Today I'd like to talk with you
about my early days at MIT and the research
that I did here. This is a long time ago. I got my Ph.D. in the
Netherlands, in nuclear physics, and I came to MIT in January
1966, almost 34 years ago. And the idea was that I was only going
to spend here one year on a postdoc position, but I liked it so much
that I never left, and I don't regret it. I joined the X-ray Astronomy
group here of Professor Rossi. Now, X-ray astronomy
has to be done from above
the Earth atmosphere or at least near the top
of the Earth atmosphere because the X rays
are absorbed by air, unlike optical astronomy
and radio astronomy, which can be done
from the ground. The kind of X rays
that we measure are not unlike those
that your dentist is using when he takes an X-ray picture. The energy range of these X rays is somewhere between one
and 30, 40 kilo-electron volts, and if you don't know
what a kilo-electron volt is, that's fine, too, but you never express the energy
of an X ray in terms of joules, because the number becomes
so ridiculously small. During World War II,
under Hitler's Germany, Wernher von Braun developed
the V-2 rockets for destructive war purposes. It was developed in Peenemuende. And after the war, the Americans used these V-2
rockets for scientific purposes, and the first rocket flights to
search for X rays from the Sun took place in 1948. And X rays were found
from the Sun. That was quite a surprise. And the power, energy per second
that the Sun puts out in X rays divided by the power in optical, which is almost
all the radiation of the Sun-- I'll give it
the symbol of the Sun-- is approximately
10 to the minus 7, so only one ten-millionth of all
the energy comes out in X rays. So, from an energy point
of view, it's very, very little. It varies a great deal, too. But it is really very little. In 1962, scientists
here in Cambridge, among them
Professor Bruno Rossi, who was a professor at MIT, and Riccardo Giacconi
and Herb Gursky-- who were working
across the street at American Science
and Engineering-- attempted to do an experiment to see whether
they could detect X rays from objects
outside our solar system. Now, the odds were very low
that they were going to succeed, and the reason is very simple. If you take the Sun and you move
it out to the nearest stars, which is typically ten
to a hundred light-years, you wouldn't stand
a chance to see X rays. In fact, the sensitivity
of the detectors in these days was too low by at least
nine orders of magnitude, a factor of one billion. To everyone's surprise-- to everyone's, yeah,
happy surprise, I should say-- they succeeded, and they
discovered an object which was later called Sco X-1. It's in
the constellation Scorpius, "X" stands for X rays, and "1" for the first X-ray
source in that constellation. The total power output
of that source was about 10,000 times more
than the Sun. That doesn't make
the source so special, because there are
many stars in the sky that radiate way more energy
than our Sun does, but what's so very special
about Sco X-1, that the X-ray power over
the optical power for Sco X-1 was approximately 1,000. In other words, the X rays are
the dominant source of energy and the optical is sort of,
let's call it a by-product, whereas with the Sun, the
optical is the main thing and the X rays is sort
of a by-product. And so the $64 question
in those days was, what can these objects be? They must be very different
from the Sun, and that's what
I want to discuss with you. When I came to MIT in 1966, there were about a dozen
of these X-ray sources known in the sky. Today there are thousands known. And they were discovered
from rocket flights. These rockets would be launched,
typically from White Sands, and they would spend
about five minutes above the Earth atmosphere. And during those five minutes
they scanned the sky, and a dozen of these sources were discovered. I joined here the group
of Professor George Clark, who is still
a professor at MIT. He was working on observations
to be made from very high-flying balloons. So we would build a telescope, and we would launch it
on a balloon, and go near the top
of the Earth atmosphere. It's not as good
as a rocket flight which gets completely
out of the Earth atmosphere, but the flights on balloons
can last way longer than five-minute
rocket flights. We could fly hours and,
if we were lucky, even days, but the price
we paid for that is that even though there was only very little atmosphere
left above us-- about 0.3 percent of
the atmosphere was left-- still that caused
