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  • Hello and welcome to dark frontiers a

  • conversation about the science of black holes

  • thanks for tuning in i'm lee billings an editor covering space and physics at

  • scientific american and our guest today is Dr. Priya

  • Nadarajan an astrophysicist at Yale university and author of mapping the

  • heavens Hi Priya hi hi everyone

  • now priya is very interested in what we might call the dark side of the

  • universe and i'm not talking about sith versus

  • jedi here i'm talking about all the mysterious

  • things that lurk out there unseen dark matter dark energy and oh yeah

  • black holes so priya again thank you for being here today now before we really

  • get into it i want to do a minor bit of housekeeping for everyone watching

  • uh if anyone in our live audience has trouble hearing

  • or seeing the discussion uh please use the chat function

  • to let us know my colleagues jeff delicio uh sonia buddha and

  • macarena carazosa are standing by to assist you and if you have questions

  • for me or for priya remember this is going to be a q a at the end

  • you can submit them to the organizers using the questions panel in your go to

  • webinar menu we're going to answer as many as we can

  • during that q a session at the end of our conversation and

  • you know speaking of audience participation

  • jeff if we could go ahead and advance the slides one uh we're gonna kick

  • things off with our first poll that's right pop quiz guys um this

  • is one of three that we're going to be doing throughout the presentation uh the

  • first question here you can see is how massive can a black hole become

  • how massive can a black hole become you're going to have about 30 seconds

  • once this kicks in uh and there will be multiple choices

  • and then we're going to read them out we're going to have priya talk about it

  • really quickly they're going to get into the meat so let's go ahead and

  • have this first poll folks we're going to see what happened

  • paul results we have i think everyone maybe can see this but i'll read it out

  • loud hey as massive as hundreds as 100 stars

  • we have four percent saying that b as massive as millions of

  • stars six percent said that see as massive as several billions of

  • stars that's a lot of stars 25 a quarter of you said that and d said

  • no limit black holes can grow indefinitely

  • 65 percent of you said that now priya let's go to the experts priya

  • i want to know which answer is right right lee do you want to take a shot

  • first so i thought the answer was d

  • black holes can grow indefinitely that's kind of what i thought but you know i

  • i've kind of cheated i had a little bit of behind the scenes uh coaching on this

  • apparently priya that's that's not the right answer

  • it's not the right answer okay so in fact um

  • it turns out you know um we did some work about 10 years ago where we showed

  • that from what we understand so far about

  • black holes they actually kind of limit their own masses so they start their own

  • growth eventually so we believe that they can grow up to

  • several billions of solar masses but then eventually stunt their own

  • growth so they don't grow indefinitely because you know

  • they are locked in into the host galaxies that they're sitting in and so

  • it's the interplay of the available gas and what's going on

  • in their larger environment and what they actually do black holes do to

  • impact the environment that they're sort of unnaturally they

  • stunt their growth so a galaxy of a given size ends up having

  • an optimal size black hole and so it

  • doesn't grow indefinitely okay okay i'm trying to imagine some

  • kind of weird scenario where there's you know a big galaxy cluster with

  • hundreds of thousands of galaxies and somehow they all just glommed together

  • and then it all feeds one big central one but

  • but maybe we'll get to that later whether or not that's feasible because

  • we first do that later because what i want to get to right now

  • is actually the first section of our talk we could have the next slide please

  • um we started out just the basics how black holes became real and i want to

  • start really quickly with an anecdote for you priya this is actually based on

  • a real life experience of mine a real friend of mine

  • every single time we get together she knows i'm a science writer and she

  • always has the same question for me which is

  • lee why does anyone think black holes are real they're too

  • ridiculous to exist it's impossible how could they be real they're too

  • radical to exist and i want to i want to raise that because

  • this is actually not that crazy of a question in fact as i understand it and

  • i'd love to hear more from you about this einstein

  • einstein who's whose theories of general relativity and special relativity

  • really inform the basis of understanding of black holes at a fundamental level

  • famously he used his intuition for all kinds of things he imagined he would

  • ride on a beam of light or uh ride in a windowless elevator he

  • had these thought experiments that he would do

  • to to help his thinking about the universe

  • um and through his intuition he would come to these great conclusions yet even

  • so he did not believe that black holes were

  • real when they kind of popped out his equations right so so how

  • is that true that is true so einstein was really

  • quite an intriguing person like and he was a real sort of hold out

  • when he came up with the most radical theories that completely transformed our

  • understanding of the universe right the fact that um you could describe the

  • entire universe on his contents and so on his general theory of relativity

  • reworking gravity um but when the implications of his theories were worked

  • out he often didn't like the implication like he didn't

  • i like the idea of an expanding universe it was a natural consequence

  • of Einstein's field equations right he didn't like that

  • why because he was you know emotionally attached to the idea of fixity of a

  • steady universe it was very disorienting to think of the universe as expanding

  • right likewise the reason he didn't like black

  • holes first of all he never expected so the black hole solution which is the

  • gravity of a very compact mass how intense it is and how it deforms the

  • space around it is an exact solution it's a simple exact

  • solution to his very complex equations he never

  • imagined there would be a simple solution so that was a surprise

  • but then it was what this solution was that he didn't like so

  • this solution as we'll see later on there are many different ways of

  • thinking about a black hole n cases what is called a singularity so

  • it's a place where all the known laws of physics

  • break down and he did not like that he said oh that's really perverse

  • and um so again it's sort of aesthetic reasons almost that he didn't like it

  • but you know eventually he sort of got around but i think what is more

  • interesting about black holes right um you know coming back to your friend

  • you know is oxygen not real just because we can't see it that's not

  • the case right i mean there are lots of entities

  • that we are happy to believe they exist whom whose existence we infer only

  • indirectly so you know black holes we infer their presence indirectly as with

  • dark matter for example right so there are lots of entities um that we

  • are used to in life there uh but you know you need incontrovertible evidence

  • and i hope today during our conversation i'll convince your friend and anyone

  • else who's skeptical that you know we really um know quite

  • well not just that they exist but also many

  • of their peculiar properties we actually see evidence for their

  • peculiar properties

  • go ahead well i was going to say you raised two really interesting points

  • there that i want to get into and one is how there are kind of different

  • definitions of black holes what is a black hole we

  • can answer those in different ways and depending on how you answer them

  • maybe that that gives a kind of different window upon you know their

  • behavior or understanding of them or how they

  • manifest in the universe and similarly uh we've talked about um how we don't

  • really see them directly and so there there are obviously ways

  • that we can study them and we're gonna get into those in much more detail later

  • uh but i wanted to kind of focus on on those things right now

  • um about just kind of getting getting the fundamentals again and maybe we

  • could maybe we could talk about that about about how there's different

  • conceptions of what a black hole is um in the

  • context of history right because people kind of

  • we forget we forget that here we are in 2020

  • and of course everyone knows the black holes are real of course black holes are

  • this thing but it's really i mean in the big scheme of

  • human history it's a very new idea and it's so radical

  • and you know and like all radical scientific ideas it you know

  • it it was not easily accepted so one of the first people

  • to really come up with the idea was chandra shikhar

  • an indian astrophysicist who in 1935 actually had worked it out a little bit

  • before um he was in cambridge england at trinity college

  • and as he was coming from india to cambridge to study

  • he had worked it out on the way on the boat

  • well on the ship on his way he had worked it out

  • that the end state of some stars would would be a very very compact dense

  • objects and when he presented this is a famous controversy known very well in

  • astrophysics where he made this presentation at the

  • royal society and you know arthur eddington was one of the famous

  • astrophysicists a senior astrophysicist of the day

  • uh refuted it because once again he didn't like these peculiar properties of

  • black holes nobody wanted to believe they existed

  • so it took till about the 1960s when we sort of had the first evidence

  • of the end states of stars so stellar corpses have you know there are

  • different kinds of courses depending on the mass

  • of the births birth mass of the star and so when they discovered

  • one of the possibilities they knew that aha so the other possibilities could

  • exist and so that's how you they really became

  • real as in observationally but you know and i think that part of

  • the reluctance um is because of what peculiar things

  • these objects are they're so enigmatic i mean this is what

  • i find so seductive about them right that you kind of think you

  • understand them there are a bunch of different ways to think about them

  • and you kind of hit a wall every which way that you think

  • you feel slightly illuminated but then you can't grasp it

  • so could we have the first slide please

  • so we need to think about it yeah yeah so it creates sort of different ways of

  • thinking about it and the reason these three different

  • ways allow you to make sense of their properties and of course

  • black holes are simultaneously everything right all these three things

  • so one way to think about it is the kind of strength of gravity that they exert

