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0:00
Imagine that you and I are on a mission to fall into a black hole. It's a bit nasty around these things! You cease to right stick stay together. We're one long string of atoms? What are we? Yeah and then the atoms get separated. If a black hole will one day end up as nothing the normal vacuum of space what happened to you and me? What happened to all of that stuff? I mean this is the central question. The laws of nature are quite clear
0:25
on this. So that's what bothered everybody for a very long time. What happened? What's the end of the story? So what happens - I'm so excited about this. The video you're about to watch is a deep dive into the cutting edge research happening right now about black holes. It's awesome. It's the extended cut of an interview that I did with famous physicist Dr brian Cox for a shorter huge if True episode all about what would actually happen if you fell into a black
0:51
hole. That episode is incredible and I've linked to it in the description but there was so much weird and mindbending stuff in this conversation that didn't fit into that shorter episode and I knew that you'd want to see it so we decided to release the full extended cut. It's just too cool and I think you'll love it so strap in because the cutting edge of black holes is wild. Great would you mind just starting us out by introducing yourself however you like. Yeah so
1:20
I'm Brian Cox and uh if you want the title the full title I'm professor of particle physics at the University of Manchester Royal Society Professor for Public Engagement in Science and Visiting Scholar at the Creek Institute in London. And how do you describe what you do every day? What I do every day physics um and that's kind of the way that I see myself if someone asks me what I do I say I'm a physicist um but actually of course most of my time now
1:50
is spent on the the public engagement side so the and I kind of fell into that accidentally but I still maybe it's a thing maybe there's some deep psychological thing going on but I never say TV presenter or whatever whatever it is I just say physicist. Well that's what I want to talk to you all about today so imagine that you and I are on a mission to fall into a black hole and we have some imaginary spaceship that can take us as far and as fast as we want so we get up
2:20
we walk outside we get into our spaceship what now what is a black hole and how do we find them so a black hole it's it's interesting that the idea or the first glimpse of them theoretically came very shortly after Einstein's theory of gravity was published in 1915 although it wasn't recognized as such at the time this glimpse but um essentially what what does Einstein's theory do it's important for what follows it's a theory of space and time and how space time which is often
2:55
described as the fabric of the universe responds warps or curves to matter and energy in the in the universe so the the equations basically the theory that Einstein published all those years ago will say give me some distribution of matter some something like a ball of matter and the equations will tell you how the fabric of the universe is distorted and by the way the force of gravity in that theory then is the response of everything else in the universe to that distortion
3:25
so Einstein would say what are we feeling now newton would say it's a force between us and the earth right but Einstein would say there isn't a force what we're responding to the distortion in the fabric of the universe created by the earth so that's John John Wheeler actually the great physicist put it beautifully he said matter tells spacetime how to curve and spacetime tells matter how to move and that's it right so that's Einstein's theory so in 1916 shortly after it
3:54
was published a man called Karl Schwarzschild remarkably managed to solve the equations uh for a perfectly spherical non-spinning ball of matter it's the simplest thing you could do which tells you how space and time are distorted by it and that's a model for a star it's the simplest thing you could do so he he solved the equations it's a remarkable thing in those equations there is a description
4:18
of a black hole although it wasn't realized at the time it's a remarkable piece simple piece of mathematics actually um so essentially what's the idea behind a black hole one way to think about it is that you could remove could you remove the star from from this fabric but leave the distortion behind so if if you if you do that you get the description of a black hole but you might say what what do you mean that how can that be formed in nature so you think about what a star
4:50
is then a star is a balancing act so it's a it's a mainly hydrogen and helium collapsing under its own gravity that's how our sun formed 4 and a half billion years ago so it's collapsing so what stops it collapsing well as it collapses the core heats up and that initiates nuclear fusion reactions in the core in in the case of our sun it's hydrogen being fused into helium that releases energy which
5:15
creates a pressure which holds the thing up so it's balancing but it needs the fuel and and it's not infinitely big of course so at some point it runs out of nuclear fuel and ultimately no more fusion reactions can occur in any star and so the star will resume its collapse so the question is well is there something that stops it because if there isn't something that stops the collapse then it will collapse without limit and and so actually if you look at the history of
5:44
physics in the 20s and 30s people were saying well we'd like to avoid this idea that the thing will collapse without limit because if it does then Schwarzschild's equation predicts some very strange things indeed and so people kind of tried to avoid it it was really actually Oppenheimer and uh his student Schneider in the late 30s just before the Second World War that showed that really with
6:09
some assumptions it looks like a massive enough star could actually collapse without limit so what does that mean collapses without limit it means that essentially it does what I said you essentially remove the star from the fabric of the universe leaving the distortion behind and the black hole the idea behind the black hole is let's say you take let's say you take the sun a star the mass of the sun and you just collapse it and you keep on collapsing it you get
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to a point when the the radius of the sun is not 700,000 kilometers which is what is in miles half a million miles i I I'm going to use kilometers because I can't remember the things in miles we can everyone can convert it afterwards right so 7,000 km and you squash it down and you squash it and squash it until its radius becomes 3 km three then there are several ways to look at this one is that on the surface the speed you'd have to travel to escape its gravitational pole it's
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called the escape velocity would exceed the speed of light that's one way to think about it so even light rays emitted from the surface if you could squash it down that far would just stay there they would not escape because they'd be trying to go at the speed of light and and the escape velocity is the speed of light and they just stop so what what happens then if a star collapses inside
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that number which for the mass of the sun 3 km is called the swatchial radius then it will collapse without limit nothing will stop it and so all you will get is essentially the geometry of space and time the curvature and that's a black hole so it's a thing that traps light in that sense so you think about that if you have this surface uh and space and time are so distorted there that if you
8:05
go in across that surface it's called the event horizon of the black hole then you can't get out you one way to think about it is you'd have to travel faster than light to get out another way to think about it is that um there's a beautiful model which is my favorite model it's called the river model of a black hole you can write the equations as as a space being like a river that flows into this thing almost like a sinkhole or something in space and the river of space flows
8:36
at the speed of light inwards on the horizon and then faster than light inside so if you imagine that you're a a photon a particle of light you're like a little fish swimming against the tide but if the tide's going at the speed of fast as you can swim the speed of light you can't get out not only can you not get out but you're going inwards towards something and this thing the the something is called the singularity you say "What is this thing the singularity?" And I think it's
