The Endless Mysteries of the Universe! | How the Universe Works | Science Channel

Added: 2026-05-05

[00:00:08]Black holes are the monsters of the universe.

[00:00:13]Terrifying cosmic beasts that devour all they encounter.

[00:00:21]But black holes scare scientists for very different reasons.

[00:00:27]They challenge our theories to the breaking point. This is at the forefront of theoretical physics.

[00:00:35]When it comes to the detailed nature of black holes, it would not surprise me if we got it all wrong.

[00:00:47]The science of black holes is so challenging that some scientists question whether they exist at all.

[00:00:58]Despite their fearsome reputation, we've never actually seen one.

[00:01:07]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 is black. And black on black is kind of hard to see in a picture of space.

[00:01:22]This is paradoxical because scientists believe black holes are born in some of the brightest explosions in the universe.

[00:01:33]Rising from the corpses of detonated stars many times larger than our sun.

[00:01:41]A star that burns for 10 million years collapses to form a black hole in a period of seconds.

[00:01:49]As it collapses, the outer region of the star hits the core, triggering a huge explosion, a supernova.

[00:02:04]We see the bang, but not what's left behind.

[00:02:10]A dead core with the enormous mass of the star crushed down into an infantessimal tiny area.

[00:02:25]From this minuscule highmass core, a black hole is born.

[00:02:32]>> The flow of gravity is so strong that nothing can escape, not even light.

[00:02:42]>> But how can scientists claim that black holes exist if we can't even see them?

[00:02:53]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.

[00:03:03]And so it's the same sort of thing with black holes.

[00:03:07]>> 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 air.

[00:03:21]>> Air is invisible. But when the wind blows, its effects can be measured.

[00:03:27]It's the same with black holes. You just need to know what to look for.

[00:03:34]>> While they emit 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.

[00:03:46]Millionderee gas gives you lots of X-rays.

[00:03:52]To find and measure these telltale X-rays, scientists turn to the New Star Space Telescope.

[00:04:02]In 2017, it spots a burst of X-rays in a cluster called 47 Tucana at the edge of the Milky Way.

[00:04:14]When scientists analyze the data, they realize they're looking at two objects orbiting each other very closely.

[00:04:26]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.

[00:04:36]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.

[00:04:50]When scientists work out the size and mass of the two objects, they find the first is the fading corpse of a sunlike star.

[00:05:00]And while the second object is tiny, it has the mass of a giant.

[00:05:07]Is this an elusive black hole?

[00:05:12]What we're talking about here is an object that is very massive, very small, very dense with intense gravity.

[00:05:19]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.

[00:05:33]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.

[00:05:46]The gravitational attraction of a neutron star is enormous, pulling in gas, dust, and asteroids. But light can still escape.

[00:05:58]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.

[00:06:14]The tiny object discovered by New Star does have enormous mass, but size and mass alone are not enough to prove it's a black hole.

[00:06:28]Cosmologists need more evidence.

[00:06:32]They can't see black holes, but is there another way? What if they could hear them?

[00:06:41]Think of two massive cars colliding.

[00:06:43]Boom. When they do, they radiate sound.

[00:06:46]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.

[00:06:57]So, by listening for a black hole crash, could scientists conclusively prove they exist?

[00:07:14]We can't see it.

[00:07:17]We can't touch it.

[00:07:21]But without spacetime, we wouldn't be here.

[00:07:25]>> Spacetime is the fabric of our reality.

[00:07:28]It shapes and governs our lives.

[00:07:30]>> If we want to understand the story of the universe, it's absolutely crucial we understand how spacetime behaves.

[00:07:38]Spacetime has been active since the beginning of everything and is the key to the evolution of everything.

[00:07:47]We have to understand spacetime in order to understand the history of the universe to understand how the universe began, how it evolved and what's going to happen in the future.

[00:07:56]>> The story of spacetime is the story of our universe.

[00:08:01]To know how the story plays out, how it will end, we need to go back to the very beginning.

[00:08:11]To a time when there was nothing, no stars, no space. A time before there was time.

[00:08:21]Then all of a sudden our entire universe was born in the big bang.

[00:08:31]>> It started in a instantaneous moment where from nothing our universe was created.

