Dimitrios Psaltis | The Black Hole Test || Radcliffe Institute

August 18, 2019 posted by


[MUSIC PLAYING] – Thank you very much, Judy,
for this warm introduction, and for pronouncing all
the Greek names right. That was very impressive. And thank you to the
Radcliffe Institute for giving me the opportunity
to be here this year. And thank you to
all the Fellows that make this year actually
remarkable intellectual endeavor. I have really, really enjoyed
the first couple of months. I also enjoyed a lot the various
talks that I attended here that other Fellows gave. And I must say that
listening to all these talks, I got very stressed preparing
mine, because they were superb. The second thing is,
because this is not really a public talk– if it
was just a public talk, I would just show nice pictures,
and movies, and telescopes, and everybody will be happy. It is not a professional
talk, because then I would show things that I really
do, which look like this. And this equation
number 56, mind you. And yet what I consider
this audience to be is a group of highly intelligent
and educated intellectuals that not only want to know
what we’re doing, but also want to understand
why we’re doing it, and what was the
process that let us make the decisions
that we’re making. So what I decided to do is that
the second part of the talk, I will focus on this wonderful,
arrogant experiment that will allow us to take
the first ever picture of an astrophysical black
hole, and allow us to test Einstein’s theory of gravity. But I will start the beginning
of the talk, the first part of the talk,
effectively giving you a history of how those
ideas were developed, because I want to
make two points. One is that some of these
ideas are truly revolutionary. And the second thing is
that, in order to make truly revolutionary ideas in science,
you only have to make small but very bold steps, that
you cannot do without. And this is the kind of point
that I will try to make. So let’s start at
the very beginning– a good place to start,
whatever it’s called. And Einstein did most
of his important work in the beginning of
the 20th century. So it’s important
to discuss what was the landscape of physics at
the end of the Victorian Age, the end of the 19th century. And it’s fair to say that
most physicists preoccupied themselves with
two general ideas. One is thermodynamics,
which is the physics of heat and engines, and the other
is electricity and magnetism, which is the physics of
cables, electricity, magnetism and light. And this was a wonderful
time to be a physicist, just like today, because
people could actually see immediate applications of
those fundamental theories that since, remarkably, brought
the Industrial Age, transportation, quality of
light– Edison’s light bulb– and telecommunications,
just like today. And people became more and
more arrogant by realizing how that works. And the end of the
Victorian Age in physics is actually marked by this very
famous speech by Lord Kelvin. And you might have
heard the name Kelvin by the Kelvin degrees
and temperature scale. His name was actually
William Thompson, and he was the first
Baron of Kelvin. He gave this talk
in April, 1900, and he talked about
the status of physics at the end of the 19th century. He called his talk
“Nineteenth Century Clouds over the Dynamical Theory
of Heat and Light.” And he starts by saying that
the beauty and clearness of the dynamical theory is at
present obscured by two clouds. What he really means
to say is that we understand all the physical
experiments that we do. We have two theories that can
tell us absolutely everything. But there are two
little problems. And that ends up being a very
shortsighted remark, because it turns out that one of
those little clouds was the beginning of special
and general relativity. The other was the beginning
of quantum mechanics, which is, of course, the scientific
revolution of the 20th century. But let’s give that to him. And I’m not going to talk at
all about the quantum mechanics aspect. I’m going to talk about,
starting from this point, why one of those clouds led
Einstein to his new theories and to black holes. And that cloud, I would like to
call the relativity of speed. This is the very
intuitive concept that what you assign as speed
to something that is moving depends on how fast
you’re moving yourself. This is something that comes
from common experience. And the easiest way to
demonstrate that is, I can be sitting on the
top of a mountain in Chile looking at the sky
in the telescopes, and then I can take
a time-lapse movie. Of course, time is moving much
faster than it naturally does. You can see the
stars and the galaxy coming around, and
moving from east to west. Now, we don’t really
believe that the stars are moving from east to west. What’s happening is that we,
ourselves, are taking part in the motion of the Earth,
which is the other way around. And we attribute this motion
as a speed for the stars, like it seems as if
the stars are moving. So what you, yourself, defined
