker 3: to be predictive enough to be able to tell apart
Speaker 3: different sources. And our claim is that the signal has
Speaker 3: very particular you know, frequencies that it's peaked at. It
Speaker 3: has angular correlations, like it depends on the angle between
Speaker 3: the arms and your interferometer. So therefore you'll be able
Speaker 3: to tell what the source of these gravitational waves.
Speaker 2: Are and what's the sort of timeline like best case
Speaker 2: scenario when you guys can build this thing and discover
Speaker 2: quantum gravity.
Speaker 3: Yeah yeah, So we've got the first bit of funding
Speaker 3: to come in and my colleague Lemacullor who's spearheading this
Speaker 3: effort here at Caltech. You know, his lab is ramping
Speaker 3: up on this. There are some technological objectives that they
Speaker 3: have to demonstrate, and they have to do R and
Speaker 3: D because they're proposing a novel readout scheme for these interferometer.
Speaker 3: What we have proposed is to have a demonstrator apparatus
Speaker 3: that would kind of scrape the signal. Okay, we're talking
Speaker 3: about two sigma kind of sensitivity in five years, so
Speaker 3: I think to really start to see this, you know,
Speaker 3: like five sigma, you're just really confident you can start
Speaker 3: to test various aspects of it. I think we're probably
Speaker 3: talking the ten year timescale.
Speaker 2: So I've read your paper. There's a lot of nice
Speaker 2: theoretical maneuvers in there. My question to you is, do
Speaker 2: you believe this is going to be real? Like you
Speaker 2: turn this thing on in ten years? Nature tales you
Speaker 2: an answer. What's your confidence that this is out there
Speaker 2: that you're going to see it?
Speaker 3: Yeah, so it doesn't seem to be going away. Let's
Speaker 3: put it that way. When you see something in a calculation,
Speaker 3: you know, you try to test it by doing a
Speaker 3: different calculation that behaves differently. You know, it has different
Speaker 3: theoretical systematics, and the kinds of things that you could
Speaker 3: mess up in the calculation are different, so on and
Speaker 3: so forth. And then you also check for whether it's
Speaker 3: in conflict with anything that you know. And through the
Speaker 3: process of doing this, you know, based on my experience,
Speaker 3: when you try to build a theory, oftentimes it'll fail
Speaker 3: and then you try to fix it up by adding
Speaker 3: other things to it. This has not been like that.
Speaker 3: If it seems like it's going to fail for some reason,
Speaker 3: it means that you should just stop and wait and
Speaker 3: try to understand what's there better, because it fixes itself.
Speaker 3: So to me, that's an indication that there's something there.
Speaker 3: It hangs together in a very self consistent way, and
Speaker 3: so from that point of view, I find it theoretically
Speaker 3: very attractive, very interesting. It's right now, I don't want
Speaker 3: to tell nature what to do. Right. Nature gets to decide.
Speaker 3: You know, there are some things that go in right,
Speaker 3: there's this fundamental fluctuations and then space time you know,
Speaker 3: needs to remember right, so there needs to be the
Speaker 3: sense in which you're losing information. And if those two
Speaker 3: things are there in nature, and we certainly know lots
Speaker 3: of analogous physical systems where that happens, then we'll see it.
Speaker 3: But at the end of the day, nature decides. And
Speaker 3: that's one of the things I really like about this
Speaker 3: problem is I can write these things down on paper
Speaker 3: and they're beautiful, and I'm understanding more things about it
Speaker 3: from a mathematical perspective. But at the end of the day,
Speaker 3: nature gets to decide.
Speaker 2: All right, Well, we look forward to hearing nature's side
Speaker 2: of the story. Thanks very much for joining us today.
Speaker 1: All right, pretty interesting. I'm super impressed you can talk
Speaker 1: to a theorist. I thought you guys spoke different languages
Speaker 1: and didn't like each other.
Speaker 2: They mostly speak in Greek symbols exactly, but sometimes I
Speaker 2: can translate. These days, I'm trying to move a little
Speaker 2: bit in the direction of theoretical physics, so it's really
Speaker 2: fun for me to talk to these folks. But yeah,
Speaker 2: they think on a whole different plane of existence. But
Speaker 2: what's really cool are theorists who propose experiments, who develop
Speaker 2: new techniques and new ideas that allow experimentalists to maybe
Speaker 2: force the universe to reveal something about its nature. And
Speaker 2: the story of this one is similar to the story
Speaker 2: of a very similar experiment, which is LIGO, the innerferometer
Speaker 2: that looked for classical gravitational waves. That was originally just
Speaker 2: a theoretical idea, and experimentalists were like, all right, let's
Speaker 2: try to build it, see if we can find it,
Speaker 2: and they did. This is like the quantum version of it,
Speaker 2: which would look for little quantum ripples in space time,
Speaker 2: basically little quantum gravitational waves. And the experiment itself is similar.
Speaker 2: It's a little innerferometer. Like shoot laser beams back and forth,
Speaker 2: see how they overlap, and see if you can catch
Speaker 2: a graviton interfering with those laser beams.
