Leonard Susskind - Why Black Holes are Astonishing

leonard the concept of black holes is something that common people can talk about today these very strange regions of very high if not infinite density and curvature and residing in the center of many if not all galaxies to a physicist and you've studied black holes why are they so important from a theoretical point of view because they lead to a major puzzle they lead to what i would call a conflict of principles the biggest events in the history of theoretical physics and maybe in physics itself is when principles that you deeply believe clash and when principles clash that's where progress is made i'll give you an example let me give you an example that i like very much and uh as uh as an example of a clash of principles you know the greeks had two different theories of the world one was the theory of the celestial planets stars and according to them the motion of the planets and the stars and everything were governed by perfectly beautiful elegant laws everything moved on perfect circles crystal spheres it was lovely it was beautiful and then there was the terrestrial and the terrestrial was ugly things fall on the floor carts get pulled by horses and if they don't get continue to get pulled they grind to a halt it was corrupt the terrestrial and the celestial two different theories of the world and they didn't coexist that persisted until galileo and galileo put an end to that by one simple observation a thought experiment the thought experiment was you take a rock which is so which is terrestrial an ugly thing and you throw it far into the air until it goes into orbit around the planet and it becomes celestial the terrestrial rock became a celestial body there must be one unified description of both we're in kind of the same situation we have two separate theories of nature which are incompatible apparently and which need to be put together one of them is the theory of the very very small quantum mechanics the quantum mechanics of atoms molecules and so forth the uncertainty principle all these marvelous things that were discovered at the beginning of the 20th century about microscopic physics at the same time physicists einstein the same guy incidentally was also thinking about the very big and the very heavy he was thinking about gravity gravity governs the very big and the very heavy quantum mechanics governs the very small two different domains of parameters two different regimes of the world just like the greeks had okay well some people have the view that there's no need to put them together there's no need to put them together one governs the very big and the very heavy other it's very small why bother looking for a theory which combines them both they're two different domains to the world then you have black holes then you have black holes black holes are galileo's rock that he threw into the air which went which belonged to both regimes black holes are objects which belong to both regimes they're big and they're heavy but they have quantum properties in fact they're very quantum mechanical objects uh the fact that they belong to both domains tells us we have no choice we have to put these two theories together we have to make sense out of them even though they appear to conflict with each other we have no choice because black holes are the entry and just like galileo's rock black holes are the entry into the world that combines them both that to me is why they're so important okay so give me a quick definition of black holes from a physics point of view and then let's see what some of the issues are when you try to combine their quantum mechanics and their general relativity yeah um okay i can of course just tell you the obvious black holes are the most dense concentrated objects that you can make when a star dies it just collapses into a black hole but let me give you a picture of a black hole which i think is more helpful in understanding what's going on it's my favorite picture and probably i probably like it so much because of my former existence as a plumber imagine an infinite big flat lake it's not very deep it's only a foot or two deep it's infinite it goes on in every direction forever forever and ever and of cour and this creature is living in this lake i call them pollywogs and they swim around they communicate they can't see all they can do they're blind all they can do is hear and they communicate with each other they see quote each other by sound and so forth and the lake just sits there perfectly nice healthy environment for them but right at the center of the lake there's a drain pipe and the drain pipe empties out below onto some very sharp and deadly rocks okay the water flows into the drain pipe very very far away the water moves slowly as you get closer and closer to the drain pipe it moves faster and faster and now imagine that the water is being sucked out so fast that at some point around this drain pipe the velocity of the water gets to exceed the speed of sound now remember these guys can only communicate by sound they can't swim faster than sounds all right so there's some place which is some circle around the drainpipe which is a point of of uh no return a point of no return once they pass that point not only can't they swim out but they can't even communicate out sound can't get out they can't get out they're isolated and what's more because the speed of the water exceeds the speed that which they can swim they're doomed they're going to crash on those rocks below that's the end but just think about one of these polywogs floating on its back down the uh down the uh down the drain when it passes the point that they'll return it doesn't there's nothing special there there's no sign post there there's no um crunch that happens to it unknowingly it just drifts past the point of no return and it's doomed okay the rocks at the bottom are the analog of the singularity that's the place where if you fall in you get killed that's the center of the black that's the center of the black hole the horizon is the point of no return you can't see anything inside or here in this case you can't hear anything from the inside if you're outside because sound just can't make it out and in black hole the analogy is light as of course light is of course light that's the nature of a black hole horizon it's a point of no return but it was always thought of as a place where if you drifted meaning fall drift now means fall if you just fell through the horizon you would experience nothing special from the point of view of somebody falling on the other hand from the point of view of somebody outside it's a barrier that nothing ever passes through why do i say that nothing ever passes through it imagine watching the polywog fall through or listening to the polywog fall through remember you can never get a sound from inside the sounds that you hear as the poliwag gets closer and closer to the point of no return take longer and longer to get out because they're fighting against the flow and so you will only hear that polywog passing the point of no return after an infinite amount of time so from your point of view they never fall through looks like it's frozen looks like it's frozen so there's this tension this tension between what is seen from the outside nothing ever falls through the horizon of a black hole on the other hand somebody falling freely through the black hole sees no point to no no sign post no anything and there's a tension a kind of conflict it's not a it's it's not a mathematical conflict it's just something funny going on where we have two different descriptions one from outside the black hole and one from falling through now this got to be a real genuine puzzle when stephen hawking and jacob beckenstein realized that black holes have thermal properties that they have the property of being warm that they glow that they give off radiation and so not only do things fall into black holes but black holes glow give off radiation give off energy and in the process of giving off energy they evaporate it takes a long long long time yeah but we don't worry about that that's plenty it takes a very long time but the theoretical physicists have all the time in the world we can talk about 10 to the 10 to the 10 to the 10 to the 10th years and in time that black hole will disappear and it's through a quantum mechanical effect through quantum mechanical effect through a quantum mechanic that's right it's a quantum mechanical effect so you see coming together quantum mechanics with the theory of the very heavy namely gravity uh so after a long long period of time everything that fell into the black hole has disappeared the black hole has disappeared nothing is left of it on the other hand there's this radiation going out carrying off the energy of the black hole why is this puzzling it's puzzling because it looks like all the information that fell into the black hole disappeared out of our universe and it's a very very basic principle of physics that information is not allowed to be lost what do i mean by information i mean by information the distinction between things are you throwing to the black hole a chicken or you throw into the black hole a duck there's information in the question of whether it's a chicken or a duck the chicken and the duck fall into the black hole the black hole eventually disappears that information is just lost from the world that's a no-no in physics a real no-no information is not allowed to be lost it may get scrambled it may get mixed up it may get hard to discern but the difference between a duck and a chicken is forever even if you chop it up and make chopped liver out of it in principle you can still recover whether it was chicken or duck black holes seem to violate that so they seem to violate some very very very basic principle of physics which i can call a conservation of information that was hawking's view things fall into the black hole it evaporates those things and all the information they carried are gone from the universe that conflicted with 300 years of physics which said that information must never be lost so you can see that was coming together was a clash of principles a meeting of the realms of the very heavy with the realms of quantum mechanics and we're now in the position where we have to reconcile this we have no choice well of course we have a choice we can just go and do something else but the curiosity keeps pushing you in the direction of trying to reconcile these things so there is a basic paradox and a conflict in our in the principles of physics that we don't understand at the moment and that conflict is the laws of quantum mechanics the laws of information the laws of physics as we've known them for 300 years says nothing must ever be lost and that must say that it cannot pass through the horizon but must be radiated back out everything that you throw into a black hole on the other hand everything we know about black holes says that things fall into the black hole and are destroyed at the singularity that is the big conflict that physics is trying to deal with and is trying to reconcile the answer whether things fall into black holes and are destroyed or whether before they actually get to the horizon they're kicked back out and radiated back out seems to have the answer that both are true i will leave you with that

