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Why is quantum gravity so hard? · Daniel and Kelly’s Extraordinary Universe

• 0:08
  Speaker 1: Hey, Daniel, how smart do you have to be to
• 0:10
  Speaker 1: be a physicist?
• 0:12
  Speaker 2: You know, it's not actually about being smart. It's more
• 0:15
  Speaker 2: about thinking that these kind of particular challenges are really fun.
• 0:19
  Speaker 1: So if you like having fun, you shouldn't be a physicist.
• 0:23
  Speaker 1: What do you mean?
• 0:24
  Speaker 2: I mean, you know, science is a very personal thing,
• 0:26
  Speaker 2: so some people might think doing integrals is really boring,
• 0:29
  Speaker 2: and somebody else might do them to relax.
• 0:31
  Speaker 1: Well, are you saying math can be relaxing.
• 0:35
  Speaker 2: It can be relaxing, and it can also be exciting.
• 0:38
  Speaker 1: You know.
• 0:38
  Speaker 2: Sometimes you're like bush whacking through the math and you
• 0:41
  Speaker 2: make amazing discoveries and you don't even have to risk
• 0:44
  Speaker 2: your life to jaguars.
• 0:47
  Speaker 1: Well you do have to worry about paper cuts, right.
• 0:52
  Speaker 2: Yeah, you know. I think there's a reason they didn't
• 0:53
  Speaker 2: make Indiana Jones a physicist.
• 0:55
  Speaker 1: Yeah. I don't think physicists could pull off the Indiana
• 0:58
  Speaker 1: Jones hat.
• 1:00
  Speaker 2: And we all have daddy issues.
• 1:16
  Speaker 1: Hi. I'm Jorge make, cartoonist and the author of Oliver's
• 1:19
  Speaker 1: Great Big Universe.
• 1:20
  Speaker 2: Hi, I'm Daniel. I'm a particle physicist and a professor
• 1:23
  Speaker 2: at UC Irvine, and I always wanted to have physics
• 1:27
  Speaker 2: adventures ooh.
• 1:29
  Speaker 1: Like real adventures, like in your couch, like, oh no,
• 1:31
  Speaker 1: I spilled my coffee.
• 1:35
  Speaker 2: I don't think you have to risk your life and
• 1:36
  Speaker 2: like get into a spaceship or even become an astronaut
• 1:39
  Speaker 2: to have physics adventures. You know, you can explore the
• 1:42
  Speaker 2: universe in your mind, make amazing discoveries, and feel like
• 1:46
  Speaker 2: you are connecting yourself to the universe.
• 1:49
  Speaker 1: I guess technically aren't like real adventures physical adventures. But
• 1:54
  Speaker 1: I guess maybe you don't want physical adventures, want physics adventures.
• 1:57
  Speaker 2: Yeah, exactly, physics versus.
• 1:59
  Speaker 1: Physical phone You don't want to make any physical exertions
• 2:03
  Speaker 1: or efforts, just like the mental kind.
• 2:07
  Speaker 2: Yeah, but you know, sometimes thinking really hard can make sweat.
• 2:12
  Speaker 2: I've definitely perspired while doing intergrals before.
• 2:15
  Speaker 1: M I see, have you ever wiped out doing integrals?
• 2:19
  Speaker 2: I've never injured myself doing math, that's for sure.
• 2:24
  Speaker 1: Well, I guess some people headaches. I guess that's sort
• 2:26
  Speaker 1: of cow injury.
• 2:28
  Speaker 2: Exactly. Migraines are a hazard of doing physics.
• 2:31
  Speaker 1: Yeah, no, I find math very relaxing, actually puts me
• 2:34
  Speaker 1: right to sleep. In fact, you should make an a
• 2:39
  Speaker 1: f for like a sleep relaxation app. And now we're
• 2:45
  Speaker 1: going to do integrals people do like the all time.
• 2:50
  Speaker 1: There you go, that's very ASMR and you will be
• 2:55
  Speaker 1: subliminally learning math.
• 2:57
  Speaker 2: The billion dollar idea right here.
• 3:00
  Speaker 1: Yeah, I can finally quit this podcast job. But anyways,
• 3:06
  Speaker 1: welcome to our podcast Daniel and Jorge Explain the Universe,
• 3:09
  Speaker 1: a production of iHeartRadio.
• 3:11
  Speaker 2: In which we do our best to make the complexities
• 3:14
  Speaker 2: and the challenges of physics accessible to you and to everybody,
• 3:19
  Speaker 2: because we think that the whole point of physics is
• 3:22
  Speaker 2: to understand the universe, and not just by a select
• 3:25
  Speaker 2: few group of people who can understand nineteen dimensional string
• 3:27
  Speaker 2: theory integrals, but by everybody. Because the end science is
• 3:32
  Speaker 2: a bunch of stories, mathematical or intuitive or in English,
• 3:35
  Speaker 2: and we want to tell those stories to you.
• 3:38
  Speaker 1: That's right. We take you through the adventures of science,
• 3:41
  Speaker 1: the wipeouts, the close calls, and the headaches of trying
• 3:44
  Speaker 1: to find out how the universe works and what are
• 3:47
  Speaker 1: place in it is.
• 3:49
  Speaker 2: Sometimes these integrals are more annoying than mosquito bites, though
• 3:52
  Speaker 2: never as dangerous as jaguars.
• 3:54
  Speaker 1: I don't think anything is this annoying is mosquitobiites. Jaguars
• 4:00
  Speaker 1: are pretty pesky too. Have you ever seen a jaguar
• 4:02
  Speaker 1: in the wild in the wild? Uh no, no, thankfully not.
• 4:07
  Speaker 1: Or do you mean like a car the car? I
• 4:11
  Speaker 1: think plenty of jaguars around here in South Pasady.
• 4:14
  Speaker 2: That's true, and those drivers can be pretty dangerous.
• 4:16
  Speaker 1: Yeah, yeah, and annoying like mosquitos.
• 4:19
  Speaker 2: You just want to swat all the jaguar drivers.
• 4:22
  Speaker 1: No, no, that's a you can get arrested for that
• 4:26
  Speaker 1: kind of thing. Yeah, exactly, Well, let me insult them
• 4:29
  Speaker 1: on a podcast.
• 4:30
  Speaker 2: When you make a billion dollars on your sleep math
• 4:33
  Speaker 2: app that you can buy yourself a jaguar and you
• 4:35
  Speaker 2: can be the same one.
• 4:36
  Speaker 1: I can buy several jaguars and a mosquito repellent.
• 4:43
  Speaker 2: Well, I was just wondering, because I know you grew
• 4:45
  Speaker 2: up in Panama, if you'd ever seen a jaguar or
• 4:47
  Speaker 2: maybe just jaguar sized mosquitoes.
• 4:50
  Speaker 1: Oh, I see, I see. That's your your perception of
• 4:52
  Speaker 1: how I grew up. I grew up in the hut,
• 4:54
  Speaker 1: in the jungle, barefoot, with my little pet jaguar next
• 4:58
  Speaker 1: to me. Exactly. I think people will live.
• 5:01
  Speaker 2: No, I'm asking paint us the picture.
• 5:05
  Speaker 1: I've seen a lot of coyotes around here in the wild. Yes, yeah,
• 5:09
  Speaker 1: those are pretty dangerous too. I guess if you're the
• 5:11
  Speaker 1: if you're small, you're a small baby or something.
• 5:14
  Speaker 2: In our neighborhood, everybody with a small dog puts these
• 5:17
  Speaker 2: spiky vests on them so the coyotes can't just snatch
• 5:20
  Speaker 2: them up and have a snack.
• 5:22
  Speaker 1: Wait, what you turn your dog into a weapon? What
• 5:27
  Speaker 1: do you mean is like spy? How dangerous are these spikes?
• 5:30
  Speaker 2: They're pretty big because there's a whole epidemic of coyotes
• 5:32
  Speaker 2: jumping into people's backyards and like grabbing these little dogs.
• 5:36
  Speaker 1: Oh wow, maybe just not keep your dog out at night.
• 5:39
  Speaker 1: Don't turn it into dangerous weapon. Like what if the
• 5:42
  Speaker 1: dog runs at a little kid or something wearing this
• 5:45
  Speaker 1: killer jacket.
• 5:47
  Speaker 2: Yeah, these coyotes are pretty brazen. It's not just that night.
• 5:50
  Speaker 2: During the full daylight, they will hop in people's backyards
• 5:53
  Speaker 2: and make off with their little shit.
• 5:54
  Speaker 1: Sues M. Sounds like me. You just just make the
• 5:57
  Speaker 1: coyotes your pets, and then that's also the problem, doesn't it. Well, last,
• 6:01
  Speaker 1: then you get a problem of jaguars coming in here
• 6:03
  Speaker 1: and eating your coyotes.
• 6:06
  Speaker 2: And somehow we have to connect all of this back
• 6:08
  Speaker 2: to physics.
• 6:10
  Speaker 1: Yeah, no, there's a physics zero of sizes right right, yes, sizes. Yes.
• 6:16
  Speaker 2: Some problems are hard, like how to protect your little
• 6:19
  Speaker 2: pet from neighborhood coyotes, and other problems are hard, like
• 6:23
  Speaker 2: how do you figure out the mathematics of the universe?
• 6:26
  Speaker 1: It's all connected, yeah, because I guess. Figuring out the
• 6:28
  Speaker 1: math of the universe has been one of the goals
• 6:30
  Speaker 1: of physics, to understand what's underlying everything that we see
• 6:34
  Speaker 1: around us, and all of the mechanics and the motion
• 6:37
  Speaker 1: and the energy that is swirling around us, what is
• 6:40
  Speaker 1: at the core of the universe.
• 6:42
  Speaker 2: Physics has two great theories, two incredible ideas that have
• 6:47
  Speaker 2: been very very successful, quantum mechanics and general relativity. But
• 6:51
  Speaker 2: bringing them together into an idea of quantum gravity has
• 6:55
  Speaker 2: been a challenge that has stood for over a century.
• 6:58
  Speaker 2: The greatest minds in physics have tried to take a
• 7:01
  Speaker 2: bite out of it, but it seems to be protected
• 7:03
  Speaker 2: by a spiky vest that's right.
• 7:05
  Speaker 1: It's been one of the hardest problems to solve in
• 7:07
  Speaker 1: physics for the last one hundred years, which makes you
• 7:11
  Speaker 1: wonder why is it so hard? What's taking so long
• 7:15
  Speaker 1: to solve this fundamental problem in physics.
• 7:18
  Speaker 2: It's because us physicists haven't just gotten off our couch
• 7:21
  Speaker 2: and bush whacked our way into the mathematical jungle.
• 7:23
  Speaker 1: Yeah. Yeah, I think that's the problem, Daniel. You need
• 7:26
  Speaker 1: to get off your couch an experiment with real gravity,
• 7:29
  Speaker 1: not just like imaginary gravity.
