Astrophysicist Explains One Concept in 5 Levels of Difficulty | WIRED

Astrophysicist Explains One Concept in 5 Levels of Difficulty | WIRED

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Hi I'm Jan 11 I'm an astrophysicist and. I've been asked to explain gravity in five, levels, of increasing. Complexity, gravity, seems so, familiar and, so everyday and yet, it's this incredibly. Esoteric. Abstract. Subject, that, has shaped, the way we view the universe on the largest scales has, given, us the strangest. Phenomena, in the universe like black holes that, has changed, the way we look at the entirety, of physics, it's really been a revolution. Because, of gravity. Are. You interested in science yes yes you are yes, do you know what gravity is it's. Something, that so, right now. There. Was no, gravity, but since there's gravity we're sitting right down on these chairs that's. Pretty, good so gravity. Wants. To attract, us to. The earth and, the. Earth to us but. The earth is so much bigger that. Even, though we're actually pulling, the Earth a little bit to us you don't notice it so much you know the moon pulls. On the earth a little bit just, like with. The ocean tides exactly. The moon is such a big body, compared. To anything. Else very nearby then it has the larger effect pulling, the water of the earth but more than the moon think about the Sun pulling, on the earth we, orbit, the whole Sun just the way the earth pulls, on the moon and causes, the moon to orbit us all of those things are adapting on you and me right now, gravity. Was too strong will we be able to get up that's, such, a good question no, we. Actually couldn't the moon gravity, is weaker can, almost float, between footsteps, if you look at the astronauts, on the moon on, the, earth it's harder because it's bigger if you go to a, bigger heavier planet, it gets harder and harder but, there are stars that, have died that are so dense that there's, no way we could lift, our arms no, way we could step, or walk but the gravity is just way too strong, do you know how tall you are I mean the force in, the force maybe for three people think that while, you're sleeping, your body has a chance to stretch, out and grab.

Crunching. You together but, when you're standing and walking or sitting that, gravity, contracts. Your spine ever so, slightly so that in the morning, you. Might be a little bit taller than in. The evening see, if it works for you. Yes. They. Say that astronauts, in space definitely. Their spinal long gates there were two twin. Astronauts. One, who, stayed here on earth, and the other who went to the International. Space Station it, was there for a long time and when, he came back he, was actually taller than, his twin brother. Yeah. And that was because gravity wasn't compressing, him all the time and he, was floating, freely, in the International, Space Station and his, spine. Just kind of elongated after, a while here on earth though he'll readjust, they'll go back to the same size have you ever heard, of how cavity was, discovered. Isaac. Newton would, ponder how, does the earth cause. Things to fall there's, a famous story that Isaac, Newton was sitting under a tree and the apple, fell, from the tree and hit him on the head and he had an epiphany and, understood. This, law this mathematical. Law for how that works I don't actually think it's a true story but. It's. A good story so Isaac Newton who realized that even if you're heavier you, will fall at the same rate as, something much lighter that, that's the same once, you hit the ground if. You're heavier you'll hit the ground with much greater force, but you will hit the ground at the same time so if we both drop down from. The plane we would both wind. At the same time but you would land heavier, yep so like a penny from, the Empire State Building will. Fall at the same rate as a bowling ball. Yeah. Amazing. Want to try it oh yeah a light object see. How light that is that's, very. Late and. A, heavy object. They. Look the same but, this is much heavier right okay, so try it just try holding your arms out and a little higher maybe give him a chance to drop and then drop them. Did. They fall at the same time did he hit at the same time so, Isaac, Newton, he was also the. One who realized, that, that's the same force that keeps the moon in orbit around, the Earth on the earth in orbit around the Sun and that's a huge leap here he is looking, at just things around him and then looks at the Stars and has, this really big realization. That, that's actually the same force so what, have you learned today. Talking about gravity, I've learned, that. The person that learned about the Apple, Newton, um he was learning about gravity, just about what he saw on this planet, I also learned, that, if you drop one light thing one heavy thing at the, same height at the same time they're both gonna drop at the same time but one's gonna drop a little heavier than the other that's, beautiful. I'm impressed. So. Rio you're in high school yeah I'm a junior and are you studying any Sciences in high school I'm taking, physics right now do you think, of yourself as curious about science well, they're like some things that interest me and how is that for me is it like depends, what. Interests, you well like I'm a gymnast, selling in physics, they talk about like force, and stuff and then I think of like how I use physics, and like my own life what's your impression of what gravity, is I, think that if there's like no gravity, I really like float everywhere, it like pulls things, down and, without it like everything, would just be like chaos, so, you're saying gravity, pulls things down yet. We've launched things into.

