The Arrow of Time, Many Worlds, & Black Holes | Sean Carroll

[00:00:00]Something that's fascinated me for decades, ever  since I could think about this question, and   that's the arrow of time. I can have photographs  of the past. I cannot have photographs that are   truly representing what happens in the future.  The early universe, fourteen billion years ago,   near the Big Bang, was in a very special,  organized state. And it's becoming more   disorganized and higher entropy, as we say, ever  since then. That's where the arrow of time comes   This is StarTalk Cosmic Queries Edition. Neil  deGrasse Tyson, your personal astrophysicist. We   got here, of course, Lord Chuck Nice. Hey, what's  happening, man? You all locked and loaded there?   I'm locked and loaded because we got queries,  man. And they're not just queries about anything   to anyone. No. They're queries on, like, cosmology  to one of our cosmologist about town yes one of my   one of our fave interviews we've got sean carroll  on the line sean how you doing man hey how's it   going lord chuck i didn't know you got a promotion  yeah if you try hard like he does you you might   get one too yeah i mean no kings but lords are  okay the occasional lord There you go. That's   right. No, I'm down with the Mill Kings, but I'm  still Lord Nice. So let's catch people up on your   trajectory through life. You spent a lot of your  professional career at Caltech over in Pasadena,   and now you joined us back on the East Coast  in Baltimore at the Johns Hopkins University.   And I've got you as the Homeward Professor of  Natural Philosophy. That's right. There aren't   that many of those. I'm basically the only one,  so it's nice. Right. This is a very retro title.   It is. Okay. I think Newton had a title of natural  philosophy. Right. Before the word physics was a   thing. Yeah. Yeah. And you're one of my favorite  people out there because not only do you bring   science to the public, which is something we  care deeply about here at StarTalk. but you have,   for me, the best combination of astrophysics  fluency, physics fluency, and philosophy fluency.   You put all that together and there's no boring  conversation you will ever have ever. True. But   we're gonna try to change that today. I'm gonna  take this as a challenge. Yeah, I bet I can do   it. So Sean, you had a couple of books recently.  I mean, you're always out there, you know, talking   physics smack with an interested public. You have  2 in a row here, Space, Time, and Motion. That's   what's left after that, right? That's a lot, yeah.  That's pretty much everything, right? But no, not   for Sean Carroll. He's got quanta and fields. Oh,  wow, look at that. Now it is everything. Space,   time, and motion, and then quanta and fields.  What's left? What's left will be volume 3,   which is complexity and emergence. That's what  I'm nearly done with writing right now. So it's   a whole 3part series called The Biggest Ideas  in the Universe, yeah. That's definitely what   that is. For sure. For sure. You know, fields is  a thing that if I didn't study physics, I'd still   think they were kind of imaginary. You go back to  Faraday, right? Who says there's magnetism there,   but there's a field. Well, can I see it? No.  But like the iron filings can see it. but i   can't right okay but if you take away the iron  father is it still there yeah and so just just   to what did it take to get everybody comfortable  with the idea of a field that's a great question   because it wasn't easy it took a while you know  um isaac newton worried about the fact that he   didn't know about the concept of fields. He said  that there was a gravitational force between the   sun and the earth, and it depends on the distance.  The inverse square law, the bigger the distance,   the less the force. But he didn't know how it got  there. How does the earth know where the sun is,   how far away it is, how massive it is? And he  said, this is over my pay grade. I'm going to   leave this for future generations to decide, which  is not the kind of thing that Isaac Newton said   very often. so it wasn't until the eighteen  hundreds so he knew something was up he knew   something needed further explanation yeah action  at a distance you know einstein famously said   spooky action at a distance for quantum mechanics  but even in newton's time there was this weird   thing how what is it that takes the gravitational  force and moves it from the sun to the earth etc   and vice versa yeah And in some way, there was an  answer there from Laplace, Pierre-Simon Laplace.   But it wasn't until Faraday, like you said, that  he starts moving magnets and watching electrical   currents pop up in a wire next to it. Like not  there, not touching it, right? Through empty   space, something happened. And the great thing  about Faraday was he was an absolutely genius   intuitive physicist. He was not the math expert  that you sometimes need to be. So Maxwell, James   Clark Maxwell came along, was a huge admirer of  Faraday and basically made it all mathematically   respectable and said, yeah, there's these things  called the electric field and the magnetic field,   and they fill all of space and you can't see them,  but we can predict what they're going to do. And   they're super duper important for explaining  everything. I think Faraday, if memory serves,   in none of his published papers does an equation  of any kind appear. That's possible. I didn't   know quite that factoid, but it's absolutely  in keeping. He was thinking about, in fact,   it wasn't even fields that he primarily focused  on. he imagined lines of force. So like out of an   electron, there's an electric field we would say  now, but he thought they were like literally lines   of force filling all of space. And Maxwell's first  papers were about trying to make mathematical   sense of lines of force. And he eventually said,  nah, it's better to think of fields with little   vectors. So like little arrows at every point,  and then the lines are sort of moving in the   direction of the arrows. that all happened in the  19th century all happened in the 19th century and   the great thing was you know if you think of the  number of different apparent phenomena in the   world that we now think of as electricity  and magnetism in action right heat light   um radio waves x-rays uh you know the magnets all  this stuff like very very different things all   explained in just 2 fields talking to each other  electricity and magnetism That's amazing. That's   completely crazy. And I was just thinking, I go  through this sort of existential moment maybe once   a month. I'm sitting there and I press a button  on my smartphone and it changes the channel on   my TV. And then I press another button, it starts  my car, which is 3 miles away. And I'm thinking,   this is magic. Yeah, basically. Yeah, and this is  why writing books is good because you write a book   and you say like, you point your remote control at  your TV and a radio wave comes out and turns it on   and you get many emails saying, that's an infrared  wave, not a radio wave. You don't know what   you're talking about. but they're all different  manifestations of electromagnetism. And so- No,   no, wait, wait, but my cell phone is not- Your  cell phone is not, but your remote control is.   