Our Universe and Others (Martin Rees)

[Music] [Applause] I'm going to start up some history and this is the best math student we ever had here in Cambridge I'm afraid it's been downhill all the way for 300 years since him but it's appropriate to start with Isaac Newton in this lecture and to remind us that as far as he was concerned this was the universe he knew about the planets the seven classical planets and he knew there were stars much further way but he didn't know how far away nor what they were made of but he famously designed his telescope and I should have said also that he was the first unifier he showed famously that the force that holds the planets in their orbit is the same that makes the Apple fall and holds us down on the ground this was his reflecting telescope of which you could see the stars and of course the stars have been known since antiquity but it wasn't until about 1860 that people realized what the stars were made of classically it was thought they were made of some special essence different from stuff on the ground but it was this chap here Sir William Huggins who was the first person to use a spectrograph and to look at the Sun and the stars and see spectral features and he was the first person to realize that the stars not only like the Sun but they're made of the same stuff as the Sun and as the earth there's nothing special about the material that makes the starry heavens Newton also must have thought about space travel this is an illustration from the English version of his book Principia it shows a kind of war being fired from mountaintop and if it's fired fast enough it's tragically curves downwards only as steeply as the earth Cosway underneath it it goes into orbit this is still I think the neatest way to explain the concept an orbital flight but Newton realized that to go into orbit you've got too far your cannonball at 18,000 miles an hour and that was not possible at his time and as people here know it was the Soviet Sputnik in 1957 which was the first object to go into orbit and ten years after that we had this iconic picture of the earth taken from my astronauts orbiting the moon the stare our moonscape contrasting with the Earth's biosphere when I got to give you a very quick tour of the universe as we now know about it starting with what we can do from space probes people haven't been any further in the moon but space probes have been to all the bodies in the solar system giving us close-up pictures of them and so if you go out in your space probe you look back at the earth from about five million miles away you see this this is a picture half earth and a half moon the sun's coming from the right and then you get to the Red Planet here it is and the lots of probes that have been and to the red planet and orbited it or landed on it here's a picture some pictures they take here's an old one missus of gourds have reminds Mars deep and this is the curiosity probe that landed on the surface of Mars about eighteen months ago it landed where that little ellipses in the top left of this big crater about 100 miles across and it's traveling around if you look very carefully you can see the tire marks there along the bottom where it's been going there's been about 30 kilometers so far and it's eventually going to climb the mountain in the middle and this is a view it's taken it doesn't look all that exciting but it's about the height of a person so this is what you would see if you were looking on Mars and this is another picture and you see the geological strata which is going to be probing going out beyond Mars the next time if we come to is the giant planet Jupiter and it's got the four famous moons that Galileo discovered and they've been observed close up they're very different this is IO which is suffers from volcanic and this is Europa which is covered in ice and there may be an ocean underneath it and this is a close-up of the ice it's clearly been melted and condensed many times then we get to Saturn this is a picture taken by the Cassini spacecraft sure Saturn with the Rings completely edge-on and this is a rather beautiful picture this shows an eclipse of the Sun by Saturn it's taken by the Cassini spacecraft when it's be on Saturn lined up at just such a distance that Saturn blocks out the Sun but the Rings are still illuminated by the Sun and really you can't quite see it the earth is about where that error is it's a long way away and light signals would take several hours to get to Saturn and that's why it's very impressive when the Europeans landed a small robot on the surface of Titan with Saturn's giant moon and the left hand and sent her pictures show what this robot sure all the way down parachuted off the surface of Titan and the right is where it landed this may look a rather nice place you can see rivers a little lake but the temperature is minus 160 degrees centigrade and those rivers are liquid ethane so it's not a very salubrious place to live I expect in the coming decades robotic probes and fabricators will pervade the whole solar system but I don't know whether people will follow this is a Harrison Smith the last man in the moon in 1972 and we don't know when people will return and for the youngest people in the audience here it's ancient history when men walked on the moon we know that the Egyptian pyramids you know the Americans went to the moon but these both seem ancient history motivated in both cases by strange national