How the Universe Works - S06E01 - Are Black Holes Real
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00:00Cosmologists are battling over the universe's greatest enigma, black holes.
00:17We've never seen them.
00:19It's near impossible to study them.
00:21And their existence challenges everything we think we know about space.
00:29The black hole represents the absolute limits of what we understand about nature.
00:35The truth is we have almost no idea what these things are and how they work.
00:41Black holes are at the very heart of cosmology.
00:44Yet some scientists question if they're even real.
00:49Black holes present lots of paradoxes and the simplest way to resolve the paradoxes
00:54would be if black holes didn't exist at all.
00:59Solving the mysteries of black holes will push our understanding of physics to the edge
01:03of reason.
01:06But it's the only way to discover if black holes really exist.
01:24Black holes are the monsters of the universe, terrifying cosmic giants that devour all they
01:46encounter.
01:48Black holes petrify scientists for very different reasons.
01:55They challenge our theories to the breaking point.
01:58This is at the forefront of theoretical physics.
02:03When it comes to the detailed nature of black holes, it would not surprise me if we got
02:09it all wrong.
02:14The science of black holes is so challenging that some scientists question whether they
02:19exist at all.
02:26Yet despite their fearsome reputation, we've never actually seen one.
02:35Black holes are everywhere.
02:36They're all over the universe.
02:37They're all throughout our galaxy, but that doesn't mean that they're easy to find.
02:42They're black, and space is black, and black on black is kind of hard to see in a picture
02:47of space.
02:51This is paradoxical, because scientists believe black holes are born in the aftermath of the
02:56brightest explosions in the universe, rising from the corpses of detonated stars many times
03:04larger than our sun.
03:09Stars exist because of an epic battle.
03:11It's a battle between the heat they generate in their cores, trying to expand them, and
03:15their own mass causing gravity, trying to contract them.
03:19But over time, it winds up losing that battle.
03:27Stars are powered by nuclear fusion in their cores, but their fuel source doesn't last
03:33forever.
03:34Eventually, when a star's power output drops, it can no longer produce enough energy to
03:40overcome the force of its own gravity.
03:44The result?
03:46The core collapses, flashes out a tremendous amount of energy in a last gasp, blows out
03:52the outer layers in a supernova explosion, and if the star has enough mass to begin with,
03:57what's left in the end is a black hole.
04:00The star that burns for 10 million years collapses to form a black hole in a period of seconds.
04:09As it collapses, the outer region of the star hits the core, triggering a huge explosion.
04:18A supernova.
04:23We see the bang, but not the remnants, a dead core with the enormous mass of the star crushed
04:31down into an infinitesimal tiny area.
04:44From this minuscule, high mass core, a black hole is born.
04:50The flow of gravity is so strong that nothing can escape, not even light.
05:00But how can scientists claim that black holes exist if we can't even see them?
05:12You could say that about the existence of the atom.
05:14We knew they had existed for decades, centuries, before we had actually seen one in some sort
05:20of imaging device.
05:22And so it's the same sort of thing with black holes.
05:26Just because you can't see it doesn't mean it's not there.
05:29Not seeing black holes, but knowing they're there is a possibility, just like we know
05:34that wind is there even though we can't see air.
05:39Air is invisible, yet when the wind blows, its effects can be measured.
05:45It's the same with black holes.
05:47You just need to know what to look for.
05:53While they emit no light themselves, black holes are tremendous sources of X-rays.
05:58And that's because as things get close to a black hole, they're accelerated by the gravity
06:02and they can heat up to millions of degrees.
06:05Million degree gas gives you lots of X-rays.
06:11To find and measure these telltale X-rays, scientists turned to the New Star Space Telescope.
06:23In 2017, it spotted a burst of X-rays in a cluster called 47 Tucana, at the edge of
06:30the Milky Way.
06:34When scientists analysed the data, they realised they were looking at two objects orbiting
06:39each other very closely.
06:45All we see is that there's a star being ripped apart and gas is spiralling down to a very
06:50dense, very dark object.
06:52So something weird is going on.
06:55As one of the objects accretes matter off the other, it causes it to emit X-rays and
07:01those X-rays can be used then to trace out the orbits and therefore extract the mass.
07:09When scientists calculated the size and mass of the two objects, they found that the first
07:16is the fading corpse of a sun-like star, and that the second object is tiny, yet this
07:23tiny object has the mass of a giant.