  • so for example a black hole the gravity is so intense

  • that not even light can escape which is why they're called black holes so

  • um so the way to intuitively think about it is you know

  • if we launch a satellite from the earth say cape canaveral or whatever right

  • for the satellite for the satellite to escape the gravity of the earth we need

  • a rocket we need um we need to blast out

  • at a velocity that's about you know 11 kilometers per second

  • yeah and so that's the kind that's why we need the boosters we need the rockets

  • right to boost it out of earth's um gravitational

  • field so if you can imagine that gives you a

  • sense of sort of the strength of earth's field

  • for a black hole that speed that you would need to launch

  • anything with is the cosmic speed limit the speed of light

  • and of course we can't speed make anything get close anything material get

  • that close to the speed of light so that's one way to think about it now

  • real quick before we go to the next slide i want to i just want to unpack a

  • few things that i'm seeing here in this slide for some of our viewers i'm

  • noticing at the bottom there's a there's a strange term here it

  • says schwartz child radius now i'm assuming Schwarzschild must be

  • a person uh but can you tell me what that is

  • yeah sure the Schwarzschild radius washer is a person

  • this was named after carl Schwarzschild he is the person who right after

  • einstein announced his theory of general relativity

  • he was uh fighting um uh is world war one in the trenches he heard

  • about the lecture worked it out got the solution of the black hole which

  • is the sort of the intense gravity he mapped out the shape of space around a

  • black hole and this was radius is sort of the sort

  • of an odd weird radius region around the

  • black hole that is also called the event horizon

  • this is the point of no escape okay so you have a black hole it has

  • this boundary called the event horizon and anything that makes it in including

  • light cannot make it back out so it's the

  • point of no return if you will for black hole and to give a sense again

  • of why is the gravity so intense and how could it be

  • and what do i mean when i say it's really dense and compact

  • so if the earth were to have the kind of gravity that a black hole does

  • all of us everything on earth would have to be crunched to the size of a penny

  • everything including all of us right now talking everything

  • yeah that's frightening it's really fantastically

  • dense incredibly dance so could we go to the next slide please

  • so another way to think about black holes and i think this is

  • what i was mentioning earlier this is what chandra shekhar came up with

  • right thinking of them understanding that they

  • are the end states of stars so if you have

  • massive stars so if you have the birth mass of a star to be about eight times

  • that of the sun or above then after finishing its life cycle exhausting all

  • its fuel it will actually explode and end its

  • life as a black hole okay okay and then the

  • way that works uh so we we mentioned that there's another kind of stellar

  • corpse earlier and i think they might be pictured here all of them we have white

  • dwarfs right white dwarfs and then there's neutron

  • stars and then black holes right that's right like black

  • peculiar properties the most peculiar properties neutron stars are intriguing

  • in their each one is intriguing in its own way

  • but you know neutron stars are just packed with neutrons

  • right and a black hole is just much more compact

  • and so a neutron star is basically as you said it's like a big giant neutron

  • essentially uh you know what it's like it's a star the size of a

  • city or something like that you know a teaspoon weighs as much as

  • i don't know and uh and then if you just throw a little bit more mass on there

  • a little bit more of something i guess like i guess i could do a couple

  • different things so we don't need to get too technical but if you threw enough on

  • there all at once presumably it would just collapse straight away to become a

  • black hole that's right if you throw enough mass

  • you could um and uh and the black hole is

  • much denser i mean i think relatively speaking a black hole is much denser

  • than a neutron star and it has this peculiar property of

  • having an event horizon right and speaking of which let's let's

  • go to the next slide as well because i think that that this is a good segue

  • here um what i want to notice here and and i

  • want you to take a lead in a moment but um we've been talking

  • about you mentioned singularities earlier about this place

  • this place where everything kind of comes together in a very literal sense

  • and all our theories break down all our understanding breaks down we

  • can't predict what happens um but that's not the same thing as an event horizon

  • so let's let's just just no no no no right

  • yeah yeah so so one of the fundamental

  • contributions that you know einstein made the reason

  • we all think um you know hero worship him

  • is this profound profound way in which he was able to link

  • mass the shape of space and motion so he sort of showed that sort of the

  • geometry or the shape of space is defined by the distribution of masses

  • so you can think of you must have all heard this analogy

  • of the entire universe as a sheet a four dimensional sheet or a tarpaulin or

  • something you know and every piece of matter in

  • the universe causes a little pothole is pock marking the universe this sheet

  • and the more massive an object the deeper the pothole the deeper the dent

  • in this sheet and remember there's nothing unlike this

  • figure where this sort of a cartoon you know there's nothing above the

  • entire universe is the sheet we are all living on the sheet so there's nothing

  • above there's nothing below so we are all confined you know all

  • celestial bodies are confined to move in this sheet and let me just point out

  • also let me add the other wrinkle right which is

  • remember the sheet is expanding all the time

  • it's not a static so i mean it's it's a complex

  • kind of you know interrelationship between the shape

  • motions and matter so if you look at how not only does the mass of an object

  • determine the shape the depth of the pothole

  • that it creates but how mass is packed in it so the density matters

  • so in this schematic you see like the sun would cause sort of a nice big

  • pothole and then you have a neutron star yeah in

  • the bottom panel and um and if you have a neutron star

  • notice it's much it's it's denser than the sun so it's going

  • to have a deeper pothole and a black hole is going to cause a

  • puncture in space-time that's how dense it

  • actually is okay so we're so

  • we're seeing black holes as you know products

  • of einstein's equations as almost like mathematical entities we're seeing them

  • as the end states of massive stars we're seeing them

  • as punctures in space-time maybe that's the same as number one um

  • and i just find it so fascinating that that you know we're talking about

  • if you compress the entire earth for instance into a black hole now that

  • would be an awfully small black hole i don't know if black holes could be that

  • small can they be that small per year they could in principle they could in

  • principle but you you compress it all and you think

  • about all the books all you know shakespeare

  • history uh uh the continents uh my memories

  • everything all of it all of the bi you know the biosphere

  • dna and it all gets compressed down to a black hole this little tiny penny shaped

  • thing and then i don't want to do that we

  • don't want that but there's another reason there's

  • another deep reason why we don't want that

  • which is the loss of information so once you cross the event horizon

  • right something bizarre happens you because no light nothing can escape

  • you we don't actually know what happens to objects that actually cross the event

  • horizon of a black hole uh but we have some ideas but what we

  • don't know is what happens to the information what is their mass what is

  • what was that object right if i fall into the black hole

  • i mean you would never know did priya have blonde hair did she have dark hair

  • was she wearing glass no you there would be nothing you would

  • know nothing you wouldn't even know there was a priya right

  • right and this is a big unsolved problem so

  • that is the peculiar nature of the event horizon because you know light is our

  • cosmic messenger all information in the universe is

  • transmitted through light we obtain that through light

  • or some other kinds of waves like gravitational waves which we will talk

  • about later right and we're just about to talk about

  • them but before before we get to that next section really quickly i actually

  • want to have there's one follow-up follow-up question there which is

  • so we compress all this stuff there's all this information that goes in

  • but what are what what are the fundamental properties of a black hole

  • you've mentioned a couple but i want to make sure we have them constrained

  • because it's only like a handful right i mean i could write it on my hand

  • yeah it's basically three properties are needed to fully define a black hole

  • it's mass it's spin whether it's spinning or not

  • and uh its charge for most astrophysical black holes

  • charge is not really a relevant quantity but so it's mass and spin so that's what

  • we're really after in astrophysics we're trying to measure masses and spins

  • because the measurements of masses and spins then we can confront that with our

  • theoretical understanding of how black holes grow and so on

  • and test our models but you know i can see why

  • that's kind of abhorrent to einstein the total loss of information the fact like

  • you can reduce all this complexity to just this bulk thing that

  • just sits right yeah so you know there's a beautiful

  • analogy that um stephen once um used and explained to me

  • and i love it and he's presented this in many talks uh this is hawking

  • so he said that you know it's like having an encyclopedia britannica

  • and looking up say you know you look up new haven connecticut and you see

  • you know where it is on the map you see the population 100 000

  • whatever right and then you you actually put the encyclopedia britannica in a box

  • in a really tight box from which nothing can escape and you burn it down

  • completely burn it down but you've caught every

  • particle of ash inside that box nothing's left that box right

  • so the information that was in the encyclopedia britannic is still in there

  • it's just no longer stored in the form of pages and printed

  • ink and so on so forth and we no longer know the act of reading like of actually