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really tempting to picture it as some infinitely dense point to which this star collapsed when you draw a map of space and time what you see really clearly is that this singularity thing is not a place in space it's a moment in time and it's in fact the end of time in Einstein's theory so so a way that I often kind of picture it or explain it to myself is that space and time have become so distorted that when you look at it from the outside they flip roles so space has be space
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has become time and time has become space in the mathematics if you put a little graphic up you'll see that the plus and minus signs in the Schwarzschild and so they they change so what you thought of as as an infinitely end place in space to which the star collapsed has become a moment in time and a way to think about why you have to go to it then it's really beautiful because um you think it becomes something that's in the future for anyone or anything that crosses the
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horizon it's in the future so it's like if you say well I want to escape this thing it's like saying I want to escape tomorrow if I said to you let's run away from tomorrow you go I can't run away from tomorrow it's in the future that's what this thing behaves like a singularity so that's the the Einsteinian description of a black hole i want to take that step by step to understand it okay
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so in order to understand this let's imagine that we choose the black hole at the center of our own galaxy yeah sagittarius A star I think it's called I know that we took this picture of it in that is by the event horizon telescope Sagittarius A star what are we looking at here so here um the light you might say what is the light in this picture so it's not from the black hole cuz we've
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just said black holes trap light it's from what's called the accretion disc around the black hole so you're to imagine really material spiraling around in orbit around this very dense object um and very violently orbiting and so it heats up and it emits light you imagine a a flat thin disc of material around the black hole but this does not look like that um what you're seeing is the
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distortion the curvature of the light rays because we said that you might think well light travels in straight lines um but Einstein's theory says that in the vicinity of this thing the fabric of the universe itself is distorted so the light the paths of the light rays follow the distortion just like that's why in an animation it looks like it's going over the top and around the sides at the same time so what you're seeing this famous image of a black hole like if you think of the
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film Interstellar that that code by the way is an implementation of Einstein's equations kip Thorne and others help them do that so so it's really a prediction so what you're seeing so imagine this disc of material around this thing like picture in your mind's eye Saturn then light rays let's say from your perspective from behind the black hole on the let's say that on the disc go around into your eyes sometimes they orbit and then go around into your eyes and they go into your
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eyes from every point on the disc from underneath and at the top and behind so you have light rays going always around this thing so so then you so you see that that's what you see as an image so there's a prediction of Einstein's theory which is a real black hole should look like that it's characteristic donut shape that you see well here's here's an image with radio telescopes of of such a thing and it looks like the prediction it is a bit blurry but you know this is at the
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center of the galaxy very difficult this was actually the second one that was imaged the first one was in a galaxy called M87 which is 55 million light years away and is bigger than this one bigger which I mean it so this one's about um 6 million times the mass of the sun give or take uh the one in M87 6 billion times the mass of the sun so this is a baby super massive black hole that we know so this is my next question our black hole how big is it compared to other
13:24
black holes that we know exist so it's a it's a smallalish one you know we And what's this it's not smallish it's a it's a super massive black hole but we think that I would say all galaxies maybe the there's an exception or so but pretty much all galaxies have super massive black holes at their centers we don't quite know why actually we we don't quite know how galaxies form in the early universe it's one of the uh things that the JWST the James Web is is is looking at in
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some detail the the new space telescope but um to good approximation all galaxies have these things um and they can be different masses so as I said the the M87 galaxy has one that's uh 6 billion times the mass rather than 6 million times the mass but how big is it so the number I always remember in my mind is the swatch radius of the sun which is as we mentioned it's the the radius you'd have to squash the sun down to make a black hole and it's 3 kilometers um so
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it goes like the mass so you can work it out so if it's six if that's 6 million times the mass of the sun then the swatcher radius is 6 million* 3 km so it's kind of easy to do the math actually so that would be the the um you might call it you have to be careful with your language but let's let's say that this thing is it gives you an idea of the size of this structure um so the the disc is outside it is much bigger i did the math with earth's radius and it seemed to suggest
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that we would become a black hole if we compressed everything on Earth and all of us into something about the size of a P yeah it's about from memory about 0.8 cm I think something like that just less than a centimeter so you're right about that that big do black holes that small exist uh no we don't think so so even black holes the mass of the sun the the sun will not form a black hole
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when it runs out of nuclear fuel it will collapse and there's something that can stop it collapsing the force of electrons yes so it's called a white dwarf star it's beautiful calculation by the way then you can do the calculation it's great so what what stops it as an aside is that as you electrons there's something called the pi exclusion principle i read about this yeah so electrons don't want to be close together roughly speaking you can say it like that so
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as you squash the thing the electrons get closer together and so they kind of separate away from each other and go into smaller and smaller little regions of space because they're trying to stay away from each other but there's also something called the uncertainty principle heisenberg's uncertainty principle so as you confine them into smaller regions they they start jiggling around faster and faster um and ultimately you can reach a limit where they're essentially trying to
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jiggle at the speed of light and they can't and so there's a limit to how much pressure that process can exert and it turns out it's 1.4 times the mass of the sun which is called the Chandra Seca limit uh so you can do that calculation but it's a beautiful calculation because you could have worked that out not knowing that stars exist all you need to know about is quantum mechanics and
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relativity and you can make the calculation what is the biggest lump of stuff that can be held up by this jiggling of electrons it turns out it's 1.4 times the mass of the sun then you look into the sky and you see there are these things called white dwarfs these collapsed stars which are and there's none more massive than 1.4 four times the mass of the sun so it's very beautiful and then you can get neutron stars which are held up by the jiggling of neutrons but ultimately if you
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go to something that's three three times the mass of the sun something like that a bit more then nothing stops it collapsing and that's when you form a black hole so the most the lightest black holes that we know of around that mass right and then and then we know of them that are 10 20 30 times the mass of the sun from collapsed stars and then these things which are millions of times or even more the mass of the sun which are the heart of galaxies so we've
