Neil deGrasse Tyson-The Great Courses- My Favorite Universe

[Music] the lecturer is neil degrasse tyson dr tyson is the first occupant of the frederick p rose directorship of the hayden planetarium he is also a teacher and visiting research scientist in astrophysics at princeton university dr tyson earned his ba in physics from harvard and his phd in astrophysics from columbia university in addition to dozens of professional publications dr tyson writes extensively for the public since january 1995 he has written monthly essays for natural history magazine under the title universe his recent books include the memoir the sky is not the limit adventures of an urban astrophysicist he is also the co-author of one universe at home in the cosmos which won the science writing prize for 2001 from the american institute of physics dr tyson's contributions to the public appreciation of the cosmos have recently been recognized by the international astronomical union in their official naming of asteroid 13123 tyson dr tyson prepared the course guide that comes with these lectures the course guide includes a detailed outline of each lecture a timeline a glossary biographical notes and an annotated bibliography to get the most out of this course you may find it useful to follow along with the outlines or review them before or after each lecture these lectures are titled my favorite universe [Music] welcome back to my favorite universe for this lecture i want to highlight what is in my judgment one of the most important discoveries of the 20th century and it relates to something that people have been thinking about across time and across culture because what has happened is if you look you read through the text religious text even scientific it won't matter look through the history of time at a culture's writings and in there you will find that people have looked up at their night sky and wondered what is their place in the cosmos where did it all begin where is it all going to end these are common themes so maybe those questions are genetically encoded within us and this particular lecture titled forged in the stars is a discussion of what is the origin of the elements that make up the human body that make up earth itself to make up everything that is not just hydrogen and helium in the cosmos most people don't know the origin of the element now how's that possible when i just told you that it's in my judgment the most important discovery in any field in the 20th century most likely you don't know about it because it was not your classical kind of discovery where you have the lone genius burning the midnight oil saying eureka in the wee hours of the morning and then being rushed by the press to find out what the discovery was and then you put headlines the next day that's not how this discovery happened so it's not a very media-friendly story in spite of its importance this particular discovery took many decades many people it uses complicated math and it's very hard to create sound bites out of it so this was the nature of the discovery of the origin of the elements and that is the subject of this lecture i can tell you now the elements in the cosmos have two primary origins one of them is the big bang itself we got all of our hydrogen and most of our healing from the big bang everything else came from stars the crucible in the center of stars this hot place in particular one variety of star that has very high mass is a star that ultimately explodes we call them supernovae mentioned several times in previous lectures supernovae if you study what how they manufacture elements and how they explode and distribute them in the galaxy you learn not only that such elements exist you get the relative mix of the elements that we find in the universe and you find out where they're distributed in the galaxy you get all that for free once you understand how supernovae work we trace that knowledge to a seminal research paper came out in 1957 it was published in the reviews of modern physics a respected journal it was titled the synthesis of the elements in stars how much more simple can you get the authors were uh margaret burbidge jeff burbridge they were husband and wife team along with william fowler and the inimitable fred hoyle himself fred hoyle's a brilliant man known for his extraordinary ideas that are occasionally correct and as a participant in this paper this was one of them one of his crazy brilliant correct ideas the 40 years prior to 1957 people had mused about whether the source of energy in the stars could be responsible for the transmutation of elements people had wondered but they had fragments of knowledge with which to make their picture so there's a limit to how far they could go but once enough time had passed and enough experimental data had accumulated burbridge burbridge fowler and oil brought together all of those pieces and implicated supernovae as the primary source of the existence of heaven elements in the galaxy and this is what made that paper significant but it took it was many decades in the making so we have to ask the question they're messy questions that led up to that messy like how do the various elements on the periodic table of elements behave when subjected to extreme pressures and temperature you can't go visit the center of the sun the son and ask and and and and bring out your your measuring devices you'd be vaporized long before you got anywhere near it you have to be cleverer than that you have to trust your knowledge of your laws of physics and you can do some experimenting but that's about it do the elements once you try to make them do you fuse them together or do big elements only break apart what do you fuse them or fission what what's the predominant process how easily is this accomplished these are questions that were driving the results of that paper when you have a reaction does it give you energy is it exothermic or does it take energy away endothermic that's a really important question with regard to the survival of a star because stars are in the business of making energy not only that the periodic table of elements itself can you explain that it's more than just a chart of boxes sitting in the front of your chemistry class this stuff there's a lot of action going on there can you start with hydrogen and helium the birth ingredients of the universe and manufacture all the other elements on that periodic table is that possible is that sufficient to synthesize everything on the table if you could do that that'd be kind of like modern alchemy that's what everyone wanted to do you read about the you know merlin characters trying to in their lab with test tubes trying to turn base metals into gold we can do that today we know how to do that the reason why they couldn't do it they couldn't do it because they were experimenting in the domain of electromagnetism they didn't know that they had to reach the domain of another force in order to transmutate the elements they had to get into the nucleus of the atom and electromagnetism doesn't go there the strong nuclear force does had they known about the strong nuclear force 500 years ago they might have known how to get into the atom and split it and make two other kinds of atoms but that's okay that's one of the basic questions in a basic attempt to understand the formation of the elements but there's another problem a very real problem that it's hard or even impossible to deduce from first principles and that is what is the what are the collision cross sections of the nuclear pathways that's fancy talk but it's not anything deeper than if you have two particles and i want to slam them together how big a target are they to each other are they small so that your aim has to be really good or do they somehow take up more space so that when you slam them together they're very likely to hit each other so we call that a collision cross-section you have an intuitive sense of this because when you're you know on the highway and they have like the wide load the double wide load coming uh double wide house that has a large cross section so they have to take tremendous precaution so that nothing collides with it in atomic physics you want to know what that cross section is you want to know it exactly so that when you start bringing your ingredients together you'll know who's going to hit who at what rate and who's going to do it the best or the slowest or the fastest and it's from that you derive the nuclear pathways of what goes on in your soup of atoms if you get the wrong sections you have no idea what's going on you might as well just give up and go home because one wrong cross-section in your calculation will give you a picture that that'll spawn all kinds of other reactions that would have never happened in the first place it'd be like going into the new york city subway system but with a map of the london subway trying to navigate through the new york city system that's just not going to work so collision cross sections are important to learn and important to know so yes this whole effort is difficult and it like i said it did not lend itself to sound bites now others yes had thought about the general problem one of the most brilliant astrophysicists of the turn of the century sir arthur eddington from a seminal book that he wrote titled the internal constitution of the stars he suspected i quote from that book i think that the suspicion has been generally entertained that the stars are the crucibles in which the lighter atoms which abound in the nebulae are compounded into more complex elements people knew the universe had complex had larger elements they knew he asked well where did it come from was the universe born that way well this is way before the big bang so they had either somehow the universe happened that way or they're made and he suspected that they were made in 1920 atomic physics was going strong there was the beginnings of nuclear physics but it was the dawn of quantum mechanics the full understanding of particles and and uh atoms and nuclei was still to come but he suspected that they were on the right track you could ask the question where else in the entire cosmos would you make heavy elements but for the centers of stars you want an exotic environment you want an environment that if you can if you have to slam light elements together to make heavy elements where is that going to happen it's not going to happen in your backyard it's not going to happen in your kitchen need something as hot as you can imagine and that's the centers of stars that was well known at the time but the real solution required the discovery of quantum mechanics in the 1920s there's no way eddington could have figured out the whole story he was missing physics physics had to be invented to complete that story quantum mechanics as i've as i've alluded to many times is the science of very small things atoms and molecules something to consider protons if you want to a hydrogen nucleus is a single proton it's the lightest element the simplest element if you want to bring two hydrogen nuclei together they're both positively charged they're gonna repel they're gonna repel and so what do you do about it well slam them together faster they'll get a little closer before they repel how do you make it go faster one way to do it is to heat the soup heated gases the particles move faster and faster and faster as the temperature rises get them close enough together so that they're within the domain of the strong nuclear force when you get them close enough together the strong force kicks in and binds the two protons together they had to cross this what we call potential barrier though this this this electromagnetic resistance this of the two positive charges this electrical resistance they had to overcome that as i said in one earlier lecture it's like taking a a toy truck and sort of rolling it up a hill but you don't give it enough energy and it just sort of rolls back and you sort of rode a little faster it gets higher up the hill and rolls back but there's a speed with which you can roll it and it hits all the way at the top and goes over that's what's going on with these protons now eddington is not stupid eddington calculated the temperature of the center of a star he got that right you don't need quantum mechanics for that he said it's about it's 10 million degrees or so then he calculated the speed with which protons much approach must approach each other in order to collide you know what temperature you got you got a billion degrees that worried him because the two temperatures were had no correspondence with they are vastly different temperatures if you want to rely on collisions in order to give you a heavier element you might be telling yourself at this point the stars