• 7:32
  Speaker 2: I'm waiting for my Indiana Jones hat to come. You
• 7:34
  Speaker 2: can't do it without the right cand of hat. You
• 7:36
  Speaker 2: have to be prepared.
• 7:37
  Speaker 1: Oh that's right, Yeah, that's right. You don't want to
• 7:39
  Speaker 1: get sunburned. It's not like there are other ways to
• 7:43
  Speaker 1: protect yourselves from the UV rays.
• 7:46
  Speaker 2: I mean, I want to understand the universe, but I'm
• 7:47
  Speaker 2: not willing to make physical sacrifices.
• 7:50
  Speaker 1: Do you also need a whip? Also?
• 7:53
  Speaker 2: I want to whip those integrals into shape for sure.
• 7:56
  Speaker 1: Yeah, there you go, crack them into shape. But anyway,
• 7:59
  Speaker 1: to be on the podcast, we'll be tackling the question
• 8:06
  Speaker 1: why is quantum gravity so hard?
• 8:09
  Speaker 3: Now?
• 8:09
  Speaker 1: Is this like hard like difficult or hard like tough?
• 8:13
  Speaker 2: If your floors are made of quantum gravity and you
• 8:15
  Speaker 2: drop your glass, man, is it gonna shatter? Yeah?
• 8:18
  Speaker 1: But how is it going to fall without gravity? Quantum mechanics,
• 8:23
  Speaker 1: I answered everything, it's gonna fall and not fall. No.
• 8:28
  Speaker 2: I love that physics has a name for this theory
• 8:31
  Speaker 2: quantum gravity, but it's just kind of like a placeholder.
• 8:34
  Speaker 2: We don't know what it is, or how it works,
• 8:35
  Speaker 2: or what the mathematics of it are. People argue about
• 8:38
  Speaker 2: different approaches. We already have a name for it, but
• 8:40
  Speaker 2: we haven't even figured it out yet.
• 8:42
  Speaker 1: Sounds unbrand for how physicists name thinks. Well, anyways, we're wondering,
• 8:46
  Speaker 1: as usual, how many people out there had thought about
• 8:49
  Speaker 1: this question of why quantum gravity is so hard? As usual,
• 8:53
  Speaker 1: Daniel went out there and God answers from real people.
• 8:56
  Speaker 2: Thank you to all the real people and definitely not
• 8:59
  Speaker 2: made up had GPT inspired bots who answered this question.
• 9:03
  Speaker 2: If you are a real person and you'd like to
• 9:05
  Speaker 2: answer future questions, please write to me two questions at
• 9:08
  Speaker 2: Danielandjorge dot com.
• 9:10
  Speaker 1: So think about it for a second. Why do you
• 9:12
  Speaker 1: think quantum gravity is so hard to solve? Here's what
• 9:16
  Speaker 1: people had to say.
• 9:18
  Speaker 3: I know that.
• 9:20
  Speaker 4: Quantic mechanic is very good explaining things that happen in
• 9:25
  Speaker 4: very smallst scale, and gravity is not very strong and
• 9:31
  Speaker 4: small as scale.
• 9:34
  Speaker 1: I don't know.
• 9:35
  Speaker 4: I think that's the reason, but I don't really I
• 9:38
  Speaker 4: don't really know.
• 9:40
  Speaker 1: I honestly don't have an intelligent answer for that. But
• 9:43
  Speaker 1: I will say this, if Albert Einstein was afraid of it,
• 9:48
  Speaker 1: that I am too.
• 9:49
  Speaker 4: It's hard because we are trying to apply our comparatively
• 9:54
  Speaker 4: super massive vantage point to these tiny particles in the
• 10:00
  Speaker 4: quantum realm that probably don't even know the difference.
• 10:05
  Speaker 2: They probably don't even know that gravity is a force.
• 10:09
  Speaker 2: Did you mean real people as opposed to chat GPT
• 10:12
  Speaker 2: or real people as opposed to physicists. I'm just now
• 10:15
  Speaker 2: realizing that was a jab.
• 10:17
  Speaker 1: I mean, like not paid actors. Okay, although maybe you
• 10:23
  Speaker 1: should include a chatgypt answer every every time we do this. Interesting. Yeah,
• 10:29
  Speaker 1: that's right now. What does chat GPT say about quantum
• 10:32
  Speaker 1: gravity being so hard?
• 10:34
  Speaker 2: Chatchipt says quantum gravity is consider challenging because it involves
• 10:38
  Speaker 2: the attempt to reconcile two fundamental theories of physics, quantum
• 10:41
  Speaker 2: mechanics and general relativity. Each of these has been incredibly
• 10:44
  Speaker 2: successful in its own but it becomes problematic when combined.
• 10:48
  Speaker 1: Whoa it just did the podcast for us? Done? AI
• 10:53
  Speaker 1: did replace? In our job? Does a chat GPT have
• 10:58
  Speaker 1: like a like a voice out. Can you have it
• 11:00
  Speaker 1: read the answer?
• 11:01
  Speaker 2: I think the free version that I have access to
• 11:03
  Speaker 2: you can't do images or voices. So no, you cannot
• 11:06
  Speaker 2: be replaced by chat jipt just yet.
• 11:08
  Speaker 1: Oh right, you still need us to read the answer
• 11:12
  Speaker 1: chat GPT gives out. I see.
• 11:15
  Speaker 2: Also, I would never rely on chat CHEPT. I asked
• 11:17
  Speaker 2: some hard physics questions sometimes and it just makes up balogney. Oh.
• 11:21
  Speaker 1: I think the news here is that you're asking chat
• 11:24
  Speaker 1: GPT for answers.
• 11:25
  Speaker 2: Yeah, I like everybody else, I was curious when it
• 11:27
  Speaker 2: came out, what you like to talk to the average
• 11:29
  Speaker 2: of the internet wisdom and follies, And the answer is
• 11:33
  Speaker 2: it's not very reliable.
• 11:35
  Speaker 1: Well, technically, neither are we, Daniel. I don't think we're
• 11:39
  Speaker 1: gonna have a hard answer here today.
• 11:42
  Speaker 2: No, but we're not going to make stuff up.
• 11:43
  Speaker 1: Well, So let's dig into this question. Why is quantum
• 11:46
  Speaker 1: gravity so hard? Why is it so difficult? Why has
• 11:49
  Speaker 1: it puzzled physicists for over one hundred years? So let's
• 11:53
  Speaker 1: start with the basics, Daniel, what is quantum gravity?
• 11:56
  Speaker 2: So chatchapt got this bit right. Quant gravity is an
• 12:00
  Speaker 2: attempt to bring together are two great theories of physics.
• 12:04
  Speaker 2: Quantum mechanics that describes things like electromagnetism and how particles
• 12:09
  Speaker 2: work and gives us a probabilistic picture of the universe,
• 12:14
  Speaker 2: and general relativity that described space and time and gravity
• 12:18
  Speaker 2: and explains things like the expansion of the universe and
• 12:21
  Speaker 2: how things move through space and time. And both of
• 12:25
  Speaker 2: these work really really well in their own regime. Quantum
• 12:28
  Speaker 2: mechanics for the small stuff, general relativity for the big stuff.
• 12:32
  Speaker 2: Quantum gravity is an attempt to have a single consistent
• 12:34
  Speaker 2: theory that works for all the stuff.
• 12:37
  Speaker 1: And these were developed independently, sort of right like, while
• 12:41
  Speaker 1: Einstein was coming up with general relativity, other people were
• 12:44
  Speaker 1: thinking about things at the smallest level and why they
• 12:47
  Speaker 1: were it quantized right exactly.
• 12:51
  Speaker 2: They were developed independently, though, around the same time, and
• 12:54
  Speaker 2: both were actually sparked by Einstein. Quantum mechanics really got
• 12:58
  Speaker 2: its kickoff from Einstein's realism that the photoelectric effect, what
• 13:02
  Speaker 2: happens when you shine a bright light at a piece
• 13:04
  Speaker 2: of metal, can only be explained by the fact that
• 13:07
  Speaker 2: photons were little packets. They were quantized. They weren't just
• 13:10
  Speaker 2: continuous beams of energy because what you saw was as
• 13:14
  Speaker 2: you turned up the energy of that beam of light,
• 13:16
  Speaker 2: you didn't get electrons with more energy boiling off. You
• 13:20
  Speaker 2: got more electrons because each one just gets one servant,
• 13:24
  Speaker 2: one photon. That was explained by saying the beam of
• 13:27
  Speaker 2: light had more photons in it, not just a brighter beam.
• 13:30
  Speaker 2: So Einstein kicked off quantum mechanics around the turn of
• 13:33
  Speaker 2: the century, and at the same time he developed his
• 13:36
  Speaker 2: theory of special relativity and general relativity that explained the
• 13:40
  Speaker 2: apparent force of gravity. And these two have been in
• 13:43
  Speaker 2: parallel development over the last hundred years, but nobody's been
• 13:46
  Speaker 2: able to bring them together into one picture of how
• 13:49
  Speaker 2: the universe works.
• 13:51
  Speaker 1: I guess maybe the question is why do you want
• 13:53
  Speaker 1: to bring them together? Like, if you have one that
• 13:55
  Speaker 1: works really well for some things and the other one
• 13:58
  Speaker 1: works well for other things, what's the need to unify them?
• 14:01
  Speaker 2: Yeah, it's a fair question.
• 14:02
  Speaker 1: You know.
• 14:03
  Speaker 2: Sometimes in life we have things that are separate, Like
• 14:05
  Speaker 2: you got one group of friends and another group of friends,
• 14:07
  Speaker 2: you bring them together, it's awkward. You don't do it again, right, Yeah,
• 14:11
  Speaker 2: they're usually a bad idea, And I guess in this
• 14:15
  Speaker 2: case there are two answers. One is philosophical and the
• 14:17
  Speaker 2: other really is experimental. Philosophically, we just think that the
• 14:21
  Speaker 2: universe probably does have a single set of laws. You know,
• 14:24
  Speaker 2: there should be one explanation for why something happens. You know,
• 14:28
  Speaker 2: the same way like when a computer program runs, it's
• 14:31
  Speaker 2: running with one source code. It's not like there's two
• 14:33
  Speaker 2: codes there battling it out. There should be one explanation.
• 14:37
  Speaker 2: And this is just sort of like a philosophical preference.
• 14:39
  Speaker 2: It would be nice if the universe had a single,
• 14:43
  Speaker 2: unified theory. It would sort of make sense to our brains.
• 14:46
  Speaker 2: That doesn't mean it has to happen. It's just sort
• 14:48
  Speaker 2: of like a philosophical preference.
• 14:51
  Speaker 1: Well, maybe explain to folks how they're separate. So, for example,
• 14:55
  Speaker 1: quantum mechanics works to describe what exactly, like the motion
• 15:00
  Speaker 1: or of little tiny particles or their interactions, or what
• 15:04
  Speaker 1: exactly does quantum mechanics do.
