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So today I will talk to you
about the research that I was

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doing in my early days here at
MIT which was a long time ago.

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I got my Ph.D.
in the Netherlands and nuclear

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physics.
And then in early 1966I came to

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MIT.
I had a postdoc for one year.

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I fell in love with MIT,
and I never left.

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So I changed fields from
nuclear physics to x-ray

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astronomy.
X-ray astronomy was a very

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young field at that time.
You can only do x-ray astronomy

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from above the Earth's
atmosphere because the x-rays

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are completely absorbed by the
Earth's atmosphere,

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unlike radio astronomy and
optical astronomy which you can

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do from the ground.
You have to get above the

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Earth's atmosphere.
So, I joined the research group

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of Professor Bruno Rossi and
George Clark.

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George is still at MIT.
And when I say x-rays,

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you have to think about the
same kind of x-rays that your

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dentists are using,
one to roughly 50 kilo electron

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volts.
All of you know,

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I hope, what a kilo electron
volt is.

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Optical light is only two
electron volts.

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So, x-rays are substantially
more energetic than optical

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photons.
It was in 1948 that American

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scientists started to explore
the outer space for which you

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need rockets.
And they used the German

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rockets, which were developed in
World War II [UNINTELLIGIBLE] by

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Wernher von Braun.
These were weapons.

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They were launched from France,
from Belgium,

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and from Holland.
And the target was London.

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They caused a tremendous amount
of misery and death with a

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flying bomb.
And after the war,

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Wernher von Braun was welcomed
into the United States.

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And for reasons that still
puzzle me to date,

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he even became a hero.
Using these rockets,

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these V2 rockets,
leftovers from Hitler,

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Germany, the Americans
discovered x-rays from the sun.

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The sun is very close,
so you would say,

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well, it's not too surprising.
Now, indeed,

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it's perhaps not too
surprising.

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And in fact,
the amount of x-rays from the

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sun is only a minute fraction of
the total energy output of the

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sun.
So if we take the power,

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joules per second in x-rays,
and we divide that by the power

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in optical light [of which?] the
sun is also ultraviolet and

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infrared.
That's all included.

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And if you do this for the sun,
then you get a number of about

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ten to the minus seven,
so, one ten millionth of the

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energy comes out in the form of
x-rays.

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It's still very puzzling,
by the way why there are so

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many x-rays.
But that's a different story.

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In 1962, scientists from
Cambridge, Professor Rossi,

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Riccardo Giacconi,
and [Herb Goersky?],

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and Frank [Poalinni?],
wanted to explore the

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possibility whether there were
also x-ray sources outside the

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solar system.
And the odds that they would

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detect something like that were
extremely low because if you

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take the sun and you put the sun
at the distance of the nearest

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stars, there is no hope on Earth
with the detectors that existed

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in those days that you would
detect the sun.

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In other words,
the detectors I think I

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recalled were about one billion
times too insensitive to see a

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solar like object at the
distance of the nearest stars.

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Yet, they succeeded.
1962, they reported the first

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extra-solar x-ray force.
It was later called Sco X1.

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Sco stands for the consolation,
X for the fact that it was an

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x-ray source,
and one because it was the

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first source in a constellation
of scorpius.

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And, we now know,
it took several more years,

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that the optical counterpart of
this bizarre object is a very

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feint, blue star.
It's about a distance of about

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1,000 light years.
And what is so unusual about

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Sco X-1?
That if you take the power in

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x-rays, and you divide that by
the power in optical,

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and you do that for Sco X-1,
then you will find

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approximately 1,000.
Now, think about it.

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In the sun, the x-ray emission
is a minute fraction of the

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total emission,
one in 10 million.

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In this case,
the optical emission is only a

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very small fraction of the total
emission.

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It's completely dominated by
x-rays.

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And Riccardo Giacconi received
the Nobel Prize for this two

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years ago, the Nobel Prize in
physics.

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And so, the big question was,
what can this be?

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How is it possible that any
object in the universe can do

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this?
When I came to MIT in early

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1966, about six of these sources
were known.

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They had been found using
rockets.

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These rockets would be
launched, spent at most five

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minutes above the Earth's
atmosphere and would reenter.

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They would make a quick sky
scan, and that led to the

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discovery in '66 of about a
total of six of these sources.

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So I joined the group of George
Clark who was preparing to

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observe x-rays from highflying
balloons.

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The advantage of highflying
balloons is that balloons can

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stay up for hours,
in some cases even for days.

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So, you have a huge advantage
over the rockets.

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You have way more time to
observe.

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But there is a problem.
And that is,

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you can never get above the
Earth's atmosphere because

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you're a balloon.
So there's always a little bit

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of residual atmosphere above
you, as little as that is.

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It absorbs most of the x-ray
spectrum.

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In fact, it absorbs everything
below 20 kilo electron volts.

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So, yes, we had way more time
but we were only sensitive to

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the energy range roughly above
20 kilo electron volts.

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Nowadays, no one flies rockets
anymore.

