>>SAAVIK FORD: Hi, I'm Saavik Ford.
>>BARRY MCKERNAN: And I'm Barry McKernan.
>>FORD: And we're going to talk to you today about black holes.
So, we've been asked to start with a little bit of
introduction on what is a black hole. So,
I want you to think for a moment about
this box that i am going to toss up here
from the surface of the Earth. Ok.
>>MCKERNAN: Good catch.
>>FORD: So, what just happened?
>>MCKERNAN: It went up and
then it came down.
>>FORD: Ok, so when it went up, what happened when it came down?
>>MCKERNAN: Well, it slowed down and then it stopped going up
and then it started to come down and
it speeded up as it came down.
>>FORD: Ok, cool. So, what would happen if I tossed this up at a faster speed?
>>MCKERNANA: it's going to go further, but it's still gonna slow down, turn around and then come back down.
>>FORD: Alright, let's try it.
>>MCKERNAN: Try it, yeah.
>>FORD: Higher catch. Ok.
What if I threw it really, really, really, really fast, like
let's just say a hundred thousand miles an hour.
>>MCKERNAN: A hundred thousand miles an hour. Well, we'd do serious damage to the Museum ceilings.
We don't want to do that, but let's imagine there were no ceilings here. Then it would go
super fast that way. Because it's made of
plastic it would suffer severe damage and
probably melt, but let's assume that it
could go through the atmosphere. It would
go all the way out into space. Assuming that's indestructible.
>>FORD: Would it ever come back?
>>MCKERNAN: At a hundred thousand miles per hour? No.
>>FORD: Why?
>>MCKERNAN: Because it's moving so fast
that the pull that's trying to bring it
back this way isn't strong enough to
bring it back. So, it can escape.
>>FORD: Ok, so that's faster than the  escape speed of the Earth.
>>MCKERNAN: That's right.
>>FORD: Ok, so could I make the escape speed of the Earth bigger?
>>MCKERNAN: It would be really difficult, but yeah you
could in principle if you squashed the
Earth. Ok, so not  that difficult. Just
take the Earth and kind of
squeeze it together so that everything
on the surface of the Earth is closer to
its center. And then the Earth would be
denser, more compact. I would have a
stronger pull from the surface.
>>FORD: Ok, so then the escape speed at the surface
would be bigger than it is now.
>>MCKERNAN: That's right, yes.
>>FORD: And what if I kept going and
squashed it and squashed it and squashed it...
>>MCKERNAN: Then the escape speed would keep going up.
>>FORD: And what happens when the escape speed gets to be the speed of light?
>>MCKERNAN: Ok, well then, you would need to throw
something away from the squashed Earth
faster than the speed of light. But wait
a minute, that's a problem.
>>FORD: Right. So, then we're going to have a thing that can't emit light and it has an incredibly
strong gravitational pull.
>>MCKERNAN: So, if i'm in space
and i'm watching you squish the Earth
and everyone on Earth is screaming, going, "Ah, why are you squishing the Earth?" and it's getting
really nasty and hot and molten, and all
the rest and the escape speed from the
surface is increasing, eventually I'm going
to get to a point where the escape speed
reaches the speed of light and then goes
beyond the speed of light. So, light trying
to get away from the surface is going to
go, "Uh." It won't be able to get away.
>>FORD: So, we're like this box when I go like that.
>>MCKERNAN: Yeah, light would behave just like this.
So,  the squashed Earth that I'll see 
getting more compressed and molten and nasty
and all the rest, eventually I won't be
able to see it because light won't be
able to get away. I've squashed something
so much that the light can't get away.
The escape speed is too much.
>>FORD: So, that's a black hole.
>>MCKERNAN: Sweet, simple.
>>FORD: Cool.
>>MCKERNAN: How big- How much would I need to squash the earth to turn it into a black hole?
>>FORD:  So, you would need to make it so small—
>>MCKERNAN: Yep.
>>FORD: —that it would be- let's see
is the island of Manhattan small enough?
>>MCKERNAN: Way smaller. You need-
>>FORD: Way smaller.
>>MCKERNAN: That's difficult.
>>FORD: Yes. It's very hard.
So, we're not going to do that.
>>MCKERNAN: So, that's what a black hole is. So, what do we work on?
