What Happens To Quantum Information Inside A Black Hole? 

It's comforting to know that ancient PBS Space Time videos will still be around even when Sag A* meets its end.

I love the idea that a future mind-uploading device requires you to breakdance on top of it while it uploads you.

"Digital Ghosts in a Dead Universe" would make a great metal album name

10:36 "(Bob) is looking for signs of the experimental qubit. But if we're being honest, he's also looking for signs of Alice." awwww, that's so cute

Alice and Bob are 2 unsung heroes of our time. They have been involved in so many experiments I can't ever count. The have been thrown into black holes, left drifting on space for years, put into the heart of a supernova, they have even been shot through the universe at the speed of light and left to experience all the negative affects from it. And they never ever complain.

Oh man, you got Dereked!

"Bob waited 10^87 years, so a week isn't so bad. " I'm now dead in this timeline. 🤣

Veritasium feels like a layman successfully reaching into the depths of science and bringing it back to us. PBS spacetime feels like a lifelong deep scientist successfully reaching out to the layman to explain things.

Veritasium Entered the chat. PBS Spacetime: “I was camping your respawn a few days ago”.

In the three body problem series (book 2) some guy fell into a black hole accidentally and they refused to pay out his life insurance because he wasn‘t dead yet from their point of reference 😂

Pancakification will be a whole new section in our text books before long and you heard it here first, from one Matt O'Dowd.

Degankenexperiment 😂 For the non-germans: he pronounced the first two syllables of Gedankenexperiment (imaginary experiment, thought experiment) backwards - honestly not that noticeable or relevant, but had me smirk, so had to say it

Thanks for the video. I’m currently falling past the event horizon of a black hole and I can confirm that it really do be like this.

I know! You either die or end up behind some bookshelves.

People saying that Veritasium beat Space Time to the punch when literally everything in Derek's video had been covered by Space Time years ago.

Cool! Another black hole video

Alice saw the tesseract and taught bob how to decipher all the qbits

This is without doubt the most engrossing and informative description of both the information paradox and equivalence for the layperson I've ever watched. Thank you Dr O'Dowd, and a special thanks to Alice and Bob for their committment to science.

I don’t really know what you’re talking about in most of this episode, but I do like the way you say it

"What Happens If You Jump Into A Black Hole?" ....To shreds you say?

This is by far the best love story I've ever witnessed on this channel.

The animations are of impressive quality for a free educational video on RU-vid, thank you PBS.

_"Spinning, whirling_ _Still descending.._ _Like a spiral sea_ *_UNENDING!!!"_* .. Rush, _Cygnus X-1_

This video demonstrates why PBS Space Time is the best science podcast in the known multiverse! Super clear presentation and analysis that makes the most difficult concepts accessible to even us non-Phd mortals!

I predict that we won't be able to decrypt the messages sent between Alice and Bob.

I just can't watch enough videos about black holes. The most fascinating thing in space.

Me: I'm smart! *Watches video* Me: I'm dumb!

I propose after the geDunkin experiment is finished we all go out for geDonuts. If we had completed a gedanken experiment instead we would all go out for fries because die gedanken sind frei.

- Being up at 4am for no reason - Sees new spacetime video notification

I'm gonna compress all the videos I've watched, that explain this "experience", into a single video from which none of the constituent videos can escape or be decoded. I love black holes. (This is the most thorough of them all; I even learned a few things.)

I’ve watched most of the PBS spacetime episodes over the years and I can say this is my favorite one. Well done PBS spacetime!

LOVING the editing and overall production on the newest episodes

3:55 Thank you! For years I've wondered how to reconcile time freezing at the event horizon with actually crossing the horizon. How could anything cross if time slows down and ultimately "stops"? Wouldn't Alice see the whole future history of the Universe flash by, including seeing the black hole evaporate before she actually falls into it? But I think this video's explanation did a good job putting these concerns to rest :)

I really appreciate all of the clever visuals

Jumping into a black hole is the secret recipe to making the best spaghetti in the universe.

Fantastic video once more. This channel perfectly fills the niche between more layman-oriented science shows like Veritasium, and full-blown physics lectures. ❤

For what it's worth, Alice and Bob are also the heroes of every computer security diagram. Their adventures are not limited to the textbooks of physics, computer science uses them too.

