Today I would like to talk to you
about some of the research that I did
during my early days at MIT.
It's a long time ago.
I got my PhD in the Netherlands
on nuclear fysics and I came over
to MIT in 1966.
I was supposed to be
here only for one year.
I had a one-year postdoc position.
But I loved it so much I never left.
I changed fields.
I joined the research group
of Professor Bruno Rossi.
Here at MIT I changed from nuclear physics
to x-ray astronomy.
X-ray astronomy speaks for itself.
You're trying to do astronomy in x-rays.
You cannot see any
x-rays from the ground
because the earth atmosphere
absorbs them completely.
So you have to go outside
the atmosphere unlike optical astronomy
and radio astronomy which
you can do from the ground.
When I use the word x-rays I'm thinking of
the kind of x-rays that your dentist would be using,
medical purposes,
about one to fifty kilo electron-volts.
And since all of you took 8.02 you
should know by now what a kilo electron-volt is.
Uh optical light is two electron volts,
where x-rays,
way more energetic than optical light.
During the Second World War,
Werner von Braun in Peenemunde
developed under Hitler Germany
destructive rockets.
They were used to destroy the Allies.
To destroy you and me.
And after the war, around 1948,
the Americans used these rockets to do science.
They also got Werner von Braun
over to this country
and for reasons unknown
to me he became a hero
They tried to observe x-rays
from the solar system.
And they found indeed that
the sun emits x-rays.
The sun is very close.
So you may say well that's not a surprise.
But it's really very unusual because
to create x-rays you need
extremely high temperatures,
which we didn't think existed on the sun.
And if you take the power that the sun puts
out in x-rays, this is joules per second now,
and you divide that by the power in the optical
and the infrared light of the sun,
this symbol stands for sun,
that ratio is about ten to
the minus seven.
So you must conclude that the sun emits largely
optical light and infrared
and that the amount of x-rays
is a modest byproduct.
Interesting as it is all by itself.
In 1962
several scientists here
in Cambridge, Massachusetts,
among them Bruno Rossi and Riccardo Giacconi
and Herb Gursky,
made an attempt to observe x-rays
from stars outside our solar system.
The odds were strongly against them.
The detectors were not sensitive enough.
If you take the sun and you bring the sun
to the nearest stars,
which is a distance say of ten light years,
then there would be no hope
that you would be able to detect x-rays
from an object like the sun.
In fact the detectors were too insensitive
by about a factor of one billion.
They tried anyhow
and they were successful.
They did indeed find
to everyone's surprise and joy,
they found x-rays from at least 
one object outside the solar system.
This object was later called SCO X one.
SCO stands for the
constellation Scorpio in the sky
and X for x-rays and one for the
first source observed in that constellation.
We now know that this object
is a faint blue star.
And what is extremely special about the object
SCO X one is,
that if you take the ratio x-ray power
over optical power
then that ratio is
about ten to the third.
Compare that with the sun.
This object, we had no clue
what it was in those days,
primarily emit x-rays,
and the optical emission is a byproduct.
Whereas with the sun it is reversed.
And so the burning question was in those days
what kind of animal is this?
It must be a totally different beast.
Something very different from our sun.
And when I came to MIT in 1966,
there were a few dozen sources
known outside our solar system.
And they were all discovered with rockets.
The rockets in those days could spend about
five minutes above the earth atmosphere
and they would quickly make a scan over the sky,
five minutes, that's all they had.
And I joined the group of uh George Clark,
who is still at MIT,
he was doing x-ray astronomy
from very high-flying balloons,
very close to the top of the atmosphere,
and the advantage of balloons was that you could
observe the sky for many many hours,
if you're lucky sometimes
even a day or more.
But on the other hand, since there is always
a little bit of atmosphere left above you,
even though there's very little, there is
still some left,
the x-rays are absorbed,
almost all x-rays below twenty kilo electron-volts
would be absorbed,
and we would not be able to see them.
But of course the compensation was that we
could look at the sky for many many many hours.
Nowadays no one is doing these
balloon observations anymore.
No more rocket observations.
Everything is done of course from satellites.
So when I came to MIT,
together with George Clark,
I developed new x-ray detectors
for these balloon observations.
Many graduate students were involved.
Many undergraduates.
It would take about
two years to build a telescope.
To give you a rough idea it would take a million
dollars in terms of 1966 dollars,
and the weight of such a telescope would be
roughly a thousand kilograms.
