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(futuristic chiming) - At first, you wouldn't notice anything, and then, suddenly everything would change radically and violently. Imagine yourself 500 years in the future. You're sitting in a ship zipping through the silent, empty void of outer space. You're traveling to explore a black hole up close for the first time in human history. You're very lucky to be able to do this. For centuries, the prospect of traveling to a black hole was absurd. The closest one is nearly 60 million billion kilometers away,
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but great advancements were made in space travel and cryogenics, and here you are, you're traveling to explore a black hole, and you're about to make a very big mistake, but we're getting ahead of ourselves. What is a black hole, really? Everyone here is either sitting down or standing. A couple of people are standing. That might not sound like a profound statement, but cosmically, it is. And if you're watching this video afterwards, chances are, you know, maybe at the home or the office, chances are you're standing at a table or sitting
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on a chair, or perhaps lying on a sofa. No matter who you are, no matter your gender or skin color, you all have one very important thing in common. You're not currently floating around the room. Why are you not floating? Because of gravity, of course. Gravity is the force of attraction between any two big objects, like between the moon and the earth, between the earth and you, but behind that simple statement lies a series of very profound insights that require us to completely change our understanding
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of the fabric of reality. So the ultimate question underneath it all is what causes gravity? So physics, as you know, this is the type of question, what causes gravity? This is the type of question that keeps physicists up at night because physics is, among other things, as you know, the search for mechanisms, you know, reasons or explanations for why certain things happen the way they do, the why and the how. And if a physicist comes to you and asks you what causes X phenomenon, and you tell her,
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that's just the way it is, she'll be very dissatisfied with your answer. And so for the longest time, the question, what causes gravity was not so well understood. And even, you know, a fairly bright guy known as Isaac Newton, even Isaac Newton, someone who managed to describe gravity quite well in the 1600s, even Isaac Newton at some point had to admit, I don't know what's going on, so in the Scholium General, he says, "I have not as yet been able to discover the reason for these properties of gravity from phenomena,
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and I do not feign hypotheses." He's saying, yeah, my equations work really, really well, but the why, I have no idea why. And for the long time people would say, okay, well, maybe gravity is just like a universal force, and that's just always been the way it is. And if you think about that, that's a little bit too close to, that's just the way it is. It is not satisfying at all. And so what is that? What is the mechanism that keeps you seated in your chair and that keeps the moon orbiting around the earth like that?
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For the longest time, again, this was not understood until Albert Einstein and Albert Einstein needs no introduction, in 1915, he finally came up with general relativity, a general theory of relativity, and came up with a mechanism that explained how gravity actually works. And it required us to completely change our perspective on the nature of reality. And so, the question is, for example, when an apple falls from a tree, what is the apple falling through? Okay, the atmosphere. So maybe that's a bad example, but what about the moon?
04:20
What is the moon falling through? Not really anything, right? But it's falling through empty space. But what is space? Again, the concept of space for a very long time was this notion that it was sort of like, you know, there's different ways that people thought about it, right? But the concept of space, for a very long time, you could think of it as the sort of background grid upon which things happen, right? So the space is not real in the sense of, you know, like a moon or the earth or you know,
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like a table or something like that, but if I want to, I can impose a grid upon it. So then I can describe things in the physical world in a better way, in a more convenient way, right? So I can impose a grid, like for example, lines on a chalkboard. If I wanna make a grid so that I can better describe some, you know, planets that are doing something with arrows. I don't know what that was, I just decided to draw it in the moment's notice. But you know, that grid is there to help me with my calculations, right?
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And you know, it's sort of like an abstract thing, and it's so abstract that if I wanted to, I could actually choose a different coordinate system if I wanted to, right? And as long as I am consistent about what I'm talking about, then it's totally gonna be okay, right? And we'll let this guy finish his work here, right? Great, very good. Oh, that's good too. Oh, beautiful. He's an artist. But again, so, you know, so space in a sense is this background grid. It's like these this metric lines that we impose upon things,
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so that then we can describe physical phenomena that are going inside. But it's not real. It's not as though I, like, draw a circle on the blackboard, it's gonna change the blackboard or the blackboard itself doesn't change the circle. But in fact, so you might think that space is like that, space is nothing. It's just some sort of like a background thing. It turns out that Einstein pointed out that this is absolutely not the case. Einstein pointed out that in fact, there's a completely wildly different way
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to think about it. In fact, the way that gravity arises, the way that gravity comes into existence, the way that the force of gravity arises is due to the presence, the fact that a presence of a certain amount of stuff within a certain volume of space actually warps and bends the fabric of space itself. So in that sense, space is actually something malleable. It's bendable, and this is a completely, this was a completely different way of thinking about the world. It was not just a, you know, a small step forward.