  • accessing the information we don't have that either

  • so the information is clearly there but it is in some form perhaps that

  • is um that we cannot recover and we don't even know how it's stored

  • so i think this is the best that you know the best analogy that i've heard

  • uh and um it's an unsolved problem by the way

  • of what really happens to the it's controversial

  • their ideas and so on but no real resolution

  • quite yet but i mean the resolution that we are inching towards

  • is exemplified in this analogy that the information is

  • likely there we just don't know quite how to retrieve it

  • so we're not completely losing it it's that's the direction in which the

  • solution is going so hold on we'll have to wait and see

  • there's always a chance you're telling me there's a chance okay good let's um

  • let's advance to the next slide i think it's another poll actually

  • because we've been talking about i think it's another poll

  • yes how do astronomers observe black holes and i gotta tell you folks

  • priya already gave you some big hints right so

  • let's go ahead and kick it off let's see what we can do we got about 30 seconds

  • which of the following is a typical way that astronomers

  • observe black holes is it a x-ray emission

  • b gravitational waves those ripples in space-time that

  • love to make headlines c stellar motions or d all of the above

  • let's think carefully about this remember black holes are black we can't

  • really see them directly yeah i think i kind of let the cat out

  • of the bag a few times didn't only only for people who are paying very

  • close attention i know some of you are are stroking your cats right now on

  • your laps i know some of you are scrolling your phones

  • shame on you you should be paying attention to me and appreciate

  • come on it's the joy of the zoom universe

  • it's true it's true we can all be connected in this in this crazy time of

  • cobit despite being isolated in our houses

  • ah yeah

  • i can't believe how many months it's been right since i

  • sort of i finished teaching a couple of months ago and it was

  • all shifted online in march but um but i think what is really fun is

  • that we're not letting this little nanometer-sized

  • virus get us down man we're still thinking about

  • black holes the universe all kinds of things about nature and the

  • wonderful things that nature offers yeah if you're going to be

  • bummed people we were more boned about falling into a black hole and never no i

  • never knowing you existed okay here we go poll results uh we have

  • x-ray emission five percent gravitational waves

  • eighteen percent c stellar motions ten percent and d

  • all of the above 67 percent a whopping 67 percent

  • the ds have it all and that is the right answer and now now

  • priya just as a segue into the next uh section

  • that we're going to talk about which is called the black hole bestiary we can go

  • ahead and get that next slide up uh i think it would be useful to talk

  • about how we have these different observables and maybe they give us uh

  • different windows onto different sorts of black holes just some some sorts of

  • black holes right now for us with our current capabilities

  • um present themselves more in one way than another when we're thinking about

  • these ways to look at them does that make sense yeah well i mean i

  • think you know um black holes first of all right they

  • come in a range of sizes yeah right and

  • their sizes determine the gravitational influence that they exert

  • as we saw and therefore the ways in which they will render themselves

  • um render their presence is going to depend on

  • the sizes of these black holes so typically we tend to see black holes

  • especially the super massive black holes these are we'll talk about in just a

  • minute uh these are some of the most massive

  • black holes in the universe like the one in the center of our own galaxy

  • and remember these gal these black holes that are sitting in the centers of

  • galaxies can be either fasting or feasting like

  • the one in the center of the milky way um the supermassive one four million

  • times the mass of the sun is actually fasting there's not much gas so that is

  • swirling in so the way we typically see black holes is when matter is swirling

  • in and it's being pulled in by the gravity of black hole

  • gas typically it gets heated and it gets it gets hotter and hotter as it's

  • getting closer and closer to the event horizon

  • starts to glow and it glows in the x-rays

  • and that's how we see most commonly feasting supermassive black holes so

  • stellar mass black holes on the other hand they are having little

  • feeding episodes so if you have a stellar mass black hole

  • that is next to a star that strays close or it's

  • bound to another star then it could start

  • feeding slowly ripping the star apart and start feeding so once again that gas

  • as it falls in so you always see sort of these dying

  • gasps of gas around black holes then for the black holes like the one in the

  • milky way we actually see them in a completely different way

  • they are not feasting so what they do is they control the

  • motions of the stars that are right nearby so can we go to the next slide

  • please so these are actually real data from the

  • center of our galaxy two different groups

  • one from reinhard genzel and the other from the ucla

  • group led by andrea getz and these are the same stars

  • whose motions are being followed and this is real data so you see the clock

  • on the top right yeah and notice that these orbits are

  • closing in so it's like the solar system right

  • we see the planets on elliptical orbits and the sun is one of the foci and you

  • know the most massive object in the solar system is the sun similarly the

  • black hole is really sitting right there and so we are able to measure the mass

  • of the black hole by looking at these orbits

  • so in the right hand panel you saw something more exciting happen

  • and that was there was a little gas blob that came close to um

  • the black hole in the center of our milky way and we

  • we thought and we were really hopeful that we might see like a feasting

  • episode we actually didn't we saw a little bit a

  • trickle in and then it just zoomed past but you

  • know it can't go too far it's in the gravitational grip of the black hole

  • it's going to come back around and what you see with the dates in the future

  • is a prediction of when that gas blob is likely to come back around

  • wow so this is the way in which we have detected

  • pretty much and measured the masses of all nearby supermassive black holes

  • it turns out that almost all the black holes supermassive ones nearby

  • are fasting so this is the way to detect them

  • but then this method doesn't work for black holes that are far away

  • because remember you have to resolve all the stars you've got to like map

  • motion of every star and that's incredibly hard

  • because the centers of galaxies if you see them are incredibly bright they're

  • chock-a-block filled with stars so you can't resolve them till you have

  • huge telescopes so next generation telescopes will allow us to go even

  • further out than we have uh to detect these fasting black holes

  • but it's the feasting ones supermassive ones that we detect

  • much more easily because you see x-ray emission and then

  • of course the dramatic new way in which we started detecting black holes

  • recently was when two black holes collide and

  • we have detected the collision of two stellar mass

  • black holes so black holes come basically

  • we categorize them in mass as in slim small stellar mass black holes

  • the end states of stars very elusive we'll talk about this more

  • in a little bit intermediate mass black holes that are

  • about a thousand to ten thousand you know forty fifty

  • thousand times the mass of the sun and then super

  • massive are black holes that are you know million to a billion solar

  • masses then you have the obese ones the ultra

  • massive black holes that are more massive than billions of solar

  • masses and the reason we you might say well

  • this looks kind of arbitrary actually it's not arbitrary the way

  • we've classified them because um they have different ways of

  • being born so stellar mass black holes we've nailed

  • it they're born and states of stars and the question of

  • course for people like me who've been working in the field for a long time is

  • try to see can you start from these stellar mass

  • black holes can we like over feed them and make them

  • intermediate mass black holes and then supermassive black holes and

  • ultra massive black holes yes that is one way

  • to do that but it turns out that there are many ways to make the first sort of

  • black hole so-called seed black holes you don't have to start with just the

  • first stars could we have the next slide please

  • now real quick just for re for our viewers so they understand when you're

  • saying that that the c black holes didn't have to come

  • from stars is that because we're talking about so early in the

  • universe that stars did not exist is that what we're talking about

  • no it was probably these these uh what are called direct

  • collapse black holes so these are basically when you have a lot

  • of gas in the early universe um and it settles down into kind of a

  • disk in the center of a galaxy no stars have formed yet in this particular

  • galaxy but you know the gas siphons in because

  • of an instability it's like you know you're sitting in your bathtub

  • and you pull the plug and you see that vortex of water going in

  • really really fast that kind of instability the pulling the plug

  • kind of instability equivalent of that loosely speaking

  • happens in the very early universe can happen and you can siphon a lot of

  • matter down very fast because that's what you need

  • to make a black hole right you need to put a lot of

  • pack a lot of matter down very very rapidly and this we believe can

  • happen simultaneously when the first stars form in the early universe

  • and we believe that this is one way to make intermediate mass black holes you

  • know a thousand to ten to the five times the

  • mass of the sun very early on in the universe and of

  • course from those you can easily build up the

  • supermassives from these direct collapse black holes

  • that are intermediate mass black holes in the very early universe you can build

  • them up very rapidly to actually make very supermassive black holes even early

  • on in the universe the reason you want to make them early

  • is because you're seeing these feasting black holes which are called

  • quasars out to the largest distances at the earliest times in the universe

  • so there's like a timing crunch you have to really kind of you know as i said if

