
20221123: How To See Black Holes By Catching Neutrinos
 06:34: ... of observatory where you need to wait until your object sets below the horizon before you can observe ...


20221116: Are there Undiscovered Elements Beyond The Periodic Table?
 15:46: ... hoping to gain new ground in humanity's journey to further and future horizons of space ...


20220824: What Makes The Strong Force Strong?
 18:07: After all, Hawking radiation is sometimes portrayed as a virtual particleantiparticle pair being separated by a black hole event horizon.
 18:15: ... is that the story about separation of virtual particles by the event horizon is a meant to be an intuitive picture of what’s happening, and has only ...
 18:28: Hawking’s original derived his radiation by calculating the disturbances on the quantum fields due to the appearance of an event horizon.
 18:42: But the event horizon changes the balance of these modes causing imperfect canceling, which looks like particles are being radiated by the black hole.


20220720: What If We Live in a Superdeterministic Universe?
 04:37: A horizontal measurement will yield a 5050 chance of being left or right for his downpointing electron.


20220622: Is Interstellar Travel Impossible?
 19:10: Otinane Yos asks whether we can be sure that virtual particle  antiparticle pairs get separated near a Black Hole event horizon.


20220615: Can Wormholes Solve The Black Hole Information Paradox?
 03:20: ... each other. If these particles get separated by a black hole event horizon before they can annihilate, one particle escapes and becomes real, ...


20220601: What If Physics IS NOT Describing Reality?
 05:53: ... left and right until it’s measured. Run it through our horizontally aligned SternGerlach divide and the electron has an even ...


20220525: The Evolution of the Modern Milky Way Galaxy
 18:41: ... field. You may have heard me say that space flows across the event horizon of a black hole at the speed of light  that’s the ...


20220504: Space DOES NOT Expand Everywhere
 11:22: ... seems to emerge from subPlanckwavelength radiation at the event horizon of a black hole. Its solution takes us to the realm of the holographic ...
 13:16: ... receding galaxies  they will spend most of cosmic time far beyond the horizon, in which case we never would have even known that we live in an ...


20220427: How the Higgs Mechanism Give Things Mass
 07:01: ... this, the vertical represents potential energy and the horizontal represents the field ...


20220420: Does the Universe Create Itself?
 16:10: ... we should expect a type of Hawking radiation from the cosmic event horizon even if the universe isn’t a black hole. All event horizons will lead to ...
 17:39: ... the stars, dark matter, other black holes, etc  then it’s event horizon is the same size as our cosmological even horizon. Sounds too much ...


20220330: Could The Universe Be Inside A Black Hole?
 01:01: In general relativity, that looks like a point of infinite density surrounded by an event horizon.
 01:07: We can think of the event horizon as the surface where the flow of space itself is like a river moving at the speed of light.
 01:15: ... nothing that not even light can travel back out from below the event horizon. ...
 01:26: The universe also has a singularity and an event horizon.
 01:46: That expansion gives us our event horizon.
 02:09: This is the cosmological event horizon.
 02:12: ... we’re still receiving light from objects that are now beyond that horizon, because it was emitted by those objects before they crossed the ...
 02:22: ... the accelerating expansion of the universe means the cosmological event horizon is closer to us than the spot where recession equals the speed of ...
 02:33: But the effect is the same: no events that happen now beyond that horizon can ever be seen.
 05:19: ... allencompassing future for the spacetime that lives beneath the event horizon in the same way that the big bang is the encompassing past for the for ...
 06:13: ... past, spacelike singularity of the white hole is surrounded by an event horizon that is the opposite to a black hole event horizon  it can only be ...
 06:24: Space flows at the speed of light across the event horizon from within.
 06:28: That’s starting to look like our universe  a past, spacelike singularity and an event horizon that can’t be crossed from the outside.
 08:36: ... thing collapses under its own gravity and an event horizon forms around it, but within the collapsing cloud the matter remains ...
 09:30: And that’s true even after the black hole’s event horizon forms.
 12:11: If we’re within the event horizon of a black/white hole, what’s on the outside?


20220323: Where Is The Center of The Universe?
 13:33: ... being a boundary to our bubble is very likely far beyond our cosmic horizon. ...


20220308: Is the Proxima System Our Best Hope For Another Earth?
 00:11: He made careful note of the position of a bright star that hung just above the southern horizon.
 00:57: By the end of the 17th century, the star had long since slipped below the horizon of Alexandria in its galactic wanderings.
 14:56: But there’s another white star on the horizon, but it’s slowly slipping away on its own orbit around the Milky Way.


20220223: Are Cosmic Strings Cracks in the Universe?
 08:50: ... We actually expect multiple nucleation events in each causal horizon, potentially leading to dozens of cosmic strings in a network ...


20220216: Is The Wave Function The Building Block of Reality?
 14:52: ... wave whose wavelength is short compared to the black hole’s event horizon can be completely swallowed, while a larger gravitational wave will get ...
 16:15: ... time dilation that we talk about approaching the black hole event horizon is not from the point of view of falling matter, but from the point of ...
 16:48: ... asks if an outsider observer sees matter slow and freeze at the event horizon, what happens when the event horizon grows? Does that matter get ...
 17:42: ... observe an object being frozen at the event horizon if the light emitted at the moment it crosses the horizon from its ...
 18:05: And so everything that went into the black hole remains frozen on the horizon.
 18:37: ... asks another related question. If we never see anything cross the event horizon, why do we say information is lost? When people talk about information ...


20220127: How Does Gravity Escape A Black Hole?
 00:00: ... very center Fact: every black hole singularity is surrounded by an event horizon. ...
 00:10: Nothing can escape from within the event horizon unless it can travel faster than light.
 01:01: ... of infinite density surrounded by this boundary of no return  the event horizon. ...
 02:56: ... light, and all of the mass of a black hole is hidden beneath the event horizon, how does its gravity get out to influence the surrounding ...
 03:07: Shouldn’t a black hole’s event horizon protect the universe from its own malicious influence?
 03:27: There’s no question here  a black hole’s gravity doesn’t care about the event horizon at all.
 04:38: Falling from very far away, an observer and the patch of space that they occupy reach light speed at the event horizon of the black hole.
 04:56: At some point it exceeds the swimming speed of any possible fish  that’s the event horizon.
 05:13: The water at the “event horizon” doesn’t know about the fall.
 06:42: If gravity is really communicated by a particle, how does that particle escape the event horizon?
 06:47: Actually, even in this picture, the event horizon has no way to halt the force of gravity.
 07:33: ... of the singularity, and they don’t have to travel through the event horizon to do their ...
 08:25: But if we’re describing the gravitational field as being built up by virtual gravitons then the event horizon is no barrier at all.
 08:39: ... this idea of sending information across an event horizon brings us to our last argument, and this one works whether we’re talking ...
 09:12: ... present mass of a black hole is hidden below the event horizon, but we can see its past mass, and it’s the gravitational effect of the ...
 09:27: As it approaches the surface that is to become the event horizon, it approaches light speed with respect to someone watching from a distance.
 09:39: It appears to freeze at the event horizon, and the light it emits becomes stretched out and sapped of energy.
 10:01: So we can still “see” the mass of a black hole  it’s imprinted on the event horizon.
 10:55: ... the black hole, but from your point of view it’s frozen on the event horizon, but is happily exerting its influence on the surrounding ...
 11:18: You interact with the local curvature of spacetime, which is produced by the past mass, which from your point of view is on the event horizon.
 11:45: By that definition the mass of a black hole is everywhere  so it’s not surprising that it can escape the horizon.
 11:53: To sum up  don’t mess around near black holes hoping that the event horizon will protect you from the black hole’s gravity.


20211220: What Happens If A Black Hole Hits Earth?
 04:26: ... then it’s probably already happened. This PBH would be small  its event horizon  its surface of noreturn  would be around the size of an atom. And it ...
 05:13: ... moon phobos  10^16 kg like a large asteroid. That gives it an event horizon the size of a hydrogen atom. At interstellar speeds it spends around a ...
 06:02: ... hole, gravity accelerates matter to incredible speeds. Near the event horizons, particles collide with each other sizable fractions of the speed of ...
 07:05: ... tiny event horizon of a micro black hole is a very narrow chokepoint for matter trying to ...
 16:33: ... space. That means that an observer falling through a black hole event horizon shouldn’t notice anything special about that boundary  except the fact ...
 18:16: ... lasts longer for fuzzballs than for regular black holes due to the event horizon being less cleanly defined. It’s also been suggested that the Event ...


