"Freeman:" "We feel it every moment of our lives." "But for physicists..." "It is the oldest unsolved mystery of the cosmos." "Why does gravity make everything attract?" "Cutting-edge theory is closing in on unexpected answers." "Could gravity be another force in disguise..." "A shadow of a holographic reality, or a rippling mirage?" "Do we, Earth, the Sun, and the stars really have weight?" "Or is gravity an illusion?" "Space, time, life itself..." "The secrets of the cosmos lie through the wormhole." "♪ Through the Wormhole 05x07 ♪ Is Gravity an Illusion?" "Original Air Date on July 2, 2014" "== sync, corrected by elderman == @elder_man" "The gravitational pull of the sun keeps Earth from flying off into space." "Earth's gravity keeps us firmly planted on the ground." "This all seems real enough." "But scientists are peering deep into the fabric of the universe and are discovering that gravity may not be what it seems to be." "Can something feel real but not actually be real?" "[ Banjo playing country tune ]" "Freeman:" "There were some days growing up when there just wasn't anything to do." "[ Clink ] So we would play simple games like target practice with rocks." "[ Clunk ]" "Gravity always worked." "No matter what I dropped, I always expected it to fall." "Physicists have their own expectations about gravity." "They believe it to be a fundamental force, an intrinsic cog in the machinery of the universe." "[ Apple thuds ]" "But experimentalist Nergis Mavalvala isn't taking anything for granted." "So, a fundamental force that -- like gravity, that describes how massive objects interact should be true anywhere you look in the universe." "Freeman:" "Isaac Newton showed that every object with mass attracts every other object with mass." "The greater the mass and the closer they are, the greater the gravitational attraction." "Over 200 years later," "Albert Einstein explained why this happens." "Space and time are interwoven into a fabric called spacetime." "Einstein believed that spacetime could bend." "This distortion is what we experience as gravity." "And that's how he understood that objects with mass attract to each other." "They follow the curvature of spacetime." "So Einstein's picture of -- of gravity was that mass tells spacetime how to curve, and then the curvature of spacetime tells mass how to move." "Freeman:" "Einstein also predicted that when all objects with mass move, they trigger tiny gravitational ripples in the fabric of spacetime." "Gravitational waves should permeate the heavens above us." "Nergis believes we should be able to detect those waves, if they are big enough." "So if I drop an apple in the middle of a pond..." "And I try to detect the ripple at the shore, it's not going to make it." "It was too small of a wave." "Freeman:" "Luckily for Nergis, bodies much more massive than apples cause a stir in the heavens." "[ Engine revs ]" "[ Whirs ]" "[ Boom ]" "Awesome!" "Whoa!" "Freeman:" "Around the cosmos, intense gravitational events, like the collision of galaxies..." "[ Explosion ] ...or the explosions of giant stars..." "[ Explosion ] ...should be sending massive volleys of gravitational waves towards Earth." "Nergis has created a way to detect them with the help of collaborators like Mike Landry." "Nergis and Mike are part of the largest experiment ever built by the National Science Foundation." "It is known as the Laser Interferometer Gravitational Wave observatory, or L.I.G.O., in this behemoth, laser beams fire down two vacuum tubes arranged in an "L" shape." "Each arm is 4 kilometers long." "The laser beams can measure the length of each arm with an accuracy of better than 1 millionth of the width of an atom." "If a gravitational wave from any intense cosmic event up to 500 trillion trillion miles away passes through the Earth, the space inside the tubes will ripple." "The lasers will detect the change, and the alarm bells will ring." "After almost a decade of listening to the heavens," "L.I.G.O. picked up the sound of crickets." "[ Static ]" "We didn't observe a gravitational wave in the initial science runs of L.I.G.O." "Nergis, Mike, and the thousands of scientists at L.I.G.O." "Have one more shot." "They're working on advanced upgrades that will increase L.I.G.O.'s sensitivity tenfold." "But there's no guarantee they'll ever get a signal." "Well, if we don't detect gravitational waves with advanced L.I.G.O., well, first, I'll cry." "But then, I think, it's actually very exciting either way." "If we don't see gravitational waves, then it's going to start off a different kind of revolution, where there'll be a lot of head-scratching about, "what is it about nature we don't understand?"" "Freeman:" "Nergis is hopeful." "In fact, in march of 2014, a group of astronomers claimed to have detected gravitational waves produced by the big bang." "But some scientists take the deafening silence at L.I.G.O." "as evidence that gravity may not be a fundamental force." "When an apple falls to the earth, something else could be pulling it down." "Physicists believe that everything in the universe, even the pulse of energy that we call force, is made from particles." "Gravity should be no exception." "[ Indistinct conversations ]" "Freeman:" "Zvi Bern is a particle