[00:08:38]>> The very definition of the moment of the big bang is that space and time were created at that instant.

[00:08:45]>> It is as far as we currently know the coming into existence of space and time itself.

[00:08:52]The infant universe.

[00:08:54]A tiny speck of energy in spaceime materializes from nowhere.

[00:09:01]Then the universe suddenly expands.

[00:09:06]The idea of inflation is that a very tiny region in an incredibly short amount of time, far shorter than a second, grew by many, many, many orders of magnitude. So imagine myself suddenly becoming the size of a galaxy.

[00:09:22]>> In a fraction of a second, the universe grew from smaller than the size of an atom to the size of a baseball.

[00:09:31]In cosmic terms, that's like a grain of sand growing almost to the size of the observable universe.

[00:09:39]>> The universe at the instant of inflation actually expanded faster than the speed of light. It seems to be a violation of everything you've heard in physics.

[00:09:47]>> You may be thinking, "Hey, hey, hey, Mr. Astronomy Guy, nothing can move faster than the speed of light." And it turns out that's kind of true.

[00:09:54]>> But the rule is nothing can move through the universe faster than the speed of light. In inflation, it's space itself that is expanding. So there is no violation. There is no paradox.

[00:10:12]inflating fast. The universe went through a phenomenal growth spurt.

[00:10:17]>> At the moment of the Big Bang, spacetime was this entity that was flying out in all directions. It was space itself that was expanding.

[00:10:26]>> But the universe didn't expand evenly.

[00:10:29]One spot in the universe was ever so slightly more dense than a spot right next to it. And we're talking about a tiny tiny fraction of a percent. One part in a 100,000.

[00:10:40]But that was enough.

[00:10:42]>> Fluctuations in expanding spaceime created areas with higher density.

[00:10:48]Inflation made these high density regions larger.

[00:10:54]And this allowed our universe to take shape.

[00:10:59]>> When parts of the universe didn't inflate quite the same way as others, all of a sudden things could start to come together.

[00:11:07]As the universe cooled, energy turned into matter.

[00:11:14]And in the denser regions, that matter started to clump together.

[00:11:20]Crucially, these regions had more mass than others.

[00:11:26]Mass bends spacetime. So, anything that is made of matter bends spaceime. And the more matter you have in one place, the more you bend it.

[00:11:34]>> In fact, I'm bending spaceime right now.

[00:11:36]When I flex, I bend it even more because of my incredibly high muscle density. I don't bend at the maximum. I don't want to destroy the Earth and the solar system, but you know, it's an effect.

[00:11:46]It's a real thing.

[00:11:47]>> If I had imaginary glasses allowed me to perceive directly to spaceime, suddenly those hidden aspects of a curved universe are as visible as the hand in front of my face.

[00:12:02]We'd see a curving grid of spaceime moving and reacting to objects within it.

[00:12:08]And we'd feel the curving of spaceime as the force we call gravity.

[00:12:15]Gravity is different from all the other forces. It is intimately connected with the curvature of spaceime.

[00:12:22]>> Something that can bend space and time has gravity. That's what gravity is. The bending of space and time itself.

[00:12:32]It's hard to visualize this, but a good analogy is a trapeze artist and their safety net.

[00:12:41]You can imagine a trapeze artist falling into a net on purpose. That net is flat and looks like a nice orderly, evenly spaced grid, but when they fall into it, they distort that grid. Well, that's a lot like space. If you have matter in space, it warps the framework. When the trapeze artist is resting in the net, they're bending that space-time grid a little bit. If you had two trapeze artists in there, double the mass in roughly the same volume, you would get a bigger dip. You have a bigger distortion. And that's how spacetime works.

[00:13:15]More mass equals a bigger curve in spaceime equals more gravity. But understanding the nature of gravity and spacetime is no easy thing.

[00:13:30]It's an idea developed by one of the greatest minds ever.

[00:13:35]>> Einstein had the idea that space itself is something something that can be bent, something that can be stretched, that we are all bound together by spacetime.

[00:13:48]>> Einstein says that space and time have a geometry. They have a life of their own.