20211117: Are Black Holes Actually Fuzzballs?
 00:39: ... surrounded by a surface from which even light can’t escape  the event horizon. ...
 01:10: And at the event horizon another paradox arises.
 02:56: The nohair theorem says that there’s no information beyond charge, mass and spin that’s observable above the event horizon.
 03:04: The information contributing to black hole entropy could be beneath the horizon, but if we add one more fact we run into serious trouble.
 04:29: But this is also the problem at the event horizon.
 04:49: ... the surrounding quantum fields, which only worked if the gravity at the horizon was relatively weak, in which case quantum gravity effects shouldn’t ...
 05:09: ... gravity starts to get quantum even above the event horizon, then it may be possible to actually encode information on that surface ...
 06:17: It turns out that though, that string theory can make sense of the black hole event horizon also.
 06:37: ... guys found a way to count the microstates on the horizon the number of possible configurations of stringy structures  the 1D ...
 06:55: ... on the mechanism by which information could be encoded on the event horizon. ...
 07:13: ... and Vafa did this for the somewhat unrealistic case of a black hole horizon with 4 spatial dimensions, but it was a decisive step and it gave us a ...
 08:06: ... how they form, or how structure can actually be supported on the event horizon. ...
 09:09: ... this is right, then black holes don’t have an empty event horizon at all, but rather a real surface that looks like a tangled nightmare of ...
 09:39: ... important at the center of the black hole but instead may pile up to the horizon ...
 10:37: The resulting fuzzball doesn’t have a local event horizon  in that there’s no single surface of noreturn.
 12:10: The event horizon is just the pair of equidistant points on the line inside of which escape is impossible.
 12:38: For a fuzzball, spacetime closes on itself at the event horizon.


20211013: New Results in Quantum Tunneling vs. The Speed of Light
 15:36: ... monopole would manifest as a black hole  in that it would have an event horizon. ...


20210921: How Electron Spin Makes Matter Possible
 15:48: ... so at most they can appear to be coming from the edge of the event horizon. ...
 16:12: ... that’s exactly why the event horizon telescope image looks like that. Although in that case the dark area is ...


20210915: Neutron Stars: The Most Extreme Objects in the Universe
 13:06: ... is growing and at some point soon it’ll form an inescapable event horizon and we’ll be stuck inside an actual black hole. Escape ...


20210907: First Detection of Light from Behind a Black Hole
 01:08: Now you might remember the picture of the black hole captured by the event horizon telescope.


20210818: How Vacuum Decay Would Destroy The Universe
 12:07: ... more comforting fact. If the vacuum does decay inside our cosmic horizon we’ll never see it coming. Because no light can outpace the ...


20210721: How Magnetism Shapes The Universe
 11:40: Intense magnetic fields live just above the event horizon of some of thses black hole, and thread the infalling disk.
 11:54: ... light surrounding the M81 supermassive black hole observed by the event horizon ...


20210707: Electrons DO NOT Spin
 06:36: ... a second set of SternGerlach magnets, but now they’re oriented horizontally. Classical dipoles that are at 90 degrees to the field would experience ...


20210616: Can Space Be Infinitely Divided?
 07:48: ... mass to produce a black hole with a Plancklength event horizon  so any attempt to measure something that small swallows it in a ...


20210525: What If (Tiny) Black Holes Are Everywhere?
 00:23: They’re surrounded by this surface we call the event horizon, whose defining property is that nothing that falls beneath it can ever emerge.
 02:06: ... a sort of a hack  a brilliant workaround  to prove that if an event horizon forms in a vacuum, then the vacuum states of the quantum fields have to ...
 02:36: In the common popsci description of Hawking radiation you often hear about particleantiparticle pairs appearing at the event horizon.
 02:45: This is misleading; for one thing, the radiation doesn’t appear right above the event horizon.
 02:52: ... of the emitted particles are about the size of the whole event horizon, so they sort of emerge from the entire region surrounding the black ...
 03:02: For the black hole left behind when a massive star dies, the event horizon is several kilometers in radius.
 04:19: One big assumption is that the space near the event horizon isn’t TOO strongly curved compared to the smallest quantum scale.
 06:41: Such a black hole would have an event horizon of around 10^35 meters  the Planck length.
 09:58: A way around this has been proposed  what if space inside black holes actually expands to a region larger than the event horizon?


20210511: How To Know If It's Aliens
 15:39: ... the bubble in a similar way to how things appear to freeze at the event horizon of a black hole. Nothing actually passes into the bubble, so the ship is ...


20210421: The NEW Warp Drive Possibilities
 03:07: Or beyond the cosmic horizon, the expanding universe is carrying galaxies away from us faster than light.


20210413: What If Dark Matter Is Just Black Holes?
 06:38: ... though these black holes would have microscopic event horizons, at those insane abundances, they’d be wreaking absolute havoc in various ...


20210309: How Does Gravity Affect Light?
 04:37: ... the event horizon of the black hole, gravitational time dilation is so strong that clocks ...
 06:10: Can this gravitational time dilation also explain the bending of a ray light traveling horizontally?
 06:15: Well, tricky  because there’s no change in clock speed in the horizontal direction  only the vertical.


20210119: Can We Break the Universe?
 04:33: ... the set of simultaneous events for a motionless observer lie on a horizontal line  all events corresponding to your notion of a given tick on the ...


20201222: Navigating with Quantum Entanglement
 04:07: And their horizontal orientation, or angle relative to lines of longitude.


20201208: Why Do You Remember The Past But Not The Future?
 04:36: Time increases upwards, while the horizontal axis is separation in space.


20201027: How The Penrose Singularity Theorem Predicts The End of Space Time
 01:17: ... by a surface where time froze. Beneath that “event horizon” all matter, light, space itself was doomed to fall inwards towards ...
 01:58: ... matter could reach the densities high enough to produce an event horizon  nor even whether such dense matter would really ...
 04:06: ... this deserve a Nobel prize? Penrose set out to show that an event horizon and a singularity would form for any distribution of matter, no ...
 05:55: ... black holes is that null geodesics beneath the event horizon that are trying to travel outwards also converge. ...


20200824: Can Future Colliders Break the Standard Model?
 14:28: ... tricky one: when two black holes merge and share a single, warped event horizon, shouldn’t there be a thin sliver of space in between that is not flowing ...
 15:06: ... general relativity calculations reveal an event horizon crosssection that’s called the pairofpants function  it looks like ...
 15:16: They start separate, and the event horizons move towards each other as they join, and at some point the event horizons touch and then merge.
 15:28: ... flow of space anywhere on the event horizon has to be exactly perpendicular to the face of the event horizon  so at ...


20200817: How Stars Destroy Each Other
 11:04: Zack Hamburg asks how we know that a black hole isn’t just a neutron star behind an event horizon.
 11:27: So what happens to the neutron star after it collapses enough to form an event horizon?
 11:32: Well, below that event horizon, we can think of space flowing downwards faster than the speed of light.
 13:37: Frank and Jim asked how the event horizons of merging black holes change just before they combine.
 13:42: ... deform into a sort of 8 or hourglass shape  in the sense that the event horizons sort of reach out to each other, connect, before coming together into an ...
 13:56: ... the shape of the stuff inside the black hole  we’re just the event horizon  the surface below which there’s a fasterthanlight flow of ...
 14:07: Here’s a simulation from the SXS  simulating extreme spacetimes group at ... that shows how the event horizons merge.


20200810: Theory of Everything Controversies: Livestream
 00:00: ... now through the core of the roll representing what i would call the horizontal tangent space which is not pointed along the tube if it was pointed ...


20200720: The Boundary Between Black Holes & Neutron Stars
 05:42: ... black holes, which by definition have an escape velocity at the event horizon equal to the speed of ...
 06:49: It’s like a phantom event horizon.
 06:52: In the case of the earth the phantom event horizon is about a centimeter in diameter.
 07:00: As you increase a neutron star’s mass, its phantom event horizon grows while its actual surface shrinks.
 12:18: ... our last two episodes we talked about how to dissolve a black hole event horizon by adding rotation or charge to the black hole, and also about ...
 12:34: ... trying to disolve an event horizon by throwoing more and more electric charge into it  Ultimantis points ...
 13:03: ... Cimo has a good one: What if I cross the event horizon just for a plank lenght and the instant later the black hole shrinks due ...
 13:34: ... watching from far away, you appear to freeze smeared out at the event horizon  and then yes, they would see your energy exit again as the black hole ...