as motion from somebody else depends on how much
you’re moving yourself. That was fine. And all the way until the
beginning of the 20th century, pretty much all
phenomena in physics seemed to obey that rule. What you measure as speed
depends on how fast you’re moving, with one little
cloud– one little exception, and that little cloud was light. There was both
experimental evidence– and for those of you who
understand what that is, I’m not going to say anything. And there was theoretical
interpretations of those experiments by another
famous physicist, James Clerk Maxwell, that demonstrated
that when you look at light and you try to
measure its speed, it doesn’t matter who
is measuring the speed. It doesn’t matter how fast
you yourself are moving. You always get the
exact same speed. And if you spend a
second to think about it, this is extremely ridiculous. It’s like I’m driving– the
Garden State is that way– I’m driving up Garden State towards
the hill, and I look outside, and I look at the
clouds, and I measure the speed with
which they’re moving backwards towards the square. I’m happy. I turn around. I start driving with the same
speed towards the square. I look out the window,
and guess what? The trees are moving with
the same speed with me towards the square. That’s what it
means that I infer the same speed of
an external object, no matter which way I’m moving. That clearly is
counter-intuitive. And this is something
that bothered scientists for a very, very,
very long time– several decades– because
nobody could come up with a way to look at science, to look
at the experimental results, and make sense out of it. And this is where Einstein came. In 1905– what we call
“The Year of Miracles,” because he wrote a number of
articles that actually changed, fundamentally, the course
of physics– Einstein introduced his Special
Theory of Relativity to remove that paradox. A side product of that theory
is his famous equation– here from his manuscript– E equals
Mc squared plus other terms that people usually
don’t think about. But I’m not going to talk
about this particular aspect. We’re talking about the
relativity of speed here. And what he said is the
following, what is, really, speed? Speed involves space and time. If I move 100 miles in an hour,
my speed is 100 miles per hour. Now I want somehow, as
I move as fast as light, the measurement of speed
and the measurement of time to conspire in such a way
that when I take the ratio, I get the exact same number,
no matter how fast I move. That’s the only way to
explain the problem. And what does that mean? It means the measurement of time
and the measurement of distance depends on the speed
of the observer. So suddenly, he took the
relativity out of speed, and he turned it into the
relativity of space and time. And that’s why he calls it the
Special Theory of Relativity. Now, that’s a little
bit of a misnomer, because when we think
about theory of relativity, not thinking too hard, we
think that Einstein said that everything is relative. Actually, Einstein did
exactly the opposite. He said, space and
time are relative, but there is
something absolute– the speed of light [INAUDIBLE]. The speed of light is fixed. And more properly,
the theory should have been called “Einstein’s
Theory of the Absolute,” as opposed to Einstein’s
Theory of Relativity. That would have been a
lot more appropriate. And it turns out that
it’s impossible to make a physical theory where
everything is relative. You always have
to have an anchor on which you base things. And this is the
anchor that he used to make his Theory of
Special Relativity. OK, nothing so far complicated. But then Einstein realized
that this is a little weird, because if everybody
measures a different space and a different time, how
can we ever agree on what is the result from experiment? If I ask you, and you give
me a result, and I ask you, and you give me a
different measurement, how can we ever compare? So he actually
became very obsessed, and you can see it
in his writings, making what became
famous, afterwards, a set of thought experiments–
Gedanken experiments– where he would play out many,
many different possibilities where he could come up with
a paradox in his ideas. And these are very famous
thought experiments, because it turns out that
there’s no paradox, because you can always come up with a very
unique way of communicating to people, and comparing
results, and discussing your relative difference
in time and space. And the big part of it
is, how do you incorporate synchronization of clocks? If we have two
clocks, and our clocks are moving at different
speed, at what point do we share information and tell
each other what is the time? So this was the whole idea
of his thought experiment. And when I was
reading that stuff, back when I was an
undergraduate student, I was always impressed
that Einstein was so obsessed with time. Everything was about
time synchronization of clocks in his writings. And apparently, I
was not the only one. Peter Galison, who is a