[00:13:53]They have dynamics. Those dynamics are what we call gravity. The more dense the region of matter, the greater the gravity, the deeper the curve.

[00:14:05]This connection is the foundation of our physical reality.

[00:14:11]>> It's the interaction between matter and energy and spaceime that created the universe that we see around us today.

[00:14:18]>> But that doesn't mean we fully understand it.

[00:14:22]There's much more that we don't know and that's frustrating. With the laws of physics, I can talk about how spacetime behaves, but it does appear to be something that stretches, that contracts, and that gravity is the embodiment of spacetime.

[00:14:37]Born in the Big Bang, space, time, and energy combined to create our infant universe.

[00:14:46]These basic materials were the foundations.

[00:14:50]But how did we get to the incredible complex structures we see today?

[00:14:56]How did spacetime build our majestic cosmos?

[00:15:05]Our entire universe was created in the Big Bang 13.8 billion years ago.

[00:15:12]Everything came from nothing.

[00:15:17]But our modern universe is a complex mosaic of matter.

[00:15:23]>> When we marvel through our telescopes at the fantastic structure of our universe and its galaxies, you got to ask where did that come from? Matter in the universe arranges itself on a vast cosmic web. Galaxies and galaxies clusters are strung out on sheets and filaments.

[00:15:42]It seems this intricate web is organized by a cosmic architect.

[00:15:48]Spacetime.

[00:15:50]It shaped everything from planets to galaxies, atoms to cities.

[00:15:58]>> The universe is made of spaceime.

[00:16:00]Whatever the substance is, time and space bound together that's expanding and creating the universe we see around us. It's everything. Spacetime is what the universe really is.

[00:16:16]Amuam Mua's surface tells the story of its journey through interstellar space.

[00:16:23]The interesting thing about OmuA Mua is its color. It's actually red. I'm standing on a surface that's a nice analog for the surface of AmuA Mua. As you look around, you see a really dark kind of shiny coating to all of the rocks. And it extends up the valley and even onto the mountains behind me.

[00:16:44]Scientists think Amuam Mua's red sheen comes from tholins, organic molecules that are the building blocks of life.

[00:16:54]How cool is it that something came out from some other origin, passed through our neighborhood, and it possessed some form of organics? That could be a possible gold mine for us.

[00:17:07]In our own solar system, distant objects like comets and asteroids also carry tholins.

[00:17:16]This happens because their surfaces are bombarded by cosmic rays and that changes the nature of the chemicals on the surface. So we think the same thing has happened to Muam Mua. It's been out there in interstellar space and been bombarded by cosmic rays over the eons.

[00:17:35]>> Galactic cosmic rays are high energy particles that tear through the universe.

[00:17:45]Interstellar space is filled to the brim with these cosmic rays.

[00:17:52]>> Things like protons and electrons or perhaps some heavier and more exotic particles that are literally whizzing through the universe.

[00:18:00]>> Some cosmic rays can travel as fast as 99% the speed of light. Incredibly fast energetic things.

[00:18:09]It takes a lot of energy to accelerate anything close to the speed of light.

[00:18:15]Cosmic rays come from many energetic and powerful and violent sources in our universe.

[00:18:24]Everything that's big and blasting generates cosmic rays.

[00:18:30]>> One of the most powerful cosmic ray generators is the death of a giant star, a supernova.

[00:18:45]A supernova is a really energetic explosion. It's so energetic that it can create all kinds of interesting things.

[00:18:54]When a star runs out of fuel, it collapses.

[00:18:58]The mass of the star crashes inwards, triggering a huge explosion.

[00:19:09]The shock wave slams into surrounding gas, amplifying magnetic fields.

[00:19:16]If you get a particle caught in there, trapped in the magnetic fields of this gas, it can bounce back and forth, be accelerated very rapidly.

[00:19:24]It goes dink like that.

[00:19:28]>> Eventually, the particle moves so fast that the magnetic field can no longer hold it.

[00:19:37]And it gets shot out at very near the speed of light.

[00:19:43]>> Cosmic rays have mass and they wreak havoc.

[00:19:50]Cosmic rays are the bullets of the universe and they are flooding interstellar space. But thankfully we're protected.

[00:20:05]Cosmic rays from interstellar space battle with another superpower.