an effect of the absorption, so we did lose X rays that
the rocket flights did not lose. But we had the great advantage
of many, many hours. To give you a rough idea
of what it took in those days-- I worked on this
with graduate students and with many
undergraduate students-- a telescope in those days, to build it cost typically
a million dollars, and it would take us
two years to build one. The balloons that we needed
to launch them were about $100,000
in those days, and the helium that we needed
to get it up was about $80.000. and the weight of such a payload was about 1,000 kilograms. These balloons would go
up to 140,000 feet and they were huge--
they were about 500 feet across. I will show you pictures
of them very shortly. It was a risky business
in that no guarantee of success. You bought the balloons. If they worked,
so much the better. If they didn't work, tough luck. There was just no way that
you could recover the money. They were very thin; the balloons are made
of polyethylene, and the thickness
of the polyethylene was thinner
than cigarette paper, so you can imagine
how easy it is to damage them, and if you don't damage them
during the launch, it's easy to damage them
on the way up, due to the jet stream
and the very cold layers of air that you have in the tropopause. So I would like
to show you now some slides, and then we will get back
to talking a little bit more about X-ray astronomy. All right, so let's see
what we have first. You see here two
of my undergraduate students. At the time they were
undergraduate students. Now they are both Ph.D.s, and some of you may think that science doesn't have
much romance, but there is a lot. They married and they have kids. This is Pat Downey; she's still very important
in working with me on PIVoT, and this is Jim Ballantine. They were working
on the electronics, which is
an enormously tedious task, to do the wiring
of the electronics. And here you see
the plant in Texas where these balloons were built. These were extremely long halls,
as you can imagine. And here the gores
of the balloon, which are like pieces
that you see on a tangerine, on the surface of a tangerine, they were sealed together
with heat sealers. And only women were allowed
to do this work because it was well known
that men are too impatient and make many more mistakes. And so only women
were allowed there. It was nothing to do
with discrimination, but simply because women
were better at doing this work. Here, a balloon comes out
of a box. This is a picture
I took in Texas, where we launched balloons from. It's nicely covered
with this pink sheet and we have this huge cloth
on the grass because the balloon is so thin that if you would put it
on the grass, it would get damaged right away. I told you, the skin is about...
thinner than cigarette paper, so it's carefully taken out
and we inspect it closely. And in this case,
there was some concern that there might be
a hole in the balloon. In fact, there was at least
a hole in the pink cover, and so we carefully inspected whether the hole
propagated deeper in. I wasn't too worried,
because this was not my balloon, but nevertheless, it's always sad if you see
some of your colleagues having a balloon
that doesn't get up. And now I'm taking you
to the desert town in Australia,
central Australia-- Alice Springs. I've launched many balloons
from Alice Springs. And now you get
a pretty good idea of what the launch
is going to be like. Here is the launch truck, and the telescope is
on the launch truck. And all of this
is empty balloon, and here is what we call
the roller arm-- it holds the balloon down, and only this part
of the balloon is going to be inflated. Here is the helium truck, and here are the inflation tubes
through which we inflate. The inflation takes place
almost always early morning or near sunset in the evening, because that's when
the wind is very calm, and we need
very calm conditions. It is this part of the balloon
that is going to be inflated, and so there's going to be a
huge force, the buoyant force-- I hope you all remember
Archimedes' Law-- of several thousands
of kilograms' force up. And so this vehicle here
weighs about four or five tons to hold it down. And when we launch the balloon,
we actually release this arm; we flip this arm up. And so you will see, then, how the balloon will make it
on the way up. So this is Alice Springs,
Australia, again. The Sun is rising,
and we started the inflation. You see here
the inflation tubes, and we put the helium
in the balloon. This is the very critical phase