goals and there was no motive to work go beyond that and in the question period I'll say more about to the manned space program that let's move on well people want to know is there any life in our solar system we don't know there might be something swimming under the ice of Europa there might be something on Mars but no one expected anything very exciting but when we look further afield then prospects have opened up one thing we've learned in the last 10 or 15 years makes the night sky much more interesting we've learned that most of the stars you see in the night sky are not just points of light there surrounded retinues of planets just as a Sun is orbited by the earth and yellow familiar planets these planets are not in general seen directly they're inferred by precise measurements of the Sun with a little star they're orbiting around and I'll just mention these two techniques they're both very simple the first comes from noting that if a planet orbiting a star then actually both the plant in the star orbit the center of mass what's called the barycenter the planet goes round in orbit the star being heavier goes round in a smaller orbit but very precise measurements of the Doppler effect of the star can detect these motions of the star only a few meters per second 10 to minus a to the speed of light due to an orbiting planet and here's an example of showing the the velocity towards us of a star over one cycle and this is a sine wave indicating a planet orbiting in a circular orbit and you can infer from such data the mass of the planet and how long its year is and by this technique many hundreds of planets have been inferred but they're mainly big ones like Jupiter and Saturn the giants of our solar system an earth-like planet would induce a motion of only a few centimeters per second in its central star too small to be detected by this technique but it is another technique transits if you're observing a star and a planet who's orbited in the plane of your line of sight then whenever the planet moves across in front of the star it would block out a bit of its light and so the brightness of the star will dip a little bit and then will dip again when the planet comes round and it's a small effect the if the Sun was seen from great distance then the earth being about 1% of its diameter the Sun one part in 10,000 of the area would produce a dip by one part in 10,000 in the sun's brightness but the Kepler spacecraft spent more than 3 years observing an area of sky and measuring writers of 150,000 stars with the precision of 1,100 and over and over again once or twice an hour and it was looking for just these are things and indeed it found hundreds of planets many no bigger than the universe and incidentally some amateurs found planets with possible fine amateurs who get access to the the time series of Kepler data on one star and in fact some amateurs found a planet orbiting a double star to be two suns in its sky that's because the automatic algorithm didn't detect something like that well these two methods are indirect we really like to observe the planets directly not just their shadows as it were and that's hard to imagine how hard let's suppose that some aliens with a big telescope we're looking at the solar system from say 20 light years the distance of a nearby star the Sun would then look an ordinary star and the earth would look and car save is nice phrase like a pale blue dot very close in the sky to its star our Sun and billions of sign fainter very hard to detect it's like looking for firefly next to a searchlight but if the alias had a big enough telescope then they could learn something about our pale blue dot because the shade of blue would be slightly different depending on where the Pacific Ocean or the landmass of Asia was facing you so they confer there were concerts and oceans the length of the day something of the seasons and climate and maybe by looking carefully at the light that there was a biosphere where we can't do that yet but within twenty years or maybe ten we'll be able to do this in particular with this instrument that European astronomers building and it they're not very good to the magical names it's called the ELT the extremely large telescope and it is indeed planned to be extremely large the mirror is going to be 39 metres across and that's I guess about twice the width of this lecture room it's not just one for you to glass it's a mosaic of hundreds of pieces of glass but instruments like that will be able to detect earth-like planets other stars and infer a lot about that this is a very exciting development will it be life on any of these planets well that's another story I'll say you know more about it let me just say that biologists are harder subject than astronomy and it'll be longer before we understand that then it takes us to understand the planets now astronomers didn't realize as he plans existed until twenty years ago when first one was found and most were found within the last decade but they weren't surprised they weren't surprised because they've known for good reason that when a star forms it goes through stages like this cartoon this is a artist's impression the star formed from a slowly spinning dusty cloud of gas as the cloud contracts under gravity it's been faster conserving angular momentum and the protostar forms and around it is spun off a dusty disk and the dust agglomeration too rocks and then into planets and this we