The balloons in those days would cost about
a hundred thousand dollars
to get us up to these high altitudes,
and we would need about
eighty thousand dollars of helium,
and you will see some slides of that.
We have to go to altitudes of about
hundred forty thousand feet.
We had huge balloons for that.
You will see one.
They have diameters of about
six hundred feet.
And the material was polyethylene.
Extremely thin to make them light-weight so
that they can go high.
The thickness of that polyethylene was about
half of one-thousandth of an inch.
Which is thinner than the saran wrap that
you have in the kitchen.
It is thinner than cigarette paper.
A very risky business to fly these balloons.
No guarantee of course that they would work.
You pay your money.
If they work that's great.
If they don't work that's just tough luck.
There is a good chance that you have a failure
when you launch the balloon.
They're very fragile.
There could be damage right at the launch.
But even if they make it up in the atmosphere
they have to go through the tropopause,
near about a hundred thousand feet
where it's very very cold,
the balloons get brittle,
and then they can burst.
And that of course would be the end then
of that balloon flight.
And that could also be the end of a PhD thesis.
Because all these flights of course were
connected with research
and therefore with PhD work
and so the tension during these early phases
of the launch were always extremely high.
Sometimes even unbearable.
So now I would like to show
you some slides.
Which will give you a good idea of what these
expeditions were like.
Oh, yeah, a classic problem.
This is nice that these -- ah, now they work.
All right,
so if I can have the first slide,
you see here Jim and Pat
who at the time were undergraduates,
they are now both Ph.D.s,
and they are working there, very tedious work,
trying to put the electronics together.
You may think that science
is not very romantic.
But I can assure you it is.
They fell in love.
They married.
They have kids.
And that's the way it
sometimes goes in life.
And so here you see the plant in Texas where
these huge balloons were made.
Uh balloons are put together sort of like
the -- the way that the tangerine is put together.
At the surface you see these gores of the
balloon.
And the sealing of these gores to make up
the balloon were -- was only done by women.
Only women were allowed
to work there.
Has nothing to do with
sex discrimination of any kind.
It just turned out that
women were more patient.
They did the work better.
They make way fewer
mistakes than men did.
That's the way it goes sometimes in life.
Here you see a balloon
coming out of the box.
Nicely protected.
In a plastic cover.
And we also have here cloth on the -- on the
grass because the balloon is so thin that
it would certainly get damaged if it touches
the grass, it's enormously thin.
This was not my balloon.
Uh we were worried that there was something
wrong with it.
You can see here the concern.
They thought it was a -- there was a hole
in the balloon.
And that if there is a hole in the balloon
there's just nothing you can do about it anymore.
You can't patch it because the hole is almost
always through many many layers.
What you're looking at here is hundreds of
layers of balloon that are folded together.
But as I said, since it wasn't my balloon
I wasn't too worried,
but of course it is never nice
if you see a failure of your colleagues.
So now I bring you to the desert town
Alice Springs in uh Australia.
Right at the heart of Australia.
And now you get a
pretty good idea of what it's like.
Here you see the launch truck.
The telescope is there.
And then you see this
enormously big balloon.
All of it is empty now and
most of this will stay empty.
This is the roller arm
which holds this part down.
This is the only part that will be inflated.
And here you see the helium truck.
And here you see the inflation tubes.
And we will let helium in from both sides
which will then gradually begin
to fill this top part of the balloon.
And you see here the roller arm in detail.
The roller arm is very important because when
this part of the balloon is being filled
it wants to lift,
it wants to go up,
and of course you have to keep it down,
you have to keep it under control.
And so this roller arm and this --
this car is loaded down with concrete.
It's very heavy.
And then just before the launch this roller
arm by command is ""fweet"" flipped over
and then as you will see later
then the balloon will make it up.
And here you see the early part of the inflation.
Helium comes in from both sides.
And so we -- we fly these balloons almost
always early morning
because then the winds are very calm.
You need extremely reliable winds.
You need to know the direction very well.
And the winds should be uh no more than something
like three or four miles per hour.
If they are stronger
you would lose the balloon.
You see here these gores
that I mentioned to you earlier.
Where the sun is behind the balloon.
Here the bubble is nearly
fully inflated now.
See here it's still going on.
Still going on, the inflation.
But we are very close
to the end of the inflation.
Here is the roller arm
and then in this direction here
five hundred feet or so
down is the payload with the truck.