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It's a complete sea change with the way we understand the world. And so now, because of that, so for example, if I put something that has a certain density, so that you can think of space in this sense as more like a, instead of just a completely empty space, it's more like a rubber sheet, right? And if I put like something heavy on the rubber sheet, it actually deforms the sheet. If I have nothing there, the regular rubber sheet, and we're all stretching it tight, if I flick a little marble across there,
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the marble goes in a straight line, right? But if I put something heavy there, like a bowling ball or this yellow thing, and then if I, now if I push something, if I flick a little marble or something, it actually, from the marble's perspective, it's going in a straight line. But from a larger perspective, it's actually falling toward the thing that is curving the fabric of space itself. This was a wildly different way of thinking about the universe. So therefore, we have our mechanism, the moon and the earth,
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for example, orbit around each other because each one is falling into constantly this, the curved spacetime of the other due to the presence of energy and matter density in space itself. This is just mind boggling, you know? And it still is to me, and it was at the time too. So this was a very, very radically different way of thinking about the world. But it gets worse because it turns out that not, the space is not just bendable, it doesn't just bend, it actually flows too. Space is something that actually can move.
08:08
Parts of space can actually move compared to other parts of space. And this has wild consequences, right? So what that means is that, so this is basically comes out of the equations of general relativity, so if you have something that has a lot of gravity, you can actually have space that's bending. It creates a kind of a sinkhole in space itself. But if there's a lot of stuff packed into a small enough volume, space itself starts to flow toward the center of that volume. So for example, if you have the sun,
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it has a certain amount of density. The denser the object, the deeper the sinkhole, and the more the space starts to bend toward, starts to flow toward the center of this volume. So the sun, for example, warps spacetime a certain way. You can think about the extent to which this space is flowing toward the center of any of these objects in the sense of how fast you would have to go to get away from the surface of one of these things, right? So for example, if you wanted to get away from the earth,
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you know, we know that you have to go pretty fast to get away from the surface of the earth, but we know that it can be done, obviously. If you wanted to get away from the sun, you'd have to travel 50 or 60 times faster than that. And if you wanted to get away from a neutron star, you need to travel at something like 60% of the speed of light. Given the fact that my astrophysics colleagues estimate that the fastest speed that a human could possibly go without getting killed is something about half the speed of light,
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and as humans, the fastest we've ever gone in anything, I think was something like 0.00001% of the speed of light, you're not gonna get away from a neutron star with our current technology. But think about what this means. If you take this to its logical conclusion, right, if I take a certain amount of stuff in space, it bends space and space starts to flow a certain amount. If I put more stuff into its fixed volume, starts to bend and flow even more. Take that to its logical conclusion. If I put too much stuff into a fixed space,
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space will bend and flow so quickly into the center that no matter how fast I move, I'll never be able to escape this flow. And this is our black hole. You can think of a black hole, like an extremely strong water drain where the water is the fabric of space itself. Imagine you're a fish in the water. If you get too close to the drain, the water will flow in inward faster than you can ever possibly swim. And even if you swim at your fastest possible speed, you'll be sucked into the drain. Likewise, if you stay far enough
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away from a black hole in space, you're okay, but there's a point of no return called the event horizon, beyond which space is flowing into the center of the object faster than the speed of light. So not even light can escape from inside of this black hole. But if that's the case, and so you've probably heard these things, I understand that for the last, you know, 10 minutes or something, I've been saying things that a lot of you have heard a million times before, but what does that really mean?
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Like if that means that space is flowing faster than the speed of light inside this black hole, beyond this event horizon, this point of no return, what does that really mean? What's actually going on inside of a black hole? That's the zillion dollar question. If you can find the answer to that, you will have about 10 Nobel prizes, I think. So two things probably jumped out at you when you see an image like this, a gift like this, right? So the first thing is that thing in the middle, a singularity.
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So you probably heard the word singularity before, but in a physics sense, what this means is that there's a point in space where you have infinite density and infinite curvature of space. I don't know about you, but I have no idea what that means. And I'm a physicist, so I have no idea what it means to have actually to, you know, to actualize that in the real world. What does it mean to have a point that has infinite density and an infinite curvature of space? Probably what this means is that there's something we're missing.
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So if you look closely at the mathematics of general relativity, and you should always look closely at the mathematics, you see these things like singularities that jump out in the equations like this. And typically when you have something that shows up in your equations that makes no physical sense, that's typically the universe's way of telling you to think a little bit harder and that you're missing something. There's something that needs to be put in to avoid these sort of catastrophic nonsense things.
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So the other thing you probably noticed about this is that inside the event horizon of a black hole, if space is flowing faster than the speed of light, if you went to travel to a black hole to study it up close, and you should definitely want to do this 'cause it would be amazing, you probably should stay away from the event horizon because if you went inside, there's no way for you to ever tell the rest of us what you learned because there's no way for you to send a signal outside of the event horizon.
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It's a one-way trip into a black hole. So that's the two things that you probably have, you know, that have jumped away from you. So again, if you wanted to go to a black hole to study it up close, and you should definitely want to do that, you definitely wanna stay away from the event horizon. And the first reason, of course, is that scientific reason, like I said. You would be a very, very bad scientist if you go one way into a black hole and you could never send your data back to the rest of the community, right?
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That's a very, you're being a bad scientific, you know, bad scientist for the community. But the second reason you would want to stay away is actually existential because the conditions inside of a black hole are such that the gravitational conditions will be such that it's likely with a certain size of black hole, it's likely that if you go into a black hole, the gravitational force on, say your feet, will be much, much different from the force on your head. And you might get stretched into spaghetti,
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so for example, the fish in the water drain, if a fish got too close to a black hole, it would probably be stretched into something resembling fish spaghetti very quickly. So existentially, you'd wanna stay away from the black hole as much as you could. So, but that's for black holes of a certain size. So for example, a smaller black holes or kind of medium-sized black holes. Yeah, the gravitational conditions as you go in will be so violent and so crazy that they would probably rip you apart or stretch you into spaghetti like this.