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launched in our spaceship we are hurtling toward this black hole could you walk us through step by step what happens from now until when we hit the event horizon yeah so the first thing to say is that it's right at the heart of Einstein's theory is something called the equivalence principle which was the idea that really led Einstein to the theory itself of gravity and um so you don't feel its pull you what you do is you just fall freely towards it so we're falling
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towards this thing now and we turn our rocket motors off um you we can't tell if we can't look outside we can't look out the windows we're just in freef fall we're just floating so it's like the atmosphere we're just there so it's fundamental to Einstein's theory it's very important actually for the problems that follow that we're going to talk about that you just freely fall towards this thing
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so we could be approaching this in this room now and we would have no clue that that that's what's happening and in fact so we're approaching the event horizon of a very big black hole like this of of a for a black hole of this mass then we wouldn't even notice something according to Einstein's theory as we fall across the horizon into the interior of the black hole so we could we would we'd fall across the horizon from our perspective in this room according to Einstein and
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we'll go for that caveat a bit later but according to Einstein into the interior of the black hole we go and we notice nothing what about when we're in the accretion desk wouldn't we be banged around by a lot of So we we get we might get in a bit of a mess as we pass through the accretion but we we're talking about the pure gravitational thing you're absolutely right it's a bit nasty around
19:20
these things so the stuff but that's nothing to do with the black hole itself it's nothing to do with the fundamental physics it's all the X-rays and all this nasty gamma all the stuff that's being um radiated from all this hot material around it so yes that would be a problem but if we had a let's say that our room this spacecraft is magically insul insulated from radiation and heat and all those things then nothing in we would go into the interior the caveat please there's
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a lot of caveats but one thing I should say is that it matters that that description I've given it's for a super massive black hole if this was a smaller black hole so smaller less massive you know a few times the mass of the sun then at some point you experience what's called tidal gravity tidal forces so those are things that raise the tides on in the oceans of the earth um it the the tidal forces we will start to in our freely falling trajectory towards the black
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hole at some point you start to get feel the tides you start to get stretched and squashed and there's a and we feel it you would start you'll feel it eventually you'll really feel it eventually because formally as you get very close to the singularity you get infinitely stretched and squashed so you really feel it i want to talk about that spaghettification yes that's my favorite but so for for for a for for a smaller black hole as you approach the horizon you
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you feel those forces but for the big ones you don't feel the tides until you've gone into the interior so we're moving toward the event horizon where somehow insulated from the messiness of the accretion disc we're moving toward it i've heard that there's a moment where the physics of this are such that if you and I are actually falling in if we look to the left and the right we would actually see the back of our own heads is that anywhere close to correct and can you tell me
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about the other little details as we approach the event horizon that you think are important well one of the biggest details one of it's not even a detail it's one of the most shocking things about this is I've described this that us falling in to the black hole and we're saying in this room where we can't see out what what do we what do we feel what can we measure and the answer is you can't
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measure anything and we don't feel anything until we get inside and we approach the singularity for this one but from the point of view of someone outside the description is very different so what would they see happening to us as we fall towards this thing so even in Einstein's theory with nothing else no quantum mechanics or anything then what they would see is time tick more slowly for us mhm so they would start to see as we approached the black hole if they could see
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our watches if we were transmitting to them or whatever it is transmitting from these cameras we were sending it out to them yeah they they would start they would see our time tick more slowly more slowly more slowly and they would see our time stop on the horizon so they would never see us fall in so from the point of view of someone outside nothing goes into the black hole ever so what do they see us imprinted there forever or does the light from us slowly fade until it would
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fade because the So also you could picture there are many ways of picturing it but you could say this light is climbing away from this through this gravitational field so it's getting stretched so we get redder and redder and redder redshifted infinitely redshifted time passes more and more slowly until it it stops on the horizon when viewed from the outside from our perspective we look at our watches they go 1 second per second and that's absolutely central there's
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nothing we Well there's something weird there but there isn't according to Einstein so it's pure Einstein's theory so that's the the first thing to say and it's a clue of to the interesting things about what we're going to talk about what follows is that there are different perspectives on what's happening here there's there's a there's a perspective from our point of view we're going
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in for a big black hole from the point of view of someone outside we never go in and that's going to be kind of important for what follows but there's nothing that's not described by Einstein's theory in in that particular bit of the description hang on let me show you something else that's really cool this is my privacy dashboard it shows how many data brokers I've gotten myself removed from
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actually our sponsor Incogn has gotten me removed from so why does this matter well data brokers make money buying and selling your information sometimes they sell to businesses so maybe I get more spam but also with more data these brokers can form a picture of you a shadow profile which they can then use in ways you might not want but incogn force many of them to remove you let me show you so they reach out to day brokers on my behalf and they say "Hey remove this person." And
24:32
then crucially they follow up to make sure that I actually got removed i really appreciate this and I think you might too if you want to try it out use the code Cleo Abram at the link below and get 60% off an annual plan now back to black holes so at this moment we cross the event horizon and we don't feel that we could have gone we could have crossed it now just now wouldn't notice and our
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experience now is completely inside the blackness of the black hole from an outsers's perspective
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gravity is increasing faster and faster and this is where we get to maybe my favorite word uh that I have learned in the process of this which is when the my understanding of this is when the gravity at your feet is so different from the gravity at your head that you begin to stretch in a very dramatic way and this is spaghettification could you explain what is happening here yeah as you get one way of thinking about it is that that the the the another way of thinking about
