Lisa Randall: Dark Matter, Theoretical Physics, and Extinction Events | Lex Fridman Podcast #403

The following is a conversation with Lisa Randal a theoretical physicist and Cosmologist at Harvard her work involves Improving our understanding of particle Physics super symmetry biogenesis Cosmological inflation and dark matter This is Al Lex Freedman podcast to Support it please check out our sponsors In the description and now dear friends Here's Lisa Randelle one of the things you work on And right about is dark matter we can't See it but there there's a lot of it in The Universe uh you also end one of your Books with a Beetles song quote got to Be good-looking cuz he's so hard to See what is dark matter how should we Think about it given that we can't see It how should we visualize it in our Mind's Eye I think one of the really Important things that physics teaches You is just our limitations but also our Abilities so the fact that we can deduce The existence of something that we don't Directly see is really attribute to People that we can do that but it's also Something that tells you you can't Overly rely on your direct senses if you Just relied on just what you see Directly you would miss so much of What's happening in the world and we can Generalize this but we're just for now To focus on dark matter it's something

We know is there and it's not just one Way we know it's there in my book dark Matter and the dinosaurs I talk about The many different ways you know this Eight or nine that we we deduce not just The existence of dark matter but how Much how much is there and they all Agree now how do we know it's there Because of its Gravitational force and individually a Particle doesn't have such a big Gravitational force in fact gravity is An extremely weak force compared to Other forces we know about in nature but There's a lot of Dark Matter out there It carries a lot of energy five times The amount of energy as the matter we Know that's in atoms Etc so You can ask how should we think about it Well it's just another form of matter That doesn't interact with light or at Least as far as we know so it interacts Gravitationally it clumps it forms Galaxies but it doesn't interact with Light which means we just don't see it And most of our detection before Gravitational wave detectors we only saw Things because of their interactions With light in some sense so in theory it It behaves just like any other matter Just it just doesn't interact with light So when we say it interacts just like Any other form of matter we have to be Careful um because gravitationally it

Interacts like other forms of matter but It doesn't experience electromagnetism Which is why it has a different Distribution so in our galaxy it's Roughly Spherical unless it has its own Interactions that's another story but we We know that it's roughly spherical um Whereas ordinary matter can radiate and Clumps into a dis and that's why we see The Milky Way desk so on large scales in Some sense yes all the matter is similar In some sense in fact dark matter is in Some sense more important because it can Collapse more readily than ordinary Matter because ordinary Matter has has radiative forces which Makes it hard to collapse on small Scales so actually it's dark matter that Sort of drives um Galaxy formation and Then ordinary matter kind of comes along With it um and there's also just more of It and because there's more of it it can Start collapsing sooner that is to say The energy density in Dark Matter Dominates over radiation earlier than You would if you just had ordinary Matter so it's part of the story of the Origin of a Galaxy part of the story of The end of the Galaxy and part of the Story of all the various interactions Through exactly I mean in my book I make Kind of sort of jokes about you know It's like when we think about a building

We think about the architect we think About you know the high level but we Forget about all the workers that did All the grunt work and in fact Dark Matter was really important in the Formation of our universe and we forget That sometimes that's a a metaphor on Top of a metaphor Okay the the the unheard voices that do The actual work okay exactly no but it Is a metaphor but it also captures Something because the fact is we don't Directly see it so we forget it it's There or we don't understand it's there Or we think it's not the fact that we Don't see it makes it no less legitimate It just means that we have challenges in Order to find out exactly what it is Yeah but the things we cannot see that Nevertheless have uh gravitational Interaction with the things we can't see Is at the uh Layman level is just mind-blowing you Know it is and it isn't because I think What it's teaching us is that we're Human the universe is what it is and We're trying to interact with that Universe and discover what it is we've Discovered amazing things in fact I Would say it's more surprising that the Mount the matter that we know about is Constitutes as big a fraction of the Universe as it does I mean we're limited We're human and the fact that we see 5%

Of the energy density of the Universe um You know about one six of the energy Density in matter that's kind of Remarkable I mean why should that be There could be anything out anything Could be out there yet the universe that We see is the significant fraction yeah But a lot of our intuition I think Operates using visualizations in the Mind that's absolutely true and it's Certainly writing books I realized also How many of our words are based on how We see the world um and that's true and That's actually one of the Fantastic Things about physics is that it teaches You how to go beyond your immediate Intuition to develop intuitions that Apply at different distances different Scales different ways of thinking about Things yeah how do you anthropomorphize Dark matter how do I just did I think I Made it the grun work workers oh yeah That's good yeah you did that's why he Get paid the big bucks with a and write The great books okay so um you also Write in that book About uh dark matter matter having to do Something with the extinction events the Extinction of the Dinosaurs um which is kind of a Fascinating presentation of how Everything is connected so I guess the Dist disturbances From the dark matter they create

Gravitational disturbances in the OR Cloud at the edge of our solar system And then that increases the rate of Asteroids hitting Earth so I want to be Really clear this was a speculative Theory I love it though I mean and I Liked it too and um and we still don't Know for sure but we can what we liked About it so let me take a step back so We usually assume that dark matter is What we being physicists that just one Thing it's just basically non Interacting in on aside from gravity or Very weakly interacting matter um but Again we have to get outside this Mindset of just humans and ask what else Could be there and so what we suggested Is that there's a fraction of dark Matter not all the dark matter but some Of the Dark Matter maybe it has Interactions of its own just the same Way in in our universe we have lots of Different types of matter we have nuclei We have electrons we have mu we have Forces um we have lots of it's it's not A simple model the standard model but But it does have some basic ingredients So maybe Dark Matter also has some Interesting um structure to it so maybe There's some small fraction and the Interesting thing is that if some of the Dark Matter does radiate and you know I Like to call it dark light because it's Light that we don't see but Dark Matter