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But there's a big caveat here, and we'll get to that in a moment. But the other thing you should be asking yourself is, okay, the thing I mentioned, and anytime your mathematics tells you something that's sort of nonsensical and doesn't make any sense, and you're like, wait a minute, this probably doesn't exist in the universe, the singularity, you should also ask, do we really know that black holes exist? Maybe this is just some kind of nonsensical thing that pop outta some equations that humans are using,
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but nature wisely avoids using because it makes no sense. So it's not just a mathematical oddity that our universe avoids. Black holes were a prediction for many years. So take, go back to 1915, like 10 years before Einstein had completely changed everything with special relativity, Brownian motion, all these things, everybody's going crazy. Then 1915, everything changed again. The guy completely ruined all of everyone's perceptions about the universe twice within 10 years. So people were catching up with this.
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And some of these first predictions that were coming out, they didn't make a lot of sense. People were like, okay, we need to check this thing, we need to check this thing. And there's also this solution that you found that led to this concept, this object known as a black hole, which is very scary and weird. And for a long time, for decades, people were like, okay, well, that's sort of an odd thing, maybe, you know, I don't know how we're gonna detect one of those things, so maybe we'll see those in a, you know,
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in a few years or something. So it was a very strange prediction for decades and decades until the 1960s when in the sixties and seventies, astronomers noticed something weird going on near Cygnus X-1, and if you can't spot the black hole on this object, you should not be feel bad because that's where the black hole is hiding. There's a black hole hiding in that light spot. So Cygnus X-1 is a double system. There's a black hole with a blue giant, super giant star next to it. And this is an artist's rendition
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of what Cygnus X-1 looks like. So it really is this. And in fact, in the sixties and seventies, astronomers started to see these hot, these extremely energetic x-rays and gamma rays that are coming away from this thing that wasn't radiating itself, but it seemed to be eating another star that was next to it. And these signals matched almost exactly what you would expect from a black hole doing this. So this is an artist rendition, of course. You might also ask, okay, this is just one example, but we also know that we can,
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so we can't actually see black holes directly, right, because no light can emerge from them, but we can see other stars orbiting completely dark, empty spots in space. There's obviously something going on there in the middle though, even though, in the middle there, even though there's no star that you can see. And as of, so the evidence for black holes is mounting and mounting. And as you know, as of 2019, we can actually see the hot gas from around a black hole as it's being twisted around this black hole,
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and this is from the Event Horizon Telescope Collaboration from 2019, one of the most amazing images that humankind has ever produced. And this is an M87, 55 million light years away. So black holes are really real indeed, and they're not even that rare. You know, it's strange because the universe seems to love these twisted vortices. There's, (clears throat) there's a big one in the center of nearly every large galaxy we've seen and our own. And in fact the closest confirmed black hole is that Cygnus X-1, which is about 6,000 light years away.
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Given the fact that our Milky Way is about 100,000 light years across, 6,000 light years is not that far away. Our Milky Way seems to be sprinkled with smaller black holes as well. And there even could be a mysterious black hole, even shockingly closer to earth. In the outer edges of our solar system, several big rocks seem to be orbiting in strange ways. Some astronomers think there's a new planet, Planet 9, that could be responsible for these wobbly odd orbits, but we see no evidence of a visible planet.
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So what if it's not, the reason we haven't seen it is that it's not a planet, but in fact is a tiny black hole about the size of an apple that was born just a little bit after the universe was formed 13.8 billion years ago, and then floated around our universe for billions of years before finally getting stuck in our solar system. So an apple-sized black hole, heads up. (attendee catches apple, audience chuckle) So an apple-sized, oh, and I wanted to show you this thing too. So actually a couple of the physicists
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that proposed this idea, because some other physicists that I know came up with the idea that maybe there's a Planet 9, then some other physicists that I know, Jakub Scholtz, this guy, he's like, well, wait a minute, what if it's actually a black hole? In their paper, if you go to their paper in the auxiliary supplementary material, that's typically the place in a paper where you put like really, really detailed calculations that nobody wants to deal with here, you know, or like the extra part of your data, the copious list,
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so in case somebody wants to replicate what you did. It's the really boring part of the paper. These guys decided to put a life-sized version of what the black hole would look like. So this is, if you printed the paper that's right there on the page, it's quite cool. So an apple-sized black hole, could you make a black hole yourself? So if you want to know what it takes to make a black hole, just grab your textbook on gravity. I assume you all have a textbook on gravity on your bedside table like I do.
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And find the black hole equation. It'll tell you for some given amount of mass, how small of a volume you have to pack it in to make a black hole. For example, to make a black hole out of the earth, you need to pack the entire thing into a volume about the size of a blueberry. Most black holes we know of, of course, are much larger than this, with masses, for example, the one in the center of our galaxy has a volume, yeah, I think a diameter that's something like 1/3 of the distance between the earth and the sun.