Space If, you ever wondered how we do that that's it like a slingshot like if you put something back enough, it'll go in the opposite direction, well that's true we do use slingshot, technology, once things are out in the solar system so for instance we use Jupiter, and other, planets so that when, some, of the spacecraft, gets close, it'll. Slingshot, around, and, it'll cause it to like speed up but mostly, around the earth gravity pulls things down so when, we want to send. A rocket into space and, we want to go to the moon. Some supplies. To the International Space Station the. Trick is to get something moving fast enough that it escapes, the, gravitational, pull of the earth have, you heard the expression what, goes up must come down it's, actually not true if. You throw it fast enough you can actually get something that doesn't come back down again and, that's basically, how rocket. Launches work you have to get the rocket for the earth to, go more, than 11 kilometers, a second, think. Of how fast it is just one breath and, it's gone 11 kilometers, if you get it to go that fast it's not gonna come back down again so you know the International, Space Station where, which is orbiting the Earth that's going, around, the earth at like 17,000. Miles an hour it has no engines anymore the engines are turned off so, it's just they're falling, forever, so once it's out there it's not coming back down as long as it's cruising like that and like does, that gravity part and it is just like in a weird way that is gravity pulling it so have you ever um had a yo-yo where you swing it around like this the, string is pulling, it in at all times but you've also given it this angular, momentum. And as long as you give it the angular momentum, pulling. It in actually, keeps it in orbit and so the earth is pulling, it in at, all times so that's why it doesn't just travel, off in a straight line it keeps coming back around, so. It's funny people think that the International, Space Station is so far away that they're not feeling gravity, and that's not the case at all they're absolutely. Feeling, gravity they're just cruising so fast that. Even though they're being pulled in they never get pulled to the surface it's, like that the. Roller coaster is really yeah you go in and it spins super fast and you can't feel it spinning fast yeah you feel pinned to that it's, exactly, like that there's, something called the equivalence, principle where. People, realized and specially Einstein that, if you were in outer space in a rocket ship and it was dark and painted and it was accelerating. At exactly, the right rate you, actually wouldn't know if you were sitting on the floor of a building around the Earth or, if you want a rocket ship that was accelerating, it's crazy, you, ever had that experience where you're sitting in a train and the other one moves and for, a second you're not sure if you're the one moving yeah, cuz I don't train, go to school I never feel like I'm moving when I'm in the chain and then I'm like wait what that's, because in some sense you're really not imagine. You're in this chain and it's going near the speed of light relative. To the platform, but, it's so smooth, then you should be in a situation in which there's no, meaning, to your absolute motion there's no absolute motion so that if you throw a ball up you might think from the outside of the platform, be, confused, that when gravity pulls that back down it's gonna hit you or something, but it'll land in your palm as surely, as if you were in your living room it's, not kind of easy so imagine you are an astronaut, you are floating in empty space you can't see anything there's no stars, there's, no earth you, can ask yourself, am, i moving just, really no way for you to tell so you would probably conclude well I'm not living so, then your friend marina comes, cruising, past you maybe she's going thousands. Of kilometers a second, and he said marina you're cruising, it like thousands, of kilometres a second you're going so fast but, she had just done the same experiment, she was just floating in space thinking. You, know am i moving there's, a no way to know which one of you is moving and there's no meaning. To the absolute motion the, only thing that's true is that you're in relative, motion that's, true you both agree you were in relative, motion and that's, clear but neither of you can say it's, actually, you who's moving and I'm stationary, I.

Don't. Even know what to say to that so. Let me tell you where. It gets really crazy. So. Let's say you. And marina are floating, in space and you can't tell, who's moving let's, say you, both see a flash of light the. Flash of light comes from somewhere you don't know where so you measure, the speed of light to be 300,000. Kilometers, per second but here comes marina and she's racing. At the light pulse as far as you can tell two cars driving, towards each other, seem like they're going faster, towards each other than somebody who's standing still relative, to one of the cars right so. You would say Oh marinas gonna measure a difference be a light but she comes back and she says nope. 300,000. Kilometers per second because from her perspective she's, standing still and the laws of physics had better be the same for her the speed of light is a fact, of nature that, says true. As the, strength of gravity, and the two of you are in this quandary because of one of you is the preferred, person, who correctly, measures, the, speed of light that, ruins, everything about. The idea of the relativity of motion which one of you should it be so Einstein decides, they. Must both measure the same speed of light how, could that possibly, possibly. Be the case and he, thinks, well, if speed, is how. Far you travel, your spatial, distance, in. A certain, amount of time then, there must be something wrong with space, and time and, he, goes from, the constancy. Of the speed of light and a respect, for this idea of relativity, to, the idea that space and time must. Not be the same for, you and, for marina and that's, how he gets to the idea of the relativity of space and time. You're. The best expression. It's. Pretty wild but that is a starting, point actually, of the, whole theory of relativity that starting. Point leads to this complete, revolution, in physics where we suddenly have a Big Bang in black holes and space-time just, from that one simple, starting point so is your impression of gravity, different than when we started the conversation yeah. Cuz like I knew, that like when I was on the train I didn't feel like I was moving but I didn't know like why, and that it, was a thing and I wasn't crazy yeah, and. It's really deep, principle. And what about the theory of gravity I don't, know like, usually what I just heard gravity, it's like for my coaches, but I didn't know like it was like all these things mm-hmm, it's like a big paradigm. So. You're in college yeah, what are you studying in college I'm a physics, major so from your perspective how, would you describe gravity, I'm taught that it's a force it's. Described, by in first law but, I also know that's a field and there's. A recent discovery with like gravitational, waves although I don't like the specific details of that about, that so when you say it's an inverse-square law yeah that means that the, closer you are yeah the more strongly you feel the gravitational, pull yeah and that makes sense yes very few things that are stronger when you're further apart yeah so you can also think of a gravitational. Field something. That permeates all, of space you, know the earth. Three stories below us, it's, not as though it's pulling, at us from a distance, right, it's we're actually interacting. With the field at this point and there's a real interaction, right, here at this point and, that's nice because people, were worried that if things acted at a distance, yes that, the way the old fashioned inverse-square force laws describes, it that it was as spooky as like mine bending a spoon that it was like telekinesis. You know if you don't touch something how. Do you affect, it and so, the first step was to start to think of gravity as a field that permeates all, of space and it's weaker very far from the earth and it's trauma very close to the earth so one way to think of this field is as a field. That's really describing a curved space-time, that, is everywhere, forget, the difficulty, of the math just the intuition comes, from two kind.

Of Simple observations, one, was, what. Einstein described as the happiest, thought of his life so right, now you, might, feel heavy in your chair and we. Might feel heavy, on the floor in our feet or or standing, in an elevator cab and Einstein, would, have said what. Does it chair you have to do with it or the floor or the elevator, those aren't gravitational. Objects, and so. He wanted to eliminate them, and one way to do the thought experiment, is to imagine standing, in an elevator, that you can't see out of a black box and imagine, the cable, is cut and you, and the elevator begin to fall so I'm free fall you're. In total free fall mhm now because things fall at the same rate yeah including the elevator and you yeah you can actually float in, the elevator if you just float, it in the elevator the two of you would drop and you might not even know it you're falling you can take, an apple and drop, it in front of you and it would float in front of you you would actually experience. Weightlessness. It's, called the equivalence, principle it, was Einstein's happiest thought that, what you're really doing when. You're experiencing, gravity, isn't, being heavy in your chair it's falling. Weightlessly. In, the, gravitational, field and that was the first step to think of gravity, as weightlessness and falling I know like as your gravity experienced, you're done with. Planes. Emily, yeah exactly yeah you, can make somebody look like they're in the International, Space Station by, flying, up in a plane and then just free-falling. The plane just drops out of the air and while. It's falling they. Will float weightlessly and there's been a lot of experiments, about it but you don't want it to end unhappily, the play has. To scoop back up yeah and then you see them become pinned to the floor oh okay, because then the plane is interrupting, their fall so, that's the first thought and then the next, is what. Is the shape that's, chase so if you are floating, in empty space really, empty space yeah and you had an apple and you, threw the Apple what shape, do you think it would chase the path well, if I threw it straight I would think it would go out straight yeah it would just go straight but if you did that on the earth what would happen it. Would just go down yeah, but it would call. It with tracing curved oh yeah tracing a heart yeah and the faster. You throw it it'll look kind of long or the arc so. The second, step to, thinking about curved space-time is to say that when things fall, freely. Around. A body, like the earth they. Trace curved, paths, as those, space-time. Itself space. Itself was, curved, and. So. So. That's the intuition that's. How I'm Stan Getz from thinking, that. Space-time is curved, from. The idea that well there's just field that permeates all of space and what it's really describing, is the curves that things fall along and from. There it's, a very long path to finding the mathematics, and the right description that's really hard but that intuition is so elegant and so beautiful, and just comes from these two simple thought experiments. That's amazing, isn't it kind of a, so. You described, learning, in a class about light the. Theory of special relativity or. Einstein is, really, adhering, to the constancy, of the speed of light and, questioning. The absolute, nature of space and time and it seems like that has nothing to do with gravity but. He later begins, to think about the incompatibility. Of gravity, with, his theory of relativity so, suppose the Sun were. To disappear tomorrow from like evil genius comes and just figures.