Would see it could radiate that and then It could perhaps collapse into a disc The same way ordinary matter collapsed Into the Milky Way disc so it's not all The dark matter it's a fraction but it Could conceivably be a very thin disc of Dark Matter thin dense disc of dark Matter and so then the question is do These exist and people have done studies Now to think about whether they can find Them I mean it's an interesting Target It's something you can measure by Measuring the positions and velocities Of stars you can find out what the Structure of the of the Milky Way is um But the fun proposal was that the solar System orbits around the Galaxy and as It does so it goes a little bit up and Down kind of like horses on a Carousel And the suggestion was every time it Goes through you have an enhanced Probability that you would dislodge Something from the edge of the solar System and something called the or Cloud So the idea was that at those times You're more likely to have these Cataclysmic events such as the amazing One that actually caused the last Extinction that we know of for sure it Wasn't so amazing for the dinosaurs or For two-thirds of the species on the Planet yeah but it gets amazing for Humans who wouldn't be what really is Amazing I mean I I do I mean I talk

About this in dark matter in the Dinosaurs I it's just an amazing Scientific story because it really is One of the real stories that combine Together different fields of science Geologists at the time or you know People thought that things happened Slowly and this would be a cataclysmic Event and also I have to say you know if You if you think about it it sounds like A story like a 5-year-old would make up Maybe the dinosaurs were killed by some Big rock that came and hit the Earth but Then there really was a scientific story Behind it and that's also why I like the Dark disc because there's a scientific Story behind it so as farfetched as it Might sound you can actually go and look For the experimental consequences or the Observational consequences to test Whether it's true I wish you could know Like high resolution details of where That asteroid came from like where in The OR Cloud why it happened is it in Fact because of dark matter it's like The full tracing back to the origin of The universe CU it humans seem to be Somewhat special but just it seems like So many fascinating events at all scales At all scales of physics had to happen For so I'm really really glad you Mentioned that because actually that was One of the main points of my book dark Matter in the dinosaurs one of the

Reasons I wrote it was because I really Think we are abusing the planet we're Changing the planet way too quickly and Just like anything else when you alter Things it's good to think about the History of what it took to get here and And as you point out it took many Operations on many different scales you Know we had to have the formation of of Structure the formation of galaxies the Formation of the solar system the Formation of our planet the formation of Humans I mean there's so many steps that Go into this and humans in some part Were the result of the fact that this Big object hit the earth made the Dinosaurs go extinct and mammals Developed I mean it is an incredible Story And yes something else might come of it But it won't be us if we mess with it Too much but it is an a grand scale Earth is a pretty resilient System uh C can you just clarify just Fascinating uh the shape of things so The shape of the Milky Ways of the Observable stuff is mostly Flat and then you said Dark Matter tends To be spherical but a subset of might be A flat disc so you want to hear about The shape of things yes yes please so Structure formed early on and now our Structure that we live in is so we know About the Milky Way galaxy so the Milky

Way galaxy has the dis you can see in a Dry dark place that's where stars and Light is but you can also measure in Some ways the dark matter and we believe That dark matter is more or less Spherically Distributed um and like we said there's A lot of it not necessarily in the dis But just because it's a sphere there's a Lot of it sitting there and the reason It doesn't collapse as far as we know is That it doesn't really it can't radiate The same way so because it can radiate Ordinary matter collapses and it's Actually because of conservation of Angular momentum it be it stays a dis And it doesn't just collapse to the Center so our suggestion was that maybe There are some components of dark matter That also radiate like I said that's far From proven people have looked for a Disc they see some evidence of some Discs of certain densities but I see These are all questions that are worth Asking basically if we can figure it out From existing measurements why not try Okay so there's not all Dark Matters Made the same this well that's a Possibility we actually don't know what Dark matter is in the first place we Don't know what most of it is we don't Know what a fraction is I mean it's hard To measure why is it hard to measure for Exactly the reason you said earlier we

Don't see it so we want to think of Possibilities for what it can be if Especially if those give rise to some of Observational consequences I mean it's It's a tough game because it's not Something that's just there for the Taking you have to think about what it Could be and how you might find it and The way you detect it is gravitational Effects on things we can see that would Be the way you detect the type of Dark Matter I've been talking about people Have suggestions for other forms of dark Matter they could be particles called Axons they could be other types of Particles and then there are different Ways of detecting it I mean the most Popular candidate for Dark Matter Probably until pretty recently because They haven't found it is something Called wimps weekly interacting mass of Particles um particles that have mass About the same as the Hig boson Mass um And it turns out then you would get About the right density of dark matter But then people really like that of Course because it is connected to the Standard model the particles that we Know about and if it's connected to that We have a better chance of actually Seeing it fortunately or unfortunately It's also a better chance that you can Rule it out because you can look for it And so far no one has found it we're