>>FORD: So, what do we work on? So,
we work on actually black holes that are
much bigger than the Earth. So, if we were
to do that sort of evil thing you know
the evil mad scientist and crush the Earth to the size of a grape, it would be very bad for
people on Earth, but the Moon would
actually not care. Or notice.
>>MCKERNAN: But wait—I thought black holes suck.
>>FORD: No, you see, black holes go around- or sorry,
the Moon is going around the Earth in an orbit. And it's being pulled on by
the Earth right now. And in that whole
process of crushing the Earth down to
the size of a grape we did not change
the mass of the Earth and we did not
change the distance from the Earth to the Moon.
>>MCKERNAN: From the center of the Earth to the center of the Moon.
>>FORD: Right, right. And so the gravitational force didn't change. The Moon is still going at
the same speed that it was and so it is still going to continue
to orbit quite happily, and it will not notice that the Earth has been turned into a black hole.
>>MCKERNAN: Wow. So, black holes [unintell]. Right.
>>FORD: We work on black holes that live in the
centers of galaxies and so a lot of
people would think, "Oh my god, the galaxy
is going to be swallowed by a giant black
hole," but actually it's not because the
stars that are in the galaxy orbit far
enough away and at the right speed to
not be sucked into a black hole. We're falling
into a black hole would be a more appropriate term.
>>MCKERNAN: So, how do we know this? What do we observe when we look at the center of
our galaxy towards where we think the
black hole is, what makes us think
there's a black hole there?
>>FORD: Ah, so we have
stars in the center of our galaxy that
are 10 times the mass of the Sun and
they are going around something. They are
being whipped around and we can actually
see them going in orbits that look just
like planetary orbits around our Sun and
there's nothing there.
MCKERNAN: So, it's as if you
saw planets- if you took our solar
system and you made the Sun go dark, but you
could see planets kind of going around
in these orbits, going [whoosh]. So, something is
making these stars that are 10 times the
mass of the Sun change direction, speed up,
slow down, speed up, slow
down. And we can figure out in the same
way that the Sun is way more massive
than planets that go around it and we can
make them whip around, the Sun can make
planets whip around. So, you have to have something
really massive to make stars that are 10
times the mass of the Sun whip around them
like this—like they're nothing. So, we can figure out that the
black hole the center of our galaxy is 4
million times the mass of our Sun and it
makes stars behave like they're just
little planets being flung around. Which is pretty
cool. We can watch that happen. If you go
on YouTube- we'll try and add a comment on
the- on the feed afterwards—a link to a
YouTube video which shows you what the star's doing.
>>FORD: Astronomoical observations. Actually there's a
black hole. Our galaxy, like most
galaxies with black holes at their
centers, are not emitting light. There's
not a lot of light being emitted from
the center of the galaxy. Black holes, of
course, emit no light themselves, but
if you had a lot of gas that was being
delivered to a black hole then the gas
would be trying to cram into the black
hole and as it falls, it's going to get into a
smaller and smaller space, rubbing up
against other gas that's also trying to
fall into the black hole and so as it
rubs up against the other gas, friction
causes it to heat up and the heat causes
it to glow very brightly and so in some
galaxies there are gas reservoirs that are
feeding black holes in the centers of
the galaxies, and those black holes, the
material that's falling into the black hole
is glowing very brightly and it's,
in fact, glowing more brightly than all the
stars in the rest of the galaxy put
together. And so when we look out we
originally saw these galaxies, we saw this
bright point in the middle that looked
like a star but we could tell when we
broke up its light into a rainbow
spectrum that it was in fact not a
regular normal star and people start to
call them something else—quasi-stellar
objects—which got abbreviated to 
QSOs or sometimes quasars if they emit
radio wavelengths. And then we realized that they
were actually enormous black holes at the
centers of galaxies that were being fed by
gas and the gas is emitting light that we can see.
>>MCKERNAN: Yeah, so, essentially even
though you can't see the black hole at
the center directly, in the same way
that you can't really- in a really good
horror movie you can't see the monster
but you can hear the screams of the
people that are being chewed up by the
monster, essentially by looking at
active galactic nuclei you can see stuff
being torn and shredded by the monster
that we can't see. So we figure out, "Ah
that's got to be a moster there because we hear a lot
of screaming."
>>INTERVIEWER (off-camera): A quick question from
Jennifer Kingslake. She says, "How do you
measure mass from just looking at
something or just from the light that's emitted?"