of the launch, because if there's
a little bit of side wind and the balloon touches
the ground, then it's all over. Then it just gets damaged, and you abort--
you don't even continue. This is a little later, when
the... we call this the bubble. This is the roller arm here, and this is all this empty
balloon in your direction, and this part is
only some 80 feet high or so. And here you see these gores
that I told you about earlier. This is all this tedious work
that is done by these women, who seal these gores together, and they all come
together here at the apex, in a big aluminum plate. All right, now you see
the situation from the launch truck site. Here you see the radar
reflectors, so we can follow
the flight by radar; you see the telescope here-- I won't go
into too many details. This is a ballast box, which contains
about 500 pounds of lead shot, which we can drop
on radio command. I believe this
was Jeffrey McClintock, who was one of my graduate
students at the time. You see here the parachute. There's a connection between the parachute
and the bottom of the balloon and we can cut that
on radio command, and then the telescope, we hope,
parachutes safely back to Earth. And here's all empty balloon,
and here is the bubble, that part
that was just inflated. They're still inflating here, but they are already tying off
this inflation tube, so we are actually getting
very close to a launch. And here is the moment
of launch. This is a moment
that no one will ever forget who watches a balloon launch. You feel ants in your pants
and butterflies in your stomach. It is a very tense moment. If things go wrong, this, in general, is the moment
that things go wrong, because when the roller arm
is here released, then this enormous amount
of free lift, the buoyant force, drives this thing up
and you get a bouncing effect. You get an oscillation of
the helium in this upper part-- we call this the mushroom. And that can already destroy
the balloon right away. The layout is such
that we always lay out so that the wind will drive
the balloon towards the launch truck. And you will see later on
why that has to be done. So the launch truck
can maneuver itself straight under the balloon, and make sure that the balloon
is carrying the telescope before we release the payload. The payload is now tied
to the launch truck. We have to wait for the whole
balloon to be off the ground before the launch truck can actually maneuver itself
under the balloon, but you already see that the...
by the exhaust that the engines
are already running. You see a close-up
here of this mushroom, and it makes
a tremendous amount of noise. It's really scary. I'm always surprised,
when I see a launch like this, that the balloon, so thin, can actually survive
this tormenting launch. So here it goes higher, comes
in this direction, as you see, getting closer and closer
to the launch truck, rises higher in the sky and, in this case
in Alice Springs, I was so close to the launch that I couldn't follow
the bubble going up much further with my camera, so the next picture
that you're going to see I took from Palestine, Texas, from which I've flown
many balloons. So it's a different launch, but it is sort of the same stage
in the launch that you see here. Alice Springs was beautiful,
by the way, always very nice, clear skies; it's a very fantastic
desert there. It's a wonderful desert town. It's really in the middle
of nowhere. Okay, so now we're in Texas and
you see here the launch truck. This is the telescope,
it was a different telescope. You see the parachute, and now the trick is for the
truck to get under the balloon. The amount of helium
that is in the balloon is only a small fraction
of the total volume. It's just enough for us
to get the free lift. We want to go up at about
a thousand feet per minute, and then, since the atmospheric
pressure goes down when the balloon goes up, the helium expands and finally
fills the entire volume, as you will see. So now is the moment,
this is a crucial moment that the launch truck
has actually maneuvered itself straight under the balloon. And now the person on
the launch truck makes sure that there is enough tension
in these lines to pick up the telescope. If the balloon were a little bit
too far ahead or too far behind and you released the telescope, then, of course, it would
pendulum into the ground and you would lose it,
so it's very important that it be done when the
balloon is straight overhead. But that alone is not enough; there must also be
enough pull on it so that the telescope doesn't