believe is the way our solar system formed and it's a generic process so we're not surprised to find that planets are common around other stars and again flashed back to Newton this is what Newton wrote in his optics he of course famously understood by planets move in little orbits but he didn't understand why all those orbits were in the same plane what we call the ecliptic whereas comets come in all directions he thought this was Providence or something like that but of course we now understand this we pushed the causal chain back and I mentioned this because that's what we're doing all the time in cosmology where understanding further back how the causal chain goes and as I'll say later in this talk we've got back not merely to endless this snow system form even to when the first stars formed but right back to the first tiny fraction of a second after the Big Bang stars are in fact quite well understood as I mentioned Huggins first showed that what they were made of and we now understand their evolution we know that all one's burn their nuclear fuel slowly the big ones burn the fuel more rapidly and shine much more brightly there the blue stars we see in the sky and they and their lives with remnants white dwarfs neutron stars or black holes and we see stars forming and dying when we look in the Milky Way this is the Eagle Nebula seven thousand light-years away where even now new stars are forming like in that cartoon and would form with dusty discs around them here are some stars dying the Sun will look like this in about six million years here's another star dying running off is out of the house and here's a more massive star that died a thousand years ago this is a famous object called the Crab Nebula which is the remnants of a massive star which exploded as a supernova in the year 1054 ad and that explosion was recorded by I guess the Chinese counterpart of astronomer royal and any Chinese readers here will notice he says that the star flared after he brightened the full moon and faded after a few weeks and now when we look in the sky we see the exploding debris nearly a thousand years later and this material eventually merged with interstellar gas well the supernova may seem irrelevant far away in long ago but as this man in particular showed they are crucial to our existence and this is why if you were to take a slice through a big star just before it explodes then if it look something like this an onion skin structure where the hotter inner layers are burned further out the periodic table hydrogen goes to helium the helium goes to carbon enter oxygen and so on and when the star runs out of nuclear fuel in the face is a crisis its core implodes and the rest is blown off to make this explosion and Fred Hoyles wonderful idea was that this material eventually mixes up and new stars condense like in the Eagle Nebula and then makes stars like the Sun and the solar system so every atom in our bodies of carbon oxygen etc was forged in a star which lived and died more than four-and-a-half billion years ago and the Sun then condensed from the gas cloud contaminated by debris from that ik only a generation of stars and indeed careful calculation suggested we have inside us atoms from many thousands of different stars which lived and died all over and work away galaxies so we are the ashes of long dead stars or for less romantic we are the nuclear waste from the fuel that makes stars shine and Fred Hoyle was the pioneer but it's a famous paper by Burbidge Burbidge Fowler and Hoyle Nesbit verbage husband and wife and they wrote this wonderful paper in 1957 which codified this whole idea of how from pristine hydrogen all the elements were made this was actually taken in Cambridge for Willie Fowler's 60th birthday and he was a train buff and that was his birthday present and this idea can explain the proportions it could explain why there's lots of carbon and oxygen why golden uranium a rare and how they came to be in our solar system and they give us a picture that our galaxy is really a sort of ecosystem where pristine gas the merely universe makes the first stars and that gas is just hydrogen helium and then that makes the rest of the periodic table and then that's recycled into a generation of other stars and our Sun was a later generation star contaminated by earlier generation and this is going on in our Milky Way galaxy which contains about a hundred billion stars if it could go two million light years away and look back at our earth people we would see something like this this is in fact the Andromeda galaxy our nearest big neighbor in space it's about it's about the same size and it's a disc viewed obliquely it's a circular disc where stars are orbiting around a central hub our galaxy would look just like this if we could get 2 million light years away and as our Sun would be somewhere out towards the edge orbiting around like that here's another galaxy this one is looked at more or less face on and galaxies are the main ingredients of the large-scale cosmic scene this is a map showing the galaxies within 500 million light-years of us and the galaxies are themselves in clusters since some clusters containing hundreds or thousands of galaxies and we and Andromeda in a few small members are innocent a stoical local group which is rather small cluster well I mentioned it we understood quite a bit about stars and we even understand quite a bit of galaxies and this might surprise you