Please make a episode about colliding blackholes and time dilation! You are saying object is like "freasing" at the event horizon. So will we see how those blackholes approach each other and eventually collide or not? Please explain! 🙏

I just want to say that although the channel is a joy anyway, I completely love these deep dive sequences. Good work, people.

I do love these sorts of thought experiments and your presentation on this one was a lot of fun to follow.

Alright, we know about Alice and Bob. But this still doesn't answer the most important question of all: Where is Waldo?

I just watched new Veritasium video about black/white holes. What a time to be alive.

This was a brilliant presentation and quite thorough, very impressive job.

Excellent video Matt, keep the hard work.

Cool! Another black hole video!

I was worried about Bob until the very end, glad it worked out in the end

Fantastic video. Thank you. Congrats!

Starting to see the vision for this arc of content, excited to see it all come together.

As another merch idea, that black hole pin design would make an awesome phone case.

There is a reasonably straightforward resolution to this paradox, but it requires that we treat the horizon as a true horizon, not a coordinate horizon - and this seems logical if we consider the effect of length contraction on Alice's perspective of her own fall. She is after all, *approaching the speed of light* as she approaches the horizon, and her perspective of the black hole (if she could see through it) is that its depth is very rapidly diminishing, and as she gets even closer its width should likewise begin to contract rapidly until the entire horizon becomes a POINT OBJECT towards which she is falling. By the time she 'reaches' the horizon, the diameter of the entire event horizon should only be a single Plank Length - the outer horizon has now *become* a singularity from Alice's perspective. All black holes have the same diameter of one Plank Length, regardless of their mass, though from a more distant perspective their Area and apparent diameter reflects their mass. Their density does NOT drop as they grow - they achieve Plank Density as the Beckenstein Bound is saturated, and their Area expands in lockstep with their mass to maintain Plank Density no matter how large or small they are. Whether a black hole is one angstrom wide, or a LY across, it has the same density, because it is now effectively a 2D sphere and its 'density' as a 2D object does not change as you add mass. Unfortunately the implication this has for the tidal forces of the black hole are somewhat dire for Alice. She will be spaghettified because she is not approaching a black hole with a diameter of billions of km - she's approaching a black hole with a diameter of one plank length. She will then be pancaked as well, but she will NOT fall through a causal barrier where Bob can no longer observe her at all because she perceives no horizon at all, nor any space to fall 'into'. Alas Bob will receive very little information about her due to the time dilation she's experiencing in this 2-dimensional pseudo-singularity, which presumably will evaporate gradually via thermodynamic processes similar to how hawking radiation is currently described. He'll still have to go through the process of recording the Black Hole's entire evaporation process and reconstruct Alice's data, because she's been scrambled about as thoroughly as anything CAN be scrambled - reflecting the maximal entropy of the black hole. Some interpretation similar to this is necessary to prevent violation of the Beckenstein Bound and prevent the formation of a true physical singularity of infinite density. In this case the surface of the black hole consists of a plank density shell of mass, with each element of it probabilistically 'smeared' across the entire horizon, as from the perspective of each particle they are in fact falling towards a singularity - though more accurately, they will see that singularity explode in their faces as they fall due to the fact that their perspective of the approach to the horizon now extends into the unimaginably distant future all the way to the point where they are re-emitted in very little pieces, their 'fall' concludes with them being blasted out into space and the infalling observer's experience would be far more akin to taking a nose-dive into a supernova than falling into a placid void. Why do I think this is the case? I mean, quite frankly it's because I don't believe that beckenstein bound violation is acceptable, nor are physical infinities, and EVERY model of a black hole interior I've ever seen postulated clearly violates the Beckenstein Bound and describes a physical infinity - while this model prevents that. That's the entire reason. Make of it what you will.

I’ve ordered the black hole pin from your store! I happened to be shopping for pins today and wouldn’t you know it, one of my favorite channels uploaded an excellent new video and they happen to have a new pin depicting one of my favorite objects! How serendipitous for us both!

I can't wait for next week!