crash to the ground vertically. And when there is any chance
that things go wrong, we just abort the flight, because the telescope is so much
more expensive than the balloon, even though the balloon
and the helium together is close
to a quarter-million dollars. And here you see it
after release: payload, parachute, empty balloon and here,
the helium. And this, from here to here,
oh, is about two-thirds of the height
of the Empire State Building. These are huge, huge balloons. And here you see it
at an altitude of 150,000 feet. This is the largest balloon that
was ever flown successfully. It's still the world record. It was a balloon with a volume
of 52 million cubic feet. This picture was taken
through a telescope. And you see here the telescope, and you can look
straight through the balloon; it's that thin. From here to here is
about 500 feet, almost 600 feet. All right, here you see
my ex-graduate student. He's now Dr. Ricker,
George Ricker. He's still a staff member
at MIT. This is in Australia. A lot of this equipment
was built by undergraduates and graduate students of mine; a lot of it we borrowed from
the balloon launching stations. And the radio data come in here, and we can command
the telescope from here. We can orient the telescope,
we can draw ballast and, very important,
we can terminate the flight. So that we rescue,
that we save the telescope when it starts drifting
out over the ocean. Because the balloon
starts going with the winds at the altitude of 150,000 feet, and those winds can vary anywhere from 20, 30 miles per
hour to up to 100 miles per hour. We try to fly only during
the days that the wind is low, and that's the case
in the spring, and in the fall,
we call that the turnaround. The winds at these altitudes
change twice per year. They change from about
100 miles per hour to the west, to 100 miles per hour
to the east. It happens in the spring
and in the fall, and that's when we try to fly,
when the winds are very low, when they are in the process
of turning around. We follow the balloon at low
altitude-- 5 --> 6,000 feet. It's a small airplane. I'm sitting here
on the airplane. This is a typical airplane that we use to hop
from airport to airport and stay as close
to the balloon as we can, so that we can give
the "terminate" command. And later we can recover
the payload, which is an adventure
all by itself. In Australia, that is much harder
than in the United States. This was El Paso, but in Australia, there are
no airports in the desert, and so that is way harder
to hop from place to place. I'm taking you to Australia now,
here is Alice Springs. And when we launched
this flight, the days before, we had balloons,
weather balloons, test balloons, which we drove
up to 140 --> 150,000 feet, and then we probed the winds
at that altitude. And the people who did that
give us good reasons to believe that the balloon would either
go sort of in this direction or maybe here, but in any case,
it would go north-northwest. And so we alerted all these
radar stations in Australia to look out for the balloon--
that we have radar reflectors, so they could give us
an early warning, because between here and here,
there are really no airports. So if we follow this
by airplane, you can land during the day
at some air strips, but really,
there are really no runways, so that's pretty dangerous. At night,
you couldn't land here. And so we were hoping
that these radar stations would give us an early alert and
tell us where the balloon is. What happened, however,
the balloon went straight down. And then it was sunset, so we don't know exactly
where the balloon is at sunset. We can't see it, but we have
radio contact with it and so we were flying
close to it. And then at sunrise we picked
it up, we could see it, and then here, when it was getting close
to forbidden zone-- because there is
commercial flights here-- we cut it down,
so we gave the radio command which separates the parachute
from the balloon. The balloon is very brittle--
it's very cold there. The balloon shatters,
breaks in pieces, comes down and, if everything goes well, then the parachute brings the
telescope safely back to Earth. This is the person that we
contacted during that flight. You try to draw the attention
of local people, and you do that by flying
with your airplane low over their house. This person lived in the desert, and his nearest neighbor was
from 70 miles away from him. He was crazy,
he was always drunk... (students laugh) he was shooting kangaroos