because we seem pretty helpless we can't do experiments and they evolve very slowly we can't do what a particle physicist does and crash together particles and see what happens at least we can't do that in the real world we can do that in the virtual world of our computer we can ask what would happen if two galaxies fell together and crash and here's a movie showing showing this these two galaxies and their computing the gravitational effect of all the stars and you get a sort of train wreck when they collide and eventually all the stars will settle down and we'll end up with one big amorphous galaxy where there were ones to disk galaxies and we can look in the sky and this is a real picture of two galaxies and having done calculations in our computer like the ones I showed you we can infer that what's happened here is two galaxies have got dangerously close and that one has pulled as a type of plume on the other and if we came back in a hundred million years or so they would have merged together and I should warn you that the Andromeda galaxy is going to crash into our Milky Way in about four billion years four billion yes not millions of be reassured and so this is something which we can do and you can also do calculation making different assumptions we could put in different amounts of gas different amounts of dark matter etc and see which fits best and one of the things we have learnt about galaxies is that as mortar than we see we see lots of stars we see lots of gas but they contain about five times more stuff in some dark for some swarm of particles that don't emit or absorb light this is the famous dark matter and every much time to go into this but as various lines of evidence one line of evidence is if you look at galaxies in a cluster then they're moving around very fast you can measure their motions and you can infer that they the cluster would fly apart and this was more mass binding it together than we see that's a dark matter and this chap here Fritz Zwicky he was the first person in 1930s to actually realize that this was a big problem the second technique for finding dark matter is that x-ray astronomers can observe hot gas in a potential well of a cluster of galaxies and for each temperature they can infer what his pressure is and how deep the gravitational potential is and here's an example of an x-ray map and from this kind of argument you can infer that's the gravitational potential of the cluster is due to more than the galaxies we see and the third method which said the Einstein was like very much which indicates it's excess mass that's illustrated here the bright objects are galaxies in a cluster about a billion light-years away the faint objects are galaxies several times further away still and you can see that some of those faint objects are elongated into arcs what's happening is that the cluster of galaxies is acting like a poorly figured converging lens because gravity bends light rays and it's distorting and magnifying some of the background galaxies and you could do the sort of optics and work out what the mass has to be in the cluster in order to produce the distortions you observe and again you find that five times as much dark matter must be and as manape see now the other important thing we have to is about the universe is that when we look at distant galaxies they're receding from us they're expanding this was a famous result of Edwin Hubble he is a heavy smoker as you can see and he was the the first person to realize that distant galaxies are receding the further way they are the faster they were seeking and at first sight people think this means we're in some special central position but it doesn't mean that and I'd like to illustrate it by imagining this this network which could go on forever and supposing that all the rods lengthened at the same rate then if you sat on one vertex you'd see the others moving away at a speed proportional to the number of intervening links and that's a good way to think of the hubble law imagine the galaxies joined together by rods it's a good model except that as you saw the galaxies aren't in a regular network there they're spread around in a complicated way there's one feature that's not reflected by this picture and is better reflected by another Shesha diagram angels and devils and that's because when we look a long way away we look far back in time and therefore if the universe is expanding we look back to a time when it seems from more compressed when the rods were all shorter so what we actually see when we look back is more like this picture whereas we look back in the past things look more and more crowded and we can look very far back this is a picture which shows a tiny patch of sky a patch of sky so small it would take about a hundred like it to cover the full moon it would look like blank with a small telescope but for the big telescope you see here hundreds of smudges each of galaxies many fully the equal of our own galaxy or the Milky Way and looking so small and faint because of the huge distance many of these are so far away that the light set out ten billion light-years away so we're looking back very far and these have a very big redshift and I'll show you one picture the very faint objects there has a had a redshift taken two years ago by this group and this is just a tracing and the reason I show this is that the strong peak the strong emission on the left hand side is the famous lyman-alpha