Still looking for it is that one of the Hopes of the large hyron collider that Was originally one of the hopes of the Large hyron collider I'd say at this Point it would be very unlikely given What they've already accomplished um but There are these um underground detectors Xenon detectors that look for Dark Matter coming in and they They are going to try to achieve a much Stronger bound than exists today just uh Take that tangent Looking Back Now What's the biggest to you Insight uh to humanity that the LHC has Been able to provide it's interesting It's both um a major victory um the Hig Bosone was proposed 50 years ago and it Was discovered the higs mechanism seemed To be the only way to explain Elementary Particle masses and it was right so on The one hand it was a major victory on The other hand I've been in physics long Enough to know it was also a cautionary Tale in some sense because um at the Time I started out in physics we had Some proposed something in the United States called the superconducting super Collider a lot of physicists I'll say Particularly in Europe but I'd say a lot Of physicists were Sayang when that the Large hyron collider would have the Energy reach necessary to discover what Underlies the standard M we don't want To just discover the we want to know

What the next step is and I think here Um people were more cautious about that They wanted to have a more comprehensive Search that could get to higher energies More events so that we could you know we Could really more definitively rule it Out but in that case many people thought They knew what would be there it would Happen to be a theory called super Symmetry so a lot of physicists thought It would be super symmetry I mean it's One of the many factors I think that Went into the fact that the large hron Glider became the only machine in town And um the super conducting super Collider would have just been a much if It really had achieved what it was Supposed to would have been a much more Robust test of of the space so so I'd Say for Humanity it's both a tribute to The ability of Discovery and the ability Of really believing in things so that You have the confidence to go look for Them but it's also a cautionary tale That you don't want to you know assume Things before they've been actually Found so you want to do things in you Know you you want to believe in your Theories but you also want to question Them at the same time in ways that You're more likely to discover the truth But it's also an illustration of grand Engineering efforts that Humanity can Take on and maybe a lesson that you

Could go even Bigger um I I'm really glad you said That though too because that that's Absolutely true I mean it's it really is An impressive it's it's impressive in so Many ways it's impressive Technologically is impressive at Engineering level it's also impressive That so many countries work together um To to do this it wasn't just one country And how it was it was also impressive in That it was a long-term project that People committed to and made it happen So it is a demonstration that when People set their minds to things and They commit to it that they can do Something amazing but also in the United States uh maybe a lesson that Bureaucracy can slow things down to Bureaucracy and and politics politics And economics many many things can make Them faster and make them slower so Science is the way to make progress Politics is the way to slow that Progress down and here we are well I Don't want to overstate that because Without Politics the you know the L Wouldn't happen either so um but you Need broccoli Um but sometimes I do think I mean You're not asking this question but Sometimes I do think when I you know Think about some of these conflicts you You know sometimes it's just good to

Have a project that people work on Together and there were some efforts to Do that with in science too to have Palestinians and Israelis work together Project called Sesame um I think it's Not a bad idea when you can do that um When you can get you Know sort of for forget the politics and Just focus on some particular project Sometimes that can work some kind of Forcing function some kind of deadline That gets people to sit in room together And you're working on a thing but as Part of that you realize the common Humanity that you all have the same Concerns the same hopes the same fears The same that you all human and that's An accidental side effect of working Together on a thing I that's absolutely True and it's one of the reasons CERN Was formed actually it was post World War II and lot of European physicists Had actually left Europe and they wanted To see Europeans work together and and Sort of rebuild and and it worked I mean They they did and it's true and I often Think that that you know one of the Major problems is we just don't meet Enough people so that everyone think Seems like when they seem like the other It's more easy to forget their Humanity So I think it it is important to have These Connections given the

Complexity all cosmological scales Involved here that led to the extension Of the dinosaurs when you look out at The future of Earth do you worry about Future Extinction events I I do think That we might be in the middle of an Extinction right now if you define it by The number of species that are getting Killed off and it's subtle but you know It's a complex system the way things Respond to events is sometimes things Evolve sometimes animals just move to Another place and the way we've Developed the Earth it's very hard for Species just to move somewhere else and We're seeing that with people now too I Mean I know people people are worried Just about AI taking over and that's a Totally different story we just don't Think about the future very much we Think about what we're doing now and we Certainly don't think enough about all The animals that we're destroying all The things that are precursors to humans That we sort of rely on it's interesting To to think whether the the the things That threaten us is the stuff we see That's happening gradually or the stuff We don't really see that's going to Happen all of a sudden I sometimes like Think about what is what should we be More worried about cuz it seems like Like with the asteroids or nuclear war It could be stuff that just happens one

Day you know when I say one day meaning Over a span of a few days or a few Months but on a you know not on a scale Of decades and Centuries because we sometimes mostly Talk about stuff that's happening Gradually but we can be really Surprised it's actually really Interesting and that was actually one of The reasons it took a while to determine What it was that had caused the last Extinction because people did think at The time many people thought that things Were more gradual and the idea of Extinction was a very was actually a Novel concept at some point you know I Mean these aren't predictable events Necessarily they're only predictable on A grand scale um but sometimes sometimes They are and and I think people were Pretty aware that nuclear um weapons Were dangerous I'm not sure people are As aware now as they were you know say 20 or 30 years ago and that certainly Worries me um I have to say I was not as Worried about AI as other people but now I understand and it's not I mean it's More that as soon as you create things That we lose control over it's scary and The other thing that we're learning from The events today is that is that it Takes a few bad actors it takes everyone To sort of make things work well well it Takes not that many things to make