physicist with a very active imagination." "He's imagining what a game of mini golf would look like if the balls were shrunk to the size of subatomic particles and ruled by the laws of quantum mechanics." "[ Whack, ball clatters ]" "Quantum mechanics is full of the strangest things you can imagine." "The concept of a particle being at one point, that becomes a very fuzzy concept in quantum mechanics." "Freeman:" "Subatomic particles are unlike anything you can see with your naked eye." "They become fuzzy when no one looks at them." "Sometimes they can appear out of nowhere... and then suddenly vanish." "Some of these appearing and disappearing particles..." "[ Whoosh ] ...transmit the fundamental forces of nature -- electromagnetism, the strong force, the weak force, and, supposedly, gravity." "I have here a golf ball." "The golf ball represents a photon." "The photon is the carrier of the electromagnetic force." "Freeman:" "The electromagnetic force attracts or repels anything with an electric charge." "The next golf balls -- these represent the W and the Z boson." "The W and the Z boson -- these are the carriers of the weak nuclear interaction." "Freeman:" "The weak force causes the nucleus of a radioactive atom to break apart." "The next golf ball -- it represents the gluon." "The gluon is the carrier of the strong nuclear interaction." "Freeman:" "The strong force binds particles together to form an atomic nucleus." "Gravity should also be carried by a particle, but no one has ever observed this so-called graviton." "In fact, when physicists try to calculate how the theoretical graviton might work, they quickly get lost in impossible math." "Gravity, unfortunately, is one of our most complicated theories in the way it interacts." "And what happens is as you do these calculations, very quickly you start encountering expressions which no computer in the world, or all the world's computer -- they couldn't possibly do those calculations." "Freeman:" "But Zvi has a trick up his sleeve to calculate whether or not the graviton exists." "Quantum theory, like mini golf, is a game of probability." "Trying to hit a hole-in-one is difficult." "There are so many ways the ball could go." "But break up the hole into smaller pieces, and things are much more manageable." "Together with some colleagues, we developed an idea that we called the Unitarity Method." "And the basic idea of that is, you take the bigger problem of these interactions, these complications, and then you chop it into smaller pieces." "And then, by solving the smaller problems and assembling it, you can do a lot better than if you were just trying to solve the whole problem at once." "Freeman:" "When Zvi and his colleagues applied their Unitarity Method to gravitons, an unexpected result came back." "What we discovered about the graviton is that, in a very precise way, it can be interpreted as two copies of gluons." "Freeman:" "Which binds the nuclei of atoms together through the strong force." "But Zvi and his colleagues believe gluons could also be responsible for gravity." "The graviton could actually be a pair of gluons." "Everything became instantly clear, like a moment of insight, the "Eureka!" moment." "This is our "Eureka!" moment, where -- where we really knew that we understood it." "And the fact that it came out that simple really was the great surprise." "We always had suspicions that something like this was true." "But that it -- it works as simply as it did -- that was really the big surprise for us." "Freeman:" "Zvi's work could mean that when an apple falls, the gravity that pulls it down is just another manifestation of the strong force." "The same force that holds the nucleus of tiny atoms together could also be responsible for holding colossal celestial bodies in orbit." "If so, the universe is awash in gluons, working together as gravitons." "Every time a pair of gluons is exchanged between massive objects, the objects move a little bit closer together." "Scientists are discovering that our assumptions about gravity may be almost completely wrong." "A whole new side of gravity could be waiting to be discovered." "In fact, we may soon discover objects that fall up." "Most physicists believe that gravity is a force that only attracts." "But cosmologists have recently discovered that galaxies appear to be pushing each other apart at an ever-increasing rate." "Perhaps it's time to reconsider what we think we know about gravity." "Freeman:" "Dragan Hajdukovic is a physicist at the European Center for Nuclear Research, or C.E.R.N., in Geneva, Switzerland." "But he does his best work when visiting his home country of Montenegro." "Dragan is using his time at home to catch up with old friends and work out a new theory of gravity -- one that involves the dangerous material in the universe -- antimatter." "I have a red apple, which is made from matter, and a blue one, which is made from antimatter." "Fortunately, it's not a true antimatter." "But if we assume that it is, look what will happen." "[ Clink ]" "[ Explosion ]" "Freeman:" "Fortunately for us, there isn't enough antimatter in the vicinity of Earth to ever blow it up." "But Dragan thinks that if there were ever such a thing as an antimatter apple, it would have an