We're now very close to a launch.
We're still in Alice Springs.
This is -- was my graduate student
Jeff McClintock at the time.
He's now Doctor McClintock.
Here you see radar reflectors which allows
us to follow the balloon -- radar.
Here you see the telescope
hanging on the launch truck.
Here is the roller arm.
All this is empty.
And here you see the parachute.
We have here a connection between the parachute
and the bottom of the balloon.
And we can control that on radio command.
We can separate that so that the telescope
safely comes back to earth.
At least that's the idea on paper.
And so now you see
the release of the bubble.
So the roller arm is up
and this bubble now takes off.
This is an incredibly fantastic moment.
This is really butterflies in your stomach
and ants in your pants.
This is the moment that
balloon can easily fail.
Very thin material, the helium goes up,
reflects against the top,
is pushed back again,
you get this peculiar mushroom shape,
it makes
an enormous sound like a storm.
The idea now is that this balloon will go
higher and higher in the sky.
Will pick up all this empty part.
This is not inflated.
As the balloon goes up in the atmosphere the
atmospheric pressure will go down.
And the helium will expand
and will fill the balloon.
And the -- the trick now is for this truck
to manipulate, to maneuver itself
under what we call the bubble.
And therefore the wind has to be in this direction
so that the balloon comes to the truck.
And then the truck tries
to get straight under the balloon.
And then the payload will be released here.
Here you see a close-up of this mushroom.
You can actually see this reflection of the
helium going up and coming back.
You can also see these gores very clearly.
It's tedious work.
By these women who
have to seal these balloons.
Enormous amount of labor goes into it.
Amount of helium as I said earlier is about
eighty thousand dollars.
About the same price of the balloon.
And here it goes higher.
We're in Alice Springs.
The cover is falling off.
Balloon is going up.
See the engine is already running.
The truck cannot move yet because if it started
to move this part of the balloon would
slide over the cloth.
There would be friction and there would be
holes in the balloon.
So this truck has to wait until all of this
is off the ground.
Going higher.
And I'm now so close to the balloon that I
couldn't continue my picture-taking from Alice Springs.
So I will jump back to an earlier flight in
the United States.
We flew these balloons in the United States
from a town called Palestine, Texas.
And so you will see then the remaining part
of the flight from Palestine, Texas.
So the balloon is now
completely off the ground.
See a little bit of gas,
well it's not so little,
but it looks very little
compared to the size of this balloon.
You see the parachute here
and here then is the connection which
on radio command we can seperate.
So now this is a very crucial moment.
The person in charge on this launch truck
has probably driven the truck
to get straight under the balloon.
And when it's straight under there they will
allow the payload to go free.
The payload is attached to this truck.
If the balloon is too far ahead and the payload
is released it will pendulum into the ground.
And if you release it too early then of course
the payload will pendulum back into the launch truck.
Both would be a disaster.
If the pull of the balloon is not enough,
for instance if a hole developed during the launch,
so if the tension is not strong enough,
you would release the payload,
it would go bang,
back to the ground.
So all these factors
have to be taken into account,
and then finally the person in charge
commits to launch,
And then there it goes.
All the way empty.
Here you see the helium.
The parachute.
And you see the -- the payload.
And here you see the balloon at a hundred
fifty thousand feet,
forty-five kilometers high in the sky.
The helium has now expanded.
The balloon is fully inflated.
And you can look straight through it.
It's only half of one-thousandth of an inch
of polyethylene.
And these are huge ducts which have openings
of about ten meters each.
And they are there because the balloon cannot
stand any over-pressure.
If there is any over-pressure
the balloon would pop
and so when the balloon keeps
rising and rising and risng
when it reaches -- reaches its maximum volume
the helium would escape at the bottom.
That's the idea of these ducts.
Here you see George Ricker who was graduate
student, my graduate student at the time.
This is in Australia.
He is now Doctor Ricker.
He's still at MIT.
He's a staff member.
And this is the kind of equipment that we
built, at least partially.
And he is checking the early results during
the ascent of the balloon.
The balloon will go up with about
a thousand feet a minute.
If all goes well, there's no leak,
it will take about two-and-a-half, three hours
to make it to altitude.
You see me here sitting on the plane that
we used to follow the balloon.
We fly of course at much lower altitudes.
Five thousand,
ten thousand feet.
We stay as close to the balloon
as we possibly can.