Professor here at Harvard, wrote a beautiful article
in 2000– many years after I was a graduate student,
and I read it soon afterwards– where he said he had
the same mission. He was sitting in
one of– maybe it was Zurich– one of the
stations in Switzerland. Einstein was living in
Switzerland at the time. And he was surprised that
Einstein was so impressed about time and synchronization. And then he looked up,
and what do you look up when you are in Switzerland? You just look at clocks
that are all moving at exactly the same time. And this is actually
from his article. This is a diagram
of the city of Bern. Actually, that’s
where Einstein lived. And all these red
circles are clocks on the city that are connected
with an underground electric circuit, to make sure they
all show the exact same time. So it is actually
not that surprising that Einstein was obsessed with
clocks and synchronization, because he woke up with them. He went to work with them. He went to sleep with
them, and it was always part of his life, which I
think is an interesting insight about Einstein and his theory. So Einstein you solved one of
the clouds of the 19th century. But very soon, he
realized that he actually introduced an even worse cloud. And that is the
fact that there is something that was not
working with his ideas, and that was gravity. Why was that a problem? Because his whole point of
making the speed of light constant required, in
his whole framework, that nothing moves faster
than the speed of light. This is one of the basic
principle of physics that we believe in today. But there is something
that does move faster than the speed of light,
and I would call it the speed of gravity. And I will explain in a
second what that means. Remember what gravity is? We have the Earth. We have the Sun. And Newton told us the
Earth and the Sun feel a gravitational interaction. One goes around the other. But the Sun is not fixed. It is not staying constant. It’s moving in the galaxy. So somehow, the Sun, with
its gravitational force, communicates that
information to the Earth. And the Earth is
coming around with it. The Earth is not left
behind from the Sun and the motion of the galaxy. And Newton told us that this
is done instantaneously. When the Sun moves a
little bit, the Earth knows immediately, and
moves together with that. But that was impossible in
Einstein’s new world theory– nothing could be
communicated instantaneously. So he spent 11 years
between 1905 and 1916, very frustrated, working
with serious problems– it was during the last two years
during the First World War, and at the time he
had moved to Berlin– trying to make a
theory of gravity that had this finite
speed of propagation. And he just couldn’t. And we know now why,
because it is impossible. It is impossible to make
a theory of gravity that agrees with all the
experimental data, and yet to have a finite
speed of propagation. And that was a big problem. And Einstein solved it
in a very Einstein way. And I will explain
that in a second. He said, “To hell with gravity.” These are the small,
bold steps that you need to make to make
scientific revolutions. He effectively said,
it doesn’t exist. There’s no such
thing as gravity. You’re going to say
here, OK hold on a second– the whole
point of science is experiments in the
world, and you look out, and make sure that
everything agrees. And I can make a
trivial experiment. I can take my keys,
and I can let them go. Why did it go down if there
is no such thing as gravity? And of course,
this was Einstein, so he had a few of those
wonderful thought experiments. And he said, this
is just an illusion. And let me explain what
do I mean by an illusion. He said, let’s think
about it thoroughly. Let’s assume that– forget
the Earth, and the atmosphere, and all the other
potential forces. Let’s take two stars, put
them far away from everywhere, and let them go. Now, you would
think that if there is no such thing as gravity,
the only thing that would happen is that they would go
down straight lines, because nothing is there
to perturb their motion. And yet, what really happens
is one attracts the other, and they eventually
crash to each other. So you and I would
have looked at that, and interpreted that the
straight lines would have gone up and down, but
the paths of the objects are actually curved. What Einstein said is that
no, these are curved lines. And the paths of the objects
are actually straight. And it’s just an illusion. But if they were indeed
straight and parallel, how is it possible that
parallel lines cross? The whole definition of parallel
lines from the ancient Greeks is that you send down two
parallel lines, like a railway, and they never cross. Is there any way to have two
parallel lines– illusions or no illusions–
that eventually cross? And Einstein was very fortunate,
because it actually is true. That is possible. And with his friend,
Marcel Grossmann that was back in
Switzerland, they actually started reading the ideas of