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it is just that the space the distortion in spaceime is is not constant over the length of your body so when we're falling in we might as well be this is the Einstein's equivalence principle in action we might as well be in flat space because the distortion it's like saying uh on the surface of the earth um if you look at a mile a square mile of the surface of the earth you don't feel the you don't see the curvature right you have to go to bigger distances to see
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that you're on a curved surface it's kind of like that so so the the distortion the difference in gravitational pull as you said or the distortion you don't feel it until it becomes very distorted or there's a big gravitational pull when you get very close to this thing And then you start to see that and actually it works it's not only stretching it's also a squashing so the way the tidal gravity works is to squash in one direction and pull in the other direction so we are getting
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so you feel it so you start to feel this strange sort of sensation of being stretched and squashed and and and as you go closer and closer to the singularity those effects become much more extreme until this so extreme that first of all you you cease to right stick stay stay together we're one long string of atoms what are we yeah and then the atoms get get separated and then the protons the quarks inside the protons will get separated and ultimately according to
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Einstein's theory the tidal forces become infinite So that in formally infinite and so everything has gone everything has been ripped apart and that this is what we call the singularity so so the whole thing kind of gets very extreme and breaks down ultimately in Einstein's picture the question that when I began this story I wanted to ask you is what is at the center of a black hole but in
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doing this research I now understand that talking about the center is also a little bit incorrect
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how should I actually think about what is happening at that singularity well I mean the the first thing to say is we don't know right so so with um we can talk about the current research and speculation um we don't really have the tools to describe it so a way to think about Einstein's theory is that as I mentioned earlier it really tells you how space and time are distorted they also kind of get mixed get up from from from the point of view of someone outside so then you'll
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see that space and time are are getting warped and distorted and as I mentioned they get so mixed up that on the horizon you see that they flip um so the thing to bear in mind for all that follows with Einstein's theory is this very central idea that if you're freely falling through space or over spacetime if you like then you don't in in the absence of these tidal effects you you really cannot tell that you're where you are in the universe if you're close to a black hole close to
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a big star orbiting around a galaxy just falling whatever it is so I think that's the key idea this so-called equivalence principle um but of course the as we said before the thing about a black hole is that why don't you see the you could say why don't I see these effects on the earth um this distortion this mixing or and you do um so you see it in GPS satellites for example so um if you
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think about what I said I said as you go closer to this thing then as viewed from the outside time passes more slowly um so you you could say well why doesn't the earth do that and it does so what you see is that time ticks at a different rate well by which I mean clocks tick at a different rate atomic clocks or biological clocks so you age at a different rate in orbit than you do on the
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surface of the Earth and you're seeing this is the same effect the the reason it gets very extreme and I should say it's quite a big effect for even near the earth so the drift is tens of thousands of nanconds per day time the the a difference in the rate that time passes at the at the orbit of a a GPS satellite and on the ground and we have to accommodate for that accommodate for that other it doesn't work but it's tens of thousands of nanconds per day even around the earth
30:07
so the question of black hole really is as you said before if I could keep the mass of the Earth the same but shrink it down to the size of a P then how much how does that distortion change as I go closer and closer to this immensely massive P thing then how does and and the that's this is the effect that that time keeps going more and more slowly from the perspective of someone outside
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until you see it stop on the horizon which as we said for the earth is around a centimeter just a little bit less um so so it's not it's kind of not in some ways this behavior of time is not it's not unique to a black hole it's just that in the black hole it becomes extreme mhm because the thing has completely collapsed so we have passed through the event horizon we have been spaghettified we have
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hit the end of time this is a question I think I know the answer to but I think it leads us into our next section the question is can we ever get back out so yeah so the the answer is according to Einstein's theory no um because you've gone to the end of time right and there's no way basically Einstein would say it's it's all over you've you know you've got ripped to bits everything has
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got ripped to bits you've gone to this infinitely distorted space and time and it's just done what would Stephen Hawking say well Stephen Hawking initially would have said in the 1970s um well we should say what did Stephen Hawking discover in the 1970s so in his words he discovered that black holes ain't so black so we've said nothing comes out of a black hole everything that goes in goes to the singularity it's gone forever because the black hole lives forever
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now Stephven Hawking um calculated that if you do some essentially quantum mechanics right so so around the horizon of the black hole so you think about what happens what does a black hole do a way of thinking about this is that in quantum mechanics which means in reality in nature uh empty space isn't empty it has a rich structure so the vacuum of space has a structure and the black hole You might say well this strange behavior of space and time it must do something and it
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does it disrupts that structure and the result is that particle photons essentially but what's called Hawking radiation is emitted from the black hole so the result and there are different ways of thinking about what's happening there's the the very precise way steven Hawking gave an analogy in his paper um and he said it's just an analogy and people get very worked up online when you talk about his analogy but Steven did write it down right so a way to picture the quantum vacuum
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um is that you can imagine particles coming in and out of existence all the time in the vacuum so in accord with the uncertainty principle they come in and out and in and out like that you can kind of picture it like that I emphas empasize it's not supposed to be a technical description but you can picture so you can picture these uh in the vicinity of the horizon you can picture that
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this structure gets disrupted you could you could have the situation where one of these particles is on the inside and one is on the outside and then we know what's happening to the one on the inside it's going to the singularity because we said the river of space is going fast in there or whatever whichever way you want to think about it so the other one is basically made real and and and
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escapes so that's a picture that Steven himself gave um but the upshot is very accurate the point is that this particle has been shaken out of the vacuum and it's now a real particle when viewed from the outside and it goes away so so that what what is that what particle's been emitted it's a temperature is glowing and so that means it's losing energy and so that means it has a lifetime so over time and these are enormous times far greater than the current age of the universe