are not the place as hot as they are it's not going to work tens of millions of degrees is far away from a billion degrees so that's why there was as much resistance as there was at that time for considering stars to be the place where heavy elements would be formed but there were the diehards that said look i don't care what you say i don't know i don't know what you're saying all i know is the hottest place i can think of is the center of the star in the whole galaxy so if you want interesting atomic things to happen nuclear things to happen if we don't look there i don't know where else to look and that's the right attitude because it turns out that the speed of protons at 10 million degrees is enough to make them collide but not by the traditional way you might think there's a phenomenon in quantum mechanics that has no analog in everyday life and it's called tunneling tunneling let's go back to my little truck i'm trying to roll it up the hill here i'm trying to shove it up the hill and it rolls back it's not enough energy in quantum mechanics depending on the ambient temperature and pressure and conditions one of these times i roll the truck a hole opens up through the mountain and it goes through the mountain and comes out the other side and it gets to the other side of that mountain for free now that'd be pretty interesting if that happened in everyday life but it doesn't it happens in the realm of the atom so that in fact these protons which are trying to overcome their repulsion some of them actually do connect at temperatures as low as tens of millions of degrees you didn't need the billion degrees but before quantum mechanics was discovered there was no way to think about that problem there was no way to answer that problem there was no solution waiting for you all right quantum mechanics tells us all right the temperature's hot enough to make your elements that's good well how about the relative amount of each element that's another kind of question to ask of the star well come the 1920s are over quantum mechanics is in the can 1931 there's an astrophysicist named robert atkinson he published an extensive paper on what was going on inside of stars and his abstract summarizes what he was after synthesis theory of stellar energy and of the origin of the elements in which the various chemical elements are built up step by step from the lighter ones in stellar interiors by the successive incorporation of protons and electrons one at a time so we already accounted for the existence of elements and he's trying to find a way where one by one you build up the elements and in his paper he talks about what kind of relative populations of the elements you would get but he didn't have a chance of getting the right answer he was missing a particle he had the right physics he's missing a particle a year later 1932 james chadwick discovers the neutron you can't talk about nuclear physics without a neutron neutrons go hand-in-hand with protons in the nucleus that's an incomplete theory you left the salt out of the soup no matter how smart you are you weren't going to get the right answer without the neutron now the fun part about neutrons is they don't have a charge so here's a nucleus with this fat positive charge and you take a neutron and toss it towards the nucleus it just walks in the front door there's nothing to resist it so you can have a nucleus feed it neutrons and build up the count of particles inside the nucleus you hand a neutron to the nucleus no it doesn't change the the species it's still hydrogen if you add a neutron to it but we have a term for it's called an isotope an isotope of an element is where you have a varying number of neutrons you keep the number of protons for this it turns out there are some elements where if you give them an extra neutron they don't like the extra neutron so spontaneously again we learn this from quantum mechanics spontaneously it's got the neutron in the nucleus and it said neutron give me a proton and an electron the charge is cancelled between a proton and electron as they are neutral in the neutron in the first place so all the charges work out so you give give the atom a neutron the neutron spontaneously becomes a proton and releases an electron so almost for free i got to have a proton join my nucleus this process is a very effective way of building up elements and it can happen swiftly there's some by the way it may be that there's some elements and not maybe it is true that there's some elements where if you give them one neutron they go unstable and they kick the whole neutron out again but if they have two neutrons they are stable and they're given the time they'll convert the neutrons to protons so think about it if you have a weak flux of neutrons into your atoms and into your nuclei if you're an atom that's unstable with a neutron it's you're not going to do anything with it if you're stable with the neutron fine you'll mutate it into the next element up on the periodic table but if you have a heavy stream of neutrons maybe you're i just gave you one and you're unstable but before you could release that neutron another one comes in now you've got two turns out the nuclear physics of it say that i'm stable with two even then i even when i was not stable with one so with two neutrons unstable now i can undergo a decay again and turn my neutron into a proton and an electron so it's simple one is called slow neutron capture the other is called fast neutron capture the only lesson there is that now there are more ways to make elements it's not just simply slamming protons together the macho way they're also less spectacular ways of building up the population of elements on the periodic table and when you combine all of those features all of those features each one of them gives you a different mix of different elements and put them all together you begin to recover the exotic distribution of elements in the cosmos you begin to recover why it is for example that on average elements with an even number of protons are more abundant than the adjacent element that has an odd number of protons that's odd those elements they needed two particles to come in you're actually slamming a helium nucleus in you get two protons right off the bat and you grow by just accumulating helium nuclei so you're hopping in in in pairs so if you look at the abundance on the chart it's high here down high down high down all the way on up fascinating and it all comes out of this analysis now what about those collision cross sections like i said you don't get those those are hard those are hard turns out just the facts of global politics we got our collision cross-sections to feed the research that went into the 1957 paper by burbridge burbridge fowler and hoyle most of those collision cross sections for the atomic nucleus came about from the research that went into the manhattan project the american program to build the first atomic bomb in the second world war when you're at war a lot of money gets spent a lot of effort gets invested not because you're curious about the science but because a weapon is being made your defense is paramount importance and a side light of that fact it's simple a side light of that fact is we got from that research once it became declassified collisional cross-sections of particles moving in and out of an atomic nucleus and it enabled those calculations in the first place now of course now that we know how to make the elements in the centers of stars in particular in the centers of the high mass stars that have enough temperature and enough pressure to keep that going right on up the periodic table of elements let's begin by asserting something that's not that's not uh strange concept that stars are in the business of making energy and to be in the business of making energy that's all they know how to do if you look at the sun the cross section of the sun what you see is down in the center that's the hottest place it's much much cooler out to the surface and it's in the core that's where all the thermonuclear fusion is going on the sun is making heavier elements it's converting hydrogen into helium but that's not the interesting elements the interesting ones are the ones that we're made of so the ones we're made of who's going to make those those are the high mass stars they start out making hydrogen into helium fusing the two fusing hydrogen becoming helium there's a loss of mass there you do the arithmetic because you start out with more mass than you end it up what happened to the mass it all got converted according to e equals m c squared and you get an enormous amount of energy from that lost mass all right so now you go hydrogen and helium high enough mass stars convert helium to carbon to take three helium atoms and get carbon out of it then if you have high enough mass it'll keep cranking this through it runs out of one element and starts manufacturing the next it goes from helium to carbon carbon to oxygen oxygen to neon and this continues and it's getting energy at every phase except it's not as efficient it blows through those heavier elements very quickly compared with how much time it's spent converting hydrogen to helium so as it moves up the periodic table one of the elements it's going to end up with is iron time to fuse iron well let's go for it all right let's collapse a little bit get the temperature up we begin to fuse iron at extremely high temperatures much higher than what the sun is doing right now or it's the centers of a star while it's converting hydrogen to helium so we start fusing iron there's a problem if you fuse iron it absorbs energy it doesn't release energy now that's a bad day for the star because a star only knows the creation of energy it's that creation of energy that supports it from collapse without a source of energy it is hosed the star has nothing to hold itself up so it gets to iron it fuses iron not only is it not good for it because it's not giving up any energy it's actually absorbing energy and the whole star destabilizes and it collapses it collapses in a matter of hours and the collapse of that star rebounds from the center in a titanic explosion that is as luminous as a billion stars and we call those objects supernovae they're visible across the galaxy across the universe you're looking at an image of a galaxy halfway across the cosmos and there's a little smudge there but there's a bright light sticking on one of the aren't spiral arms of a spiral guy that's a supernova that just went off these things happen in real time when they happen you can watch them get brighter and watch them get dimmer as the explosion runs its course well this is such an active place that in fact there's a lot of free neutrons running around in proton so not only did you manufacture elements on way to iron there's a lot of manufacturing going on after iron as well slow neutron capture fast neutron capture a little bit extra fusion on the side it is a trove of enriched chemical elements one such remnant of a supernova is the crab nebula the crab nebula is the remnant of a star that exploded and was seen it was seen by the ch well actually we think everyone on earth saw it but it was only recorded by the chinese a.d 1054 july 4th in the records of the chinese records there's a new star in the sky we now know that was a supernova we now know it was that supernova making this residue this explosive remains this 2000 year old remains which we have poetically referred to as the crab nebula now you notice on it there are these fibrous parts to it each one of those areas contains a strand of heavy elements lifted out of the center of the star and brought into the galaxy for the rest of the galaxy to share so that subsequent stars that are made can make something other than the central star like planets and even people the next one it's another supernova remnant cassiopeia a we happen to call it it's in the constellation cassiopeia cass a you notice the once again you'll see how sort of explosively thrust forth these gas clouds are and if you analyze the chemical composition of each of those strands you'll find very high iron and oxygen and carbon and all the things all the elements that we've come to know and love as living beings that thrive on the heavy elements of the cosmos so you combine all this and what berber's burbidge fowler and hoyle did was they took the well-tested tenets of quantum mechanics combined it together with the physics of explosions and the latest collisional cross-sections for the atomic nuclei and they also looked at all the nuclear pathways that can result based on those input parameters combine that also with basic stellar evolution theory and when you've done all of that and you've done your homework you account for not only