line of hydrogen which is normally in the far ultraviolet 1216 angstroms but here it stretched in wavelength by a factor more than eight into the infrared so this is an object very far away Internet if this is not difficult galaxy it's a galaxy which has in its center a spinning accreting black hole it's a quasar and the quasar gives much more lights and brightens the gas so we can see a clearer picture and so what's going on in the center of the galaxy is some something like this gas swirling in to a central black hole but again that's another story if it could look even further away further back we look back to a time before there were any galaxies but again as I spent everyone knows we do have evidence for what the universe was like at the early pre galactic era and most famously from this spectrum here this was discovered 50 years ago that the whole of space is pervaded by weak microwaves which have the spectrum of a thermal radiation view a blackbody at 3 degrees above absolute zero and this radiation is the relic of the universe's hot dense beginning the universe was once much hotter and denser than the inside of a star and as it expanded then the radiation got diluted the wavelengths got stretched and they shifted right down to the microwaves and we're left behind with justice and evidence if this kind and others has led to this standard time chart for expanding universe and we can look back we when we look back into the most distant galaxies we're looking back to about a billion years the microwave background photons were last scattered when universe's 300,000 years old we have fairly good evidence actually about one second that that's when nuclear reactions made hydrogen helium deuterium in the proportion if we observed ever confidence back back there but why stop there if you go back to about a nanosecond then every Park in the universe would have an energy equivalent to what you can produce at CERN in Geneva further back still you'd become more uncertain but as a nanosecond everything we now see with our telescope out to those distant galaxies would have been squeezed down to the size of our solar system I'll say more about in a moment but one thing which people often ask when they're presented with this idea is doesn't it seem contrary to the famous second or third eye nameks because we say the early universe was amorphis a hot fireball and now it's structured with galaxies stars and us doesn't this seem contrary to what the second law supposed to do - watch out structure well the answer is that gravity has funny properties as far as as far as thermo knowledge is concerned if you have the early universe not completely smooth but some regions a bit denser than average then as universe expands those dense bits will lag behind more and more and eventually will condense out and this movie shows a region of the universe and the expansion is subtracted out but you see early on the density contrasts aren't perceptible but as it expands then the over dense regions lag behind more and more and I should say that the blue is the dark matter the red is the is the atoms which are going to make the stars so this is how a galaxy forms from an initial slight over density in the early universe it's not contrary to thermodynamics is just that gravity enhances density contrast doesn't wash them out and this story has a lot of corroboration because when we look at the background radiation we can observe how smooth the universe was when it was three hundred thousand years old that's when it's ready to last scatter and this famous picture from the Planck satellite shows the fluctuations the scales but some patches are warmer than average by one part 100,000 some are cooler than average by that amount and the important point is that in the simulation I showed the fluctuations initially weren't put in at random they were put in from here and it's a great vindication of this idea that the fluctuations observed when universal 300,000 years old under the action of gravity will develop into structures which match what we see today and this is a one diagram due to max tegmark which summarizes all this the the theory says that the density fluctuations move mean square on different scales should follow this line and we have lots of data looking at galaxies and clusters etc and they follow pretty well so the idea that the early fluctuation seen in the microwave background are the seeds for all the structures we see is strongly indicated so there's a step by step emergence of complexity and I've talked about gravity forming the first structures but it's fairly clear that other things had to happen because the step steps that led to us involve forming stars heavy elements etc and what I want to do now is to say something about what the prerequisites are in getting from a simple beginning to us and the reason I'm going to do this is that we can then ask suppose you tweak some of the numbers would this change things I'm going to end up by saying maybe there are places where the numbers are different but for the moment if you don't like that idea you can reassure yourself it's just like you know what kind of fact or history where people asked what would happen if Britain didn't fight the first world war if the Brits behave better in 1776 or if it has Troy didn't wipe out the dinosaurs etc it's kind of fact of history and you can ask in the same spirit what would it be like if the universe evolved differently