ESL Video Quiz: TOEFL Quiz 7: Black holesQuiz #: 12133
Listening Comprehension from TEDGlobal
Quiz by Danielle_BIA
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This transcript is copied from the one that
appears on the Ted.com website for this particular
talk. The transcript on the Ted website is
interactive, so if you want to listen to a
particular part of the lecture again, you may want
to view it on the Ted website.
Here is the copy of the transcript:
How do you observe something you can't see? This
is the basic question of somebody who's interested
in finding and studying black holes. Because black
holes are objects whose pull of gravity is so
intense that nothing can escape it, not even
light, so you can't see it directly.
So, my story today about black holes is about one
particular black hole. I'm interested in finding
whether or not there is a really massive, what we
like to call "supermassive" black hole at the
center of our galaxy. And the reason this is
interesting is that it gives us an opportunity to
prove whether or not these exotic objects really
exist. And second, it gives us the opportunity to
understand how these supermassive black holes
interact with their environment, and to understand
how they affect the formation and evolution of the
galaxies which they reside in.
So, to begin with, we need to understand what a
black hole is so we can understand the proof of a
black hole. So, what is a black hole? Well, in
many ways a black hole is an incredibly simple
object, because there are only three
characteristics that you can describe: the mass,
the spin, and the charge. And I'm going to only
talk about the mass. So, in that sense, it's a
very simple object. But in another sense, it's an
incredibly complicated object that we need
relatively exotic physics to describe, and in some
sense represents the breakdown of our physical
understanding of the universe.
But today, the way I want you to understand a
black hole, for the proof of a black hole, is to
think of it as an object whose mass is confined to
zero volume. So, despite the fact that I'm going
to talk to you about an object that's
supermassive, and I'm going to get to what that
really means in a moment, it has no finite size.
So, this is a little tricky.
But fortunately there is a finite size that you
can see, and that's known as the Schwarzschild
radius. And that's named after the guy who
recognized why it was such an important radius.
This is a virtual radius, not reality; the black
hole has no size. So why is it so important? It's
important because it tells us that any object can
become a black hole. That means you, your
neighbor, your cellphone, the auditorium can
become a black hole if you can figure out how to
compress it down to the size of the Schwarzschild
At that point, what's going to happen? At that
point gravity wins. Gravity wins over all other
known forces. And the object is forced to continue
to collapse to an infinitely small object. And
then it's a black hole. So, if I were to compress
the Earth down to the size of a sugar cube, it
would become a black hole, because the size of a
sugar cube is its Schwarzschild radius.
Now, the key here is to figure out what that
Schwarzschild radius is. And it turns out that
it's actually pretty simple to figure out. It
depends only on the mass of the object. Bigger
objects have bigger Schwarzschild radii. Smaller
objects have smaller Schwarzschild radii. So, if I
were to take the sun and compress it down to the
scale of the University of Oxford, it would become
a black hole.
So, now we know what a Schwarzschild radius is.
And it's actually quite a useful concept, because
it tells us not only when a black hole will form,
but it also gives us the key elements for the
proof of a black hole. I only need two things. I
need to understand the mass of the object I'm
claiming is a black hole, and what its
Schwarzschild radius is. And since the mass
determines the Schwarzschild radius, there is
actually only one thing I really need to know.
So, my job in convincing you that there is a black
hole is to show that there is some object that's
confined to within its Schwarzschild radius. And
your job today is to be skeptical. Okay, so, I'm
going to talk about no ordinary black hole; I'm
going to talk about supermassive black holes.
So, I wanted to say a few words about what an
ordinary black hole is, as if there could be such
a thing as an ordinary black hole. An ordinary
black hole is thought to be the end state of a
really massive star's life. So, if a star starts
its life off with much more mass than the mass of
the Sun, it's going to end its life by exploding
and leaving behind these beautiful supernova
remnants that we see here. And inside that
supernova remnant is going to be a little black
hole that has a mass roughly three times the mass
of the Sun. On an astronomical scale that's a very
small black hole.
Now, what I want to talk about are the
supermassive black holes. And the supermassive
black holes are thought to reside at the center of
galaxies. And this beautiful picture taken with
the Hubble Space Telescope shows you that galaxies
come in all shapes and sizes. There are big ones.
There are little ones. Almost every object in that
picture there is a galaxy. And there is a very
nice spiral up in the upper left. And there are a
hundred billion stars in that galaxy, just to give
you a sense of scale. And all the light that we
see from a typical galaxy, which is the kind of
galaxies that we're seeing here, comes from the
light from the stars. So, we see the galaxy
because of the star light.
