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