well what are the prerequisites the first important one is gravity I mentioned it's important in enhancing density across contrast to make the first stars and galaxies but it also of course holds together the Stars I'd like to put this up this is my favorite pedagogical diagram it shows radius along here and mass up here and there's a proton and there to the right that's an atom roughly the mass of a proton and larger by the fine-structure constant to the minus two that's about the Bohr radius 10 minus 8 centimeters this is black hole line slope 1 on a log-log plot and the black hole with the size of a proton has a mass of ten to thirty eight protons that reflects how weak gravity is now if you take solid objects then they're roughly the density of one atom per per unit volume and you could imagine building up sugar lumps people asteroids etc and this is getting close to the black hole line I can't go up forever and what happens is that an asteroid isn't affected by gravity then you get to a planet grab as you can make it round and you get up to the mass of Jupiter then gravity cannot only make a driver could start crushing it and if you made it mine a bigger than Jupiter it would be crushed and would have to turn into a star and so this is a nice way of indicating that we're where stars exist they exist when gravity starts becoming strong and you can see that it's about ten to fifty seven times the mass of a proton now the reason I show this picture is that this picture would look more or less the same if you made gravity stronger except that the scale will be different if that whole line would go down and so you could have a small scale speeded up universe where gravity was stronger but it's fairly clear to have an interesting universe there's got to be many powers of ten between the microscale and the scales where gravity is important and and that's why gravity has to be very weak otherwise creatures like us would be crushed by gravity so a long-lived universe where there's a big gap between the cosmic scales and the micro scales requires that there's a very big number you can think of this as the ratio of the electrical and letti and gravitational forces between say two protons that's about 10 to the 38th that's that big number and any instant universe has to have a very big number like that but just to provide so sometimes you read in pop in the book says gravity is finely tuned it's not finely tuned so got to be weak in fact if gravity was ten times weaker you might have even better universe because there'd be even more time and stars would be even bigger and animals could get even bigger before being crushed etc so there's nothing optimum but gravity has to be very weak second requirement is more matter than antimatter if the big bang with symmetric and matter antimatter then as it cooled you get lots of radiation but no atoms it's got to be something which provides your favoritism between matter and antimatter we have some ideas and then go into them further also we need non-trivial chemistry we need a periodic table we need this for two reasons we need it in order to have nuclear fusion to power the stars and also to allow complexes like us to exist because if it's only hard you shouldn't be no complex chemistry and certainly no organisms and this requires a balance between two forces the electric force which disrupts the many protons in a heavy nucleus and the nuclear force that binds them together so this is another case of apparent fine-tuning and we need have stars probably two generations of stars because the first ones make the first heavy elements and then the second ones can form planets around them because the heavy elements are there already also we need a tuned cosmic expansion rate we write that here this this shows the the size of the universe ly the rods on the in in the Asia picture against time and the universe expands and then it goes like this now if the density is too high then universe collapses very soon otherwise it may go on expanding forever and there's a important number called Omega which is a density on average of the stuff in universe / critical density the critical density is one where kinetic energy and gravity just balance the universe just has enough energy to go in expanding forever now the actual values of something like this the ordinary baryons I thought my atoms about 4% of the critical density Dark Matter provides another 25 or 30 percent so we have point three altogether and so that's enough to give us a universe and whatever picture which goes on expanding forever but decelerating but we had a big surprise in 1998 it was found that the universe wasn't merely not slowing down very much if it actually speeding up so the gravity which everything exhausted everything else must be being outweighed by an extra mysterious force latent in empty space itself which causes an acceleration and that means that we have we follow the and the universe is following something like that like this it may have decelerated early on but now it's accelerating and this force later in empty space is a deep mystery we won't understand it till we understand the granule in the nature of space on a tiny tiny scale and the other thing we need is nonzero fluctuations in the early universe which there was no fluctuation at all then even now after thirteen billion years the universe will be cold neutral hydrogen with no stars and no galaxies just very briefly to think