in the desert. There is no windshield here. He would drive 60 miles an hour, and then he would chase
these kangaroos and shoot them. And he had a crazy game
which I didn't like at all. He would put the dog
on the roof-- and he gave me
a demonstration once; I was with him in this truck-- and he would drive 60 miles
per hour, would slam the brakes, and the dog would
catapult through the air, and then he would say, "You can't teach
an old dog any new tricks." We encountered wonderful animals
during recovery: koala bear, quiet, peaceful, very lazy,
unlike most 8.01 students. (students laugh) And then, when we got closer
to payload-- it took us a day and a half
to get to the payload-- there was this nasty iguana,
he was about six feet long. And let me tell you,
I was really scared. It scared the hell out of me. But of course I didn't want
to show that, so I said
to my graduate student, "There's no problem,
these animals are harmless; you go first." (laughter ) And he did, and it turns out
they are harmless, and during the entire four hours
that it took us to recover the payload
and get it on Jack's truck, this animal was just sitting
still, didn't move at all. That is his way of thinking
that we don't see him, then. Beautiful animals,
these iguanas. The aborigines eat them, it's
very precious food, by the way. So here you see Don Brooks, who was an American
who came with me. He was an electronic expert, and this is Alice,
which was the wife of Jack. You see the payload here,
tumbled over, but it's in good condition. The crash pad is there purposely
to protect against the impact, to get a lower deceleration, and, of course, it's okay that that cardboard crash pad
is destroyed-- that's the whole idea. And then when you come back a
few days later in Alice Springs, it's... nothing happens,
ever, in Alice Springs. I mean, it's completely
a hole in the ground. So this front-page news,
"Perfect Balloon Launch" and "1,000 watch the start
of a space probe"-- they called it a space probe. I had a long interview
with this news reporter, and I told him that the reason
why we have to go high is because of the absorption
of the X rays in the Earth atmosphere, but the article
didn't get that across. They said, "They fly balloons because then they're
closer to the stars." Well, I suppose
that is close enough, but it really missed the issue
of the absorption of the X rays, which of course... that's
the reason why we have to go up, not because we want
to get closer to the stars. Okay, so now I'll go back
to the blackboard, if I can find my way. So between 1966
and roughly late '70s, I had about
20 successful flights from the United States, from
Canada, and many from Australia. Now, we also had some problems,
we had some bad luck. Twice during my flights,
the balloons popped. 70,000 feet,
there is the tropopause; it's very cold, -70 degrees. There are jet winds
and they beat on the balloon, and then the balloon can burst. And when that happens, we don't have enough time
to terminate the flight-- we can't separate the parachute
from the balloon; it happens all of a sudden--
and then, in general, the parachute gets entangled
and then you get a free fall. So the payload
is entirely destroyed, and that happened twice. But we were lucky enough
that at several occasions we made some
interesting discoveries. During the early years
of X-ray astronomy, we discovered
five new X-ray sources, And some of these sources
that we saw from balloons were highly variable. We noticed an X-ray flare; the X-ray intensity went up
by a factor of three or four on as little time
as ten minutes. And that was completely new
in those days, and that could not have been
discovered from rockets, because the rockets themselves are only five minutes
above the Earth atmosphere, and they're not looking
at one source all the time. They are scanning the sky,
because their objective was to find as many X-ray
sources as they could. But we were up
sometimes 26 hours, so we had plenty of time to look
at one portion of the sky for a long time, for hours on,
and so it was not an accident that we discovered
these flaring events which lasted up
to ten minutes and longer. We also discovered an object
which we called "gx1+4"-- the number has to do with
where it is in the sky-- and we noticed,
much to our surprise, that the X rays seemed
to fluctuate in a periodic fashion,
2.3 minutes. At the time, we had no clue
what that meant, but later,
as you will see very shortly, it became clear that that was the rotation
period of a neutron star. So the big question was,
in the early days: What are these objects? And this is something that