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for any black hole that we know of in the universe because we don't know of tiny ones so all the ones we know of the immense lifetimes in excess of 10 to the^ 100 years for these super massive ones right it's ridiculous times but ultimately they have a lifetime and that means one day it will be gone and space will be all nice and normal again right all you will have left will be the Hawking
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radiation that's been emitted over these eons of time okay I think something is missing from my understanding of the universe i agree with you something's missing in my understanding as well and everybody else's but here's what I think is missing from mine right now so all of this matter has including us passed through the event horizon ended up at the singularity it is something in there at the end of time so yeah and it would increase the mass of the black hole and the
35:24
black black hole would grow right because you've gone in um and also I think that I know that every law that we have about the universe says that information is conserved if a black hole will one day end up as nothing the the normal sorry the normal vacuum of space what happened to you and me what happened to all of that stuff i mean this is the central question so um Steven's initial calculation 1974 um was that this radiation the Hawing radiation is informationless it's
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information free so it's not gone away it's not disappeared the black hole it's turned into the radiation right um and but his calculation said there's no information in that it's what's called thermal purely thermal not surprising if you think about it because it's been kind of shaken out of the vacuum of space so it's certainly you would think got nothing to do with the stuff that falls
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in this is something to do with the horizon it's not anything to do with the singularity this stuff so so out it comes and the calculation is very clear so that would suggest as you said that the information any record of anything that fell in will have been erased the energy will be the same right the energy is conserved the the black hole hasn't vanished it's turned into radiation but the
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radiation contains no trace of anything that fell in that is weird um as you said because if you think let's imagine you might say well what's the difference what if I get this piece of paper and set fire to it it goes and there's just stuff ashes and radiation yes but in principle then if you could just measure everything which you can't but if you could then you could reconstruct the information on the page and you can see why because it's got something what's happening when
37:26
you burn it is chemical reactions and and there's oxidation whatever it is and but you can trace everything back so you can say all the atoms and things are still around and yeah and there it is in a black hole you can't so so it wasn't surprising what was surprising is that that it appears there is a calculation that tells you that black holes erase everything and as you said the laws of nature are quite clear on this information does not get destroyed in
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the universe it gets scrambled so you can never reconstruct it but it doesn't get destroyed so that's what bothered everybody for a very long time h what happened what's the understanding
38:07
so the end of the story we're not at the end of the story yet but in 2019 a series of papers were published one of the papers was by Jeff Pennington and the other one was by another group of authors um but those papers suggest that Steven missed a bit in his calculation very subtle he could have never seen it yeah i mean that's why it took you know 50 years right to see what he'd missed but it turns out that the radiation is not information free at the end of the process
38:42
then all the information about everything that fell in is imprinted in the radiation as you would normally expect um but you might but then it becomes interesting because you say well okay so there's a calculation this mathematics that was done but you say well what's the picture then what happened because I I understand if I if I take this and throw it into the black hole it's gone across the horizon it's gone to the singularity it's definitely true that nothing comes out of
39:11
the black hole you know but so so how is this information about this thing getting imprinted in the radiation And it turns out that it's not just so initially quite a few people thought well it must be just right at the end when when it's all quantum gravity and all this stuff so the thing's just about to disappear back into the universe and it's getting hotter and hotter by the way as it gets smaller and smaller so there's more and more it's getting more and more violent
39:37
and something weird happens and everything comes out but um it was known for a long time his work by a great physicist called Don Page that the problem about this the about the information and and and and the structure of space these problems occur about halfway through the black hole's life so that it's called the page time so the problems with the information structure of this thing occur
40:08
way before you should be thinking about quantum gravity and a load of weird stuff so so there was a big challenge to physics it's like we should be able to calculate stuff when the black hole is halfway through its life there's nothing weird at the horizon but yet something weird appears to be happening the picture of the thing is breaking down um and so then we could skip if you want to the So what is what is the picture of this and I should emphasize the health warning here this
40:36
is ongoing research and there is no agreed upon picture and there's some other subtleties we'll talk about in a minute but but a picture of what's happening is this is related to it's related to something called the holographic principle at some level we think and um so um another discovery that was made in the 70s which is quite interesting by quite I'm using it in the American sense it's
41:01
very interesting is that um so Jacob Beckinstein um one of the pioneers along with Steven Hawking calculated what's called the entropy of a black hole so this thing has a temperature steven calculated the temperature that is inscribed on his gravestone in Westminster Abbey his equation for the temperature of a black hole so it's a huge fundamental discovery temperature um is now we understand it now we didn't when we first introduced the concept we didn't know
41:30
about atoms and molecules then we did and we we realize that temperature is about how fast the the component parts jiggle around of a thing right so in this water here you heat it up the molecules are jiggling around faster that's what temperature is this thing a black hole think about what this is i said the description of it is just space and time that's all it is it's geometry so immediately
41:54
we've got the temperature of a geometry not a temperature of a thing made of stuff but just a temperature of space right so that's kind of interesting and then Beckenstein calculates that this thing has an entropy which implies that it hides information from us there's there's a structure there's information in there the entropy turns out to be equal to equal to the surface area of the event horizon in what's called square plank lengths so this is a remarkable idea that
42:29
you can think of the horizon and I said remember it doesn't really it's not really there from our perspective i said we could be falling through it now wouldn't notice anything according to Einstein in we go but then you look at it from the outside and it looks like there's information encoded on the horizon in pixels that are one plank length in size so what does that mean information's encoded on in space somehow on the surface and also by the way it's weird isn't it if I said
43:01
how much information is contained in this room you would say well it's to do with the volume of the room right it's the library it's how many books can I fit in the library this is saying no at a fundamental level the amount of information in this room is is is determined by the surface area of the room not the volume so it's almost as if nature has said you can paper the outside of the library with the pages of the book but there is it's almost like there is no interior
43:28
to this thing so you start to get these hints that there's something very strange going on um what is happening here um so we've got this picture now which was in the 1970s of this thing that has a temperature and an entropy which are calculated and so that kind of is suggestive of substructure but structure of space and time what do we mean by that so nobody knows nobody knew that right so