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But with a mass that's like several million times that of the sun. And there are much, much larger black holes out there too, with masses that are billions, tens of billions of times that of the sun and with volumes that will encompass our entire solar system. But it's really actually that easy. The equation is fun. We can actually do some fun things with this equation. We could just take any mass and look at it. So this is our equation. Let's imagine we wanted to make a black hole out of a proton.
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A proton is already very, very small, 10 to the power of minus 15 meters, completely diminishing already, right? If you wanted to make a black hole out of a proton, you need to pack it into a volume that's 10 billion, billion times smaller than the Planck length. Given that the Planck length is the smallest physically meaningful distance that quantum mechanics allows us to define, a proton is safe. You're never gonna make a black hole out of a proton. What about you? Can we make a black hole out of you?
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If you wanna make a black hole out of you, we would need to pack you into a volume that's one ten-billionth the size of a proton. Given that this is a thousand times smaller than the current most powerful distance resolution on earth, which is the Large Hadron Collider, you are also safe. We're not gonna make a black hole out of you. What about the sun? If you wanna make a black hole out of the sun, you'd have to smash it into a diameter, into a sphere that has a diameter that goes from about Westminster Abbey to Dalston.
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Okay, so I know that sometimes during, you know, high tourist season, Trafalgar Square gets so dense that it sort of seems like it would make a black hole, but no, it's not ever gonna be close to the volume, the density you'd need to make a black hole like, you know, packing the entire sun into Dalston. What about something really big? What about the observable universe? So recall that space bends and flows, right? We established that earlier. We know that space is not something that's static. It actually is malleable and it bends and it flows.
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This happens also on large scales. You may have heard the statement a zillion times before. The universe is expanding, and it's true, we know that the universe is expanding, but expanding into what? Nothing. Space itself is expanding. For example, two galaxies in our solar system, or sorry, two galaxies in our universe are like two pins stuck into this rubber sheet. Then the rubber sheet is being pulled from all directions. From this perspective of an ant on the sheet, nothing happened to make the pins move.
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Space itself is moving, the sheet, and the distance between them is increasing. So we know that this is happening to our universe, and because of that, we also know that the particular way, so, you know, for example, if you, because of the particular way that the universe expanded right at its birth, we also know that there are parts of the universe that are currently unobservable to us. They are outside of the so-called observable universe. This is a consequence of the way that universe expanded
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right at the moment to the Big Bang, right before the moment to the Big Bang, in fact. So there are parts of the universe that are so far away that we can't see them, okay? So the observable universe is defined as this sphere that's around you. Every single one of you in fact has a slightly, slightly different observable universe. There's a sphere around you that's composed of all the stuff that has had a chance to send a light signal that you could receive. But beyond that, we have no idea how big the entire universe is.
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So the observable universe right now is 93 billion light years in diameter. But the entire universe, we have no clue. It could be infinite, in fact, it could be something smaller, we don't know. So we can't, it's very difficult to estimate the amount of stuff that's in their entire universe, but, you know, my astrophysics, physics colleagues are quite good. We can estimate the amount of stuff that's in the observable universe, right? So if we take all the stuff that's in the observable universe, right,
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we add up all the protons, all the neutrons, all the electrons, all the stuff that makes up you and me and potatoes, and then we also add up all of the neutrinos. Every single one of you has about 10 trillion neutrinos from the sun zooming through your thumb every second. We add up all of these particles as well, and we add up all the dark matter and we add up all the photons and we add up all the gravitational waves, we get an enormous number. We get something that the universe is probably, the estimated total mass is something like
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10 to the power of 54 kilograms. Then if we look into what this, so, again, we just use this equation, we can figure out what it would take to make a black hole out of the entire observable universe. And if you do this calculation, you find that it would take a diameter that's three times the diameter of the current observable universe. Let me double check my calculation here. (audience laugh) Okay, so, okay, so, okay, astrophysical numbers are, you know, subject to uncertainties, maybe we got the number a little bit wrong.
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So let's imagine that even the number is twice as much as the estimate here. Let's imagine that in fact the universe is something like 10, you know, five times 10 to the power 53, it's half of what we thought it was. This will make more sense, right? So then if you wanted to make a black hole out of the entire observable universe, you need to pack it into a sphere that's about a little larger than the current observable universe. Do we live inside an enormous black hole? When I first came across this calculation,
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I was dumbfounded. It made no sense. Is it possible that the interior, that our entire universe is the interior of an enormous black hole? I mean, it doesn't make any sense because I thought that black hole's, you know, from science fiction, these twisted vortices that suck in entire stars and they make, you know, Christopher Nolan movies crazy, you know, things like that. So I thought that's what black holes were. And this made no sense to me. It turns out that it depends upon, so, you know, I thought that it makes no sense
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that we would be inside of a black hole because you would just get twisted, you would get torn to shreds, just like we talked about. It turns out that as long as the black hole is large enough, you would be okay. You would be okay inside the event horizon of a black hole. So it depends on the size, so, you know, for example, if you happen to fall into a black hole that was the size of the one that would go around London, the solar mass, black hole within, you know, from Westminster Abbey to Dalston, you'd have no chance.