the existence but the distribution and the relative abundance of elements in the cosmos and it was in 1957 where it was determined something about us that i will never forget and i will tell as many people as i possibly can and you might have expected it all along but it remains true not just figuratively but literally that we are all stardust [Music] welcome back to my favorite universe today we're going to tackle a pretty hot topic and that's the one that involves the search for planet in our galaxy now the search for planets takes on many takes on many dimensions you might think well why search at all we're pretty sure the the galaxy the milky way galaxy our galaxy has planets in it because the sun has got eight or nine planets before the 1990s our eight or nine planets were the only known planets in the galaxy right now we happen to be rising through 100 100 planets known outside of the bounds of the solar system and that number of planets were not discovered just because people were curious whether there were planets elsewhere in the galaxy i i always had the confidence that if we look do we find them not not a no problem there what's really going on here is that we're trying to find worlds that could support life and if the sun is an ordinary star not too bright not too dim not too hot not too cold not too massive not it's ordinary and it's got all these planets and one of them has life on it then imagine other ordinary stars in the galaxy so it becomes an exercise of not just simply filling catalogs of planets as we've done before searching for particular types of stars or even particular types of galaxies we're looking for places that could have life now suppose for every ordinary star no matter how many planets it had one of those planets had a uh was capable of supporting life as we know it so how many planets might that be in the whole galaxy well how about the whole universe well the 100 billion stars in the galaxy 100 billion and in the whole universe there's about 50 to 100 billion galaxies you take those two numbers and multiply them that gives you the total number of ordinary stars that's a one followed by 21 zeros one sextillion most people nearly everyone has never had an occasion to greet that number one sextillion but let me give you an idea how big it is it's 100 000 times bigger than the total number of all the sounds ever uttered by all human beings who have ever lived that's how big that number is that's how many stars there are in the universe and perhaps that's how many planets there are capable of supporting life unless of course we're unusual that's an unpleasant thought i'd like to think that we're common but let's take a look to get a sense of how many stars there are in this view we have a tiny piece of the sky pointed towards the center of near the center of the milky way galaxy it's towards the constellation sagittarius sagittarius gets the award for having the most stars in it because those are the star the stars that trace out the constellation sagittarius are sitting in front of our sight line that goes straight to the center of the galaxy and all these stars are crammed in there that's just stars and one patch of sky in our own galaxy imagine summing that up for the whole galaxy and across the universe so dare we think that we're alone now it turns out to theorize about there being other worlds other places where you might find life to theorize about that wasn't always met with support or praise in fact in the year 1600 there's brother giordano bruno he was a monk uh he lost his life how did he do that well it wasn't really his fault and the square of flowers in rome the catholic church burned him naked at the stake naked at the stake and had he just been an ordinary heretic they would have just ordinarily they just would have burned him without removing his clothes but he was an impertinent and pertinacious heretic why what was his crime he suggested that the universe must be infinite because if it were not infinite then it would have to be here instead of there and how could it be in one place and not another because if it's finite it's leaving out some space over here so his philosophical sensibility told him that the universe must be infinite because it can't be one place and not another number one number two from the vastness of this infinite universe he concluded there must be many worlds beyond earth just like earth and he was rather animate about this this got him into deep trouble enough trouble for him to be burned at the stake it was against scripture against catholic interpretations of scripture in fact there's a statue to him in the square flowers in rome he's posed with his robe on my last time in rome i had a pilgrimage to that statue thinking to myself they don't do that anymore fortunately because i'm talking about planets beyond earth all the time and not only is that work praised but it sometimes makes headlines so so how do we find these planets well it's a challenge it's a challenge you don't just look up and say there's a planet it takes it takes much more than that because these planets these planets are in orbit around a host star that's vastly greater in brightness than the planet itself and so there's a challenge there to challenge you have to detect something that may be only 1 100 millionth the brightness of the host star it'd be like it'd be like you know catching a firefly and putting it in one of those beams one of those hollywood searchlight beams when they have when there was a movie opening just toss the the firefly into that beam step back and say where's the firefly show me the light of the firefly it's there but you have to somehow you have to somehow block the rest of the light eclipse it occult it somehow suppress it so that the light of the firefly can show up a little better now it turns out many planets as well as uh solar systems in formation give off principally infrared light as opposed to visible light all they do is reflect visible light but they give off infrared on their own and in fact they are more brighter compared to the host star in the infrared than they are and they're visible so what you might be able to do is whip out an infrared telescope and in that way the contrast is not a hundred million you might gain by a factor of ten maybe it's only one ten millionth the brightness instead of one one hundred millionths the brightness these are tactics we use still crude but they're tactics nonetheless and in combination with that in fact we found a way i don't know if you've ever seen a total solar eclipse if you go see one you're out there the sun is there the moon goes in front and right when the moon fully covers the sky goes dark stars come out you have blocked the light of the host star enabling you to see things that are much dimmer that were always there you just didn't notice them before because things were too bright we have figured out a way to within the optics of the telescope put an eclipsing disc in front of the host star blocking out most of its light enabling us to see whether there's any signature of a tiny bit of light nearby possibly in orbit around the host star you hope that and that's not just some chance juxtaposition of another star in the background that would just be bad luck by this method by this method in fact one of the first things discovered was one of these orbiting disks of a solar system in formation and that was the star vega one of the brightest stars in the nighttime sky visible throughout the evening sky in the summer or very early morning hours the wee hours of the morning in the winter vega goes right overhead at night for most residents of solar eclipse i've seen one in my life unforgettable i was off the coast of northwest africa some years ago and what happens is of course in a solar eclipse the sun moves uh the moon moves in front of the sun eclipsing the sun occulting the disk of the sun and when that happens when the light of the sun is removed you see the glowing outer atmosphere of the sun which is not bright enough to reveal itself when the sun is visible when the disk of the sun is you have to blot out the disk of the sun then you see the solar corona the crown of the sun the soul of corona we didn't know what it we didn't know much about it for a long time it's like what is it it's just some glowing gas what is it that's what we assumed but we didn't know much about it then we study the light take the light pass it through a prism and study the features of the light the component colors you pass light through a prism it breaks up that light into its component color just the way sunlight moves through raindrops and makes a rainbow red orange yellow green blue violet sunlight is composed of those colors do the same thing for the solar corona and you can study what kinds of elements are present in that corona and we found a signature of an element nobody had ever seen before in fact we invented a name for we call it coronium because we didn't know what it was there was no laboratory counterpart for it we didn't know where it might fit in the periodic table of elements we didn't know anything because we had this placeholder name for it turned out under the low density conditions in the outer atmosphere of the sun under very high temperatures millions of degrees iron iron emits a signature of light highly ionized iron we're talking about iron that has lost most of its electrons ionized heavily ionized it has a spectral signature unmistakable but we had never ionized iron that much before we had never subjected it to low density environments such as the corona before we had no way to understand it until we figured out that it was under very low density and very high temperature and so the solar corona is an example of something that's quite visible and quite beautiful but is also very very low density in the cosmos by the way if we're much higher density it might be more visible even with the sun in the picture you really have to blot out the sun in order to see the corona temperature the corona millions of degrees millions that's how hot she had to be to ionize iron as much as it got ionized and the surface of the sun is a mere 6000 degrees it remains a mystery how you can get something that hot sitting above something that cold major frontier of solar physics we have ideas but no true consensus just yet let's keep rising up how about the asteroid belt we all have a favorite vision of the asteroid belt as this as the shooting gallery all right don't dare take your spaceship through the asteroid belt they'll come and knock out your your antenna it'll be really dangerous now wait a minute let's look at the facts take all the mass of the asteroid belt and combine it together and weigh it how much is it what do you get you get about two or three percent of the mass of the moon of our moon no it's not some jumbo planet and all its got shattered and all its debris is still there it's like two and a half percent of the moon now of all that mass put 75 percent of it in only four asteroids and take the rest and scatter it for a hundred million miles wide in an orbit around the sun one and a half billion miles around that's the asteroid belt now it's still more dangerous to go through the asteroid belt than to not go through the asteroid belt but it is much less dangerous than most people ever think of it to be much lower density of material than we ever credited for having and of course our four spacecraft that have gone to the outer solar system and have left beyond the orbit of pluto that's pioneer one in pioneer 10 and 11 and voyager 1 and 2 those 4 spacecraft travel through the asteroid belt without incident just as we suspected let's keep going out into space by the way the [Music] density of matter in interplanetary space we learned what it was on earth uh sea level what was it a quintillion uh quintillion molecules per cubic centimeter out in interplanetary space we're down to 10 10 per cubic centimeter there's a vacuum for you there's a vacuum and that's about the best laboratory vacuums uh the best we can get with laboratory vacuums here's a here's a sketch of one uh the compartment that has the little circular uh illus the circular parts that's inside that cavity is where they're pumping on that cavity to remove as many molecules of air as they can so that you have as low a density environment as possible and then you could do interesting experiments particularly with the excitation of molecules but in the vacuum of interplanetary space there are other objects that are quite visible but are also quite thin comet tails comet tails comet tails are long big comet tails a good like comet halley 100 million mile long tail recently comet hail bob and comet yakitaki in the last several years those comments