Yeah, that's for sure. Yeah, exactly. Who knows  these things? I can't keep track of these things.   I'm just a theoretical physicist. It's funny,  while you guys are talking, I'm sitting here   with my iPad and I'm taking my finger and moving  the screen up and down. Into this void above your   Right. And it's exactly the same thing. Isn't that  the electromagnetic field on my finger? Basically   everything is the electromagnetic field. Other  than gravity, it's all electromagnetism all the   way down. We live it. We just live it. There's  a hugely important philosophy of science lesson   here, because like Neil said, you can't see the  electric field or the magnetic field, but they're   clearly everywhere. Like we have equations that  describe them exactly and make predictions and fit   all the data. Therefore, we accept that they are  there. You don't need to see them with your eyes   to have evidence that they're part of reality. i  i do a whole stand-up bit about that which is i   try to beat that into chuck every time when i say  the universe is under no obligation to make sense   to you which believe it or not i can't accept you  must make sense to me because i am the center of   all things And so, Sean, what's this latest  paper we have you publish here, co-authored,   what Hawking radiation looks like as you fall into  a black hole? Was that something that needed to be   addressed? It's something that I've worried about  for decades. And honestly, really? Yeah. Well,   so here's the question. There's 2 things  that we think are true about a black hole.   One is if you're standing very far away and you  look at the black hole, Stephen Hawking says,   black holes give off radiation. OK, not that much  radiation, admittedly, especially for a big black   hole. But he, again, has an equation that predicts  exactly how much you should see. The second thing   is we have this feeling, no one's ever done it,  but we have this belief that if you fall into a   black hole, you see nothing special when you cross  the boundary, when you cross the event horizon,   right? It just looks like ordinary empty space  everywhere. But these 2 statements seem a little   bit contradictory because if I'm standing far  away and I see radiation coming out and then I   just fall in, I stop my rocket ship or whatever  and let myself fall in, you know, Neil, that you   should see that radiation get blue shifted. It  should look brighter and brighter and more and   more energetic. So why does it turn off when you  hit the event horizon? What actually happens? What   is it that you see? And it turns out this is a  hundred percent, you know, implicit in all the   equations that we have, but took a lot of work to  actually pull it out. and the answer, in a very   short, slightly oversimplified form, is: there is  high intensity radiation when you're crossing the   event horizon, but you're moving so fast that  you don't have time to observe it, so it looks   to you like there's nothing there. wow, yes, mean  in a heisenberg uncertainty principle way, you,   you don't have time to observe it. yeah, exactly  right. and by the way, before i forget, i got to   give huge credit to chris shaloo, my co-author, on  this. he was a grad student at harvard who, just   graduated, did all the heavy lifting on this and  other projects, and he, he was fantastic. so he,   you know, he got the right answer after other  people's got it wrong. graduate students, they   have to have an exception to the slave amendment  in the constitution for graduate students. right,   just so you know. why are they not? are they only  35ths of a person, 35ths of a scientist. yeah,   35ths of a scientist. is that what they are? his  graduate student did the heavy lifting. right   there it is, but see, that's the whole. sean is  in the bahamas and his graduate- and that's the   way it should be- sean already did his work. okay,  that's the whole idea. i would have loved to do   the heavy lifting, but for his sake, i needed to  let him have that experience. you know how it is,   of course. yes, i mean, listen, leonardo da vinci.  uh, when he painted, he had artisans that worked   under him. yeah, and they do a bunch of the work.  yeah, and then you come in and you sign your name.   No, but I will tell you this: that in physics  and astronomy, in our journals, it is not our   tradition to put our degrees after our name, the  way it is in the social sciences. And why is that?   Well, I made up a reason why that's good, but  I don't know if it had different origins. And,   Sean, you might have some insight here If you  didn't otherwise know the people. you have no idea   who is senior, who is junior, because a brilliant  idea can come out of anybody, even your students.   And so there's no reason to segregate who's got  title and who doesn't? because a brilliant idea   is a brilliant idea, no matter the package, and a  stupid idea is a stupid idea No matter who comes   up with it. Yeah, I like that. I like that system.  It's very egalitarian. I like that. Nope, that's   Ready, Do you harbor questions about the universe,  Some long-held bit of cosmic curiosity lingering   within you, Or questions about anything at all?  You can become a Patreon supporter of StarTalk.   You have access to our exclusive question line on  our website. Those are where we draw our questions   for our cosmic learning, Some of the most extreme  objects in the universe. StarTalk is a work in   progress where we every month come up with some  new way, some new idea, some new understanding of   how to bring the universe down to Earth. We could  not do that without your support. Thank you And,   as always, keep looking up, Sean. one other thing  before we get to our Patreon questions, Something   that's fascinated me for decades, ever since  could think about this question, And that's the   arrow of time. Some of my earliest books in middle  school were all about time and how it passes. and   why do we know? time is going forward rather than  backwards, And you have some emergent thinking on   that. I absolutely do. This is something that was  hugely important for me in my career. The arrow of   time is just the fact that the past and future are  different from each other. I can have photographs   of the past. I cannot have photographs that are  truly representing what happens in the future.   AI's, got your future, Lord Chuck can do it, but  I can't do it. So you know why is this true? And   Aristotle wouldn't even have had the question:  Like they're just different things, Like why is   an elephant not an orange? Like what are you even  asking? But once Isaac Newton comes along with his   theory of physics, suddenly the distinction  between the past and future disappears,   Like these equations. don't treat the past and  future differently, Right, Yeah. So it turns out,   long story short, that it's a cosmology question.  The early universe, fourteen billion years ago,   near the Big Bang, was in a very special organized  state, And it's becoming more disorganized   and higher entropy, as we say, ever since then.  That's where the arrow of time comes from. But OK,   why was the early universe like that? And that we  don't know. So I wrote a paper about this 20 years   ago and I'm revisiting that now to try to improve  upon it. We still don't know, but I do think we're   making progress in that direction. Well, I hope  so. after 20 years, That's not very long in the   history of the universe, You know what I was going  to say. Excuse me, I'm pulling the universe on me.   