  • you start with a stellar mass seed you have to like

  • overfeed it and that's kind of challenging so but if you start off with

  • a seed that's already intermediate mass then it's very easy to account for the

  • supermassive black holes so here i just want to plug some recent

  • work a paper that i just wrote so we wrote a paper

  • a set of papers showing how you could make these direct collapse black holes

  • more than 10 years ago but we recently realized that there was

  • another way to make an intermediate mass black hole

  • so if you form a cluster of early stars you

  • form one of the stars actually gives you a little stellar mass black hole

  • this little black hole could be bouncing around and eating a lot of gas feasting

  • and kind of you know be a complete glutton

  • and it could become an intermediate mass black hole in the very early universe so

  • that we worked out and i recently realized i just submitted

  • a paper a little while ago where you know there's no reason that

  • cannot happen later on in the universe so basically continually

  • you could form these intermediate mass black holes

  • so you might say oh this is all great we're making all these intermediate mass

  • black holes the problem is we're not seeing them in

  • that stage so it's almost like you know you have

  • the photo album of black holes you're seeing pictures in

  • infants say you're seeing nothing in teenage and

  • early adult years and then you're seeing kind of

  • the older like midlife and the geriatric black holes

  • so there's kind of a gap and so you know that is one of the open

  • kind of puzzles could we have the next slide please

  • i mean to be fair i i tried to kind of mask my aqua adolescence too so you know

  • no one can find those pictures of the internet i hope

  • yeah like you thought maybe that we should yeah maybe that's not a bad idea

  • right we should all hide our pictures but you know i don't know i think you

  • know i was actually kind of that was the time that i was actually

  • pretty so that's too bad i don't have many photos but um anyway

  • so this you know this is i wanted to show this

  • movie uh because it's such a beautiful visualization it's

  • from one of my recent phd students angelo riccarde what it the these are

  • this is how black holes grow so black holes grow by

  • eating gas by feasting and they also by colliding with each other

  • so what you see here is the life cycle of black holes you

  • start out as a cluster and that's a cluster that you have to start out with

  • you see them growing by eating gas and colliding with each other

  • and then eventually what you see at the um

  • when they reach the top that is the real data of nearby black holes

  • like our milky way is a point on that graph

  • in that black cluster on the top the milky way black hole is there

  • and so this shows you the sort of dissects how black holes grow

  • over cosmic time so this is a time lapse if you will

  • of the assembly of a black hole so an individual black hole the supermassive

  • one that we see today has had a very complex history by

  • growing not just feasting but also merging with

  • many many other black holes and the reason these kinds of you know

  • so it's a story line right so this is we've built this of course it's based on

  • science and physics but this is a model that tells you how you can end up

  • to explain everything that we see now so one of the intriguing things that we see

  • is that that's what is shown on the right panel the size of the black hole

  • that you find in the center of a galaxy in the central

  • galaxy is adjusted to the size of the galaxy

  • somehow the galaxy and the black hole know about each other as it were

  • and this is where we come back to the start to our first question

  • they somehow know about each other so they regulate

  • each other so in some sense the inner part of the galaxy

  • kind of controls the feeding of the black hole

  • and as i'll tell you in a minute black holes actually kind of

  • burp and so they're they sort of burp and they

  • emit and push stuff around in galaxies as well

  • so there's a kind of intimate relationship

  • between the um the inner part of a galaxy

  • the star is in the inner part of a galaxy and the black hole

  • and so we've been trying to work out you know as astrophysicism

  • is this just a mere correlation or is this actually causation so that's the

  • deep question we actually think it's

  • causation and that's what these models show you

  • yeah you know this is kind of you know i i'm kind of an exoplanets and

  • astrobiology guy i mean i really love black holes and fundamental particles

  • all that stuff too but you know my heart isn't big dumb objects like

  • that you can live on right but i don't want to fall into it and just become

  • a wisp of plasma i hopefully can i want to talk to aliens

  • um and i was wondering the relation to that is

  • with this is um you're talking about this correlation that might exist

  • this causation that might exist um between the central black hole the

  • central supermassive black hole and the the environs the outer parts of the

  • galaxy the whole galaxy as a whole so i mean doesn't that kind of mean

  • you're cause i think people would want to say a lot of times

  • oh who cares who cares about black holes sure they're out there doing their thing

  • they're feasting they're famine they're they're fasting they're

  • they're burping but who cares they're so far away it doesn't affect your life

  • here at all but i mean isn't there if if a central black hole in a galaxy

  • controls the size of the galaxy and can burp out

  • things and cause problems doesn't that kind of to some degree mean that i mean

  • we might actually we might actually have the central

  • supermassive black hole the milky way uh to thank for our being here to some

  • degree or maybe or maybe maybe it's just that you know

  • uh there is a relationship that's a great question because i mean

  • i think okay first of all let me like we are very safe

  • we in the solar system are very safe from the central black hole of the milky

  • way so let's not worry about personal fate but absolutely i mean i

  • think that in many ways um we thought that black

  • holes because you know in the grand scheme of a galaxy right

  • i told you they're massive and all of that or if you take the milky way the

  • mass of the black hole is 4 million times the mass of the sun

  • the mass of the galaxy is 10 to the 12 it's a million times more so the black

  • hole is really tiny it's you know in the grand scheme

  • of things however it punches more than its weight in terms of

  • what it does to regulate the galaxy so we used to believe when we first

  • measured these masses right of black holes from the motions of stars like we

  • just saw people thought okay you know what they

  • can't count for much they're the mass budget of a galaxy they're

  • nothing it turns out and so they must play a very marginal role so it turns

  • out that actually they punch much more than

  • their way they play a central role in shaping galaxies

  • so one could kind of poetically say that

  • you know maybe we wouldn't even be here if black holes and galaxies the

  • centers of galaxies were not so tightly locked in right that the galaxy

  • and its ultimate shape and its fate and what it looks

  • like is shaped by the central black hole wow and and just

  • one more thing i know we're actually moving on to the next section soon but i

  • i want to clarify for people very quickly in this animation on the bottom

  • right uh these little these little red dots

  • those are those all actual data points or those things from simulations these

  • are black holes so this is the growth history of a black

  • hole that will end up and what you really see there on top is

  • the clock and when it says zero that actually corresponds to today

  • so that's what you want to end up today so so this is explaining

  • and you know and what you see these models are very very rich

  • so we can take a slice in different moments of time

  • and we can match it to what black the black holes that we

  • detect in the universe at each of those epochs so the model that i'm showing you

  • is one that is well calibrated that is the best to date

  • that explains all the data that we see okay and so that's the black hole best

  • area and some we have the stellar mass stellar corpses which can get a little

  • bigger we have intermediate mass ones we're going to talk a little more about

  • those i think they're kind of mysterious and strange the adolescent period the

  • awkward adolescents and black holes of super massive black balls then the ultra

  • massive black holes the really the really big boys

  • that are that are the centers of galaxies and and is that is that

  • the entire best area are there any other is there room in there anywhere

  • there is room for another sort of speculative kind of black hole

  • um and you know i kind of said uh you know could they be tiny when you ask me

  • so there's speculation that in the

  • primordial universe very very early universe prior to the universe

  • becoming matter dominated where prior to even sort of the fireball

  • stage very early sort of seconds of the universe's

  • life you could have formed a population of

  • primordial black holes right and they would be tiny tiny black

  • holes they would form and so but you know we

  • don't have i mean it's speculative we don't have evidence for them and you

  • know we're constantly trying to figure out because

  • you know these little black holes primordial black holes you know that

  • formed if they were more than 10 to the 15 grams

  • they could have survived anything that was less than 10 to the 15 grams if it

  • formed with a birth mass of less than that early on

  • it would have evaporated by now we'll come to that later

  • uh but the other guys could have survived and maybe

  • they could have grown maybe they could have formed they could even be the

  • precursors of some of these supermassive black holes

  • so but that that is still pretty speculative because you know from

  • that epoch in the universe we don't actually get any direct or even indirect

  • data at the moment okay so um that is

  • speculative so before we uh before we move on to

  • like you know one of the ways in which intermediate ma black holes were

  • actually unmasked recently it's like totally exciting

  • because i wanted to talk about a little bit of personal history and personal

  • anecdote right um which is you know we always think

  • about so this correlation that we just saw in the previous slide

  • between the size of the black hole and the

  • galaxy the stars in the galaxy the inner regions of the host galaxy

  • i mean one of the things so that result was published in 1998 and

  • i was a graduate student at cambridge cambridge england and i realized at that

  • point that if this little black hole had to somehow

  • impact this large galaxy because remember

  • this washed shield radius is tiny tiny tiny around the black hole okay

  • it is it is well well well inside when we saw the milky way the stars moving

  • around remember those toys are well outside this watch child radius

  • almost a million times outside so the swastika radius is like a tiny region so

  • right so if the black hole that is you could

  • think of the black hole as the event horizon

  • how does it have a reach that far out into the galaxy right

  • so the first speculation so you know i was young and bold and radical and i

  • said okay you know what we should there is a way in which we've

  • always been fixated thinking about black holes as

  • things getting swallowed into black holes

  • what if this energy that is being emitted as matter is falling into the

  • black hole could be tapped somehow and be used to

  • push gas out it behaves like a piston that energy could be you know put in and

  • you know assembled into like a piston outside well outside the black hole then

  • i realized there was enough energy to push gas out to very large distances

  • in fact even outside the galaxy outside the stars for

  • sure and so that was a revelation i thought

  • oh that's really cool so that's a burp so black holes can actually burp so it's