were great comments great in the sense in the classical sense of the word great you get a few of those in a lifetime so make sure you put in your schedule to go see them as many of us did but what of a comet tail as visible as they are what are they made of they're still thin they're still thin but they're remarkably visible for their thinness they reflect sunlight and they emit light of their own for having been excited by the high-energy photons from the sun so yes it's a visible stream not very high density yes it's it's it's more dense than interplanetary space but not by much in fact the comet pioneer fred whipple once described comet tails as the most ever made of the least the thousand times the density of interplanetary space which is still small compared with ordinary earth air now let's let's quantify how thin the comet tail is even though it's as high dense even though its density is high compared with intertime interplanetary space let's take the whole tail collapse it to atmospheric density and ask how much do you have that's a good way to compare take the 100 million mile long tail say let's make atmospheric pressure out of that you could fill a cube maybe a half a mile on the side that's all it is red whipple is right it's the most ever made with the least now when people first invented tools of spectroscopy studying the light from objects and looking at what material was in them what spectral signatures permeated the analyzed light they found that comet tails had cyanogen in them cn a deadly poison would kill you post haste well when comet halley came around in 1910 people didn't know the public didn't know about how low dense how low the density of the comet tail was and how it was barely any no matter what you found it's not enough to do anything to anybody what scared people is that we knew that earth was passing through the hell of the tail of haley's comet it passed through the tail it actually did intersecting some of that gas people got all worried charlatans were selling anti-comet pills making a lot of money on people's fear but if you knew a little bit about how thin it was you would have saved your money so knowing about rarefied phenomenon in the cosmos can be good for your pocketbook on occasion that takes us to something not widely appreciated let's look at the density of the sun it's gaseous but you might think it's like really dense it is very dense in the core less dense on the surface but if you averaged it it's about the density of water so too are human beings that's kind of interesting water has a density of one gram per cubic centimeter human beings we basically float or sink or somewhere in between so that puts us at about one gram per cubic centimeter the sun is actually a little heavier the sun is just a little heavier 1.4 grams per cubic centimeter so if you took a scoop of the average material of the sun it would sink in your bathtub but it would not sink that fast what happens in 5 billion years the sun is about to die in five billion years it swells up becomes a red giant it becomes a hundred times bigger in diameter than it is now of course it didn't add there's no mass that got added to it so the average density drops it drops to one millionth its current density by then the surface of the sun is very close to earth that's bad bad day on earth the atmosphere would evaporate the oceans would come to a rolling boil they too would evaporate life would vaporize ignoring those complications what we find is that the material of the sun as rarified as it is earth is trying to orbit the sun plowing through some of that material and that resists the motion of this of earth and earth ends up losing its orbital energy and spiraling into oblivion down into the center of the sun that's going to happen in five billion years so as rarified as the sun becomes as a red giant it will still impede the motion of the earth let's go to interstellar space we go beyond the planets beyond the oort cloud of comets that surround the solar system interplanetary space it's even less dense there by the way the nearest stars are far our space probes pioneer 10 and 11 and voyager 1 and 2 at those speeds it'll take 25 000 years to get to the nearest star it is far it is far it is big it is empty there you'd be hard-pressed to find a couple of atoms a couple of atoms for every few cubic centimeters this one tenth what it is between the planets much better than most vacuums ever created on earth gas clouds that live between the stars they are rendered visible much like comets that come near the sun gas clouds in the galaxy are rendered visible if they're near a star the star reflects light off of them and they get excited and release light from their own atoms and molecules and we use special telescopes to detect this in these gas clouds there was another spectral feature that no one knew about and no one had ever seen before in the nebulae of space we found an element we didn't know what to we call it nebulae they ever hear that nebulae what did nabillion turn out to be it turned out to be oxygen in a peculiar state of temperature and density what kind of density very low density the kind of densities that were very uncommon on earth and very hard to create in the laboratory although now we can do it routinely interstellar space it's got stars it's got gas clouds it's pretty empty between those places however empty that is it's emptier between the galaxies between the galaxies that's empty that's empty there's no dust there's no stars there's no planets there's no moons there's no comets there's no comet tail it's empty it's as empty as you could possibly find and there you get one atom in a cubic meter a cubic meter another way to think of it is take a 200 000 kilometer cube how many atoms are in that 200 000 kilometer cube about the number of atoms in the air in your refrigerator that's empty that means most of the entire universe is a vacuum as i said earlier the cosmos loves a vacuum now you might have heard that the universe is expanding there's not enough mass in the universe to exhibit enough gravity to halt that expansion and bring it back but we can ask the academic question now academic question how much mass would it take to balance the expansion of the cosmos how much density of matter is required for that well it's got to be more than the one particle per cubic meter how much more okay 10 particles per cubic meter if the whole universe had 10 particles per cubic meter that would be enough to halt the expansion and send us right back from whence we came that doesn't sound like much it's not that much just more than what it is now but what again of this notion of perfect vacuum suppose there were no particles any does that even have any meaning how do you do that how do you does such a place exist well we see from our favorite vacuum pump once again that we're making great strides to try to create the vacuum of interstellar space intergalactic space it may be impossible but let's imagine the day comes where we can create the perfect vacuum like the perfect storm the perfect vacuum inside of this cavity what would we measure turns out the field of quantum mechanics a very successful branch of physics discovered in the 1920s added to in the decades that followed that's a description of nature on its smallest scales it tells us what atoms and molecules are doing which is wholly unfamiliar to the kinds of things we see happen in everyday life all kinds of new forces take effect forces of quantum mechanics new forces of physics new behavior of material turns out quantum mechanics predicts that in the vacuum the vacuum can't really be a vacuum that it's seething with these things called virtual particles which are particle pairs matter and antimatter pairs that pop into existence annihilate and then go out of existence again and they live too short a period of time to measure their existence now this just sounds like a cartoon it sounds like science fiction sounds like just not science fiction it sounds like fiction but so much else that quantum mechanics has predicted has come true that no one has the audacity to assume that this prediction is somehow going to be false when everyone else turned out to be correct so what does that mean if particles are popping in and out of existence in the vacuum you can measure what effect this has on the environment and it creates a pressure a pressure a density of energy that has an opposite effect of gravity instead of bringing things together it pushes things apart we call this the vacuum energy and you know something if you calculate how much vacuum energy is in the entire cosmos you get a number that's kind of insane some something's wrong with our calculation but we do calculate that in fact there's a pressure on the cosmos that will never let us recollapse no matter how much matter we would ever find recently we discovered that in fact the universe has a kind of uh kind of an anti-gravity pressure operating on it it's a recent discovery a few years ago this is this famous dark energy nobody knows where it came from nobody knows what it's made of but we know it's there this this vacuum energy from these seething virtual particles predicted by quantum mechanics could explain that except that when you calculate it the numbers are wildly different from each other so we don't know what the problem is it remains a frontier of our understanding of the cosmos but we feel a little better that there is something that we know about that can create this negative pressure on the universe forcing it to expand exponentially into the future even though the calculation doesn't work out it's far too much compared with what we see but it's encouraging because before we had these ideas we there was not there's nothing we could conceive of that could resist the titanic forces of gravity on a cosmic scale well is there some limit to this nothingness i talk about empty space but i still use the word space i'm still saying there's a thing called space suppose there was a place where there wasn't even space what is that maybe that's outside of our universe if there is such a concept because if space there is nothing then perhaps outside of our universe where there is no space there's not even nothing what shall we call that we would call it nothing nothing [Music] welcome back to my favorite universe today following up on an entire lecture devoted to that which is rarefied in the cosmos today i want to talk about that which is dense or more generally a discussion of what it means for things to have density at all some of my favorite objects in the universe have some of the highest density of anything it's a mind-boggling range of measured densities in the cosmos in fact it spans 40 powers of 10. now it reminds me when i was a kid what's our first way we understand things that are dense and things that are not there's the old joke which weighs more a ton of feathers or a ton of lead and you said oh of course a ton of lead of course no no they weigh the same because they're both a ton but what that the fact that some people answer that question wrong is indicative of the fact that many people think of the weight of things not so much for how much it weighs but for how dense it is there's some intuitive sense of density and we can define density mathematically it's just a ratio of the mass of the object divided by its volume it's very simple that is the density it could be grams per cubic centimeter could be could be kilograms per cubic meter these are densities it's the unit of densities now there are other kinds of densities that aren't strictly measured that way one of them is the human resistance to common sense that's a kind of density he's dense or she's dense or okay we all know what that means although if you took it literally it would mean they had more gray matter than others with the same size head if they're denser it means they have more brains so maybe there's an occasion to reverse that insult another way to think of density is population statistics you're not thinking of a volume density you think of an area density how many people per square mile live here in manhattan my home uh borough of new york city at mid-afternoon is 100 000 people per square mile in manhattan that's very high my wife who's from alaska has nearly zero people per square mile the whole state has a half a million people tops very low density populations other ways to think about the term density now in the universe again thinking about mass density the range is so large we hinted to some of the thinner densities in the previous lecture intergalactic space has the lowest density of all one atom per cubic meter can't get much lower density than that but at the other extreme one very common form of high density