It's a fourth of your life, a fifth of your  life. The universe will wait. We know which   direction time moves, because certain phenomena we  would never see in reverse, Like some famous ones   are. you drop blob of ink in water, the ink will  disperse. You never see that happen in reverse.   So if you're just eavesdropping on a scene and  no one told you which way time was pointing,   just by phenomena that occur, you should be able  to figure it out, like you never see an orange   fall up from the ground back onto a branch.  right, but these are. you know, you're picking   good examples, but, let's admit, you're picking  the easy examples. we understand those examples,   but i can make a choice about what to do right  now. like i could make a choice right now. this   is the most boring podcast ever. i'm going to  storm off and not do it. you won't be the first,   but i can't make a choice now not to have come  on the podcast in the first place. i can't make   a choice that affects the past. oh, look at that.  and and so that's right there, at the boundary of   physics and philosophy, both the physicists and  the philosophers need to, uh, think about this,   and again, we have good ideas, but there's  still work to be done. wow, and now, now   you got me thinking about the multiple timelines  and, uh, parallel universes and every single like,   if we are on a call it a trajectory, every  choice that we make, all those other possible   trajectories from this point of decision still  continue forward, just without me. so what the   hell is happening there? I'm not sure if I'm  quite on board with your explanation there of   what's going on. Well, no, what I'm saying is I'm  being way too literal, Sean. What I'm saying is   the possibilities of all those trajectories. Well,  to the many worlds, hypothesis, Is that what that   is? Is that what I'm talking about? Well, it's  pretty close. I mean many worlds- is like really   down to earth and equation based- There's the  Schrodinger equation of quantum mechanics- tells   you what set of things happen in the future and  which don't. And it's not because you're making   a decision creates a difference between one world  and another, It's because of entanglement in the   underlying quantum structure of reality. But we  do have reason to believe that there are other   copies of the universe where things turned out  a little bit different. and uh, it's. it's. uh,   you can all. if you have an iphone, you can  download an app called universe splitter,   and i'm a big, i'm a big salesman for this app  here and i don't get any commission or anything.   but i think everyone should get it, because when  you're torn between 2 decisions, um, you know,   should I take this job or should I just continue  living a life of leisure? You can ask the phone   and it will plug in these 2 options and it will  send a photon down a beam splitter that has a   fifty, fifty chance of saying you should take the  job or you should just stay sitting around, And   then there'll be. if you do it, if you obey the  instructions, you know there will be a world in   which you did the other thing. That brings me to  my question. You got a question before we start,   Before we even start, And specifically, Did you  pay your 5 dollars a month? I'm going to owe you   In another world. he did In another world I  paid That's funny. All right, Sean, this is   specifically for you. okay, This question? That  is your question. It's my question. first of all,   you have to be familiar with this experiment In  the double-slit, delayed choice quantum eraser   experiment. okay, I know you've been worried  about that one. yeah, No, I really am, Sean, I'm   dead serious. I told Neil this is keeping me up at  night. Sean, that's a thing. It's a thing. Yeah,   it's a thing right. It's the double-slip, delayed  choice quantum eraser experiment. Okay, I'd never   heard of this. Okay, go, Okay. And maybe it's all  I don't know. maybe it's I read about it and I'm   just like. this is crazy. Maybe it's BS? I don't  know. Give it to me All right. So it appears that   the photons know they're being measured Like they  actually know. All right. So the signal photons   and the idler photons show a constructive wave  interference pattern when they're not measured by   a detector. But there's 4 detectors and they can't  know because of the way the detectors are set up,   because the photons are split and some go on not  to be measured and some go on to be measured. OK,   now here's the deal. There is no wave interference  pattern when they are measured. So this makes it   look like they know when they're being observed.  I can understand the whole explanation of   entanglement. I don't know why I should say I  can accept it. I don't know why. it's easy for   me to accept that, but I can just accept it.  What I can't know or understand is a particle,   knowing that it's being watched. So are there any  hypotheses that would explain why this thing with   no. So that's just that your query there sounds  like you just need the double slit experiment.   I'm still distracted by the rest of what it's  called. No, this is beyond the double split,   double slit experiment because, well, maybe I'll  let Sean explain it because I can explain it,   but I'm probably not going to do,  explain it as eloquently as Sean,   the way the experiment is set up. Yeah, the  double slit experiment is, of course, a famous   thought experiment originally. They wrote down  what it should do long before they ever did it   to illustrate the magic of quantum mechanics. You  have a single electron going through 2 slits. It's   detected somewhere on the other side. And it's  just a dot. You can't figure out that much from   the dot. But you do that many, many, many times.  And what you see is an interference pattern.   It's like a wave. went through the 2 slits  and waves go up and down depending on how far   away they are from the target and so a wave going  through one slit can interfere with the wave going   through the other slit and that's what you see in  the distribution of dots on the detector screen   so The miracle occurs when you observe which  slit the electron goes through, right? Because   in the Copenhagen usual way of thinking about  quantum mechanics, when you observe the electron,   you collapse its wave function. It's no longer  going through both slits. You saw it go through   one or the other and magically the interference  pattern goes away because indeed you changed the   electron dramatically by observing which one  it goes through. So that's all standard stuff.   Everyone and their mom learns that in kindergarten  when they're learning about quantum mechanics. The   delayed choice quantum eraser experiment is  a hilariously convoluted elaboration of that,   which makes people feel bad for no good reason  at all. Oh, thank God. I actually wrote a blog   post about it called The Notorious Delayed Choice  Quantum Eraser, where I make fun of people who are   trying to make you lose sleep because of this.  