  • not like the matter that goes in gets filled out it's on the way

  • the stuff that is getting heated and you get energy that energy can be

  • tapped in mechanically if you will you can think of it like a piston and then

  • there's a piston that pushes all the gas that's around

  • and it turns out that those gas blobs i realized

  • that those gas blobs would glow in a very special way there'd be large

  • bobs of gas and they would actually glow in the x-ray because they would cast a

  • shadow on the cosmic microwave background

  • radiation which is sort of the relic radiation from the big bang you

  • know that's like a thermal radiation that is all around us even now we are

  • bathed in it and early on in the universe this is

  • because the universe is expanding this radiation has been cooling

  • and today it's at three degree kelvin it's very cool it's not that's why we

  • don't even feel it we're bathed in it but we don't feel it right

  • and um and it's measured extremely well very very accurately

  • but earlier in the universe it was hotter and so this hot gas is actually

  • hotter than that relic radiation and it will cast a shadow

  • and that that shadow should be measurable but this i wrote this paper

  • in 1998 right after because i was trying somehow

  • to link the scales the small scale with the big scale

  • but we didn't have an instrument we didn't have

  • we needed the alma array the atacama alma array

  • and chilly telescope yeah it's a very large radio telescope many many dishes

  • many frequencies and you needed this real

  • wide span in frequencies to actually make this measurement and i was super

  • excited because like less than two years ago they actually

  • detected one of these first gas blocks and what is exciting about it is this

  • blob is almost eternal relatively speaking it

  • lives on forever so your quasar might have feasted

  • and then be in a fussing mode which means we wouldn't see it

  • and if it's far away we wouldn't be able to see the motions of the stars like in

  • our galaxy yeah but this glowing blobs of

  • gas would hang around and linger and so you would detect the presence of

  • a quasar that was actively feasting ten to the

  • eight years ago if you see these blobs this kind of

  • reminds me of like uh you know maybe i i ate mexican

  • yesterday and then i just you know i we should released on

  • okay okay but i mean the point though is is that you could actually you could

  • track the the quasars activity the big black

  • holes activity through time and kind of see almost

  • excavate previous meals and things you see

  • exactly wow so does that mean does that mean that you could in some

  • way maybe constrain or figure out um how quasars uh

  • affect some this part of evolution of galaxies because obviously you need that

  • gas you need that gas to form stars right

  • that's right or prevent it from forming if it's too

  • too hot it'll prevent the formation of stars

  • that's actually what we think happens and

  • that's how black holes limit their own growth

  • so you have this gas that is too hot so you prevent the formation of stars

  • right and so you keep this hot halo of gas

  • around and the gas is not going to get um um

  • you know it the gas gets fed in and then there isn't

  • enough time to replenish for gas to come in from the outside to

  • repopulate the inner region and so that is how black holes could

  • stunt their growth they could eat all the gas

  • and then the um evacuate the central region basically

  • and then it may take too long for the gas to fill

  • in and the other way in which so you know you raised a really important point

  • that's worthwhile mentioning that we believe that quasars turn on and off

  • episodically so they feast and then they fast they feast and then

  • they fast so once a black hole is exhausted all the gas

  • right around it it can get rejuvenated because it takes

  • gas for gas to trickle in it takes a very long time but there's another way

  • in which you can get gas if it goes quack with another galaxy so we believe

  • that galaxies are constantly colliding in the early

  • universe and when they collide you know their stars

  • collide their dark matter kind of goes past and

  • the gas collides the black holes collide and the black

  • holes could collide merge produce what we call

  • the gravitational waves which basically shake up

  • that fabric of space-time that sheet of space-time gets tremors when the two

  • black holes merge they become one and then you could have

  • the lot of the gas still there so it can start feeding again

  • it can grow it can grow by merging and then

  • also a big feeding episode because when these two galaxies merge there's a lot

  • of gas that ends up right in the center wow

  • will we go to the next slide please yeah i think the next slide might be

  • another poll let's see what's poll number three folks if you're

  • still listening i hope you're with us how does a black hole die let's let's do

  • it let's get to it how does a black hole die let's choose

  • one of the following uh choice a it vanishes suddenly due to quantum

  • fluctuations b it tunnels into another dimension

  • or universe c it evaporates over almost incalculable periods of time

  • or d it doesn't black holes last forever which is it gonna be

  • i'll give you a hint priya mentioned it earlier

  • were you paying attention i hope so yeah i think this question is something

  • that people love to speculate about because

  • you know we are so um hardwired to think about life and

  • death and that you know the sense of of our

  • own experience of life on earth that you know the sort of the notion of

  • eternity i mean i think in a way that's what is so incredible about the cosmos

  • right the kinds of time scales that um where um astronomers are talking

  • about um they feel like eternity compared to

  • sort of our lifetimes right well you know they say they say eternity

  • uh uh you know it lasts a really long time especially

  • towards the end right that's right

  • but yeah so it sounds like i've always had the impression that

  • you know black holes seem like these we can probably go ahead and put up the

  • results i think um and we'll keep talking as we

  • as we look let's see so okay poll results a advantage is suddenly due to

  • quantum fluctuations only six percent said that

  • b it tunnels into another dimension or universe three percent said that c it

  • evaporates over almost incalculable periods of time

  • 69 said that nice d it doesn't black holes last forever

  • 22 oh the 69ers have it that is great that is the right answer it evaporates

  • over almost incalculable periods of time uh priya tell us more yes so for example

  • that is the right answer and for example um a black hole that's the mass of our

  • sun like 10 to the 30 uh 33 grams or so

  • that will last it will evaporate over 10 to the 64 years

  • that is like super eternity as far as we are concerned because even on

  • even you know by cosmic scales right the age of the universe is 13.8

  • billion years that's 10 to the 10 years roughly speaking

  • so and of course the more massive a black hole the longer it will take

  • so the as i mentioned earlier the only black holes that could have evaporated

  • formed and evaporated sort of without a trace already or ones that might have

  • been born at the big bang with a mass that is

  • less than 10 to the 15 grams okay remember the sun is 10 to the 33 grams

  • to just give you a feel right so those tiny tiny black holes are the only ones

  • that could have evaporated so essentially black holes are eternal

  • they're going to be there it'd be here forever

  • so so i just got i just got i got to wrap my head around this real quick bro

  • go to the next section i know i know we're kind of running out of time almost

  • but i think we can still pack a lot of this stuff in

  • um so we're talking about these things evaporating and i understand

  • you mentioned steven stephen hawking earlier and obviously this is due to a

  • process called hawking radiation which we can get into a little bit if

  • you want to but but i had a question so everyone always talks about this slow

  • sedate evaporation and i guess maybe it speeds

  • up and gets more intense the smaller the thing gets

  • um but what happens at the end like does it just kind of fizzle out

  • or does it just explode at the very end like what is it like do we know

  • well we don't know we think it becomes a singularity

  • we're back again to a singularity right okay okay the so-called naked

  • singularity where maybe it's just it's my mind's being blown right now okay and

  • the other thing is just these time scales again so

  • so you mentioned that that that black holes are going to be around practically

  • forever anything above stellar slow mass and

  • size um but then the super massive ones i mean

  • you you you quoted a huge number that made my brain hurt

  • uh the 10 to the 64th yeah 10 to the 64th

  • is a is a solar mass black hole right these chinese stellar mass

  • black holes we're already talking about just you know those that are detected by