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No one flies balloons anymore.
We do everything from

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satellites 365 days a year.
So, I developed and I built in

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1966 x-ray detectors with the
help of graduate students,

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with the help of undergraduate
students who take about two

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years typically to build such a
telescope.

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In today's dollars,
it cost about $3-$4,000,000 to

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build such a telescope.
So, you need substantial

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funding.
And, the weight of the

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telescope would be 1,000 to
2,000 kg.

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The balloon that we needed,
you'll see a picture of it,

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with today's dollars,
would be about $300,000.

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And the helium that is needed
to inflate the balloon to

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inflate only a small portion of
the balloon, something like

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$150,000.
So this was not really a cheap

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endeavor.
The diameter of the balloons,

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when they are fully inflated,
is 500 feet.

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They are made of polyethylene,
and they are extremely thin

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because you need a low weight
balloon.

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Otherwise, you can't get very
high.

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In fact, the skin is thinner
than cigarette paper,

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15 ° thin.
You can look straight through

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it.
It's a very risky business.

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You pay a lot of money.
There's no guarantee that it

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will work.
You try your luck.

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If it doesn't work,
tough luck.

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You get no money back.
They could easily be damaged

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because they are so thin.
There's a good probability that

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you damage the balloon right at
the launch.

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You'll see why.
It's very difficult to launch

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these balloons.
But even if the launch is

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successful, when the balloons go
up they go through the

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tropopause, which is near 70,000
to 100,000 feet where the

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temperature is very low,
-70°C.

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And there are jet winds.
And so, the low temperature

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makes the [NOISE OBSCURES] break
the balloon.

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And, when that happens,
we do have a parachute.

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You will see the parachute that
is supposed to bring the payload

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back.
But when the balloon bursts,

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in general it interferes with
the parachute.

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And the whole thing comes down.
You have a freefall and you

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lose your payload.
And with that,

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when such a balloon bursts,
often the Ph.D.

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thesis also pops because there
are always Ph.D.

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students involved in these
experiments.

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It's very dramatic if a student
has worked two years on these

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payloads and then there is a
freefall, and you destroy the

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payload.
And it has happened,

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believe me.
So, what I want to show you now

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is a series of slides that give
you an idea of what the balloon

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launch is like.
So, if we can make it very dark

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for this, and we shall turn this
off, then you will see here the

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first slide.
These were two undergraduates

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of mine: Pat Downy and Jim
Valentine.

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They were working on the
electronics.

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Astrophysics is a very romantic
feud.

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They fell in love with each
other.

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They married.
They have kids.

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They're still in touch with me.
And Jim got his Ph.D.

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with me.
He stayed with me as a graduate

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student.
And, here you see the plans in

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Texas were these huge balloons
were built.

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This hall is 1,000 feet long.
Balloons are put together in a

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way like a tangerine is put
together with [gores?].

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They are sealed.
And that work was only done by

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women.
It has nothing to do with sex

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discrimination of any kind.
But it just turns out that

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women are more accurate workers
when it comes to this than men.

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They make fewer mistakes.
That was the only reason.

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You can imagine just a small
mistake and you can have a leak

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in the bulletin,
and that is the end of it.

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So, we lay these balloons out
on an area in general close to

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an airport.
You put cloth on the ground

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because the balloon is so thin
that if the balloon touches the

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grass, it would definitely rip.
And the balloon is packed in

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some plastic.
You see that.

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That is the pink sheet that you
see that the balloon is inside.

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There are hundreds and hundreds
of layers of balloon inside

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there.
And it's taken out of the box

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very slowly.
People inspect it because

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there's always a possibility
that there is some damage during

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transport.
And here, I take you now to a

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desert town in Australia,
the heart of Australia,

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Alice Springs,
where you see already the first

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layout of the balloon.
Here's the launch truck.

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The payload is there,
and the balloon is also there.

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You see the parachute shortly.
And all of this is empty

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balloon.
And it's only this top part

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that is going to be inflated.
We launch these balloons

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typically early morning when the
wind is very low.

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You also need a very consistent
wind.

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You'll see shortly why.
The direction of the wind

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should not change by more than
10 or 20°.

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If it does, you lose the
balloon.

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00:12:47,017 --> 00:12:50,877
You need the wind to push the
balloon towards the launch

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truck.


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00:13:00,000 --> 00:13:02,734
So this is, then,
the launch truck and then.

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00:13:02,734 --> 00:13:04,896
And this is all this empty
balloon.

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00:13:04,896 --> 00:13:08,267
And it will remain empty
because as the balloon rises,

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00:13:08,267 --> 00:13:11,320
the gas will expand.
And so, it is only this part

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00:13:11,320 --> 00:13:13,736
here that will be inflated with
helium.

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00:13:13,736 --> 00:13:15,708
You see one inflation tube
here.

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00:13:15,708 --> 00:13:17,743
And so, that part wants to go
up.

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And so, you have to hold it
down, of course.

