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