Out A way to evaporate, the Sun in, Newton's, understanding. Of gravity we would, instantaneously. Know about it all the way over here at the earth and, that's incompatible. With, the concept, that nothing, can travel faster than, the speed of light no information. Not even information about the Sun could possibly, travel faster than the speed of light so, we shouldn't know about what happened to the Sun for a full eight minutes, which, is the time it would take light to travel to us and so, he begins to question, why gravity is so incompatible. With, relativity, but he already knows he's thinking about space and time in relativity, so then he gets to his general, theory of relativity where. He realizes. If, I eliminate everything. But, just the gravitational. Field of let's say the earth and I look at how things fall and I see that they follow curves, well, then he realizes that space and time don't, just contract, or dilate that, they can really warp that, they can bend and that they can curve and then he finds a way to, make gravity. Compatible. With relativity, by saying if the Sun were to disappear tomorrow the. Curves, that the Sun imprinted, in space-time would. Actually, be into ripple and those are the gravitational, waves and they would change and, they would flatten out because, the Sun was no longer there yeah Matt would take the, light travel, time to get to us to tell us that the Sun was gone uh-huh and then we would stop orbiting, and just travel along a straight line Wow. Wow. Well. Let's hope it doesn't happen. So. What do you think you walk, away with what do you think you learned well. More. About the intuitions, behind, the concept, because. We already just do, the problems, but, sometimes, you get lost in a math but, like speaking, like this it's really helps, build by intuition yeah it just, it does for me too so thank you. So. You're getting your PhD in physics that's, right theoretical, high-energy physics, basically, the physics of really really small fundamental. Things so what would that have to do with gravity or astrophysics. Well what I'm looking at is states of matter that might exist inside neutron, stars so when a star dies if, the star is massive enough there's a huge explosion called a supernova and. The, stuff that's left behind that, doesn't get blown away collapses. Into a tiny compact blob, called a neutron star so, what I love about neutron, stars personally, is that they're, kind of City sized that's, right about the size of a city so you're imagining something more, than the mass of the Sun yep, or about the mass of a Sun condensed, to the size of a city dense enough that one teaspoonful, would. Weigh about a billion tons here, on earth no that, makes, the, gravitational. Field. Incredibly. Strong that's, right around the neutron star so, what would happen if we were on a neutron star because. We. Would immediately be, crushed into the ground I think our bodies would be shred into their subatomic particles. So what's the connection between neutron. Stars and black holes so, as I understand, it a black hole sort of like a neutron, stars big brother it's more intense if, you have so much matter when. A star is collapsing, that it can't hold itself up it, collapses. To a black hole and those, are so dense that space-time, breaks. Down in, some way or another black holes are so amazing, that where, the neutron star stops and there's something actually there, right, there's a material there, if it's so heavy it becomes a black hole so it keeps falling once. The, event. Horizon of the black hole forms, which is the shadow the, curve that's so strong that not even light can escape the, material keeps falling and like you said maybe. Space-time, breaks down right at the center there but whatever happens, the, star is gone that. Black hole is empty so in a weird way black holes are a place, and not a thing so, is there a sensible, way to talk about what's inside a black hole or is that should you think of it as there.

Is No space-time. There. Isn't a sensible, way to talk about it yet and that probably, means that's. Where Einstein's, theory, of gravity. As a curved space-time is beginning to break down and we need to take the extra step of go took some kind of quantum, theory of gravity and we. Don't have that yet so even though the black hole isn't completely understood, we do know that they form, astronomically. That in the universe things, like neutron stars form and things, like black holes form, the consequences. Are very. Much, speaking. To this curved space-time so, for instance of two black holes orbit, each other they're, like mallets, on a drum, and they actually cause space-time. To ring and it's very much part of gravitation, the ringing of space-time, itself we, call gravitational, waves and that's what's what's something Einstein, thought about right away in 1915. 1916, he was thinking about that those, waves are very exciting, for me too because neutron, stars orbiting, each other also give off gravitational. Waves and we might be able to get some data about neutron. Star material from, that kind of signal yes the ring space-time, also like a drum and you can record, the sound of that ringing after, a billion years when it's traveled, through, the universe but then the next thing that happens, is those neutron stars collide, and because. Of this incredibly. High energy, state. Of matter which you study it, becomes, this, firework. Of. Different. Explosions. It's really quite spectacular that's right in fact when we recorded, that for the first time with gravitational waves we then pointed telescopes, at it and were able to see it optically, as well and that gave scientists. A lot of data yeah it was um to, my knowledge the, most widely. Studied astronomical. That in the history of humanity Wow, so where the gravitational, waves were recorded, and they realized, oh this sounds like you can reconstruct, the, shape and size of the mallets of the drum from the sound these, sounds like neutron, stars colliding not black holes and so like you said there was a trigger for, satellites. And experiments, all over the world to, point roughly. In the direction that the sound was coming from so from your point of view I mean they're like two superconducting. Giant magnets, colliding. An experiment, you could never do on earth that's, just of the most tremendous, scales, and, peculiarities. Of matter absolutely I've heard statistics, like many earth masses worth of gold or created, forged, in the yeah, a neutron, star collision that. That we used to think that most elements. In the universe were created, in supernovae, which is when stars explode because. There's so much violent, activity. At the center that you need that kind of energy to, create new, elements the way you do in a bomb it's basically nuclear fusion sure, but we now think that that kind of fusion happens, when two neutron stars collide. If. You think about it you have two massive, blobs of neutrons when, you smoosh them together, you've, got neutrons, colliding, it creates, the conditions, where new elements, can be created, that's amazing it's literally populating. The periodic table yes, we now think that most, of the heavy elements after, some, number are. Created in neutron star collisions, so, you are, already a PhD, student you know a lot about gravity, but what do you think you've taken away from this conversation well I've definitely taken, away that the way that we think about gravity, today is very different, from how Newton thought about it and. That even though we have a very good understanding there's. Lots of things that we don't fully understand, there's still a lot of questions to be answered which i think is really exciting see, you're a scientist, isn't. The best part being able to ask the questions oh yeah. So. We've been talking about gravity, from, Newton, and, celestial. Bodies the earth the, moon pulling, on each other in the conventional, sense of gravity, being an attractive force to, the. Earth creating. Curves in space-time then, we moved, on to just diffuse. Seas. Of energy, and space-time, as, the. Real universe, and gravitation. Is really just, talking. About space-time, in general and here, we are and, you're. Really hardcore in theoretical physics where would you take the, exposition of gravity, from, that point well one thing is quantum mechanics quantum mechanics, it's the most accessible theory in the history of science it, explains the, most, different phenomena the most precisely yet, many.