a famous mathematician who died extremely young,
Bernard Riemann, who wrote an entire thesis
in the mid 1800s called, “On the Hypothesis
which Underlies Geometry.” And Mr. Riemann asked
the simple question, is there any way to make
two parallel lines cross? Now, it doesn’t get
more abstract than that. He didn’t have much
luck explaining that in cocktail parties and stuff. But it ended up being
extremely, extremely useful for relativity. And it’s actually
very easy to see why. I’m going to make a
little demonstration here, how is it possible to make
two parallel lines cross. So here’s a blowup of
the map of the Earth. You can recognize the top
part of the North America. This is the equator. This is the country of Ecuador. And what I can do is, I can
take two straight parallel lines coming out of the Equator. How do I know they are parallel? Because they’re both
perpendicular to the Equator. This is the definition, if
you want, of a parallel line. They look nice and parallel. They don’t cross. But then if I zoom
out, they do cross. They do cross at the pole. All the lines that start
parallel in the Equator do cross at the pole of the
Earth, and the other side as well. And what that demonstrates–
which is actually the whole idea of
Riemann’s geometry– is that the fact that
parallel lines do not cross is true for flat space. If you draw two parallel
lines down there, they will not cross, assuming
that this continues to be flat. But the minute the space–
or space-time, if you wish– gets curved, then things
that start as parallel lines eventually cross. And this was the
remarkable intuition that Einstein had– that
what gravity really is, is not a force–
an interaction– between particles. But the minute you put a massive
object in the middle of space, that massive object
curves space-time. It warps space-time
such that the future of all the other
objects turn towards that big, massive object. So the reason
gravity works is not because things interact
with each other, but because they have
turned their futures towards each other. This was the fundamental
intuition that Einstein had. Now you’re going to say we
had a little bit of discussion of parallel ways of
interpreting the same reality. Is that any better,
than Newtonian gravity other than the fact that
you can make it have a finite propagation speed? And there was an
obvious interpretation. If it is parallel
lines that appear bent, then anything that follows
parallel lines is going to appear attracted by
gravitational objects– by anything else. And light, for
example– light always goes along straight
lines– will also appear as it gets attracted
by massive objects. Newton did not say
anything about light would not even care
about massive objects. Light has nothing
to do with gravity. But Albert Einstein said, no,
light follows straight lines. And all straight
lines are curved. So in this particular
demonstration, if you put the Sun in
the middle of space-time and you make a curve, and
you put a star behind it, then the light from
the star gets bent as it goes next to the Sun. And since we don’t know
that this is happening, we assume– this is
our interpretation– that the location of the
star has actually moved. So the central object acts
like a gravitational lens, because it lenses the light
to move it in a different way. That was a prediction that was
written in the original paper by Einstein in 1916. And very quickly, within three
years– and the only reason it took three years
is because people were waiting for the
First World War to be over– English astronomers,
British astronomers, went out to a place
where they could have a solar eclipse–
because of course, if you want to see stars
during the day behind the Sun, you need to have
the Sun blocked. This is the negative from
one of those pictures. This is the Moon. This is the solar corona. And those circles–
you can probably not see them– those are
the location of the four stars that they
measured, their position. They compared with the
positions six months earlier when the Sun was in a
very different place. And they show a displacement. Not only that, the
displacement was exactly what Einstein predicted
in his theory, given the mass of the Sun. And that was remarkable news. Suddenly, we start talking
about curved space-times. We’re talking about
space-times to begin with, and Einstein
becomes very famous. And even New York Times reported
the wonderful discovery. “Lights All Askew
in the Heavens,” The New York Times
title, “Stars not where they seemed or were calculated
to be, but nobody need worry”– whatever. “A Book for 12 Wise Men,”
and “No more in all the world could comprehend
it,” said Einstein, when his daring
publisher accepted it. Of course, this is all bogus. None of that ever happened. He published it. He gave a talk,
and he published it in the Proceedings of the
Prussian Academy of Sciences. But it made a better story. It’s The New York Times. But the punchline is that
this is, of course, what made Einstein a worldwide icon. And you can see his face,