Speaker 1: Hmmm, because the gravit times would be sort of like
Speaker 1: bedding space, is that the idea? Because gravity can't interact
Speaker 1: with footon or candy.
Speaker 2: Gravity doesn't interact with photons in a sort of Newtonian
Speaker 2: way because photons have no mass, but gravity does bend space,
Speaker 2: and photons move through that bend space, and so yeah,
Speaker 2: you're exactly right, Like a little gravitational quantum fluctuation the
Speaker 2: kind she's looking for, would affect the shape of space
Speaker 2: for one of these beams and would sort of knock
Speaker 2: a photon out of the path. And that's what they're
Speaker 2: looking for.
Speaker 1: The idea is that like a graviton would pop out
Speaker 1: of nowhere, it pops out, it bends space around it,
Speaker 1: and maybe it will deflect the photon. Is that the idea.
Speaker 2: That's the idea. But it's not one single graviton that
Speaker 2: would be totally invisible. It's this effect where a lot
Speaker 2: of gravitons are working together. And the super duper weird
Speaker 2: thing is that this effect only happens when you're making
Speaker 2: a measurement. It's a quantum effect. It comes from not
Speaker 2: being able to see the whole universe. So she's imagining
Speaker 2: space filled with all these gravitons, and when you make
Speaker 2: this measurement, it can only be affected by like a
Speaker 2: certain bubble of the universe, a bubble of the universe
Speaker 2: that's like close enough to you that light can travel
Speaker 2: to you. Because you create this information horizon, you limit
Speaker 2: like the wavelengths of these gravitons, and so only some
Speaker 2: of them can talk to your experiment, and that's what
Speaker 2: creates this weird effect. And I'll be totally honest, there's
Speaker 2: a lot of math there that I just don't even understand.
Speaker 2: But she's been trying to prove to herself that this
Speaker 2: works or that this doesn't work, and the math just
Speaker 2: keeps holding together no matter how she probes it. So,
Speaker 2: as you heard maybe in the interview, she really believes
Speaker 2: this is real.
Speaker 1: And so the idea is that you could maybe build
Speaker 1: this experiment on a tabletop like it could be, you know,
Speaker 1: a small experiment to prove a huge thing like quantum
Speaker 1: gravity exactly.
Speaker 2: Lego classical gravitational wave experiment is like kilometers long and
Speaker 2: cost billions of dollars. This would be like meters long.
Speaker 2: You literally could build it in a lab in the
Speaker 2: basement at Caltech, and if it works, they could see
Speaker 2: quantum gravitational effects on these beams of light and they
Speaker 2: could prove that gravitons are out there and that they're
Speaker 2: dancing together to make these little tiny ripples in space time.
Speaker 1: Cool. Well, she's welcome to hang out in my backyard
Speaker 1: and do the experiment here. That could be exciting.
Speaker 2: I don't think she wants her experiment it sprayed by
Speaker 2: the hose or like doused with water.
Speaker 1: Balloons, yeah, or have screaming kids running all around it
Speaker 1: that you usually it tends to make gravitons shy.
Speaker 2: I tend to dampen the effects of your experiment.
Speaker 1: All right, well, pretty exciting. Thank you to doctor Catherine
Speaker 1: Zurich for talking about her research. What does this all mean, Daniel,
Speaker 1: Are we far or near proving the idea of quantum gravity?
Speaker 2: I think we're still pretty far from figuring anything out.
Speaker 2: The theorists are working hard and making progress all the
Speaker 2: time about building their theories. But now it's exciting that
Speaker 2: we have experimental efforts which maybe in the next five, ten,
Speaker 2: fifteen years could provide us with really valuable clues to
Speaker 2: tell us, Oh, gravity is classical or nope, gravity is
Speaker 2: quantum mechanical. You better figure it out. That would be
Speaker 2: really powerful indication for sort of which direction to go theoretically.
Speaker 2: And I love this dance between experimental and the radical physics.
Speaker 2: You know, the ideas flourish and then experiments kill them,
Speaker 2: or sometimes experiments discover something weird which inspires lots of
Speaker 2: new theoretical ideas. It's really beautiful to see the interplay
Speaker 2: of these two different avenues of exploration. It's like a
Speaker 2: theoretical tango exactly. Even the physicists don't really know how
Speaker 2: to flirt. And I think the tango is pretty flirtatious.
Speaker 1: All right, Well, it sounds like the answer is stay tuned.
Speaker 1: In theory, it might be ten to fifteen years, but
Speaker 1: in reality, who knows. It could be that we may
Speaker 1: never answer this question, or it could be that we'll
Speaker 1: answer it within our lifetimes.
Speaker 2: That's right, we could be flirting with understanding or confusion.
Speaker 1: We hope you enjoyed that. Thanks for joining us, See
Speaker 1: you next time.
Speaker 2: Thanks for listening, and remember that Daniel and Jorge Explain
Speaker 2: the Universe is a production of iHeart Radio. For more
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