People Would still say we don't understand even. The basics of it so when we think about quantum mechanics we think about particles. And their quantum charges and in. The Fineman way the way that Fineman taught us you know they come in and they exchange, a force carrier and then they come out again so that's how we think of an electron, and light scattering, for instance or something like that and the language that einstein, gave us is so different it's completely, geometric, it's all of this space-time and it's also unnecessary. Yeah. For me the beauty, of the theory of gravity is is the way Einstein formulated, as a theory of geometry, of curved space, and time I think like you that's one of the things that really pulled me into it is there really space-time or are we just using. Unnecessary. Language. Because it's elegant, and we like it and it's beautiful well I think there is really space-time in the sense that it's a description that works really well so there has to be something right about it I mean if we're gonna talk about what's really really, underlying, that yeah and we're gonna put quantum mechanics into the mix then. There should be some quantum, mechanical, wave function for, space-time. You should be able to take two different space times and. Add them together because one of the crazy things about quantum mechanics as you know is its, together yeah any two states any two possible. States, of the world you can just literally put a plus sign between them and that's a that's a sensible state that's a good state make sense so do you think there's. Some sense in which we shouldn't be thinking about individual, universes, individual, space-time so we should be thinking about superposition. Subspace limbs yeah, I think so I think if you were to go far, enough back in the history of the universe back to when it was very very dense very small and when, quantum mechanics was certainly important then, it must, have been like that I mean if we believed that the dominant, standard, model of cosmology something. Had to produce the density perturbations, the, things that seeded all the galaxies and stars and, us everything else in the world so there's a galaxy over there let's say and not over there so, how did that happen why is there galaxies they're not there in, the standard theory as you know that was a quantum event a random event yeah and it doesn't mean that it happened they're not there because you flipped a coin it actually happened in both places there's got to be a wavefunction where. In one branch of the wavefunction its galaxies they're not there and in the other branch it's the opposite, so when we're talking about, the. Multiverse, or the Big Bang we are really, talking about gravity, ultimately, and we're talking about how a theory of gravitation which we now think of as a theory of space-time, has, a quantum. Explanation. Has a quantum paradigm, imposed on it that will help us understand, these things and we don't have that yet one of the things that I think is so amazing is that the terrains, in which we're going to understand, quantum gravity, are very, few, right, it's the Big Bang because that's where we know that quantum, and gravity. Both were, called, into, action and there's, black holes one, of the most interesting discoveries, of course Hawking's, discovery. Kick-started. A kind. Of crisis. Right, in thinking, about why quantum mechanics and gravity were, so knocking. Heads it was one of the most beautiful.

Examples Sure, yeah it's it is a beautiful, beautiful idea so first of all to be totally clear that we've never observed Hawking, radiation which, is what he predicted directly, I don't think very many people doubt that it's there but yeah, discover it mathematically that's when, you, have a black hole it's, got an event horizon it's got a surface, which, is a point of no return if you fall through that surface no. Matter what you, have no matter how powerful the rocket you got even, if you beam a flashlight, back behind you in the direction you fall from nothing, escapes not even like it all gets sucked in and spaghettified. And destroyed, at the singularity or, something something happens it, doesn't get out but in quantum mechanics you can't, really pin down the location of something precisely, if, you try to pin down an electron and a tiny circuit, and a microchip sometimes. You discover that it's not actually there and then your computer crashes this is Eisenberg uncertainty principle, in, reality, you can't precisely say, where, the electron is and you, can't precisely say how, quickly, it's moving exactly, yeah so when you get the blue screen of death that might be because of quantum physics you. Know you try to pin something down near a black hole well it's a surface it's got a particular radius for a round black hole and you want to say something is inside or outside well you can't absolutely say that quantum mechanics and this kind of uncertainty produces. A, radiation. Which. You can think of is pulling some of the energy out of the, black hole the black hole was formed out of some mass and there's an energy in that you, can think of pulling some energy out of that and sending it off to infinity in the form of particles being emitted yeah, and what Hawking found is that it's a thermal spectrum it looks like a hot, or not so hot for a large black well but like an oven the kind of radiation that comes out of it right cast-iron this idea, that the darkest, phenomena in the universe actually is forced, to radiate, quantum particles, it's. Pretty wild I think everyone understood that the that, it was a correct calculation. But. I don't think a lot of people understood the implications, that, it meant something really terrible was happening because. This black hole which could have been made of who knows what is. Disappearing. Into these, quantum particles, which in some sense have nothing to do with the material who went in so do you think that's a big crisis the black hole evaporates, the information, is lost it's. A crisis, because, of some of the details of it but I would say the way you just described it I mean if I build a big bonfire or, an. Incinerator, and I throw an encyclopedia into, it good, luck reconstructing, what was in that inside the information is lost for all practical purposes practical, fair yes so this is a huge crisis because either quantum, mechanics is wrong and as you described, it's the most accurately, tested, paradigm in the history of physics how could it be wrong right or, the. Event horizon is, letting information, out and violating, one of the most sacred, principles, of relativity one thing about quantum mechanics is that anytime, you have a state of the world in another state of the world you can you can literally add them together and get a third possible, status, raises. That sounds and so, if you're gonna have a quantum theory of gravity. Then we can't really talk about there being a black hole or not a black hole right. Or an event resident, or not innocent, because we could always take a, state.