>>FORD: Ah, so we don't necessarily
measure it- There are indirect ways
of measuring it from just the light, but in
the case of, for example, the center of
our own galaxy—the black hole there—it's
very direct. So, it's the same way that we
measure the mass of the Sun or the mass
of the Earth or the mass of Jupiter.
If you have an object in orbit around
another object, then the speed that it goes
at basically tells you a
combination of the mass of the object
that's pulling on it and the distance
from that object. And so if we can
measure the speed of the orbits—we just
have to watch the thing go around—to
measure the speed accurately, and then
you measure the actual distance from the
object to where it seems to be being
pulled on from, then the only thing that
you don't know is the mass of that
object and then you can figure it out.
>>MCKERNAN: Yeah, good question.
>>INTERVIEWER (off-camera): So, how do you study
black holes and how is the study of
black holes changing? How are we getting
sort of better looks at them as the
years go on?
>>FORD: Ok, so actually we've
gotten some really exciting new tools to
look at black holes just in the last
year. So, there are many kinds of black
holes. So, there's the black holes that live in the centers of galaxies, and then there's
also black holes that live in our own
galaxy. For example,
in Cygnus X1 has a black hole that we
can see because it is destroying its
partner. It's a binary- it was a binary
star. So, you have a black hole and a regular
star and they're going around and the
black hole and the regular star have
changed their structure and distance in
a way that allows a black hole to suck
material off of the normal star.
>>MCKERNAN: Right. They're doing a dance of death, essentially.
The outer layers of the star are being
stripped and feeding the monster and so
we can figure out what the monster is
doing by watching the star being eaten alive.
>>FORD: And we think that stars like
Cygnus, stars that end up as black holes
like Cygnus X1, that they come originally
from very massive stars that end their
lives in supernovae and become- and leave
behind this remnant core that becomes a
black hole. Those are called stellar mass
black holes because they come from stars,
as opposed the supermassive kind that we
study in the centers of galaxies. And
last year the LIGO team detected for the
first time gravitational waves that came
from the merger of two stellar mass
black holes. And this was a very exciting
type of discovery and a completely new way
of looking at the universe, really. Up
until last year the way that astronomers
knew about the universe was almost
exclusively from the light that came to
us. So, we could look at what we saw and
then light would come to us and we could
analyze it and we have lots of tricks
for- ways of using that to get a
greater understanding of our universe,
but gravitational waves are a completely
different type of wave and people have
made the analogy that it's like before
we could see, but we could not hear. And
now with gravitational waves, we can hear
the universe. So, gravitational waves come
from- Actually here, let me make one. See, I
accelerated a mass. However, because
gravity is a very weak force I did not make a very strong gravitational wave, and no
one has a hope of ever detecting that
one. But if you have very large masses
and that are very compact, like black
holes, that are orbiting around each
other, then they generate very strong
gravitational waves that we now have
incredibly sensitive detectors to detect. And we basically were very excited to be
able to use this new window on the
universe to understand stellar mass
black holes. And in the coming years
there are plans for similar
gravitational wave detectors. LISA is
basically as the European and
hopefully joint with NASA plan to do a
space-based observatory that will let us
look at supermassive black holes and
their gravitational waves.
>>MCKERNAN: Yeah, one of the
big surprises from the LIGO result was
that the very first gravitational wave
detection came from two black holes that
were overweight compared to the black
holes that we see in our galaxy, the
stellar-mass black holes that we see in our
galaxy, and everyone was kind of
surprised by that and it means we have
to think about, well how could you make two slightly overweight- actually
significantly overweight stellar mass
black holes close together and have
them merge so that we could detect this. And we, in fact, among- we have a model that
we're working on that explains the merger
of these stellar-mass black holes in terms of
a swarm of black holes, small-mass black
holes in the centers of galaxies
around supermassive black holes and so
essentially, as Saavik said, if there's a
gas disk around a supermassive black
hole, there are also stars and small-mass
black holes also around this- the
supermassive black hole in the centers of
galaxies. Some of them will end up in
the disk. There's so much mass of gas in
the disk that you will slam some of
these small-mass black holes together
and so you can quite quickly build up
large amounts of black holes. And so one of
the things that we're really excited
about over the next two years is we want
to see if LIGO will detect
more of these overweight stellar-mass
black hole mergers. If they do, it would
suggest that some of these mergers
happen around supermassive black holes
in our- in our universe. So, that's super
exciting work we're-
>>FORD: And that's directly
reacted to our research.