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You would be ripped to shreds immediately, but with a large enough black hole, you'd be okay. It would still be a one-way trip. Once you cross this event horizon, there's no way for you to go back out. But it's entirely possible that our observable universe right now exists on the interior of an enormous black hole. This is very weird, I admit, but black holes are more than astrophysical oddities. If you think about it, they're actually profound statements about the limitations of knowledge. So what is a black hole actually doing?
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Think about what it does, right? A black hole separates the world, separates the physical world into two regions, right? You have the inside the black hole and you have outside the black hole. And this event horizon becomes a barrier, a border, this thing that's always in the distance, right, that you can never reach. No matter how quickly you travel, you'll never be able to get to that thing, right? We know that black holes eat things and grow. So for example, that big, that artist rendition I showed with the swirling black hole
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sucking in the blue giant, that black holes or event horizon grows the more stuff that it sucks in. So we know that that event horizon is growing. If you are on the inside, this thing would be growing too. So this horizon, think about what it means to be inside of a black hole. You'd have this point of space, this region of space that you could never get to, and it's always receding from you, a horizon, right? So, so yeah, just to, you know, show, so you have stuff that's falling into the black hole, the event horizon grows, right?
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But think about what it then means for you right now on earth. Think about your situation right now here on earth. If you look out into space, there is a horizon very, very far out, away, beyond which we cannot see, but we know there's stuff there, but we can never possibly see. And no matter how quickly you travel, you'd never be able to get to it. That's remarkably similar to what you would experience on the inside of a black hole. And it turns out that the mathematics, and again, you should always look closely at the mathematics,
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it turns out that the mathematics of the interior of a black hole is almost identical to the mathematics of the exterior of the black hole. You have to do some stuff with the infinity is going to infinity versus zero, but yeah, trust me, you don't have to trust me, you can look it up in your textbook on gravity that you have on your bedside table, but this is, you know, this is actually what is going on. Mathematically, it makes sense as well. So there's also another reason to think that this is, oh,
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so yeah, and for example, I wanted to show off my nice animation here, right? So as stuff, come on, show me here, there we go, yeah, okay. It's the same thing, right? You have things that are coming, so this edge of the observable universe is getting bigger and bigger, and for example, if you were able to live for, you know, hundreds of millions of years, there would be stuff that would be coming into your view that you couldn't see before. So this is very similar to what you would experience inside a black hole.
30:58
So the other thing that's interesting about this, right, is that we know that the universe is expanding and then if you run the clock backwards, like the YouTube slider of the universe, you just run it backwards, right, eventually that means that everything far, far back in time, if everything's expanding now, everything had to be packed into a tiny dense little point at some point. And that's kind of similar to what you would expect from a singularity in the middle of a black hole. So does this mean that our universe,
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which was born from a black hole, 13.8, or was born from a Big Bang 13.8 billion years ago, was our universe actually birthed from a black hole in another universe? And does that mean that black holes in our universe are doing the same thing? Every black hole could be a seed to a new universe. And I see the looks on your faces as well, and I completely agree with you, because at the end of the day, I am an experimentalist and these ideas are wonderful. I love these fantastic ideas. I love to read about them.
32:09
I love to have theorists go crazy and show me their new papers, and look, this is happening. At the end of the day, I need to see some evidence. And it currently turns out there is scant possibility for us to gather any evidence for this hypothesis in the foreseeable future. So at the end of the day, so again, this is not, this may sound very science fictiony, right, but it's actually, at the end of the day, you can find solutions, again, going back and looking closely at the math, you can find solutions that show that inside
32:38
of a black hole, it doesn't have have to be just this singularity point, right, that, you know, that makes no sense. It's like this mathematical nonsense. In fact, as you've heard, and again, I mentioned Christopher Nolan movies, right, in fact, there are solutions that if you go into a black hole, it's not just a point, but it could be a so-called wormhole or an Einstein-Rosen bridge. And in principle, this wormhole could be a wormhole from here, from our universe to some other point in our own spacetime within our own universe.
33:03
Or it could be to another universe within a multiverse, or it in fact could be a little seed to another universe. But again, I'm an experimentalist, how are we going to test this? Here's the problem. Can we test the universe-in-a-black-hole idea? First of all, the way you test it is to go into a black hole and find out. As we mentioned, that's a one-way trip. I really, really thank you for doing that, but I'm never gonna be able to learn what you learned. So that's, you know, that's a one-way trip.