43:57
um then so there's there's another thing that come another property that then comes which is why I was careful about us falling in and I kept saying from our perspective nothing happens so you got this Hawking radiation is it's coming from the disruption of the vacuum in in one picture so it's all there and it's coming away and it's climbing away from this black hole it's losing energy and
44:21
so it's very low temperature by the time you're far away from this thing but you go in towards it then towards the horizon if you lowered a thermometer down from far away so our friends are in a spacecraft now you got a thermometer and they lower it down the the idea is that it sees hotter and hotter temperatures because you're getting closer and closer to this horizon so you've got like a fishing rod around your sending a thermometer down and it's going hotter and hotter
44:48
from the point of view of someone outside from the point of view of someone falling in there's no temperature at all right you don't see anything you don't feel anything you don't measure any Hawking radiation you just go in because you're in freef fall so from your perspective nothing's happening from the outside you've got this high temperature so from the outside a description of what happens to us is is different the description of what happens to us is that we get vaporized
45:18
we never get spaghettified as I said you know the other thing is we never go in right so what happens to us we get vaporized before we cross the horizon and and all the all our ashes and all the stuff comes out and it and you could collect it and you could say well that's fine great it's just like burning a piece of paper but Einstein's theory is absolutely clear that from our perspective we go in we get spaghettified so the from our perspective our demise the description
45:48
of our demise is spaghettification y from the exterior perspective the description of our demise is incineration right so which one is it so you say Well come on you either get incinerated or spaghettified which one the the modern view at some level that is much debated goes all the way back to work by Leonard Suskin and Jared Tahuft and some others uh it's called black hole complimentarity and the idea is that both pictures are correct from different points of view this is
46:23
relativity in action right so the idea is both pictures are correct now so there is some it's not as simple as that and that even is not simple because there's other stuff going on here but so you might say well okay so I understand sort of this idea that the information essentially you're saying that from the outside things fall in and get scrambled up and they're somehow stored close to the horizon and you can obviously imagine these bits of information coming off as the
46:56
Hawking radiation no nothing really goes in and so it's fine i understand how that stuff got out but then the question is what happens what's the description of that from the from our perspective going in then because we definitely went in right so we we we've gone in from our point of view what happens what's the other description are there two of us no that's a great question it's a So
47:21
you might say well we get copied then so there's a copy of us mate it's like blender suskin calls it a quantum xerox machine I think so he says is it a quantum xerox thing this do we get copied so but there's a very fundamental theorem in quantum mechanics called the no cloning theorem which says you can't copy a quantum state you can't copy the information and this is fundamental so you this is
47:45
a problem in quantum computing when you're trying to it's another whole other discussion so so it's not copied so what's the description so this is where we get speculative and we're trying to understand what the mathematics is saying one perspect one description of the of our perspective is yeah we get we go to the singularity we get all scrambled up and then it looks like the interior of the black hole is in some sense the same place as the exterior you could almost picture
48:18
wormholes opening up from the interior of the black hole to the exterior and very naively you imagine our bits of information going through the wormholes and coming out again so so you we end up outside the information ends up outside in the Hawking radiation how it gets there is it really wormholes that are connecting the interior to are we really is that what's happening there is a an
48:48
idea that's been around for a long time again due to Suskin and others called ER equals EPR which is Einstein Rosen equals Einstein Podolski and Rosen okay so Einstein Pidolski and Rosen very famous paper was about quantum entanglement in the 30s they wrote this paper and it was it was they were very concerned about if you have these um quantum systems that are so-called entangled systems then
49:15
you can make a measurement on this thing over here and instantly this one which might be a light year away can will will have to right configure itself the the classic example is a quantum coin so you can have a quantum coin in a in a quantum state which can be heads tails plus tails heads let's say that with a one over root two whatever so if it's in that state then it means that's a full description of the state heads tails plus tails heads it means that if you separate these
49:48
quantum coins they could be an electron right you separate the quantum coins to a large distance then it's still the case that this thing is in this what's called a superp position of heads and tails then you you you make a measurement of it you make an observation whatever that you want to describe it and if that one comes up heads then you know this one is tails even though before you did anything to this they were both in this rather this this entangled state right
50:18
so so that bothered everybody Einstein Rosen and they said maybe there's something else going on and whatever but we think that's the way the world is now Einstein Rosen is the paper that Einstein and Rosen wrote noticing that the Schwarzfield description of a black hole has a wormhole in it so it's a wormhole so Einstein Rosen is a wormhole e is quantum entanglement So there's been this idea that that the same picture of this so you can picture quantum entanglement somehow as a
50:48
wormhole between the two things linking them yeah so this was an idea from a long time ago and um so the picture of a black hole is kind of similar to that in that you we we're developing this picture where the interior of the black hole when viewed from one perspective is the exterior of the black hole when viewed from another perspective and so you you're what you're getting there we think and
51:17
this is and again even as I say this there are papers being written saying different thing it's it's really cutting edge stuff but I think what everybody agrees on pretty much is that you're seeing something called the holographic principle at work here so it's a dual description of nature so there's a very famous paper by Maldesina called it's called the ads CFT conjecture and I think it
51:42
I'm right in saying it's the most cited paper in all of theoretical physics is and it's a it was a very particular model of of quantum mechanics on a surface that's what CTF conformal field theory it means it's quantum mechanics on a surface and a and a precise proof that there's a jewel description that this quant Quantum theory describes an interior geometry of space which is was not there in in the in the in the surface theory and the space is called an ads space antid
52:16
space or whatever but it's a very it's a perfect a proof that there's a onetoone description between these two things and I I think it's fair to say that the black holes the these strange apparent paradoxes are telling us that such such a thing our universe can be described in such a way so there are different ways of describing the same physics um and they're radically different ways ultimately the thing is it shouldn't be so problematic in that in both descriptions the
52:52
information comes out mhm but the way it came out is radically different depending on your point of view and I so I think most people would say we're seeing glimpses of of of some kind of holographic they're called holographic by the way because if you think of um what does it mean to have a complete description of of a higher dimensional thing so let's let's be very concrete this room so let's say that this principle is correct and many people think it is by the way for this for
53:25