Well, they got to me. They got it. I know. Yeah,   we're all victims here, I think. So the way that  that works is rather than just observe whether   the electron goes to the left slit or the right  slit, what do you mean by observe? You mean like   maybe you with your eyeball or with some measuring  apparatus have detected it. But what if you just   entangle it a little bit, like you entangle it  with one little particle, okay? Then unlike if   you personally had observed it, you could imagine  unentangling it. You could imagine not letting   it get the interference pattern destroyed, or  you could imagine having it be destroyed. And   you can make that decision after the electron's  been detected. That's the slightly spooky part,   right? That's the spooky part, right. If you were  the kind of person who spoke a language of the   electron goes through one slit or the other slit  and it's making a decision, then what Chuck says,   like, it sounds like the electron's decision was  affected by what I did after it was detected. Oh,   my God, how could that be? But if you just talk  the language of a wave going through and becoming   entangled and taking quantum mechanics seriously,  all of this is a hundred percent what is predicted   by the Schrodinger equation of quantum mechanics  without anything, knowing anything, anything,   making any choices or anything going backward in  time. Okay. So there. So there you have it. It's   hard to explain, but I encourage people to Google  notorious delayed choice quantum eraser and they   will find my blog post and they will hear it  all. And this is literally the chapter I wrote   for my book, Something Deeply Hidden, which is all  about quantum mechanics. And I read the chapter   and said, this is too much. Like they don't need  this. Like this is too complicated and specific.   So I just made it a blog post instead. Excellent.  And where do we find that blog post? On my blog,   preposterousuniverse.com slash blog. But again,  if you just Google delayed choice quantum eraser   with the word notorious in front, my blog post  will come up first, I promise you. Preposterous   universe. Preposterous universe. That's  preposterous. Exactly. All right. All right. Well,   that was good, man. All right. Thanks for that.  You got your 0 dollars worth. Yeah, I got my 0   dollars worth. It's a perfect example of rather  than trying to demystify quantum mechanics,   sometimes people try to mystify it. They try  to make it sound even more confusing than it It   is. Quantum mechanics really is confusing. You  don't need to make it sound more confusing than   it is. And that's what this is doing. And you're  absolutely right. Well, that's cool, man. Well,   I appreciate that. Okay, here we go. This is  from our buddy, Kevin the Sommelier. And he says,   if dark matter were a wine varietal would  we be at the I know it exists but I can't   quite describe it stage or the I swear I taste  something but everyone thinks I'm making it up   stage that's funny on a more grounded note for the  holidays Gaia Promis. Gaia. Gaia. Gaia. Oh, Gaia,   like Gaia. No, G-A-J-A. Gaia. It's Italian. Okay.  Gaia Promis, which is a beautiful Super Tuscan,   will pair incredibly well with any roast that  you are planning. And you're familiar with this   Gaia Promis. It's expensive, but it's good. Okay.  All right. And you say that like, Chuck, you can't   afford it. That's what you just said. That's what  I was hearing. That's all I heard. You were like,   Gaia, yeah, Chuck, you cannot afford it. Okay.  So maybe one day I'll pour it for you at my   house. In the meantime, keep dreaming. But the  yellow tail is good, Chuck. Don't worry. You'll   love it. 2 buck Chuck. 2 buck Chuck. I'm down.  So in that question, I guess he's trying to,   using his wine expertise, trying to probe  what's really going on with dark matter,   dark energy, or both? No, he said dark matter.  Just dark matter. Yeah. Yeah, could you just   put some anchoring onto, because a lot of  people think we're just making it up. Yeah,   making it up. Yeah. And does it have to be a  particle? Ooh, ooh, that's a tougher one. You   made it tough. I had an easy question. You threw  me a softball there. See, this is what I do,   man. You just saw what I did with that stupid  delayed double slit thing. I can't help. And Neil,   we just did this earlier. We were together earlier  before you got on. And he was just like, Chuck,   you overthink everything. Just calm down, bro.  Look, back in my day, when Neil and I were young,   it was perfectly okay to think that maybe  dark matter didn't exist. In other words,   there were absolutely things going on in galaxies  and clusters of galaxies, things like that,   that looked like dark matter, looked like there's  more matter in a galaxy than we could attribute,   account for just by counting the stars and the  gas and the dust. But maybe there was something   weird going on with gravity. Maybe Isaac Newton  and Albert Einstein didn't have the last word.   We've long since passed that phase of the  development of cosmology, and I think a lot   of people haven't caught up because we've learned  so much more from the leftover radiation from the   Big Bang, the cosmic microwave background, from  gravitational lensing with clusters of galaxies,   from the growth of structure of galaxies and  clusters and things like that. Dark matter exists.   It's really there in some form or another. Now,  maybe gravity is also modified. That's perfectly   OK. But yeah, there's something called dark  matter. Is it a particle? Well, we got to admit,   we don't know what it is. The range of  possibilities goes from some tiny fraction   of the mass of an electron to the mass of the moon  or something like that. I mean, there's a very,   very large range of possible masses that dark  matter could have if it's a particle. Could it   be something beyond a particle? It always could  be like this. Anything is possible. But boy,   it really acts like particle like like we know a  lot about where the dark matter is, how much of it   there is, how fast it's moving. It looks exactly  like some massive, slowly moving particle that was   sort of still stationary in the early universe  and started moving ever since then. So it's our   job to go actually find it. Then we'll know.  So 2 small points. Whenever given the occasion,   I don't describe it as dark matter because we  don't know it's matter, but it is definitely dark   gravity. That's what it is, right? because we're  tracking gravity at every turn that we say we're   measuring the dark matter so i think that's the  the most honest way to describe it because because   sean you've seen this they're they're they're uh  clickbait there's a news article that says uh you   know scientists may be wrong about dark matter of  course it might not be matter yeah But we can't   be wrong about dark gravity because we don't know  what it is, right? I'm just trying to distinguish   that. No, I think that's a perfectly good and  fair distinction. But I think that I would just   be more conclusive about what we do know. You  know, like I said. And you sounded very good   there. You sounded really good. In the eighties,  what we were discovering was there's more gravity   at the edges of galaxies than there is that you  would predict from the stars, et cetera, inside.   