That Had an event resident in a state that doesn't or has the event arise in a slightly different position maybe and, add them together so, the existence, or position, of an event horizon can't possibly be you know determined, as a fact any more than the position of electron is determined so I think that's the loophole that's a nice way of looking at it so that you're not actually violating. Classical. Relativity. Once you're in a regime where, the wavefunction. Has really. Peaked around a very, well-defined stage, that's right I'm whenever one, of the most exciting developments, in the last 10, or 20 years is called holography, and it's called holography because, a hologram, is a two-dimensional, surface it creates a three-dimensional image it's got sort of 3d information built into it right and this in a fundamental, way really has that freedom for a higher dimensional, information built, into it yeah it's exactly the same as this, theory of gravity in more, dimensions yeah so one of the things I like to think of it with holography is that I can pack a certain amount of information a black hole I mean you can literally think of it as like returning things into it let's say I have information and some volume and I'm under the illusion that I can just keep packing information. In that volume as much as the volume will contain, eventually. I'll make a black hole and I'll find out what the maximum amount of information I can pack into anything in the entire universe is what I can pack on the area right and since areas projecting. The illusion, maybe a volume, maybe, the whole world is just a hologram it's not a principle that only applies to black holes it's saying, that, if, this theory of quantum gravity is correct then this, whole three dimensionality, is an utter illusion, really, the universe is two-dimensional. And. As practically speaking you mentioned before in a conversation, that it's really interesting the Heisenberg uncertainty principle is a practical, limit now in microchips, yeah, and if we make microchips month much smaller than they already are even, as they already are it, causes, errors because you don't know that backgrounds, and if holography if, this limit on how much information you can ever pack if that ever became a limit as far as we know that's an absolute limit right we started off with with clay tablets. Not so much information mm-hm. Then. We had written stuff it's getting better and say computers with thin paper that's even better CDs. Dancer, dancer inflation eventually make a black hole at some point you try to fill. Up your encyclopedia. With knowledge and you get. The. Most knowledge you ever have would only be a two-dimensional on a two-dimensional surface right as big as the universe and then you're done yeah so you know it's not likely. That we're gonna hit that limit any time soon do you think it's possible that. Gravity. Is really, ultimately, just. Quantum, mechanics, and doesn't, exist, at all in the fundamental, ways that we've been talking about so far like the. Newtonian way and the space-time way that those are just these, kind of macroscopic, illusions. And sometimes I talk about in terms of temperature temperature is. Not a thing there's no single, thing called temperature, right it's a macroscopic, illusion.

That Comes from the, collective, behavior really, quantum behavior of random motions of atoms and is, it possible at the whole of gravity, is like some kind of emergent, illusion. From what's. Really quantum phenomena, underlying it well if we buy the idea of holography then, absolutely, that's for sure that's, what that's what it's telling us well though which side is the illusion in which that is the reality right they're the same I mean temperatures still great to talk about it doesn't mean we shouldn't talk about temperature, I mean we should absolutely adjust, your thermostats, and talk, about temperature but if we look at it closer and closer and closer we realize there's not a thing in the world that has a quantum, value. Temperature. Yeah isolated, and so maybe there is no such thing as. Gravity. Isolated. From, quantum mechanics right, so I guess with the wholly graphic description we've got two sides which are which are actually secretly the same on one side there's definitely, not gravity on the other side well, it's a quantum, theory of gravity whatever that means but, the point is you can get it out it's equivalent to this neighborhood deficit, that's like just saying there's the the idea of a dual description, is just saying there's a perfect, dictionary between these two descriptions and so to pull, a bore which one's real is silly it's like saying as French wheeler is English real right yeah, no I like to give is if, you take some extra dimensions and. You, compactify, them let's say just one all that is it's exactly prevalent, if whatever particles, you had whatever fields you had in your original theory before you, just add an infinite Tower of new particles with certain properties that are all easy to calculate for, me it's a question of which one which description is most useful I mean, if you want to say gravities and it's all quantum that's great but then you fall down the stairs and bang your head.

Like. This description. That. Works pretty well you don't go to the doctors, yeah I forgot, certain defensible cost plus series of fluctuations. So. There's so many open questions the fact that there, are all these fundamental, issues that we really don't understand, but. On the other hand there's all of these moving parts that fit together so neatly there's definitely, something that's working here but, ultimately what, is going to emerge from that what structures lying under it we just don't know but I think the. Fact that there are so many fundamental. Questions that we just don't know the answer to that is an opportunity that's exciting. It's great thanks so much for coming it's, really good to have you here thank you very much John it was my pleasure I. Hope. You learned something about gravity, you hadn't thought of before and I hope even more that it provoked some, questions, so thank you for watching.

2019-12-23 15:38

Show Video


The child made me smile

I just know the high school student is just like me and lowkey doesn’t understand

haaaaaaa tricia takanawa not a news reporter no more

What about quantum knot theory, where gravity is an emergent phenomena? What about gravity behaving differently at galactic scales, which determines the shape of galaxies and postulates dark matter? Where do those fit in?

The biggest library in the university would be a blackhole. Given the quantity of light that falls in to them they are like faint CCD cameras taking million megapixel videos in uHD.

Level 1:child Level 2:teen Level 3:college student Level 4:grad student Level 5:expert Level 100000000 :flat earther

the teen was definitely getting a little lost at the end but to be fair i don't really think space time relativity is high school level physics haha. she seems really nice tho

What I learned is, that you can't talk about gravity without talking about relativity

She's so great in explaining one of the ambiguous concept in Physics.

omg her teaching energy reminds me of Carla Lalli Music but for astrophysics

Bonet explains gravity Me at that age: Scratch scratch...