unusual gravitational property." "It is quite possible that antimatter falls up." "Freeman:" "Dragan suspects that antimatter and matter repel each other, and that hidden pockets of antimatter could be responsible for pushing the universe apart." "Physicists use the term "quantum vacuum"" "to describe the space that fills every corner of the cosmos." "Don't let the name fool you." "It's bubbling with microscopic activity." "At every point in the quantum vacuum, tiny, innocuous pairs of matter and antimatter particles are popping in and out of existence." "They exist for a split-second before annihilating each other." "There are billions of billions of billions of billions and -- let's stop, we can continue -- of pairs in the metacube of the quantum vacuum." "So they must play a role in theory of gravity." "Freeman:" "Think of the quantum vacuum like a typical Montenegrin town." "Every particle of matter always dances with a partner." "When a pair of tiny dancers pops into existence, the gravity of the matter is cancelled out by the antigravity of the antimatter." "So, normally, no matter how many pairs of particles and antiparticles are created, the resulting gravitational effect is zero." "But the quantum vacuum doesn't always exist in a vacuum." "The universe is filled, after all, with giant islands of matter called galaxies." "If you put matter inside, it spoils the symmetry, and you have gravitational effects." "[ Native folk music playing ]" "Freeman:" "At the end of a Montenegrin folk dance, male dancers are drawn into the center to form a massive structure." "The female dancers are pushed outwards." "[ Cheers and applause ]" "In Dragan's theory, tiny particles of matter and antimatter in the quantum vacuum follow the same steps." "Galaxies are made of matter." "They pull in the matter in the quantum vacuum and push its antimatter away." "So there's slightly less matter and slightly more antimatter in the space between galaxies." "So the quantum vacuum becomes gravitationally repulsive and galaxies are pushed apart." "Physicists can see this galactic drift happening." "They are not sure where the energy that is causing it comes from, so they call it dark energy." "But Dragan thinks dark energy is gravity's hidden dance partner." "Many physicists tried to explain the existence of dark energy." "But once again, no one knows what's dark energy." "Now, what's -- what's the simpler solution -- to invoke dark energy, or to assume gravitational repulsion between matter and antimatter?" "[ All shout in native language ]" "[ Cheers and applause ]" "Freeman:" "Dragan's theory is controversial." "But we may soon find out if gravity has a repulsive alter ego." "Back at C.E.R.N., Dragan's colleagues are using the Large Hadron Collider to produce antihydrogen." "If it falls up, we may finally have an explanation for dark energy." "Or it could be another false step on the road to understanding gravity." "I think that our understanding is incomplete." "If you try to explain astronomical phenomena by our best physics, it's a disaster." "Freeman:" "Our gravitational theories are broken." "Neither Einstein's theory nor quantum physics explains all of what we observe." "Is gravity a trick of the mind?" "Or, perhaps, gravity is what's real, and reality itself is the illusion." "We use to believe the Earth was flat." "But seafarers proved this was an illusion." "The horizon only looks flat because our planet is so large," "Change your perspective by flying high enough, and you can see the curvature of Earth." "If gravity is an illusion, can we find a new perspective on it and see it for what it is?" "[ Seabirds calling ]" "Freeman:" "Princeton University's Herman Verlinde is soul searching." "Multiple experiments have shown that Einstein's theory, that gravity is the warping of space and time, appears to be correct." "But an equally powerful theory, quantum mechanics, says that Einstein's theory cannot explain what gravity is made of." "Einstein told us that if you move through space, you don't notice it, because space is empty." "It's not made out of anything." "But quantum theory tells you that actually, that there must be a granularity to space, just like this sand." "Freeman:" "Einstein's theory says that the particle that carries gravitational force, the graviton, must float on the completely smooth surface of empty space, like the surface of the sea." "But according to quantum mechanics, space is not smooth at all." "It is made up of little grains, which make for a bumpy ride." "It is a disagreement that has plagued physicists for over a century." "But Herman is beginning to think both theories may be correct, because reality itself may be deceiving us." "Einstein famously got into trouble by thinking that reality should really exist, uh, and he called that an objective reality." "But in physics, we know that the -- the world is not quite what it seems." "Freeman:" "Objects that travel at the speed of light, like a photon or a graviton, will see a dramatically different version of reality." "When the ball approaches the speed of light, something very strange happens." "The rest of the world seems to become shorter." "And the faster the ball goes, the shorter the rest of the world becomes, until it