this room so we have a surface surrounding the room the walls let's say and there's a theory that lives just there and it describes fully everything that's in the room in that sense we're holograms we are both holograms because we're described by a theory that is is lives on a I mean a hologram is a piece of film like that and it has a perfect a perfect hologram would be a perfect 3D image
53:50
would be encoded in the surface so it's a perfect 3D image encoded in this 2D surface that really is the picture that we're talking about here you can see it with the event horizon and all the information on the outside and is the interior there and what happens so it it looks like we're flipping between descriptions of the world does it feel sometimes like that story about um a one-dimensional being meeting a two-dimensional being and trying to describe their circumstances
54:21
and then a two-dimensional being and and that we are somehow like seeing the weirdness of our own experience of dimensionality and that there is as you said if someone were viewing us from a higher dimension they would see this somehow clearly and I don't really know what I'm asking no no I know you I know you it's It's kind of Yeah you What we seem to be struggling with Yeah is as you said it's this it's almost the same struggle as as a two-dimensional being trying to understand
54:53
a three-dimensional world or actually we do the it's it's three-dimensional beings trying to understand the four-dimensional world which is what relativity is we struggle with it it's hard for us um so this is a level of abstraction further it's it's we I think one I saw described someone described the black hole as why are we glimpsing this deeper structure of nature in it when we think about black holes which are real things and I saw someone say it's almost as if
55:24
this thing slices through space and leaves the building blocks of space dangling right at the horizon so you can it's this strange behavior of of of which gravity has created this situation where you start to come face to face with the with what with theru with the structure of space and time so we we're we're talking about as I said before that in retrospect the hints were there with temperature and entropy and information in space encoded enclos encoded in space why so it
56:01
looks like we're trying starting to glimpse a theory of the underlying structure of space and time so this goes by the name of emergent spacetime um so the picture really would be that you have uh a description which looks by the way for all the world like a network of cubits which is what a quantum computer is right so it looks like there's a description of the universe that doesn't have space and time in it but is just a network it's just information and that goes
56:31
all the way back to John Wheeler we mentioned him once the great physicist John Wheeler who had an idea he used to call the it from bit so it is this and bit is information that's what it looks like we're seeing so our window onto this deeper theory call it quantum gravity if you like it has been very simple questions actually about black holes which are real things that very simple question
56:58
from Steven Haw that's why Hawings calculation is so important because it's the first glimpse of a problem with our picture of the world and it's a very very precise glimpse so it's a precise question does this thing destroy information or not if it doesn't how does the information get out that's a simple question but it's lead and is still leading which is why I'm waving my hands around a lot you you would get a different different pictures from any any you know expert
57:35
who does the calculations i'm not one of those right but if you talk to someone who does the calculations they would not be certain about the physical picture i think it's fair to say
57:46
so is it fair to say that the specific research into black holes the specific questions that we're asking about black holes are helping us unlock much much deeper questions about the universe as a whole exactly it's very well put and it's wonderful and and one of the wonderful things is that so the the techniques that you develop as a let's say a PhD student or a posttock or someone who's working in this area the techniques that you develop to try to understand what the black
58:17
hole is doing are the same techniques you need if you want to understand how quantum computers work so and that's a real engineering question as well as a fundamental physics question you know we're trying to build these things we we haven't built one that works in the way we want to yet but obviously Google and Microsoft and IBM and and others are pouring a lot of money into it because they're profoundly powerful devices but the insight a lot of insight now into how
58:48
they might work and how we might build them is coming from this field of emerging spacetime so it's the best example I know of you know if you say to a funding agency um I want to I want money from to to work on black holes they go well they they fortunately they do give us some money but they often will go what is it any use I mean what the point you know a black hole does anybody care um the lesson is that when you study nature and you go to the you try to study things that you
59:24
don't you don't understand that uh pose profound questions about our understanding of physics in this case then you are I would say likely to learn something useful because you're studying reality and this is the best example of that that I know for because you're studying completely collapsed stars or the things at the hearts of galaxies and you're gaining insight into quantum computers and networks of cubits and quantum information it's it's the most wonderful thing
59:57
it's incredible so there's one thing I should mention for a complete um picture for people who are listening or watching and want to know more there's a thing called which is really important called the firewall paradox as it was initially introduced and I I it hasn't been resolved yet and and the problem is that um when you when you disrupt space in this way that the black hole does
1:00:27
and when you try to understand what's happening to the information and and and then tangled particles that are on one side of the horizon and the other side you know the hawking radiation that we discussed earlier um you can you do bump up against a problem which is that maybe this description of the equival equivalence principle and of us freely falling across the horizon from our perspective into the interior of the black hole maybe that doesn't quite stand up so maybe
1:00:59
you're getting a challenge to the this basis of general relativity maybe it's really true that actually there isn't an interior of the black hole maybe it's really true that you don't go in there isn't an in right and so it's um you'll see if you go and search through the literature that the the the firewall paradox was that these papers were big papers in this debate and then some people
1:01:26
thought they were solved and other people thought they weren't solved and I think it's fair to say that the jury is still out on these issues but what what the thing to emphasize is it's really there's something really fundamental going on here and it probably it almost certainly is associated with this the nature of space and time themselves or spaceime it's associated with geometries of
1:01:51
space time maybe wormholes maybe those kind of things so it's tremendously interesting and and you'll see it's just wonderful to dig into this subject because it's so so it develops so fast and our understanding is developing so fast but the the last thing to say is it does look like that that there is a description of the universe that looks like a giant quantum computer in some sense which does not imply that someone built it right it doesn't simul but it does suggest
1:02:29
that there's a good description of reality that you can write down in the form of some kind of network of entangled cubits which are presumably the plank length inside who know maybe even that's not obvious right but something like that you know I came into this story thinking that I was going to tell about falling into a black hole and then eventually get to the cutting edge of research into black holes and that that was going to be the end of the story but actually what I'm
1:02:58
hearing you say now is that the cutting edge of research into black holes is actually a cutting edge of research into the universe itself and that is just incredibly exciting it's it's really beautiful isn't it they're the window like almost like the Rosetta stones that allowing allowing us to translate between different pictures of the universe all right so that's probably where we end yeah if I rewind for a second one of my questions to just connect things that
1:03:27