07:27Could this be an elusive black hole?
07:31What we're talking about here is an object that is very massive, very small, very dense
07:36with intense gravity, but it turns out there are lots of different ways to create an object
07:42like that.
07:43There is another type of ultra-dense object out there in the universe called a neutron
07:47star.
07:52Neutron stars form in the same way we think black holes form, when stars die and explode.
07:58But then neutron stars collapse down into a tiny ball of matter.
08:03The gravitational attraction of a neutron star is enormous, pulling in gas, dust and
08:10asteroids, but light can still escape.
08:16Black holes and neutron stars are kind of cousins, but in the case of a neutron star,
08:21it didn't have quite enough mass to collapse out of control, so you can sort of think of
08:26it as just barely hanging back from collapsing into a black hole.
08:30It's easy to see the difference between a neutron star and a black hole, because a neutron
08:34star has a surface you can see light coming off of, and if something falls at it, it crashes
08:39into the surface, whereas black holes have no surface.
08:46The surface of a neutron star is a place you don't want to be.
08:52The enormous mass of the tiny body means its gravity is huge, and it would crush a human
08:57in an instant.
09:00But the mass of a neutron star is still smaller than that of a black hole.
09:09There's an upper limit to how massive they can be, just under three times the mass of
09:13the sun.
09:14They simply can't get any bigger, because then they would collapse to form a black hole.
09:18So if we see an object out there that's not emitting light, and has more than three times
09:22the mass of the sun, it can't be a neutron star.
09:29The tiny object discovered by Neustar does have enormous mass, but size and mass alone
09:36are not enough to prove it's a black hole.
09:39Cosmologists need more evidence.
09:45They can't see black holes, but is there another way to find them?
09:50What if they could hear them?
09:55So all astronomy is based on the electromagnetic spectrum.
09:59Looking at visible light, x-ray light, radio light.
10:04Gravitational waves are something else entirely, and they're generated from some of the most
10:09exotic processes in our universe.
10:12So they give us a direct look at these processes that we simply can't see.
10:19We simply can't see it, but we can hear that through gravitational waves.
10:26Gravitational waves give us this wonderful window into the universe.
10:33Think of two massive cars colliding.
10:35Boom.
10:36When they do, they radiate sound.
10:37And then we can tell whether or not that collision occurred, and maybe even how far away it was.
10:41It's like that when black holes collide.
10:50So by listening for a black hole collision,
10:55could scientists conclusively prove they exist?
11:19Black holes are gravitational giants of the universe.
11:29But we've only found circumstantial evidence that they exist.
11:35For definitive confirmation, cosmologists are listening for proof in the hidden world
11:40of gravitational waves.
11:45There are gravitational waves going through this room all the time.
11:48Every time I move my hands like I just did, I create gravitational waves.
11:52The problem is, gravity is so weak that you don't detect those gravitational waves.
12:00In order to detect those disturbances in space and time, you have to have cataclysmic events
12:06involving massive objects.
12:11Black holes are some of the densest objects in the universe, so we should be able to hear
12:16and measure the waves created when they collide.
12:22LIGO, the Laser Interferometry Gravitational Wave Observatory, listens for waves that can
12:28come from over a billion light years away.
12:35In 2017, LIGO heard an enormous crash.
12:43Two very massive objects collided at near the speed of light in one of the most energetic
12:48events that we have ever witnessed in the history of humankind.
12:55Two ultra-heavy, ultra-dense objects whirled around each other, hurling powerful gravitational
13:02waves through space.
13:05The closer they fell towards each other, the more gravitational energy they threw out.
13:11Finally, they collided in one of the most violent events in the universe.
13:21The smash sent out immense gravitational waves that rippled across intergalactic space, until
13:27eventually LIGO detected them.
13:34Listening to a gravitational wave is like listening to a musical instrument, if it's
13:39making certain tones or certain vibrations, you can figure out the size of the musical
13:45instrument, the type of the musical instrument, who's playing the musical instrument.
13:52The thing that's really amazing about the LIGO detection is it allowed us to measure
13:56the mass of these objects and how quickly they coalesced together.
13:59So we actually have an idea how dense they must have been.
14:03And with modern physics, we say, well, it has to be a black hole.
14:07But the question is, have we missed something?
14:13The information gathered by LIGO is groundbreaking, but some scientists think that the gravitational
14:19waves could have come not from black holes, but from something even more mysterious.