matter is a white dwarf here we have a white dwarf ngc 3132 in this image what you see here is the this nebulosity is the shell of a star that had been released into space in its dying days laying bare the hot dense core of what was once of what was once the center of thermonuclear fusion it is densely packed atomic matter it's small and it's hot enough to be glowing white we call it a white dwarf to get the density of a white dwarf what you need to do is take the sun or slightly less mass than that and cram it down into something the volume of the earth that's how high the density is of a white dwarf but it's not the king of high density objects that goes to my favorite object the neutron star a neutron star can be seen in the center of this image this particular image is of the crab nebula crab nebula is the remnant of a star that has once that some time ago exploded exploded spread its guts into the galaxy enriching the galaxy with heavy elements elements the kinds of elements you and i are made of the kind of elements you find that are the active ingredients of life and of planets and of asteroids and comets we'll learn more about that in an entire lecture devoted just to that subject but right now what's of interest here is not the crab like nebulosity of this the crab nebula but in the center of the crab nebula there's a pair of stars one of those is the exact center of that explosion and that is the neutron star an object so dense well first let's use the sun analogy take the sun cram it down into a volume about a dozen miles across far more dense than a white dwarf let's get a more earthly analogy let's take let's take a thimble full of that neutron star like we gave this example in the unbeing round lecture take a thimble full of it and ask what will it take on earth to to sort of balance that on a scale so you put it on one end of a see-saw and then the other end of the see-saw stack a herd of 50 million elephants then it'll just about balance that's how crammed together the material in a neutron star is that's dense that's as impressively dense as anything i have ever thought of but the things that go on when you're dense if you're small dense and have a lot of mass packed in there you have a high surface gravity and high surface gravity wreaks havoc on its immediate environment if a gas cloud comes a little too close it'll take the gas cloud and draw it in in such a way that it'll spiral around the central point in inner regions spinning faster than the outer regions creating friction when you create friction you have heat when you have heat you have high temperature and high luminosity you have these spiral areas look like toilet bowls but we have an official word for it it's called an accretion disk it's a system by which these small dense objects consume matter because they're so small the matter doesn't fall straight down on it most of the time it misses and hits the disc on the side the disc is a way to release energy so that it can descend into the oblivion not only do you find accretion disks around neutron stars but you'll also find them around black holes another favorite object in the cosmos that is small and has a high surface gravity now the density of things in the cosmos uh like as i said earlier this ranges 40 orders of magnitude but let's start with simple things that we know and love and understand let's start with water water has a density of one gram per cubic centimeter in fact it was originally defined that way water is a very important part of our culture it makes sense that we would find uh metric measures based on that other things have about the density of water frozen methane frozen [Music] ammonia in fact frozen water has slightly less density than liquid water we know that because ice cubes float icebergs float not much above water but it's floating big old ice sheets in the polar regions they're floating on top of liquid oceans there's a moon of jupiter europa has ice sheets that shift around we think there's liquid water underneath from that evidence that all floats those those frozen objects in this in out in space it's easy to freeze methane and and and frozen carbon dioxide and comets are made of this stuff comets and by the way when they come too close to the sun the sun evaporates them and helps to make the tail that we know and love but in the core the actual chunk of comet in the head of the comet is made of these frozen gases approximately the density of water on earth we get rocks i could have had a picture of any rock because a rock i'm not a geologist so one rock is the same as another rock here we have a rock formation in the southwest rocks kind of density to rocks have rocks they range anywhere from twice that of water maybe up to five times out of order earth's crust is mostly rock it's mostly rock let's keep going up there is anything more dense than rock sure there is there are metals more dense than rock iron iron two or three times the density of rock fact i have a sample of iron right here this slab of iron in fact is a meteorite from the canyon diablo meteor crater arizona it's a fragment of a fragment of the larger meteorite that made that crater this is very heavy it is very dense it's hard for me to convince you of that until i do this this weighs 15 pounds about three five pound pegs of sugar imagine that iron 90 iron 10 nickel 4.6 billion year old piece of the origin of the solar system iron yes there's some iron on earth's surface but you know where most of the iron went down to the core of the earth because iron is heavier it is denser than rock so we have iron and nickel in our core rock on our surface air above the rock in the early stages of earth when it was partially molten the heavy things fell to the center the lighter things rose to the top yes there's still some mixtures among them but the core is predominantly this crust is predominantly rock by the way there's stuff denser than iron out there platinum is much denser than iron so is gold one of my favorite scenes is you watch movies where there's some gold heist and they finally get into the vault and they just grab the gold bricks and they start they're tossing them to each other and walking out with with their backpack full of listen no no no as heavy as this is gold is heavier denser denser if they're trying to run if they're trying to steal some bricks of bricks of gold they're going to have to be like going like this one at a time they're not running out with satchels full of this stuff iridium also very high density the densest thing out there however is osmium make the world's best paperweight although this would do a pretty good job too by the way a cubic foot of osmium weighs about as much as a buick by the way you want to know how dense these things are now another way we know you can reason that earth would have its heavier materials in its center but there are other ways we know this because of earthquakes that take place periodically on earth's surface it sends these seismic pressure waves through the earth and depending on the density of material it will refract the seismic wave at one angle or another depending on where the earthquake was and what the run of density is as you go from the crust down through the center and you have enough of these stations measuring the time delays you can construct what the profile of density is for the earth and what do we get core has a density of about 12 times that of water it's up there with the heavy metals crust has a density of about three the whole average for earth is about five and a half grams per cubic centimeter about five and a half times that of order as i'd already noted we so often use the word heavy when we mean dense there are plenty of things you would pick up in your life that weigh much more than this like uh what's a good example a nice well-stocked bag of groceries is going to weigh more than this but you're not saying oh my gosh you don't freak out by lifting it because the size seems commensurate with the weight that is not so with this this is some of the most amount of dense material anyone would ever pick up usually when we have iron in the world uh extracted from iron ore on our crust we would extrude this into i-beams and things where you get strength but with low mass so we hardly ever build anything with slabs of iron this large or slabs of steel that chunky so getting back to the old feathers and lead question if i just come out and tell you lead weighs more than feathers even though that sentence is not scientifically precise you know what i mean by that you know what i mean led way's more you're not going to say you know you didn't say that right oh i don't know what you mean you're confusing me you know what i mean by that i mean lead is denser than feathers that's really what i mean fine no ambiguity there but if you want to play that game watch out because there are cases where that phrasing fails you i'll give you a good example you go to the store you go to the dairy section there's skim milk there's half and half and then what else is there heavy cream the word heavy is on the on the labeled on the on the carton you ever done the experiment heavy cream is lighter than skim milk it floats on skim milk those of you who are older out there in the audience will remember that they would deliver milk unhomogenized to your house the cream was on the top the only way heavy cream can go to the top is if it is lighter than the rest of the milk that's below and you skim it off and what's left is of course skim milk because you stole all the cream for your strawberries so it fails there plus here's another example the qe2 one of the biggest ocean liners the world has known it weighs 70 000 tons it's lighter than water of course it's lighter than water otherwise it would sink it doesn't sink so the total volume the total mass divided by the total volume is less than one same is true of course with battleships and aircraft carriers and everything else that's out there that is really massive and heavy now now i have fun with density let me give you some other tidbits some other density tidbits you ready okay we say that on earth hot air rises well of course that would only happen if there's gravity it would hot air would not know where to go where there's no gravity out in space in the space shuttle when they're weightless in orbit around the earth if the air is hot it just stays there okay unless they blow on it but here on earth hot we say hot air rises but really you could just as legitimately say cold air sinks because it's not simply that it's hot it's that when you heat air it becomes less dense and less dense things rise and more dense things sink so i'd like to sort of broaden the vocabulary and spread the word instead of saying oh hot air rises cold air sinks what else do we have i got a good one uh we already know that solid water is less dense than liquid water okay fine there's another one dead fish are less dense than live fish how do we know this because they float belly up for at least some part of the time after they have died whereas the other fish are just happily moving around now if the fish is sort of neutrally buoyant it means a live fish has a density of one and a dead fish has an entity less than one hence the posturing with its belly up in the tank and we mentioned this in an earlier lecture but it's worth repeating that the density of the of water the density of human beings and the density of the sun are all about the same sun is a little bit more dense but it's all about the same one gram per cubic centimeter so i'd like to think so many people flock to the beach during the summer time and we enjoy water there's some there's some communing going on i think we know that we're the density of water and we're made mostly of water and we want to sort of get close to it there's something there's something deeper going on than just oh i want to play in the sand my favorite planet the planet saturn has a distinction among all planets in the solar system for being the only one whose average density is less than water so that if you had a bathtub and you took a scoop of the average material of saturn it would float in your bathtub and it's the only planet that would do that so when i was a kid i always wanted instead of the rubber ducky that you play with in the bath i wanted a little rubber saturn so because i knew that saturn would float unless no one has come up with such a product but i encourage you to out there in audience land there's a limit to density even beyond that of neutron stars it's what happens at the center of a black hole now the size of a black hole by convention is described as the size of its event horizon that's that boundary if you pass it you're never coming out because you have to