of counterfactual universes the fluctuation amplitude is about ten to the minus five and that gives rise to galaxies if you had this number cue which measures the amplitude if that was bigger say ten to the minus four you get a universe where galaxies were much big you would have huge disk galaxies sounds at times bigger than ours if we've more than ten to minus three that wouldn't be so good because then huge clouds would collapse early on and you have universe of huge black holes a violent place on the other hand if the fluctuation were even smaller than actually our tens minus six then we have an anemic universe the fluctuations might eventually produce some galaxies but they wouldn't be very dominant and so this fluctuation amplitude is something which must be within a certain range between about 10 to the minus 4 and 10 to the minus 5 will be best with these various numbers the basic forces the fluctuation amplitude etc so when were these key numbers imprinted and this is a again from max tegmark this depicts how the other data which tells us what the koel mullick parameters are the density and then we theorist turn our handle and try to see what this implies well I want to put up a hazard sign here because everyone agrees that the answer to the question of why the universities decline the way it is why it's got the ingredients it has lies very early on in the universe I said that we can infer back with some confidence to a nanosecond when the energy of particles was about that's produced in the LHC but we now have to go back much much further still to the time when observable volume was squeezed down not merely to the size of our solar system but to the size of a tennis ball and I like this picture here which showed that rather nicely and this is the era when we believe the universe inflated and when men the fluctuations were established and justly update up to date then maybe some of you heard that at four o'clock today there was an announcement that the fluctuations in the microwave background had a special property which has been determined by particular experiments which tells us that in the early universe there were not just density fluctuations but there was some transverse waves of gravitational waves and this is an important diagnostic and this gives us I think much better arguments than we had before for taking seriously this huge extrapolation back to any universe was the size of a tennis ball that's when quantum fluctuations gave rise to these fluctuations we now see he spread across the sky in the microwave background now let's widen our horizons still further this is the picture I showed again showing the limits of our of our universe and we know that the laws of physics are almost certainly the same everywhere we can see there's a big debate where I think there might be slight differences more or less it looks as though the atoms in those galaxies I like the ones here on earth the strength of gravity to say but if we ask how uniform universal of physical constants laws really are we've got to realize that this domain the domain we can observe may be a tiny fraction of what there is there's a sort of horizon around us set by the maximum distance like kind of traveled since the Big Bang but that's not a real horizon any more than the horizon you see around you if you're in the middle of the ocean is a real horizon you don't think the ocean naturally stops just beyond your horizon and likewise with the universe and it could be that our universe goes on much much much further indeed most people would guess it goes on at least a thousand times further that's because the gradient across the part we can see it's very small if you look as far as you can in that direction in that direction things don't differ by more than one part in 100,000 but it could go on much much further still to produce the domain so vast that all combinatory options are fulfilled and there's another lecture on somewhere out there with another set of people here and you will have an avatar and it may be some consolation that if you make some mistake your avatar may get things right far away but that's not all all this is the aftermath of our big bang and Accord to subversive inflation our Big Bang isn't the only one and here's another cartoon there's the valley within a horizon here the galaxies far beyond that but this may be just one bubble one domain in a universe cosmos that's much much much larger and so these different domains which may I'll be governed by different laws and had different constants may actually exist it may be kind of factual but they may actually exist and I think one of the key questions that will have to be answered is which part of this decision tree is correct how many big bangs are there this is just one or other many if there are many are they all governed by the same physics when they cooled down or is that a variety to some end up a little different gravity different repulsive force in space different atomic physics etc we don't know but this is something which I think since today but I'm more optimistic about being able to settle this because we now have fermer reasons for taking seriously the very early stages when these conditions are imprinted but if there was one big bang then that's a root fat etc if there are many and if they got by different laws then of course we have the possibility that we are not in a typical cosmos we are in a typical member of the