My intuition tells me that the quantum bit (and Alice, for that matter) isn't duplicated or deleted, but it being in the blackhole and emitted in Hawking radiation are different moments in time. I'm sure it's a gross oversimplification and me not fully understanding the math and physics, but i can't shake the feeling either. Guess I'm waiting like Bob lol Now that i think about it some time later, i wonder what digitized Alice would remember once Bob is done reassembling her. What was her subjective experience of it all? Would she remember falling past the event horizon? I wonder why or why not...

Six stars of the Northern Cross In mourning for their sister's loss In a final flash of glory Nevermore to grace the night - Rush

This was the 1st RU-vid channel I subscribed to and it's still going strong

This video is amazing!❤

i think Alice never got pass the event horizon, falling down she starts to experience such a huge time dilation, that on her time scale the black hole starts to evaporate. I also think because of a such curved space the black hole (or is it the stuff that fell in? i feel like there's no difference, black hole is the stuff that fell into it) will start to glow. in that case black holes are frozen in time (for the outside observers) collapse, and that huge collapse is hidden by Einstein's squeezing of space to very thin slice of what we perceive as the event horizon. The crossable event horizon doesn't exist, instead it's a place where collapse of the black hole is happening, while the singularity is the moment in time to there they fall/when the black hole evaporates. This is the intuition i got from watching these wonderful videos for years

My first thought here was: Wouldn't Alice witness the end of the universe due to time dilation before crossing the event horizon?

This video came out just days after Derek's video and such perfect timing! A whole week of black hole pondering!

Didnt realize the thought experiment about falling into black holes had so many variations and variables based on perspective. My mind engine bes starting up and my thoughts be rumblin to life! Love it when videos do that! :)

Why can't Alice's perspective of falling into the singularity be equivalent to Bob's perspective of the hawking radiation emitted? Ie. the singularity is a projection of the inevitable future of the evaporation of the black hole by Alice's perspective? As I understand it this would make sense with what the spacetime diagram interior to the blackhole tells us?

Now do one about what an infinite universe with infinite mass and infinite energy would look like. PBS infinite series mentioned it in one of their last videos. The Mandelbrot set or another fractal pattern seems like it would immerge is such a situation due to infinite mass and energy requiring infinite black holes. I think the we could be inside of a black hole that is a part of a russian nesting dolls of black holes that is also part of a fractal of black holes and parallel universes and black holes could be the pathway to other universes. It certainly makes more sense that "dark energy"

Thanks for this. I’m planning to jump into a black hole for my birthday and I’m wondering what to expect.

Wow, Matt ending on a cliffhanger for once, what a pleasant surprise. Super cool episode, this is exactly the kind of stuff I wonder about sometimes (on a way lower level than this obviously). Something about the event horizon is so final, it would be a miracle if physics could find a way to peer past it. Maybe in a couple hundred years! or a couple thousand...

Great video as always. But there's something that I've been wondering for a while and still haven't been able to find an answer. Time dilation gets more and more extreme the closer you get to the event horizon as you mentioned, so in this case if Alice were to "hang around" close to the EH for a minute in her own relative time and then managed to get back to Bob far from it a way longer time would have elapsed for him and the "universe outside". That I understand. But the problem is about her perspective as she goes through the event horizon. Wouldn't time dilation tend to infinite as she gets closer and closer? If so, as soon as she were to cross it, her relative time dilation would be so extreme that any amount time she experienced inside the EH would be equivalent to a tremendous amount of time "outside"(maybe infinite?). And if so any time she spent there would mean that enough time outside had passed that the black hole would've evaporated, so she wouldn't ever be able to actually experience what it's like to be inside one, as all the space inside would get instantly smaller and she'd be crushed against the singularity before just "popping out of existence". This has been leading to so many sleepless nights for me 😅. Please tell me what I've been doing wrong or missing. Thanks!

Love that you’re covering this topic. For most of my adult life I’ve pondered what would happen if you we’re to dive into a black hole (and survive the tidal forces/radiation etc). I came to the conclusion that, theoretically, you’d never reach the “singularity”. By the time you reached the point in space that could be considered a singularity, the black hole would have dissipated and you’d find yourself back in normal space, albeit a few thousand billion years in the future.

these black hole related episodes are my absolute favorite! 🖤

Outstanding Dr. Matt.

How freaky-cool is it , that the event horizon is a "border" between now & the literal Far far far future. 😁🌏☮️

This episode smells of Leonard Susskind.