exceed the speed of light to do so that's why they're called black because light can't even get out even at the breakneck speed of 186 000 miles per second so it's black you go you cross the event horizon you're never coming out well by the way if black holes eat the vent horizon gets bigger and bigger and bigger but what happens to the material the material kept collapsing down and there's no known force to prevent the continued collapse until all of that matter ends up at a singular point at the center of infinite density and we call that sensibly the singularity the center of a black hole i don't even know what infinite density means but i can tell you that our laws of physics that describe a black hole when we carry the matter down to its center all those laws of physics lose their applicability we're in desperate need of a new theory of physics in order to explain the singularity so when i say it has infinite density i'm just i'm just professing my ignorance because the community has not yet come to a replacement theory for general relativity of albert einstein that gave us black holes in the first place there's a missing piece stay tuned for that so that's infinite density infinite density now density there's some mysteries to density there's some mysteries my favorite mystery is that a can of diet pepsi floats and a can of regular pepsi sinks that we may never understand that one by the way the only way you know this because there aren't many occasions to float cans of soft drink is if if you hang out very late at a party when everyone else has gone home and the big cooler that used to be filled with ice is now what it's now water with ice cubes the remaining ice cubes of course afloat on top and so whatever cans of soft drink are left you get to determine whether they have sunk or whether they're floating and take a look you'll see the diet pepsi is floating and the regular pepsis have sunk that's one of my the most profound mysteries i know but let's look at ways that things can behave the black hole is odd because it eats all the material goes to a singular point of infinite density and the event horizon grows okay that's kind of peculiar but let's let's think about the classical way you would add material to something let's think of marbles in a box add more marbles you've increased the volume and you've increased the mass you've increased the both terms in the equation for density mass divided by volume if one increases at the same rate as the other one the density is the same just divides out so a bag of marbles this big has the same density as a bag of marbles this big same it weighs more has more volume but it's got the same density fine but would that happen with other ingredients that that's a marble those are marbles but let's suppose we took down feathers let's try that experiment dump a bunch of down feathers in a box figure out how much mass it is look at its volume now take that same amount and add it to what you just put in that box what is the volume is it twice as much no of course not because it's down feathers and the down feathers at the bottom are feeling the weight of the extra feathers on top and they're getting squished so that the act of adding down feathers to a previous supply of downed feathers makes the ensemble more dense than how it started so you could in fact double the mass and not double the volume that's of course a general behavior of squishy things earth's atmosphere is squishy it's compressible at the lower atmosphere is much under much higher pressure than the upper atmosphere half of all the molecules of earth's atmosphere are below below three miles half of all the air that's why astronomers always running to mountaintops trying to get above as much air as possible and the next next best thing to do launch something into orbit we're above 99.99999 percent of all the air molecules that could interfere with your observations and of course where does earth's atmosphere end there's not there's not there's not a signpost that says you are now leaving earth's atmosphere nasa suggests that they leave the atmosphere they suggest so because they say oh we're up in orbit and you see clouds way below you of course it makes sense to think of it as that but no no earth's atmosphere goes for thousands of miles and only at thousands of miles does the density equal the density of interplanetary space then you could say i can't tell the difference anymore between being closer to earth and being out there between the planets that must be the edge of earth's atmosphere thousands of miles out and even at the distance that the space shuttle flies in the space station how high up do they fly anywhere between two and 400 miles up surely there's no atmosphere there no not the case here's the space station with all its solar panels in orbit in orbit around the earth and up there even though it's a couple hundred miles up there still remain atmospheric molecules and this thing is plowing into them and what effect does that have it slows down its orbit in fact the space station and every object in low earth orbit couple hundred miles up it has to go up with extra fuel to keep itself boosted so that it doesn't just fall out of orbit because once it starts falling there's no stopping it because you fall a little bit you're you've descended to a region of the atmosphere that has much higher density of particles than where you would once were so then you fall faster for a little bit more now you're in a dense or you fall faster yet the decay of your orbit is exponential and you know what's in it when it's especially worse you may have heard or remember that the sun goes through cycles 11 year cycles of activity so every 11 years the energy level of the sun kicks up a little bit the notch then it descends back down and evidence of that you see sun spots you see extrasolar flares and prominences and all kinds of turbulent features on the on the sun's surface that extra activity warms earth's atmosphere and the act of warming it makes it swell up and reach out farther from earth's surface than it normally does so anytime you're going through solar maximum as we call it solar max all of your satellites orbiting between two and 400 miles are at risk higher risk than they would otherwise be for having their orbit decay so you have to be careful when you launch satellites you have to watch for the density of the atmosphere and watch for what the sun is doing it's not insurmountable you just give a periodic boost but you have to know you got to do that one of my favorite cases of this is the largest thing ever deorbited you may remember is a few years back the mere space station the mere space station built by the soviet union followed on by the russians after 1989 the mir space station was getting long of tooth the russians agreed to participate in the international space station which is up there now the first permanently occupied space platform and so what do you do with it you de-orbit it how do you de-orbit it those rockets that normally use you use to keep it buoyant up there face them in the opposite direction facing the opposite direction will slow down the orbit drop it to a lower altitude where it exposes itself to more air and it descends rapidly and you can do this with such precision plus fortunately there's this big wide open hair area here called the pacific ocean you can start a deorbit burn out here over asia and by the time that thing comes tumbling out of orbit you've aimed it straight for the center of the pacific ocean so it doesn't fall on somebody's head a lot of pieces burn up in the atmosphere the rest just descends into the bottom of the pacific we think of it as littering the pacific but there's no comparison to how much uh lost ships from the past 400 years at the bottom of the pacific ocean a little space space debris is nothing compared with how many lost voyages of explorers of the past 400 years we can find at the bottom of all the oceans even in areas very close like cape cod bay for example so the lesson here i think is we you need to think creatively about the concept of density and how it plays out in your life it's not just some abstract scientific concept they're very valuable tools you can bring with you by thinking of density thinking of how density reveals itself in different materials in how you um why is it for example that certain you know take for some automobile accident some cars survive accidents better than others in almost every case it's because they not only weigh more they're also denser the denser the material the more it is likely to survive and encounter with something else that is passing through little things like that and also i i'll leave you with this one case i was in a in a restaurant in pasadena california one of these coffee houses that also serves evening desserts you go to your fancy restaurant and then you do your dessert at one of these places and one of my favorites is hot chocolate with as much whipped cream as could possibly be balanced on top so i ordered the whipped cream with i ordered the hot chocolate with whipped cream and it came and then came the whipped cream and i didn't see it came the hot chocolate i didn't see any whipped cream at all so i asked the waiter i said waiter there's no where's the whipped cream you know what the waiter said i couldn't believe this the waiter said oh it must have sunk to the bottom and i'm thinking either the laws of physics are different in this restaurant from the rest of the universe or you are mistaken mr waiter he's so i'm sure he said i'll show you so he went back and got some blobs of whipped cream came up plopped it in and you know what it did it sunk for a split second because it had momentum popped right back up and sat up and i said thank you for putting whipped cream on my hot chocolate [Music] welcome back to my favorite universe today the subject is one of the most fascinating in all the cosmos today we're going to talk about black holes and just as by way of statistics when i'm out in the public and people know that i'm an astrophysicist it's one of the top three questions i get asked usually i get asked something about the search for life in the universe next is about the big bang third right up there with those two is what is a black hole are they dangerous what will it do to me if i run into one well first yes they are dangerous they will kill you post haste and they're also extremely fascinating because they wreak havoc on the environment in which we find them i've titled this lecture death by black hole because well we can first describe how a black hole would kill a person that's fascinating unto itself but this havoc that gets wrought upon the environment by black holes throughout the galaxy and throughout the universe in a way is killing stars and gas clouds and so it's really just a lecture about death and destruction caused by black holes for human beings it's quite a spectacular way to die if how to choose a way to go i'd say launch me into a black hole because what it does is it rips you apart atom by atom and you go in as a stream of matter right down to this bottomless abyss and we'll detail that a little more in just a moment but first of all what is a black hole a start at the beginning what is a black hole black hole is a region of space within which the escape velocity has exceeded the speed of light an escape velocity is a magic speed with which if an object were launched at that speed it would escape the object forever it makes sense that for higher gravity objects they have very high escape velocity velocities but for a black hole that escape velocity is the speed of light and you know the speed of light is the fastest stuff we know of in the whole universe so if light can't get out nothing gets out nothing hence the name black hole it's black because nothing comes out it's a hole because if you fall in you're gone sensibly identified now what this business about escape philosophy let me let me give you a sort of a grassroots discussion of escape velocity i'll use my shoe for this demo if you don't mind if i take my shoe and toss it i play catch with myself i toss it up it goes up about a foot and comes the foot and comes back i threw it at a particular speed in order to reach one foot above my head but if i threw it at a higher speed it goes even higher it takes longer for it to return to me if i keep doing this constantly increasing its speed i can do the experiment and find out at what speed will the shoe never come back if you do that experiment you'll find that that speed is about seven miles per second converted in in speed limit language it's 25 000 miles per hour extremely fast you don't encounter this in everyday life and that's probably what led to the adage what goes