20200630: Dissolving an Event Horizon
 00:44: ... surrounding all of this weirdness is the event horizon: the surface around the central singularity where the inward flow of ...
 01:07: The event horizon sounds dire, but it’s a blessing.
 01:30: ... that every gravitational singularity must be surrounded by an event horizon. ...
 02:12: So let’s talk about how to dissolve an event horizon, although I would ask you to please not try this at home, for the sake of all of physics.
 02:45: With nothing to counter the inward flow of space, an event horizon is inevitable.
 03:25: It’s like the eye of the storm, and it’s separated from the surrounding cascade by the inner event horizon.
 03:34: As the spin of a Kerr black hole increases, the spacetime waterfall is beaten back, and so the inner horizon grows.
 03:40: At a certain very high rotation rate, the inner and outer horizons merge  which means they both vanish.
 04:20: ReisnerNordstrom black holes also have an inner horizon, interior to which space and time seem normalish.
 04:28: The more electric charge you drop into a black hole, the larger its inner horizon becomes.
 04:33: And just as with the rotating black hole, at some point the inner and outer horizons become one and vanish and you’re left with a naked singularity.
 04:43: ... the tipping point  when the inner and outer horizons are right next to each other  you have what we call an extremal black ...
 05:59: The outer event horizon will shrink until it meets the inner horizon, and again you have an extremal black hole.
 06:21: Hawking radiation is a direct result of there being an event horizon.
 06:58: ... will always stop a gravitational singularity being stripped of its event horizon  but it doesn’t tell us the physical ...
 08:06: ... black hole  one that’s rotating nearly fast enough to lose its event horizon  the gas near the event horizon orbits entirely riding on the carousel ...
 10:23: In fact the field itself will always generate enough mass to prevent the black hole from losing its event horizon.
 11:16: ... two episodes  building black holes in the lab with analog event horizons, and Roger Penrose’s conformal cyclic ...


20200622: Building Black Holes in a Lab
 00:16: ... about never being able to see inside one beneath the inescapable event horizon. Then there’s the fact that real black holes are, thankfully, very far ...
 03:05: ... where the waterfall of space equals the speed of light is our event horizon. No information  from fish or astronauts or anything  can reach us from ...
 03:26: ... this was just an illustrative, evocative example of the power of event horizons. But by 1982 he realized the two situations had much more in common, ...
 04:08: ... analogous situations and found a number of systems with event horizons Analog theoretical black holes are all very well, but their real value ...
 04:55: At some point the flow is faster than the waves  and that’s your analog event horizon.
 05:01: ... the speed of ripples on the tank surface you again have an analog event horizon. ...
 05:25: ... these event horizons, physicists can look for black holelike behavior. For example, Hawking ...
 08:04: ... through the black hole’s ergosphere. That’s the region around the event horizon where the circular flow of space becomes irresistible. It turns out that ...
 11:04: ... appear for a variety of systems—you just need some sort of apparent horizon then finding bonafide Hawking radiation for one system should tell you ...


20200527: Does Gravity Require Extra Dimensions?
 13:29: ... episode we talked about the strange regions that lie beneath the event horizon of a rotating black hole  regions like the Carter time machine and the ...


20200518: Mapping the Multiverse
 00:52: Back then we stopped short of dropping below the event horizon  the boundary of no return.
 01:11: The inescapable surface surrounding a black hole is called the event horizon.
 02:29: ... the event horizon is the ergosphere  a place where the fabric of space itself is whipped ...
 02:40: The event horizon itself is distorted  wider at the equator than the poles, just like the rotating earth.
 02:50: Now let’s peek below the event horizon to see how weird things really get.
 04:20: This leads to a second event horizon called the inner horizon.
 05:14: ... can try to turn around and go back the way you came through the inner horizon, or you can plunge through the singularity ...
 07:41: Also, there are no horizons above you  there’s a straight path to the outside universe  but it is certainly not our universe.
 07:50: For one thing, the lack of event horizons above you means the ring is a naked singularity, as well as being repulsive.
 08:54: At this point we can follow an outgoing geodesic across the inner horizon.
 09:03: That outgoing flow continues above the inner horizon, and will eventually eject you past the outer horizon.
 11:10: ... fact of your utter obliteration before you ever cross the inner event horizon in the first ...
 11:21: The complete Penrose diagram for the Kerr spacetime has not one, but two inner event horizons leading to two parallel wormholes.
 11:38: ... correspond to the region below the event horizon but they’re causally disconnected, the difference being that time flows ...
 11:50: So you have this messed up situation where the direction of the flow of time actually clashes at the inner horizon.
 12:34: So if you do get to the inner horizon you are in a bath of energy on par with the Big Bang.
 13:30: You have the same inner horizon and chain of wormholeconnected universes.


20200511: How Luminiferous Aether Led to Relativity
 16:14: ... Virgo supercluster. That's so weird to think about  the most distant horizon we can see is the same size as the local bubble of galaxies. Those poor ...


20200504: How We Know The Universe is Ancient
 14:28: Devansh Rana asks Can wormholes exist without black holes ie without the event horizon.
 14:36: ... the Schwarzschild wormhole definitely has an event horizon  although at its most open, entering the event horizon ejects you into ...
 16:15: ... the distance has to be short. When the wormhole is fully open, the event horizons of the wormholes ...
 16:44: Pass thorugh one event horizon at that instant you exit the other.


20200428: Space Time Livestream: Ask Matt Anything
 00:00: ... I hope you're okay Bob which is that when mass is just about the event horizon of a black hole is the event horizon slightly distorted due to the ...


20200422: Will Wormholes Allow Fast Interstellar Travel?
 03:45: ... of the wormhole is actually a sphere the size of the Schwarzschild event horizon. If we could pass through that event horizon we would not find ourselves ...
 04:50: ... are blended and mutated so that time doesn’t freeze at the event horizon, which itself becomes this 45 degree path. Sublightspeed paths are ...
 05:17: ... if you travel to the left at any possible speed you’ll find the event horizon and enter the interior of the black hole. But what if you could travel ...
 05:43: ... has narrowed in width and passing through means crossing two event horizons  and the region in between is the inside of a black hole. When that ...
 11:03: ... circumstances like on quantum scales or if they’re hidden by an event horizon. Combined with Roger Penrose’ cosmic censorship conjecture, which ensures ...


20200331: What’s On The Other Side Of A Black Hole?
 00:00: ... maps are useless inside black holes. At the event horizon  the ultimate point of no return as you approach a black hole  time ...
 01:53: ... It even works inside the black hole  beneath the inescapable event horizon. Although it works in both these regions, the Schwarzschild metric canNOT ...
 02:56: ... the tortoise, and a plummeting cartographer would fall through the event horizon. The event horizon is just a coordinate singularity like the earth’s ...
 03:14: ... measure of distance that becomes infinitesimally compact approaching the horizon. That compactification cancels out the infinite stretching of time so ...
 03:46: ... coordinates, and they revealed that the singularity of the event horizon was an illusion. Kruskal–Szekeres coordinates improved our map by ...
 04:14: ... very clear what parts of the universe are accessible. Close to the event horizon, even a lightspeed path has only a narrow window of escape. Once inside ...
 07:32: ... imagine you could travel at infinite speed  then you could take these horizontal which would dip beneath the event horizons and emerge in the mirror ...
 09:00: ... if you can travel faster than light you’ll emerge from the same event horizon that swallowed you. Or you can plunge faster than light towards the ...