Now, there are a few relatively exotic galaxies. I
like to call these the prima donna of the galaxy
world, because they are kind of show offs. And we
call them active galactic nuclei. And we call them
that because their nucleus, or their center, are
very active. So, at the center there, that's
actually where most of the starlight comes out
from. And yet, what we actually see is light that
can't be explained by the starlight. It's way more
energetic. In fact, in a few examples it's like
the ones that we're seeing here. There are also
jets emanating out from the center. Again, a
source of energy that's very difficult to explain
if you just think that galaxies are composed of
So, what people have thought is that perhaps there
are supermassive black holes which matter is
falling on to. So, you can't see the black hole
itself, but you can convert the gravitational
energy of the black hole into the light we see.
So, there is the thought that maybe supermassive
black holes exist at the center of galaxies. But
it's a kind of indirect argument.
Nonetheless, it's given rise to the notion that
maybe it's not just these prima donnas that have
these supermassive black holes, but rather all
galaxies might harbor these supermassive black
holes at their centers. And if that's the case --
and this is an example of a normal galaxy; what we
see is the star light. And if there is a
supermassive black hole, what we need to assume is
that it's a black hole on a diet. Because that is
the way to suppress the energetic phenomena that
we see in active galactic nuclei.
If we're going to look for these stealth black
holes at the center of galaxies, the best place to
look is in our own galaxy, our Milky Way. And this
is a wide field picture taken of the center of the
Milky Way. And what we see is a line of stars. And
that is because we live in a galaxy which has a
flattened, disk-like structure. And we live in the
middle of it, so when we look towards the center,
we see this plane which defines the plane of the
galaxy, or line that defines the plane of the
Now, the advantage of studying our own galaxy is
it's simply the closest example of the center of a
galaxy that we're ever going to have, because the
next closest galaxy is 100 times further away. So,
we can see far more detail in our galaxy than
anyplace else. And as you'll see in a moment, the
ability to see detail is key to this experiment.
So, how do astronomers prove that there is a lot
of mass inside a small volume? Which is the job
that I have to show you today. And the tool that
we use is to watch the way stars orbit the black
hole. Stars will orbit the black hole in the very
same way that planets orbit the sun. It's the
gravitational pull that makes these things orbit.
If there were no massive objects these things
would go flying off, or at least go at a much
slower rate because all that determines how they
go around is how much mass is inside its orbit.
So, this is great, because remember my job is to
show there is a lot of mass inside a small volume.
So, if I know how fast it goes around, I know the
mass. And if I know the scale of the orbit I know
the radius. So, I want to see the stars that are
as close to the center of the galaxy as possible.
Because I want to show there is a mass inside as
small a region as possible. So, this means that I
want to see a lot of detail. And that's the reason
that for this experiment we've used the world's
This is the Keck observatory. It hosts two
telescopes with a mirror 10 meters, which is
roughly the diameter of a tennis court. Now, this
is wonderful, because the campaign promise of
large telescopes is that is that the bigger the
telescope, the smaller the detail that we can see.
But it turns out these telescopes, or any
telescope on the ground has had a little bit of a
challenge living up to this campaign promise. And
that is because of the atmosphere. Atmosphere is
great for us; it allows us to survive here on
Earth. But it's relatively challenging for
astronomers who want to look through the
atmosphere to astronomical sources.
So, to give you a sense of what this is like, it's
actually like looking at a pebble at the bottom of
a stream. Looking at the pebble on the bottom of
the stream, the stream is continuously moving and
turbulent, and that makes it very difficult to see
the pebble on the bottom of the stream. Very much
in the same way, it's very difficult to see
astronomical sources, because of the atmosphere
that's continuously moving by.
So, I've spent a lot of my career working on ways
to correct for the atmosphere, to give us a
cleaner view. And that buys us about a factor of
20. And I think all of you can agree that if you
can figure out how to improve life by a factor of
20, you've probably improved your lifestyle by a
lot, say your salary, you'd notice, or your kids,
And this animation here shows you one example of
the techniques that we use, called adaptive
optics. You're seeing an animation that goes
between an example of what you would see if you
don't use this technique -- in other words, just a
picture that shows the stars -- and the box is
centered on the center of the galaxy, where we
think the black hole is. So, without this
technology you can't see the stars. With this
technology all of a sudden you can see it. This
technology works by introducing a mirror into the
telescope optics system that's continuously
changing to counteract what the atmosphere is
doing to you. So, it's kind of like very fancy
eyeglasses for your telescope.
Now, in the next few slides I'm just going to
focus on that little square there. So, we're only
going to look at the stars inside that small
square, although we've looked at all of them. So,
I want to see how these things have moved. And
over the course of this experiment, these stars
have moved a tremendous amount. So, we've been
doing this experiment for 15 years, and we see the
stars go all the way around.
Now, most astronomers have a favorite star, and
mine today is a star that's labeled up there, SO-
2. Absolutely my favorite star in the world. And
that's because it goes around in only 15 years.
And to give you a sense of how short that is, the
sun takes 200 million years to go around the
center of the galaxy. Stars that we knew about
before, that were as close to the center of the
galaxy as possible, take 500 years. And this one,
this one goes around in a human lifetime. That's
kind of profound, in a way.