we have discussed in 8.01 and I will go over it
very briefly again, but we discussed it and you even had
some homework problems on it. These objects
are X-ray binaries, whereby one object
is very compact-- which could be a neutron star, or in some cases
even a black hole-- and the other object,
the other star, is a normal nuclear burning
star, something like our Sun. And they are
very close together. They are so close together
that the matter, which is here, is attracted by the neutron star stronger than it is attracted
by the star itself, and so it starts to find its way
to the neutron star. This is a binary system,
so they go around each other. This matter cannot
just go in radially, but it would spiral in slowly and find its way
to the neutron star. Strangely enough that we still don't understand
how it makes it, but it does make it,
ultimately, to the neutron star, and this is, then,
what we call the accretion disk. This is the accretor
and this is the donor. The donor provides the fuel that finds its way
to the neutron star. And if you take
a little bit, mass m, and you drop that
on a neutron star-- and a neutron star has mass capital M, say,
and radius capital R-- then the kinetic energy
that is released at impact is something that all
of you should be able to do. That is the following: mMG divided by R
equals one-half mV squared. This is the gravitational
potential energy that becomes available if an object of mass little m
crashes onto the star. The surface of the star
has a radius capital R; the mass of the star
is capital M. And that is converted
to kinetic energy, which is one-half mV squared,
so this is the speed at impact. Of course, it's always
independent of little m, and so you can calculate
that speed and that speed is horrendous
for a neutron star, the reason being that
the radius of the neutron star is so absurdly small;
it's only ten kilometers. It is roughly
100,000 times smaller than the radius of our Sun. The mass of the neutron star is
comparable to that of our Sun-- a little larger,
but it's comparable. But it is the radius
which is so small, and that's why
you get a huge speed at impact which is about half
of the speed of light. And this kinetic energy
is converted to heat-- for the same reason
that when we drop something here on the floor, that the kinetic energy
ultimately goes into heat-- and so it heats up the surface
layers of the neutron star, and the temperature becomes
horrendously high, ten to the seven,
ten to the eighth degrees-- 10 million,
100 million degrees-- and at that
very high temperature, almost all the energy, almost all electromagnetic
radiation comes out in the form of X rays. The Sun has a temperature
of only 6,000 degrees; most of it comes out
in the form of optical light, but when you go
to 10 million degrees, that's no longer the case. The spectrum shifts
in favor of the X rays. The amount of energy that
is released is horrendous. To give you some feeling for
that, if you take a marshmallow and you throw a marshmallow
from a large distance onto a neutron star, then the energy that is
released, which is this energy, is comparable to the energy
that was released of the atomic bomb
that was thrown on Hiroshima. So that tells you something about the enormous
gravitational forces that are at work on the surface
of a neutron star. We know now
what these systems are; the evidence is overwhelming. We have observed the rotation
of the neutron stars. The 2.3 minutes that we found, we now know is the rotation
of the neutron star. These neutron stars have
a strong magnetic field, and the matter that accretes
onto the neutron star reaches the magnetic poles. In 8.02 you will see...
you will learn why this plasma, which is highly ionized, why that cannot just reach
the neutron star anywhere, but it is forced to only enter
the neutron star near the magnetic poles, and if the neutron star rotates, then the magnetic poles
can rotate like this. And when you are on Earth, you see X rays, X rays,
X rays, no X rays, X rays and so you see pulsations. And so these pulsations
have been seen from many neutron stars now, from many
of these binary systems. It's very clear
that it's a binary system. If you are in the plane
or near the plane of the orbits of the two stars, then the neutron star can go
behind the donor, and then you don't see
any X rays, because the X rays are
then absorbed by the donor. And then you see an X-ray
eclipse, so the X rays vanish. So you would see the pulsations,
strong X-ray signal, and all of a sudden, boom--