so okay maybe gravity is different but nowadays we  see gravity where there is no ordinary matter to   cause it so it's not just that the strength of  gravity is different but it's just pointing at   something which we're not seeing so that's dark  matter you said a moment ago that we might one   day need to modify gravity, but that's not what  we need to do to explain dark matter. It is not   sufficient to explain dark matter. We need extra  stuff out there. And just to be super duper clear,   because again, much like the notorious  delayed choice quantum eraser experiments,   sometimes people like to make things harder to  understand rather than easier to understand.   There are people who have theories of modified  gravity where they've changed Einstein's theory   of gravity and they say: I can explain away the  dark matter. But when you look at their theory of   gravity, in addition to changing gravity, they've  also put new sources of gravity in there which are   just dark matter. Right, They break the universe  in the process. Right, Okay, All right. Well,   that's okay. Kevin, kudos for the creative work.  posing of the question, This is Rachel Ambrose And   Rachel says: Hey, Rachel, here from Austin Texas,  I'm a big fan of Sean and his Mindscape podcast.   Thank you, Rachel. I've been thinking Product  placement. Rachel says I've been thinking about   the spinning universe hypothesis which says the  entire universe may be rotating very slowly as a   whole. This was hypothesized to help resolve the  Hubble tension, But I was thinking, if it's true,   could this also help us explain dark energy  as a kind of centrifugal force or centrifugal,   depending upon how you want to say it? Yeah, look,  I'll be super honest, and my honest answer is:   I don't know. That sounds like a research program.  Rachel should write a paper about this and submit   it to the physical review. I doubt it is the  slightly longer answer, because if the universe   is rotating, then that tends to break the isotropy  of the universe. Okay, God bless you for that.   I'm going to fill you in. Isotropy just means  things look the same in every direction. Okay,   Statistically the same. Statistically the same.  Not exactly the same, Of course, That's right. So   there's not like a big hotspot in one side of  the universe and cold spot on the other side.   It's more or less the same average everywhere,  with little fluctuations around it, And we have   that right now. That's exactly what we have. And  so once you start messing with that, can you mess   with it like just a little bit, and maybe no one  has noticed yet. Sure, You absolutely always can,   it's hard to do that. And the theory that we  have for dark energy with Einstein's cosmological   constant just does a really good job at fitting  the data in a sort of a simple, direct, blatant   kind of. here I go kind of way. So I'm absolutely  open to creative new things like the rotation of   the universe, but where would that come from? I'm  not sure. And what other effects would it have   on observable quantities? I'm also not sure.  So that sounds like a work all right. okay,   very cool. i think we still need verification  that that net angular momentum is manifest   everywhere. i think it was just one pocket of the  universe where they made this measurement and and   so you would need to do more of the universe to  reveal this. right, if it's really true. so it   was just. it was the. it's. which way that spiral  galaxies are rotating right? yeah, if there's a   net rotation in one direction over here, that says  there's something going on in the whole universe.   right, because these houses wouldn't know about  each other necessarily. there's something. so   they would be. they're responding to the same  thing exactly. that's the case, correct? i got   you right? okay, by the way, there's just just to,  so people know that you got to be careful about   these things. there was one study that had the  brilliant idea. it was a while back, so we didn't   have. you know, the computers weren't there. they  had volunteers. it was like citizen science. they   said: here are some pictures of spiral galaxies.  You go through them, you citizen scientists,   and tell us: are they counterclockwise spirals  or are they clockwise spirals? Okay, And you   would expect it to be 5050, but it was like 7030.  Like there were a lot more clockwise spirals than   counterclockwise spirals And the astronomers  were like: oh my God, what is going on. And   someone had the brilliant idea: Okay, let's give  the same galaxies to the same people, but let's   take the mirror image of them, Let's flip them.  Ah, what a smart idea. And guess what? It was   still 70 percent clockwise galaxies. It was people  were seeing things. People are not unobjective.   Get people out of the equation. So you just just  got to be careful about all of these things. Sean   Browning says: Hello, this is Sean Browning from  Hood River, Oregon. In a previous episode, Neil   stated that if you were to fall into a black hole,  that would see the future of the universe unfold,   Or you would see the future of the universe  unfold Behind you. Right? Yeah, yeah. If that's   true. then what would happen if the black hole  finished evaporating before the universe ended? so   i think that the details matter here. you know we  there's a way that we have a thinking about black   holes. that is sort of idealized, like i wrote a  textbook on general relativity and you can learn   about black holes in there. but the real world  black holes are messy and you know they're made   of stuff and things like that. so i don't actually  think it's true that you see an enormous amount of   the past of the universe in a real black hole. in  a real black hole, you would, as as as you know,   get spaghettified and die very quickly, right? so  right, if you're in a real evaporating black hole,   you would first get spaghettified and then  you would be released as a stream of- uh,   black body radiation, and you're not. none  of you is experiencing anything here. so,   uh, yeah, i think that you don't need to  worry, like you would. you would see more,   maybe, than you would if you were outside the  black hole. but there's no apparent paradox   that you're going to see the whole thing gotcha.  okay, you're, uh, very literal, sean. um, but   the point is, if your time is slowing down as you  near the black hole, then the time for the rest   of the universe is speeding up. right, i mean you  see, sorry, you, you're normal, and the rest of   the universe is speeding up. so i think that's the  foundation of this question. that is, however long   it takes you to fall into the black hole. will the  universe, or vice versa? will one of them live out   their days before the other one finishes? what  it's doing I think you should always think about.   I mean what I would like is that everyone in the  world thought about space-time diagrams And light   cones. you know your past light cone, which is  the set of all things that can send light signals   to you without moving faster than the speed of  light. it never covers the whole universe. It   never sees the future. It never sees things that  are further away from you than the speed of light   can get. That's true, whether you're inside a  black hole or not. So there's some quantitative   question about how much you see. but there's no  worry. you're seeing things that didn't happen yet   before the black hole evaporated. Wow, That's a  bummer. You're the one falling into a black hole.   