Things go wrong it's it's the issue with Disease you know we can find out what Causes a disease but to make things Better is not necessarily that simple Sometimes it is but for things to be Healthy a lot of things have to work for Things to go wrong only one thing has to Go wrong and so it's amazing that we do It and the same is true for democracy For democracy to work a lot of people Have to believe in it a few bad actors Can destroy things sometimes so a lot of The things that we really rely on are Delicate equilibrium situ some of you Know and there is some robustness in the Systems we try to build in robustness But a few extreme events can sometimes Alter things and um I think that's what People are scared of today in many ways They're scared of it for democracy They're scared of it for peace they're Scared of it for AI I think they're not As scared as they should be about Nuclear weapons to be honest um I think That's a more serious danger Than People Realize um I think people are a little Bit more scared about pandemics than They were before um but I still say They're not super scared about it so You're right there are these major Events that can happen and we are Setting things up so that they might Happen and we should be thinking about Them the question is who should be

Thinking about them how should we be Thinking about them how do you make Things happen on a global scale because That's really what we need it certainly Shouldn't be a source of division it Should be a source of grand Collaboration probably wouldn't that be Nice yeah I just wonder what it would be Like a to be a dinosaur it must have Been beautiful to like look at the Asteroid just enter the atmosphere until Like everything just man would I that Would be one of the things I would Travel back in time to you know that's Also one of the things that I think you Probably could do with virtual reality I Don't think you have to be there and get Extinct just experience it I think There's something you know it's an event You're just watching you're not doing Anything you're just looking at it so Maybe you could just recreate it I Actually heard that there a there's a Nuclear weapon explosion experience in Virtual reality that's good to remind You about like what it would feel like I Have to say you know so I got I got an Award from the Museum of nuclear history And technology in the southwest and I Went to visit the museum which turned Out to be mostly a museum of nuclear Weapons and the scary thing is that they Look really cool you know it's true that You have that yes this is scary but you

Also have this this is cool feeling and I think we have to get around that Because you I kind of think that yes you Can be in that but I'm not sure that's Going to make people scared has it have They actually asked afterwards are you More or less scared yeah that's a good Uh it's a really good point I mean That's a good summary of just Humanity In general We attracted to creating cool stuff even Though it's can be dangerous and Actually that was the really interesting Thing about visiting that museum Actually I it was very nice cuz I had a Tour from people who had been working There in the cold war and actually one Or two people from the Manhattan Project It was a very cool tour and you just Realize just how just the thing itself Gets you so excited I think that's Something that sometimes these movies Miss just the thing itself you're not Thinking about the the overall Consequences and it was kind of like in Some ways it was like the early Silicon Valley you know people were just Thinking like what if we did this what If we did that and you know not keeping Track of like what the peripheral Consequences are and you definitely see That happening with AI now I mean I Think that was the moral of the battle That just happened that you know it's

Just Full Speed Ahead which gives me a Really great transition to another quote In your book so you you write about it The experience of facing the sublime in Physics and you quote uh Ryan AR quote for beauty is nothing but the Beginning of Terror which we are still Just able to endure and we're so OD Because it's serenely disdains to Annihilate us it's pretty intense it I Think applies to nuclear weapons but it Also I mean at a more mundane perhaps Level I think it applies you know it's Really interesting one of the things I Found when I wrote these books is you Know some people love certainty you know Scientists kind of many Revel in Uncertainty it's not that you want to be Uncertain you want to solve it but You're at this Edge where it's really Frustrating because you don't really Want to not know the answer but of Course if you knew the answer that would Be it would be done so you're always at This Edge where there's you're trying to Sort things out and there is something Scary you don't know what is you don't Know if there's going to be a solution You don't know if you're going to find It so it's not something that can Destroy the Earth it's just something That you do on your individual level but Then of course there are much bigger Things like like the ones you're talking

About where they could actually be Dangerous the stuff I do I just want to Be clear I'm doing theoretical physics Not very dangerous um but sometimes Things end up having bigger consequences Than you think yeah But dangerous in a very pragmatic sense But isn't it still in part terrifying When you think of a just the size of Things like the size of dark matter like The the the power of this thing in terms Of Uh is is potential gravitational effects Just this Co cosmological objects of a Black hole at the center of our galaxy So this might be where why I'm a Physicist or why I differ from other People because I'm not such a big fan of Humanity in some ways some ways I am but The idea that we were everything would Be really boring to me I love the idea That there's so much more out there that There's a bigger universe and there's Lots to discover and that we're not all There is wouldn't it be disappointing if We were all there is yeah and the the Full diversity of other stuff it's Pretty interesting you know we have no Idea how much there is you know we we Know what we what we can observe so far So the idea that there's other stuff out There that we yet have to figure out It's exciting well let me ask you out There

Question okay so if you think of like Humans on Earth life on Earth as this Pocket of complexity that Emerged you know and there's a bunch of Conditions that came to be and there's Uh darwinian Evolution and however life Originated do you think it's possible There's some pockets of complexity of That sort inside dark matter we can't See well so that's Possible um I chemistry and biology Evolving in different ways and that's One of the reasons we suggest I mean It's not the reason but it would be true If there were the type of interactions We suggest I mean it would need more Complex ones and we don't know um I will Say that the conditions that give rise To life and complexity they're complex They're unlikely um so it's not like There's great odds that would happen but There's no reason to know that it Doesn't happen it's worth investigating Are there other forces that exist in the Dark matter sector that's exactly so the Dark Matter sector doesn't have all the Forces of the the standard model of Physics right as far as we know it Doesn't have It might have it at some low level but It could have its own forces just like The dark matter might not experience our Light maybe it has its light that we Don't experience so there could be other