It's not always so -- not always easy,
certainly not in Australia.
And so we keep an eye on things
and if necessary
we can terminate the balloon flight
by giving a radio command
so that the parachute comes down.
Certainly when we get close
to the ocean of course
that is necessary if you don't
want to lose the payload.
The data -- data come back via radio,
so we wouldn't lose the -- the data.
You get very sick,
by the way in these planes.
If you're sitting for eight or ten or twelve
hours or longer in these planes as I have,
I learned actually a little bit of flying
which is quite easy with a plane like this.
Here you see a map of Australia.
Here's Alice Springs.
Uh we fly pro-balloons weather balloons at
a hundred forty thousand feet every day
to find out in what direction
the balloon would be drifting.
And we had all reason to believe that the
balloon would drift somewhere in this direction.
And we alerted these radar stations.
These circles here are radar stations.
Because there are no
airfields here in Australia.
And so we knew we
couldn't follow the balloon.
We would probably lose it.
And therefore we alerted
these radar stations.
They could tell us then
when the balloon was in sight.
And that would allow us then
to cut the balloon,
cut the payload loose and,
and make the recovery.
Instead, the balloon
went straight south.
So the predictions by the weather balloons
were not too accurate.
The balloon went straight south.
And here there was sunset and then we don't
know too precisely where the balloon was.
Remember this was
in the nineteen seventies.
And so we were uncertain.
But here about twenty-six hours later
when we were getting
close to Melbourne
which we were not allowed to enter the air space here
between Sydney and Melbourne
we cut the balloon loose.
That means we separate
the payload from the balloon.
The balloon is very brittle.
It's extremely cold up there.
The balloon then fractures and comes down
in many pieces
and if everything worked well
then the parachute opens
and brings the payload
safely back to earth.
And then comes the big problem how are you
going to recover the payload.
You're in the middle of nowhere.
This balloon came down, this payload came
down in the desert.
And there are no airports.
At least the chances are that you
are a few hundred miles away from
the nearest airport with your payload.
So what you do then is the following.
You try to find a house close to where the
payload is located.
We locate the payload.
We see the stuff come down.
Radio beacons on the payload.
And then you fly over that house many many
times in a very obnoxious way.
You make a -- a lot of noise.
You fly over very low.
And so the people who -- whose next neigh-
neighbor is probably se- seventy miles away
from them know what
you're trying to tell them.
You're trying to draw their attention.
And they know what that means
is that they want you
they want you to meet them at the airport.
Whatever airport means.
It's sometimes just a strip in the desert.
You can't land there at night but you can
land there during the day.
And so that's exactly what we did.
We drew attention to the house
of this guy Jack.
He was -- he was a complete nut,
he was always drunk, was a crazy guy.
And so we went to the airstrip and we waited and indeed after fifteen hours he showed up with this truck.
Uh there is no windshield here in the --
in the truck.
And he used to shoot kangaroos there.
He would go sixty miles per hour on the desert
floor and he would -- he would shoot kangaroos.
And he gave me a demonstration.
Uh he put his dog on the roof.
He would go sixty miles an hour.
And he would slam the brakes.
And then the dog would catapult
through the air, the poor dog.
And then all he would say is oh, you can't
teach an old dog any new tricks.
And he seemed to enjoy that.
When we go after the payload the plane, the
recovery plane, is in the air.
Takes off from that airstrip and we have contact
with the recovery plane.
They and only they can see
and know where the payload is.
From the ground
of course you can't tell.
And so they maneuver
you to the payload.
And so of course Jack's
help was invaluable.
We needed him.
That was independent from the fact that the
man was a little strange.
On these recoveries
you encounter many animals.
You see a koala bear here.
In a l- eucalyptus tree.
Very lazy animal.
Unlike most of you.
And then when we came close to the payload
there was this animal,
a goanna,
six-feet tall goanna.
I'll tell you,
it scared the hell out of me.
And uh I didn't want to show that
to my graduate student
who was with me and I said to him
look you know these a-
these animals are completely harmless,
why don't you go first.
This animal was no farther than four feet
from the payload.
And so my graduate student went first
and the amazing thing is
during the ten hours
that it took us to recover the payload
and put it back on Jack's truck
this animal never moved.
It was just sitting there completely still.
This is their way of
trying not to be noticed.
And so here you see the payload.
This was Alice, was Jack's wife,
this is Tom Brooks,
he came from the United States,
he was an electronic expert.