becomes flat like a plane." "Freeman:" "If you were a graviton, you would be convinced that you were always standing still and the entire universe was a flat sheet in front of you." "We observe particles in our reality moving in linear paths." "But from a particle's point of view, there may be no such thing as moving at all." "In the late 1960s, mathematician Roger Penrose proposed a new way to see the world." "He said that particles that move at the speed of light, like photons and the theoretical graviton, experience an alternate reality he called twistor space, where points are lines and lines are points." "In twistor space, the path that the graviton travels become points, so it's a new set of coordinates for space and for time." "Freeman:" "The idea of a hidden reality seemed preposterous 50 years ago." "But a more recent idea in physics suggests Penrose was ahead of his time." "It is a theory physicists call the holographic principle." "The holographic principle is the idea things that we see in space are actually sort of a reflection of some other reality on holographic screen." "It's as if the actual reality is sitting on the walls of this room." "Freeman:" "Herman is marrying these two ideas into twistor holography." "It's a reality-bending theory where Einstein's gravity and quantum mechanics get along just fine." "Einstein's theory requires that the graviton move through smooth space." "But in twistor holography, the path of the graviton's movement is a point." "It doesn't matter whether the graviton is floating on water or sand, because in this reality, the graviton stays completely still." "If Herman is correct, gravity is real in an altered reality." "And what we experience as reality could be an illusion, constructed from something else." "It's kind of like watching a good TV show." "You might not realize that an invisible group behind the scenes created it." "Man:" "And cut." "Uh, tell me about reality in life and in physics." "In physics, reality is sometimes not unique and sometimes not..." "Objective and sometimes deceptive." "You're sitting here and you're real to me, but who knows?" "Maybe someone is tricking me." "If gravity is the universe's greatest mirage, then it must be created from something." "A groundbreaking theory now argues that gravity could be another form..." "[ Flames whoosh ] ...of pure heat." "[ Lighter clicks ]" "The ancient Greeks believed that fire was a fundamental element of the universe." "But thermodynamics, the study of how microscopic objects create macroscopic effects, proved the Greeks incorrect." "Fire is a phenomenon created from the furious motion of hot atoms." "Now a bold new theory is setting the world of physics ablaze." "It suggests that, like fire, gravity is a thermodynamic mirage." "Freeman:" "The science community is heralding a recent discovery as one of the greatest revelations of gravitational physics, and it's all thanks to this man." "No, it's not Herman Verlinde." "It's his identical twin brother, Erik." "Well, as a child, Herman and I discussed a lot about what we found interesting." "When we would read something, we would talk about it, and -- and we shared our excitement in physics." "Freeman:" "Erik and Herman lived similar lives in Holland." "They both got their PhDs in physics from Utrecht University and even married two sisters." "But Erik's parallel path would take a dramatic turn when a little chaos showed up at his doorstep." "[ Door closes ]" "I was vacationing, and I came back from a run, and I came into my apartment, and then I saw someone had broken in and stolen my car key, my laptop, my passport." "Many things got lost." "Freeman:" "Physicists use the term "entropy"" "to describe the amount of chaos in a system." "Entropy in the universe is always increasing." "In physics and in life, things naturally go from order to disorder." "Turning entropy into order requires energy, just like when Erik had to expend energy to restore order to his house." "While dealing with the unexpected chaos," "Erik was hit with a flash of inspiration." "There is a deep connection between entropy and gravity." "[ Flames whooshing ]" "Imagine traveling to the surface of a neutron star where the intense gravity would make you weigh" "140 trillion times more than you do on Earth." "[ Groaning ]" "Freeman:" "It's enough to significantly raise the entropy of the atoms inside you." "[ Continues groaning ]" "Freeman:" "As objects fall toward a massive body, they experience an ever stronger gravitational pull." "And so their entropy also goes up." "What I realized is that what causes gravity is that the apple, when it's here, has less entropy than when it's down on the floor." "And nature tries to increase entropy, or tends to increase entropy." "This is why, if I let go of the apple, it will try to get as much entropy as possible, and this is why it's falling." "Freeman:" "Erik believes that objects with mass feel the force of gravity because the universe is increasing the amount of disorder, or entropy, deep inside them." "A force that is created from entropy is nothing new to physicists who understand thermodynamics." "In fact, the entropy inside a hot air balloon will lift you up into the sky." "[ Flames whoosh ]" "Erik:" "So a hot air