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Second, there's no distinguish, there's no experiment I can come up with currently that would enable me to distinguish that hypothesis from some other hypothesis, right? So if I say, oh yeah, well, you know, we live in an enormous black hole, okay, that's, okay, you know, part of the equations show me that that's indistinguishable from our current experience. But how am I gonna distinguish this from some other hypothesis? There are possibly some ideas floating around, but they're not so good. Third, what does it mean for me
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to contact another universe? I would love for someone to give me a coherent answer to this question, because the ideas that come up make no sense. For example, what if I have a measuring device here in my universe that works really, really well, like a ruler or you know, some kind of extra device or something, and I take it into another universe where carbon atoms don't exist. The measuring device is useless. So this is just one extreme example. But at the current, you know, currently, there's not a way for us
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to even test this hypothesis. So I love these ideas, I love them, I want more ideas like this, but I also want ways to be able to test them. However, I am actually excited for the future because I know from history, I would never want to underestimate humanity's ability to come up with ways, ingenious ways to test impossible ideas, so, you know, I'm hoping somebody in this room will actually end up studying physics and find the way that we can test this hypothesis. So again, at the end of the day, we, you know,
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we don't have a way to currently test this, but one thing we haven't covered at all so far is how black holes come into existence, because if we knew that, in principle, we could design an experiment to make some, or maybe to, you know, to figure out how we could study this in a sort of indirect way, we don't have to directly go into a black hole, just like we don't have to directly, you know, the Higgs boson, we discovered it, the LHC. You'll never hold the Higgs boson in your hand, but we can indirectly see that the Higgs boson
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exists based upon its decay properties, things like that. So maybe there's an indirect way to understand whether or not we live inside of a black hole, but first we need to figure out how to make black holes. Okay, so how does the universe make black holes? How does the universe make these twisted vortices, these things that are so dense that they nearly puncture the fabric of spacetime? Well, one way is when an enormous star dies. After billions of years, a star can exhaust the fuel it needs to burn.
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And with a final hurrah, it explodes, and then with no more nuclear fusion to push it outward,, gravity wins and the whole thing collapses in on itself. And this collapse can be so severe that it creates a black hole. So, you know, so what you can see from this is that you need, and also, so let me mention the apple-sized black holes. So the gentleman's black hole right here that he's holding. So these apple-sized black holes could in principle be something called primordial black holes. That means that these could be black holes
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that were caused right back near the moment to the Big Bang, right when the universe was born. And the particular way that the universe expanded could have popped a bunch of these primordial black holes into existence and they just sprinkled around the universe for billions of years, right? So you can see that you need an extremely violent event to make a black hole. It's why you can't make a black hole out of you or an apple or anything, or the earth. So is it possible that we could make black holes
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where I work at the Large Hadron Collider? So the Large Hadron Collider is a 27 kilometer circular tunnel on the border of France and Switzerland, about a hundred meters underground. And in this tunnel, we use superconducting magnets colder than outer space to accelerate protons to almost the speed of light. And then we slam them into each other millions of times a second. On the particle level, this is a pretty violent event. And in fact, it's theoretically possible that we could make tiny, minuscule versions of black holes
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that would evaporate immediately. But I know of what some of you might be thinking, and I see some of the looks on your faces as well. Let me make very clear to you, there's no way we're ever gonna make a regular black hole at the large Hadron Collider for the following reason. You should actually know this by now, since you've seen the first, you know, the first half of this talk, right? Black holes are all about gravity, right? And so, we're colliding protons together at the Large Hadron Collider, right?
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Protons get close to each other and they start to experience interactions, forces, and we only know of four known forces, fundamental forces in nature, right? We have three that, you know, the strong force, the weak force, the electromagnetism, and then gravity, right? But they're not all the same strength, especially at the particle level. When two particles get close together, some are more powerful than others. So when two protons get close together, the strongest force we know of is called the strong force.
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It's a very nice name for it, so let's give that a strength of one. Compared to the strong force, the force of gravity is 10 to the power minus 39. There's no possible way that, in fact, we completely neglect this when we do calculations at the Large Hadron Collider of proton collisions, the force of gravity between two protons is completely negligible compared to the others. So there's no possible way we're gonna make a black hole out of two protons at the Large Hadron Collider. Oh, and then I wanted to show you,
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show off my nice animation here. Pow, black hole, no. (audience laugh) Just to hammer it home. But recall that the reason why, well, I'll put it this way. The reason why we physicists are obsessed with and fascinated by black holes is because they could help us answer one of the biggest open questions, long-standing questions in physics. And this is the question as to whether gravity and quantum mechanics have anything to do with each other. In physics, we have two fantastically good theoretical models that have withstood essentially
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all of our experimental tests. The first is called general relativity, which we talked about, which governs how gravity works on very large scales. And the other is called quantum field theory, quantum mechanics, which governs the world of the very small. Each of these by itself ranks among the most impressive intellectual achievement of humankind. But there's a problem. When we try to naively combine these two theories, hoping for a more fundamental theory of the universe, everything breaks. We get nonsense answers like infinite energies
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or probabilities greater than one. (imitates explosion) When this happens, like I said, this is the universe's way of telling us to think harder. And it's entirely possible that the reason that we don't fully understand how gravity and quantum mechanics work together, is that in fact there's something we're missing about the fabric of space itself. So one of the ways that gravity could, because basically the question is, why is gravity so weak compared to the other forces? Why is that? If you were some kind of, you know,,
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logical universe-making person, you would never make it that way. It makes no sense that you'd have one force that's all the way down here and all these other forces that are up here, right? So one of the ways that this could happen is if in fact our understanding of the fabric of space needs to be changed. In fact, one of the ways that we could be missing the fact that gravity is so weak compared to the other forces is if gravity in fact exists in extra dimensions of space that are tiny and curled up at every single point in space.