And you see here the payload.
It looks heavily damaged but it really isn't.
This is crash pad which protects the payload
when it impacts.
And an impact crash pad worked very well,
very little damage to this payload.
And then you come back after
several days to Alice Springs.
Alice Springs is a hole in the ground.
Nothing ever happens there.
And so obviously you make it to the front
page of the Centralian Advocate.
Perfect balloon launch, thousand
watch start of space probe.
They think of this as a
space probe which is fine.
Balloon professor is back in Alice.
They called me there
the balloon professor.
I gave -- I gave several talks there for high
schools and for uh the Rotary Club.
So I was a sort of a local celebrity.
I talked to the uh news reporter
for several hours.
And when you read this story you won't believe
the nonsense but that's all a detail of course.
OK, that's enough for the slides for now.
I had si- about twenty successful s- flights.
Between nineteen sixty-six and nineteen eighty.
Many from the United States.
From Canada and also from Australia.
Where that's a s- southern hemisphere
which covers part of the sky that we
cannot see from the United States.
I had two free falls.
Two of my balloon burst on the way up
in the tropopause.
We were unable to separate
payload fast enough
and then the whole thing,
parachute gets entangled.
You get a free fall.
Big hole in the ground and that's
the end of the telescope.
And it was.
Twice did I lose the telescope completely.
But I was lucky and I made several interesting
discoveries during those successful flights.
We discovered very early on
five new x-ray sources.
None of them had ever been seen with the rockets.
And several of these sources, that was the
really new thing, were highly variable.
They changed their x-ray intensity
on a very short time scale.
We noticed even one source went up by a factor
of three in about ten minutes.
And that of course could not
have been discovered with rockets.
Because the rockets were only above
the atmosphere for five minutes.
They would quickly scan the sky and
so there's no way they could
discover variability on a time
scale of tens of minutes.
But with balloons you can do that.
So it did pay off that we were watching the
sky sometimes for ten, twenty hours.
Our -- my longest balloon flight was actually
twenty-six hours.
We also observed x-rays from one source which
we named GX one plus four.
Which stands the one plus four stands for
the position in the sky.
And um this showed a
periodic signal in x-rays.
For about two point three minutes periodicity.
At the time we had no clue
what that meant
but later of course we understood
the significance of that.
And you will understand that
also very shortly.
How significant that was.
So what kind of objects are they?
They are very very
different from the sun.
And we now know what they are.
These objects are binaries.
Binary stars.
One star is not unlike the sun,
it's a normal nuclear burning star.
And it is in orbit with a neutron star or
in some cases a black hole.
They go around each other.
And if they are close enough together
it is possible that the matter of this star
is attracted by this neutron star
with a larger force than
it is attracted by the star itself.
And if that's the case this matter doesn't
want to stay here.
But wants to go to the neutron star.
Now of course the matter has angular momentum
because they goes around.
So it cannot free fall
to the neutron star.
But it would spiral in and slowly make its
way to the neutron star.
And we call this an accretion disk.
And we call this the donor,
provides the fuel for the transfer
of mass onto the neutron star.
And let's assume that the neutron star has
a mass capital M and has a radius R.
And let's assume that we dump some matter,
little m, onto the neutron star.
Well all of you should remember from 8.01
that you can calculate very easily
the speed with which this matter
reaches the neutron star.
The kinetic energy, one-half mv squared,
must be equal to mMG over R.
We had this equation on the blackboard last
lecture when we discussed cosmology.
It was the same equation.
This is the speed with which the matter will
fall onto the neutron star.
If this is the mass of the neutron star and
this is the radius of the neutron star.
You lose the -- the mass as you always do.
And so you can calculate this speed.
This speed is horrendous because the radius
of a neutron star is so ridiculously small.
The mass of a neutron star is very comparable
to the mass of the sun.
A little larger.
But not much larger.
But the radius is a hundred thousand times
smaller than that of the sun.
It's only ten kilometers.
And as a result of that
the speed with which
the matter hits the neutron star
is about one-third of the speed of light.
When it hits the neutron star this kinetic
energy is converted to heat.
It will heat up the surface layers
of the neutron star
and increase the temperature to about um ten million,
a hundred million degrees,
and at such high temperatures,
the star would emit almost all its radiation
in x-rays and not in the optical.
Our sun is relatively cold.
It's only six hundred six thousand degrees.