  • the ligo collaboration they've crashed into each other

  • gravitational waves these guys would live for

  • more than 10 to the 65 years they would live forever

  • so i mean not only are black holes littered

  • of every size littered everywhere in the universe

  • tiny to the ultra massives they're also gonna basically

  • hang around and be there they're gonna witness everything

  • i mean i've heard some people speculate that you know people think about like

  • proton decay the idea that the protons that are inside your atoms

  • um are act in the nucleus of your atoms are actually going to

  • decay at some point because they're not stable and so

  • no one has i think the time scale picks that exactly but i've heard that you

  • know the notion is that eventually they're going to decay which is bad news

  • because then that means you just crumble apart into nothingness um so i

  • guess in some sense falling into a black hole

  • a supermassive black hole would be really

  • bad because you would you would die um but on the other hand you would kind of

  • stick around maybe longer than anything else

  • right sure i mean it depends on what you want

  • to do with your life when you're sticking

  • around right i mean right i if i can't have my

  • favorite flavor of frozen yogurt i mean life is pretty pointless

  • pardon a silver silver lining every cloud yeah

  • that's right if you cannot if you cannot enjoy doing science

  • listening to music um uh creating and uh enjoying art we

  • can't do any of these things what's the point of living forever i mean

  • really yeah it's true so we should probably move on right let's move on

  • that's right we're gonna have to go to kind of hyper speed we're gonna have to

  • go faster than the speed of light now guys it's part three what comes next uh

  • and let's actually go ahead and go to the next slide from here

  • um we need to talk about next generation facilities and observations that are

  • gonna really tell us even more about black holes

  • the real dark frontiers so let's look at this

  • uh so uh coming back to this elusive stage of intermediate mass black holes

  • um so it turns out right so you might think as i've

  • gone on and told you you know super massive black holes in the centers of

  • galaxies and so on it turns out that's where we were

  • looking for intermediate mass black holes so we were looking at tinier

  • galaxies we thought okay you know what we know that the scale of the black

  • holes and that of the galaxies they kind of scale that correlation

  • so to find an intermediate mass black hole all we have to do is to look at a

  • you know fainter and fainter tinier and tinier

  • galaxy like a dwarf galaxy you know wimpy galaxy right

  • that that is hard we're starting to have hints

  • but the thing that we have missed and this was the very exciting detection

  • and measurement now of the mass notice that little circle on the left

  • hand panel so that is an image of a taken from

  • chandra space telescope and the hubble space telescope overlaid

  • x-ray and or and optical image and notice the little

  • circle that is off-center yeah that's the intermediate mass black

  • hole so we've been looking in the wrong places they're not

  • necessarily at the center they're kind of wandering around in the outskirts

  • of other galaxies that actually host supermassive black holes

  • these guys likely off off-center then you might say hey how are we seeing them

  • we're seeing them because they're glowing in the x-rays they're still

  • feeding so some of them are feasting so you spit

  • them up second this one was actually detected

  • because of something called a tidal disruption event

  • a star sadly skirted close by and got completely ripped apart and so that

  • flare was detected oh that's that's uh that's that's that's

  • disturbance into the force right there i'm guessing the planets probably uh

  • wouldn't have a good time if their star got totally disrupted right that would

  • be bad yeah that would not be a fate that you

  • know i would i would you know we're worried

  • about the story itself let alone the hanging on planets right and else

  • possibly on those planets yeah we should probably let's move on but

  • yeah so so we've seen some intermediate mass black holes though it

  • sounds like and now there's other ways to study them with some next-generation

  • stuff right that's right so i i mentioned earlier

  • right so one way to detect black holes and measure their masses and their spins

  • right is to actually detect gravitational

  • waves from their collisions so the ligo collaboration already

  • detected the collisions of nearby stellar mass

  • black holes little ones and the the collisions of these

  • supermassive black holes which are going to be in the centers of galaxies

  • when they collide we will see a tremor uh in space time similar tremor but it's

  • at a lower frequency so we actually have to be above the

  • earth's atmosphere in a satellite so could you start the animation on the

  • bottom

  • yeah and so this is what is planned the europeans and nasa esa and nasa are

  • planning what's called a laser interferometer

  • space antenna so a configuration of three satellites and

  • they will actually measure the collisions of supermassive black holes

  • distant ones and the collision of a supermassive and

  • an intermediate mass black hole so that's another way in which the

  • intermediates are going to come into view

  • very i mean this is an experiment that we think will fly in the 2030s or so

  • and actually i'm part of the nasa leasing science team

  • you know trying to generate you know models and understanding of

  • what all should be detectable one of the interesting things about

  • supermassive black hole collisions is because these supermassive black holes

  • tend to be in the centers of galaxies there's lots of gas and stars and so on

  • when they collide we will actually see simultaneously

  • signatures in other parts of the electromagnetic

  • spectrum because these gravitational waves are not part of our usual

  • electromagnetic spectrum they are waves that travel at the speed

  • of light because remember they're almost like they're waves in space-time itself

  • so they're different from light right but the nice

  • thing about supermassive black holes and it turns out some stellar mass black

  • holes as well might have these counterparts we might

  • have telltale signatures that we can see in the x-ray and optical

  • and infrared so we might be able to actually nail

  • down where the collision is happening because

  • we'll see and a siren go off before the actual collision

  • happens so that's the goal and of course james webb space telescope

  • is in there and i'm of course deeply invested in it because

  • uh once the telescope launches hopefully on

  • october 31st next year uh we should be able to it will bring into view the

  • first black holes the very first black holes that likely

  • formed in the universe so if this idea of direct collapse black

  • holes are forming these intermediate-sized black holes

  • from the get-go bypassing the formation of a star remember from the gas and the

  • vortex analogy if those are really there we

  • will see them so that's what i'm most excited about

  • because that's a window that's going to open uh first

  • wow that is so exciting and amazing i hope we managed to

  • be around for to see these things launched hopefully you know james webb

  • next year and i i hope lisa in the 2030s you know we'll see it's pretty advanced

  • tech but uh i'm sure we can do it we put people on

  • the moon right so why not i mean and you know that the tech for

  • lisa is already there was a test um uh a prototype and the prototype

  • performed better than expected than specifications

  • so very optimistic it's only now just sort of money and

  • time that we need to build everything and test everything

  • um and launch i think that is that was our last slide

  • wasn't it i think we have one more which is just

  • want to know more i want to plug your amazing book mapping the heavens the

  • radical scientific ideas that reveal the cosmos

  • excellent and then of course anyone who's interested in black holes could

  • also check out scientific american in our

  • ongoing comprehensive coverage um so we're a little over time only by

  • two minutes though we went hyperdrive we nailed it no no i

  • um i i want to add that you know i was very very excited that um

  • scientific american sort of asked me to write about these first black holes

  • and um it was an article that came out in february 2018 but i think it was

  • republished in this volume that you have

  • exciting discoveries in black holes so that's an excellent volume to get a

  • special issue that's an excellent one to get

  • there are many uh interesting articles about the frontier what's happening

  • uh for black holes of all sizes i think we're gonna have to have you back pretty

  • soon to write an update because it sounds like we're gonna

  • have to rewrite the textbooks at some point the next couple of years again if

  • james webb goes up and because i think that's my excuse so i have this

  • report physics reports is very prestigious um

  • place where we'd write review articles and so you know i've been

  • waiting they've been asking me they asked me invited me to write a review on

  • black holes and i said oh just wait there's this one

  • new discovery and you know i heard a hint about this you know i heard a rumor

  • let me wait and i'll hand you that article once i put that in

  • but it's been non-stop i mean we've had these wonderful discoveries like pretty

  • much you know even the last few months like every few weeks

  • there's something exciting right so yeah it's crazy so

  • more things to come and now now the part that maybe some folks have been waiting

  • for you get to ask for your questions not

  • just me with my dumb questions you could ask questions i'm going to read them off

  • we're going to handle as many as we can we have until

  • um

  • can you hear me but now i can hear you and i lost that question i don't know if