Kinds of forces I mean there could be Other kinds of forces even within our Sector that are too weak for us to have Discovered so far or that exist at Different scales than we know about I Mean we detect what interacts strongly Enough with our detectors to detect so It's worth asking and and that's one of The reasons we build big colliders to See are there other forces other Particles that exist say at higher Energies at shorter distance scales than We've explored so far so it's not just In the dark matter sector Even in our Sector there could be a whole bunch of Stuff we don't yet Know so maybe let's zoom out look at the Standard model of pH of particle physics How does Dark Matter fit into first of All what is it can you explain what the Standard model is so the standard model Of particle physics is basically tells Us about Nature's most basic elements And their interactions and so it's the Substructure as far as we understand it So if you look at atoms we know they Have nuclei and electrons nuclei have Protons and neutrons in them protons and Neutrons have particles called quirks That are held together by something Called the strong force they interact Through the strong force Force the Strong nuclear force there something Called the weak nuclear force and

Electromagnetism so basically all those Particles and their interactions Describe many many things we understand That's the standard model we now know About the higs boson which is associated With how Elementary particles get their Mass so that pce of the puzzle has also Been Completed we also know that there are Kind of a weird array of masses of Elementary particles there's not just The up and down but there are heavier Versions of the up and down cork charm And strange top and bottom there's not Just the electron there's a muon and a Tow there are particles called neutrinos Which are under intense study now which Are partnered with the leptons through The weak interactions so we really do Know these basic Elements and we know the forces we know I mean when we're doing particle physics Experiments we can usually even ignore Gravity except in exceptional cases that We can talk about so those are the basic Elements and their interactions Dark Matter stands outside that it's not Interacting through those forces so when We look at the world around us we don't Usually see the effects of dark matter It's because there's so much of it that We do and it doesn't have those forces That we know about but the standard Model has worked spectacularly well it's

Been tested to a high degree of Precision people are still testing it And one of the things we do as Physicists is we actually want it to Break down at some level we're looking For The Precision measurement or the energy Or whatever it will take where those Where the standard model is no longer Working like not that it's not working Approximately but we're looking for the Deviations and those deviations are Critical because they can tell us what Underlies the standard model which is What we really want to see next where Can you find the the places where the Standard model breaks down like uh what Are the what are the places you can see Those tiny little deviations so we don't Know yet but we know the kinds of things You wouldn't want to look for so one Obvious place to look is at higher Energy um we're looking at the large Hron collider but we'd love to go beyond That higher energies means shorter Distances and it means things that we Just couldn't produce before I mean eals Mc^2 so if you have a heavy particle and You don't have enough energy to make it You'll never see it so that's one place The other place is precision Measurements if you you know the Standard model has been tested Exquisitly so if been if it's been

Tested 1% you want to look at a tenth of A percent MH and there are some Processes that we know shouldn't even Happen at all in the standard model or Happen at very suppressed level and Those are other things that we look for So all of those things could indicate There's something beyond what we know About which of course would be very Exciting when you just step back and Look at the standard model the quirks And all the different particles and Trinos And isn't it wild how this like little System came came to being creates is Underpin depends everything we see Absolutely and that's why we'd like to Understand it better we want to know is It part of some bigger sector um why are These particles why do they have the Masses they do yeah um why is the hibos On so light compared to the mass it Could have had which we might have even Expected based on the principles of Special relativity in quantum mechanics So that's a really big question why are They what they are and and they Originate there's like some mechanism That created the whole thing that's one Of the things we're trying to study why Why is it what it is I mean even just Like the mechanism that creates stuff Like the way a human being is Created From a single cell it's like embryo

Genesis like the whole thing like bu you Build up this Thing all of it this whole thing comes To be from just like a don't forget it Is interacting with the environment sure Okay right right right it's not it's Right it's important well that's a Really good question is how much of it Is the environment is it just just the Environment acting on a set of Constraints And uh like how much of it is just the Information in the DNA or the Information how much is it in the Initial conditions of the universe or uh Versus the the some other thing acting On it these are Big questions these are Big questions in pretty much every field Um you know we for the universe we do Consider it you know it's everything There is by definition but people now Think about it is it one of many Universes um and of course it's a Misnomer but could there be other places Where there are self-contained Gravitational systems that we don't even Interact with so but those are really Important questions and the only way We're going to answer them is you know We go back as far as we can we try to Think theoretically and we try to think About observational consequences it's All we can do one interesting way to Explore the standard models to look at

Your uh fun nuanced disagreement with With Carlo Relli when you talked about him writing In his book electrons don't always exist They exist when they interact they Materialize in a place when they collide With something else and you wrote that Well I'll just read the whole thing cuz It's it's kind of interesting stocks may Not achieve a precise value until they Are traded but that doesn't mean we Can't approximate their worth until they Change hands similarly electrons might Not have definite properties but they do Exist it's true that the electron Doesn't exist as a classical object with Definite position until the position is Measured but something was there which Physicists use a wave function to Describe it's a fascinating Nuance Disagreement so do electrons always Exist or not does it does a tree fall in The forest if nobody's there to see it So I like to think of the universe is Being out there whether or not I mean it Would be really weird if the only time Things came into existence was when I Saw them or I measured them there's a Lot of weird I mean I could believe that The Middle East doesn't exist because I'm not there now I mean that would be Kind of ridiculous I think we would all Agree on that so I think there's only so Much that we can attribute to our own