• 15:07
  Speaker 2: Quantum mechanics describes everything about tiny little particles, their motion,
• 15:11
  Speaker 2: their interactions, what's going to happen, what's not going to happen.
• 15:15
  Speaker 2: If you have, for example, two electrons and they're interacting
• 15:18
  Speaker 2: with each other, quantum mechanics tells you what's going to happen.
• 15:21
  Speaker 2: You make two electron beams, you shoot them at each other.
• 15:24
  Speaker 2: Quantum mechanics tells you the probabilities of what will come
• 15:27
  Speaker 2: out of those collisions, or if you replace an electron
• 15:30
  Speaker 2: with a muon, or you put in a quark or
• 15:32
  Speaker 2: a proton or whatever. Quantum mechanics is the rules of
• 15:35
  Speaker 2: all of those interactions, and the standard model of particle
• 15:38
  Speaker 2: physics what we talk about on this podcast all the
• 15:41
  Speaker 2: time that has been super successful in explaining the structure
• 15:45
  Speaker 2: of matter deep down inside the atom and why everything's
• 15:48
  Speaker 2: bound together and how that all works. That's all quantum mechanics.
• 15:51
  Speaker 2: It's all fundamentally quantum mechanics. Every little bit of it
• 15:55
  Speaker 2: is quantum mechanical, and it's quantum mechanical because it paints
• 15:58
  Speaker 2: this picture of how the universe works that's very different
• 16:01
  Speaker 2: from the way that our universe seems to work, the
• 16:04
  Speaker 2: one on our level you know about baseballs and planets
• 16:07
  Speaker 2: and basketballs, where things have like smooth paths. It tells
• 16:10
  Speaker 2: us that fundamentally the universe follows very different rules. That
• 16:13
  Speaker 2: quantum objects, tiny little bits, only have probabilities to go places.
• 16:18
  Speaker 2: They don't have smooth.
• 16:19
  Speaker 1: Paths, right, thinks are kind of fuzzy down at the
• 16:22
  Speaker 1: microscopic level. What does general relativity do exactly?
• 16:25
  Speaker 2: So general relativity explains space and time and gravity, So
• 16:30
  Speaker 2: it says that what Newton described as a force of
• 16:33
  Speaker 2: gravity is actually just objects moving through curved space. Newton
• 16:37
  Speaker 2: imagined space was absolute as this backdrop of the universe,
• 16:41
  Speaker 2: and then things with mass had a force between them.
• 16:44
  Speaker 1: You know.
• 16:44
  Speaker 2: He famously explained that apple dropping and also the Moon
• 16:47
  Speaker 2: orbiting the Earth unified in his law of gravity as
• 16:51
  Speaker 2: an attraction between mass. But Einstein tells us that that's
• 16:54
  Speaker 2: not the case. General relativity tells us that actually things
• 16:57
  Speaker 2: are just moving through the curvature of space. Space itself
• 17:00
  Speaker 2: is curved when mass is nearby, and that changes the
• 17:04
  Speaker 2: natural inertial path of objects. Objects will move in what
• 17:08
  Speaker 2: looks like curves even without accelerating.
• 17:11
  Speaker 1: And space gets spent by gravity, right or gravity is
• 17:14
  Speaker 1: the bending of space.
• 17:15
  Speaker 2: Right, base gets spent by energy in a very complex way. Essentially,
• 17:20
  Speaker 2: mass is a kind of energy, so it helps bend space,
• 17:22
  Speaker 2: but it's not the only way you can bend space.
• 17:25
  Speaker 2: And gravity is sort of a fuzzy term. Now, it's like,
• 17:27
  Speaker 2: are you referring to the Newtonian force, which isn't really
• 17:30
  Speaker 2: part of our picture anymore, or you're talking about the
• 17:32
  Speaker 2: whole theory of general relativity as an explanation for it.
• 17:35
  Speaker 2: But you know, what we describe as gravity things seeming
• 17:38
  Speaker 2: to fall down, is explained by Einstein, is things just
• 17:42
  Speaker 2: following the curvature of space.
• 17:44
  Speaker 1: Which which gets a curved because of the presence of energy. Right,
• 17:48
  Speaker 1: that's the basic and that effect. Basically you can sort
• 17:51
  Speaker 1: of lump it into the idea of gravity.
• 17:54
  Speaker 2: Yeah, exactly. And we had a whole podcast digging deep
• 17:57
  Speaker 2: into like why gravity isn't the force and how if
• 18:00
  Speaker 2: you're moving along with the curture of space time you
• 18:02
  Speaker 2: don't feel any acceleration even if other people see you,
• 18:05
  Speaker 2: like moving in circles or moving towards the center of
• 18:08
  Speaker 2: the Earth, all that kind of stuff. It's a really
• 18:11
  Speaker 2: fascinating different way to think about how the universe works.
• 18:14
  Speaker 2: It tells a very different story from Newton's, but it
• 18:17
  Speaker 2: mostly describes really really big stuff because you need a
• 18:19
  Speaker 2: lot of mass to curve space, and that curvature is
• 18:23
  Speaker 2: kind of gentle, so the effect of that curvature is
• 18:25
  Speaker 2: hard to measure, especially compared to these quantum forces, which
• 18:28
  Speaker 2: are extraordinarily powerful in comparison.
• 18:32
  Speaker 1: All right, so now maybe paint us a picture of
• 18:34
  Speaker 1: how they are not unified, Like can I just have
• 18:38
  Speaker 1: some quantum particles interacting in a gravitational field? Or can
• 18:43
  Speaker 1: I just have the path of a quantum particle bent
• 18:46
  Speaker 1: by the bending of space and time. What doesn't these
• 18:49
  Speaker 1: two things do together? Like, what are scenarios in which
• 18:52
  Speaker 1: they exclude each other?
• 18:53
  Speaker 2: Yeah? Great, And so this is sort of like number
• 18:55
  Speaker 2: two reason why we want to unify them, because in
• 18:58
  Speaker 2: some situations they disc Like we talked earlier about having
• 19:01
  Speaker 2: separate theories of the universe, Maybe that's cool, but it's
• 19:04
  Speaker 2: not cool if they're talking about the same phenomenon. If
• 19:07
  Speaker 2: you're asking them the question what happens here? Most of
• 19:10
  Speaker 2: the time you can keep them separate because for tiny,
• 19:12
  Speaker 2: little particles you can ignore gravity. Gravity is very very
• 19:15
  Speaker 2: weak for little particles, and for really big stuff, quantum
• 19:18
  Speaker 2: effects mostly average out. You don't need to know quantum
• 19:20
  Speaker 2: mechanics to predict the path of a baseball. But in
• 19:23
  Speaker 2: some scenarios they do disagree, things like what happens inside
• 19:27
  Speaker 2: a black hole. That's a scenario where you have really
• 19:30
  Speaker 2: powerful gravity so gravity can no longer be ignored. And
• 19:34
  Speaker 2: things are very very small because we think that things
• 19:37
  Speaker 2: are super compressed inside a black hole, so quantum effects
• 19:40
  Speaker 2: are important. So super duper massive, very very tiny objects,
• 19:46
  Speaker 2: quantum mechanics and general relativity disagree about what happens there,
• 19:50
  Speaker 2: and so the universe can't have a contradiction. Two theories
• 19:52
  Speaker 2: tell different stories. They can't both be right.
• 19:55
  Speaker 1: But I guess I mean, in like an everyday scenario,
• 19:57
  Speaker 1: do they work together? Sort of like a man imagining
• 20:00
  Speaker 1: say a microscopic particle, like an electron out there in
• 20:04
  Speaker 1: the near Earth orbit and it's floating out there in
• 20:07
  Speaker 1: space close to the Earth. Does it get pulled by gravity?
• 20:10
  Speaker 1: Is it going to fall down to Earth? Is there
• 20:12
  Speaker 1: a problem with me trying to use quantum mechanics to
• 20:16
  Speaker 1: model how it falls to Earth?
• 20:17
  Speaker 2: Yeah, so you might think, can't we just test quantum
• 20:20
  Speaker 2: gravity and figure out what the answer is, which one's
• 20:23
  Speaker 2: right by looking at the gravity of a tiny quantum particle, Right,
• 20:26
  Speaker 2: So that's what you're asking. What happens for the Earth's
• 20:28
  Speaker 2: gravity on an electron?
• 20:30
  Speaker 1: Yeah? Like, do the two theories break down or do
• 20:32
  Speaker 1: they agree on their normal conditions that are not inside
• 20:35
  Speaker 1: of a black hole.
• 20:36
  Speaker 2: So the two do not agree about what happens to
• 20:38
  Speaker 2: an electron in the Earth's gravitational field.
• 20:41
  Speaker 1: They don't.
• 20:42
  Speaker 2: They don't, but they're not both relevant at the same time.
• 20:45
  Speaker 2: It's a little tricky, and the issue is how do
• 20:47
  Speaker 2: you calculate the gravitational field of the electron, or even
• 20:50
  Speaker 2: the gravitational force on the electron, or the effective curve
• 20:54
  Speaker 2: space however you want to say it, because that depends
• 20:57
  Speaker 2: on where the electron is. Electron is a little quantum
• 21:01
  Speaker 2: particle with quantum effects. Then maybe it has like a
• 21:04
  Speaker 2: fifty percent chance to be at this altitude and fifty
• 21:06
  Speaker 2: percent chance to be at that altitude, in which case
• 21:09
  Speaker 2: it would feel different amounts of gravity. And so how
• 21:12
  Speaker 2: do you calculate the gravity on a quantum particle, We
• 21:15
  Speaker 2: don't know. The theory of quantum gravity would tell us
• 21:18
  Speaker 2: how to do that, but general relativity doesn't tell us
• 21:21
  Speaker 2: how to do that. General relativity requires that you know
• 21:23
  Speaker 2: where the electron is, and so it ignores its quantum nature.
• 21:29
  Speaker 2: So if the quantum nature of the electron is important,
• 21:31
  Speaker 2: it's doing quantumy stuff, then we don't know how to
• 21:34
  Speaker 2: calculate the force of gravity.
• 21:35
  Speaker 3: On it.
• 21:36
  Speaker 2: But also we can't measure the force of gravity on
• 21:39
  Speaker 2: a tiny little object because the force is so small
• 21:42
  Speaker 2: because its mass is so small.