And so the sun has most of its
radiation in the optical
but when the temperature becomes ten million,
a hundred million degrees,
the maximum of the
emission is in x-rays.
To give you a little bit respect, a little
bit of insight, for this incredible power,
for this incredible gravitational pull of
the neutron star, because R is so small,
if you took a marshmallow
and you threw a marshmallow from a large
distance onto the surface of a neutron star,
at impact the energy that
is going to be released is comparable
to the energy of an atomic bomb
as was thrown on Hiroshima
near the end of
the Second World War.
Neutron stars have very
strong magnetic fields.
And the matter that flows from the donor
onto the neutron star is ionized.
It's plasma.
It's charged.
And as you remember from 8.02 when
you have a charged particle in a magnetic field
there is the V cross B term.
The V cross B force.
And the V cross B force will then spiral these
charged particles around the magnetic field lines
and they would end up
near the magnetic poles
not unlike the solar wind
when it reaches the earth,
these charged particles enter the
earth atmosphere near the magnetic poles,
giving rise to aurora
as we discussed earlier.
And so you end up on the neutron star
with two hot spots
where this matter slams into the neutron star.
At the magnetic poles.
And if the axis of rotation of the neutron
star is not the same as the --
[Audience noise]
[noise] Hi, Nastasia.
[laughter]
Lewin: I remember your name.
[laughter]
Lewin: You were in my 8.01 class.
[laughter]
Lady: [unintelligible]
Lady: And [unintelligible]
..., is this a physics lecture?
Lewin: Ask the students, I don't know.
[laughter]
Lady: Very handsome and charming.
Lady: Ok
[laughter]
Lewin: So are you.
[laughter]
Lady: [unintelligible]
Lewin: I think you
know the answer to that one.
Lady: OK, I think we have the right person.
[background noise]
Lady: We have a song to sing
to you.
Lady: [unintelligible]
[tone]
Singing: When I was young,
I never needed anyone.
Singing: but speeking words of wisdom,
times have changed.
Singing: Maxwell's equations are too hard
for me, so please just help me, help me.
All singing: Help.
Singing: I need somebody.
All singing: Help.
Singing: Not just anybody.
All singing: Help.
Singing: You know I need someone.
All singing: Walter.
Singing: When I was younger,
so much younger than today.
Singing: I never needed anybody's
helping any way.
All singing: "toop te toop"
Singin: But now these days are gone,
I'm not so self-assured,
now I've found my homework's hard, no nothing anymore.
Singing: Help me if you can I'm failing now.
Walter Lewin, can do physics, I don't know
how.
background singing: "toop te toop"
I am on pass-fail, don't let me down.
[class laughter]
Singing: Won't you please please pass me.
background singing: "pam pam pam"
When I was dumber, so much dumber than today,
I didn't know about Lentz, Maxwell or Faraday.
Singing: But now those days are gone, I'm
not so dumb no more.
Thanks to eight oh two
and our daring professor.
Help me if you can I'm failing now.
Singing: Walter Lewin, can do physics, I
don't know how.
background singing: "too too"
I am on pass-fail don't let me down.
Singing: Won't you please please pass me.
Walter Lewin thanks for
teaching eight oh two.
Now it's summer and we will all miss you.
Singing: You've been crazy and
we've all had a good time.
So in your praise the Muses' voices chime.
Singing: Eight days a week.
I love you.
Eight days a week.
Lewin: beautiful, one more.
Is not enough to show I care.
Singing: Thanks for all you've done,
I'll pass somehow.
E and M can't get me down, I know this now.
I'll miss pass-fail next year, this I vow.
Lewin: get some water
Singing: Won't you please please pass me,
pass me, pass me, ooh.
Lewin: I have tears in my eyes.
[applause]
So that was very nice.
So now you won't have any time to fill out
your evaluation form.
[laughter]
So I was going to um
talk to you about the um
show you some evidence
for these binary systems.
So we have these hot spots on the neutron
star and as the neutron star rotates then
and the axis of rotation doesn't coincide
with the dipole, magnetic dipole axis,
you're going to see hot spot, hot spot, hot spot,
hot spot, and that explains then the x-ray pulsations.
Uh you can also see in
some cases x-ray eclipses.
If the neutron star as seen from the earth
hides behind the donor star
which is much bigger then
all the x-rays are absorbed
and so the x-rays stop completely.
You go into an x-ray eclipse and a few hours
later you come out of the x-ray eclipse again.