Powers of seeing so and the whole system Doesn't come into being because I'm Measuring it and so what is weird and This isn't even a disagreement about the Standard model this is agreement about How you interpret quantum mechanics I Mean I would say that those wave Functions are real I mean one of the Things that forget that particle physics Does that Quantum field Theory says is That electrons can be created and Destroyed it's it's not that every Electron has to be in the universe I Mean there can be I mean that's what Happens at collider is particles get Created and destroyed um but that Doesn't mean that if I electron in an Atom it's not there it's certainly there And we know about it its charge is there So physics is a kind of way to see the World so what at the bottom what's the Bottom Turtle like what do you have a Sense that there's a bottom reality that We're trying to approximate with physics I think we always have in our head maybe That we'd like to find that but I have To I mean I might not seem so but I Think I'm kind of more humble than a lot Of physicists I'm not sure that we're Ever going to get to that bottom level But I do think we're going to keep Penetrating different layers and get Further I just wonder how far away we Are you know we all wonder that um it's

Not even like what's even the measure of How far away we are I mean one way you Can measure it is just by our everyday Lives in terms of our everyday lives We've measured everything in terms of What underlies it there's a lot more to See and so part of it has to do with how Far we think we can go I mean it might Be that the nature of reality changes so Much that even these terms are different Maybe we measure you know this the Notion of distance itself might break Down at some point but also to push back On the we've measured everything maybe There's stuff we haven't even considered As measurable for example Consciousness Or did There's there might be stuff just like You said forces unseen undetected so It's an interesting thing so one and This is often a confusion that happens So there's sort of the fundamental stuff Underlying it and then there's sort of The higher levels what you know what We'll call like an effective Theory at Some level you know so we're not always Working I mean when I throw a ball I Don't tell you where every atom is I Tell you there's a ball and so there Might be different layers of reality That are built on ter terms of the Matter that we know about in terms of The stuff we know about that and when I Say we've measured everything I I say

That with a grain of salt I mean I Measure everything St so so there's lots Of phenomena that we don't understand That we but often there are complex Phenomena that will be given in terms of The fundamental ingredients that we know About but that is an interesting Question because yes there's phenomena That are at the higher level of Abstractions that emerge but maybe like With consciousness there is far out People that you know think that Consciousness is um panus right that It's there's going to be almost like a Fundamental force of physics that's Consciousness that prates all matter Right usually when you have a crazy Sorry okay when you have a far out Theory yes the thing you do is you test All the possibilities within the Constructs that exist yeah so you don't Just jump to the most far out Possibility I mean you can do that but Then to see if it's true you either have To find evidence of it or you have to Show that it's not possible without That uh and we're very far from that I Think one of the criticisms of your Theory on the dinosaurs was that it Requires if I remember correctly for Dark matter to be weirder than it Already is and then I think you had a Clever response to that can you can you Remind I'm not sure I remember what I

Said then but I mean we have no idea how Weird dark batter is I mean it's based On everyone thinking they know what dark Matter is I mean so weirder than it Already is I mean it's not already Anything thing we don't know what it is So there's no normalization Here so dark matter do we know that if Dark Matter varies in density it Certainly does in the universe just like I mean so for example there's more dark Matter in galaxies than there is between Galaxies so it it it clumps I mean it's So it's it's matter so it's distributed Like matter it is matter it does Clump But the the uh the full details of how It clumps and the complexity of the Clumping it's understood Pretty well people do simulations there I mean where where people are are always Looking for things including us as Particle physics it's sort of at small Scales are the deviations on small Scales so that indicating other Interactions or other processes or Interactions with Barons that is to say Normal matter that we don't understand But on large scales we have a pretty Good understanding of Dark Matter Distribution you were part of a recent Debate on quote cons science uncover Reality let me ask you this question Then uh what do you think is the limits Of

Science I'm smart enough to know I have No idea and also it's not even clear What science means right because there's The science that we do which is partical Physics we try to find fundamental Things and figure out what their effects Are there science like biology where you Know it's at a higher level the kind of Questions you ask are different the kind Of measurements are different um the Kind of science that's going to happen In the sort of more numerical age I mean Or even AI or like what does it mean to Answer a question does it mean that we Can predict it does it mean we can Reproduce it so I think we're coming up Against sort of the definition of what We mean by science as human beings so in Terms of the science that we can do I I Don't think we'll know it until we get There um you know we're trying to solve Hard problems and you know we've made Progress I mean if you think of how much Science has advanced in the last century Or century and a half it's incredible I Mean you know we didn't even know the Universe was expanding at the beginning Of the 20th century we didn't know about Quantum mechanics at the beginning of The century we didn't know about special Relativity that's a lot in a relatively Short time on depending on how you think Of time um so I think it would be Premature to say we know the limitations

And at various points throughout that History we thought we solved everything Or declared or at least various people Various people exactly yeah declared That we've solved everything uh so this Also a good place to maybe could you Describe the difference between top down And bottom up approaches to theoretical Physics that you talked about in the Book so you could try to jump in and say I have a theory that I think is so Perfect um that I can predict everything From it or at least predict some salian Features from it that's top down that Would be top down bottom up is more like You know like the questions we just ask Why your masses what they are we measure Things we want to put them together mhm And usually a good approach is to Combine the two if you ask a very Specific question but combine it with The methods of knowing that there could Be a fundamental Theory underlying it Sometimes you make progress I mean some You know the community tends to get Segmented or fragmented into people who Do one or the other but there are Definitely times I mean some of my best Collaborations have with with people who Were more top down than I am so that we Come up with interesting ideas that we Wouldn't have thought of if either one Of us was working individually would you Say the truly big leaps happen top down