But it's the key to this experiment. The orbit
tells me how much mass is inside a very small
radius. So, next we see a picture here that shows
you before this experiment the size to which we
could confine the mass of the center of the
galaxy. What we knew before is that there was four
million times the mass of the sun inside that
circle. And as you can see, there was a lot of
other stuff inside that circle. You can see a lot
of stars. So, there was actually lots of
alternatives to the idea that there was a
supermassive black hole at the center of the
galaxy, because you could put a lot of stuff in
But with this experiment, we've confined that same
mass to a much smaller volume that's 10,000 times
smaller. And because of that, we've been able to
show that there is a supermassive black hole
there. To give you a sense of how small that size
is, that's the size of our solar system. So, we're
cramming four million times the mass of the sun
into that small volume.
Now, truth in advertising. Right? I have told you
my job is to get it down to the Schwarzchild
radius. And the truth is, I'm not quite there. But
we actually have no alternative today to
explaining this concentration of mass. And, in
fact, it's the best evidence we have to date for
not only existence of a supermassive black hole at
the center of our own galaxy, but any in our
universe. So, what next? I actually think this is
about as good as we're going to do with today's
technology, so let's move on with the problem.
So, what I want to tell you, very briefly, is a
few examples of the excitement of what we can do
today at the center of the galaxy, now that we
know that there is, or at least we believe, that
there is a supermassive black hole there. And the
fun phase of this experiment is, while we've
tested some of our ideas about the consequences of
a supermassive black hole being at the center of
our galaxy, almost every single one has been
inconsistent with what we actually see. And that's
So, let me give you the two examples. You can ask,
"What do you expect for the old stars, stars that
have been around the center of the galaxy for a
long time, they've had plenty of time to interact
with the black hole." What you expect there is
that old stars should be very clustered around the
black hole. You should see a lot of old stars next
to that black hole.
Likewise, for the young stars, or in contrast, the
young stars, they just should not be there. A
black hole does not make a kind neighbor to a
stellar nursery. To get a star to form, you need a
big ball of gas and dust to collapse. And it's a
very fragile entity. And what does the big black
hole do? It strips that gas cloud apart. It pulls
much stronger on one side than the other and the
cloud is stripped apart. In fact, we anticipated
that star formation shouldn't proceed in that
So, you shouldn't see young stars. So, what do we
see? Using observations that are not the ones I've
shown you today, we can actually figure out which
ones are old and which ones are young. The old
ones are red. The young ones are blue. And the
yellow ones, we don't know yet. So, you can
already see the surprise. There is a dearth of old
stars. There is an abundance of young stars, so
it's the exact opposite of the prediction.
So, this is the fun part. And in fact, today, this
is what we're trying to figure out, this mystery
of how do you get -- how do you resolve this
contradiction. So, in fact, my graduate students
are, at this very moment, today, at the telescope,
in Hawaii, making observations to get us hopefully
to the next stage, where we can address this
question of why are there so many young stars, and
so few old stars. To make further progress we
really need to look at the orbits of stars that
are much further away. To do that we'll probably
need much more sophisticated technology than we
Because, in truth, while I said we're correcting
for the Earth's atmosphere, we actually only
correct for half the errors that are introduced.
We do this by shooting a laser up into the
atmosphere, and what we think we can do is if we
shine a few more that we can correct the rest. So
this is what we hope to do in the next few years.
And on a much longer time scale, what we hope to
do is build even larger telescopes, because,
remember, bigger is better in astronomy.
So, we want to build a 30 meter telescope. And
with this telescope we should be able to see stars
that are even closer to the center of the galaxy.
And we hope to be able to test some of Einstein's
theories of general relativity, some ideas in
cosmology about how galaxies form. So, we think
the future of this experiment is quite exciting.
So, in conclusion, I'm going to show you an
animation that basically shows you how these
orbits have been moving, in three dimensions. And
I hope, if nothing else, I've convinced you that,
one, we do in fact have a supermassive black hole
at the center of the galaxy. And this means that
these things do exist in our universe, and we have
to contend with this, we have to explain how you
can get these objects in our physical world.
Second, we've been able to look at that
interaction of how supermassive black holes
interact, and understand, maybe, the role in which
they play in shaping what galaxies are, and how
And last but not least, none of this would have
happened without the advent of the tremendous
progress that's been made on the technology front.
And we think that this is a field that is moving
incredibly fast, and holds a lot in store for the
future. Thanks very much. (Applause)
TOEFL Quiz 7: Black holes
Quiz #: 12133
1. What is the lecture mainly about?
2. According to the lecturer, why can't we see a black hole?
3. Which of the following is NOT one of the three characteristics used to describe a black hole
4. According to the lecture,
5. The Schwarzschild radius is useful because it provides all of the following information EXCEPT
6. According to the lecture, a supermassive black hole
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