balloon contains molecules." "Those molecules are moving." "They want to increase the entropy, and this they can do by getting more space inside the balloon." "And if the balloon expands, it actually can do so by moving up." "Freeman:" "The hot air inside the balloon tries to increase its entropy by pushing outward and upward." "This results in an emergent force called buoyancy." "Buoyancy is not a true force." "It's created from the entropy of air molecules." "Erik thinks that gravity is also created from the entropy of something else, perhaps from disorder in the very fabric of space and time." "Erik doesn't yet know what it is created from, but he feels sure gravity cannot be a fundamental force of the universe." "From the fact that I can derive gravity from changes in entropy, that basically means we have to think about gravity in a different way." "Instead of assuming it as a fundamental force, we can now view it as something that can be emergent." "Freeman:" "We might find out the truth about gravity if we could feel it more intensely." "This is impossible on earth, where gravity is weak." "But there is a place in the universe where gravity reigns supreme... inside a black hole." "Here, it may completely incinerate matter in a wall of gravitational fire." "[ Clanking ]" "In unusual situations..." "[ Inhales sharply ]" "[ High-pitched voice ] You don't always get what you expect." "But sometimes unusual situations lead to new insights." "[ Normal voice ] To find the truth about gravity, physicists are studying it in a place where they expect it to..." "[ Inhales sharply ]" "[ High-pitched voice ] ...Act very strangely." "[ Engine roars ]" "Freeman:" "Physicist Sean Carroll has a lot on his mind." "Gravity is the hardest problem in physics, and he's tackling it head-on." "The fact that gravity is hard was a surprise to everybody." "We're really gonna need a breakthrough, a different way of thinking about gravity." "Freeman:" "Physicists know where to look for new insights about gravity..." "Inside a black hole." "These cosmic monsters form when stars collapse." "The entire mass of the star is compressed into a single point..." "[ Explosion ] ...where gravity reaches its theoretical maximum." "Surrounding every black hole is an invisible, intangible shell known as the event horizon, the point beyond which not even light can escape the black hole's gravity." "No one knows what actually exists on the other side of this boundary." "The gravitational field in that region of space is so strong that it's a one-way ticket." "You can go in, and you can explore around inside, but you can never come back out." "Freeman:" "Theoretical physicists like Sean turn to their imaginations for answers." "Suppose you're astronaut Alice, a daring cosmic explorer willing to take the plunge into a black hole." "Our current best theory of gravity says that you wouldn't even notice there was an event horizon." "There are certain cherished principles that we like to hold on to." "One of them is simply called no drama." "You could pass right through the event horizon, and it wouldn't look any different than any other place in the universe." "So there's no drama when you're near the black hole." "Freeman:" "Physicists have long believed that when you cross the event horizon, nothing dramatic happens until you're deep inside the black hole, and the rising gravitational intensity turns you into human spaghetti." "But scientists are learning that this time-honored story might not hold up." "The laws of gravity may break down at the event horizon." "Inside, gravity could be something entirely different or not exist at all." "Physicists started to notice contradictions after calculating how particles in and around black holes connect to each other through a process called entanglement." "Entanglement says I could have two electrons, and I don't know what either one of them is doing, but if they're entangled by measure one, and I see it's spinning clockwise, then I know instantly the other one is also spinning clockwise." "Freeman:" "Another cherished principle of physics states that particles are strictly monogamous." "They can only entangle with one partner at a time, no matter what." "But our understanding of the physics of black holes seemed to imply that particles at the event horizon needed to have more than one entangled partner." "This was a scenario no physicist was willing to entertain." "[ Flames whoosh ]" "So this is what we call the Almheiri-Marolf- Polchinski-Sully paradox, after the four Santa Barbara physicists who proposed it." "Freeman:" "The four physicists proposed a dramatic solution to the paradox." "It was time to let go of the cherished principle of no drama at the event horizon." "In fact, something very dramatic happens." "[ Flames whoosh ]" "If you went to a event horizon of a black hole and visited there, you would be incinerated by a wall of fire." "Freeman:" "Black holes may be surrounded by a wall of fire so powerful that it either incinerates any particle going into it, or perhaps incinerates the very fabric of space and time." "If there's a firewall, that means there's -- somehow, there's a boundary." "There's an edge." "And when you hit that region, we're not sure what happens." "It seems