I want my kids to be as smart as that 8 year old lol, but knowing my luck they will still wonder how to peel a banana at that age.

does gravity push or pull on us? Edit: i am kinda dumb so sorry if that is a silly question

Like, I can't believe like, I was like, a teenager once.

dim the lights and slide a table with a candlelit dinner between last two

why do they always get really stupid teenagers for all of these


she is pretty bad in explaining things in basic language, the words she uses need more time to be processed/understood for people who dont use them outside of school. maybe she does give them time and simpler explanations but the editor cut them out, but i doubt it. only at the harder levels does she start explaining properly.

Yeah, i think she went way too advanced on the teen level. No fault on the teen.

They should get some Indian teen...


Please make some videos on cosmology like this

Sorry to be that guy, but she made a mistake at 6:52. We do not need rockets to go 11 km/s because they are continuously propelled, not just launched with an initial velocity. The concept of escape velocity is often butchered in explanation and that makes me sad :(

ello past







None of them talked about the Graviton!

I think one of these with a philosopher would be really interesting

This is insane. It finally makes some sense.


I can sense even at the hardcore PhD level, they are being very considerate of viewers like us.

God I love astrophysicists

love this

8:45 a better way of explaining that conversation might be that we don't feel movement. We feel acceleration, we feel the friction of the wind on our face... but we don't feel motion. So crusing down the highway at 60mph you may feel the bumps on the road but you are only pushed into the seat backs while accelerating to 60mph

the thumbnail girl looks like Kim kardashian


3:30 in Argentina. This is what I waste my time in.


I love how Janna subtly just shows how much smarter she is than everybody

Her relative motion example to the teen took me until 3rd semester physics(Quantum mechanics) to hear. I can imagine how amazing her stories to understand concepts would be if she was my professor. I made it through the first half of the expert discussion then...slowly became 2 steps behind on understanding. Still love this discussion.

I love the College student's outfit so much

Friend: Why did your pc crash? Me: quantum mechanics

The expert needs to move his head!

I really hope the kid goes into the science field

Why is the child smarter than the teen they really picked a dumbass teenager

This was amazing. Thank you!

No one: The college student: “mhm...” “nyeh”

That teenager checked out and didn't understand anything within like 5 minutes

topic, gravity, astrophysicist: what would happen if two black wholes spin each other, i totally loose myself in the conversation.

The teen is less intelligent than a child, wow.

Did anyone hear her saying," Hi, I'm Gen eleven" ?

The child's reaction to the morning/evening height experiment was adorable. If they did that with the 16 year old her brain would have gone kaboom like an austin powers fembot.

So like I stopped watching after like the teenager like said like like 10 times

The 4-year-old is smarter than the Junior...

Lol the professor and that smug look in the beginning of their conversation

I’m serious! The First two are the same person in different times

5 Levels of Vaccine?


This reminded me I really have no idea what physics is about. Her talking sounded like gibberish and I tried so hard to understand again but I really only understood up to the child. As soon as she got to the teen it was BEYOND ME

*Level 6* : Flat-earthers

You forgot Flat Earth Level!

I cannot remember the name of the theoretical physicist who said the infinitesimally small and infinitesimally large are the same thing . His theory says that when the universe expands to the limit where particles are stretched so far they can only be energy, and unable to interact, the universe would collapse and start all over. My question: Is the universe in that theory acting like a wave and collapsing to a point particle, and then expanding again. I watch your videos and find they help in my struggle to understand what theoretical, and astrophysicist are discovering about the nature of everything.

Last guys top lip doesn't move when he talks. Creepy



What gravity is Gravity isnot a force but you can think is a force Please stop brain wash

Is it just me or was first one smarter than the second one?

The teen is like dumber than like the child like

The last chat was very interesting. Wish there was a talk show you guys had where you just invite experts in their respective fields to talk to one another for 30 minutes about the state of science. Best way to learn what's the cutting edge and provokes one to question what they know.

Good thing the teen is good looking, otherwise good luck in life

I know some of these words

I'm happy that I was able to follow up through the grad student. During the discussion with the theoretical physicist, you said "Feynman" and then "wa wa wa waaa wa wa wa wa" like in Peanuts. But I am trying. But frankly, I like my objects to be where I put them. Probably this is why my field is nursing informatics and not physics.


The little girl was smarter then the teen

i would love to have that lady as a teacher

Lol she is not speaking at high school level. That girl was completely lost and I don't blame her

As a fleshy meat bag human i can say 100% that humans are not designed to go faster than a run

Thank God for beautiful educated woman

Last dude looked like he was ready to science debate warfare lol. #jakehunter88

If am travelling at the speed of light... How do i calculate the speed of light.. do i calculate it as 0 kms /s

no one: college girl: "yea" "mhm"

Renormalizing eg, put out your finger to see which way the wind is blowing and fit it in your equation #theory. Mathematics isn't Physics. Space doesn't exert any force. Vacuum is a medium. Mass is an energetic variable so G/ gravity can't be a constant.

Why does the little girl and the teen look like the same person but years later

I hope not

Flat Earthers triggered in the first minute.

The level one girl is so smart

In my opinion the professor didn't explain these concepts accurately. The purpose of these videos is not to talk in professional language, but to convey the ideas and concepts of certain subjects (in this case astrophysics) intuitively and as far as possible in simple language - especially in the first two levels. She was talking about fields and waves with the teenager, a fairly bad approach. The concept of gravity is quite abstract for "ordinary" people who are unfamiliar with these things. And please stop degrading the teenage girl or compare her with the 7 years old girl. That's ridiculous and disrespectful towards the teen. Before ridiculing the teen, first you should blame the Professor for her lack of accurate conveyance accessibly the subject. A significant amount of Professors lack the accessible conveyance of the subject matter, just because they are used to these things for more than 20 years. I appreciate the courage and intelligence of all these five persons, especially the first two, as they're not used and engaged to this subject!

Real scientists use metric

Bruh I did projects on string theory in the third grade don’t even try.

This 8 year old is actually very smart, my faith in humanity is restored.

I knew the concepts tillthe middle of 4 with average interestin the matter, super fun and i learned a lot of new concepts, please do this more often

meanwhile there are BILLIONS of delusional religious people praying to some magic fairies lelelel what is this world


do one on LIGHT

I really loved how the experts explained their ideas together. I obviously didn't understand it all, but was surprised how much I was following it.