14:31It's possible that what we identify as black holes in our universe are really another object
14:37like gravastar, possible.
14:40There's a capital P on that possible.
14:49A gravastar is what scientists call an exotic compact object.
14:55This bizarre theoretical body has exactly the same mass and gravitational pull as a
15:00black hole, but it's made of exotic matter.
15:05A gravastar would be impossible to see with the naked eye.
15:09And because it forms differently to a black hole, it has a strange, incredibly dense surface.
15:22In the formation of what we think of as a black hole, the catastrophic gravitational
15:27collapse of a dense object, maybe it doesn't go all the way down to become an infinitely
15:32dense point.
15:33Instead, maybe there's some interaction that prevents the formation of the black hole.
15:38And instead you have a tight little dense ball, which is what we call a gravastar.
15:45So the LIGO data could be the signature of two black holes colliding, but it also could
15:51be the signature of two gravastars colliding.
15:55Right now, we can't tell them apart.
16:08So for now, gravitational waves have led to a dead end in the hunt for black holes.
16:15We can't be completely sure that we're hearing them, and we already know we can't see them.
16:22But what about the mayhem they leave behind?
16:29Even though you can't see the black hole itself, it's going to leave behind a trail of destruction,
16:35and that is something you can see.
16:43So could analysing such devastation lead to solving the mystery of black holes?
17:07This is the Hydra A galaxy cluster, 840 million light years from Earth.
17:16It's a region of space filled with galaxies and dense intergalactic gas.
17:21But a dark and massively destructive force is at work here, one that's blasting holes
17:29in the gas that are bigger than the Milky Way.
17:35Could a black hole be responsible?
17:39It's almost as if an intergalactic bomb has exploded to blow these cavities out.
17:45Some of these cavities are tens or even hundreds of thousands of light years across, and to
17:50create a cavity that large requires an immense source of energy and a powerful engine that's
17:57driving it.
18:03To discover what's creating the cavities, scientists combined images taken at different
18:09wavelengths.
18:13They revealed something remarkable.
18:18The cavities are being carved out by enormous jets emanating from a galaxy in the centre
18:24of the cluster.
18:32Because the jets are ejected from the galaxy, they can actually slam into the surrounding
18:38material and form a shockwave, which we can see.
18:43And eventually that jet inflates a bubble, a cavity, and that cavity grows as it's inflated
18:49with hot, dense plasma ejected from this jet.
18:56These jets are incredibly large and incredibly powerful.
19:00This is like a Death Star, but for real.
19:02It's much, much more energy than it would take to blow up a planet.
19:05Carnage like this could have been created by the energetics of a supermassive black
19:09hole in the form of a quasar.
19:15Supermassive black holes are millions or even billions of times the mass of the sun.
19:22They can be voracious overeaters, cramming in huge amounts of matter.
19:32As gas and dust swirls towards the supermassive black hole, it rubs together, causing friction
19:39that heats the material to millions of degrees Celsius.
19:50The magnetic field around the black hole forms the material into twin jets that spin
19:55out at enormous speeds for hundreds of millions of light years.
20:04And this is the birth of a quasar.
20:10The jets emitted by black holes are not only incredibly high energy, but incredibly intense,
20:15because more energy than is emitted in almost any other object is observed from quasars.
20:21Those quasars are intense enough to vaporize objects that they hit.
20:26They're deadly rays from space.
20:27They almost sound like science fiction objects.
20:33In order to generate that much energy, what kind of physical process do you need?
20:39And pretty much the only answer we have is a giant black hole.
20:45Quasars are absolutely really, really convincing smoking gun evidence for the existence of
20:50black holes.
20:57The awesome power of a black hole can explain the vast mysterious cavities in the galaxy
21:02cluster Hydra A.
21:05So can we now say conclusively that black holes are real?
21:19Although there is great evidence for black holes, we have to keep questioning whether
21:23they're real.
21:24As a scientist, I'd much rather have questions I can't answer than answers I can't question.
21:35One question scientists are struggling to answer is how black holes work.
21:49Can cosmology solve such a simple yet incredibly complex question?
21:56There's always going to be details that we still have yet to figure out.
21:59That's true for black holes right now, and a lot of the problems are big ones, like how
22:03do they even exist, how do they behave?
22:06I mean, we're seeing that the math doesn't seem to work out, so there is an issue here.
22:13To show that black holes exist, scientists need to solve the mathematics.
