Cosmologists are battling over the universe’s greatest enigma black holes we’ve never seen them it’s near impossible to study them and their existence challenges everything we think we know about space the black hole represents the absolute limits of what we understand about nature the truth is we have almost no idea what these things are and how they work black holes are at the very heart of cosmology yet some scientists question if they’re even real black holes present lots of paradoxes and the simplest way to resolve the paradoxes would be if black holes didn’t exist at all solving the mysteries of black holes pushes our understanding of physics to the edge of reason but it’s the only way to discover if black holes really exist [Music] [Music] black holes are the monsters of the universe terrifying cosmic beasts that devour all they encounter but black holes scare scientists for very different reasons they challenge our theories to the breaking point this is at the forefront of theoretical physics when it comes to the detailed nature of black holes it would not surprise me if we got it all wrong the science of black holes is so challenging but some scientists question whether they exist at all despite their fearsome reputation we’ve never actually seen one black holes are everywhere they’re all over the universe they’re all throughout our galaxy but that doesn’t mean that they’re easy to find they’re black and space this black and black on black is kind of hard to see in a picture of space this is paradoxical because scientists believe black holes are born in some of the brightest explosions in the universe rising from the corpses of detonated stars many times larger than our Sun the star that burns for 10 million years collapses to form a black hole in a period of seconds as it collapses the outer region of the star hits the core triggering a huge explosion a supernova [Music] we see the bang but not what’s left behind a dead core with the enormous mass of the star crushed down into an infinitesimally tiny area
[Music] from this minuscule high mask or a black hole is born the flow of gravity is so strong that nothing can escape not even light [Music] but how can scientists claim that black holes exist if we can’t even see them [Music] you could say that about the existence of the atom we knew they had existed for decades centuries before we had actually seen one in some sort of imaging device and so it’s the same sort of thing with black holes just because you can’t see it doesn’t mean it’s not there not seeing black holes but knowing they’re there is a possibility just like we know that wind is there even though we can’t see there [Music] air is invisible but when the wind blows its effects can be measured it’s the same with black holes you just need to know what to look for while they admit no light themselves black holes are tremendous sources of x-rays and that’s because as things get close to a black hole they’re accelerated by the gravity and they can heat up to millions of degrees million-degree gas gives you lots of x-rays to find and measure these telltale x-rays scientists turned to the new star space telescope in 2017 it spots a burst of x-rays in a cluster called 47 to Kanna at the edge of the Milky Way when scientists analyzed the data they realize they’re looking at two objects orbiting each other very closely all we see is that there’s a star being ripped apart and gas is spiraling down to a very dense very dark object so something weird is going on as one of the objects accretes matter off the other it causes it to emit x-rays and those x-rays can be used then to trace out the orbits and therefore extract the mass [Music] what scientists work out the size and mass of the two objects they find the first is the fading corpse of a sun-like star and while the second object is tiny it has the mass of a giant is this an elusive black hole what we’re talking about here is an object that is very massive very small very dense with intense gravity but it turns out there are lots of different ways to create an object like that there is another type of ultra dense object out there in the universe called a neutron star neutron stars form in the same way we think black holes form when stars die explode but then collapse down into a tiny ball of matter the gravitational attraction of a neutron star is enormous pulling in gas dust and asteroids but light can still escape black holes and neutron stars are kind of cousins but in the case of a neutron star it didn’t have quite enough mass to collapse out of control so you can sort of think of it as just barely hanging back from collapsing into a black hole the tiny object discovered by NuStar does have enormous mass but size and mass alone are not enough to prove it’s a black hole cosmologists need more evidence they can’t see black holes but is there another way what if they could hear them think of two massive cars colliding boom when they do they radiate sound and then we can tell whether or not that collision occurred and maybe even how far away it was it’s like that when black holes collide [Music] so by listening for a black hole crash could scientists conclusively prove they exist you [Music] black holes are gravitational giants of the universe but we found only circumstantial evidence that they exist to make it a slam-dunk cosmologists are listening for