>>MCKERNAN: Yeah, so we work on it.
>>INTERVIEWER (off-camera): I want to jump in with a couple of questions from our Facebook audience.
From Abby Stoms who asks, "Can a
black hole die?"
>>MCKERNAN: Yes, in principle.
>>FORD: So, yes. We think that
a black hole can, in fact, die. But not right now.
>>MCKERNAN: Have patience.
>>FORD: So, basically,
this is one of the things for which
Stephen Hawking is most famous, is this
idea called Hawking radiation. And it
comes from the idea that if you have a
black hole that it can evaporate. And
this comes from the idea that in our
universe right now in this room and
everywhere where you are—your room—there
are particles that are popping into and
out of existence. And we have
experimental evidence for this. There's a-
there's an experiment you can do called
the Casimir effect that can detect these
particles that pop in and out of
existence. And this is totally legal as
long as they pop in and pop out in a
very short amount of time. The problem is
that they're doing this in every cubic
centimeter of the universe-
>>MCKERNAN: Including
right here.
>>FORD: And also right around the
edge of the event horizon of the black
hole. And so we said, right, that the- a black
hole is formed when the escape speed
from the surface of an object gets
bigger than the speed of light. So, the-
this effect where particles pop into and
out of existence don't- doesn't care that
there is this surface. And so particles
will pop into and out of existence
perhaps across this surface. The problem
is that the gravitational pull on one
side is in and strong and it's slightly
less strong on the other side. So, you could
have a situation where a particle pair—
and they always come in pairs and
that's why they come- they
pop out, may recombine. And they always
do this as this pair, and as long as they
pop out and recombine as a pair, that's legal.
>>MCKERNAN: But if one goes inside the horizon- the
event horizon of a black hole and the
other's left outside they can't come back.
>>FORD: They can't find each other in legal time to make it all ok, and conserve energy. And so what actually
happens is something else has to
conserve the energy that was spent to
create the particle pair. And what is
going to pay the bill is the mass of the
black hole. So, that particle that escapes
carries away a fraction of the mass of
the black hole and-
>>MCKERNAN: The particle-
Basically, the particles that pop into and
out of existence are real only for a
tiny, tiny fraction of a second, but if
you've just taken one away the other one
can't annihilate with its partner. It's
become real. It exists. It can no longer
go away. It's real and it scoots off, away
from the black hole. So, you've just had-
you have a stream of particles moving
away from the black hole. So, the black
hole looks like it's emitting a whole
bunch of particles and the energy to
make those particles real and stay real
comes from the black hole. So, the black hole is losing energy as it radiates the
stream of particles away. So, if there's
nothing else feeding the black hole and
if you leave the black hole alone,
eventually it's going to lose- keep
losing energy, keep losing energy. It's gonna shrink. It'll get smaller and smaller and smaller...
And eventually, it will explode in a flash.
Problem is, black holes have gravity. They can
pull on a lot of material and space
isn't really that empty. And so there's
enough radiation, there's enough mass that
these things will stay in existence
until a very long time from that.
>>FORD: And, in fact, even if you had a completely
isolated black hole in the middle of nowhere,
away from all matter in the universe,
right now the light that pervades the universe,
that is left over from the Big Bang, is
hot enough that any dying star that will
produce a black hole, that black hole's
mass is big enough that it will divert
the light from the cosmic microwave
background radiation and it will prevent
that black hole from evaporating. However,
as time goes on, the universe is getting
more and more spread out and that radiation
is decreasing in temperature. So,
eventually the light will not be able to
balance out even stellar-mass black
holes. So, if you have very, very, very small
black holes that we don't actually know
how to make, like black holes the size of
the Earth, those could have evaporated by
now, but we don't- we don't see any yet, so-
>>MCKERNAN: So, if you're a black hole and you're watching, it's okay you got lots of time.
>>INTERVIEWER (off-camera): I do not think we could wrap up on something more mind-bending than that,
so I think that's where we're going to leave
this one. Thank you guys so much for
making time for us today.
>>MCKERNAN: Yeah, absolutely.
>>FORD: Sure.
>>MCKERNAN: Thank you.