20200324: How Black Holes Spin Space Time
 00:21: ... if that mass is sufficiently compressed, the metric predicts an event horizon  a spherical surface where the fabric of space cascades downwards at ...
 01:01: ... own galaxy, and we’ve even taken an image of a black hole with the event horizon telescope. But none of these real black holes are particularly well ...
 01:44: ... place, and the rotation of any latter object sucked through the vent horizon. Even nonrotating objects will affect black hole spin if they fall in at ...
 03:54: ... tempting to just say that some physical thing deep beneath the event horizon is rotating. But if nothing can escape the event horizon, how can that ...
 04:11: ... fact, how can any effect of gravity extend from beneath the event horizon? In fact it sort of doesn’t actually. Both the gravitational field and ...
 04:26: ... are selfsustaining holes in the fabric of spacetime. Space at the event horizon cascades downwards, dragging more space behind it, sort of like how ...
 05:49: ... common misconception, as long as you don’t get too close to the event horizon it’s possible to orbit a black hole in a perfectly stable way. For a ...
 07:27: So yeah, you can orbit “safely” pretty close to the Kerr black hole’s event horizon.
 07:33: ... above the event horizon is a particularly bizarre region called the ergosphere. There, frame ...
 07:50: ... situation here is actually similar to the state below the event horizon where space moves downwards faster than light. In the math, that ...
 08:34: The ergosphere extends all the way down to the event horizon.
 08:37: ... the event horizon in the rotating case is not spherical  it’s squished at the poles, like ...
 08:54: ... I know you would love to drop below the event horizon right now  but you’re going to have to wait. There’s still plenty to do ...
 09:17: ... exactly the right point, one half will go plummeting through the event horizon while the other is ejected from the ergosphere and escapes. In fact it ...
 12:11: ... And we'll encounter all of these when we drop below the event horizon into the deeper weirdness of the Kerr ...


20200316: How Do Quantum States Manifest In The Classical World?
 02:56: ... have to choose what angle you want to measure it at  say, vertically or horizontally. ...
 03:12: ... and you might see that the spin axis is pointing up or down, measure horizontally and it’ll be pointing left or right. Weirdly, if you measure vertically ...
 04:28: ... basis, each particle starts in a state of both up AND down, while in the horizontal each is left AND right, etc. The only thing we know is there’s this ...
 09:28: ... into the details here  suffice to say that it's possible to recover the horizontal spin information, even if it’s more straightforward to recover the ...
 11:48: ... up the experiment  whether you align your magnetic field vertically or horizontally. But ultimately they are states that can survive contact with the ...


20191217: Do Black Holes Create New Universes?
 03:38: ... it sort of bounces  but unable to exit the event horizon of the black hole, it forms a new region of spacetime, effectively ...


20191021: Is Time Travel Impossible?
 08:51: The maelstrom of spinning spacetime may generate closed timelike curves deep down below the event horizon.


20191015: Loop Quantum Gravity Explained
 15:49: ... the black hole rotates more than a certain amount then the event horizon evaporates, exposing the singularity to the universe  or in the case of ...


20191007: Black Hole Harmonics
 01:30: Imagine it: two event horizons – two roughly spherical black surfaces that are literal boundaries to our universe.
 02:06: The event horizon seems to define the surface of the black hole, but really it’s the fabric of spacetime itself that’s vibrating.
 03:46: The harmonic oscillations of 2D surfaces – like drum skins, bells, or the event horizons of black holes – are a good bit more complex than in 1D.
 03:56: In the case of the event horizon, or any sphericalish surface, we break down the oscillations not into sine waves but into spherical harmonics.
 04:33: They leak away over time, but while present they warp the shape of the event horizon.


20190930: How Many Universes Are There?
 15:30: I'm also echoing some of the thoughts of Alan Stern, lead scientist on the new horizons mission to pluto.


20190923: Is Pluto a Planet?
 11:11: Don’t think of this as a demotion  it’s more of a ... reincentivizing horizontal pivot.


20190826: How To Become an Astrophysicist + Challenge Question!
 10:46: ... a rate for inflation Let's assume the minimum rate needed to explain the horizon and flatness problems That is all distance is increased by a factor of ...


20190725: Deciphering The Vast Scale of the Universe
 00:49: ... has grown enormously – to include countless galaxies to the cosmic horizon and almost to the beginning of ...
 08:27: ... the art modern observatories like the Gemini Observatory, and the Event Horizon Telescope, and LIGO, which we visited throughout this ...
 08:55: If we continue exploring – if we keep looking deeper and further – who knows what we’ll find in the expanding horizons of space time?


20190715: The Quantum Internet
 06:54: They’re entangled so that they have opposite polarization, say one is vertical the other is horizontal.


20190701: Thorium and the Future of Nuclear Energy
 17:49: ... definitely doesn't extend more than 10 or so times the black hole event horizon Oppie asks why NASA isn't dedicating more resources to investigating all ...


20190606: The Alchemy of Neutron Star Collisions
 02:47: ... of the newly formed black hole presumably doomed to fall into the event horizon but that same beta decay process the converted neutrons back to protons ...


20190509: Why Quantum Computing Requires Quantum Cryptography
 05:48: ... for example, pass a randomlypolarized photon through a horizontal polarization filter like a polarized sunglasses lens and the photon will ...
 14:46: ... episode on the galaxy without dark matter, and our coverage of the event horizon telescope's black hole ...
 15:56: Tommy Barlow states that nor the event horizon, nor anything near it, was imaged.
 16:02: The event horizon is invisible.
 16:05: But the telescope DID resolve down to the size of the event horizon.
 16:16: That's probably around 5 times larger than the event horizon, in the case of this quasar.
 16:21: ... seriously, the "light from outside the event horizon because we can't actually see the event horizon telescope" doesn't have ...


20190501: The Real Science of the EHT Black Hole
 00:18: ... first ever actual bona fide photo of a black hole, made by the Event Horizon Telescope and revealed to the world in a press conference on April ...
 01:00: It has an estimated mass of several billion times that of the Sun, which gives it an event horizon larger than the solar system.
 03:31: ... to observe the event horizon of the M87 black hole you need to resolve one onemillionth of 1% of one ...
 04:24: ... Event horizon Telescope  the EHT  consists of nine radio observatories across the ...
 05:58: The event horizon itself is the point where even outwardpointing light can’t escape the black hole.
 06:04: For a nonrotating black hole the size of the event horizon is called the Schwarzschild radius, and it’s proportional to the mass of the black hole.
 06:13: But we’re not seeing the event horizon here – that’s somewhat smaller.
 09:37: ... found that monster has a mass over 6 billion solar masses, with an event horizon that's about 1/5 the size of that ring– so that’s VERY big, even by ...
 10:54: ... this actual picture given to us by the Event Horizon Telescope and its brilliant team – finally hits us with visceral reality ...


20190424: No Dark Matter = Proof of Dark Matter?
 00:03: ... is indeed a reasonable interpretation of the universe beyond our cosmic horizon as a hologram living in the light bill quite reach us trapped at the ...


20190410: The Holographic Universe Explained
 02:41: And while it’s waiting to be radiated, that information should be encoded on the event horizon.
 02:52: Things that fall into a black hole do actually experience crossing the event horizon and being inside the black hole.


20190403: The Edge of an Infinite Universe
 00:46: It’s boundary is called the particle horizon.
 01:02: ... define its horizon like we might build a little picket fence around our little patch – ...
 01:17: We visited these cosmic horizons in one of the early Space Time episodes.
 01:22: To review: the particle horizon defines the limit of the visible past, and there’s also cosmic event horizon defining the limit of the visible future.
 04:20: ... spacetime –to the edge of an infinite universe or across the event horizon of a black ...
 04:49: The first efforts were designed to allow physicists to cross the event horizon of black holes – mathematically.
 05:03: ... the artificial infinity – the coordinate singularity  of the event horizon – also something we’ve discussed (the phantom singularity) It was Roger ...
 05:31: We’ve used these before to understand black hole event horizons, but these were originally conceived to understand the boundaries of the universe.
 05:49: ... horizontalish contours are our old time ticks  moments of constant time across the ...
 15:45: ... all, if space is expanding faster than light at the event horizon, that should counter the lightspeed flow of space into the event ...
 16:01: ... stackexchange, in which he suggests that the shrinking cosmological horizon could merge with the event horizon to produce a global state where ...


20190328: Could the Universe End by Tearing Apart Every Atom?
 07:02: ... to the nearest inaccessible region of space is called the 'cosmic event horizon'. Now, if the expansion is accelerating, then, over time the distance ...
 07:38: But as long as we have a nice gravitationally bound galaxy to live in, the cosmic event horizon can never shrink to a size smaller than that galaxy.
 07:47: ... dark energy there's no protection from the encroaching cosmic event horizon. ...
 08:10: That is the big room scenario, It happens when the cosmic event horizon is smaller than the smallest possible structure.
 09:26: ... this doesn't mean that the cosmic event horizon is quite inside the Milky Way just yet, just that the effect of phantom ...
 09:38: In fact, at this point the cosmic event horizon is still about 200 million light years away.