Pls, more physics related videos

Much more interesting than I thought!

0:49 sarcasm 100

Smart people cross their legs

26:19 He is mirroring her.

The girls must be sisters

So what have you learnt today? The little girl: Well, gravity is the geodesic is a 4 dimensional Hausfroff differential manifold in which when a metric is imposed gives rises to geodesic and objects that aren't experiencing any force move along the geodesic. P.S: The little girl is really intelligent. And so are you!!

The reason they cast her was because she was beautiful other than she got nothing. You know who it is

One great thing about physics is you can simplify it so people like me can understand, One bad thing is you simplify it all you want I ain't gonna understand s**t

The final section was so interesting, I had to listen to it twice.

Halfway through the video, I realized the astrophysicist looks like a young Helen Mirren.

Loved this video

3:39 No ma'am that's now how gravity was discovered. That's how Issac Newton discovered that its the same force that makes heavenly bodies move the way they do also makes things fall on earth. It's not like one day we found a new guy, rather we just came to know that our good old friend is also the culprit behind the heavenly movement.

4 seconds into the expert section and I'm bored out of my mind. Next time I am having trouble sleeping I'll come rewatch this. Y'all are way over my head!

Oh yea I will like throw a pebble in the air look up wait a few second wait where did it go

She is an incredible teacher! Love her descriptions and examples.

Expecting a 16 year old to understand a crappy, incoherent, explanation of special relativity that is really more about believing rather than explaining, is like expecting any child to have zero difficulties in the whole of college. Honestly, special relativity just doesn’t make much sense when you use words, but math makes easy logical connections that is so fundamental to your understanding, after all Einstein’s relativity theories are heavily based on geometry. Plus, I don’t think most people here understand it either, rather they have heard the same story over and over and finally memorized it rather than learning the truths and equations. And the child is always exceptional as they are looking for children who actually want to participate in a logical discussion.

The amount of times that teenager said "like" made me physically wince.

It’s best summarized this way What-everyone How-college student Why-experts, sort of

That is one smart 8 year old. I probably didn't know of tides at that age.

All nightmare before Christmas ghouls explained!!!! Including how they all died pleaseeee

I love her voice, its so calming, thank you miss Janna

Yeah, I was thrown after the explanations to the child. I'm okay with that, I'm aware my skills lie elsewhere.

Fantastic video.

The strangest thing in the Universe is why the GOP keeps protecting Trump.

Lol you actually see the point the teen loses all interest in the conversation haha

Ngl, I just about understood the kids one lol then I shut off

fake news! the earth is flat !

She's the female version of Neil deGrasse Tyson.

Not a compliment

This has made me even more confused than I've ever been.

"One of the things that really pulled me into it" I love subtle scientific humor

I accidentally printed on the same page twice and made a black hole

"max absolute limit" = max amount of data we can ever know is the size of our universe. The fish reaches the end of its aquarium...Its like saying one pixel difference makes an identity. hmm...lot of info to take in here...

Turns out gravity is just wind pushing us down universally, no math needed.

Intelligent Falling. It is God who pushes us down onto the Flat Earth so we don't hit the solar light bulb.

If you can understand a simple formula you can essentially understand that certain factors are the only contributing things, you can expand on that very same simple formula (lets say ideal conditions in a nutshell) to then explain the smaller details that play a factor. Also i was kinda of expecting her to begin with explaining constant freefall (correct me if im wrong on this, but it was what I was taught in school)... as well as maybe explaining reference/baseline.

I'm always disappointed with the grad students found for these videos. They must be first or second years because they really don't have anything good to add to the conversation.

If the physics lady and the level 5 guy had babies, they would find the 4th dimension

Did i understand that right? So black holes cant be a problem for physics because of quantum physics you can allways say theres a state for things in which there is no black hole?

mhmm, yeah, mhmm, yeah...

Lmao, he kept rambling on and on!

The astrophysicist and the theoretical physics expert would make an annoying date match. She would leave that table in nanoseconds.

The teen was stupid, clearly gave up on science at a young age>

"I go on the train everyday when I go to school but I never feel like I'm moving and then I'm like, 'wait, what'?" Absolutely genius

High school was dumb. College was awkward.

These videos are great i always learn something

Gravity doesn’t exist… It is a theoretical force. Problem solved this person is an idiot

so if the gravitational waves imply softer & stronger gravity against us, does it ever get negative?

"Shaped the way we view the universe" and shaped the universe ;-)

For some reason I understood the higher level stuff better than the simple stuff

I’m almost certain the professor at the end is a cyborg

He's certainly pretty enough.

35:04 Shots fired .

hold me down forever like gravity

LIke, like how about stop, like using, like the word, like, like, for two, like minutes?

Gravity seems so simple yet I never quite understood it scientifically.

College Physics Major: "I''m taught that Gravity is a force" Really?

wow, this is one of the most interesting ones! the last bit was super interesting and mind blowing really, the stuff they say

The two quacks at the end, Levin and the "expert" are real real quacks. They live in fantasy land and have no idea of what gravity is. They just know how to do math but have no clue of anything else. And it's because they hold their religion, which was invented by Einstein and the other quacks, they hold it as divine. Then they go to the other 4 people in the clip and they brain wash them with their religion to keep the quackery going. That's how religion spreads itself, by misinformation under the umbrella of authority and the title of science and philosophy. This was a total waste of time and a piece of junk.

Could you enlighten me about their, "misinformation," and tell about gravity in the real sense then please?

This teenager seems a bit slow

the 16yr old kinda looks like emily wills

college student: "nnyah"

Janna Levin was my physics professor at Barnard College! And now I’m getting my PhD in physics right now

Level 1 Level 2 Level asian Level 4 Level 5

I’m pretty sure the little girl had a better understanding than the teen

The little girl is smarter than most of my coworkers :-(

The last two dudes were teaching her

that expert is a creep why is he looking at her like that I'm-

We stan a woman in STEM

The young girl was so smart and poised. Like a brilliant, respectable young lady.