22:18If they can't, they might fail to prove that black holes exist at all.
22:33Cosmologists say the universe is filled with black holes, big and small, but the evidence
22:52for them is not conclusive.
22:58Even if there is nine out of ten pieces of evidence for black holes, there's still one
23:02piece of evidence left.
23:04That can open it up to the possibility that maybe black holes don't exist.
23:14Scientists instead look to the theoretical science of how black holes work.
23:21They're thought to have the super-dense, collapsed core of a star at their centre, around which
23:27is a sphere known as the event horizon, a place where the rules of physics go haywire.
23:39The event horizon in many ways cuts a black hole off from the rest of the universe.
23:43Whatever comes in can never come back out.
23:47It's almost like an invisible line in space.
23:51It's not until you try to turn around and leave that you realise you're never going
23:56to escape.
24:03It's Einstein's theory of general relativity that tells us nothing can leave a black hole.
24:10This set of rules governs the giant structures of the universe, galaxies, star systems and
24:19planets.
24:21General relativity is Einstein's theory of gravity.
24:23It is the all-encompassing theory that describes everything we know about gravity and how the
24:28universe on large scales functions.
24:32However, the universe also functions on small scales.
24:42Everything in the universe, including us, is made up of tiny bits of matter.
24:48These are governed by another set of rules known as quantum mechanics.
24:53Quantum mechanics allows you to understand at the smallest levels, at the smallest scales
25:01of what builds our universe.
25:04Weird stuff happens in the quantum world.
25:06It defies all intuition and one of the weird things that can happen is you can have the
25:10empty space of a vacuum creating particles.
25:19In the quantum world, tiny particles can pop spontaneously into existence.
25:25They're drawn to each other like magnets and when they collide, they annihilate each other.
25:33So you've got two particles, they can pop out of the vacuum and they annihilate very
25:39quickly.
25:40They don't break any laws.
25:41Now in the vicinity of a black hole, though, things get kind of complicated.
25:50Gravity is usually too weak to affect the particles in the quantum world, but physicist
25:58Stephen Hawking theorized that at the event horizon of a black hole, the normal rules
26:05don't apply.
26:09What Stephen Hawking realized is that if you have a pair of particles that pop up at the
26:13edge of a black hole and one gets sucked into the black hole, then the other is forced to
26:18become a real particle in our universe.
26:20In order to do so, it takes energy from the black hole and in that mechanism, black holes
26:26very slowly over time lose mass.
26:30With Hawking's work, we see that the black hole will eventually evaporate and that was
26:35a complete shift in how to think about black holes.
26:40According to Hawking, when black holes aren't consuming material, they actually shrink by
26:46emitting thermal radiation or heat.
26:50In this process, a black hole will eventually evaporate completely and this creates a problem
26:56for scientists.
26:59Everything in the universe, from atoms to planets to spacecraft, carries information
27:06about what they're made of, how fast they're going and where they've been.
27:12The laws of physics state that this information can't be lost from the universe and that should
27:18apply to black holes.
27:23So if an object passes through the event horizon of a black hole, all the information about
27:28that object becomes part of the black hole itself.
27:37Once they've fallen in, all you can see is a heavier black hole and you don't know what
27:42fell in if you weren't watching it.
27:44Well, that doesn't violate the laws of physics, but if the black hole continues to evaporate
27:49and evaporate and evaporate with thermal radiation and then disappear, then all you have afterwards
27:54is thermal radiation.
27:55You have no information, even in principle, about what fell in.
27:59And that information is precisely what quantum mechanics says can't be lost.
28:07The thermal radiation coming out of a black hole contains no information.
28:13If this continues until the black hole evaporates, then all the information is completely lost
28:18when the black hole disappears.
28:24If that information disappears, then the laws of quantum mechanics are violated and we don't
28:31know how to solve that paradox yet.
28:36We're trying to combine quantum mechanics and relativity and the first time we're able
28:41to do it leads to this giant mess that calls into question fundamental assumptions about
28:47the way the universe works.
28:49So that's kind of a problem.
28:55This problem is known as the information paradox.
28:59And if scientists are going to find an answer to it, they'll have to unify their two theories.
29:05General relativity, the rules of the large, and quantum mechanics, the rules of the tiny.
29:13So far, that's proved impossible.
29:17I think the single biggest embarrassment in physics is that we have these two theories,
29:24general relativity describing the big and quantum mechanics describing the small, that
29:29simply don't get along.