proof in the hidden world of gravitational waves there are gravitational waves going through this room all the time every time I move my hands like I just did I create gravitational waves the problem is gravity is so weak that you don’t detect those gravitational waves in order to detect those disturbances in space and time you have to have cataclysmic events involving massive objects black holes are some of the densest objects in the universe so we should be able to hear and measure the waves created when they collide LIGO the laser interferometry gravitational wave observatory listens for waves that can come from over a billion light-years away [Music] in 2017 LIGO heard an enormous crash two very massive objects collided at near the speed of light and one of the most energetic events that we have ever witnessed in the history of humankind to ultra-heavy ultra dense objects whirl around each other hurling powerful gravitational waves through space the closer they fall toward each other the more gravitational energy they throw out finally they collide in one of the most violent events in the universe the smash sends out immense gravitational waves that ripple across intergalactic space until eventually LIGO detects them listening to a gravitational wave is like listening to a musical instrument if it’s making certain tones or certain vibrations you can figure out the size of the musical instrument the type of the musical instrument who’s playing the musical instrument [Music] the thing that’s really amazing about the LIGO detection is it allowed us to measure the mass of these objects and how quickly they coalesce together so we actually have an idea how dense they must have been and with modern physics we say well it has to be a black hole but the question is have we missed something the information gathered by LIGO is groundbreaking but some scientists think that the gravitational waves could have come not from black holes but from something even more mysterious it’s possible that what we identify as black holes in our universe are really another object like gravis star possible there’s a capital P on that possible a [Music] grab a star is what scientists call an exotic compact object this bizarre theoretical body has exactly the same mass and gravitational pull as a black hole but it’s made of exotic matter a grab a star would be impossible to see with the naked eye but because it forms differently than a black hole it has a strange incredibly dense surface in the formation of what we think of as a black hole the catastrophic gravitational collapse of a dense object maybe it doesn’t go all the way down to become an infinitely dense point instead maybe there’s some interaction that prevents the formation of the black hole and instead you have a tight little dense ball which is what we call a graviton so the LIGO data could be the signature of two black holes colliding but it also could be the signature to grab the stars colliding right now we can’t tell them apart
[Music] so for now gravitational waves have led to a dead end in the hunt for black holes we can’t be completely sure that we’re hearing them and we already know we can’t see them but what about the mayhem they leave behind even though you can’t see the black hole itself it’s gonna leave behind a trail of destruction and that is something that you can see by picking apart a cosmic crime scene can scientists finally solve the mystery of black holes [Music] [Music] this is the Hydra a galaxy cluster 840 million light-years from Earth [Music] it’s a region of space filled with galaxies and dense intergalactic gas but a dark and massively destructive force is at work here and it’s blasting holes in the gas that are bigger than the Milky Way could a black hole be responsible it’s almost as if an intergalactic bomb has exploded to blow these cavities out some of these cavities are tens or even hundreds of thousands of light years across and to create a cavity that larger requires an immense source of energy and a powerful engine that’s driving it to discover what’s creating the cavities scientists combine images taken at different wavelengths they reveal something remarkable the cavities are being carved out by enormous Jets emanating from a galaxy in the center of the cluster as the Jets are ejected from the galaxy they can actually slam into the surrounding material in form a shock wave which we can see and eventually that jet inflates a bubble cavity and that cavity grows as its inflated with hot dense plasma ejected from this jet these Jets are incredibly large and incredibly powerful this is like a Death Star but for real it’s much much more energy than it would take to blow up a planet carnage like this could have been created by the energetics of a supermassive black hole in the form of a quasar supermassive black holes are millions or even billions of times the mass of the Sun they can be voracious Overeaters cramming in huge amounts of matter as gas and dust swirl toward the supermassive black hole it rubs together causing friction that heats the material up to millions of degrees Fahrenheit the magnetic field around the black hole forms the material into twin Jets that spin out at enormous speeds for hundreds of millions of light years [Music] a quasar is born the Jets submitted by black holes are not only incredibly high energy but incredibly intense