and pretty much all countries have one way or another of
demonstrating how much they appreciate what he has done. And this is what I usually show
to talk about Einstein being an international icon. But Halloween two days ago, I
was actually walking around, and I saw a crazy
scientist dressed up with white hair going like that. And everybody said,
you’re a crazy scientist. And Einstein is a household
name and household appearance. So this was where all
his ideas came from. And you see, there are
small steps, trying to solve one problem at a time. And you’re getting deeper
and deeper and deeper down the drain, literally, by
making more and more bold moves to make everything work. But fundamentally, then,
you look at experiments. And the experiments either
confirm or contradict you. And for the case of
Einstein, they confirmed it. But then very
quickly– actually, two years after the discovery
of the development of General Relativity– people started
realizing that you can even take that theory to
a higher extreme, and say, what if I take a
star and I crunch in so much– I make it so small
and dense– that when the light from the
star comes out, it gets bent so quickly that
it turns back into the star? So not even light can escape. And this is the whole idea of
a black hole– an object so collapsed, so dense, so
gravitationally strong, that it does not even
let light escape from it. And now I’m going
forward 80 years, because there was an
incredible amount of work– both theoretical and
experimental– people really arguing and fighting
this notion, because it’s in the
extremely counter-intuitive. Do black holes exist? Do black holes get formed? How would we know
we see a black hole? But with the advent
of modern telescopes and modern technology,
there has been multiple ways of finding circumstantial
evidence that black holes do really exist in the universe. And in my mind, by far the best
one– until very recently– was the black hole in the
center of our own galaxy. And how do we know that
such an object exists? When you take a picture with
a very large telescope– this is from the Caltech
telescope in Hawaii– in a particular
wavelength all the way at the center of the galaxy,
you see a number of stars. And don’t worry
about the colors, because those colors are meant
to demonstrate something else. And if you take the picture
in 1995, and then one picture every now and then,
then what you see is that all the stars– all the
[INAUDIBLE] seven, eight, nine, ten, and so on– all the stars
orbit a very common center, that of course, has
an asterisk here, but there’s nothing there. No light comes out of there. This is just to demonstrate
what that common center is. And you can do the
usual physical exercise that an undergraduate
student can do, and infer how big is the mass of
that object in the center that attracts all of them
and makes them orbit? It turns out to be
equal to 4 million times the mass of our Sun. Now, you have an object
here that is 4 million times the mass of the Sun. And yet it’s not as
luminous– not as bright– than any of those single stars. You want to confine 4
million stars in a place and make them not
produce any light. And it turns out
that the only object that we know astrophysically,
or physically, or that we can envision, that
could do that is a black hole, and nothing else. So this has been,
until very recently with the discovery of
gravitational waves, which I’m going to talk about, the
best evidence that there is something very massive
and very black– a black hole– in the
center of our galaxy. So what we wanted to do is,
like Thomas from the Bible, is we wanted to see
it to believe it. So several of us
decided that we really want to take a picture of that–
an actual, real picture of that empty space in the middle of
the galaxy, and see is it really a black hole. And of course, the
question that you’re asking is, how big that
hole is meant to be? What it will be like taking
a picture of a black hole? And I can give you numbers, but
they’re so small that I cannot even understand what
these numbers really mean. And the simple similarity
that I can make is, I can say it’s equivalent of
taking a picture of a doughnut. So this is a hole, and
there’s stuff around it. This is what [INAUDIBLE]
we expect the black hole to look like. However that doughnut is
going to be extremely small, because it’s really far away. It’s at the center
of the galaxy. And how small would it be? It will be like taking a
picture of that doughnut on the surface of the Moon. This is how small
it’s going to look. Now, I’m sure you’ve all
seen pictures of the Moon. And I don’t think you’ve
seen anything as small as a doughnut in any of them. No current camera,
no current telescope, has a resolution like that. And therefore, you
had to think way out of the box of how
to make that happen. Keep that thought. The second thing is,
what does it really mean to take a picture
of a black hole? What is a hole? What would the hole look like
on an empty sky, on a black sky? It will look like nothing. You won’t be able