it's gone. And then a few hours later,
it starts up again when the neutron star reappears,
reemerges from the donor star. So that picture
is all very clear, but I do want to show you
at least a sketch of what we think such a system
would look like, which is just the next slide, and maybe the person
in the booth... Oh, I can do it from here. So this is
what it sort of looks like. You see the donor
there on the left, and you see here
the neutron star, or it could be a black hole-- that's sort of the same idea,
you would not be able to tell-- and you see how
the matter swirls in. Of course
this is a not a real picture, this is a sketch made
by an illustrator. We know many hundreds of these
systems in our own galaxy, and, of course, there are many
in other galaxies as well. I discussed with you in 8.01 that if you measure the
Doppler shifts of these stars-- and if you are lucky,
you get the Doppler shift both from the pulsations
of the neutron star and from the optical lines
in the donor-- that you can even find the mass of both the star here
and the star there. And if the mass
becomes horrendously high, as in some cases,
then you have to conclude that you are dealing
with a black hole. And you had a problem
on section... in one of
your homework assignments. I'm no longer flying balloons, because all the work
that I do nowadays is done,
of course, from satellites. You get 365 days per year data, you are always
above the Earth atmosphere, so that's clearly the way to go. And I have used European
satellites, Japanese satellites, and nowadays I am using
the Rossi X-ray Timing Explorer, which is an American satellite, and Chandra, which was
launched early this year, which is
the biggest thing in town. In 1975, here at MIT we had
our own satellite, called SAS-3. And we operated SAS-3 from
the Center for Space Research, which is Building 37,
where my office is, 365 days per year,
24 hours per day. And in '75, Josh Grindley,
from Harvard, and John Heise
in Utrecht, the Netherlands, discovered something
which we call an X-ray burst. And an X-ray burst
is a phenomenon that you see the X-ray signal
become very strong all of a sudden. In about one second,
it becomes ten times stronger, maybe 20 times stronger
than it was before the burst, and it peters out on a time
scale of about a minute or so. And we were very lucky
at the time, 1976, with SAS-3, that we could do research
on these X-ray bursts and within a year or two, we discovered eight more
of these burst sources. And it is largely
through that work-- that observational work
that took place-- and through the work,
theoretical work by Professor Paul Joss,
who is still at MIT-- that we now know
what causes these X-ray bursts. They are nuclear bomb explosions
on the surface of neutron stars. What happens is that on
the surface of the neutron star, the matter that accretes-- which is largely hydrogen
and helium from the donor-- becomes very hot,
it becomes very dense, and at that high temperature
and at very high densities, you get thermonuclear fusion. And a reaction
that can take place is that three helium nuclei--
helium-4-- fuse to form carbon-12, and when that happens,
energy is released, thermonuclear energy
is released. This reaction is extremely
sensitive to temperature. When energy is released,
the temperature goes up. When the temperature goes up, the reaction rate goes up,
then the temperature goes up, then the reaction rate
goes up even further, and the whole thing
gets out of hand and that's why it is
a thermonuclear explosion. We call it
a thermonuclear flash, so it is an uncontrolled,
runaway process. And the bomb explosion
that occurs on the surface
of the neutron star would be a billion
times a billion-- a billion times a billion, ten to the 18 times
more powerful than hydrogen bombs
that we can make here on Earth. We speculated early on that when you see
an X-ray burst in the sky, that you may be able to see also
an optical flash in the sky. The donor stars
and the accretion disk emit optical light. It's very faint,
there are very faint sources, but you can see them
from the ground with your optical telescopes. And we had reasons to believe when an X-ray burst occurs, when an X-ray burst occurs, and I'll tell you why we
believed that was the case. If you have here
a neutron star and here you have
the accretion disk, and if the bomb
explosion occurs-- these red wiggles
are the X rays-- then the stuff that goes
straight to the Earth you will see. But there are other X rays
which go in this direction, and the heat of the disk...