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And so gravity in fact, leaks into these other dimensions. And if we had some magical measuring device that would allow us to somehow measure gravity in these other dimensions, we would measure it as being just as strong as the other forces. This would be amazing, and, you know, to give a few more details, so basically what it means is that the Planck scale, so I mentioned this thing called the, you know, the Planck energy or the, you know, the Planck length and things like this. Basically this is a length and energy and a time scale
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that is sort of a limitation based upon the constants of nature, and what it means is that the radius of a sphere, for example, again, remember that it takes a certain amount of stuff you have to pack into a certain volume to make a black hole, right? But that radius, again, that Schwarzschild radius, the radius part in the equation that I was showing you, right, that radius, the definition of a radius changes when you have extra dimensions of space. And in fact, what it can do is it can make this Planck level, this Planck scale
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which is the place at which gravity and quantum mechanics have something to do with each other, it could actually bring it down, it actually lowers what this Planck scale is, and it makes it so in principle, you could make a black hole out of two small things with very, very small amount of, with a much smaller energy. So this is entirely possible, but what it means is that we're not gonna make a regular black hole at the Large Hadron Collider. In principle, we can make a tiny minuscule object that would snap and pop into existence
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and snap into the extra dimensions and wobble around a bit before then decaying and they're evaporating into things that hit our detector. And it, you know, so this is a not an artist rendition, but this is a simulated version of if we were to make a miniature black hole at the Large Hadron Collider, it might look something like this. So again, this is, you might be a little bit weirded out by this that we might be making miniature black holes, but just to, you know, I don't want to go, I'm getting a little bit late here,
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but you know, just wanted to show that in fact this is nothing to worry about, because even again, if even if we make a miniature black hole at the Large Hadron Collider, it would evaporate immediately, one of these miniature things. And it would be fantastic if we did because it would help us understand how gravity and quantum mechanics work together and help us maybe answer one of these biggest open questions in science. But if you're still worried, don't worry at all because the Large Hadron Collider,
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we use very high energies at the Large Hadron Collider, but we are no match for nature. Nature has much higher energy collisions going on all the time, just a few kilometers above your head. So above your head right now are cosmic ray protons coming in from outer space all the time and they come into the upper atmosphere and they smack into the atoms that are up in the upper atmosphere. And so, if these collisions are happening all the time at energies that are much higher than the Large Hadron Collider, and you've never actually,
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you never experienced a quantum black hole that suddenly comes into your body and decays and eats you up. This has never happened. So if this has never happened in the upper atmosphere, it's not going to happen to Large Hadron Collider. And I think for the sake of time, I'm gonna go through these, oh, show off my nice animations. So they can actually do different things. So if we saw these quantum black holes, these miniature black holes in the upper atmosphere, they would do certain things, blah, blah, blah.
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They could in fact pass all the way through the earth and go through the other side. Or in fact, they could decay right near the surface of the earth and we might be able to detect these. So in fact, there are experiments, they're looking for things like this, but okay, even though we could potentially make miniature black holes at the Large Hadron Collider, we haven't seen any evidence of them yet. Oh, this is very sad. But maybe we just need a bigger collider. Discussions are currently underway for the potential
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successor to the Large Hadron Collider, the future circular collider, which would be a hundred kilometers around and could reach energy seven times that currently used. But will that be big enough? Maybe quantum gravity, the holy grail, this thing we've been searching for, maybe quantum gravity is waiting for us on the moon. So last time I was here at your illustrious institution, I gave an entire talk about the concept of a Big Bang machine on the moon. And I talked in very kind of rough details
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about what it would take to build a collider that goes around the moon and why we would wanna do this. And just a few months ago, in fact, a colleague and I finally wrote a paper about this concept. And it turns out, so you might think it's like building a, you know, so a colossal collider around the moon will be, where the Large Hadron Collider is 27 kilometers around, a circular collider on the moon will be 11,000 kilometers around and could reach energies a thousand times that currently used.
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And I see the looks on some of your faces too. So building a collider around the moon to potentially make miniature black holes on the moon. Is this crazy? And in fact, I went through the exercise with a colleague of mine and it turns out it's not crazy. In fact, there's no show stoppers to building a circular collider around the circumference of the moon. All the technology is there, all of the sciences there. It's just a matter of scale. It's just a matter of development and scale. A circular collider on the moon sometime in the 22nd century
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is something that might sound impossible, but it's just regular impossible. It's not impossible impossible, and regular impossible, we can do. Regular impossible is only impossible right up until the moment somebody makes it possible. So is that going to be big enough though? Maybe a moon collider is not large enough for us to make these miniature black holes. Again, the whole point here is that we wanted to understand how, what, like what's actually going on inside of a black hole? How do they get made?
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Maybe we could find a way to make them in a laboratory so we could understand them. The place where we'd really, really, really want to go to understand this completely is called this Planck scale, right? And the Planck energy, again, if these extra dimensions don't exist, then the Planck scale is something extremely high. This is an energy at which gravity and quantum mechanics must have something to do with each other. Again, the larger collider you build, the bigger the energy, the better chance you have to discover things
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like miniature black holes. So, but the problem is to make a Planck scale, Planck scale collider, by some estimates, you'd have to build a collider that circles around the outer edge of the solar system. Clearly we're going to need some major innovation to do that. And by the time our society, our civilization advances far enough to be able to build a super structure like that, we'll probably also have mastered interstellar space travel. And at that point, 500 years from now, you will be sitting in your ship zipping through the void
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of empty space to explore a black hole. One day, in your ship, you get tired, you notice that you should arrive at your destination in about 20,000 years. So you decide to take a 20,000 year-long nap. And as you lie down on your cryogenic bed, you very slightly bump your ship's accelerator. You don't even notice. After 20,000 years you awake, fix yourself a cup of coffee, and you notice from your gravitational sensor that something is very wrong. You appear to be too close to the event horizon of your black hole.
