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