20190306: The Impossibility of Perpetual Motion Machines
 12:43: ... check out the Destination Pluto, which follow the timeline of the New Horizons mission from its inception to the date of its close encounter with ...


20190207: Sound Waves from the Beginning of Time
 06:26: We call this the sound horizon. And at recombination, it was around five hundred thousand light years.


20190116: Our Antimatter, Mirrored, TimeReversed Universe
 13:52: ... everyone, I'm back from the event horizon, aka. Australia, which as you know is the CPT inverted version of Canada. ...


20181107: Why String Theory is Right
 15:26: More accurately, the conducting plates create a horizon in what would otherwise be a perfect infinite vacuum.
 15:33: In fact, you create two horizons between the plates and one horizon on the outside.
 15:40: Those horizons perturb the vacuum which can lead to the creation of very real particles, as in Hawking radiation.
 15:47: ... in the Casimir effect, the double horizon between the plates restricts what real particles can be produced there ...


20181031: Are Virtual Particles A New Layer of Reality?
 00:21: Near black holes, virtual matter and antimatter pairs are separated by the event horizon to create Hawking radiation.
 11:11: ... virtual matterantimatter pairs being separated by the black hole event horizon, allowing one of the pair to escape to beautiful freedom and ...
 11:28: In his actual mathematical derivation, he instead talks about vibrational modes of the quantum vacuum being cut off by the event horizon.


20181018: What are the Strings in String Theory?
 13:50: ... what it would take to compute a universe simulation on the event horizon of a black ...
 14:03: Roman R. asks whether computation at an event horizon would experience massive time dilation relative to an outside observer.
 14:17: Really, you can't read off the result of an event horizon computation until practically ever.


20181010: Computing a Universe Simulation
 04:13: ... at the Bekenstein bound, essentially, your memory device is the event horizon of a black ...
 04:50: Again, the Bekenstein bound is the event horizon surface area in Planck areas with an extra factor of a quarter that Stephen Hawking figured out.
 05:24: The radius I gave is the Schwarzschild radius, the radius of the event horizon of a nonrotating neutral black hole, again, like Switzerland.
 11:34: It's not clear that programming and then reading out from an event horizon computer is even possible.


20180920: Quantum Gravity and the Hardest Problem in Physics
 05:57: ... put into that region of space would make a tiny black hole with an event horizon one Planck length in ...
 13:20: A few of you asked why it is that the surface area of a black hole's event horizon must always increase and how mass and radius can actually decrease.
 13:46: Event horizon radius is proportional to mass.
 14:30: In that process, the event horizon only changes shape.
 15:56: Clue you'll need to go beyond the formula for the Bekenstein bound in terms of event horizon surface area.


20180912: How Much Information is in the Universe?
 01:57: Jacob Bekenstein figured this out by realizing that the entropy of a black hole is proportional to the surface area of its event horizon.
 07:25: And for black holes, that entropy is the Bekenstein bound, the number of Planck areas on its event horizon.
 07:32: Because the information about the black hole's previous state is lost, to fully describe it, you need to fully describe its event horizon.
 07:54: Its event horizon is around 12 billion meters, giving it a surface area of 10 to the power of 90 to 10 to the power of 91 Planck units.
 09:05: ... limit, it would immediately become a black hole with an event horizon as big as the current cosmic ...
 09:42: Essentially, they're computing on the surface of a black hole event horizon.


20180905: The Black Hole Entropy Enigma
 02:47: He described a mechanism by which the information contained by infalling particles could be preserved on the event horizon of the black hole.
 04:49: At the instant the star collapses far enough to form an event horizon, it becomes a black hole.
 06:02: The surface area of a blackhole event horizon can never decrease, at least not according to general relativity.
 06:53: That's the surface area of the event horizon.
 08:37: ... is going to be directly proportional to the surface area of the event horizon. ...


20180823: How Will the Universe End?
 03:13: ... all galaxies beyond the Virgo Supercluster outside of the cosmic event horizon. ...


20180704: Will A New Neutrino Change The Standard Model?
 11:22: So, how does that work with the cosmic event horizon?
 11:26: Shouldn't regions outside the event horizon be lost to our access and therefore impossible to trace backwards?
 11:33: The statement about the retraceability of the universe doesn't actually care about event horizons, whether cosmic or black hole.


20180620: The Black Hole Information Paradox
 00:22: [MUSIC PLAYING] Black holes are engines of destruction that remove from our universe anything that crosses their event horizon.
 02:01: The inescapable event horizon shields the outside universe from any other influence within the black hole.
 06:07: The motivation for this idea is the fact that, from the point of view of an outside observer, nothing ever actually crosses the event horizon.
 06:14: For the outside universe, everything that ever fell into the black hole remains frozen in time and smeared flat over that horizon.
 07:25: From the point of view of an observer falling into the black hole, they aren't frozen at the horizon.
 08:44: He did a more careful calculation of the effect of infalling material and found that it doesn't exactly freeze above a completely static horizon.
 08:52: Rather, it distorts the horizon, creating a sort of lump at the point of crossing.
 10:00: ... how the information on an event horizon gets attached to Hawking radiation is still contentious, but a number of ...
 10:09: ... tunneling from within the black hole could interact with the holographic horizon and carry information back out into the ...
 14:49: ... with the interior or just the persistence of the field at the event horizon is a matter of ...


20180613: What Survives Inside A Black Hole?
 00:16: ... body, a point of hypothetical infinite density surrounded by an event horizon. ...
 00:34: At that horizon, time is frozen and the fabric of space itself cascades inwards at the speed of light.
 00:40: Nothing can travel faster than light, and so nothing can escape from below the event horizon not matter, not light, not even information.
 01:45: ... charge, and angular momentum should emerge from beneath the event horizon. ...
 02:20: Nothing beneath the event horizon can influence the exterior universe because no signal can escape the event horizon to carry that influence.
 02:44: Given the oneway nature of the event horizon, the loss of this information makes some intuitive sense.
 03:06: So how does mass, electric charge, and angular momentum communicate their influence across the uncrossable horizon?
 04:20: ... the case of the event horizon, the outside universe can't see the mass inside the black hole, but that ...
 04:32: In fact, the space at the event horizon is already falling inwards.
 05:22: In the case of a black hole, at least a nonrotating one, the event horizon is a closed spherical surface with a singularity at its center.
 06:04: This means that the electric field above the event horizon of a black hole remembers all of the electric charge that fell through that surface.
 06:12: Black holes act as though their charge is spread across the event horizon.
 09:14: It dramatically changes the shape of the event horizon and the orbit of anything nearby.
 09:19: ... it will either add or subtract from this flow of space above the event horizon. ...


20180502: The Star at the End of Time
 10:25: ... the Event Horizon Telescope has now detected radio emission from pretty close to the event ...


20180411: The Physics of Life (ft. It's Okay to be Smart & PBS Eons!)
 12:26: Moma the Belly Dancer asks whether this means that the expansion of the universe also causes an event horizon?
 12:34: The cosmic event horizon is that service from beyond which we can never obtain new information.
 12:39: We can actually see that horizon today.
 12:59: ... horizon should produce a type of Hawking radiation, but its wavelength would be ...
 13:09: On the other hand, during the inflationary epoch in the extremely early universe, the cosmic event horizon was very close to every point.


20180404: The Unruh Effect
 00:06: Every time you accelerate, put your foot on the gas, quicken your step, get out of your chair, you generate an event horizon behind you.
 00:58: It creates a type of event horizon.
 01:00: As we saw in our episode on horizon radiation, the presence of horizons distorts the quantum vacuum in a way that can create particles.
 04:50: This means that any events happening to the left of that diagonal line will never affect the accelerating observer, which sounds pretty horizonlike.
 04:59: In fact, the active acceleration does create a type of event horizon called a Rindler horizon.
 05:06: ... physicist Wolfgang Rindler who, by the way, also invented the term event horizon. ...
 05:20: The Rindler horizon flows at a fixed distance behind a constantly accelerating observer.
 05:28: The distance of a Rindler horizon is inversely proportional to acceleration.
 05:32: The larger the acceleration, the closer the horizon.
 05:36: All parts of the universe beyond that horizon are out of causal connection with the Rindler observer as long as they continue to accelerate.
 05:46: Even momentary acceleration generates a Rindler horizon.
 05:54: It's like the projected future acceleration gives you a Rindler horizon in the present.
 06:00: And just as with Hawking radiation, that horizon cuts off your access to certain fundamental frequency modes of the quantum vacuum.
 06:53: This is because that distant point of spacetime is smoothly connected to the spacetime near the horizon.
 07:00: The only observers who don't see Hawking radiation are those plummeting in freefall towards the event horizon.
 10:12: ... acceleration can get that high, and that's right above the event horizon of a black ...
 10:21: If you hover close enough to that event horizon, you would actually be bathed in Unruh radiation.
 10:29: ... between the Unruh particle seen by someone hovering at the event horizon and the particles of Hawking radiation seen by a distant ...