And that's what I would like to show you the
evidence for, which came in the early seventies.
With the satellite Uhuru,
the first slide is the basic idea.
No, that was right, uh John,
John, go back to that picture,
yeah so you see here this is of course a sketch,
this is not the real thing.
Here you see a star.
Not unlike the sun.
And then here you see the neutron star.
In some cases a black hole.
And then the matter is being
sucked off this star
because the gravitational force
in this direction is larger.
Forms the accretion disk and
ends up on the neutron star.
And the next slide is then
the discovery of the early
convincing discovery of a
pulsating system.
The rotation of the neutron star.
This time scale here is about
one-and-a-quarter second.
And the data are here,
this is the data,
unfortunately in this publication this --
this very bold line dominates almost the data,
but the idea being very clear
that the x-ray signal,
this is the strength of the x-ray signal,
this is time,
one-and-a-quarter seconds, is oscillating
one-and-a-quarter second periods.
And that's the rotation
of the neutron star.
And the next slide shows you of the same object,
it's called Hercules X one.
You see on a very different
time scale this is days.
You see that the x-ray eclipses,
that the x-rays disappear completely
when the neutron star
goes behind the donor.
And the orbital period is
one point seven days.
The x-rays disappear completely.
And so this picture is well-established.
It's beyond a shadow of a doubt we do know
what these objects are.
With no one is flying balloons anymore
I make all my reserv- o- observations
nowadays from satellites
using European satellites, Japanese
satellites and American observatories.
Lately we have the Rossi
timing explorer in orbit
and also Chandra which is
the biggest thing in town.
And now between nineteen seventy-five and nineteen
seventy-nine we were so fortunate here at MIT,
that we had our own private x-ray observatory.
It was called Sas three.
It was an all-MIT operation.
We uh maneuvered it from my building, Center
for Space Research, Building thirty-seven,
twenty-four hours a day,
three hundred sixty-five days per year.
It was at that time that Josh Grindlay
and John Heise
had discovered using a Dutch satellite,
believe it or not,
they had discovered that
one x-ray source showed sudden x-ray bursts.
The x-ray intensity would rise
in about a few seconds,
would become ten,
twenty, thirty times stronger,
and then over a time scale
of maybe a few minutes,
the x-ray intensity would peter out again.
And we had this Sas three observatory which
ideally suited to do research in these x-ray bursts.
And we discovered within two years
eight new burst sources.
And I think it's fair to say that it's largely
due to our observational work
and also to the theoretical work
by Professor Joss
who was at MIT and still is at MIT
that we now understand
what these x-ray bursts are.
They are huge nuclear bomb explosions on the
surface of a neutron star.
The matter that falls onto the neutron star
is largely hydrogen and helium.
Because that's the matter of this star.
And the density and the temperature on the
surface of the neutron star
is so high that you get nuclear reactions.
And three helium four nuclei
can fuse to carbon twelve.
And then energy is released.
And this nuclear reaction is very unstable,
is extremely sensitive to the temperature.
If the temperature goes up
the reaction rate is higher,
more energy is released.
Temperature goes up.
When the temperature goes up the reaction
rate goes up and so on, more energy is released.
And the whole thing
gets out of hand.
You get a thermonuclear runaway
as we call it.
You see a thermonuclear flash.
A gigantic bomb explosion
on the surface of the neutron star.
These bomb explosions are about eighteen
orders of magnitude more powerful
than the most powerful hydrogen bombs
that we can build on earth.
So this -- this layer, this fresh accreted layer,
goes up in one huge bomb explosion
and then new material is accreted and a few
hours later you will see another bomb explosion.
So you can see several of
these x-ray bursts per day.
The optical counterparts of these stars,
these binary systems,
uh are very faint, but you can
see them from the - the ground
with uh optical observatories
from the ground.
And we had reasons to believe at the time
that simultaneously with an x-ray burst
you might actually
observe an optical burst.
And I'll give you the reasons
why we believed that.
You see here the neutron star
and you see the accretion disk.
And let us assume that there is
an x-ray burst going on now.
These red wiggles is uh are x-rays
from the x-ray burst.
The earth is in this direction.
So these x-rays reach the earth first.
But there are x-rays
which go in this direction.
They are absorbed by the disk.
And then the disk heats up.
To thirty, forty thousand degrees.
And starts to emit optical light.
Some of that optical light will go in the
direction of the earth.