as more energy than its emitted and almost any other object is observed from quasars those quasars sir intense enough to vaporize objects that they hit their deadly rays from space they almost sound like science fiction objects in order to generate that much energy what kind of physical process do you need and pretty much the only answer we have is a giant black hole Jets are absolutely really really convincing smoking-gun evidence for the existence of black holes the awesome power of a black hole can explain the vast mysterious cavities in the galaxy cluster Hydra a so is the case closed can we say conclusively that black holes are real although there’s great evidence for black holes we have to keep questioning whether they’re real as a scientist I’d much rather have questions I can’t answer than answers I can’t question [Music] one question scientists are struggling to answer is how black holes work our cosmologies greatest minds in danger of flunking out there’s always going to be details that we still have yet to figure out that’s true for black holes right now and a lot of the problems are big ones like you know how do they even exist how do they behave I mean we’re seeing that the math doesn’t seem to work out so there is an issue here to show that black holes exist scientists need to solve the math if they can’t they might fail to prove that black holes exist at all cosmologists say the universe is filled with black holes big and small but the evidence for them is not conclusive [Music] even if there is nine out of ten pieces of evidence for Michaels there’s still one piece of evidence left that can open it up to the possibility that maybe black holes don’t exist [Music] experts instead look to the theoretical science of how black holes work they’re thought to have the super dense collapsed core of a star at their center around this is a sphere known as the event horizon a place for the rules of physics go out the window the event horizon in many ways cuts a black hole off from the rest of the universe whatever comes in can never come back out it’s almost like an invisible line in space it was not until you try to turn around and leave that you realize you’re never going to escape it’s Einstein’s theory of general relativity that tells us nothing can leave a black hole this set of rules governs the giant structures of the universe galaxies star systems and planets general relativity is Einstein’s theory of gravity it is the all-encompassing theory that describes everything we know about gravity and how the universe on large scales functions but the universe functions on small scales to everything in the universe including us is made up of tiny bits of matter these are governed by another set of rules known as quantum mechanics
[Music] quantum mechanics allows you to understand at the smallest levels of the smallest scales of what builds our universe weird stuff happens in the quantum world it’s it’s it defies all intuition and one of the weird things that can happen is you can have the empty space of a vacuum creating particles in the quantum world tiny particles can pop spontaneously into existence they’re drawn to each other like magnets but when they collide they annihilate each other so you’ve got two particles they can pop out of the vacuum and they annihilate very quickly that doesn’t break any laws now in the vicinity of a black hole though things get kind of complicated gravity is usually too weak to affect the particles in the quantum world but British physicist Stephen Hawking theorized that at the event horizon of a black hole the normal rules don’t apply what Stephen Hawking realized is that if you have a pair of particles that pop up at the edge of a black hole and one gets sucked into the black hole then the other is forced to become a real particle in our universe in order to do so it takes energy from the black hole and in that mechanism black holes very slowly over time lose mass with Hawking’s work we see that the black hole will eventually evaporate and that’s a that was a complete shift in how to think about black holes according to Hawking when black holes aren’t eating material they actually shrink by emitting thermal radiation or heat in this process a black hole will eventually evaporate completely and this creates a problem for scientists everything in the universe from atoms to planets to spacecrafts carry information about what they’re made of how fast they’re going and where they’ve been the laws of physics state that this information can’t be lost from the universe and that should apply to black holes so if an object passes through the event horizon of a black hole all the information about that object becomes part of the black hole itself once they fallen in all you can see is a heavier black hole and you don’t know what fell in if you didn’t weren’t watching it well that doesn’t violate the laws of physics but if the black hole continues to evaporate and evaporate evaporate with thermal radiation and then disappear then all you have afterwards is thermal radiation you have no information even in principle about what fell in and that information is precisely what quantum mechanics says can’t be lost the thermal radiation coming out of the black hole contains no information it’s a blank slate if this continues until the black hole evaporates then all the information is completely