[00:20:12]Our own bodyguard in the solar system, the sun.

[00:20:18]We think of the sun as the source of energy and warmth for Earth, the giver of life. But it's also protecting us in ways you might not be aware of.

[00:20:29]The sun emits a stream of charged particles called the solar wind.

[00:20:35]The particles hurdle out past the planets at more than a million miles an hour, but they do eventually run out of power.

[00:20:48]There's this region where the solar wind grinds to a stop. It's plowing into this material between the stars and eventually slows and stops.

[00:21:00]The solar wind carries the sun's magnetic field with it, forming a bubble around our solar system.

[00:21:09]>> We call that the heliosphere. Helio for sun and sphere for this giant magnetic field.

[00:21:14]>> It acts basically like a shield protecting us from these galactic cosmic rays. If that weren't there, the radiation levels hitting the Earth would actually increase.

[00:21:25]So in a real way, the sun is protecting us from the dangerous environment of interstellar space.

[00:21:32]>> The heliosphere protects us from the majority of cosmic rays, but some still make it into the solar system.

[00:21:41]Fortunately for us, Earth also has its own defense mechanisms.

[00:21:47]We have our magnetic field that can redirect the lowest energy cosmic rays.

[00:21:53]And we have our nice thick security blanket of an atmosphere which absorbs most of the high energy cosmic rays before they even get a chance to reach us here on the surface.

[00:22:12]Cosmic rays from interstellar space can alter DNA and cause diseases. But without them, we might not be here at all.

[00:22:23]Even that tiny fraction of cosmic rays that makes it through our atmosphere to the surface of the earth can have a profound influence on the evolution of life.

[00:22:32]Cosmic rays can damage the DNA that carries the information of life.

[00:22:39]When those molecules are broken apart, the atoms altered by collisions with cosmic rays. The information carried is changed. That's a mutation. That's what drives natural selection. So life and we ourselves are deeply connected to uh interstellar space around us.

[00:23:00]>> But interstellar space is also home to much larger objects. Objects that could wipe out life altogether.

[00:23:14]Our solar system races around the center of the Milky Way at 143 m/s.

[00:23:28]At its center, the sun, just one of around 200 billion stars in our galaxy.

[00:23:35]>> We're not living in the isolated bubble all on our own here in the galaxy. We're living in a swarm, a neighborhood of other stars.

[00:23:46]And the movement of all these stars can have farreaching effects on our solar system.

[00:23:54]Beyond the planets and our heliosphere lies the Orort cloud.

[00:24:06]Right on the border of true interstellar space, >> the or cloud is the remnants of the formation of the solar system. Small, icy, dirty bodies, aka comets.

[00:24:24]>> The comets in the Orort cloud are so far out, they're only weekly bound to the sun.

[00:24:31]They spend most of their lives perfectly happy, orbiting the sun lazily in their frigid depths. But every once in a while, they can be perturbed.

[00:24:44]Our sun is moving through interstellar space and so are other stars.

[00:24:51]As our sun orbits the galaxy um and encounters other stellar neighbors, inevitably there's going to be one that's going to pass through or near our or cloud.

[00:25:02]The gravity of a nearby star could disrupt the or cloud, sending showers of comets barreling through the solar system.

[00:25:19]Some of them could strike Earth.

[00:25:36]Comets falling down into the inner solar system is something that we really want to pay attention to. That could actually be dangerous to life here on Earth. So, one of the things we do is look out into the galaxy and see if any stars are going to be coming nearby anytime in the near future.

[00:25:52]With a new space observatory called Gaia, astronomers keep watch over millions of neighboring stars in our galaxy, tracking their movements through interstellar space.

[00:26:07]>> So, what's the next star that's going to pass the Earth? And it turns out we may know. There's an orange dwarf. It's called Giza 710.

[00:26:17]>> In 2018, new data shows Giza 710 is on a collision course with our Orort cloud.

[00:26:26]>> It's going to kick up a lot of dirt, kick up a lot of dust, and it might be bad news for the inner solar system. We might get a lot of unwanted visitors.

[00:26:45]The story of the Big Bang is based on the discovery that the universe is continuously expanding. And if we go back in time, this leads to one inescapable conclusion.

