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I'd like to try to tell you something
about current research in black hole
physics, a current mystery.
So there won't be an answer here,
and that's one of the greatest parts of science.
It's an open, endless field.
So I'm going to be telling you about something we're working
on trying to understand.
To do it I need to tell you a few things about black holes.
The first thing I need to do is describe to you
that there are two types of these black holes.
There are the ordinary black holes, which
are formed when stars collapse.
And these individual stars turn into objects that weigh about 5
to maybe 50 times as heavy as our own sun.
If you want to know more about those come to the lecture
that we'll have a little later this afternoon.
And I can tell you more about those kinds of objects.
But for the moment, I want to tell you
about something different-- supermassive black holes.
These titanic beasts weigh millions to billions of times
as much as our sun.
And therefore, when the centers of galaxies collapse,
so the whole cores of galaxies like our Milky Way collapse
down to a very small object the size of our solar system,
containing enormous amounts of mass.
These objects are interestingly enough
pervasive in the universe.
Now, while a galaxy like our Milky Way
may have thousands of ordinary black holes,
it has one also of these supermassive black holes.
Our particular supermassive black hole is kind of pathetic.
It's only about 4 million times the mass of our sun,
but it's in our galactic backyard.
And so we can see the thing.
I show you here a photograph taken with a Keck telescope
in infrared light, looking down into the heart of our galaxy.
Now, if you take a little look at that box at the center.
We'll blow that up and show you an animation that
was made-- really a movie-- taken from observations
made over a number of years of the stars moving
in the central parts of our Milky Way by our colleagues.
These stars, as they move to the center of the galaxy,
are whipping around and orbiting around a, well,
little yellow cross, which isn't actually in there in the sky.
But if you zoom into that region where the yellow cross is
and track the stars over a period of more than a decade,
you can see them moving about an invisible object.
And as they orbit this object over this period,
you can see as they get close they whip by this thing.
And with the incredibly large mass that this object has,
you can see the motions of these stars
on very human time scales.
It's dramatic proof that there's an invisible mass of object
there.
It's a supermassive black hole.
Now, as I say, many galaxies have these objects,
and other galaxies seem to have even bigger black holes.
Intriguingly, the mass of the black hole in the center
seems to correlate with how big the galaxy is.
And that leads me to something interesting,
I say there's one deep in the center of the galaxy,
and that seems to be almost universally true.
That is a puzzle.
And the puzzle is for us because of a second fact
that I need to describe to you, and that
is that in the history of the cosmos over billions of years
galaxies have merged.
They've combined.
You started out with smaller galaxies,
and over time they build to bigger and bigger ones.
And those mergings are dramatic processes.
And these galaxies, perhaps very much like our Milky Way,
like these two shown here come close into space.
And then the gravitational attraction between the two
starts to distort and work the galaxies.
And as they get stretched and distorted
over this period of billions of years,
the central regions actually fly past each other, crash
together, and over time they merge into a single big blob.
One good question you might have-- here I
am, trying to describe to you something that takes
billions of years to occur.
How do I know that this is true?
I haven't had tenure for that long,
so I couldn't say from personal experience.
But what happens in astronomy is we do it statistically.
You can take pictures of the night sky,
and we see pictures that look exactly like this, or indeed,
like earlier phases in the process.
So we can see the game playing out.
Not in an individual galaxy, but across the universe
and galaxies over space and time.
Now, the end product of this, as you
can see those two blobs in the center coming together,
is what we call a merger.
OK.
Now, you might think from what you learned as a kid,
or maybe before you were born in Star Wars,
that all this interesting stuff happened in a galaxy far,
far away and a long, long time ago, but not so.
It's going to happen here.
It's in the future, in fact.
In our case, our own Milky Way Galaxy,
you can see on the right hand, stage-right side of the panel,
a picture of the Milky Way as you might
see from a dark desert sky.
And on the left side is a photograph
of the Andromeda Galaxy at about four billion years.
So don't worry, there's plenty of time.
The gravitational attraction between these two galaxies
is going to bring them together and we
will have a merger event.
We will merge with Andromeda.
So it's a process that's ongoing in the universe.
It's an interesting process because if we pay attention
back to those black holes that I described at the beginning,
if each of these galaxies had a black hole
in the center and the two things got together
and they make a new galaxy.
And that galaxy has one black hole--
there's the title of my talk-- where
has the other black hole gone?
So in this merging between all these black holes
boiling down 20 different components into one,
we have a mystery.
Something happened to that black hole.
OK.
There's essentially two possible explanations
that have been proposed.
And if you remember that animation showing you
the violent crash between two galaxies,
it seems plausible that one of the two
might have just gotten slung out and head off
into intergalactic space.
And so maybe only one was left behind.
But the alternative, and personally I
think more interesting possibility,
is that not only the galaxies get together and connect
and merge, but the black holes themselves do the same.
So here, again, we see the galaxies combining.
But if you look down into the product galaxy,
you'll see these two nuclei, these two massive objects
swinging around and orbit around each other.
And again, over a long period, we
don't know exactly how long in this case,
but it's hypothesized to be many millions of years,
the two black holes will orbit and get
closer and closer together.
And finally, as they speed up in their orbit
they will come so close that they
will start orbiting on time scales of years
and finally merge into a single massive object, a heavier
black hole.
Thereby preserving the one hope for one galaxy
that we saw before.
Well, that's an interesting puzzle.
And I would argue that we don't actually know the answer
to what's going on yet.
One thing we would very much be surprised by
is if this was easy to do.
There's a lot of energy in that orbit.
And to get those two things together and get
them to combine is difficult.
Theory, in fact, suggests that it's quite difficult.
And that there should probably be
many cases where it didn't work and they were stalled.
Yet, as I told you, we only see one black hole
in these galaxies.
So something got rid of the other one.
Well, if this stalling occurred, we
should see many close pairs of black holes.
And that's not something we do see.
Therefore, it was kind of fun a few years ago
when some colleagues and I went out and were
doing a survey of galaxies.
We managed to find one example where
there were two black holes, supermassive,
several billion solar masses, only about
20 light years apart.
We said, wow, we're onto something.
So we checked another-- it's in this rather undistinguished
galaxy name-- 4C plus 3.11.
Astronomy's done a really good way of doing that.
But when we went and looked at another several thousand
galaxies, it was still the only one of this sort.
So there's something special about this object.
And I and my colleagues are chasing this down right
now to see why this one was caught
in the act of merging instead of getting the whole job done.
And by learning what's special about this galaxy
we hope to learn about the process as a whole.
Why this is an exception to the grand rule.
I want to close with one final thought.
And that is I referred earlier to the energy involved
in the orbit, the gravitational energy that
must get released to bring these two black holes together.
A stupendous amount of energy, it far
exceeds the sort of energy you get
by the ordinary stars in the galaxy over periods
of billions of years.
And that energy must go somewhere.
We believe it goes into what are called gravitational waves.
These are, if you will, distortions or ripples
in space time that propagate throughout space, traveling
throughout the universe, carrying
the power and the information about this cataclysmic merger
event that might have occurred very far away.
Interestingly enough, those waves
are buffeting at us right now as we speak,
passing through the earth.
The signal-- a distinct cataclysm in the cosmos.
And one of the grand quests of astrophysics
that we're hoping to succeed in the next few years
is to detect such gravitational radiation,
to see the distortions in space time,
and to see the signals of the most exotic and energetic
objects in the universe that are being destroyed out
at cosmic distances.
So that's one of the great things we hope to do,
and Stanford is involved in that quest.
And I thank you for your attention.
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