  • that was the trick that you kind of stretched it out and i

  • had to figure it out i think it fell into a blackberry that

  • shifted the question could you um repeat the question sure it's from

  • ariel bach it is what is your favorite theory about

  • what happens beyond the event horizon um

  • i think okay so look there's in in in science right i mean science is like

  • a creative process so there's like the real

  • and the imagine so we take little leaps of imagination

  • and then we figure out and work out the physics right so let me just quite

  • clarify now that when we talk about anything inside the horizon it is pure

  • speculation pure imagination okay so this is not

  • scientifically a realistic option or a solution the thing i find most

  • fascinating is when you fall into a supermassive black hole

  • the possibility that you know that singularity

  • could take you places right that it could potentially

  • tunnel you and take you into a brand new universe i think i saw matthew

  • mcconaughey the most important i think i saw matthew

  • mcconaughey do that one time yes that's right that's right in

  • interstellar yes yeah uh but i have to say sometimes

  • right this universe and this earth i get really depressed right when i see you

  • know the the reluctance to accept climate change

  • and all the anti-science stuff i was like you know what i feel like tunneling

  • out into a different universe man there are times when you really want

  • that portal as well well folks i want to see more questions

  • from you only have one of them coming up which gives me a space to ask another

  • question right now here it is uh if you fell into a black

  • hole uh i've heard some people say that you'd be

  • able to see something really strange uh on the on the sky as you were falling

  • in is there do you know what i'm talking about

  • right so first of all um when you fall into a black hole it kind of

  • depends uh on your point of view so if you are looking at things as a

  • distant observer whereas you are the sad person who is

  • hurtling in right so you see different things

  • but the intriguing thing is that if you are the person falling

  • in so the person who is sitting outside will basically see you

  • hurtle towards the uh event horizon and the light from you getting redder and

  • redder and redder and then you will basically

  • freeze for them they won't be able to see anything

  • right the minute you cross because no light can escape so they cannot see you

  • anymore so this is the distant observer whereas meanwhile you the sad person

  • who's falling in unfortunate fate you will see weird things when you fall

  • into the horizon you would see the light from behind you bent

  • because light is bent in strong gravity this phenomenon is called gravitational

  • lensing so you would see the weird effects of

  • light bending you would see um you would see

  • everything getting redder than getting white and then you would

  • see the cosmic microwave background it will flash by

  • so you'll see very bizarre optical um i wouldn't call them illusions but i

  • guess yeah illusions is the right word yeah you would see all these weird

  • optical effects what about time dilation anything weird

  • there yes of course time i mean you know time and space switch in terms

  • of their place in the mathematical equation once you cross even horizon

  • so um basically it would take you once again it depends on

  • the person was really far away it would it would take

  • in from that person's point of view you're frozen it would take an infinite

  • amount of time for you to actually fall into the singularity from their

  • point of view right but their point of view doesn't count as much because

  • you're the sorry person who's falling in right

  • right so once again if you are falling in uh

  • to a stellar mass black hole remember it is much more compact

  • it's a schwarzschild radius or the event horizon is smaller

  • the size of the event horizon is directly proportional to the mass of a

  • black hole so if a black hole is small the event

  • horizon is even tighter so then and and the force the forces are

  • intense right so as for a stellar mass black hole

  • when you're getting close aside from all the optical effects

  • one thing that's going to happen is that the let's say you're falling in head

  • down first the difference in gravity between your

  • head and your toes is going to be so strong that you're going to get

  • stretched out so you would be spaghettified it's a

  • technical term spaghetti i mean i always want to be taller

  • i mean so that doesn't sound like that's one way i think this is like super

  • desperate right this is super desperate yeah but

  • then if you are falling into a supermassive black hole it's

  • not as dramatic but you know you will become

  • you will eventually you will become ashes whatever that mean you will not

  • exist basically let's not go there is like what's going

  • to happen to every individual atom in your body

  • it's a pretty sad and violent kind of uh death nasty stuff we got another one

  • from shintanu harad what is the mass of the biggest black

  • hole that we know oh the biggest black hole

  • that we know is in a nearby what is called a

  • brightest cluster galaxy it's a galaxy that's the

  • center of a cluster of galaxies and a cluster of galaxies is about a

  • thousand galaxies held together by the gravity of dark matter

  • and i believe the latest number that is published that i know

  • is a few times 10 to the 11. there's an uncertainty in

  • that mass so i mean i would say that for sure there

  • are black holes that are 10 times 10 to the solar masses

  • that's is that is that like hundreds of billions

  • 10 billion 10 billion for sure i mean there are uncertainties

  • in masses there are some there is one that's claimed to be

  • uh closer to 100 billion okay okay um here's another one from an unnamed

  • an anonymous submitter are gravitational waves quantized

  • like waves are they evidence of gravitons

  • um they're not quite evidence for gravitons

  • um gravitational waves um we believe propagate through gravitons

  • um loosely so you know we think about uh you know we think about light as

  • photons uh similarly we think that um gravitational waves

  • have gravitons associated uh with them we do not yet have um you know from the

  • measurements that we have currently made uh we can reconcile that picture it

  • doesn't mean that we've tested that picture

  • or that we've detected them okay so gravitons

  • we think they're there but but but we haven't we haven't found them yet

  • there's no detector that said ping ping ping graviton is detected for sure okay

  • um okay wow we just got a bunch oh just a huge number came in

  • let me sift through them very quickly um why are black holes in the center's

  • galaxies i think we covered that a little bit

  • um unless you have more to say there's many more um let's see what do they want

  • to um that was it just why are black holes

  • in the centers of galaxies i think but i think yeah i think we kind of covered

  • that a little bit i mean they're quite

  • massive they end up they either form there we know that many

  • of them form there to start with because i told you that

  • you know the formation of the black hole and the growth of the black hole is

  • intimately tied to the assembly of stars in the center of a galaxy

  • so some of them are born there some of them end up there because of mergers

  • they get you know twice and sent to the center

  • and then we also believe that if you had that you know that intermediate mass

  • black hole that we saw that was wandering in the outskirts

  • we think it will eventually wander in and get pulled into the centers it could

  • take a long time um and then some black holes could be

  • lingering around right but in general you either make

  • your way in wandering or during a collision or you're born

  • there okay okay very cool so we still it's

  • there's there's a couple of different options

  • uh here's a fascinating one from uh caleb

  • eridani uh what do you think will be the next

  • multi-messenger method that most greatly influences our knowledge

  • lisa neutrinos radio waves gravitational wave decryption

  • or something mathematical wow that is a there's a lot of options there that's a

  • lot of options i'm just going to pick my favorite

  • um i think my favorite so multi messenger uh

  • is basically this idea that you would see a phenomenon

  • in many many different wavelengths obviously with some time delays because

  • of the processes that produce them so for example one my favorite

  • is lisa and the fact that you would see merging black holes you would detect the

  • gravitational waves from these merging supermassive black holes

  • or a supermassive and an intermediate mass black hole

  • um or a supermassive and a tiny stellar mass black hole so you could see all

  • these combinations um with lisa um that that is my favorite

  • um multi-messenger phenomenon because in this case

  • we believe that one of the ways in which though when two

  • galaxies merge their black holes are going to merge so every uh

  • black hole in the center of a galaxy has a feeding disk of gas that is sitting

  • around it and that's called the attrition disk

  • and so when the second black hole during a merger

  • is brought in and threatening to the center it will fall

  • plop into the accretion disk so that is gas

  • and this gas will torque and drive these two black holes together in the final

  • final stages right so remember black holes are so

  • tiny that when two galaxies merge they're gonna miss

  • they're not gonna you know you're not gonna be able to hit them and make them

  • stick as it were and collide right on right

  • they're gonna kind of miss gonna get you know trapped around each other

  • grinding closer and closer and then it's gonna plop

  • into that feeding disc of the bigger one and so the secondary will slowly kind of