we call it X-ray heating. And the disk locally
would get a temperature of maybe 30 --> 40,000 degrees
and brightens in optical. So we expected
that we should see that effect, that effect of X-ray heating. But our goal was
even more ambitious. You see, the optical light
that comes from here must be delayed than the X rays
that go straight to the Earth, because first the X rays travel
in this direction, and then the optical light goes
in this direction. So if I put my pencil here,
this is the extra path that the electromagnetic
radiation is going, and that takes time. And if that takes one second, then that means this distance
is one light-second. If it takes 20 seconds, this
distance is 20 light-seconds. So our goal was to actually
measure, for the first time, the dimensions of the inner part
in the accretion disk. And so we organized
a worldwide campaign in 1977. 17 countries were contributing,
44 observatories, and we told them that we were going to look
at a particular star in the sky, a very faint star, which
was this X-ray binary system. We would record with SAS-3
the X-ray burst, and we wanted them to record--
in the radio, in the infrared, wherever possible,
in the optical-- whether they would see
a change in the appearance, in the optical
or in the radio appearance. In the summer of 1977,
we saw 110 bursts from a particular burst source. None of them were seen in the optical or in the radio--
zero results. We did it again in 1978
and then we succeeded. This was in collaboration
with Josh Grindley from Harvard, Jeffrey McClintock, who was
my ex-graduate student-- he is now also at Harvard-- and my good friend
Jan van Paradis from the University
in Amsterdam, who worked with me
at the time here at MIT. This was a splashing result-- simultaneous observation
of an optical flash with an X-ray flash-- and it was on the cover sheet
of Nature, which is a very prestigious
journal in which people publish, so we were extremely happy. I do want to show you
the simultaneous events but not the 1978 event, but I'll show you one
that is more impressive that my colleague
Holger Pedersen observed a year later, when he did
the optical work from Chile. This is that observatory
that I mentioned to you earlier. I have been there many times
myself. It's in La Silla,
2,400 meters above the ground. This is where
the atmospheric pressure is only three-quarters
of an atmosphere, and where you can't boil
a soft-boiled egg because the temperature
of boiling water is only 92 degrees. Okay, here is the optical flash
that Holger observed, and we were looking
with the Japanese satellite, which was called Hakucho. SAS-3 I think was no longer
operating at the time. You see here the time
of the optical signal, so this is the strength
of the system in quiescence and then the X-ray burst occurs, and we see here clearly
an optical flash, and here you see the X rays
from Hakucho. They look very similar, but
now comes the interesting part. Of course, I have scaled them up
here on the view-graph so that they have
the same height. That's just artificial,
of course. But now comes
the interesting part. If I overlay them, then look at the blue line,
which is the optical-- it's clearly delayed
relative to the X rays, and that was our goal. And this was really
a very clean observation, cleaner than
our 1978 observations, and if you move
the optical back-- or the X-ray forward,
whichever you want-- by about two seconds, then
they almost exactly overlap. And so this was
conclusive evidence that the dimensions of the disk, of the accretion disk
around these neutron stars, had typical dimension
of about two light-seconds, of that order. We suspected that,
for other reasons, but nevertheless, this was
the conclusive evidence. The bad news is
that during my past term that I was lecturing 8.01, I have been able to do nothing
but 8.01-- no research at all. And I think you have all the
right to feel guilty about this. (class laughs ) Very guilty. Now, the good news is
that I enjoyed it, and whether you like it or not, you were on my mind almost
all the time, day and night. I had even nightmares about it, and a typical nightmare that
I would have is the following. I would come into 26.100, but I lost my lecture notes
and I couldn't find them and I was completely
ill-prepared, but I would start
my lectures anyhow, and you would start
laughing at me and I would wake up in sweat. I don't think we need Freud
to explain that dream. Now, I have enormously enjoyed
lecturing 8.01, and in a way,
you have touched my life and I trust that I, too,
have touched your life. Now, I make myself no illusions. I am sure that you will very
soon forget Kepler's Third Law, although I hope it won't be
before Monday, when we have the final. (class laughs) And you will probably also
forget how to properly apply the conservation
of angular momentum. But perhaps you will always
remember from my lectures that physics can be
very exciting and beautiful and it's everywhere around us,
all the time, if only you have learned to see
it and appreciate the beauty. And surely, when you go
on your first monkey hunt, wearing your own safari hat, or when you will orbit the Earth and you want to throw a ham
sandwich to your friend, you may be thinking of me. And I hope those will be
happy memories. I wish all of you the very best, and I thank you
for attending my lectures. (class applauds) LEWIN:
Thank you.