29:31Nature obviously knows what it's doing, but we just don't have a single unified explanation.
29:38The past few decades, as we've tried to describe black holes fully, the more work we've done,
29:47the more of a tangled mess we get.
29:50It will take at least one brilliant mind to figure this out.
29:57Maybe someone already has.
30:01The classic black hole has a shell-like event horizon beyond which nothing can escape.
30:08And according to the information paradox, information is lost when a black hole evaporates.
30:15But there is one radical, mind-bending idea.
30:20What if black holes have hair?
30:23What Hawking and his collaborators pointed out is that black holes can have soft hair.
30:30And if that's true, that would be a way of encapsulating all the information that went
30:34into the black hole.
30:38Scientists propose that these so-called hairs somehow store the information of whatever
30:43has fallen into the black hole.
30:47The information is then imprinted on the thermal radiation emitted as the black hole evaporates.
30:57These hairy black holes could solve the information paradox, but they're entirely theoretical.
31:07These are really new ideas and people are still trying to figure them out.
31:11So it's hard to give a really good explanation of something that is...
31:13This is theory in the making.
31:17People are working this out now.
31:22Solving the information paradox is pushing science to its limits.
31:28But an even bigger problem is sitting at the heart of a black hole.
31:35The big problem about the idea of black holes is that at its center, there is what we call
31:40a singularity.
31:41At this singularity, matter has infinite density and space is infinitely curved.
31:49That's not something that really sounds like physics.
31:56Singularities do not exist in nature.
31:59And when they appear in the mathematics, that's a signal that you're doing something wrong,
32:04that you're incomplete.
32:06It's like the ultimate curse word.
32:09And Mother Nature doesn't like it when we curse.
32:13To complete the mathematics and so understand black holes, cosmologists must tackle one
32:20of science's most mind-bending concepts, infinity.
32:34Black holes, they're thought to be a fundamental part of the universe.
32:55But scientists are not only struggling to explain how they work, they're struggling
33:01to prove that they even exist.
33:05So black holes have some problems.
33:08Every time we try to think of something, some creative mathematical solution to describe
33:14black holes fully, it breaks down and we just can't make any progress.
33:19We can't make reliable predictions, we can't compare to observations.
33:25What is the solution?
33:28We honestly don't know.
33:33Scientists are finding that both general relativity and quantum mechanics go haywire at the edge
33:39of a black hole.
33:42But inside a black hole, things could be even stranger.
33:58When a giant dying star collapses, the mass of the star falls in and keeps falling in,
34:05crushing down into an infinitely small point.
34:12This is called the singularity.
34:17Singularity is troubling because although it sounds cool and scientific, it's really
34:21just a fancy word of saying, oh, we have no clue what happens here.
34:26The way our physics describes black holes, when they form, you're taking a finite amount
34:31of mass and you're collapsing it down and its volume should shrink all the way to zero.
34:38But that means it has infinite density and infinite gravity and that doesn't make sense.
34:48If you make a prediction and the answer is infinite, it tells you there's something wrong
34:52with your prediction because we've never seen an infinity in the universe today.
35:01Once again, quantum mechanics is at the heart of the problem.
35:05Have you ever thought about the term quantum mechanics and thought about what those words
35:12mean?
35:13Well, everything in the universe is broken up into tiny little units.
35:16There really is a basic unit of energy, a unit of time, even a unit of space that cannot
35:23be divided any further.
35:25There's a limit to how small things can be.
35:30The smallest unit of space in the universe is what's known as a Planck length.
35:37If you took a human hair and blew it up to the width of the observable universe, one
35:42Planck length would be about one-tenth of a millimetre.
35:47If there is a universal limit on the smallest size, then something infinitely small can't
35:55exist.
35:56Well, if infinity doesn't exist, then singularities don't exist.
36:00And if singularities don't exist, then Einstein's theory of general relativity is not correct.
36:06The simplest thing to do is to say, well, let's choose some new equations.
36:10Let's change Einstein's theory of gravity somehow.
36:13Let's invent what we would call exotic speculative physics.
36:22This speculative physics has led scientists to invent the idea of the Planck star.
36:33If you passed one in space, it would look just like a black hole.
36:41But a Planck star doesn't have a singularity at its core.
36:48Maybe things can't collapse down to less than the Planck length, because maybe you get stuck
36:54with this sort of Planck-sized nugget that stabilises things, keeps everything finite.