I think the only problem the teen had with expressing herself was she used the word “like” too often which made her come off as less intelligent, but I think she did just fine

That gymnast can get it anytime

35:37 - I'm going to use that next time I'm at the doctor. Doctor: "Did you fall down the stairs?" Me: "No, the Heisenberg Uncertainty Principle caused a series of fluctuations and thus injured my head!"

I don't think the 16-yr old understand anything technical about the discussion. She did look like she was turned on though.

So what material are black holes made of?

As far as I understand, the question loses all meaning when applied to black holes because matter at infinite density has no form as we conceive it. That is, we conceive of elements, in solid, liquid, or gas form, made of atoms, which are made of sub-atomic particles. All that works until you get to neutron stars, which are broken down below the level of atoms (no elements, no solids, no liquids, no gas, just the crushed components those things are made out of). And past the level of neutron stars you don't even have those sub-atomic particles; maybe you have nothing at all (well, nothing except mass itself), or maybe you have something we don't know about.

holy snap: 11:18 is probably the best, most concise explanation of relativity I've ever heard. Amazing scientist and excellent teacher.

*what you thought sience was:* infinite dimensions *what sience actually is:* maybe there's like two dimensions

Finally someone who bothered explaining more to the kid than just the bare minimum. Everyone I've seen has been like 'you know about this? yeah okay well next level then,' like a kid somehow isn't able to understand more than that

At one point I half expected the two experts to say 'so yes, we ARE in the matrix!'

Do you know what a matrix is?

I’m gravitated towards Janna.

Lets go to Six Flags Like Rollercoasters place or whatever.

I'm flat earther

When she said her name, it sounded like she said jan 11

Great series of video

And now stringtheory

Gravitational waves travel faster than light?

No. It travels with speed of light

Please help clear up my confusion... If the elevator and the passenger are falling at the SAME rate, then why don't the feet of the passenger remain on the floor? The elevator must be falling faster.

Do flat earthers go to heaven? Dislike yes : like button no

Okay I am in 5th grade and it's ôo to see the high schooler that takes physics class be so non intelligent it's like omg a "slingshot" is called a gravitational assist and rockets accelerate similar to an explosion but slower and the liquid or solid rocket fuel rapidly turns into gas propelling the rocket upward

That expert gives me Ben Shapiro energy and idk why

There's something about his cadence, but that's about it. (Thankfully.)

Like stop like using the word like

This longer version is great, to not be so superficial

The teenager girl looks just like Kim Kardashian lol

She : So what do u know about gravity? Me : pretty much everything.. She : really, So whats your major? Me : umm.. Interstellar was a really educational movie.

that eye contact form the last guy is creepy af. he got the same eye contact the entire time ahahah

Gravity doesn't exist - every Flat Earther knows that.

top gun charlie?

Indoctrination at the finest

The girl is Kim K except smart and young

Expert has bedroom eyes

10:00 this is award worthy

Teen: "so like...swipe right?"

Shred into our sub atomic particles.

Sting Patrick Harris

Janna Levin and Matthew Kleban I send it they would make an amazing power couple.


Lasy guy is a great quantum Ventriloquist

You omitted level negative infinity: conservatives. It is impossible to teach a cuntservaturd anything.

A 16 year old, studying physics - not aware of escape velocity.

So, a penny and a bowling ball would fall at the same speed? Somebody please tell me the wind would be a factor

Rate not speed

I want more of that last conversation.

Time doth not exist! Movement does!

Gravity is the enemy of all illusions .... Force of mutual attraction will bring you right back to earth. - Biokonstructor.

Okay that little girl is so smart and well spoken!

Mhm yeah mhm yeah mhm yeah

Really great job through all five levels

Nothing sexier than smart women. .

The first one says yes to questions she doesn't know the answers to. Reminds me of engineering students pretending to know the answer to something instead of just saying no because they're scared to look dumb.

dude... why did the teenager talk like a child, while the child talked like a grownup...?

I got the red ring of death in my Xbox 360 because of quantum mechanics

You know that massive magnetic field around the planet? Yea? Well so called gravity is nothing more than magnetic acceleration. BOOM! solved, end of story.

take a drink everytime the college student says "nyeah" or "mmhmm"

what is spacetime?

Second girl be like , I'm like... and like and usually I like. So yeah like you know like.

I want more expert on expert talks about theoretical physics. I don't understand much of it, but it's amaaaaazing.

The teen was so annoying

what is the name of the physicist that spoke last?

This was the most honest, straightforward discussion about gravity I’ve ever heard, especially at the end.

best description i have encountered about what is going on inside and event horizon: there isnt a sensible way to talk about that yet.

I love this episode. Who loves this, too?

Is it just me or was that teenager high?

gallileo understood that the period of the pendulum was a mere function of the length of the pendulum ...

9:54 we lost her...

The 8 year old is very smart. I can tell she does and will continue to perform brilliantly in school

How about a feather and a bowling ball? Wouldn’t a feather’s rate be significantly lower than the ball as it’s considerably less weight?

this is so strange to watch. as a space enthusiast this concepts are so familiar, it's strange to see even a college student struggle with the simplest concept of free-fall and space-time fabric. i wonder why, after 100 years, this is not common knowledge already. it's not even difficult to explain/show. who doesn't know the the experiment with the sheet and a ball in the middle that creates a dip? kindergarten kids can grasp that. p.s. well, not even the "expert level" was really that, all very mainstream, you can read/watch that in every mag/doc available. experts would mention that gravitational waves have been recorded multiple times, yes (so its claimed), but the idea with pointing the telescope at it has worked only ONCE and could since not be repeated, so ppl have started questioning the data mining method (you have to calculate out noise...its complicated). no expert level talk here.

After lvl 4 it's no longer about explaining, its a freakin intellectual battle about who knows the most

How about String theory in 5 levels of difficulty?

I understood everything until the expert came

I loved the explanation to the college student and the conversation with the professional especially.. wonderful video

Dont make videos over 10 minutes, I won't watch them

Level 5 = Boss

i was good at the first one. past that, i'm gone.

fake, there are no girl physicists

tell me why the teenager is more ignorant than the kid

i officially fall into level 0

This confirms I’m just below elementary school level.

3:39 wasn't Galileo the first to find out that all objects fall at the same rate in free fall?

Nobody: College student: Mmhm. Yeah.

OMG the asían girl ni the 3rd level doesnt stop saying "mmhm" and "yeah" and im getting triggered

socks with heels... why?

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