Like Einstein Einstein was not a top Down person in the beginning he was um Special relativity was very much him Thinking about you know they were Thought experiments but he was very much You know the original theory about Relativity is something like on the Nature of electromagnetism he was trying To understand how Maxwell's laws could Make sense when they were you know seem To have different symmetries than what We had thought they were so he was very Much about bottom up person and in fact He resisted top down for a long time Then when he tried to do the theory of General relativity or the general theory Of relativity whichever you want to call It um incorporating gravity into the System where you need some feedback then He was helped by a mathematician who had Developed some differential geometry and Helped him figure out how to write down That and after that he thought top down Was the way to go but he actually didn't Make that much progress um but certainly So I think it's you know naive to think It was just one or the other in fact a Lot of people who made real progress Were rooted in actual measurements well Speaking of mathematicians uh what to Use a difference because you've had a Bit of foot in both uh between physics And Mathematics in the way it helps us Understand the world well there's to be

Frank there's a lot more overlap in Physics and math I think than has been I Mean well maybe not more but there's Certainly a lot but I think again the Kinds of questions you're asking are Usually different um mathematicians like The structure at itself physicists are Trying to concentrate on to some extent On the consequences for the world um but There is a lot of overlap the string Theory is an example there's certain Theories where there's a a certain kind Of mathematical Beauty to it there's Also you know there's also some really Cool ideas that you get in particle Physics where you can describe what's Going on and connect it to other ideas That's also really beautiful um you know I think I think basically insights can Be beautiful um you know they might seem Simple but sometime and sometimes they Genuinely are and sometimes they're Built on a whole system that you have to Understand before I mean if you actually Saw Einstein's equations written out in Components you wouldn't think it's so Beautiful you write it in a compact way It looks nice uh what do you think about The successes and the failures of string Theory to what degree do you think it's Succeeded to what degree is it not Succeeded yet or has failed I think to Talk about any science in terms of Success and failure often misses the

Point because there's not some absolute Thing and I think I do think that Strength theorists were a bit overly Ambitious not overly ambitious but a Little bit overly arrogant in the Beginning thinking they could solve many Problems that they weren't going to Solve that's not to say the methods and Um advances in strength Theory don't Exist and um but they certainly weren't Able to immediately solve all the Problems they thought they could solve But it has given us tools it has given Us some insights um but it becomes Almost a sociological question of like How much it should be one or the other I Do think that you can get caught up in The problems themselves and and Sometimes you can get caught up in the Methods and just sort of do other Examples so the real physics insights Often come from people who are thinking About physics as well as as math because You mention AI is there Hope that AI might be able to help Find some interesting insights I mean Another question another way to ask this Question is how special are humans that Were able to discover novel insights About the World that's a great question um and it Depends what kind of insights and we're Going to find that out I mean you know It's because it's hard to think about

Something that doesn't quite exist yet I Mean I could just think about something Take a step back you know it's a little Bit like trying to understand three four Dimensions you go back to three Dimension you know so go to something You can imagine so you can sort of say a Lot of the things in a very different Level about the internet you can say you Know has the internet helped do things And that you know it definitely took on A life of its own in some sense but it's Also something that we're able to tame You know I know that I myself wouldn't Have been able to write books if the Internet didn't exist because I wouldn't Have had the time to go to the library And look everything up and um it helped Me enormously and in some sense AI could Be that in a very nice World it could be A tool that helps us go a step further Or than we would and a lot more Efficiently and it's already done that To some extent or it could be like the Parts of the internet that we can't Control that are ruining politics or Whatever so and there's certainly a lot Of indications that can do that then There are even bigger things that you Know people speculate about about AI Being able to do its own things but in Terms of actually figuring things out um You know we're in the early Stages yeah there's several directions

Here one is like on the theorem prover Side so wolf from alpha where everything Is much more precise and when you have Large language model type of Stuff one of the limitations of those is It seems to come up with convincing Looking things which we don't know if It's true or not right and that's a big Problem for physics so large languin Models are more or less like Generalizations of stuff that we have so The question is so there's still Breakthroughs in AI waiting to happen And maybe they are happening and maybe They'll be good maybe not but that's not Quite the same I mean maybe just some in Some cases it's just pattern recognition That leads to important things but Sometimes it could be something more Insightful than that that I can't even Put my finger on so it forces us to I Mean we don't really understand how Smart we are we don't understand how we Think about things all that well Actually but one thing is true that we Are a lot more efficient right now than Computers and coming up with things we Require a lot less energy to do that so If computers figure out how to do that Then it's going to be a totally Different ball game so and so there are Clearly kinds of connections that we Don't know how we're making but we are Making them and so that's going to be

Interesting so um you know I say we're In early stages but this is changing Very rapidly but right now I don't think That it's actually you know we Discovered like new laws of physics but Could in the future maybe it can it will Raise big questions About what is special about humans that We don't quite appreciate you know with There could be things that are like that Leap of insight that happens truly novel Ideas that could potentially be very Difficult to do So there are sort of abstract questions Like that there's also questions of how Is it that we can address to some extent You know how will AI be used in the Context of the world we live in which is Based on you know at least our country Is based on capitalism and a certain Political system and how will global Politics deal with it how will our Capitalist system deal with it what will Be the things that we focus on doing With it how much will researchers get Control of it um to be able to ask Different sorts of questions I mean you Know while it was starting out people Were doing these kind of toy problems But what will it actually be applied to And what will it be optimized to do There's there's a lot of questions out There that it's really important we Start