• 21:44
  Speaker 1: Sounds like a great situation and maybe one that we
• 21:48
  Speaker 1: need a little bit more time on. So let's dig
• 21:50
  Speaker 1: into that scenario and dig into well, how exactly these
• 21:54
  Speaker 1: two theories don't match up, and then we'll get into
• 21:56
  Speaker 1: a little bit of the math that makes it so
• 21:58
  Speaker 1: hard to integrate the tube. So let's do that. But
• 22:00
  Speaker 1: first let's take a quick break. All right, we're talking
• 22:16
  Speaker 1: about why quantum gravity theory that unites quantum mechanics and
• 22:20
  Speaker 1: general relativity it's so hard to come up with and
• 22:24
  Speaker 1: to make these two theories play well together. And so Daniel,
• 22:28
  Speaker 1: we're talking about a scenario in which I have an
• 22:31
  Speaker 1: electron in near Earth orbit. It's out there in space
• 22:34
  Speaker 1: above the atmosphere, and I'm trying to figure out what's
• 22:37
  Speaker 1: going to happen to this electron. Is it going to
• 22:39
  Speaker 1: fall to Earth? What path is it going to take
• 22:42
  Speaker 1: as it falls to Earth? And you're saying that it's
• 22:45
  Speaker 1: hard to theoretically predict what's going to happen, right, because
• 22:48
  Speaker 1: it's definitely going to fall if I put an electron
• 22:50
  Speaker 1: on your Earth, right, like it's going to do something,
• 22:54
  Speaker 1: but we don't really have a good theory to predict
• 22:56
  Speaker 1: what it's going to do.
• 22:57
  Speaker 2: Is that what you're saying, we can't be very very
• 22:59
  Speaker 2: precise about its predictions. We can be approximate. Like, there's
• 23:02
  Speaker 2: two approaches you can take. You can say, I'm going
• 23:05
  Speaker 2: to ignore the quantum mechanical part of it. I'm just
• 23:06
  Speaker 2: going to treat the electron like it's a tiny little
• 23:09
  Speaker 2: rock or a tiny little ball. I'm going to calculate
• 23:12
  Speaker 2: its gravity and I think about how it's basically in
• 23:15
  Speaker 2: orbit around the Earth. And you can do that and
• 23:17
  Speaker 2: you get very good predictions, and you can calculate how
• 23:20
  Speaker 2: things boil off the top of the atmosphere or they
• 23:22
  Speaker 2: fall to Earth, or whether they're in stable orbits or not.
• 23:25
  Speaker 2: So basically, ignore the quantum nature of the electron, treat
• 23:29
  Speaker 2: it like a tiny classical object and do gravity on it.
• 23:32
  Speaker 2: That's one approach.
• 23:33
  Speaker 1: But then you're saying it's hard to know how much
• 23:36
  Speaker 1: gravity is applied to the electron because of quantum mechanics,
• 23:40
  Speaker 1: or is it hard? Can you just say, like the
• 23:42
  Speaker 1: electron has this much mass and it's a little tiny rock,
• 23:45
  Speaker 1: and so that's how much gravity's going to feel or
• 23:48
  Speaker 1: is that at some level wrong?
• 23:49
  Speaker 2: Well, that's at some level wrong because you're ignoring the
• 23:52
  Speaker 2: quantum nature of the electron. You're treating it like a
• 23:54
  Speaker 2: tiny rock, and it's not a tiny rock.
• 23:56
  Speaker 1: Yeah, but I guess. I mean, like, if you do
• 23:58
  Speaker 1: treat it like a rock, do you get something wildly
• 24:01
  Speaker 1: off or do you get something that seems to be
• 24:03
  Speaker 1: pretty exact?
• 24:04
  Speaker 2: You get something that works pretty well. As long as
• 24:06
  Speaker 2: it doesn't have any interactions. As long as that electron
• 24:09
  Speaker 2: is not interacting with any other particles, it's mostly just
• 24:12
  Speaker 2: ignoring them, then yeah, you get something that's correct.
• 24:15
  Speaker 1: Like it's going to follow the same path as a
• 24:17
  Speaker 1: little rock.
• 24:18
  Speaker 2: Yes, as long as it's not interacting. But if it's
• 24:20
  Speaker 2: in a soup of other electrons and charged particles and
• 24:22
  Speaker 2: it's interacting with those, then boom, it's quantum nature becomes
• 24:26
  Speaker 2: important and those quantum effects dwarf gravity. They completely take over.
• 24:32
  Speaker 2: So you can either ignore the quantum effects and just
• 24:34
  Speaker 2: do the gravity, or you can ignore the gravity and
• 24:37
  Speaker 2: just do the quantum effects. For an electron, only one
• 24:41
  Speaker 2: of those is relevant at a time, and that's why
• 24:43
  Speaker 2: you don't need quantum gravity to think about electrons. You
• 24:46
  Speaker 2: can do either quantum mechanics or gravity. They're never both
• 24:50
  Speaker 2: relevant at the same time.
• 24:52
  Speaker 1: But I guess to an approximation. So then when do
• 24:54
  Speaker 1: you get into trouble? Like when is it a problem
• 24:56
  Speaker 1: that these two are not unified? But what's the scenario?
• 24:58
  Speaker 1: Like it's in gravitaational orbit around the Earth, the electron
• 25:02
  Speaker 1: is and it's sort of a little bit interacting with
• 25:04
  Speaker 1: another electron. Then it's like we don't know what to do.
• 25:07
  Speaker 2: As long as it has a quantum interaction, that's just
• 25:09
  Speaker 2: going to dominate because the quantum forces are so much
• 25:12
  Speaker 2: more powerful than gravity. You know, they're like ten to
• 25:16
  Speaker 2: the thirty times as powerful as gravity.
• 25:18
  Speaker 1: But like what if it's like ten to thirty one
• 25:21
  Speaker 1: times further away, wouldn't it be at the same level
• 25:24
  Speaker 1: of gravity.
• 25:25
  Speaker 2: I mean gravity falls with distance just like quantum forces do. Right,
• 25:28
  Speaker 2: So it's not a matter of distance, it's a matter
• 25:31
  Speaker 2: of the mass to charge ratio. Like if you have
• 25:34
  Speaker 2: two electrons, they feel a very strong electromagnetic repulsion because
• 25:38
  Speaker 2: of their charge. They don't feel a very strong gravitational
• 25:41
  Speaker 2: attraction because of their mass. The electromagnetic force there is
• 25:44
  Speaker 2: always more powerful at any distance.
• 25:47
  Speaker 1: I thought maybe the scenario you're trying to paint was like,
• 25:49
  Speaker 1: I have an electron out there in space, and it's
• 25:52
  Speaker 1: been attracted by gravity to the giant Earth, but maybe
• 25:55
  Speaker 1: it's also sort of being repelled by another electron that's nearby,
• 25:59
  Speaker 1: and so then we don't know what's going to happen.
• 26:01
  Speaker 2: You have a scenario we have a single electron orbiting
• 26:03
  Speaker 2: the Earth, and then some very distant electron is very
• 26:06
  Speaker 2: gently pushing on it with the same power as the
• 26:09
  Speaker 2: gravity of the entire Earth.
• 26:11
  Speaker 1: Yes, Is that like the scenario that you run into
• 26:14
  Speaker 1: problems or can you still handle it?
• 26:16
  Speaker 2: No, that's a cool idea. That's a scenario where gravity
• 26:19
  Speaker 2: and quantum forces are at the same level, and so
• 26:22
  Speaker 2: you can't ignore one of them. You have to take
• 26:24
  Speaker 2: both into account, and we don't know how to do
• 26:27
  Speaker 2: that prediction. That kind of experiment is also pretty hard
• 26:30
  Speaker 2: to realize because you need an isolated electron affected by
• 26:34
  Speaker 2: only one other electron that's super far away, So it's
• 26:37
  Speaker 2: not like practically something we could set up. Otherwise that
• 26:40
  Speaker 2: would be really awesome. It would tell us something about
• 26:42
  Speaker 2: quantum gravity.
• 26:43
  Speaker 1: Yeah, yeah, But that's a physical problem about a physics problem,
• 26:46
  Speaker 1: so you know, are couch surfing and Dana Jones doesn't care.
• 26:49
  Speaker 2: The other way to tackle this is to say, well,
• 26:50
  Speaker 2: what if you have a really really massive quantum particle
• 26:53
  Speaker 2: particle that is feeling quantum forces but actually has enough
• 26:57
  Speaker 2: mass that it's gravity can't be ignored, and that's when
• 27:00
  Speaker 2: you end up at.
• 27:00
  Speaker 1: A black hole? All right? So can you be more
• 27:03
  Speaker 1: specific about what the problem is, like we don't know
• 27:06
  Speaker 1: how to tell what the particle is going to do next,
• 27:09
  Speaker 1: or we can't predict how it's going to interact. Can
• 27:11
  Speaker 1: you describing words what the problem is at the indecentnarios?
• 27:14
  Speaker 2: The problem is that our two theories, general relativity and
• 27:17
  Speaker 2: quantum mechanics make different predictions about what's going to happen.
• 27:21
  Speaker 1: What do you mean, like one theory says that the
• 27:23
  Speaker 1: electron is going to turn right and the other theory
• 27:24
  Speaker 1: says the electric is going to turn left.
• 27:26
  Speaker 2: Yeah. For example, general relativity is a classical theory, and
• 27:29
  Speaker 2: so it assumes electrons have a very definitive location at
• 27:32
  Speaker 2: every point in time, whereas quantum mechanics says no, this
• 27:35
  Speaker 2: is probabilistic, and you can get things like interference. General
• 27:38
  Speaker 2: relativity says, I'm ignoring all that interference stuff, and it's
• 27:41
  Speaker 2: going to make a prediction based on treating the electron
• 27:44
  Speaker 2: like it's a little rock flying through space. So they're
• 27:46
  Speaker 2: going to come up with very different predictions for what's
• 27:48
  Speaker 2: going to happen. General relativity doesn't allowed for like entanglement
• 27:51
  Speaker 2: or any of the other important quantum effects that totally
• 27:55
  Speaker 2: control what happens to an electron.
• 27:57
  Speaker 1: I see, because quantum mechanics also might change which direction
• 28:00
  Speaker 1: the electron goes.
• 28:01
  Speaker 2: Absolutely, yeah, Electromagnetism is a quantum effect, right, Those forces,
• 28:06
  Speaker 2: all the forces in the universe that cause acceleration, the
• 28:08
  Speaker 2: weak force, electromagnetism, the strong force, These are all quantum effects.
• 28:12
  Speaker 2: They all operate on the probabilistic wave functions that control
• 28:16
  Speaker 2: these particles. General relativity ignores those. So you ask quantum
• 28:20
  Speaker 2: mechanics and gr what's going to happen to this electron.
• 28:23
  Speaker 2: They disagree about what's going to happen.
• 28:25
  Speaker 1: Well, maybe this is the point where we have to
• 28:27
  Speaker 1: get more into the math, because you know, as a
• 28:30
  Speaker 1: lad person, I might just say, like, can you just
• 28:32
  Speaker 1: add these two things, like why can't just have a
• 28:35
  Speaker 1: particle that gravity pulls on its average position, and so
• 28:39
  Speaker 1: the average position curves according to gravity, but then where
• 28:43
  Speaker 1: it is exactly might be fuzzy due to quantum mechanics.