[00:27:00]So if you run the clock backwards and let the universe get younger, it'll get smaller and smaller and smaller and then everything is basically compressed into one point.

[00:27:08]The Big Bang story claims this point was infinitely small.

[00:27:14]But scientists have not been able to prove the existence of such singularities.

[00:27:20]>> In our universe, we see all galaxies receding from all other galaxies. On average, imagine if you were looking at trains leaving a station. If you ran the clock backwards, the trains would converge to the same station. Now, did these trains come from the same station?

[00:27:40]Probably. Did they come from the exact same platform? Probably not. You can't fit all the trains onto the same platform. But while physicists can't prove everything came from an infinitely small and dense singularity, they're convinced the observable universe did expand from one small point. And this point was incredibly dense and incredibly hot. Imagine that you and a bunch of friends are in a very large room and you're all hanging out and it all seems normal, but now you're all crammed together in a very small elevator and it starts to feel much warmer because you're interchanging all this heat. It's sort of like that in the early universe. Everything is smashed together. Everything is very hot.

[00:28:25]It makes sense theoretically that this period was intensely hot. But how do you prove it?

[00:28:32]How do you take the temperature of the early universe which began 13.8 billion years ago?

[00:28:41]You can't. But you can take the temperature of the coldest part of the universe now. So if you go away from all the stars and get away from all the galaxies, you might think space was infinitely cold, absolute zero. But it turns out it's not. Empty space has a temperature of roughly 455° Fahrenheit below zero, 5 degrees higher than absolute zero. Where did these mysterious extra 5° come from?

[00:29:13]Big bang believers thought they had the answer.

[00:29:16]They claimed this faint trace of heat was left over from the incredibly hot infant universe. Getting the proof took decades, but it came in 1964 by pure accident.

[00:29:32]>> Pensius and Wilson were Bell Labs engineers, and they were given an assignment to measure certain radio signals for the idea of sending wireless signals via telephone so rural areas could have telephones.

[00:29:44]>> They used a radio antenna shaped like a giant horn. The problem was, no matter where they pointed this horn, they kept hearing kind of a static, just a radio noise coming from every direction.

[00:29:57]And they thought, okay, maybe it's a a satellite. But it didn't match up with any satellite positions. There's a nearby army base and they called up the army base said, "Hey, are you broadcasting at this frequency?" And they said, "No, we're not." Thought maybe it's pigeon poop. Well, you know, there are these pigeons nesting inside the antenna, and their droppings are creating this noise in your telescope.

[00:30:21]So, they actually went inside and scraped out all these pigeon droppings.

[00:30:24]But no matter what they did, the noise remained.

[00:30:27]>> They tried everything they could to remove this background noise, and they finally realized it was coming from the sky. It was real.

[00:30:36]>> What they were hearing was not radio waves, but a different form of radiation. microwaves, a heat signature left over from the Big Bang.

[00:30:50]They had discovered the cosmic microwave background, a ghostly snapshot of the early universe.

[00:30:59]Different colors highlight subtle variations in temperature.

[00:31:04]The cooler blue areas will develop to form stars and galaxies.

[00:31:10]The warmer orange areas will eventually make up the vastness of intergalactic space.

[00:31:17]>> The cosmic microwave background is a literal baby picture of our universe.

[00:31:23]It's the equivalent of a picture of you when you were 7 seconds old.

[00:31:29]>> We can date the cosmic microwave background to 380,000 years after the Big Bang. The temperature here is estimated to be 5,000° F.

[00:31:43]But how hot was the Big Bang? As we run the clock backwards, the universe gets smaller and the temperature increases.

[00:31:56]We know what the temperature of the cos microwave background was, but prior to that time, we know the universe was getting smaller and smaller and smaller, and therefore it had to get hotter and hotter and hotter. But can we find out how hot?

[00:32:10]In the early universe, it was much smaller, denser, and hotter than it is today. And in fact, it was so hot it could fuse hydrogen into helium.

[00:32:21]25% of the mass in the early universe is fused into helium in this time scale of just a few minutes.

[00:32:28]So, it's trillions and trillions of times more than the amount of fusion that's going on in the sun.