Nice explanation 💪🏻

Massively interesting episode!

MUUUURPH

"....Bob can do this, because he has nothing but time on his hands." Bob should have thought of that before he dropped Alice into an infinitely endless expanse of black nothing with a singularity at its center.

Great episode, feels like I am waiting near the event horizon for next week an eternity from my perspective.

Great job 👍👍 makes sense why it's been a wait, but worth it

General relativity and quantum mechanics will never be combined until we realize that each individual observer is observing them both at different moments in time. Because causality has a speed limit (c) every point in space where one observes it from will be the closest to the present moment. When one looks out into the universe they see the past which is made of particles (GR). When one tries to look at smaller and smaller sizes and distances, they are actually looking closer and closer to the present moment (QM). The wave property of particles appears when we start trying to predict the future of that particle. It is a probability wave because the future is probabilistic. Wave function collapse is what we perceive as the present moment and is what divides the past from the future. GR is making measurements in the observed past and therefore, predictable. It can predict the future but only from information collected from the past. QM is attempting to make measurements of the unobserved future and therefore, unpredictable. Only once a particle interacts with the present moment does it become predictable. This is an observational interpretation of the mathematics we currently use based on the limited perspective we have with the experiments we choose to observe the universe with.

it's spaghetti time!

Damn! What a cliffhanger😧 can't wait for the next episode 👏

Matt thankyou so much! I have been dying for this series for years now! You are spot on! Cant wait for Bob and Alice to have a couple of blackhole mass sized piles of bell pairs 😉 I am AMPS…I mean AMPED for the next one!

I'm confused. It seems to me like there is a simpler contradiction driving this weirdness, present without anything quantum (except Hawking radiation): In Bob's frame of reference, Alice takes infinite time to cross the event horizon. Also in Bob's frame, the black hole Hawking evaporates in finite time. So, Alice never crosses the horizon in Bob's frame. If we then ASSUME Alice crossed in her own frame, then of course the frames contradict. I can't do the necessary math, but intuitively, Alice should never be able to cross, even in her own frame of reference. Time dilation would make it so the horizon shrinks away from her (via Hawking evaporation) before she reaches it. The outro (15:28) says Alice not crossing would break the equivalence principle, so my question is, how? (Is this the "firewall at the horizon?")

0:11 It's "Gedanken," not deganken or whatever that was

Love the breakdancing mind copier 👌

Good follow-up to the episode of Star Talk from a couple weeks ago!

wheres the new video dog

You die, saved you 20 minutes

this was such a great video

Fantastic episode, Matt! Thank you!!

Am I first?!

Thank for the video 🎉

6:57 Top notch animations (and I mean this both sarcastically AND sincerely)

There are a few things I don't understand : -If time accelerates to basically infinity when you get close to the event horizon, then shouldn't the black hole have evaporated entirely before you reach its horizon ? -If photons from the event horizon appear to have more and more energy as Alice accelerates towards the black hole, shouldn't she be disintegrated before reaching it ? -Why can light inside the black hole go towards its outside (like the light from the cube going towards Alice's eyes when both have crossed the horizon) ? Shouldn't things only be able to move towards the center of the black hole ? It feels contradictory because if things can't get out of the event horizon, then it should be even worse when you get closer to the center, right ?

Awesome! :D Great episode. I have 2 questions: 1. I thought it was impossible to fly **directly** into a black hole? I was of the understanding that black holes are all “frame dragging” the space around itself in a sort of 3D whirlpool. 2. The fact that photons have to “struggle” out of the gravitational well was covered. So, shouldn’t Alice’s image have been red-shifted until she disappeared as she crossed the event horizon? (**ASIDE:** BTW, not trying to nitpick! I just want to know if stuff I’ve learned elsewhere still holds. I know these _may_ have been omitted either by oversight or because it would needlessly overcomplicate the thought experiment.)

Thank you 🙏

Thank you for talking me out of jumping into a black hole.

I liked both this and Veritasium's video. But I really liked this video's analogy!