20180315: Hawking Radiation
 00:50: ... create a hole in the universe, a boundary in spacetime called an event horizon that could be entered, but from beyond which nothing could ...
 02:03: ... a black hole, sometimes, one of the pair will be swallowed by the event horizon, leaving the other free to escape and taking its stolen energy with ...
 04:11: But spatial curvature can mess with the balance of the underlying quantum field modes by introducing horizons.
 04:18: Horizons cut off access to certain modes of the quantum fields, disturbing the balance that defines the vacuum.
 05:17: It emerges barely ahead of the forming event horizon.
 06:55: They are nudged off their narrow escape path and so are lost behind the forming event horizon.
 07:29: It produces particles that also have wavelengths about as large as the event horizon.
 07:53: More directly, it's proportional to the surface area of the event horizon.
 08:07: OK, so what about the whole picture of particle/antiparticle pairs being pulled apart by the event horizon?
 08:27: And for the escaping modes, there exist a corresponding set of modes linked by quantum entanglement that are trapped behind the event horizon.
 08:51: Remember that Hawking radiation has wavelengths the size of the event horizon, the size of the entire black hole.
 09:09: Not from specific points on the event horizon.
 09:12: In fact, an observer in freefall through the horizon sees nothing.
 09:28: When you turn on your jet pack and hover a fixed distance above the horizon, then you do see particles.
 10:17: ... radiation by thinking about particles escaping from beneath the event horizon through quantum ...
 10:32: ... that we'll want on the same world line becoming separated by the event horizon. ...


20180124: The End of the Habitable Zone
 11:00: Existenceisillusion asked about the nature of the thermal particles produced in horizon radiation.
 11:09: Actually, all quantum fields are affected by the presence of a space time horizon.
 11:56: If the event horizon swallows one half of a virtual matter antimatter pair, then how does the black hole lose mass?


20180117: Horizon Radiation
 00:54: But relativity can throw some weirder things into the mix, specifically the idea of a horizon.
 01:01: ... example, the event horizon of a black hole, out of which no information can travel or the ...
 01:11: And a strange type of apparent event horizon even appears when we accelerate.
 01:17: Generally speaking, a horizon is a boundary in space time from beyond which no influence can pass.
 01:29: As weird as the space times with horizons may be, the statement from before holds.
 03:55: ... when an observer who sees a horizon tries to write down these equations, in order to preserve the laws of ...
 09:46: OK, so what happens when we add a horizon to our infinite quantum field?
 10:18: If we introduce an event horizon, then we lose access to some momentum modes, while other modes behave very differently.
 10:26: ... to create and annihilate the same particles as we had in an infinite, horizonless ...
 11:04: So you can be that horizon's drum skin to produce oscillations that are consistent with the infinite, unhorizoned skin.
 11:23: It appears to be bathed in thermal particles particles that don't exist for an observer who doesn't see that horizon.
 11:53: So this year, keep your eye on the horizon the event horizon and the strange things it does to the quantum contents of space time.


20171122: Suicide Space Robots
 08:33: Pioneer 10 and 11, Voyager 1 and 2, and New Horizons are all on trajectories that will fling them into interstellar space.


20171025: The Missing Mass Mystery
 12:21: ... control faster than the speed of light, can't they escape the event horizon of black holes via Hawking ...


20171004: When Quasars Collide STJC
 01:49: The largest have event horizons that would envelop most of our solar system.


20170920: The Future of Space Telescopes
 08:19: However, it would be able to see xrays right down to the event horizons of super massive black holes in distant galaxies.
 12:42: ... stars don't manage to stop before the core is smaller than its own event horizon, forming a black ...
 12:56: So it's reasonable to imagine that neutron starlike conditions exist extremely briefly before the event horizon forms.


20170830: White Holes
 01:31: ... of inward flowing space time with a one way boundary called the event horizon, from inside of which nothing can ever ...
 01:45: It also has an event horizon, but that horizon prohibits entry, not exit.
 03:03: We've talked quite a bit about the bizarre behavior of space and especially time at and below the event horizon of a black hole.
 03:24: ... perspective of an outside observer, any events occurring at the event horizon, including folding into it, happen infinitely far in the ...
 05:17: If we place an eternal black hole far to the left, then the future left boundary represents the black hole's event horizon.
 05:26: Any movement the left brings you closer to that event horizon.
 05:30: The event horizon itself is a 45 degree line.
 05:39: ... at that 45 degree angle takes infinite time to escape the event horizon, and the region beyond that line represents the interior of the black ...
 06:32: ... instead of flowing towards the singularity, it flows away, and the event horizon is now a barrier to entry, not to ...
 06:51: Imagine that something in our past was traveling at the speed of light and trying to reach the past event horizon.
 07:03: Oh, it'll reach an event horizon, but only the event horizon of our future, where it plunges into a regular old black hole.
 07:13: Remember that all of this is from our perspective far from the event horizon.
 07:17: We can never see anything cross the horizon.
 07:30: ... the past region of the eternal black hole has an event horizon that's a barrier to entry, but also light rays within that region must ...
 08:11: The past singularity and past event horizon are infinitely far in the past from our point of view.


20170816: Extraterrestrial Superstorms
 08:48: This is perfect for measuring cloud heights and also to record horizontal cloud drift, which gives us wind speeds.
 12:59: Now, draw some horizontal lines.


20170810: The OneElectron Universe
 10:59: ... universe are drifting very slightly towards a point beyond the cosmic horizon. ...


20170802: Dark Flow
 01:05: ... may be moving ever so slightly towards the same point beyond the cosmic horizon. ...
 07:11: In fact, it's toward something beyond the cosmic horizon in the same direction as these constellations.
 08:38: Things that are now beyond the cosmic horizon were close enough to affect us gravitationally.
 08:58: ... if, somewhere beyond our cosmic horizon, there exists a region of much more stuff, a different bubble of ...
 09:40: ... influence of a neighboring region of the greater universe, beyond the horizon of observable ...


20170405: Telescopes on the Moon
 10:30: Last week on "Space Time," we talked about time space, the way time and space, which rolls, beneath the event horizon of a black hole.
 10:52: You could transform the gravitational potential energy of things falling towards the event horizon into usable energy.


20170329: How Time Becomes Space Inside a Black Hole
 00:06: ... and time that occurs in the mathematics when we drop below the event horizon of a black ...
 03:14: That rs is the Schwarzschild radius, the radius of the event horizon.
 03:20: Very far from the event horizon, the Schwarzschild interval becomes the good old Minkowski interval.
 03:29: ... if an object gets close to the event horizon, so r just a little bit bigger than rs, that stuff in the two brackets ...
 03:41: But as long as you are outside the event horizon, time behaves itself mostly.
 03:56: Things change radically below the event horizon when r gets smaller than rs.
 04:10: Below the event horizon, there is only one way to maintain the respectable causal progression expected of a wellmannered temporal entity.
 06:12: As you approach the event horizon of a black hole, more and more light rays are turned towards the event horizon.
 06:57: This entire diagonal line represents the event horizon.
 07:06: Our entire future light cone encompasses more and more of the event horizon.
 07:20: Meanwhile our past light cone now encompasses light that has been struggling to escape from just above the event horizon since the distant past.
 07:30: But we still see nothing from below the horizon.
 07:34: Yet as soon as we passed the horizon, everything changes.
 07:49: At the moment of crossing, light rays from the event horizon itself are suddenly visible.
 08:20: ... first formed the black hole, emitted long before we entered the event horizon. ...
 08:48: This is light that entered the event horizon after we did and appears to reach us from above.
 09:47: Below the event horizon, there's still a sense of spatial upness and downness.
 10:28: In fact the Schwarzschild metric really gives two separate spacetime maps in a single equation, one for above and one for below the event horizon.