Channel: Closer To Truth

Views: 552,733

Rating: 4.8891797 out of 5

Keywords: closer to truth, robert lawrence kuhn, Why Anything, why black holes are important, Leonard Susskind, Black Holes, dark energy, space and time, understanding black holes, what is a black hole, are black holes real, what do black holes do, what is at the center of the galaxy, Theoretical Physics black holes, philosophy of science, cosmology black holes

Id: d_XuFkVdAYU

Length: 13min 30sec (810 seconds)

Published: Sun Jun 13 2021

Reddit Comments

[u/FluffPudd16]

This man gives me strong John Malkovich vibes

[u/gebmozko]

What I don't get is he says black holes evaporate, therefore any matter once goes in the black hole eventually comes out in the form of radiation. How does that fit with the loss of information theory?

[u/International_XT]

One thing worth noting: you definitely should notice your passage through a black hole's event horizon, even if it's a very big black hole with a huge event horizon radius, simply due to the fact that once you're past the horizon, ANY signal (including the signals sent by your own neurons) can only travel TOWARDS the center of the black hole, never out from it. In other words, say you're passing through the event horizon face-first; now your frontal cortex right beneath your forehead will never again be able to pass signals to your brainstem at the base of your skull. Likewise, your eyes would no longer be able to send signals to the visual cortex at the back of your skull.

It's a quite literal mindfuck.

[u/I-seddit]

Also want to add the Susskind is my favorite author for explaining things - I love his books. He's at the Feynman level, imho.

[u/Explorer928]

Question 1: Is Hawking Radiation a basically mathematical theory that tries to explain why information is not lost when matter falls into a Black Hole, or is it an observed phenomenon for which Hawking has provided a theoretical explanation? Have we actually observed Hawking Radiation?

Question 2: If objects falling into a Black Hole appear to move slower in our frame of reference, and never actually disappear beyond the event horizon, why don't we see a bright mass of matter surrounding every former star that collapsed into a Black Hole?

[u/SquarePegRoundWorld]

I think my favorite part of the video is around the 12:20 mark when he claims we have no choice, then says of course we can do something else but draws a blank on an example of something else to do other than search for the truth being driven by curiosity. I am glad people like him exist.

[u/famschopman]

I wonder what would happen if you let a segment of a large chain or rope enter the black hole and then pull it back. Will it be cut off or return as a whole.

[u/chipirons]

I'm confused. He says the "information" entering the black hole is lost. That it is destroyed in the singularity of the black hole.

And because physics says information cannot be destroyed, we need a [unification] theory for both physics and quantum mechanics.

Yet it seems that information is still in the black hole...where the matter and energy of the collapsed star(s) also is.

So it seems we're unable to locate that fallen information, but it's not really "lost". Right?

What am I missing? Or am I about as dense as a black hole?

[u/Phrankespo]

Something I'm really confused about, is he saying that if I'm outside an event horizon and I'm watching something pass the horizon, that object I'm watching would look like it's frozen there forever? Like if I watch it for 20 years it'll still be there?