subset in which complexity could arise and we could bear in mind that many universes may be governed by laws that don't allow any complexity there still warn or stare and as it were and to make another historical analogy here's Kepler's famous picture where he thought that the planets were in orbits whose sizes were related to the platonic solids a beautiful idea we now don't believe that anymore and I think we may be due for a similar transition in our thought now so at the top planets we now think that there's nothing special about the earth orbit the earth is just one planet around one star all we can say is it's in the temperature range where water can exist otherwise we wouldn't be here there's nothing more special about it than that so similarly our Big Bang may turn out not to be special we're not in a simplest universe which would have a zero cosmological constant and that critical density we are in a universe that allows complexity to arise but it may be a typical and properly allowed model just as we're on a typical and perfectly large planet so Copernican demotion will go one stage further if we learn not meted if we're on one one star and one galaxy but we're in the aftermath of just one Big Bang in many and so the interesting part we want to address is what parts of parameter space allow interesting complexity I'll jump over this bit but I want you to finish with this picture which is a I think the logo which I would choose for this research area it's or a Boris and it shows the small scales on the left and the large scales on the right this is the world of the quantum this is the world of gravity we know that there are many left-right links we know the everyday world of people and mountains is determined by atomic and molecular structure we know halfway up there's a link atomic nuclei determined how stars evolve and higher up still the dark matter house galaxies together is some kind of sub-nuclear particles made in the Big Bang but most of science can get by without the unification of the small and large this is the domain of quantum and this is where Einstein holds sway chemists don't need to worry about the gravitational forces between the atoms in a molecule with their 38 powers of 10 weaken the electric forces conversely astronomers don't need to worry about the quantum fuzziness in your wit of the planet because the fans got so many particles image but we do need a unification and we need it in particular as symbolized gastronomically at the top here if you ought to understand the very very early universe and if you have to understand the nature of space and time themselves so we have these two frontiers the very large and the very small mostly they're disjoint in their applications but we need to unify them to understand the very beginning but before leaving this picture before finishing I want to note that there's a third frontier and two very bottom the very complex and the most complicated things we know in the universe are these things we are complicated we have a large compared to atoms we have layer upon layer of structure but we are very small compared to stars so we're not crossed by gravity in fact we are Midway the geometric mean as the mass of the Sun and the mass of a proton is about 50 kilograms within a factor of two of the mass of everyone here I would guess and so we are midway and so we need to understand not just the atoms were made of but the Stars have made those atoms and this third frontier the front of complexity is where 99% of scientists work and this is a famous picture by you Robert Hooke who was Newton's least favorite colleague and had run the first microscopes and I show this because it reminds us that even a flea is far more complicated for how to understand than an atom or a star so its complexity and not sheer size that makes things hard to understand and I mentioned this because I think it's appropriate to end on a modest note we are hoping that we will have theories to understand a fundamental physics and what relates together the four fundamental forces we want to extend the unification which started with Newton continue through Faraday and Maxwell and we hope will eventually lead to the synthesis at the top of that picture but even when that's done that'll be the end of a important strand in science and a great intellectual achievement but it will be irrelevant to 99% of scientists who are working on the frontier of complexity it won't help them at all to disentangle their complexities and I think the other nice analogy is with the game of chess if you've never seen chess being played you look at some people playing you can refer what the rules are but knowing the rules is just the beginning of a long progression from novice to Grandmaster and similarly what physicists are doing is learning the basic rules and trying to understand them but even when we understand them fully then seeing how they apply and how they've led over 13.8 billion years to the chain of complexity growing from atoms mr. stars to planets and then to a marvelous biosphere that's an unending quest and that's perhaps a good note on which to finish thank you very much [Applause] [Music] [Applause] you [Music]

Channel: PhilosophyCosmology

Views: 57,687

Rating: 4.837728 out of 5

Keywords: Philosophy of Cosmology, University of Oxford, University of Cambridge, Martin Rees (Author)

Id: KXJkZNw-azs

Length: 49min 54sec (2994 seconds)

Published: Wed May 07 2014