[00:32:38]Extremely high temperatures are required to fuse hydrogen into helium. Scientists estimate fusion started 100 seconds after the Big Bang when temperatures reached 1 billion° F. During the very first fractions of the very first second of the Big Bang, some estimate the temperature could have reached 250 million trillion trillion degrees Fahrenheit. But what sparked this massive release of energy here at the birth of the Big Bang?

[00:33:14]The initial moments of our universe are a source of frustration because it'd be really nice to know that, but also a source of curiosity. This is the frontiers of physics. This is where we're really pushing things to try to understand the fundamental aspects of reality.

[00:33:35]But even if we can back up the Big Bang story by proving the way everything came from a tiny hot point, there's still another problem. Where did everything that made up that tiny point come from?

[00:33:50]You can't get something from nothing, right? We all know that. Except it looks like we got everything from nothing. The entire universe seems to have appeared out of nowhere. How can that work?

[00:34:03]Time travel inspires incredible journeys of science fiction, and traveling to the past would be the ultimate vacation.

[00:34:13]>> If I could time travel into the past, I would love to experience ancient Rome at the height of the Roman Empire.

[00:34:22]>> I would travel 13 billion years in the past and I would watch our galaxy form.

[00:34:27]Well, I can tell you if I were a time traveler, I would love to show up for Stephen Hawkings party.

[00:34:33]But is this actually possible? Could we ever travel back into the past?

[00:34:38]>> If we could travel back in time, the possibilities would be endless. But backwards time travel also causes mystifying temporal paradoxes.

[00:34:51]>> Even in science fiction, time travel is all about paradoxes. Is it possible that you could influence your own past? And the most simple way of putting this is the grandfather paradox.

[00:35:01]>> What if you could go backwards in time and kill your grandfather?

[00:35:06]>> In that case, how could your parents have been born? How could you have ever been born?

[00:35:11]>> But if you were never born, then you didn't exist. How did you kill your grandfather?

[00:35:16]You just run in circles. It it doesn't make any sense. It's logically impossible.

[00:35:22]It seems like the laws of the universe will not allow you to travel back in time.

[00:35:28]>> But maybe there's a loophole.

[00:35:32]>> There could be a way to travel back in time without creating a paradox. Thanks to the way that space and time are linked.

[00:35:44]Once you believe in four-dimensional spaceime, you begin to conceptualize reality as the whole four-dimensional thing, which you then call the block universe. It's like a four-dimensional block of stuff. The different slices are different moments of time.

[00:36:03]In the block universe, the past, present, and future coexist.

[00:36:10]But if you could step outside of this entire framework and see this block universe, you would see the entire history of the universe from time zero to time infinity sitting in front of you.

[00:36:23]From dinosaurs roaming the earth 150 million years ago to humans colonizing the solar system hundreds of years in the future and Hawkings party for time travelers back in 2009.

[00:36:41]In the block universe, all of history exists simultaneously.

[00:36:48]Astrophysicist Paul Sutter explains.

[00:36:52]You can think of the block universe as a film reel where the past and future already exist. They're just frames on the same film. All the frames already exist. They're just right there. But we experience them in a particular order and in a particular direction based on, you know, a particular turn of the handle.

[00:37:16]Just like a handle turning a film reel, time flows from past to future. But since every moment in time exists as a frame somewhere on this reel, then surely we can visit them.

[00:37:33]If the idea of the block universe is really true, that makes time travel more understandable and more possible. We just need to find a way to get to different parts of the real. To do that, we have to find a way to travel through time.

[00:37:52]We know planets and black holes curve spacetime, but Einstein's equations reveal that really massive objects moving around each other can drag spaceime into a loop.

[00:38:07]>> The regions of our universe most likely to harbor the greatest possibility for something crazy like time travel is in the most extreme regions of space-time curvature.

[00:38:17]You could imagine a very complicated situation where you had enough mass and and it was moving in such a way that you could twist space up on itself.

[00:38:26]Theoretical objects called naked line singularities could do just that. Like the hearts of two black holes, but stretched out infinitely.

[00:38:38]Two naked singularities moving close to each other could create a looped path through space-time called a closed timelike curve.