• 28:46
  Speaker 2: Yes, so what you're trying to do right now is
• 28:47
  Speaker 2: come up with the theory of quantum gravity. You're trying
• 28:49
  Speaker 2: to say, can I get all the good bits of
• 28:51
  Speaker 2: general relativity and all the good bits of quantum mechanics
• 28:54
  Speaker 2: and smooth them together to make a theory of quantum
• 28:57
  Speaker 2: gravity that makes one prediction? Right? And that's the topic
• 29:00
  Speaker 2: of the episode. Why is that so hard? And people
• 29:03
  Speaker 2: have been working on it for one hundred years. It
• 29:05
  Speaker 2: sounds straightforward, but there's a bunch of reasons why it's
• 29:08
  Speaker 2: actually quite tricky. One of them is the problem you
• 29:10
  Speaker 2: just mentioned, which is this question of like space and
• 29:13
  Speaker 2: time and probability. You know, quantum particles don't have definitive locations,
• 29:18
  Speaker 2: and general relativity doesn't allow for probabilities in space time.
• 29:23
  Speaker 2: What you just described is like, well, what if the
• 29:25
  Speaker 2: electron is allowed to have a probability being here or there?
• 29:28
  Speaker 2: And so we just say, like space has a probability
• 29:31
  Speaker 2: of being curved here and a probability of being curved there. Right,
• 29:35
  Speaker 2: that's like a pretty deep change to how general relativity works,
• 29:39
  Speaker 2: and the mathematics of it breaks down, Like general relativity
• 29:42
  Speaker 2: doesn't allow for those probabilities?
• 29:45
  Speaker 1: What do you mean, like doesn't allow it? Like you
• 29:47
  Speaker 1: just don't know how to write it down, or like
• 29:50
  Speaker 1: you get nonsensical answers, or it's like trying to fit
• 29:53
  Speaker 1: a square peg in a round hole. You know, am
• 29:55
  Speaker 1: I trying to use fractions to you know, compute things
• 29:57
  Speaker 1: to certain decimal point or something.
• 30:00
  Speaker 2: Yeah, the mathematics of general relativity is hard.
• 30:02
  Speaker 1: You know.
• 30:02
  Speaker 2: It took Einstein like ten years to figure out how
• 30:06
  Speaker 2: to wrangle these equations to make any sense of them.
• 30:09
  Speaker 2: He had this idea that maybe space was curved and
• 30:11
  Speaker 2: that was explaining what gravity really was. But to make
• 30:14
  Speaker 2: the math work to even Einstein in a decade, and
• 30:17
  Speaker 2: it takes a lot of people a lot of time
• 30:19
  Speaker 2: to understand and to wrestle with the equations, which turn
• 30:21
  Speaker 2: out to be really really complicated. It's not like one
• 30:24
  Speaker 2: equation for a general relativity that says more mass means
• 30:27
  Speaker 2: more curvature. It's a matrix of equations. There's like sixteen
• 30:31
  Speaker 2: coupled equations which are really hairy, and if you add
• 30:35
  Speaker 2: to those the probability that space is may be curved
• 30:38
  Speaker 2: here and maybe curved there, it increases the complexity exponentially,
• 30:42
  Speaker 2: and it makes those equations impossible to even write down.
• 30:44
  Speaker 2: We don't know how to write down equations that both
• 30:47
  Speaker 2: describe space as a curvature of this differential geometric manifold
• 30:52
  Speaker 2: and allow for probabilities, like we just don't have the
• 30:55
  Speaker 2: tools for it. It might be that somebody out there
• 30:57
  Speaker 2: is developing some cool theory probabilistic manifolds that later will
• 31:02
  Speaker 2: be able to slip in to build the theory of
• 31:04
  Speaker 2: quantum gravity. But it's like we need a power tool
• 31:07
  Speaker 2: and all we have is a handsaw.
• 31:08
  Speaker 1: Yeah, Like you say, like if we just don't have
• 31:10
  Speaker 1: the right tools that will both fit a square peg
• 31:13
  Speaker 1: and a round hole.
• 31:14
  Speaker 2: And we don't know if we're missing the right tool,
• 31:16
  Speaker 2: Maybe that's just the wrong direction, right. It could be
• 31:18
  Speaker 2: that that's not the right way to try to build
• 31:20
  Speaker 2: the theory of quantum gravity. But a lot of times
• 31:23
  Speaker 2: it is the case that progress in mathematics is limiting
• 31:26
  Speaker 2: progress in physics. Einstein was only able to build general
• 31:29
  Speaker 2: relativity because the theories of differential geometry had been developed
• 31:33
  Speaker 2: like ten fifteen years earlier by mathematicians who didn't care
• 31:37
  Speaker 2: at all about gravity or physics. They just thought it
• 31:39
  Speaker 2: was cool to think about like wiggly shapes in their mind,
• 31:43
  Speaker 2: and so this kind of stuff happens all the time.
• 31:45
  Speaker 1: I see you're saying it's all the mathematicians fault, like
• 31:48
  Speaker 1: they're holding you back. Man.
• 31:49
  Speaker 2: Math is the language of physics, and in the end,
• 31:52
  Speaker 2: it's mathematical problems that are preventing us from building theories
• 31:55
  Speaker 2: of quantum gravity. And what you described is basically trying
• 31:59
  Speaker 2: to make space quantum mechanical. You can also go the
• 32:02
  Speaker 2: other direction, and you can say, well, what if we
• 32:04
  Speaker 2: try to make gravity itself quantum mechanical. What if we
• 32:07
  Speaker 2: try to describe the theory of gravity as a quantum
• 32:10
  Speaker 2: force instead of this whole crazy curvature of space and time,
• 32:15
  Speaker 2: And then you run into a completely different kind of
• 32:17
  Speaker 2: mathematical mm I see.
• 32:18
  Speaker 1: I think what you're saying is like each of these
• 32:21
  Speaker 1: two theories work, but only if they ignore each other.
• 32:23
  Speaker 1: Like quantum mechanics assumes that space doesn't bend and there
• 32:27
  Speaker 1: is no gravity. Basically gravity doesn't exist. To quantum mechanics,
• 32:31
  Speaker 1: and general relativity assumes that things are not fuzzy at
• 32:34
  Speaker 1: any level exactly, which works for a lot of situations,
• 32:37
  Speaker 1: but in some situations you have to take them into
• 32:41
  Speaker 1: account both at the same time.
• 32:42
  Speaker 2: Exactly, and it's amazing that both of them work. They
• 32:45
  Speaker 2: tell very different stories about what's really happening in the universe,
• 32:48
  Speaker 2: and in almost every scenario you can ignore one.
• 32:51
  Speaker 1: Right.
• 32:52
  Speaker 2: It's like you have two friends that are like different
• 32:54
  Speaker 2: kinds of movies or something, and most of the time
• 32:56
  Speaker 2: you can just ignore one friend and listen to the
• 32:58
  Speaker 2: other friend. But sometimes they have opinions about the same
• 33:01
  Speaker 2: kind of movie, and you're like, well, is this movie
• 33:03
  Speaker 2: good or bad? I don't know who to listen to.
• 33:05
  Speaker 2: And for general relativity and quantum mechanics, currently they only
• 33:08
  Speaker 2: overlap in places we cannot see inside black holes, so
• 33:13
  Speaker 2: we don't know who's fundamentally right, or if either of
• 33:15
  Speaker 2: them are right.
• 33:17
  Speaker 1: I see. It's like having a friend who only watches
• 33:19
  Speaker 1: sci fi movies and then having another friend who only
• 33:22
  Speaker 1: watches romantic comedies. And usually you can have perfectly good
• 33:25
  Speaker 1: conversations with either of them. But let's say science fiction
• 33:29
  Speaker 1: wrap the comedy comes out. Now there's true.
• 33:31
  Speaker 2: Exactly exactly right. Now you can't hang out with your
• 33:34
  Speaker 2: friends anymore, right, boom the universe.
• 33:36
  Speaker 1: You gotta stop watching movies. You can stage your couch
• 33:40
  Speaker 1: and just do physics and math all day. Has this
• 33:45
  Speaker 1: already happened to you, Ben?
• 33:47
  Speaker 2: I think Passengers was not a sci fi romantic comedy.
• 33:50
  Speaker 1: Oh there you go, And that was very controversial.
• 33:53
  Speaker 2: Man, exactly.
• 33:54
  Speaker 1: Nobody likes that we don't have the math tools or
• 33:58
  Speaker 1: framework or theories that let us tackle these two things
• 34:01
  Speaker 1: at the same time. What are some other ways that
• 34:03
  Speaker 1: make it hard to unify these two things?
• 34:05
  Speaker 2: So a really popular approach is to trying to make
• 34:07
  Speaker 2: a theory of quantum gravity that has a graviton. Say,
• 34:11
  Speaker 2: you know, Einstein, that was cute. We like your idea
• 34:14
  Speaker 2: of Kurt space time. It's pretty, but maybe it's just
• 34:16
  Speaker 2: fundamentally the wrong direction. Maybe if you zoom in, what
• 34:19
  Speaker 2: really is happening is that gravity is a force and
• 34:21
  Speaker 2: it's exchanging gravitons, and I mean, then we can describe
• 34:26
  Speaker 2: the whole theory of gravity back sort of as like
• 34:28
  Speaker 2: a quantized version of Newton's theory. And people have tried
• 34:32
  Speaker 2: to do this because that'd be pretty right if we
• 34:33
  Speaker 2: could just like add gravity to the standard model and
• 34:36
  Speaker 2: have another particle and ignore this whole curvature business. That
• 34:39
  Speaker 2: would be cool.
• 34:40
  Speaker 1: So that would be going back to the idea that
• 34:42
  Speaker 1: gravity is a force, not a bending of space and
• 34:45
  Speaker 1: time exactly.
• 34:46
  Speaker 2: So to tell philosophically a very different story from what
• 34:49
  Speaker 2: Einstein is telling us about how the universe works, they
• 34:52
  Speaker 2: would say there is no curvature. There are these tiny,
• 34:54
  Speaker 2: little invisible gravitons being passed back and forth. The same
• 34:57
  Speaker 2: way that like electromagnetism you think about in terms of
• 35:01
  Speaker 2: electric fields that are sort of like virtual photons being
• 35:04
  Speaker 2: passed back and forth, you can think of gravity in
• 35:07
  Speaker 2: terms of gravitons being passed back and forth. So that's
• 35:10
  Speaker 2: one direction. The problem is nobody can make that math
• 35:13
  Speaker 2: work either. When you do the calculations there and you
• 35:16
  Speaker 2: ask like, well, what happens if you try to collide
• 35:19
  Speaker 2: two black holes together, or even two protons together, you
• 35:22
  Speaker 2: get nonsense answers. You get answers like, well, the probability
• 35:26
  Speaker 2: of this happening is one hundred and fifty percent, and
• 35:28
  Speaker 2: the probability of that happening is one thousand percent, Just
• 35:31
  Speaker 2: like numbers that do not make sense you can't have
• 35:34
  Speaker 2: probabilities greater than one. But that's what these calculations spit out.