  • spiral in in that disc and that entire process

  • of spiraling in because it's gas it's going to be glowing we're going to be

  • able to see it feeding it'll feast because it's

  • going to be embedded in gas so we'll see a precursor to the actual

  • collision in electromagnetic components we might

  • see it in optical infrared radio and so on and then after

  • the two black holes merge remember there's still a lot of gas around

  • in galaxies so after that there could be a

  • superfeast there could be a gluttonous feast where basically

  • all the gas that's left over in the center is going to go slack

  • into the very massive black hole so you could see

  • various stages and that's why it's my favorite in the multi-messenger

  • you could see precursors you would then see so you would see precursors that

  • would alert you a week before a year before months

  • before then you would see the actual

  • gravitational waves and then after that you could see a

  • post-cursor another feeding episode of the very

  • massive merged black hole now feeding and

  • evacuating all the gas that's right around it

  • so it's you know you would get an observation that

  • is spread out over a couple of years and we can

  • pin down so many properties of the system

  • so that's why that's what i find super fascinating that

  • is super fascinating and also i just want to say i want to lobby

  • for you making those technical formal terms both plop and thwack

  • when we see this i want to i want to hear i want to see those in papers

  • so okay we've got to i'm going to get a bit of a reputation right like

  • burp with quack i mean i have to kind of improve my

  • um lingo now this is a here's an interesting one um

  • it's from lisa holt she asks how big a role

  • does dark energy play in a black hole's feeding slash fasting process

  • interesting question so it turns out that

  • dark energy actually plays no role because at some level right dark

  • energy if you will is this large-scale uh countervailing gravity

  • kind of entity in the universe so once a galaxy forms right it's like a

  • little closed box it's its own little universe

  • remember the galaxy itself is not expanding

  • right so remember dark energy is what we think is propelling the

  • accelerating expansion of the universe so once you form a galaxy you have

  • separated out from the expansion of the universe

  • which is why you know our galaxy is intact right it's not being spread out

  • and expanded out our galaxy is intact the milky way

  • and and we know that right even nothing is expanding like the milky way is not

  • expanding the solar system is not expanding because we have

  • so this is what we mean when we say a galaxy has formed it has separated out

  • it has decoupled from the largest sale cosmic expansion

  • and it's the larger scale cosmic expansion

  • where dark energy is relevant and that is what is driving it

  • on very small scales inside these objects are called collapsed

  • objects in the universe a galaxy is a collapsed region of the universe

  • which means that basically it's an airtight region that

  • is its own entity and so the and you know the black holes are sitting in the

  • center of the galaxy so by and large they are really not

  • affected by dark energy of course the early

  • formation of structure etc happened in the backdrop

  • of dark energy being around except that now we know

  • from you know dark energy was discovered only in 1998 right the accelerating

  • expansion of the universe but we also know that in terms of

  • cosmic epoch right that dark energy most likely was always around as the what we

  • call the cosmological constant it's one of the constituents of

  • the universe it's like the baseline energy of the

  • universe if you will but that it was constant over time and

  • that the universe went through stages where the energy of the radiation

  • dominated then matter dominated and then late in

  • the universe several billion years ago like five billion years ago or so

  • is when we believe our universe became dark energy dominated

  • so dark energy did not play a starting role

  • it was kind of wading in the wings and has only sort of

  • taken off in terms of its effect in the universe of course

  • it's deeply important for the future of the universe of course

  • yeah and actually that's a follow-up question very quickly uh there's more

  • here but one that's bringing to mind for me is

  • you know we talk about the different scenarios that exist for the future of

  • the universe long-term we talk about the so-called big rip the big crunch and i

  • guess maybe the big chill the big grip being where

  • dark energy somehow accelerates run away exponentially and then

  • you know next thing you know your atoms are being pulled apart ah i'm dead and

  • everything's terrible uh there's the big crunch where somehow

  • it ends up reversing you know as if as if maybe there's enough matter in the

  • universe to pull it all back together via gravity doesn't sound like that's

  • gonna happen uh and then it goes back to the

  • singularity almost uh and then you have the big

  • chill which is you know where it's almost like a flat universe and

  • everything just kind of severely spreads out you get the

  • kelvin style heat depth however um if there was a big rip

  • what would that do to black holes do we know

  • well um well we we believe that you know one way the uh the

  • the big rip uh could impact black holes i mean you know

  • it it it depends it depends on the nature of dark energy in detail so

  • um i think it would be um we would have to work with a particular model to sort

  • of work out what the consequences would be

  • but what is very clear i think is the one

  • of these three possible deaths the one that is easiest to visualize

  • for black holes and galaxies and so on is the big chill

  • so that's the one in which basically the universe is going to expand

  • dark energy is going to cause the expansion to speed up and so the

  • distances between galaxies is going to grow very very

  • dramatically it'll be a very lonely and isolated

  • universe and so the black holes themselves will

  • be intact in the centers of galaxies but nearby the distance between nearby

  • galaxies is going to grow so dramatically large wow

  • so folks we only have about 10 minutes left but i think that's time for enough

  • questions i'm going to try to squeeze in some more

  • pre are we good on time you've got some more time yeah well i have a few more

  • minutes so actually here we go so uh here is one

  • from juan pablo salazar it is is there any theoretical basis

  • for wormholes and would they be in any way related

  • to black holes

  • well um no um i mean you know you can think of if i want to be generous i can

  • say that you know um we can think about wormholes this

  • sort of this possible this imaginative thing where i said that you know you

  • could tunnel into possibly another universe

  • so that would be the wormhole um and i mean that's what we saw in interstellar

  • right which was i mean in every aspect i love interstellar except for this

  • one thing where you know there's this little wormhole near saturn or whatever

  • right that they go go through um but

  • um yeah no i mean they're speculative i wouldn't

  • um yeah they're speculation mathematical speculation where's kip

  • thorne when you need him kip kip log into the chat let's talk uh okay

  • i think kip kip would also agree that they are

  • speculative he he i mean i think he would like them

  • to exist i mean so would i as i said right

  • portals i also want to be mathematically so i mean

  • maybe in that universe somewhere out there that can happen um let's see

  • uh now here's one from oh oh i don't even know how to read this one hold on

  • i'm gonna do skip on that one uh here we go um

  • from hamad yusuf will spooky action at a distance we're gonna have to define that

  • um will spooky action at a distance uh hold true when an entangled particle

  • goes inside the black hole this is a good one

  • all right this is a good one so let's just define split the action at a

  • distance so that's quantum mechanics i think that

  • is um spooky action at a distance it refers to

  • the very smallest scale phenomena in which we have fundamental uncertainties

  • so facts things like the uncertainty principle heisenberg's uncertainty

  • principle right so where you cannot simultaneously measure

  • the position and the velocity of a particle

  • and and the kind of domain in which this separation

  • between the observer and the observed kind of

  • neat separation breaks down so the very act of measurement

  • influences what you the system that you are measuring it tweaks the system right

  • so uh so you know i'm going to interpret this question as sort of a big picture

  • question which is that you know um right now we

  • have not yet we've unified all the forces in the universe right the

  • four forces in the uh the four forces that we know exist in

  • the universe electromagnetism the strong force the weak force and gravity

  • right so the goal is to unify all the forces

  • and to but so far um gravity which is the force that governs

  • the larger scales and is relevant over the cosmos as we discussed today

  • has not been integrated with quantum mechanics which is

  • the physics of the small so we don't yet have a theory of quantum gravity a

  • theory of quantum gravity which would integrate these microscopic whatever

  • phenomena with cosmic level sort of phenomena so

  • a force that acts on those ranges that is the kind of theory you would need

  • to actually explain what would happen to have a good quantum description

  • of what would happen if a particle follows and falls into a black hole so

  • one you know so this hawking radiation that we talked about earlier right

  • so one of the kind of it's still sort of classical thermodynamics but

  • that is kind of gives you a hint of the kinds of things we're talking about so

  • you know vacuum is not actually empty right so vacuum is basically particles

  • and anti-particles whizzing and coming together destroying each

  • other and then kind of you know so they're called there's a constant whirl

  • of particle anti-particle pairs and um and the hawking radiation and

  • this black hole evaporation all of that has to do with the fact that right

  • around the horizon when you have these particle anti-particles pairs

  • you could occasionally have one particle go in um into the horizon

  • whereas the anti-particle comes out right so you're

  • actually losing energy if you will so um i think all i can say

  • is that you know we don't quite have a relativistic

  • and a quantum level deep understanding yet to talk about the

  • quantum mechanics of particles that fall into

  • but you know there are people working on it and there has been quite a lot of

  • progress made and in fact um you know hawking was

  • working on it till his last few days you know

  • i i think that's a beautiful answer because you

  • you answered actually about four or five different other questions in the queue

  • right there uh and unfortunately unfortunately everyone

  • we are out of time at this point um i wanted to just tell everyone you know

  • that i hope you had fun i had a lot of fun uh i hope you had some fun priya and

  • thank you thank you it's fun right so so you've

  • shared your outstanding knowledge with us today and

  • uh and everyone out there watching it again thank you and

  • i just i want to say i have to plug uh at scientific american we look forward

  • to seeing you all future webinars uh and uh in the

  • meantime i really hope that you can uh follow us on twitter

  • instagram and facebook also priya has twitter and it's a hot place for

  • astrophysics news let me tell you check it out yeah and also i

  • um i have a webpage if you want to read more a little more technical stuff on

  • the papers and the work and look i love black holes and

  • so it was the sheer pleasure for me to talk about something that

  • i know it's really strange to say this but i really care about black holes

  • i think maybe maybe again why are we here we could just be the product of the

  • burp of uh

  • yes and looking forward to many new discoveries meanwhile uh stay safe

  • everyone all right i think i'm breaking up i'll

  • see you guys i'll see everyone have a good one bye

Hello and welcome to dark frontiers a

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Geek Out Session: What do we really know about Black Holes?

  • 7 1
    joey joey に公開 2021 年 04 月 28 日
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