There's other bummers you have to worry about.  Let me tell you: This is true. It's not your   biggest bummer. It's not the biggest bummer, All  right? This is Shadow Dominic. And he says: Is his   name Shadow Dominic? Yeah, All right, He says:  Hi, this is Dominic from Madison, Wisconsin.   If you could instantly know the answer to one  currently unsolved physics problem, no strings   attached. Was that a pun? Stay away from strings  theory. Was that a pun? What are you doing, bro?   He says: which one would you pick and why? I mean  the cheating answer is yes, Tell me the theory of   everything. Then I could like work out everything  else from that. Let me, let me double up behind   you. What gives you the confidence that a theory  of everything even exists Because the universe   exists? No, no, no, I'm not accepting that.  No, don't tell me, No, So You should accept the   universe, Neil. It could be that quantum physics  and general relativity in this universe never come   together and there's nothing that would ever bring  them together. That's just how the universe is.   We're invoking a philosophical bias of beauty  to even assert that there is one theory of   everything. No, that's not true. The universe  exists. There is something that happens, right,   We don't know the best way of summarizing what  happens. The best way of summarizing what happens   in the universe might just be to literally list  everything that happens. That would be a terribly   uninformative theory of the world. We think we can  do much better than that. I'm not saying that the   theory of everything is simple or elegant or fits  on a napkin or a T-shirt or anything like that,   But there is some full and complete description  of the universe. The fact that you said it could   be a very long account of everything that happens,  but if it's everything that happens, then that's   everything that happens. So that could be just a  really messy, ugly, not philosophically beautiful   theory of everything, Because there's a page  in the book for everything that happens. Right,   Because I'm very much in agreement with the  philosophy that we should not be going around   telling the universe how to behave. We can't  decide ahead of time what the universe is   like. It's absolutely possible that the ultimate  explanations are not that simple. I actually don't   believe that. I think the ultimate explanation  probably will be really really simple. I don't   know. yeah, What if you can't access that  information Like it is non-accessible on this   plane of existence? but the answer is there. Or  you're saying: are we just too stupid as humans?   That's another way to ask your question. Yeah,  I guess it is. Sean, do you think that we have   sufficient intellect to even get there. Yes, Okay,  Oh, All right, That's encouraging. Oh man, loving   him some humble sapiens there. Okay, I mean,  again, we don't know. But what I like to do is   just think about how far we've already come. You  know, a hundred years ago, we had just finished   getting quantum mechanics. We didn't know about  the expansion of the universe. We certainly didn't   know about quantum fields in the Standard Model  and all these things. And we didn't know about   other galaxies yet. It happened in in in 19 26.  Holy moly. Yeah. Of course, it's possible we'll   never get there, but the rate at which we've  been learning things, and this is just the   past hundred years. Think about the past thousand  years and ten thousand years from now, we're going   to know a lot. If we're here. Yeah. If we are  here. One of the hypotheses that I thought was   intriguing was that as we measure the limits of  whatever the limits are of what we're measuring,   we might reach a cutoff like in a Truman Show  example where he goes to the horizon, but the   horizon is a painted sky. So we look at like the  energy levels of gamma rays or we find a cutoff   that would have no natural explanation other than  that somebody programmed this and they had to put   in a limit because they couldn't put infinity  into their software. yeah so i think that's   it's it's fascinating to imagine at a slightly  more detailed careful level what are the ways   that we could become convinced that we're not ever  going to find a simple theory of everything that's   another way to think about it yeah and things  like that are you know maybe we get evidence   that we live in a simulation or that you know that  that that the laws of physics are different from   place to place in time to time and so even if we  get them figured out here we won't know them some   other way but again what What's amazing to me is  how rock solid and reliable the laws of physics   are. Like we're able to extrapolate them way  past the environments in which we invent them   and they still work. I just gotta put one example  on the table, which is big bang nucleosynthesis.   This is my favorite example of exactly this. A  bunch of scientists, mostly in the 20th century,   figured out the rules of nuclear physics,  the rules of gravity, the rules of cosmology,   and they realized the universe is fourteen billion  years old, and they extrapolate these rules back   to when the universe was one minute old. And they  make a prediction for how much hydrogen and helium   they should be, and they got it right. Like, what  in the world? That's the impressive thing to me.   Like, we can figure this stuff out. Fantastic.  Well, that's very encouraging. And there's another   example I got from Rich Gott. George Gamow makes  a prediction based on this early nucleosynthesis   of the universe that there should be a residual  temperature of the universe. And that would be   the temperature pretty much, not pretty much, that  would be the temperature everywhere. Everywhere,   everywhere. So think of the level of extrapolation  this required. And he said the temperature would   be ten degrees. Okay? Okay. So then we finally  measure it. It's 3 degrees. Wow. Okay, so you can   say he was an idiot He really crapped the bed The  standards were lower back then chuck it was a very   But rich got said that's like predicting that a  tenfoot flying saucer would land on the lawn of   the White House, but it was a 3foot flying  saucer instead. Right, yeah. That's pretty   wild. It's still, oh my gosh, it's a flying  saucer. No, that's super impressive. Yeah.   All right, very cool. Okay, so you helped  me, talked me off the ledge there. You know,   every now and then I just wake up skeptical  that we're going to ever figure anything out.   And what we have figured out, I need to be more  impressed with that. People are impatient, yeah,   I know. It'll take some time. Thank you. So  what else you got? All right, this is Shota,   I'll say Zidzi Guri. Zidzi Guri. I'm betting that  is so not right, but go on. Easy for you to say.   All right, Shota Zidzi Guri. Anyway, hello, this  is Shota from Georgia. What a name like that.   