Thank you for the great material - it kept me busy for a few days :) In my opinion, there is no paradox if we assume that Alice never crossed the event horizon. The paradox arises from extending the equivalence principle to areas we should never reach. It's like in Einstein's thought experiment, pondering too long whether we are falling in an elevator or floating in zero gravity - reaching the Earth's surface will solve the puzzle. Alice will perish in a fraction of a second, just before reaching the event horizon. In a sense, she will collide with matter suspended in 'timelessness' that forms the history of the BH, constantly falling but never reaching the event horizon. This will only happen if Bob starts the colossal task of shielding Sagittarius A* with a screen cooled almost to 0 K, shielding background radiation and other radiation sources quickly enough. He knows that if he doesn't do this quickly enough, from Alice's perspective approaching the EH will cause the flow of time of the universe she leaves by falling into the BH to accelerate beyond imagination - thus her back will absorb massive amounts of background radiation photons, which falling in the BH's gravitational potential will increase in intensity and energy as they approach the EH. I imagine that the EH is a mathematical construct where spacetime reaches the speed of light. Beyond this space, we lose the ability to interact. From the perspective of a distant observer, how can anything cross the boundary if time stops? In my view, as long as the falling object can somehow interact with the universe, it 'must wait' until the end of such a relationship before making the move to cross the event horizon - as if the universe with its possible influence on Alice's fate must first be resolved before she could cross the EH. I wonder if in some sense, falling spacetime near the EH does not stretch and millimeter-sized areas in one reference frame cannot be light-years deep, creating a volume for the matter above the EH to solidify - a true hologram. However, the most important thing that would influence the matter would be the gradient of time flow - I imagine that it could generate effects similar to pressure (similar to countless photons on Alice's back), but it is difficult to imagine being in a place where a second passes near the legs while years pass near the head. At the same time, I would like to see if someone has calculated and what the calculations of quantum effects look like for two areas with such different time flows, or if quantum tunneling could not explain the effect of black hole evaporation? Only what I just described is the opposite of the thesis that black holes have no hair... :| The above solution seems not to generate paradoxes, a problem arises elsewhere: we know that BHs can form in finite time, grow, and merge with each other. While the above scenario prevented crossing the EH. Wouldn't it be an elegant solution to assume that only spacetime can overcome the speed of light limits, thus wrapping falling objects near the EH? Additional mass of a box with an electron and Alice at some point in finite time will increase the EH, creating a disturbance spreading at the surface, pushing out the EH surface with submerged matter above it. Similar phenomenon to the surface tension of liquids. Unless, the BH evaporates with Alice before they merge with the EH. I would like to be able to calculate this. :) The disturbance of the event horizon as a result of merging BHs should also emit Hawking radiation - after all, spacetime curvature is its source - it would be nice if we someday had detectors near merging BHs :) It seems that the above description does not generate paradoxes - yet I have devised a thought experiment that still disturbs my well-being: above, I assumed that it is not possible to cross the EH in finite time - it only submerges the matter that would have fallen onto it. However, couldn't we create a black hole in a way that does not require crossing the EH? For example, placing Alice inside a large empty space. Then surround her with an amount of water that would fill the entire solar system. Do this in such a way that the water circulates on certain orbits, preventing too rapid a gravitational collapse and gentle experimental conditions. In such a situation, can we describe the process of BH formation from the center of mass (future singularity;) ) and the immediate cessation of future BH matter on itself, or do the equations indicate that the EH will form outside, cutting off its influence on the universe? Piotr Pyrka :)

Loved the animations

Off all the answers, the firewall is my favorite. Can’t wait to see how it breaks everything!

Regarding the last question about unitarity: My guess is, that unitarity is probably not broken, because the qubit never reaches the singularity. Because of the exponentially increasing time dilation they would travel through time at most until the end of the black holes life, never reaching the point of singularity completely. That would mean antiparticles falling into the black hole via hawking radiation would eventually catch up in the inner horizon with the matter falling in and annihilate each other. Assuming the inner horizon exists and it actually does allow particles to choose a path other than directly into the point (or ring) of singularity, like the Kerr black holes suggests. But that does not explain how the energy created by this, can leave the black hole. But who knows what particles are created when matter and antimatter are colliding at those speeds and what properties they have 😂 Please only take this as speculation and nothing more ^^ Either way Im glad these videos are always so interesting and are leaving room to speculate on how it all works. Keep up the great videos!