• 35:38
  Speaker 1: Like if you assume a gravitin exists, then you get
• 35:40
  Speaker 1: these weird answers, but more fundamentally, like you're ignoring general relativity, right,
• 35:45
  Speaker 1: You're ignoring things that a lot of experiments have verified
• 35:48
  Speaker 1: that Eisin was right, and so you're sort of not
• 35:51
  Speaker 1: really solving the problem, right, are you.
• 35:53
  Speaker 2: Well, you'd have to think about it as an upgrade
• 35:54
  Speaker 2: to general relativity. You'd have to reproduce all the predictions
• 35:57
  Speaker 2: of general relativity. So you need to develop a theory
• 36:00
  Speaker 2: of gravitons, which when you zoom out, looks a lot
• 36:03
  Speaker 2: like general relativity, and that people actually can do. There
• 36:06
  Speaker 2: are theories of quantum gravity that involve graviton exchange that
• 36:10
  Speaker 2: when you zoom out, look a lot like general relativity.
• 36:13
  Speaker 1: So space time is not being bent.
• 36:15
  Speaker 2: Yeah, they tell a different story, but they make the
• 36:17
  Speaker 2: same predictions about like the motions of objects.
• 36:20
  Speaker 1: Including things like gravitational ways and frame dragging and all that, all.
• 36:24
  Speaker 2: That kind of stuff. The problem is what happens at
• 36:27
  Speaker 2: the small scale when you try to think about like
• 36:29
  Speaker 2: when two particles scatter against each other, or when two
• 36:32
  Speaker 2: black holes are being eaten, then this theory breaks down.
• 36:35
  Speaker 1: But generativity still works for those.
• 36:37
  Speaker 2: General relativity still works for those. But we think it's wrong, right.
• 36:40
  Speaker 2: General relativity, we think is giving the wrong answer for
• 36:43
  Speaker 2: what happens when two protons collide or what's at the
• 36:46
  Speaker 2: heart of a black hole, because it's ignoring the quantum
• 36:48
  Speaker 2: mechanical effects. You try to build a theory of quantum
• 36:51
  Speaker 2: gravity that has gravitons and explains all of general relativity
• 36:55
  Speaker 2: and gives you a gravitational quantum mechanical prediction for what
• 36:59
  Speaker 2: happens when two particles collider, two black holes collide, Then
• 37:02
  Speaker 2: you get all sorts of nonsense. You get all sorts
• 37:04
  Speaker 2: of infinities that we don't know how to wrangle.
• 37:07
  Speaker 1: M I see, So graviton not a great ivid you
• 37:10
  Speaker 1: or one that hasn't worked so far?
• 37:12
  Speaker 2: Yeah, exactly. There are also sort of fundamental problems we
• 37:16
  Speaker 2: just don't know how to solve for quantum gravity, like
• 37:19
  Speaker 2: deep inconsistencies between the picture of the universe we get
• 37:22
  Speaker 2: from general relativity and the picture of the universe we
• 37:25
  Speaker 2: get from quantum mechanics about how physics should work. That
• 37:28
  Speaker 2: we just don't know how to reconcile. Well, what do
• 37:30
  Speaker 2: you mean, Like what like a deep principle in physics
• 37:32
  Speaker 2: is this idea of locality that things should be near
• 37:35
  Speaker 2: each other to affect each other, that you shouldn't have
• 37:38
  Speaker 2: like an electron over here affecting something really really far
• 37:41
  Speaker 2: away in the universe, especially in quantum mechanics, and in
• 37:46
  Speaker 2: quantum mechanics, we have this deep connection between the distances
• 37:49
  Speaker 2: between things and their energies. Like the reason that we
• 37:53
  Speaker 2: use the Large Hadron Collider to study really really really
• 37:56
  Speaker 2: tiny things is you need really high energy to study
• 38:00
  Speaker 2: really small distance scales. Like things that have a lot
• 38:03
  Speaker 2: of energy interact with each other very very.
• 38:06
  Speaker 1: Closely, because if things have low energy, then they don't
• 38:10
  Speaker 1: interact with the things around them.
• 38:11
  Speaker 2: If things have low energy, then they can interact with
• 38:13
  Speaker 2: stuff that's further away. Another way to think about it
• 38:16
  Speaker 2: is in terms of the wavelength of stuff. You know
• 38:19
  Speaker 2: that you need like really high energy photons to see
• 38:22
  Speaker 2: really really small stuff. With lower energy photons they have
• 38:25
  Speaker 2: a longer wavelength you can't like resolve small details. That's why,
• 38:29
  Speaker 2: for example, when you want pictures of really really tiny stuff,
• 38:32
  Speaker 2: you use high frequency photons you go beyond that to
• 38:36
  Speaker 2: use like electrons to take pictures of atoms, for example.
• 38:39
  Speaker 2: So you want to see the universe on a really
• 38:41
  Speaker 2: small scale, you need to use really high.
• 38:43
  Speaker 1: Energy probes, maybe to say high frequency instead of high energy.
• 38:47
  Speaker 2: Yeah, energy and frequency very closely connected. In quantum mechanics,
• 38:51
  Speaker 2: you need very high frequency stuff to see really short distances.
• 38:55
  Speaker 1: Okay, okay.
• 38:55
  Speaker 2: In general relativity they have the opposite relationship. As you
• 38:59
  Speaker 2: add energy to something in general relativity, then its influence
• 39:03
  Speaker 2: grows to longer distances. So quantum mechanics, higher energy means
• 39:08
  Speaker 2: shorter distances. In general relativity, higher energy means larger distances.
• 39:13
  Speaker 2: Like think about what happens to a black hole's radius.
• 39:16
  Speaker 2: As you add energy to a black hole, black hole
• 39:19
  Speaker 2: gets bigger, you add more energy, black hole keeps getting bigger.
• 39:23
  Speaker 2: The short styled radius, the distance from the singularity to
• 39:26
  Speaker 2: the event horizon, just keeps growing as the black hole
• 39:29
  Speaker 2: gets more massive. So, somehow general relativity doesn't have the
• 39:32
  Speaker 2: same relationship between energy and frequency and the distances involved.
• 39:37
  Speaker 2: They have like this deeply opposite relationship. This might sound
• 39:41
  Speaker 2: like sort of weirdly philosophically handwavy to you, but it
• 39:45
  Speaker 2: sort of tells.
• 39:45
  Speaker 1: You about it not at all. I don't know what
• 39:48
  Speaker 1: you mean.
• 39:50
  Speaker 2: But the reason it's important is that it tells us
• 39:52
  Speaker 2: that these two theories have like a fundamentally different sort
• 39:54
  Speaker 2: of philosophical foundation. Like one of them is very very local,
• 39:58
  Speaker 2: the other one is very non local. So when we
• 40:00
  Speaker 2: go to make a theory of quantum gravity, we're like, hmm,
• 40:03
  Speaker 2: these two things are kind of like very different. How
• 40:05
  Speaker 2: do we bring them together? It's like, how do you
• 40:07
  Speaker 2: get your science fiction fan friend and your rom com
• 40:10
  Speaker 2: fan friend together into a single movie If they have
• 40:13
  Speaker 2: just like really opposing needs for pacing and jokes and
• 40:17
  Speaker 2: whatever in the movie, it might be fundamentally impossible. If
• 40:20
  Speaker 2: these two things are so deeply in conflict.
• 40:23
  Speaker 1: It sounds like you're just kind of maybe saying the
• 40:25
  Speaker 1: same thing we talked about before, which is like, you know,
• 40:28
  Speaker 1: quantum gravity is good for things that are small, and
• 40:32
  Speaker 1: general relativity is good for things that are really big,
• 40:35
  Speaker 1: but there is a certain overlap between them, and that's
• 40:37
  Speaker 1: where you get into trouble.
• 40:39
  Speaker 2: Yeah, exactly, But I think there's one more layer there, Like,
• 40:41
  Speaker 2: imagine you have a singularity, so at the tiny little
• 40:44
  Speaker 2: spot with a huge amount of mass. Right, so it's
• 40:48
  Speaker 2: quantum mechanically important but also has a huge amount of mass.
• 40:52
  Speaker 2: General relativity says it affects things really really far away
• 40:55
  Speaker 2: because it creates a black hole. Who's event horizon can
• 40:58
  Speaker 2: be really really far away. Quantum mechanics says, no, it
• 41:02
  Speaker 2: can only interact with stuff really really nearby, because that's
• 41:04
  Speaker 2: really really tiny frequency.
• 41:06
  Speaker 1: Well, I would say that it can only interact quantum
• 41:08
  Speaker 1: mechanically with things that are close by, but then it
• 41:11
  Speaker 1: can interact through gravity or generativity for things that are
• 41:14
  Speaker 1: far away, like way back to the same spot.
• 41:19
  Speaker 2: Yeah, exactly, we're back to the same spot that if
• 41:21
  Speaker 2: you have quantum gravity, then you don't know can it
• 41:23
  Speaker 2: only interact nearby in its vicinity or can it interact
• 41:27
  Speaker 2: far away as well? Gravity says far away, Quantum mechanics
• 41:31
  Speaker 2: says nearby. We don't know what quantum gravity says. It
• 41:33
  Speaker 2: seems like maybe impossible to come up with a theory
• 41:36
  Speaker 2: that satisfies both m.
• 41:38
  Speaker 1: Just like it's impossible to come up with a good
• 41:40
  Speaker 1: sci fi rom com.
• 41:42
  Speaker 2: Exactly, they're fundamentally opposed.
• 41:45
  Speaker 1: All right, Well, let's dig into some of the other
• 41:47
  Speaker 1: ways that make it hard to unify generalativity and quantum mechanics.
• 41:52
  Speaker 1: But first, let's take another quick break. All right, we're
• 42:06
  Speaker 1: talking about quantum gravity. Can we unite quantum mechanics and
• 42:11
  Speaker 1: general relativity which has gravity in it? So far, Daniel,
• 42:15
  Speaker 1: you're saying it's really hard.
• 42:16
  Speaker 2: It's really hard. Some of the smartest people in the
• 42:19
  Speaker 2: universe have tried for decades and failed.
• 42:22
  Speaker 1: In the universe. That's a big claim.
• 42:25
  Speaker 2: Well, the smartest people in the universe, it could be
• 42:28
  Speaker 2: aliens out there much smarter than us. I don't know.
• 42:30
  Speaker 2: Are they people though? Are aliens people?
• 42:33
  Speaker 1: Well, you're assuming that the smartest people on Earth have
• 42:35
  Speaker 1: become physicists.
• 42:36
  Speaker 2: Oh that's a good point. Yeah, I know that people
• 42:39
  Speaker 2: out there who are like hedge fund bros.