20170202: The Geometry of Causality
 00:32: Recently, we've been talking about the weirdness of spacetime in the vicinity of a black hole's event horizon.
 00:38: Very soon, we'll be dropping below that horizon to peer at the interior of the black hole.
 10:23: ... to some incredible predictions when we try to calculate the sub event horizon interval of ...
 12:22: One idea is that the inside of an event horizon is composed of a ball of raw strings, a socalled fuzzball, and that no infinite density exists.


20170125: Why Quasars are so Awesome
 04:43: However, a lot of it never makes it below the event horizon.


20170119: The Phantom Singularity
 02:39: An example of a frame dependent singularity that might be familiar to space time viewers is the event horizon of the black hole.
 07:51: If you haven't guessed, this bad behavior corresponds to the event horizon.
 08:00: Imagine an object sitting at the event horizon but not moving, so its delta r would be 0.
 08:12: The entire space time interval for a nonmoving point at the event horizon is 0.
 08:26: So does that mean time doesn't pass for an object hovering at the event horizon?
 08:33: Time certainly doesn't pass at the event horizon.
 08:40: ... the prohibition against objects experiencing time at the event horizon is actually a prohibition against objects spending time at the event ...
 08:59: At the event horizon the only way to get a nonzero space time interval is to have a nonzero delta r.
 09:08: An object at the event horizon has to change its distance from the black hole to keep its clock ticking.
 09:14: That means falling below the event horizon.
 09:42: ... a photon exists in a single instant, and so it can hang out at the event horizon, which also only exists at 1 infinitely stretched out ...
 09:54: The act of crossing the event horizon is where this singularity really starts to behave badly.
 10:13: ... and delta r, doesn't let us trace a world line smoothly across the event horizon. ...
 10:27: That horizon is a coordinate singularity, just like Kelsey talked about.
 10:55: Anyway the upshot is that it's really a breeze to drop through the event horizon, both physically and mathematically.
 11:03: Of course, once inside the event horizon, we still have that central singularity to deal with.
 11:34: ... theory, we have to go back to that coordinate shift at the event horizon. ...


20170104: How to See Black Holes + Kugelblitz Challenge Answer
 02:49: The Event Horizon Telescope is right now in the process of mapping space around the Milky Way's Sag A star black hole.
 03:36: Now, the actual event horizon is even smaller than this insane resolution, but EHT isn't finished yet.
 03:43: ... it will actually see the dark circular shadow of the Sag A star event horizon. ...
 04:28: ... the Event Horizon Telescope and microlensing studies, and of course, more LIGO ...
 05:12: It has enough energy to produce a black hole with a mass of 100,000 suns and an event horizon that almost reaches the moon's orbit.
 06:37: Eventually, it gets small enough for the event horizon to form.
 06:41: On the Penrose diagram, that horizon extends both forwards and backwards in time.
 06:46: This is because there are regions of the universe that are doomed to end up in the singularity even before the true event horizon forms.
 07:25: ... in a volume smaller than its own Swarzschild radius, an event horizon forms as spacetime flows faster than the speed of light towards that ...
 07:53: Even after the true event horizon forms, there remains this shrinking patch of normal flat spacetime.
 08:15: We generate a perfectly reflective sphere at approximately half the radius of the eventual event horizon.
 08:24: The light shell passes the moon's orbit, and a true event horizon forms.
 08:51: See, once the event horizon forms, all paths below it lead to that singularity, even outgoing light paths.
 09:01: ... that's seen as the future light cone of everything below the event horizon leading to the singularity, even of the reflected light ...
 09:19: As long as the event horizon has not yet formed, the incoming light can be stopped.
 09:39: Just above the sphere, which is only a bit larger than that event horizon that was going to form, the spacetime curvature is pretty insane.
 09:53: ... of the near Kugelblitz to appreciably increase the size of the event horizon. ...


20161221: Have They Seen Us?
 13:18: A couple of weeks ago, we looked back into black holes and studied the nature of the event horizon.
 13:27: Luca$sino asks what we would really see if we followed the monkey directly through the event horizon?
 13:33: Would it appear ahead of us despite previously appearing frozen on the event horizon?
 13:41: The light from the instant of the monkey's crossing of the event horizon is eternally trapped at that horizon.
 14:57: ... light emitted by everything that fell into it as it crossed the event horizon. ...
 15:49: The event horizon itself just wouldn't apply within your warp bubble.
 16:31: ... the event horizon, you need to travel at the speed of light relative to the black hole's ...
 16:49: ... happen to get exactly the right size for the event horizon if you use Newtonian gravity to calculate the distance at which the ...
 17:06: But the concept of escape velocity at the event horizon is still as meaningful as it is from the surface of the Earth.
 17:33: So once inside the horizon, avoiding a singularity is like avoiding next Tuesday.


20161214: Escape The Kugelblitz Challenge
 00:14: ... infinitely stretched spacetime in the vicinity of a black hole's event horizon. ...
 02:11: If the star's core collapses to a size smaller than its own Schwarchild radius, then the event horizon forms, engulfing what's left of the star.
 02:21: Below that horizon, but above the stillshrinking surface of the star, spacetime takes on the mad properties of the black hole interior.
 02:40: Outside the black hole, the event horizon becomes the new edge of the universe on our Penrose diagram.
 02:47: The shape of spacetime outside the horizon warps to make this diagonal line, a line of constant radius, the radius of the new black hole.
 03:09: There's a region where all forward light cones only include the singularity, even before the true event horizon forms.
 03:18: For anything in this region, there isn't enough time to clear the impending event horizon, even traveling at the speed of light.
 03:27: On the Penrose diagram, we should extend our effective event horizon backwards to include that space.
 03:34: This invisible horizon of doom grows as the star shrinks and finally merges with the true event horizon.
 03:41: Any observers within this extended event horizon are cut off from any future causal connection with the rest of the universe.
 04:59: So the kugelblitz event horizon forms just after the shell of light passes the moon.
 05:33: Space inside the collapsing kugelblitz would remain comfortably flat until the collapsing shell overtakes it, even after the event horizon forms.


20161208: What Happens at the Event Horizon?
 00:06: At the event horizon of the black hole, space and time are fundamentally changed.
 00:16: But there is a powerful tool in physics that can give us real intuition into the true nature of the event horizon.
 00:29: Black holes, objects with densities so high that there's this region, the event horizon, where the escape velocity reaches the speed of light.
 00:38: ... that falls below the event horizon can never escape and is lost to the universe forever while we see ...
 00:50: And anything that happens below the event horizon stays below the event horizon.
 01:38: For example, are objects falling through the event horizon really physically frozen there from the point of view of the outside universe?
 01:48: Would you see the entire future history of the universe playing fast forward at the instant that you crossed the event horizon?
 02:08: It's a special type of spacetime diagram designed to clarify the nature of horizons.
 03:10: It has a point of infinite density, the singularity, and an event horizon a bit further out.
 03:16: ... time so that light rays appear to crawl out of the vicinity of the event horizon before escaping to flat spacetime, no longer following 45 degree ...
 03:32: As it approaches the event horizon, its future light cone bends towards the black hole as fewer and fewer of its possible trajectories lead away.
 03:42: Below the event horizon, all possible trajectories lead towards the singularity.
 04:48: ... lines represent fixed locations in one dimension of space and red horizontalish lines are fixed moments in ...
 05:51: Its event horizon becomes the end of the line in that direction.
 05:56: The future cosmic horizon on the Penrose diagram is replaced with a plunge into a black hole.
 06:03: The compactified grid lines there now represent the stretched spacetime near the event horizon.
 06:41: Once you're beneath the horizon, your future light cone still represents all possible paths that you could take.
 07:25: The progress of the monkey appears to slow to a halt very close to the event horizon, and the final signal at the moment of crossing never reaches us.
 07:45: First, what would happen if the monkey remembered to fire its jet pack at the last instant before reaching the event horizon?
 08:15: ... is probably doomed to a graceful reverse swan dive through the event horizon, watching the entire future history of the universe play out above it at ...
 08:54: ... it could instead hover above the event horizon, then it would see the universe in fast forward, although that view would ...
 09:04: Watching the monkey frozen on the event horizon is going to make us feel a bit guilty, after a while.
 09:24: ... would see it suspended above the horizon as we're racing to meet it, but it will always appear to be just a ...
 09:35: ... the monkey isn't actually above the horizon for infinite time, it only appears that way to us because as long as ...
 09:50: In order to see that crossing, we would have to cross the event horizon ourselves.