to see anything. But the galaxy is
never really black. There’s always clouds. There’s always plasma. There’s always hot matter
that is orbiting around. And all those stars that
orbit the black hole spew out a lot of
that hot matter. So if you look at the right
part in the electromagnetic spectrum, you might be
able to see the silhouette. You might be able to see
the shadow, the hole, in the bright background
to the diffuse emission. And when I was talking before
about how would it look, of course, nobody
had any intuition. But I think a lot of you might
have a little intuition of how a black hole would look, for
those– I guess this is not the right crowd for this joke. So in Interstellar is a movie. Movies are things
that you go and watch. [LAUGHTER] We have a few movie
directors in this audience. But there was a movie that
became pretty well known in the sci-fi genre a
couple of years ago. It’s called Interstellar with
a couple of famous actors. And what they did is eventually,
they go into a black hole. And this is what
the black hole is supposed to look in their case. There’s a lot of
plasma material around, and the shadow of the
black hole in the middle. Now, for some reason they wanted
to make it very realistic. And they asked one
of the, actually, fathers of black
hole astrophysics– his name is Kip Thorne
from Caltech– to help them out, write all the
various equations, and do the real calculation
with Einstein gravity and lensing and everything
of how black hole looks like. I don’t know why, because
when I go to the movies, I don’t care how
realistic it looks like. When I go to Harry
Potter, I’m not asking why a hippogriff works. Maybe Michelle
likes those things. I don’t know, she gave a talk
about that the other day. But what I found
very fascinating, related to our work, is this
quote from a Wired Magazine discussion about the
movie Interstellar, where they say that some
individual frames took up to 100 hours to render,
the computation overtaxed by the bendy bits
of distortion caused by the Einsteinian effect
called “gravitational lensing.” Each one of those frames
in the movie– and this is high frame rate– took
them more than 100 hours to render with the
correct physics. And that has actually
been a problem that we’ve been dealing
with, because we want to know what would the black hole
look like in order to train our telescopes to look at them? And we were always
confined by the fact that computers were
not fast enough. Because what you
want is, you want to send millions and millions
and millions of light rays into a simulation, just like
a weather simulation of what happens around the black hole,
and turn it into an image. And that was extremely
computationally complicated. So something that we did
that made a very, very big difference at the University
of Arizona in our group is, we realized that each
one of our computers– and even each one
of our cellphones– actually has a
specialized microchip that allows you to do very, very
quickly rendering of images. It’s called a graphics
processing card, a graphics card. And because of
computer gaming, they have actually been
developed so quickly that if you care to do nothing
else other than rendering, they can do it remarkably fast. So what we did is,
we hacked into it. And we added a black hole. Why not? And this is the
hand of our post-doc who actually did
most of that work. And we let the graphics card of
the computer do the rendering. So we provided the information,
and had the graphics chip of the computer to do it. And our code became–
our computer algorithms– became so fast that now, we
don’t show animations that come out of our simulations. We actually show, running in
real time, the simulations. And not only that, but they’re
so fast that [INAUDIBLE] who does that cannot type fast
enough commands in order to make them– to stop it,
start it, and everything else. So he has a little interface
he has called a “leap pad.” It is like the X-Box
that you can actually control the simulation
with his hands, like Minority Report style. And so you will see
what will happen. This is the black hole. It’s on YouTube as
well, with some music, if you want to see it. So this is the black hole. And he’s going to throw a
wall of 1,000 light rays. And as they go close
to the black hole, they’re going to
get scattered away. And he’s going to stop it,
explore it, do various things that he wants to do. So there he is. This motion says,
“Start the simulation.” There’s the one–
million light rays. He stopped it. He you can turn it around, all
real time as the calculations happen. He can zoom it in. These are all the lights. This is the black hole. It’s about to cross
into the black hole. It turns very red,
gets scattered, gets a particular structure. So he can move it all around. He can go forward in time. He can go backwards in time. This is the kind
of ways that we’re using this particular
interfaces, and the extreme rapid algorithms
that we developed to understand what happens to light