[00:38:48]>> A closed timelike curve is a very special kind of path through spaceime where you have some starting point and you start moving through spaceime just like you'd advance in frames in this piece of film. And it just so happens in a closed timelike curve that your ending frame is exactly the same as your beginning frame. So as you move through space, you start moving into your future, but you also move into your own past. And you end up at exactly the same point where you started both in space and in time. And you've closed the loop.

[00:39:31]>> With closed timelike curves, you may be able to visit your own past by looping space time. But traveling in the block universe has a big drawback. You can never alter the past.

[00:39:48]If this block universe idea is correct, this movie real universe that all of time exists all at once, that solves the grandfather paradox. You can't go back in time to kill your grandfather because you haven't. You never will. You never will have done it. You can't do it because it didn't happen.

[00:40:09]Time travelers in a block universe can't change history. So since we know that no one attended Stephven Hawkings party, no one ever will.

[00:40:23]By investigating time travel, scientists are unraveling mysteries of our universe. But one question remains unanswered.

[00:40:33]Why does time seem to only run in one direction?

[00:40:38]>> How is it then that we remember the past but we don't know the future? This seemingly obvious question turns out to have its explanation in the origin of our universe. Shockingly, the passage of time isn't set in stone.

[00:41:01]Time can be bent, slowed, even frozen.

[00:41:07]But our experience of time seems fixed.

[00:41:12]Time only flows in one direction.

[00:41:16]>> There just is a direction to time in a way. There's not a direction to space.

[00:41:20]There's no difference between up, down, left, right, forward, backward, but there's still a difference between yesterday and tomorrow.

[00:41:27]>> Why does time seem to run forwards and not backwards?

[00:41:32]>> So many things in our everyday life only make sense in one direction of time. You break an egg, it doesn't suddenly become an egg again. You scramble an egg, it doesn't become whole. You know that there's there's sort of directions of things.

[00:41:46]>> This arrow of time seems to be linked to the chaos and disorder we see in our day-to-day lives.

[00:41:54]Best explained perhaps over a coffee.

[00:41:59]If I have a mug of coffee, there's only one way for all the little bits and pieces of the mug and the liquid in the coffee to be in this shape, and it's right here in front of me. The mug is in what's called a highly ordered state.

[00:42:14]>> But if I shove it off the table and it smashes into a million pieces, we'll never see all those pieces and the bits of liquid reassemble into the shape of the mug again.

[00:42:28]We know the shattered mug won't reassemble itself.

[00:42:34]In scientific terms, the disorder or entropy of the coffee mug increases but never decreases.

[00:42:43]And across the universe, entropy always increases. Just like across the universe, time flows from past to future.

[00:42:54]Everything in the universe is gradually becoming more and more disordered.

[00:43:00]But why?

[00:43:02]>> We never really think about broken eggs reassembling themselves. And that actually may go all the way back to what the conditions of the Big Bang were like.

[00:43:14]>> 13.8 billion years ago. Spacetime rapidly expanded from a tiny point.

[00:43:23]In the blink of an eye, the universe was born. This marked the first moment of time.

[00:43:32]Our current understanding of the universe is that there was a time zero.

[00:43:35]There was a moment that the universe came into being and that is the big bang.

[00:43:41]>> The big bang seems to have been an incredibly low entropy state. Everything was very ordered, very dense and very hot.

[00:43:50]So there was really nowhere for entropy to go but to increase from that state.

[00:43:56]At time zero, the universe expanded from a highly ordered dense speck of energy.

[00:44:05]380,000 years later, the first atoms formed. Gradually, gas began to clump together.

[00:44:15]something like 200 million years later that the first stars formed and then those formed into galaxies sometime after that.

[00:44:23]As the universe ages and expands, it becomes more and more disordered.

[00:44:29]Galaxies move further and further apart.

[00:44:33]In trillions of years, disorder will rule.

[00:44:37]Star building gas will run out. No new stars will form.

[00:44:43]When the last stars die, the universe will become cold and dark.

[00:44:52]On the search for alien worlds, we've uncovered plenty of the strange, the scary, and the incredible.

[00:45:00]But we still haven't detected anything remotely like our planet.