• 42:41
  Speaker 1: Yeah, or cartoonist maybe you know. I'm just saying you're
• 42:45
  Speaker 1: sort of making a general assumption.
• 42:46
  Speaker 2: Here are you saying you're not a physicist? I think
• 42:49
  Speaker 2: years in basically you're a physicist by now.
• 42:52
  Speaker 1: Oh, if that's true, then I have a diploma for
• 42:56
  Speaker 1: you to sign. Yeah.
• 42:57
  Speaker 2: I think we gave you that podcast diploma physics.
• 43:00
  Speaker 1: Right, I'm so waiting for that in the mail. It
• 43:02
  Speaker 1: must have gotten lost. I guess you sent it, right.
• 43:05
  Speaker 2: But we sent it. Yeah, you should definitely get a
• 43:07
  Speaker 2: physical copy. You do like a discount at dollar.
• 43:09
  Speaker 1: Stories like this is an imaginary diploma again for an
• 43:13
  Speaker 1: imaginary field of study. All right, So it's hard to
• 43:18
  Speaker 1: combine these two big theories. We talked about how the
• 43:22
  Speaker 1: math makes it really difficult. The philosophy of them make
• 43:24
  Speaker 1: it really difficult. What are some other ways that make
• 43:26
  Speaker 1: it hard?
• 43:27
  Speaker 2: People argue about the fundamental story of space and time
• 43:30
  Speaker 2: that quantum gravity will have to tell us, because quantum
• 43:33
  Speaker 2: mechanics and general relativity really do tell very very different stories.
• 43:37
  Speaker 2: Here in quantum mechanics, we have kind of like a
• 43:39
  Speaker 2: Newtonian view of space and time. We say space and time,
• 43:43
  Speaker 2: they're backdrops, they're absolute, they're fixed, and we put quantum
• 43:47
  Speaker 2: fields on top of that space and time. We assume
• 43:50
  Speaker 2: that space and time already exists somehow, and we say
• 43:53
  Speaker 2: that this quantum fields in that space and time, and
• 43:56
  Speaker 2: those fields operate and they're part of space. But we
• 43:58
  Speaker 2: don't ask about where space comes from or what it is.
• 44:02
  Speaker 2: That's kind of stuff. It's like a fixed background, we
• 44:04
  Speaker 2: call it. But in general relativity, space time is dynamical.
• 44:09
  Speaker 2: It's not like fixed can bend and twist, and it
• 44:11
  Speaker 2: doesn't like exist inside some other kind of space. We
• 44:15
  Speaker 2: have this way to like calculate the relative distance between points,
• 44:19
  Speaker 2: but space itself is not like some new field that's
• 44:23
  Speaker 2: sitting inside some sort of meta space or superspace or
• 44:26
  Speaker 2: subspace or some other kind of space. It tells us
• 44:30
  Speaker 2: that there is no fixed background. So people describe general
• 44:34
  Speaker 2: relativity as background independent, like the universe. Space itself is
• 44:38
  Speaker 2: not sitting inside some sort of deeper box, and which
• 44:43
  Speaker 2: is kind of weird because it makes you feel a
• 44:44
  Speaker 2: little like unmoored from the foundations of reality.
• 44:48
  Speaker 1: But I guess maybe couldn't you just have quantum mechanics
• 44:52
  Speaker 1: sit on top of a bendy stretch hey space time,
• 44:56
  Speaker 1: you know, sort of like you know, the planet Earth
• 44:58
  Speaker 1: of the Sun or moving through space time and they're
• 45:01
  Speaker 1: getting they're flowing through the curvature and all the bending.
• 45:05
  Speaker 1: Couldn't you have quantum fields also kind of right on
• 45:09
  Speaker 1: top of the curves of space time. Mm hmm.
• 45:11
  Speaker 2: And I love how you just like casually suggesting these solutions,
• 45:15
  Speaker 2: which are like the whole areas of research.
• 45:18
  Speaker 1: Working for twenty years, Daniel, come on, I mean I
• 45:22
  Speaker 1: just spent five to twenty minutes talking about this, and
• 45:24
  Speaker 1: I already know how to come up with the answer.
• 45:27
  Speaker 2: You're basically a business so long. No, I don't mean
• 45:30
  Speaker 2: to make it at all. I mean that these are
• 45:32
  Speaker 2: actually really clever ways forward, and these are exactly the
• 45:34
  Speaker 2: things that people are working on at the cutting edge.
• 45:37
  Speaker 2: It turns out to be complicated. You can do quantum
• 45:40
  Speaker 2: mechanics on curved space, you can put these fields on
• 45:44
  Speaker 2: curved space time, and mostly it works as long as
• 45:48
  Speaker 2: the curvature is small, So like if they're a little
• 45:51
  Speaker 2: bit curved, things are fine. As soon as the curvature
• 45:54
  Speaker 2: becomes large, you get back to all those crazy infinities
• 45:56
  Speaker 2: that we can't wrangle in all sorts of nonsense predictions.
• 46:00
  Speaker 2: So the quantum mechanical theory breaks down if the curvature
• 46:03
  Speaker 2: is really really high. So we just don't know how
• 46:06
  Speaker 2: to do those kinds of calculations.
• 46:08
  Speaker 3: Mm.
• 46:09
  Speaker 1: And that includes time, right because like in general relativity,
• 46:13
  Speaker 1: time can slow down and time can speed up. And
• 46:15
  Speaker 1: so you're saying you don't know how to do that
• 46:17
  Speaker 1: in quantum mechanics.
• 46:18
  Speaker 2: Exactly, we don't know how to do that in quantum mechanics. Again,
• 46:20
  Speaker 2: when the curvature is high, for what they call weak
• 46:23
  Speaker 2: field gravity, where it's like a very small effect gravity,
• 46:27
  Speaker 2: then we can do those calculations and even feel the
• 46:29
  Speaker 2: influence of gravity, not just negligible gravity, just weak gravity.
• 46:33
  Speaker 2: But when gravity gets strong, which is what it does
• 46:36
  Speaker 2: near a black hole, where we think these two things
• 46:38
  Speaker 2: are both important, we don't know how to do those calculations.
• 46:41
  Speaker 2: That's when quantum mechanics on curve space breaks down because
• 46:45
  Speaker 2: of all these infinities we can't grapple with all.
• 46:47
  Speaker 1: Right, Well, as we mentioned, we're not quite experts in
• 46:51
  Speaker 1: this topic, but we don't Maybe we will talk to
• 46:54
  Speaker 1: one of the smartest people in the universe who is
• 46:56
  Speaker 1: tackling this problem directly.
• 46:59
  Speaker 2: That's right down to Nathan, whose father is a listener
• 47:02
  Speaker 2: of the podcast and listen to the podcast mostly so
• 47:05
  Speaker 2: you can have a hope of understanding his son's research.
• 47:08
  Speaker 2: Nathan is a physics brad student at Arizona State University,
• 47:12
  Speaker 2: a well known department with experts in cosmology and theories
• 47:16
  Speaker 2: of quantum gravity, and he was kind enough to spend
• 47:18
  Speaker 2: five minutes talking to me about his research and so.
• 47:21
  Speaker 1: Here at Daniel's interview with Nathan Berwick, Quantum Gravity researcher.
• 47:26
  Speaker 2: So then it's my pleasure to welcome Nathan to the podcast. Nathan,
• 47:30
  Speaker 2: please introduce yourself and tell the listeners who you are.
• 47:33
  Speaker 3: Yes, I'm Nathan Berwick. I'm currently a first year PhD
• 47:39
  Speaker 3: student at Arizona State University studying physics and cosmology and
• 47:46
  Speaker 3: hoping to continue doing so and move into ideally stuff
• 47:51
  Speaker 3: along the lines of quantum gravity.
• 47:52
  Speaker 2: Awesome. Well, we often talk on the podcast about how
• 47:56
  Speaker 2: science is just a bunch of people following their curiosity
• 47:59
  Speaker 2: and pushing forward the forefront of knowledge. So tell us
• 48:02
  Speaker 2: what is it about physics and gr that drew you in,
• 48:05
  Speaker 2: that made you decide you want to spend your life
• 48:08
  Speaker 2: on this question.
• 48:10
  Speaker 3: That's a really good question. So I think part of
• 48:13
  Speaker 3: it is just I always kind of grew up as
• 48:15
  Speaker 3: a curious kid, and you know, when I was younger,
• 48:18
  Speaker 3: I always knew I wanted to be like a scientist
• 48:21
  Speaker 3: in air quotes. But eventually I sort of narrowed in
• 48:26
  Speaker 3: on physics, and I think as soon as I learned
• 48:29
  Speaker 3: about like the most basic concepts in general relativity, sort
• 48:32
  Speaker 3: of the idea that space time is one great big
• 48:35
  Speaker 3: thing and gravity is just the curvature of that it
• 48:38
  Speaker 3: was just immediately alluring, especially when you start to consider
• 48:43
  Speaker 3: how gravity is, like are most interacted with force at
• 48:47
  Speaker 3: least sort of perceptually. You know, we talk about it
• 48:50
  Speaker 3: a lot. You don't really talk about how electromagnet doesn't
• 48:53
  Speaker 3: really plays a role in your life very often. But
• 48:56
  Speaker 3: it's also very poorly understood in general, and there's still
• 49:01
  Speaker 3: like a lot of really big open questions to be
• 49:04
  Speaker 3: answered for general relativity. And I think that sort of
• 49:07
  Speaker 3: openness is very much an invitation to explore, which I
• 49:11
  Speaker 3: really like.
• 49:12
  Speaker 2: So you're not going to give your dad any credit
• 49:14
  Speaker 2: for encouraging you to study physics.
• 49:17
  Speaker 5: I think, I think I definitely should.
• 49:20
  Speaker 3: Yeah, No, he definitely like fostered my curiosity in physics
• 49:24
  Speaker 3: growing up. Both my parents did, but my dad was
• 49:26
  Speaker 3: always fascinated in physics, and so from just like really
• 49:30
  Speaker 3: small things, you know, we'd always find questions on the internet,
• 49:33
  Speaker 3: you know, whether it be a bad string theory, which
• 49:35
  Speaker 3: is always like a very big, you know, media topic
• 49:37
  Speaker 3: and whatnot. But he definitely played a very big role
• 49:40
  Speaker 3: in me wanting to do physics.
• 49:42
  Speaker 2: So a lot of people say that general relativity, once
• 49:45
  Speaker 2: you fully understand it, is deep and beautiful. Do you
• 49:48
  Speaker 2: have that kind of esthetic reaction to it?
• 49:51
  Speaker 3: Yeah, I think a lot of the beauty of it
• 49:53
  Speaker 3: comes from the idea alone that it's all just curvat true,
• 49:58
  Speaker 3: that gravity is in essence just curvature of this big
• 50:02
  Speaker 3: manifold which you may or may not well, which you
• 50:05
  Speaker 3: can't really visualize. But that, for me is the part
• 50:08
  Speaker 3: of the esthetic that general relativity has is just like
• 50:13
  Speaker 3: a breakdown into a more fundamental concept.