close to a black hole, and make appropriate
predictions. Now I think this goes
on for [INAUDIBLE]. He has forward in time,
backwards in time. It’s mesmerizing, isn’t it? Imagine doing it yourself. So sometimes, I come
and do it here in front. But I usually fail, because
while you’re talking, you’re [INAUDIBLE]
and it doesn’t work. So I just thought the YouTube
video is a little better. And the joke that
we had– we actually had a press release about
Interstellar when it came out– is that it’s great
with that movie, but we can literally
do it faster just by waving our hands. Waving your hands
in science means without putting any effort. In any case, a
science, internal joke. So of course, we didn’t want
to just use our laptops, because if we can do
it so fast in a laptop, imagine if you actually built
thousands of those graphics cards, and make a supercomputer
out of those graphics cards. So we built a special-purpose
supercomputer just for that project. And this is people
putting it together at the University of Arizona. And when we put it
together, we actually became number seven in
the fastest supercomputing per wattage of power used in the
world, which we were very, very impressed. No other institution other
than Cambridge University, and definitely not an
institution in the United States had, at the time, a
supercomputer that was so fast. And we were very
happy, because now we can throw the plasma
into the black hole. We can throw the light
into the black hole. We can throw everything,
and the kitchen sink, and calculate a truly
realistic simulation of what we think the black hole
in the center of the Milky Way will look like. This is weather-like. This is turbulent. This is very variable. This is hydrodynamic. This is full of clouds
and everything else that you can imagine. You see also, we change the
orientation of the observer, so you see it
moving up and down. You see primarily the motion
of the streaming of the plasma as it goes around. You see the black
hole shadow, which is right here in the middle. And you see those
two big– that’s the black hole [INAUDIBLE]
because it is very rapidly rotating. And it gets some of the matter,
and some of the matter blows out. And these are the
kind of simulations that we’re looking at. It turns out that by looking
very carefully at what that shape and the
size of that shadow is, this is where we’re doing all
the tests of Einstein’s theory, because that shadow tells you
what the gravitational field around the black hole is. And by doing all
those measurements, we can see whether Einstein
was really 100% right or not. But one of the
reasons that we did all of those
numerical experiments is because we
wanted to know what is the optimal place in the
electromagnetic spectrum to look at? Because it’s not enough
to have a big telescope. And I haven’t told you
anything about telescopes. We’re doing theory here. It’s not enough to see what
the black hole will look like. You also need to look
at the right place. And there’s a number
of coincidences that need to happen, if you wish. The atmosphere needs
to be transparent. And the atmosphere is not
transparent in all parts of electromagnetic
spectrum, otherwise the x-rays from the Sun will
have completely evaporated us a long, long time ago. The galaxy needs
to be transparent all the way to the center. And again, that puts
a lot of constraints. And the plasma
around the black hole needs to be very transparent. And if you look at a
slightly different part of electromagnetic spectrum–
and this is, for example, at 3 centimeters
wavelength– then you wouldn’t see the shadow
of the black hole that I showed you before. But you see this fog
of plasma around it. And this is a result
of our simulation. But this turns out to be
the very first project that I did as a post-doctoral
fellow here at Harvard, when I came in 1997. This is a very dear
project to my heart. One reason– because I love
black hole astrophysics. And the other reason is
because I did it together with a graduate student
at the time, who ended up being my future wife. So this is a very,
very wonderful project. And she is, actually, right
now giving a similar talk at the University of
Chicago, across the country. But this is what you
would have gotten if you made the observation
at 3 centimeters. But if you look at
a different part of the electromagnetic
spectrum, what you see is the fog becomes more,
and more, and more, and more transparent. And if you pick the wavelength
of observation right, you will see the black
hole shadow popping out. So the reason that we did all
those computational numerical models is to actually
identify what is the optimal place
to look at that shadow. But again, theory– in
theory, everything is fine. In practice, it isn’t. And what made the next
big technological leap, or next big transition, is the
demonstration in 2008, using only three telescopes–
one in Hawaii, one California, and
one outside of Tucson, combined together
as a big telescope, the size of this
entire triangle– by a number of people