[00:45:06]For all these treasures that we've been digging up, we haven't found the crown jewels, a planet similar to Earth.

[00:45:17]>> Finding an exoplanet with conditions suitable for life takes a lot of luck.

[00:45:23]Sifting through these exoplanets looking for something that's habitable for life is like an interstellar dating app.

[00:45:33]If we have molten iron rain, that's definitely out.

[00:45:37]>> You see toxic atmosphere and you swipe and you see red giant and you swipe.

[00:45:41]This one's like, "Oh, too hot, too cold, too small, too thick an atmosphere."

[00:45:45]>> The UV ray. No, no, no. Doesn't even have a star. Like, it's just not working again and again and again.

[00:45:53]>> When it comes to finding life, there is one basic element that everyone agrees is necessary. There is a phrase that we use whenever we talk about the search for life elsewhere. Follow the water.

[00:46:07]>> And now we think there could be lots of worlds out there that do contain water.

[00:46:12]But is there a catch? Could they hold too much water?

[00:46:18]A 2019 study suggests the Milky Way might contain many worlds with thousands of times more water than Earth. Many of these planets are a bit smaller than Neptune. We call them sub Neptunes.

[00:46:33]>> What they found were these sub Neptunes.

[00:46:36]Planets smaller than Neptune but bigger than Earth. Unlike any planets we'd seen before.

[00:46:42]>> We think we found such a planet just 40 lighty years from Earth in the constellation Afiaakus.

[00:46:50]Scientists have nicknamed the planet the water world. GJ1214b could be one of these sub neptunes with more water than we would know what to do with.

[00:47:01]>> So far, we're not too sure what GJ1214b looks like. Though Earth is called the blue planet, it's only 05% water by mass.

[00:47:14]As much as 70% of GJ1214b's mass could be water.

[00:47:21]The planet is thought to have a rocky core, strange oceans, and a hot, steamy atmosphere of water vapor.

[00:47:29]>> We spent a lot of time looking for very small amounts of water to establish whether or not a planet could even be habitable. And so, it's kind of amazing that we just found this planet that was essentially nothing but water.

[00:47:43]Unlike Earth, GJ1214b most likely has no complex arrangement of water and land masses.

[00:47:51]>> The lack of interaction between stable land masses and a and a healthy long-term stable ocean might really be a killer. And you might need that land interacting with that water to have a good location for life.

[00:48:06]>> We think life began in the oceans, but it needed chemicals from rocks to start.

[00:48:12]Without the interaction between land and oceans, life might not have evolved.

[00:48:19]Not only is there no land sea relationship on GJ1214b, evolution here may be limited in another way. Earth's oceans are replenished with chemicals from hydrothermal vents thousands of feet down on the seabed, GJ1214b's ocean floors, or thousands of miles deep.

[00:48:43]Right at the bottom of these incredibly deep oceans, you've got very high pressures. You've got so much water above you and you got very cold temperatures. You're really being shielded from any incoming solar radiation or sunlight. So water itself could turn to ice.

[00:48:59]>> Most ice on Earth is called ice one.

[00:49:02]When ice is subject to increasing pressure, its categorization number goes up. We think the ice on GJ1214b is ice 7, the type of ice we believe to be on moons like Enceladus and Europa.

[00:49:18]On GJ1214b, we believe ice 7 seals off the seabed, preventing potential nutrients from the rocky core from passing into the ocean.

[00:49:29]>> We've been following the water. That's been the the key to trying to understand astrobiology. And then we we find these worlds where it's too much of a good thing. There's too much water for perhaps for life to exist. So it's certainly one of those things that a little you need, but maybe too much is bad, too.

[00:49:47]>> We need to find worlds with just the right amount of water and land for life to evolve.

[00:49:53]GJ1214b looks like a dead end, but the hunt goes on.

[00:50:01]Space is big and I like the idea that it's not just for us. So, I'm hopeful.

[00:50:06]Whether it will be in my lifetime or my daughters, I don't know. But I'm hopeful.

[00:50:10]>> As we continue to probe the cosmos, we've discovered one hopeful distant object, a moon. But this exom moon is a monster. It's four times larger than Earth. So, how did it get so big?