lost when the black hole disappears if that information disappears then the laws of quantum mechanics are violated and we don’t know how to solve that paradox we’re trying to combine quantum mechanics and relativity and the first time we were able to do it leads to this giant mess that calls into question fundamental assumptions about the way the universe works so that’s kind of a problem the problem is known as the information paradox and if scientists are going to find an answer to it they’ll have to unify their two theories general relativity the rules of the large and quantum mechanics the rules of the tiny and so far that’s proved impossible I think the single biggest embarrassment in physics is that we have these two theories general relativity describing the big and quantum mechanics describing the small that simply don’t get along nature obviously knows what it’s doing but we just don’t have a single unified explanation the past few decades as we’ve tried to describe black holes fully the more work we’ve done the more of a tangled mess we get it will take at least one brilliant mind to figure this out maybe someone already has the classical black hole has a shell like de Vence horizon beyond which nothing can escape and according to the information paradox information is lost when a black hole evaporates but here’s a radical mind-bending idea what if black holes have hair what Hawking and his collaborators pointed out is that black holes can have soft hair and if that’s true that would be a way of encapsulating all the information that went into the black hole scientists propose that these so-called hairs somehow store the information of whatever is fallen into the black hole the information is that imprinted on the thermal radiation emitted as the black hole evaporates these hairy black holes could solve the information paradox but they’re entirely theoretical these are really new ideas and people are still trying to figure them out so it’s hard it’s hard to give a really good explanation of something that is but this is uh this is this is theory in the making people are working this out now solving the information paradox is pushing science to its limits but an even bigger problem is sitting at the heart of a black hole the big problem about the idea of black holes is that a center there’s what we call a singularity at this singularity matter has infinite density and space is infinitely curved that’s not something that really sounds like physics [Music] singularities do not exist in nature and when they appear in the mathematics that’s a signal that you’re doing something wrong that you’re incomplete it’s like the ultimate curse word and Mother Nature doesn’t like it when we curse to complete the math and understand black holes [Music] cosmologists grapple with one of Sciences most mind-bending concepts infinity [Music] black holes they’re thought to be a fundamental part of the universe but scientists are not only struggling to explain how they work they’re struggling to prove they even exist so black holes have some problems every time we try to think of something some creative mathematical solution to describe black holes fully it breaks down and we just can’t make any progress can’t make reliable predictions we can’t compare to observations what is the solution we honestly don’t know
[Music] scientists are finding the general relativity and quantum mechanics go haywire at the edge of a black hole but inside a black hole things could be even weirder [Music] when a giant dying star collapses the mass of the star falls in and keeps falling in crunching down into an infinitely small point [Music] this is called the singularity the singularity is troubling because although it sounds cool and scientific it’s really just a fancy word of saying oh we have no clue what happens here the way our physics describes black holes when they form you’re taking a finite amount of mass and you’re collapsing it down and it’s volume should shrink all the way to zero but that means it has infinite density and infinite gravity and and that doesn’t make sense [Music] if you make a prediction and the answer is infinite it tells you there’s something wrong with your prediction because we’ve never seen an infinity in the universe today once again quantum mechanics is at the heart of the problem [Music] have you ever thought about the term quantum mechanics and thought about what those words mean well everything in the universe is broken up into tiny little units there really is a basic unit of energy a unit of time even a unit of space that cannot be divided any further there’s a limit to how small things can be the smallest unit of space in the universe is what’s known as a Planck length if you took a human hair and blew it up to the width of the observable universe 1 Planck length would be about 1/4 thousandth of an inch if there is a universal limit on the smallest size then something infinitely small can’t exist well if infinity doesn’t exist and singularities don’t exist and if singularities don’t exist then Einstein’s theory of general relativity is not correct the simplest thing to do is say well let’s choose some new equations let’s change in Stein’s theory of gravity somehow let’s invent what we call exotic speculative physics [Music] this speculative physics has led scientists to invent the idea of the Planck star [Music] if you passed one in space it would look just like a black hole but a planked star doesn’t have a singularity at its core maybe things can’t collapse down to less than the Planck length if maybe you get stuck with this sort of plaque size nugget it stabilizes things keeps everything finite so we’re a singularity is at the center of a black hole a Planck sized nugget is at the center of a Planck star a Planck star is just like a black hole except it plays by the rules of quantum mechanics the problem is Planck stars are just another exotic theory the reason there are so many exotic alternatives the black holes is because you can write down a gazillion different postulate of mysterious new kinds of matter and say suppose this kind of weird stuff exists then maybe that could explain the data problem is there’s no evidence that any of that kind of stuff exists [Applause] scientists are getting creative as they try to prove that black holes exist they’ve imagined strange objects and used exotic workarounds but one black hole idea is the strangest of all what if we are living inside of one it’d be the most mind-blowing thing ever if we were actually living inside a black hole and the crazy thing is it could happen it could be true you
[Music] black holes are full of theoretical holes scientists say they’re out there but we can’t see them Matt says there’s a singularity at a black holes core but in nature these don’t exist the rules we use to understand the universe simply don’t seem to apply to black holes so where does that leave us maybe a classical black hole with an event horizon described by general relativity just isn’t the proper description of the physics [Music] [Applause] this lack of understanding opens the door to some outlandish theories that aim to show black holes can’t exist but one idea stands out as the strangest of them all in our understanding of the universe there are two places where everything seems to break down what is inside the heart of a black hole the other is what happened right before the Big Bang and some people have wondered if these two things could be linked it sounds crazy but there’s actually a model that you could put together in which all of our observations of the universe are entirely consistent with us actually being inside of a black hole how would that work well a black hole is supposed to have a tiny dense region at its core containing trillions and trillions of tons of matter there is a theory that as matter is crushed into the center of a black hole it actually reaches a point where it can be crushed no further an event like this could in fact lead to a big bang when the collapsing matter in the black hole reaches a maximum density it bounces back expanding outwards in a cataclysmic explosion the matter gradually cools over time to form atoms building galaxies stars and planets if that sounds familiar it’s because it’s just like the universe we see today this is one idea for how a universe like ours is formed then in fact we all live inside of a black hole that was created this way within our galaxy alone there are tens of millions of black holes and just think inside each one of them could be a baby universe waiting to be born that’s incredible a weird side of a black hole that exists in the universe that has other black holes that has other universes which exists inside of a black hole it goes on and on and on and infinitum once again this is all theoretical but it’s a compelling idea I mean are we actually existing inside of a black hole I don’t know but you know sometimes when I’m sitting in traffic waiting to get home it feels like time is stretched out infinitely I don’t think that you should lose sleep at night wondering if we’re actually inside of a black hole the answer is probably not but because black holes are just pushing at the edge of what we understand about nature they are the perfect illustration of everything that we don’t know about the universe and that is a lot [Music] so what do we know we can’t see black holes we can only find circumstantial evidence of them they violate the laws of physics that predict them they may even be hairy so do they actually exist so I will tell you that right now in modern physics we have no idea what is going on inside the heart of a black hole where there black holes and the true sense really exists at all but the wonderful thing is that physics is now taking us down paths that we would never have imagined before lack of evidence of how black holes work is not evidence against the existence of black holes it’s just evidence of lack of understanding if you ask me what I believe I’d have to tell you that I don’t believe in black holes I believe in something which behaves like black holes it could be that all the objects in our universe that we currently identify as black holes aren’t really black holes but if I were to bet I would bet on black holes I think they’re the simplest explanation yep they have a lot of problems that we have to resolve but I do I believe in black holes whatever it is that we’re seeing it smells like a black hole walks like a black hole it quacks like a black hole it’s a black hole right now black holes are the best explanation for what we see out there but if we can find a way to unify our theories we might finally prove they exist and discover a whole lot more about how our universe works.