led by Shep Doeleman, who is the PI of the
project right now, who turned those three
telescopes to the black hole in the center of the galaxy. It was small enough. It was only through telescopes
very close together. They could not take
an image, but they could measure the size. And they demonstrated
that the size is comparable to the
black hole’s shadow, and everything in
between is transparent. And that has been a
remarkable demonstration, that all these ideas, all
those theories, everything that we all talked about,
is actually something that has a possibility of working. Was I right? Not “will work,” “has a
possibility of working.” So a number of
people got together at MIT Haystack
Observatory in 2010. Shep, in the meantime,
moved to Harvard. He is now at
Harvard Observatory. But of course, the more
people got excited, the more and more people
joined the collaboration. So when something
becomes hot, everybody likes to be part of it. So we hosted the inaugural
meeting in Tucson in 2012– nice,
beautiful weather. The next one was in November,
2015 in Canada– horrible snow. The third meeting is actually
the week after Thanksgiving here in Cambridge,
Massachusetts, down the river someplace, where
we’re all getting ready for that experiment. Now of course, as
the group of people becomes bigger and
bigger– and these are people from all universities
all around the country– I found that has been extremely
complicated and remarkable to get different people
from different backgrounds, ethnic backgrounds, social
backgrounds, and everything else, agreeing on how to
do a single experiment. And to quote my
wife, I would say that I don’t mind dealing
with the complex mathematics of the universe. It’s people that I don’t get. And if you actually work on
the sociology of science, please come and
study us, and tell me what the answer is at the end. But somehow, we manage to
make this whole thing work. Now, the reason that, of course,
it took some time since 2008 is because they used
only three telescopes. We’re not building
new telescopes, because these are extremely
expensive processes, and nobody has the money. But people go to every
single radio telescope that exists around the
globe, and outfit it with the proper equipment
to do that observation in the proper wavelength. And this is one
of our colleagues, Dan Marrone, his finger
pointing out the radio receiver. Everything is gold plated. Everything is highly
calibrated, highly put together. You build it at the
basement of the university. Then you transport
it to the telescope. This is the Large Millimeter
Telescope at Volcan in Mexico. And this is Shep Doeleman,
the PI of the project, inserting something in one of
the mirrors in the telescope, trying to get it ready. This is from The New York Times. Some of the telescopes are
easier to reach than others. Judy talked about the
one, the ALMA in Chile, which is almost at the
base camp level of Everest. It turns out that
there’s one that is even more difficult to reach,
and that is the South Pole telescope, which is actually at
the very center of Antarctica. It’s a wonderful place
to do observations, if you can make it there. And this is our colleague,
Dan [? Maron, ?] who shows that he is
really good with weather. He left his parka out, and
this is the South Pole. And this is the
little instrument that he was building before. And that’s his graduate student. And we were all joking that
he didn’t leave him there at the end of the day. So what happened
in the last 8 years is a remarkable convergence
of about 150 people all around the world to
build all the telescopes that are required, all
the instruments that are acquired, literally
transformed them by hand, and installed them in
every single telescope that is required. And that’s how we got
together the Event Horizon Telescope, which has telescopes
anywhere from Hawaii– those two– Northern California,
Tucson, Arizona, Mexico, ALMA in Chile, the South
Pole Telescope, which is down here, APEX,
which is also in Chile, and two telescopes in the
French Alps and in the Pyrenees. And all of those telescopes
are, as we speak, completely outfitted
ready to turn. And the only thing that they’re
waiting for is for the galactic center to come from the backside
of the sun, because right now, the galactic center is behind
the sun, and you cannot see it. And that will happen
this coming April. So we are at the point
where everything is ready. We’re all keeping
our fingers crossed. We’re hoping that
very soon, this is a simulated image that will
come out of the Event Horizon Telescope, given the best
that we know about how the instruments
are going to work, and how the weather
is going to work. We’re keeping our
fingers crossed. And we’re hoping
that very soon, this won’t be a result of
simulation, but this will be a real first
picture of a black hole in the center of the galaxy. Thank you very much. [APPLAUSE] [PLAYING MUSIC]

No Comments

Leave a Comment

Your email address will not be published. Required fields are marked *