I know about the universe maybe is what is the relationship we think between the singularity at the center of a black hole and the point that big banged oh so you it's a it's a very good question that this thing so the the singularity in inside a black hole in Einstein's theory is the end of time and in Einstein's theory alone of the evolution of the universe there is another singularity at the other end of time which is the big bang singularity which you might call you might
1:04:03
be tempted to call the beginning of time um it's a very different singularity um black holes are um maximally scrambled information right the highest entropy things we know of in the universe and one of the great mysteries about the origin of the universe is it's a very low entropy state um broadly speaking you can if you want to know more about that Sean Carol actually writes a lot about this so it's it's there's a lot of interesting to say but it's a different kind
1:04:34
of singularity um and so are they related is is the question i I would say yeah our understanding of them surely is um and and but but it's really not clear and when you add quantum mechanics in then you begin to ask questions about whether the big bang sing if there is such a thing as a singularity in the past is it what is it is it the beginning of time there's a thing called the no boundary idea that Hawking and others had and you know again all bets are off right but I think
1:05:08
it would be good i I think not would be good it it is correct to say that I'm sure the understanding of the two is related but we understand neither at the moment level I'm sure that I am garbling fragments of understanding from things that you have explained but does it feel related at all that there is the hologram principle the hologram paradox that you described that we can be described as two dimensional that there can be a twodimen dimensional kind of description of
1:05:44
a huge amount of mass threedimensional mass that fell into a black hole and that we can all is it possible that we are all living on the outside of a black hole right now what is the question I'm trying to ask no I I think I it's a great question i I think there is a question so there is a cosmological horizon there are different kinds of cosmological horizons and it is a a a very good
1:06:13
question that but people don't really even know how to phrase it that but could it be that you can describe the whole universe in terms of a quantum theory living on a boundary of some description and I think the guess is yes the the guess but we don't even know what we mean by the boundary right um it's one of the problems here so the the reason that Maldesina was able to show this is this works for this very specific thing called ads space is because there's a boundary you can
1:06:52
identify in that particular geometry whereas our universe is dissa right there isn't it's not obvious what you mean by a boundary it's not really obvious what question you know as you said it's hard to find the words it is we don't really know what question we're asking But that rough picture that there could be a a theory of a quantum theory somehow this network of cubits that gives rise to geometry space time is accepted broadly right so we but when you try
1:07:30
to get into the detail of it it's only been fully realized for a very specific model which is kind of a toy model it's not our universe so it could be that our universe does not admit that description could be could be that but it's certainly beyond us the moment technically but it's a wonderful thought and actually there's a paper uh recently that so people are beginning to use the the Google chip the Willow chip um which is a a very powerful it's not a quantum computer
1:08:00
it's a protoquantum computer kind of thing so and people have started to use it because what it is is a load of cubits that you can entangle and you can set it up and it's very well controlled so whilst we don't know how to do quantum calculations on the thing really what you can do is try to say well could I set them up so that it's like space emerges from them and there was a paper recently where something that was described in the paper as a one-dimensional wormhole was
1:08:34
made it wouldn't be in our universe this thing it would be but it kind of emerged from this structure and that's the kind of picture we're trying to get who it's a it's a good paper it's been peer- reviewviewed it's a controversial paper you'll see loads of stuff online but it's worth looking at those papers and by the time this is uh sent out there might be some other paper a lot of people are working on it so and also actually building little clocks little quantum clocks tiny
1:09:01
clocks and because we don't even know what a clock is right at the most fundamental level because we don't know what time is so so we we're talking about very fundamental questions about reality in this research but it all came from these that right which is how cool is that it came from thinking about those things it's so cool and thank you for taking that question so seriously it's a great question no it's and I I think it's really instructive that um neither of us because
1:09:32
nobody has the language when you get into the real detail you're talking about mathematics ultimately um and even the mathematics is not complete so we're talking about research at the cutting edge and it takes a long time i mean even quantum mechan you think about quantum mechanics if we were talking about quantum mechanics this is a hundredy old theory that's very well tested and very well understood but when you try to put it into language to speak about it it's much
1:10:02
easier to write down the equations or something I mean even Feinman says that in the fman lectures when he talking about this thing called the double slit experiment it's a great description of the way quantum mechanics behaves but ultimately fman says you know mathematically it's easy this I can write down a a one line which tells you how this interference works and all those things but then
1:10:24
if you try to say well what does it mean what happens what does it mean for our picture of reality then we're still arguing about that now but the maths is easy this is even worse because here the maths is really difficult and we have no idea what it's telling us really about reality
1:10:42
one of the things that I really admire about you and your work is that you have this sense of wonder at the universe and I think that the thing I've most appreciated about it and I'm sure I speak for a lot of people is that you give that sense of wonder back to people you say "Hey don't you remember what it was like when you were a little kid looking up at the sky?" And actually in fact as you learn more about the universe that sense of wonder can increase not decrease and I
1:11:12
think that's really special and I think that people need that and first I just wanted to say thank you oh thank you and second my last question for you is how do you feel when you go outside and look up at the night sky given what you know oh it's a wonderful question and um you kind of alluded to it earlier that um the the the sense of awe and the sense of mystery and beauty increases
1:11:47
the more you know about what you're looking at it doesn't decrease the mysteries the the number of mysteries increases the more you look at it and it is incomprehensible even if you think about the Milky Way so it's a beautiful thing to look at so anybody watching this if it's a clear night and you're away from the city lights you can go we've all looked at the Milky Way at some point
1:12:11
if even that if you try and picture what it is and you learn that it's a galaxy of what 200 billion maybe 400 billion suns what does that mean 200 billion suns most of them have planets so it'll be trillions of planets takes light 100,000 years to cross that thing none of that is is I think you you can't internalize even that and that's kind of a simple thing cuz it's just a galaxy and we know
1:12:44
about galaxies and then you talk about as we've talked about the center of that galaxy lies this thing the super massive black hole and then we don't know we we don't know what that thing is you can look towards it so you can look towards Sagittarius espec the further south you are the easier it'll be to look at but you can look in the direction of the center of the galaxy so you can go out if you if you're not too far north and look at that right you can look at it with your
1:13:14
eyes and um yeah and wonder about it so that's what I I really believe i know Richard Feinman said it many years ago many other people have said it but the more you understand about nature the more mysterious and magical it becomes it's a beautiful place then thank you so much for your time
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