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You look out the window and you see an enormous profoundly black disc in space, the light from the stars and galaxies behind it twisted and deformed. You stare into the center of this void, a cosmic eye staring back at you. And it's the emptiest thing you've ever seen. Your jaw drops and your eyes widen, and then you realize that you're not sure if you've passed the event horizon yet, the point of no return. You double check your gravitational sensor and it says you're not there yet. You have five seconds to blast away.
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You jump from your chair, leaping toward the controls for the rocket, spilling your hot coffee on your hands, burning your fingers. You scream and fall to the ground. And by the time you get up, it's too late. You're passing the event horizon of a black hole. You nearly stop breathing, your mouth feels like sand, and you close your eyes. You can't believe that this could possibly be happening. You open your eyes and look out the window again, and everything looks about the same. The disc is getting a little larger,
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but otherwise, everything is the same. You feel the same, nothing is different. You think maybe your estimate was wrong, maybe there's still time to escape. You triple check your gravitational sensor and no matter which direction you pointed, it says you are pointing toward the center of a black hole. And then you know for sure you've passed the event horizon. You are inside a black hole, a calm terror settles over you. How did you get in this situation? Very slowly, while you were asleep, the conditions of the universe around you were changing
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nearly imperceptibly, and then suddenly it was too late. Floating in your ship inside of a black hole, what do you do? You might start thinking of possible escapes, I mean, you know logically it's impossible to go back outside of a black hole, but you think maybe there's something you missed. You go through all the options. I mean, maybe all the clever scientists were wrong and there's some way to escape that hadn't been anticipated. You start fantasizing that maybe if you just wait long enough or you just get lucky,
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you will eventually pass back over the event horizon and could zoom away from the black hole, back to the way things were, back to the world you once knew, and as you're fantasizing, you look down and you see that your feet are drifting away from you and your legs are being stretched into long, thin spaghetti-like strands. And then suddenly, your shoes are so far away that you can't see them anymore. And in that split second, before you reach the center, you realize two things. One, that no one really knows what happens
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in the center of a black hole. And two, there is no going back. The only way out is directly through the black hole. Sometimes reality becomes twisted seemingly beyond recognition. And right now, as the oceans burn and the global temperature rises, and as literal fascists have returned to our politics, and as so many of us are dead from a mismanaged pandemic, you would be forgiven for thinking that we had fallen into a societal black hole. In retrospect, it happened so slowly, almost imperceptibly while so many of us were asleep.
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And then everything changed. But just like in a black hole in space, the only way out is through. Our current societal black hole is a golden opportunity to construct a better world and to think in radical new ways to do so. But we'll need to dig deep, very deep, just like when we'd study a black hole in space. If you want, if we were able to go to a black hole in space or create one in the laboratory, we would be able to study the fabric and structure of spacetime to understand what creates these twisted vortices.
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And right now, we can use the current contemporary situation to collectively understand and study the fabric of society itself to understand what leads to these societal black holes. How do the social structures that we have around us lead to such societal black holes and how can we upend them? For example, there's more than one reason why we should consider building a large collider, a circular collider around the moon. The first is scientific. I want to know what amazing discoveries might be in this data, even though I'll be long dead.
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We'll all be long dead by the time this thing ever starts taking data, because such discoveries like understanding quantum gravity could completely change our perspective on reality. But there's another reason. Our society is addicted to repeating its mistakes. And right now, we seem to be on the verge of simply giving space, the moon, and Mars to wealthy private individuals and corporations whose only interests are extraction, exploitation, and profit. This is bad because this extractive and exploitative mindset
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here on earth is leading to the destruction of humanity due to anthropogenic climate change. Instead of allowing like encouraging a moon rush, for example, the moon should be protected in perpetuity against commercial exploitation. And a project like this, a circular collider on the moon sounds insane, but it's a project that's mounted solely because our species is curious about nature. Centering projects like this, when we explore anything, not just space, but anything humanity explores, centering projects like this would remind us
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that the moon belongs to everyone. The moon belongs to everyone, space belongs to everyone. We need to finally definitively abandon systems, for example, that lead to situations where a few dozen individuals can have as much wealth as billions of the rest of us combined. We need to abandon these systems. If that sounds impossible to you, keep in mind, gravity creates black holes and gravity is simply a law of nature. But the social, political, and economic structures that lead to societal black holes,
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that lead to things like extreme wealth inequality, racist policing, or climate emergencies, these are not laws of nature. They're human-made, which means they can be human-unmade. We need to finally find the courage to unmake these structures. We owe it to future humans because of course it won't be you 500 years from now traveling to a black hole to explore it up close. It's up to us to fix our societal inequities, and it's up to us to make sure that humanity isn't crushed into oblivion, so that your great, great,
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great, 25 greats, granddaughter can push forward into the unknown to understand better humanity's place in this vast universe. Thanks. (audience applauding)
