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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