like maybe what happens is that whatever is there is not space and time anymore." "It's still quantum mechanics, but it's not good old gravity and spacetime as Einstein would have understood it." "Freeman:" "Past the black hole firewall, gravity could take on an entirely new form." "If we could find out exactly what that form is, we may learn the true nature of gravity everywhere else in the cosmos." "[ Whoosh ]" "Seeing the event horizon of a black hole was once thought to be impossible." "But this astronomer thinks he has a shot at it." "He's building the largest telescope the world has ever seen." "26,000 light-years away, there's a place where we could learn the true nature of gravity." "It's the supermassive black hole at the center of our galaxy." "Astronomers think this hole in space is not much bigger than our sun." "Seeing something that size so far away would take a telescope the size of our planet." "So..." "Why not build one?" "Astronomer Shep Doeleman's career was launched when he answered the call to adventure and landed... here." "What excited me about this particular brand of radio astronomy was that you got to travel the world, and I said, "well, that's for me." "I definitely want to go out into the field and do that."" "And then when I got here, they said "well, largely, that work's been done."" "Freeman:" "Shep does most of his work trapped in his office, where he often escapes by daydreaming..." "About being the first astronomer to observe a black hole." "[ Paper crinkles, whoosh ]" "It's one of the hardest problems in his field, because astronomers can only observe objects that radiate light." "When you ask yourself what a black hole looks like, you -- you really have to begin with, why do we see black holes at all?" "By definition, they should be invisible." "Freeman:" "When light enters a black hole, it's gone forever." "But not all of the light around a black hole gets sucked in." "Some of it bends around the event horizon, creating a shadow image of the black hole." "That image could reveal how gravity behaves at the event horizon." "But by the time the light reaches us, the signal is so diluted that shep would need a telescope thousands of miles across to pick it up." "So..." "He set out to build one." "[ Camera shutter clicks ]" "Shep is traveling to exotic locations around the world, coordinating a massive international collaboration." "In the spring of 2015, nearly all of the world's high-precision telescopes will point towards the center of our galaxy." "So at the center of our galaxy is an extraordinary object." "It's a supermassive black hole." "And because it is so massive, and because it's relatively close to us, we have a shot, we have a chance to resolve it." "Freeman:" "To resolve an image of this black hole," "Shep's team devised a method that turns a collection of individual telescopes into one virtual telescope the size of our planet." "Well, right now I'm in the center of the earth, represented by this ball field." "And we're gonna see water flying out of a nozzle, and you can think of that as light coming from a cosmic object, say a black hole." "And a single telescope can only capture a small amount of that data." "We're gonna put telescopes around the entire ball field, and they're gonna capture all the water flow and sample, very comprehensively, all the data that we need to make an image of the object." "[ Horn honks ]" "[ Whirring ]" "Freeman:" "When matter falls into a supermassive black hole, it spews radiation out into space." "Shep is trying to reconstruct the shape of the light as it leaves its source." "It's just like water leaving a nozzle." "The further it travels, the more the spray spreads out." "But if enough collectors are spread out over a wide enough area, the amount caught in each one would allow you to reconstruct the shape of the nozzle." "The nozzle of the hose is spraying information out." "With a single telescope, or a single cup, you can only record or capture part of the information coming from the nozzle." "But with many cups spread out all over the field, you sample the full information field from the object you're looking at, in this case, the nozzle." "And you can recreate and understand what was happening when the water left that very small volume." "Freeman:" "Shep's planet-sized virtual telescope should have enough resolution to determine the gravitational physics at the edge of a black hole." "Gravity is a theory." "It works very well on the earth, but we haven't put it to ultimate tests." "We haven't put it to the test where gravity is dominant, at the edge of a black hole." "So this is one place where gravity could conceivably break down." "And it's very important to test these theories, because it's the only way we understand the nature of reality, really, the only way we understand the fundamental basis of what we believe about the universe." "Gravity feels real." "It holds all of us to this little rock we call home." "But gravity may not be what it seems." "If gravity is an illusion, then it's time to call into question everything we think we know about the cosmos." "Only when we let go of what we feel to be correct can we taste the real truth." "[ Crunches ]" "== sync, corrected by elderman == @elder_man"