And so what this means now is that there is
a delay between these x-rays,
these - this optical light
and these x-rays.
Because look.
This path length is longer to us
than this path length.
And therefore we were hoping not only
to be able to see an optical flash,
which would mean
the x-ray heating of the disk
with the optical light
from the disk,
but we were also
hoping to see the delay.
If you can measure the delay,
if you see a one-second delay,
it would mean that this geometry was
roughly one light-second from here to here.
And so we were very ambitious.
We organized a worldwide campaign
in the summer of 1977.
With Sas three we were going to observe one
particular x-ray burst source in the sky
for two weeks on.
And we were asking observers from the ground,
optical, radio, infrared observers,
to keep an eye on that object
and also observe that,
all the time,
as long as they could.
Seventeen countries contributed.
Forty-four observatories contributed.
Participated.
And during these two weeks we saw
one hundred and ten x-rays bursts from this object
with Sas three, none was --
were observed in the optical,
none were observed in the radio.
In nineteen seventy-eight we tried it again
and we were successful.
This was a collaboration with Joss Grindlay
who was at the time at Harvard
and my graduate student Jeff McClintock
who was at that time
already Doctor McClintock.
He also was at Harvard
and he still is there.
It was a smashing success.
We made it to the cover of Nature,
which is a rather prestigious uh scientific journal.
It was covered by the New York Times
and by many newspapers in the world.
So we were the first to be able to detect
simultaneously an x-ray burst
and an potical burst
from this binary system.
Uh the data that I want to show you are not
the nineteen seventy-eight data.
But they are data from a year later
because they have better quality.
We learned how to do it of course.
These are the x-ray data
not from Sas three.
Because Sas three was no longer
in orbit in nineteen seventy-nine.
This was a Japanese satellite Hakucho.
And so here you see the times in seconds,
and here you see the x-ray intensity
and you see here the x-ray burst as we observed it
with the Japanese observatory Hakucho.
And here you see the optical data which were
taken by my friend and colleague Holger Pedersen.
He used a European southern
observatory in -- in Chile.
And he observed clearly
an optical flash.
This is the intensity of the light
before the x-ray burst
and then you see an incredible increase
and then you see a decay.
I have plotted these so that they
both have the same height.
That of course is artificial.
But it makes it easier
to compare the two.
And so now comes the acid test.
If now I put one on top of another then you
see very clearly that as we expected all along
that the optical signal is indeed delayed
relative to the x-ray signal.
And you can now do the measurement
that we hoped we could do.
You shift one curve on top of another and
if you shift it by about two seconds
then they're almost carbon
copies of each other.
And so we succeeded then
for the first time
to measure the geometry of the accretion disk
around these neutron stars.
They have radii of
very roughly two light-seconds.
Light had to travel two seconds more,
first x-rays, x-ray heating
and then the optical.
Two light-seconds is about twice the distance
from the earth to the moon.
So these systems are amazingly compact,
very small indeed.
Needless to say that
during the past term
I haven't been able to do any research,
no science at all,
8.02 has swallowed up
everything that I had to offer and more.
And I think you should feel sorry
for my graduate students.
And I think you have all the right
to feel guilty as well.
[laughter]
Uh you were on my mind all the time
and not only at MIT
but also at home,
in my living room, my kitchen,
when I took showers,
and you even very often
appeared in my dreams.
Believe me, it was hell for my --
for my significant other,
who is in the audience, Susan,
it wasn't very nice for her.
Frankly, my life will change after today.
I make myself no illusion however.
Most of you will quickly forget
all four Maxwell's equations
and you will forget all about induction
and about nonconservative fields.
I hope for you though that
it will not be before next week.
[laughter]
But surely when you will see a rainbow
you won't be able
to resist to check
that the red is on the outside
and the blue is on the inside.
It is a disease for which there is no cure.
And if you carry your personal
polarizers with you,
then you will want to verify that indeed
the bows are strongly polarized,
and if you do that I'll be very proud of you.
And if that is all you will
ever remember of 8.02,
long after you've forgotten all about
Ampere, Gauss, Faraday and Maxwell
Long after you don't even remember
how to spell their names.
If that's all you will ever remember,
I will have achieved something
that has enriched your live,
and you will remember me.
And I hope those will be happy memories.
Thank you for attending my lecture.
[applause]
[cheering applause]
Lewin: thank you thank you
[applause]