up must come down but that adage is more just for some everyday experience if you look at the physics of the cosmos there is a speed with which you can launch something so it will never come back rendering that childhood remembered phrase incorrect you might go back and tell your teachers that so it makes sense that the escape velocity must somehow be correlated with how much gravity there is on the object from which you're launching on the orb from which you're launching your object let's take a very low mass thing like a comet comets are small a big comet might be 100 miles across the escape velocity on a comet is about one meter per second you can calculate that out that's about two miles an hour you can walk that a brisk walk is three miles an hour so if you ever found yourself on a comet and just started walking fast watch out because you'll just sort of go you'll propel yourself into orbit around the comet and by the way that's why comets are very dirty things in the solar system anything gets jostled loose on it it flies away and the comet loses it it's this big garbage heap and it strews garbage behind it every time it comes around the sun in fact that's the cause of meteor showers earth plows through the debris of these things and it rains down on earth's surface it's because it has a low escape velocity but what's next we can go to the moon the moon has more mass and more gravity than a co than a comet for the moon the escape velocity is about two and a half kilometers per second that's relatively fast that's fast that's about what is that one one and a half miles per second i can't throw anything that fast you need launch vehicles to do that even on the moon for uh nasa did that of course we landed on the moon and then escaped back out of the moon they had to be launched from the moon at that speed otherwise they would have fallen back to the moon let's keep going as we know earth 25 000 miles per hour the sun escape velocity is about a little more than 600 kilometers per second about 400 miles per second that's fast that's really really fast it's very hard for things to just sort of escape the solar system because the sun has this near eternal lock upon them because of such a high escape velocity but what it means is what that number means is if you're standing on the surface of the sun and you want to say goodbye solar system if you don't have that speed you're going to fall right back to the sun from whence you came now yes mass matters high mass objects generally have high gravity but size matters too size matters too the more compressed an object is the closer you can get to its center the closer the surface of that object is to its center and the force of gravity is related not only to the mass but what is your distance to the center that is the force of gravity and so black holes and other dense objects in the cosmos they might not even have that much mass a black hole might have the mass a few times the mass of the sun got plenty of stars with that mass but it's because the black hole is small that its surface gravity is high and the escape velocity is high and it can plunge down in close back on itself preventing light from escaping because its escape velocity has exceeded the speed of light now this region of space you know we think of it as a surface but it's not a solid thing it just happens to be within which you don't come out is the point of no return we have a word for that this point of no return rather poetically described as the event horizon the event horizon of a black hole now you might ask where did all this come from did we just make this stuff up no no someone you've heard of before albert einstein when he introduced the theory of relativity back at the turn of the century 1905 that was special theory of relativity concerning motion in straight lines non-accelerating motion and then 10 years later 1915 published in 1916 he came out with a general theory of relativity which described motion of any kind accelerated motion including gravity and the acceleration of gravity that is known as einstein's general theory of relativity and in that he described the force of gravity not purely as a force of attraction between two objects that's very newtonian newton's theory of gravity thinks of attractions in that way what einstein did was said no gravity curves space and when objects move they're moving in response to that curved fabric of the universe and there's something called an embedding diagram which indicates this for a black hole here we have imagine space you know we live in three dimensions plus a dimension of time that gives us four and it's hard to imagine that warping so let's let's by the way we would be able to see it if we were higher dimensional creatures looking down on the space in which we live so let's let's do that but for our case let's create a two-dimensional universe this sheet of rubber with a grid on it and let us warp that sheet of rubber so we're living inside that sheet of rubber and there's the warp that we're talking about that is the kind of warp that exists except in higher dimensions in the universe we live as described by einstein but here because of our feeble human minds we can only imagine it in the two-dimensional case notice the warping of space so at the center of that funnel is an object of very high gravity and if it's a black hole that funnel is so long you're never climbing out of that to escape back into the grid of the rest of the cosmos so that's a way to think about what's going on by the way if there were two objects in orbit around each other you'd have two dimples imagine a cluster of stars there'd be dimples all over all responding to their own distortions of space and of time i'll add that uh there's there's a phrase uh i first heard it from john archibald wheeler who's a student of albert einstein summarizing all of general relativity and he said matter tells space how to curve space tells matter how to move and for me that summarized all of the philosophical underpinnings of the theory of general relativity now why would something collapse down into a black hole in the first place well it turns out usually things are supported from collapsing under gravity they could be chemical forces it could be thermal forces the the movement of molecules gravity is trying to squeeze it down but they're moving fast and so it prevents it from collapsing so those are forces but if you have so much mass you can actually overcome that and cram cram matter down until it's sort of atom next to atom that's possible you can do that we have states of matter that are just that we call them white dwarfs then you can cram it down even more until it's not just cheek to cheek atom to atom but it's nucleus to nucleus if you do that you get a neutron star that's the densest matter we know neutron stars if they're rapidly rotating and we see them that way we would label them as pulsars now what happens if it has so much gravity that this pressure of neutrons can't even support against them that's all she wrote we know of no force of nature that can support an object against the gravitational forces of let's say a ball of mass ball of mass 5 6 10 times the mass of the sun once that starts making its nuclear fuel and it begins to collapse nothing is left available to support it against the collapse and the matter just keeps going and go it descends through its own event horizon that's when it disappears from view and as far as we know all the mass collapses down to a single point of infinite density zero volume that's kind of absurd what does that even mean infinite density i i don't know what that means but what i do know is that general relativity the most successful theory of gravity ever put forth fails at that singularity so we know that it's a theory that is incomplete even though everything is predicted has been true it's an incomplete theory and we're searching string theory is one of them one of the ideas put forth that'll give us a handle on the singularity deep inside but it's for all these reasons that black holes are one of some of the most romanticized of cosmic objects they're mystery they're danger they're mysterious they're dangerous and they're basically fertile for storytelling particularly in the realm of science fiction so why don't we take a feet first dive why don't we take a feet first dive into a black hole and see what happens well as i'd already mentioned as you're descending towards the black hole the force of gravity is growing exponentially but all that means is you fall faster and if you're in free fall you're actually weightless so you wouldn't care that you're descending towards a black hole because you wouldn't notice it just yet you'll just be falling it'd be like falling towards earth or falling towards anything so that's not what kills you it's not the high gravity that kills you you know what it is it's the difference in gravity between your feet which is closer to the black hole than your head there's a difference in gravity as i stand here on earth between my feet and my head i barely notice it because my height is very small compared with the radius of earth you can ask how much stronger is the gravity at my feet than at my head well look at the size of earth and look at my height that's basically nothing so so i that if that i don't even notice that difference but if you're descending towards a black hole and black holes can be tiny things if the size of the black hole and your height are comparable imagine in a limiting case i've got a six foot diameter black hole and i'm about six feet and i'm like falling towards the black hole my feet are twice as close to the black hole in my head you calculate what force that is that my feet are feeling an acceleration towards the black hole that's four times the acceleration of my head i begin to stretch my body begins to feel like something is like i'm on a rack or made of rubber of course i would a rubber man i would just stretch according to these forces but i'm not made of rubber i'm made of human flesh and that has force limits by the way these these forces the this force that stretches you as you descend towards a black hole we have a word for it's called tidal forces it's the same word as in the tides on earth earth feels tides from the tidal force of the moon one side of the earth is closer to the moon than the other in a measurable way and so the oceans feel a little bold they're drawn a little more on the near side of the earth than on the far side of there so that there's the stretching of the oceans in response to the tidal forces of the moon now if the moon can do that to earth imagine what a black hole can do so what happens i begin to descend and eventually these tidal forces exceed the chemical bonds of human tissue and as i descend there's a point where i can no longer resist it and i snap into two pieces a lower segment and an upper segment and i keep falling and then sure enough those two segments feel enough of a title force that they snap into two pieces and then those four pieces each snap into two pieces and it goes from one to two to four to eight to sixteen and this just continues eventually you are completely snapped into countless pieces of biological matter as you descend towards the black hole now it gets worse than that it turns out space and time have collapsed onto a black hole this is what general relativity describes for us it squeezes down into the black hole and so here i am occupying a space that is in fact getting narrower and narrower and narrower like a funnel so not only am i stretched head to toe i am squeezed shoulder to shoulder it's as though my body is being extruded i think of like it's like toothpaste being extruded through the the hole in a toothpaste tube that's what's going on as you fall towards a black hole it's also like if i don't know if you've ever had one of these homemade spaghetti making machines you take the the the semolina dough and you knead it and put it up and put it in the machine you squeeze it and out the other side comes these long strands of spaghetti in fact this phenomenon is officially known as spaghettification it's what happens to matter that's descending into a black hole now black holes eat what happens to them well turns out they get bigger it's true with most things they you get bigger black holes it turns out get bigger in exact proportion to how much mass they have consumed so if a black hole is of a given size and a given mass and then each its own mass again the equivalent of its own mass again then it becomes twice that size beats three times as much it'll become three times that size the arithmetic of uh relativity demonstrate why this is so and again the size is referring to the event horizon size of the event