20161116: Strange Stars
 12:12: ... also not see the triangle curve over in the sense that there would be no horizon. ...


20161012: Black Holes from the Dawn of Time
 09:28: A billionton black hole has an event horizon around the size of a proton, so it would pass through the planet as though the Earth were made of air.


20160929: Life on Europa?
 09:45: ... a verticallyaligned measurement device and a second particle with a horizontallyaligned ...


20160921: Quantum Entanglement and the Great BohrEinstein Debate
 05:43: If we measure horizontally, it will be left or right.


20160615: The Strange Universe of Gravitational Lensing
 08:00: The lightspeed flow of spacetime at the event horizon results in old light paths pointing inwards.
 08:10: Light falling below the event horizon is lost forever, so we don't describe it as being lensed.
 08:16: But just outside the event horizon, we find the most extreme gravitational lensing in the universe.
 08:23: The photon sphere hovers at about half, again, the height of the event horizon.


20160413: Will the Universe Expand Forever?
 08:14: Over billions of years, they will slip beyond our cosmic horizon, leaving only darkness beyond the local region of the Milky Way.


20160406: We Are Star Stuff
 12:35: It's happening right now in regions of the universe beyond what we call the Hubble Horizon, which is 13.7 billion light years away.
 12:44: It's happening below the event horizon of black holes.


20160323: How Cosmic Inflation Flattened the Universe
 01:19: This smoothness of the CMB is called the horizon problem.
 01:25: The horizon problem isn't the only troubling feature of the CMB.
 04:16: This flatness problem is just as much of an issue as the horizon problem.
 05:58: ... neatness with which this inflation solves both the horizon and flatness problems really has most cosmologists thinking that ...
 13:05: However, the shape of the event horizon of the black hole itself does depend on spin.


20160316: Why is the Earth Round and the Milky Way Flat?
 12:45: It has always come from our cosmic horizon, but that cosmic horizon expands.


20160302: What’s Wrong With the Big Bang Theory?
 07:05: Another way to say this is that those edges of the universe have always been beyond each other's particle horizons.
 07:13: Here's a nice review episode on cosmic horizons.
 07:25: This serious issue is called the horizon problem.
 08:28: This whole idea fixes our horizon problem.
 11:22: ... before the word "universe." Everything that we can see to our cosmic horizon, so the observable universe, was once compacted into something smaller ...


20160211: LIGO's First Detection of Gravitational Waves!
 05:49: The signals are expected to carry information about the strange warping of space in the region of the event horizon.


20160127: The Origin of Matter and Time
 05:41: So what this means is that there's no single preferred vertical time axis, or indeed, horizontal space axis.


20151216: The Higgs Mechanism Explained
 07:46: AFastidiousCuber wants to know how a black hole can grow if anything falling into it appears to freeze before it crosses the event horizon.
 07:55: ... although an outside observer can never witness anything cross the event horizon, as something falls to the horizon, the light it emits is red shifted ...
 08:06: ... infalling stuff does vanish, and the event horizon that an outside observer sees does grow because anything falling into ...
 08:21: Casterverus would like to know if the potential event horizon that we talk about is the same thing as the Schwarzschild radius.
 08:29: The Schwarzschild radius is the radius of the event horizon of a nonrotating black, and it depends on the mass.


20151209: How to Build a Black Hole
 07:58: And yet, below the star's surface, their lurks the potential event horizon, the surface of infinite time dilation.
 08:05: Now, the event horizon doesn't actually exist as long as the neutron star stays larger than the would be horizon.
 08:12: However, if we can increase the mass of the neutron star, the actual star shrinks, and the event horizon expands.
 08:22: There's a mass where the radius of the neutron star and the event horizon overlap.
 08:28: At this point, the event horizon actually comes into being.
 08:38: But what happens to the star when it slips below its event horizon?
 09:46: But on our timeline, nothing ever happens beyond the event horizon again.
 09:54: Beneath the event horizon, there is no corresponding now.
 10:10: To us, there is only the event horizon.


20151028: Is The Alcubierre Warp Drive Possible?
 01:51: ... below the event horizon of a black hole, spacetime cascades towards the central singularity ...


20151007: The Speed of Light is NOT About Light
 11:17: RedomaxRedomax asks what you would see if you traveled 18 times the distance to the particle horizon to come back to where you started.
 11:26: ... that number, 18 times the particle horizon, only applies if the universe has positive curvature, making it a ...


20150930: What Happens At The Edge Of The Universe?
 01:47: We call this the particle horizon of the universe.
 01:56: Anything inside the particle horizon is referred to as the known universe.
 02:33: To travel to the particle horizon, we need to move through expanding space.
 02:51: Just as black holes have event horizons, so too do universes.
 02:55: ... event horizon of a black hole is that point beyond which we can never receive ...
 03:23: We can ever get to anything beyond the cosmic event horizon because that space will be moving away from us faster than light before we reach it.
 03:33: The event horizon of the universe is actually closer to us than the particle horizon.
 03:37: Given our best measurements of cosmological parameters, we think that the cosmic event horizon is around 16 billion light years away.
 03:56: As our universe expands, more and more of it will cross the event horizon and eventually almost all of that will be lost to it forever.
 04:22: But for now, let's just assume we have a nice Alcubierreclass warpship and we burn the mass energy of entire stars to chase the particle horizon.
 04:36: Remember, the particle horizon is just defined by the limit of our current view.
 04:43: Wait a minute, and my particle horizon expands.
 04:46: Travel to the particle horizon instantaneously and you'll see the Milky Way as a cute baby CMB blob on your new particle horizon.
 05:37: So if it's true, what happens if you cross the particle horizon?
 06:46: ... to travel an absolute minimum of 18 times the distance to the particle horizon to get back to where you started, assuming expansion froze for the whole ...


20150923: Does Dark Matter BREAK Physics?
 07:48: ... and others asked whether a monkey falling through a black hole's event horizon should see the entire future history of the universe happen in the ...
 08:02: ... would approach infinity as the monkey drew extremely close to the event horizon. ...
 08:59: ... that a freefalling monkey is eternally frozen on the event horizon with respect to a distant observer, shouldn't the black hole evaporate ...
 09:14: ... see anything weird about space at the moment it crosses the event horizon. ...
 09:30: The idea of the event horizon as this boiling hot sea of Hawking radiation isn't right.
 09:43: So at the instant of crossing the event horizon, the monkey is not actually bathing in this stuff.
 09:54: ... falling through the event horizon, the monkey's clock, its universe, now contains events that happen at the ...
 10:03: The monkey's horizon crossing corresponds to a time when the black hole exists.
 10:13: And with that radiation comes all of the remaining photons that the monkey emitted before crossing the horizon.
 10:18: But the moment of horizon crossing is never seen.


20150819: Do Events Inside Black Holes Happen?
 04:25: OK, this final batch of events for all objects that enter that black void taken together is called the event horizon of the black hole.
 04:34: The horizon is not just a spherical surface in space.
 06:48: As long as he stays outside the horizon, he can use rockets to hover or move radially outward just like on Earth.
 08:20: That means that to external observers, black holes are black because light that gets emitted just outside the horizon is redshifted into invisibility.
 08:37: ... you know that it's the black hole mass divided by the volume inside the horizon, then ...
 09:00: By the way, bigger black holes also have smaller tidal effects near their horizons.
 09:48: However, the horizon forms first in the interior of the star and then expands.
 09:52: So to external observers, most of the matter never crosses the horizon.
 10:05: ... black hole that didn't form from anything, a spacetime that has an event horizon even though there's no stuff anywhere, including behind the ...


20150715: Can You Trust Your Eyes in Spacetime?
 04:18: ... on either diagram, I could also have drawn lines that are more horizontal than 45 degrees, but they wouldn't be world lines because to be present ...


20150603: Is Gravity An Illusion?
 02:11: Relative to the car's interior, you will accelerate backward, even though you can't identify any horizontal forces on you.


20150408: Could You Fart Your Way to the Moon?
 06:54: ... fact, far from their horizon, which in this case would be just centimeters since the horizon is ...


20150401: Is the Moon in Majora’s Mask a Black Hole?
 03:50: And I put some lower limits on the radius of the planet, based on the fact that we don't see its horizon curving in the distance.