Maybe it's Soviet Georgia. I mean, Georgia, it's  not Soviet anymore. Yeah, it's not Soviet anymore,   but yeah, I know what you're saying. uh he  said uh or shota says regarding the many   worlds interpretation is it simply a mathematical  framework we use to describe quantum behavior and   apparent wave function collapse or do you think it  represents an actual physical process in which the   universe truly branches into separate real worlds  yeah how real are these these other worlds? Yeah,   it's a great question. So the way I like  to think about it is we have an equation,   the Schrodinger equation. You can read  about it in my book, Quanta and Fields,   by the way. And the equation makes predictions  for what's going to happen. And the the simplest   reading of the prediction is that the universe  branches into these many copies slightly changed   because in one the electron is spinning clockwise  and the other it's counterclockwise. But otherwise   the universe is the same. And we live in one of  those possibilities. We don't see the other ones,   and it seems to fit the data. So then you say,  OK, what about all those other possibilities?   Should we take them seriously as real? And the  answer is, if you take our possibility as real,   then you have 2 choices. Number one, either  you take the other possibilities as also real.   You treat them equal. They're all there in the  equation. I'm going to take them seriously. Or   you tell me why I shouldn't treat the other  ones as real. And there's a long history of   people trying to come up with reasons why the  other worlds aren't there. Disappearing worlds,   theories of quantum mechanics, as Ted Bund  has called them. It turns out to be hard.   It turns out to be awkward. It doesn't fit in well  with modern physics. You can try to do it. It's a   free country. Go nuts. Invent all the theories of  quantum mechanics you want. Or you can just say,   yeah, they're there. I don't care. They don't  bother me. They don't take up the resources we   have in our world or anything. It's not grabbing a  beer in the middle of the night from your fridge.   And just correct me if I'm wrong, when we think  of what's called the Copenhagen interpretation,   that's the many worlds hypothesis, correct? No,  that's the opposite. The opposite. So describe   the opposite. Copenhagen is the one that was  invented by Bohr and Heisenberg back in the   day. And Bohr is the only one who's Danish in that  pair. Yes, that is correct. So why does he get the   name? Why does his hometown get the name of the  idea? He was slightly older than everybody else,   and he founded the Institute for Theoretical  Physics in Copenhagen, where all this work was   done. So. Well, there you go. He had the money.  Heisenberg was a postdoc at this time, and Bohr   was a famous physicist. So Copenhagen puts in the  laws of physics the notion of measurement. It says   that when you make a measurement of something,  a dramatic effect occurs where the wave function   that describes the system completely changes. The  real hardcore Copenhagen philosophy is that there   is no such thing as what the system is doing  before you measure it. The whole of reality   consists of nothing but measurement outcomes.  That's what Copenhagen actually says. I remembered   learning that, but I didn't connect it with the  Copenhagen interpretation. And if we were divided   into camps, that's my camp. It took me a while  to grow accustomed. Let me restate what you just   said. It doesn't even make sense to talk about it.  unless there's a measurement of it. To talk about   a state of a system, unless you can measure it.  And then the measurement is the reality of what   things are. Did I oversimplify that? No, that's  actually quite good. And I'm all about, like,   the universe doesn't care about you and your  measurements. The universe is just out there,   and I'm just going to believe in it. Why not? The  universe is impersonal, and the word measurement   should not appear in the fundamental laws  of physics. that's cool, I like that,   I like that. All right, this is Rory L. who  says, hello, Dr. Tyson, Lord nice Dr. Carol,   Rory from Colorado here, the happy recipient  of a gift subscription. Wow, somebody gave him   a Patreon gift subscription. Hey, guess what? You  guys can do that too. Give a gift subscription to   somebody else, buy him a year of Patreon. And  by the way, that's a cheap ass subscription.   It really is. but you can make somebody really  happy for not so much money. Okay. Dr. Carroll,   which science discipline do you rely upon the most  when studying a possible multiverse or is it a   combination? Thanks. And throw philosophy in there  as well as a branch of thought. Why not? Yes. I   presume that the question is referring to like,  is it mostly a philosophical question or is it   mostly a physics question? Or is it mostly quantum  physics or relativity physics? How about that? Add   that in there. Right. Different parts of physics  are also very relevant here. But this is exactly   why I chose to title myself a professor of natural  philosophy, because I don't. think that there's a   boundary or a dividing line between the philosophy  of it all and the physics of it all. You are never   going to invent quantum mechanics by sitting  around in your armchair thinking about how the   universe would be, right? You needed experiments  that gave you, giving you data that you couldn't   otherwise account for. And that's what led us to  quantum mechanics. then quantum mechanics leads   us to this idea of many worlds happening every  time you make a quantum measurement and philosophy   becomes super important for accounting for that  for understanding that the physicists bless their   hearts have done a terrible job most of them just  live in denial and don't even wanna think about   this. And it's kind of an embarrassment, but the  philosophers have at least taken up the challenge.   I don't think that we're done yet. I don't think  we have a perfect understanding of everything.   That's good for me, full employment for me and my  graduate students. We still have things left to   do, but I think you need them all. There's no one  answer there. As everyone from the South knows,   if you say bless his heart, the next thing isn't  gonna be a nasty insult. Well, yeah, in the South,   bless his heart means F you. They got it down to  a science. The most polite F you possible. Bless   his heart. Bless his heart. This is John Mayer.  He says, hi, I'm John from San Diego. And listen,   every episode I listen to, it's my jam. Thank  you, dear Dr. Tyson and Dr. Carroll. Can advertise   that? What? StarTalk, it's your jam. It's my jam.  He says, Dr. Tyson, Dr. Carol, and the energetic   Lord Nice, please help me understand how a photon  traveling between stars experiences no time,   yet also has a wavelength pattern and changes over  space and time. Is the wavelength a function of   the photon or is it a display function of the  space-time medium within which it is traveling   within? Or is it even something else? What a great  question. people love this question I get this   question it's uh and they don't like the answer  which is that photons don't experience things   they're just single particles electrons don't  experience things either I think that we get in   trouble because we are complicated creatures right  with senses and memories and things like that and   so we have a feeling for what it is like for time  to pass and that wouldn't apply if we were moving   at the speed of light because then no time would  pass but When we talk about the wavelength of a   photon or its path through the universe, we're  not talking about its inner experience. We're   not talking about its first person point of view.  