horizon well what that means is black holes can actually be any size depending how hungry they've been in their lives and not all black holes will kill you before you descend through the event horizon ones will but really really big black holes really really big black holes the tidal forces right at the event horizon are relatively low and so they're less damaging to human biology as you descend in it's and by the so what that means is it's the low mass black holes the ones that are the smallest that'll do the worst damage to you as an unsuspecting visitor it's because the rate and change of the gravity that's the tidal force the rate of change of the force of gravity gets significant as you near its center in both cases you'll get ripped apart no matter what because there's some distance from the center where this will happen to you the difference is that with a small black hole it'll happen before you get to the event horizon and everyone will get to see this happen whereas in a big back hole black hole you'll descend through the event horizon you'll get ripped to shreds and no one will know except for you but you won't be able to tell anyone about it of course so now suppose you get really really really big black holes well let's go back let's go to the real universe i was describing ones that might eat a person but let's get to ones that do some real damage in the universe the real universe you have stars that don't necessarily travel alone a lot of stars that travel in pairs call them binary stars when you have stars and pairs typically one will age before the other and part of its aging means it becomes a red giant it swells up as it gets bigger some of its material gets a little too close to the neighboring object if the neighboring object is a black hole black hole is going to it's going to eat it that's not just going to eat it it's going to flay the red giant as it expands into its space and we have an image of this first there's a person descending down into the black hole that person is not having fun you see the spaghettification in progress by the way that would happen to any material going in it's just more explicitly conveyed when you're a human being but notice in this image we have a star a red supergiant that has become big and bulbous and in that effort some of its material has gotten a little too close to its companion that little companion is that little dot off to the left there's a disc of material around it that is the collection area of the red giant gas and it feeds the material it feeds the hole in the center it feeds the event horizon if we take another look at this go close up notice that their jets coming out above and below there's so much material trying to descend into that black hole and it gets so hot because there's friction from all the gas rubbing against itself descending down into the center that that friction heats up the gas and that that energy is trying to escape somehow it can't escape through the plane so it pops out the top and the bottom and this is a classic image of a black hole with a disc of material around it in companion with a red giant and jets tossing uh being spewed up and below above the plane and below the plane at those temperatures that it reaches it starts radiating ultraviolet light and especially x-rays a sure giveaway of this kind of system is you look out in the universe at a star that you think is mining its own business whip out an x-ray telescope and that's one of the brightest objects in the sky say something's going on there it's a disc of material heated coming down to a small central area see if we have another look at that there's the disk of material once again but now from above the plane you see the black hole in the center and you just want to avoid that really you don't want to come anywhere near this thing they're black holes bigger than this these black holes are maybe 10 times the mass of the sun and they're wreaking havoc on its neighbor star they get bigger than this though they're black holes in the centers of galaxies we know this because we've looked at galaxies galaxies of stars we're talking about systems that have 100 billion stars in their center is a supermassive black hole in some cases a billion times the mass of the sun a billion times they're enormous they're this they're these black holes can be so uh can wreak such havoc on their environment that whole star clusters can get eaten here in the previous case of the of the blue supergiant or it could be a red giant but any large swelled up star in the case of the large star and the neighboring black hole it's only beating one star at a time you go down to the center of the galaxy this black hole is so large it has the capacity to dine on enormous gas clouds and star clusters of voracious appetite and it too will make an accretion disk of titanic proportions in fact the radiation coming out of these accretion disks is so significant that in fact it can in on occasion outshine the entire galaxy in which it's embedded in fact such objects when they were discovered we didn't even know what they were we said what is this they emitted not only high energy radiation they also emitted radio waves in fact they were discovered using radio telescopes and most of the energy came from a tiny little spot not spread out like you'd see in a full galaxy and so when these were discovered they were they were they weren't really stars because their energy profile didn't match that of stars so we called them quasi-stellar and they gave off radio waves so quasi-stellar radio objects quasars turns out these were galaxies with supermassive black holes sitting at the edge of the universe that has such high energy coming from them they said and it's coming from such a small spot that if you look at it in the night sky say oh that's another star sitting up there when in fact the thing is hailing from the edge of the cosmos extraordinary extraordinary now you know what happens it's possible to get so big and to have your event horizon stretch out so far that no longer are you able to rip things apart because your tidal forces are very shallow and if you rip if you're trying to when you rip things apart that's what makes the accretion disk this disk of material that feeds the center that's where the friction takes place if you're so large and you don't get ripped apart as you descend then you get eaten whole you get eaten whole if you're eating whole no radiation comes out no high energy x-rays gamma none of that so it's possible for a black hole to get so big that it shuts off this mechanism we think some quasars that have turned off since the beginning of time have done just that by the way there's another way to turn off and that and that's if you've eaten everything in your environment and nothing comes close to you anymore that's another way to turn off you just ran out of food well the latest evidence suggests that perhaps all galaxies have black holes in their centers some more massive than others some galaxies that have super massive black holes if those black holes are kind of quiet we suspect that those galaxies in the distant past would have looked to us like quasars so it's a shift in paradigm of how we think of galaxies in modern times when i first started school it was well there's this kind of galaxy and there's that kind of there's a quasar in that it's all the same kind of galaxy just slightly different properties within it it's not a different species they're all the same species just with different different different mass of the black hole in the center slightly different rotation rates and by bringing them all together in one intellectual construct it enables you to more thoroughly examine how it is that these objects differ because you're already grabbing onto a core of what is the same the fact is they're all galaxies milky way our galaxy of course it's got a black hole too no it's not as big as the biggest you know it's not as you know but it's kind of like an ordinary black hole our black hole if you have black hole envy all right our black hole is about a million times the mass of the sun turns out that's not big enough to have ever really looked like a quasar in the early times but it is big enough to kind of disturb the middle of the galaxy and their colleagues of mine whose entire research program focuses on exactly what's going on in the center of the galaxy you track the the motion of stars that get almost too close to the black hole the gravity is so high they get pulled in very fast you see stars kind of minding their own business and then they come a little too close to the black hole and you see their speed increase dramatically speeds that you don't find anywhere else in the galaxy that tells you that there's a lot of mass in a small volume these are some of the ways we have deduced that every galaxy we've ever looked at has evidence for a black hole in its center so black holes they're things to respect in the cosmos and i'd like to summarize their danger with with just a rhyme i suffer through this rhyme that i once composed because i felt compelled to do this because black hole i couldn't get black holes out of my head and it goes something like this in a feet first dive to this cosmic abyss you will not survive because you surely will not miss the tidal forces of gravity will create quite a calamity when you're stretched head to toe are you sure you want to go your body's atoms you'll see them we'll enter one by one the singularity will eat them and of course you won't be having fun [Music] welcome back to my favorite universe in this particular lecture we're going to review things that can bring an end to earth titled this one ends of the world now typical references to the end of the world that you might hear people might think a rampant virus that totally decimates an entire species if not some other species then perhaps humans themselves there's a lot of talk about especially in the days of the cold war about global thermonuclear exchange bringing an end to earth or perhaps just the extent to which we are destroying the environment that's often how you think of people talk about the end of the world but these save the earth slogans are really they don't mean save the earth that's not what they mean earth is going to be here no matter what we do earth the planet it's actually egocentric to say let's save earth when in fact you're only really talking about saving homo sapiens earth was here before us it's here now it's going to be here long after we are gone and a memory in the fossil record so what i want to talk about are real scenarios that put the entire planet in jeopardy i can think of three three scenarios for this lecture one of them well all of them actually we actually won't live long enough to see so then why am i talking about it well because the formulations of astrophysics allow me to tell you about them so why not maybe if our species lives long enough our very distant descendants will care about what these prognostications are but the laws of physics and the phenomenon in the cosmos tell us what is going to happen in the very distant future and let's start off with the death of the sun following the death of the sun we'll get to the collision between our milky way galaxy and our nearest neighbor the andromeda galaxy and a third of these will be the heat death of the cosmos let's start with the sun we know what the sun is made of okay made of gas hydrogen and helium mostly and some residual of other materials that's fine but what we have are the colleagues of mine who specialize in stellar evolution you give them the the birth ingredients of a star and they crank it through a machine and they look through the forces and the thermonuclear interactions and and the the whole life cycle of a star and what they do is produce what they'll call them this but they produce what are basically actuarial tables for the stars this one is this old and it's going to last for another this many years and it's got this kind of health and the black hole is a little too close to it it's going to have an early death they figure this stuff out let's look at the actuarial statements for the sun the sun was born about 5 billion years ago giving birth not only to itself but to the entire solar system we date earth back to about 4.6 billion years the sun is going to live another 5 billion years so we are exactly midway we're middle aged we now i'm referring to the sun how about the sun's structure well let's look inside

Channel: TurdLok

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Keywords: Neil DeGrasse Tyson, astrophysics, universe, multiverse, black holes, Neil degrasse Tyson, table of elements

Id: T_hhQFuwzX8

Length: 131min 57sec (7917 seconds)

Published: Thu Aug 04 2022