• 50:15
  Speaker 2: So if gr is so gorgeous and it works so well,
• 50:19
  Speaker 2: all these experiments confirming Einstein all the time, how can
• 50:22
  Speaker 2: it be wrong? I mean, is it wrong in the
• 50:25
  Speaker 2: way that like Newton's gravity is wrong where it was
• 50:28
  Speaker 2: telling fundamentally the wrong story about what was happening but
• 50:31
  Speaker 2: we didn't really notice until we dug into the details.
• 50:34
  Speaker 2: Or is it mostly telling the right story just needs
• 50:37
  Speaker 2: some like corrections and band aids here and there.
• 50:40
  Speaker 5: Yeah.
• 50:40
  Speaker 3: So that's a really good question, and I think it
• 50:43
  Speaker 3: really depends on how much of the last one hundred
• 50:46
  Speaker 3: years you're willing to disregard, and you know that that
• 50:50
  Speaker 3: has certain consequences but also benefits. But I think the
• 50:54
  Speaker 3: most generally accepted perspective right now is that it needs
• 50:59
  Speaker 3: because general relativity, as this idea of gravity being caused
• 51:05
  Speaker 3: by space time curvature, is so so rigorously tested at
• 51:10
  Speaker 3: the moment, you know, we keep trying to knock Einstein
• 51:13
  Speaker 3: down and find errors in his theory somewhere, but every
• 51:17
  Speaker 3: single time like he just was right. And so that's
• 51:22
  Speaker 3: a difficult thing to try and find errors in. But
• 51:25
  Speaker 3: there's obviously still big questions. So quantum gravity is probably
• 51:30
  Speaker 3: the largest area of questions regarding general relativity, and a
• 51:36
  Speaker 3: lot of that has to do with just quantum mechanics
• 51:39
  Speaker 3: not playing nicely when you try and think about how
• 51:41
  Speaker 3: gravitational fields work with it. General relativity doesn't have any
• 51:44
  Speaker 3: fundamental understanding of what probability distribution looks like. There's no
• 51:49
  Speaker 3: like wave function understanding when you start getting into the
• 51:54
  Speaker 3: smaller regimes for general relativity.
• 51:56
  Speaker 2: So that's a really big topic, like general relativity and
• 51:59
  Speaker 2: quantm mechanics. What are you actually researching right now? Like
• 52:02
  Speaker 2: what are you working on today today?
• 52:04
  Speaker 3: So throughout the last semester, I've been working on what
• 52:09
  Speaker 3: are called scalar tensor theories, which I actually think you
• 52:13
  Speaker 3: guys did an episode fairly recently on no hair theorems.
• 52:16
  Speaker 3: For black holes, and that's very much what I'm working
• 52:20
  Speaker 3: on at the moment, just showing that there's no sort
• 52:24
  Speaker 3: of scalar hairs for certain types of theories.
• 52:29
  Speaker 2: Cool. So then I'm gonna ask you to speculate unscientifically.
• 52:33
  Speaker 2: What do you think is inside a black hole?
• 52:37
  Speaker 5: Oh? Boy?
• 52:38
  Speaker 3: What do I think is inside a black hole? Speculative
• 52:41
  Speaker 3: answer is, well, if it's a big black hole, there's
• 52:43
  Speaker 3: probably stuff sort of floating around in there being eaten up.
• 52:46
  Speaker 3: But I like to think that there's this sort of
• 52:51
  Speaker 3: taboo in physics of natural infinities. They aren't there're usually
• 52:56
  Speaker 3: when there's an infinity, it's something horribly horribly wrong. And
• 53:00
  Speaker 3: that was sort of the big hub up when they
• 53:02
  Speaker 3: figured out that black holes could be a thing that
• 53:04
  Speaker 3: actually existed, was that it just felt wrong that there
• 53:08
  Speaker 3: could be such a dense object that's just so infinitely packed.
• 53:12
  Speaker 3: I like to think that it is just, you know,
• 53:15
  Speaker 3: a perfectly oh infinite density singularity, just because I think
• 53:20
  Speaker 3: it'd be very wild and wacky and cool to have
• 53:24
  Speaker 3: this very strange yet natural like divergence or infinity just
• 53:30
  Speaker 3: existing in the universe.
• 53:32
  Speaker 2: Do you think the singularity inside a black hole is
• 53:34
  Speaker 2: as dense as boba, you know, these chewy blobs of
• 53:38
  Speaker 2: death that people inexplicably like shooting up their straws as
• 53:41
  Speaker 2: they enjoy and otherwise relaxing beverage. I happen to know
• 53:44
  Speaker 2: your father agrees with me on this.
• 53:49
  Speaker 5: He sent me a clip of this just the other day.
• 53:54
  Speaker 3: I certainly hope that boba is it not at all
• 53:57
  Speaker 3: comparable to singularities. But I suppose you never know what
• 54:01
  Speaker 3: that would be, the hell that my dad would die on.
• 54:05
  Speaker 2: I think, well, I think maybe your dad should open
• 54:08
  Speaker 2: up a black hole boba shop that sells black hole boba,
• 54:11
  Speaker 2: super dense, super dense little chunks of destruction. All right, Nathan,
• 54:16
  Speaker 2: thanks very much for taking some time to talk to
• 54:18
  Speaker 2: us about your work on the forefront of understanding of
• 54:21
  Speaker 2: general relativity, and good luck figuring it all out.
• 54:24
  Speaker 5: Thank you very much. Thank you for having me.
• 54:27
  Speaker 1: All right, interesting chat there, pretty controversial about both boba
• 54:32
  Speaker 1: and black holes.
• 54:34
  Speaker 2: You know, this is the cutting edge of food science
• 54:36
  Speaker 2: and quantum gravity.
• 54:38
  Speaker 1: That's right, and sugary drinks, yes, But it's interesting that
• 54:45
  Speaker 1: he seems to follow pretty strongly on the general latuty side.
• 54:48
  Speaker 1: He thinks inside of black holes are singularities.
• 54:50
  Speaker 2: Yeah, I've seen this in a lot of quantum gravity theorists,
• 54:53
  Speaker 2: that they are intoxicated by the beauty of general relativity.
• 54:58
  Speaker 2: It's sort of hard to overstate the this has on people.
• 55:01
  Speaker 2: When they are able to import Einstein's equations into their
• 55:04
  Speaker 2: mind and they can see the universe in terms of
• 55:06
  Speaker 2: this differential manifold, they feel like the scales have fallen
• 55:10
  Speaker 2: from their eyes and they're seeing the universe the way
• 55:12
  Speaker 2: it really is, and it has to be true. So
• 55:14
  Speaker 2: they want to preserve this vision of the universe as
• 55:17
  Speaker 2: having curved space time. It's not exactly religious, but it's
• 55:21
  Speaker 2: almost like a spiritual experience.
• 55:23
  Speaker 1: Oh my goodness. This is coming from, of course, a
• 55:26
  Speaker 1: quantum mechanics particle researcher. Right, you're trying to say that
• 55:30
  Speaker 1: the other side are a bunch of religious zillids because
• 55:34
  Speaker 1: your side is obviously right.
• 55:35
  Speaker 2: I mean, they like boba, so so you can't trust
• 55:38
  Speaker 2: them at all.
• 55:40
  Speaker 1: Well, boba are like particles, so I would have thought
• 55:42
  Speaker 1: they're fuzzy particles, so I would have value you be
• 55:44
  Speaker 1: a big fan.
• 55:45
  Speaker 2: No, they're macroscopic, man, They're dominated by gravity.
• 55:48
  Speaker 1: Mm. Well, I wonder if maybe there is a singularity,
• 55:52
  Speaker 1: you know, and maybe it's also fuzzy at the center
• 55:55
  Speaker 1: of it. It's just that maybe in a singularity, space
• 55:57
  Speaker 1: is so compressed or so squish together, that maybe the
• 56:02
  Speaker 1: quantum certainties also get squished down.
• 56:04
  Speaker 2: Maybe we're gonna solve quantum gravity right here on this
• 56:06
  Speaker 2: podcast today.
• 56:08
  Speaker 1: Yeah, yeah, let's do it. Why wait one hundred years,
• 56:11
  Speaker 1: let's come up with that romantic comedy sci fi movie
• 56:14
  Speaker 1: right now?
• 56:14
  Speaker 2: Why is quantum gravity so hard? Because nobody asked a cartoonist.
• 56:18
  Speaker 2: There you go, boom, question answered, boom.
• 56:23
  Speaker 1: Done. Let's go tackle something harder, like cancer or democracy.
• 56:31
  Speaker 1: All right, Well, you're on one side, people like Nathan
• 56:34
  Speaker 1: are on the other side. Sounds like we still have
• 56:36
  Speaker 1: a long way to go.
• 56:37
  Speaker 2: We do have a long way to go, and there
• 56:38
  Speaker 2: are deep philosophical and mathematical hurdles to overcome. We want
• 56:43
  Speaker 2: to have a single theory of the universe, one that
• 56:45
  Speaker 2: gives us a complete picture of what's really happening the
• 56:48
  Speaker 2: source code for the universe. But so far none of
• 56:51
  Speaker 2: our attempts actually compute.
• 56:53
  Speaker 1: And so it's an active area of research, maybe waiting
• 56:56
  Speaker 1: for the next person to tackle this and possibly solve it,
• 57:00
  Speaker 1: and that could be you out there asking these questions,
• 57:03
  Speaker 1: listening to this podcast, wondering how the universe works.
• 57:06
  Speaker 2: I hope that future genius doesn't suffer an unfortunate boba accident,
• 57:10
  Speaker 2: choking to death before they can share their insights with humanity.
• 57:14
  Speaker 1: Oh my gosh, it's a little bit dark there, a
• 57:18
  Speaker 1: little bit dark. I think you need a pretty large
• 57:20
  Speaker 1: bubba ball. There's choking.
• 57:22
  Speaker 2: Then it's boba relativity.
• 57:25
  Speaker 1: That's right, the answer, the choking hazard of a boba
• 57:28
  Speaker 1: is really all right. Well, stay tuned. You hope you
• 57:32
  Speaker 1: enjoyed that. Thanks for joining us, See you next night.
• 57:39
  Speaker 2: For more science and curiosity, come find us on social
• 57:42
  Speaker 2: media where we answer questions and post videos. We're on Twitter, Discord, Instant,
• 57:47
  Speaker 2: and now TikTok. Thanks for listening, and remember that Daniel
• 57:51
  Speaker 2: and Jorge Explain the Universe is a production of iHeart Radio.
• 57:54
  Speaker 2: For more podcasts from iHeart Radio, visit the iHeartRadio app,
• 57:58
  Speaker 2: Apple Podcasts, or wherever you listen to your favorite shows.