We're talking about what it looks like to us. The   photon passes us by. It has a wavelength. It  has a path. it's all about us it's all about   us in that case it's all about us well even better  it's all relative as einstein would have said it's   relative from what the observer sees relative  to the observer yeah and it's also about the   math because that's the thing that people that  the photon is a particle but the math is why it   doesn't experience time it has no mass and it's  traveling at the speed of light so there is no   time There just isn't. That's right. It travels  through a path in space time that takes it no time   from its point of view. Right. Okay. However, if  you look at muons, a muon knows when to decay. Oh,   that's right. So it must have an internal  clock. For you to just say a particle,   and you even mentioned electrons, doesn't  care about time. If I'm a decaying particle,   I do care about time. Ooh. So there. You guys got  to stop anthropomorphizing elementary particles.   You want to say, you ready to go? You ready to  jump out of this one? That's right. Let's go   together. Hold hands. That's great. That's great.  Yeah, there is time that the extent of a muon's   path through the universe does include a passage  of time, but the muon as an elementary particle   has no hopes or desires. Sorry. Poor muon. All  right, time for one last question. All right,   this is Ben Grund or Grund. He says, hello, smart  people. Ben from Novi, Michigan. I think Sean   helped me understand entropy best in his books  and lectures. Charles Liu made an interesting   comment recently that time can be measured by  entropy. I was wondering, entropy creating time   in some strange way? Thanks for all the blown  gaskets. Brain gaskets, brain gaskets. Let me   add some extra punctuation in that question.  If entropy goes up with the passage of time,   allowing us to deduce the arrow of time by seeing  systems left to themselves as we watch entropy   rise within them, could it be that entropy goes up  no matter what the universe is doing, even if it   were re-collapsing? or is the entropy intricately  connected to an expanding universe? So that even   if we, so even if we re-collapsed, the entropy  would continue to go up. As far as we know,   there's no connection between whether entropy goes  up and down and whether the universe is expanding   or contracting. You could have it anyway. There's  an empirical fact about our universe that entropy   was low when the universe was relatively tiny  and has been going ever since. But if it started   collapsing, we expect that entropy would still be  going up toward the future. And that's part of the   actual answer to the question, which is that you  have to distinguish between time And the arrow   of time, time could exist without an arrow. And  the fact that entropy provides time with an arrow   doesn't mean that it explains or accounts for  time itself. Like here in this room where I am,   if I take my coffee mug and I let it go, it  falls to the ground. It falls down. It doesn't   fall up. There's an arrow of space here in the  room. There's a clear distinction between up   and down. But it's because the Earth is beneath  my feet. It's not because the Earth is creating   space. The Earth is just distinguishing between  2 different directions in space, up and down.   That's what entropy does for the universe. I  like that. That's great. That was really good,   man. But wait, but if we re-collapse and the  whole universe then occupies a small volume,   isn't that necessarily a lower entropy universe?  What? What? Don't leave me hanging. We have to end   now. You can't leave me hanging like with a nope.  We gotta go. All right, bye everyone. He was like,   take it easy. Bye, have a beautiful time.  We learned from statistical mechanics where   you have this like blob of gas. and you  just let it go, and then it will expand,   and its entropy will go up because there's  more places the particles can occupy,   and it's not contained within a volume. If the  universe is shrinking, the whole universe is in a   smaller and smaller and smaller volume how could  you tell me that that universe can also have an   increase in entropy do you think eggs are going  to start unscrambling just because the universe   starts shrinking yes please don't think that neil  degrasse tyson okay no no that makes sense all   right all right yeah all right the eggs are broken  they're in they're in the bain-marie and that's   it yeah you've scrambled them up all right Just in  case there's anyone who really wants it out there,   I mean, Neil's given a very good reason to think  that entropy should go down. And the secret for   a little bit more technicality, because this is  the end of the podcast, we should care about phase   space, not space. That is to say, we should think  about both the positions of particles and also   their velocities. so the positions of particles  are indeed going to get squeezed together if the   universe collapses but the velocities are going  to spread all out and do crazy things and increase   the entropy of the whole configuration and in fact  it's going to look wildly inhomogeneous with black   holes and empty regions and it's going to look  nothing like our early universe looked Wow. Man,   you're good. I'm old. I've done this before. Not  my first rodeo. What can I tell you? So, Sean,   Quantum and Fields, that was 20 24. That was 20  24. And the one I'm supposed to be finishing up   right now is called Complexity and Emergence.  20 26. Nice. All right. Nice. Look at that. And   after that, that's the trinity of the universe.  You're done. Yeah. You go to Bahamas. I figure   like the trilogy format worked well for J.R.R.  Tolkien. It should work well for me, too. So I'm   waiting for the adaptation. And then you escape  to Middle Earth and all as well. Right. We find   you online with the Preposterous Universe. That's  right. And a lot of my work these days, you know,   talking out there is in the podcast format with  the Mindscape podcast. Yeah. mindscape and how   often do you do you drop those every single week  wow nice very nice very nice and at least some of   your fans came through us to get back to you again  absolutely like you know delighted by that podcast   bros yeah well sean delighted to see you again uh  next time i'm in baltimore i'll give you a call i   was there actually a few months ago i just forgot  to call you It's going in your permanent record.   He's like, you know, you don't really have to  tell me that. That's true. You know, that's   something you can keep to yourself. You know what  I mean? Yeah. Yeah. A hundred percent. Next time   we're on the podcast, he's going to be saying,  yeah, I went back to Baltimore again. I forgot.   Strange how I forget every single time. Sean,  delight to see you once again. All right. Good   stuff. This has been StarTalk Cosmic Queries. Yet  another cosmology edition. I love these. No end of   people's curiosity. Chuck, you doing good? Always  a pleasure, man. This was so much fun. All right.   Neil deGrasse Tyson, your personal astrophysicist.  Until next time, keep looking up. So,