37:02So where a singularity is at the centre of a black hole, a Planck-sized nugget is at
37:09the centre of a Planck star.
37:14A Planck star is just like a black hole, except it obeys the rules of quantum mechanics.
37:21The problem is, Planck stars are just another exotic theory.
37:29The reason there are so many exotic alternatives to black holes is because you can write down
37:35a gazillion different postulated mysterious new kinds of matter and say, suppose this
37:40kind of weird stuff exists, then maybe that could explain the data.
37:44The problem is there's no evidence that any of that kind of stuff exists.
37:57Black holes are full of theoretical holes.
38:01Scientists say they're out there, but we can't see them.
38:06Mathematics says there's a singularity at a black hole's core, but in nature, these
38:11don't exist.
38:13The rules we use to understand the universe simply don't seem to apply to black holes.
38:20So where does that leave us?
38:23Maybe a classical black hole with an event horizon described by general relativity just
38:29isn't the proper description of the physics.
38:37This lack of understanding opens the door to some outlandish theories that aim to show
38:43black holes can exist.
38:47But one idea more than any other is the strangest of them all.
38:58In our understanding of the universe, there are two places where everything seems to break
39:02down.
39:03One is inside the heart of a black hole.
39:05The other is what happened right before the Big Bang.
39:08And some people have wondered if these two things could be linked.
39:14It sounds crazy, but there's actually a model that you could put together in which all of
39:19our observations of the universe are entirely consistent with us actually being inside of
39:24a black hole.
39:27But how?
39:29Well, a black hole is supposed to have a tiny, dense region at its core containing trillions
39:35and trillions of tons of matter.
39:40There is a theory that as matter is crushed into the center of a black hole, it actually
39:44reaches a point where it can be crushed no further.
39:47An event like this could in fact lead to a Big Bang.
39:53When the collapsing matter in the black hole reaches a maximum density, it bounces back,
40:01expanding outwards in a cataclysmic explosion.
40:08The matter gradually cools over time to form atoms, building galaxies, stars, and planets.
40:18If that sounds familiar, it's because it's just like the universe we see today.
40:27This is one idea for how a universe like ours is formed, that in fact we all live inside
40:32of a black hole that was created this way.
40:35In our galaxy alone, there are tens of millions of black holes.
40:40And just think, inside each one of them could be a baby universe waiting to be born.
40:44That's incredible.
40:45Are we inside of a black hole that exists in a universe that has other black holes that
40:52has other universes which exist inside of a black hole?
40:55It goes on and on and on ad infinitum.
40:57However, once again, this is all theoretical.
41:02But it's a compelling idea.
41:04I mean, are we actually existing inside of a black hole?
41:08I don't know.
41:09But you know, sometimes when I'm sitting in traffic waiting to get home, it feels like
41:12time is stretched out infinitely.
41:17I don't think that you should lose sleep at night wondering if we're actually inside of
41:20a black hole.
41:21The answer is probably not.
41:23But because black holes are just pushing at the edge of what we understand about nature,
41:29they are the perfect illustration of everything that we don't know about the universe.
41:33And that is a lot.
41:43But what do we know?
41:46We can't see black holes, and we can only find circumstantial evidence of them.
41:53They violate the laws of physics that predict them.
41:56And they may even be hairy.
42:00So, do they actually exist?
42:09So I will tell you that right now in modern physics, we have no idea what is going on
42:13inside the heart of a black hole, whether black holes in the true sense really exist
42:17at all.
42:18But the wonderful thing is that physics is now taking us down paths that we would never
42:22have imagined before.
42:25Lack of evidence of how black holes work is not evidence against the existence of black
42:32holes.
42:33It's just evidence of lack of understanding.
42:38If you ask me what I believe, I'd have to tell you that I don't believe in black holes.
42:41I believe in something which behaves like black holes.
42:47It could be that all the objects in our universe that we currently identify as black holes
42:53aren't really black holes, but if I were to bet, I would bet on black holes.
43:00I think they're the simplest explanation.
43:02Yep, they have a lot of problems that we have to resolve, but I do, I believe in black holes.
43:11Whatever it is that we're seeing, it smells like a black hole, walks like a black hole,
43:15it quacks like a black hole, it's a black hole.
43:20At this moment in time, black holes are the best explanation for what we see out there.
43:28But if we can find a way to unify our theories, we might finally prove they exist and discover
43:35a whole lot more about how our universe works.