Addressing uh what to you Is the most beautiful unsolved problem In physics in cosmology what you is Really exciting if we can unlock the Mystery of in the next few Decades um so is it what's the most Beautiful unsolved problem or what is The most beautiful unsolved problem I Think we can make progress On oh boy we make progress on in the Next few Centuries there's you know this most of The questions the big questions have to Do with what underlies things how things Started what's at the base of it there's Also just basic questions like how that You asked earlier how far will science Take us how much can we understand Um there are questions like how we got Here um what underlies it are there you Know but also I mean there's really deep Questions like you know what fraction Are we actually seeing if there are These other forces if there is another Way of seeing the world are there G you Know universes beyond their own they're If they're so totally different how do We even comprehend them I mean how do we Detect like what would we even think About them So there's a lot about trying to get Beyond it's always just getting beyond Our limited vision and limited Experience and trying to see what

Underlies it both it's small scales and At large scales we just don't know the Answers I mean I'd like to think that We' understand more about Dark Matter About Dark Energy about are there extra Dimensions things that we actually work On but there probably a lot beyond what We work on that's yet to be Discovered yeah understanding the extra Dimensions Piece will be really interesting thing Totally I mean if it is you know how the Universe went from higher Dimensions to What we see it do you know are the extra Dimensions present everywhere I mean you Know one of the really interesting Pieces of physics we did that I talk About my first book War passages is Finding out that there can be a higher Dimension but only locally do you think There's a gravity of a lower Dimension So it could be like only locally do we Think we live in three dimensions it Could be higher Dimensions is different This it's not actually the gravity we Have but it there's all sorts of Phenomena that might be out there that We don't know about all sorts of Evolution things time dependence that we Don't know about um and of course that's From the point of view of particle Physics from the point of view of other Kinds of physics we're just beginning so Who knows yeah if the physics changes

Throughout is is not homogeneous Throughout the universe that's that Would be weird I mean you know for the Observable universe it's the same but Beyond the observable universe who knows What ADV advice would you Give uh you've had an exceptional career What advice would you give to young People maybe High School College on how to have a career that you Can be proud of and a life that you can Be proud of I think the weird thing About being a scientist or an academic In general is you have to believe really Strongly in what you do while Questioning it all the time you can't You know and that's that's a hard Balance to have sometimes it helps to Collaborate with people but to really Believe that you could have good ideas At the same time knowing they could all Be wrong that's that's that's a tough Tight rope to walk sometimes but to Really um test them out um the other Thing is sometimes you know if you get Too far buried you look out and you Think oh there's so much out there and Sometimes it's just good to bring it Back home and just think okay can I have As good idea as the person next to me Rather than you know the greatest Physicist who ever lived but right now Like you said I think there's lots of Big issues out there and and it's hard

To balance that and Sometimes it's hard to forget the role Of of physics but I think you know Wilson said it really well when he said You know when they were building fmy lab It was like this won't defend the Country but it'll make it worth Defending you know it's just the idea That you know in all this chaos it's Still important that we still make Progress in these things and sometimes You know when major world events are Happening it's easy to forget that and I Think those are important too you don't Want to forget those but to try to keep That balance because we don't want to Lose what it is that makes hum special So that's the big picture would you also Lose yourself in the simple joy of Puzzle solving yeah yeah I mean I that's We all like solving puzzles and actually One of the things that drives me in my Research is the inconsistencies when Things don't make sense it really bugs Me and I just will go in to you know Different directions to see how could These things fit together so it bugs you But that motivates you yeah totally Until it doesn't because I think I have This underlying belief that it should Make sense even though the world comes At you in many ways and tells you Nothing should make sense but if you Believe that it makes sense then you

Look for underlying logic and I think That's just good advice for everything To try to find like why is it the way it Is I think you know we talk I talked About effective theory in my second book Not going have in store a lot you know It's sort of rather than ask the big Questions sometimes we just ask the Questions about the immediate things That we can measure and we can like I Said we can sometimes tell one that We'll fail but we can have these Effective theories and sometimes I think You know when we approach these big Questions it's good to do from effective Theory point you know why do I find this Satisfying why is the world we have the Way it is we think things are beautiful That we live in I mean you know I'm not Sure if we had different senses or Different ways of looking at things we Wouldn't necessarily find it beautiful But I have to say you know it is kind of Fantastic that no matter how many times I see a sunset I will always find it Beautiful it's like I don't think I ever See a Sunset and say h whatever you know It's just always beautiful you'll always You know and so there there are things That humans you know clearly resonate With us but you know we were maybe Evolved that way that's about us um but In terms of figuring out the universe It's kind of amazing how far we've

Gotten you know we have discovered many Many wonderful things um but there's a Lot more out there and I hope I hope we Have the opportunity to keep going and With effective theories one small step At a time just keep unraveling the Mystery but also having in mind the big Questions but doing one small step at a Time exactly yeah looking out to the Stars you said the sunset for me it's The sunset the sunrise and just looking At the stars it's wondering what's all Out there and having a lot of hope that Humans will figure it out right I like It Lisa well thank you for being one of The Humans in the world for uh that are Pushing it forward and uh figuring out This beautiful puzle of ours and thank You for talking today this is amazing Thank you thanks for listening to this Conversation with Lisa randelle to Support this podcast please check out Our sponsors in the description and now Let me leave you some words from Albert Einstein the important thing is to not Stop questioning Curiosity has its own Reason for Existence thank you for listening and Hope to see you next Time

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