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And, you do that with a very
heavy weight,

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00:13:23,085 --> 00:13:25,947
tons of concrete.
And there is this roller arm

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00:13:25,947 --> 00:13:30,869
that holds the balloon down.
And then, when the balloon is

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00:13:30,869 --> 00:13:33,189
being launched we slipped his
arm up.

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00:13:33,189 --> 00:13:36,927
We do a little radio command,
and then the whole thing goes

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up.
So this is a very critical part

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in a launch.
As I said earlier,

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00:13:41,051 --> 00:13:44,790
you want your payload back.
The balloons are not allowed to

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fly over certain areas where
there is heavy plane traffic

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even though the balloons fly way
above the planes.

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We fly them at 140,000 feet.
And airplanes fly only at

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30,000 feet.
But you are not allowed to fly

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over those lanes.
So when you get close to that,

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you're forced to terminate the
flight.

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And you do that on radio
command.

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00:14:06,665 --> 00:14:10,031
And there are scripts here that
break those cables.

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And then, the parachute comes
down.

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And the balloon is so brutal to
cause it's cold there that the

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balloon shatters and pieces and
comes down all by itself.

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And we often,
though not often,

231
00:14:22,217 --> 00:14:26,055
we always find the balloon.
Also, we terminate the flight,

232
00:14:26,055 --> 00:14:30,095
of course, when we get close to
water because we would not be

233
00:14:30,095 --> 00:14:34,000
able to recover the payload over
water.

234
00:14:34,000 --> 00:14:37,805
So here, you see the beginning.
We are back in Alice Springs.

235
00:14:37,805 --> 00:14:40,152
We see the beginning of the
inflation.

236
00:14:40,152 --> 00:14:43,768
So here is this roller arm
here, and this part is going to

237
00:14:43,768 --> 00:14:47,764
be inflated, helium truck there.
We have to truck the helium and

238
00:14:47,764 --> 00:14:50,492
from the United States:
very time-consuming,

239
00:14:50,492 --> 00:14:53,473
very expensive too.
It was done by the Air Force

240
00:14:53,473 --> 00:14:55,059
for me.
They pay for that,

241
00:14:55,059 --> 00:14:58,041
which was very nice.
And here, you see a typical

242
00:14:58,041 --> 00:15:00,641
situation.
We fly these early morning when

243
00:15:00,641 --> 00:15:03,432
the wind is very calm and very
steady, which,

244
00:15:03,432 --> 00:15:08,000
by the way, happens only maybe
a few days per month.

245
00:15:08,000 --> 00:15:11,163
You can't fly every day,
so you sometimes wait for a

246
00:15:11,163 --> 00:15:13,272
long time for the right
conditions.

247
00:15:13,272 --> 00:15:15,940
And so, you see here the
inflation going on,

248
00:15:15,940 --> 00:15:18,483
and then the sun is just about
to come up.

249
00:15:18,483 --> 00:15:22,205
You see the [gore?] lines here.
I mentioned to you that these

250
00:15:22,205 --> 00:15:26,114
balloons are put together like a
tangerine done by the women who

251
00:15:26,114 --> 00:15:29,215
seal them with filament tape to
give them strength.

252
00:15:29,215 --> 00:15:33,000
And you see those gore lines
here very clearly.

253
00:15:33,000 --> 00:15:35,980
And now, the bubble is up.
So the roller arm is right

254
00:15:35,980 --> 00:15:37,872
here.
Inflation is still going on.

255
00:15:37,872 --> 00:15:40,509
You can see that,
and the sun was just about to

256
00:15:40,509 --> 00:15:42,229
rise.
I waited for that moment.

257
00:15:42,229 --> 00:15:45,611
I thought it was very romantic,
you know, just at the moment

258
00:15:45,611 --> 00:15:48,420
that the sun comes up,
at the apex of the balloon,

259
00:15:48,420 --> 00:15:51,286
you know, terrific.
But my real favorite picture is

260
00:15:51,286 --> 00:15:53,407
this one, which I took a little
later.

261
00:15:53,407 --> 00:15:55,184
Now, the sun is a little
higher.

262
00:15:55,184 --> 00:15:57,248
And the inflation was still
going on.

263
00:15:57,248 --> 00:16:02,268
This is still Alice Springs.
And so, you see the inflation

264
00:16:02,268 --> 00:16:06,806
tubes, and you look at these
gores, this incredible tedious

265
00:16:06,806 --> 00:16:11,422
work to seal a polyethylene
together with the filament tapes

266
00:16:11,422 --> 00:16:15,256
hundreds and hundreds and
hundreds of these gores.

267
00:16:15,256 --> 00:16:18,229
And here, now,
we are very close to the

268
00:16:18,229 --> 00:16:20,889
launch.
You see here at the time my

269
00:16:20,889 --> 00:16:23,393
graduate student Jeff
McClintock.

270
00:16:23,393 --> 00:16:25,193
He's now Dr.
McClintock.

271
00:16:25,193 --> 00:16:28,557
He's at Harvard.
You see here the telescope,

272
00:16:28,557 --> 00:16:33,629
and then the empty balloon.
And then you see the portion

273
00:16:33,629 --> 00:16:36,715
that is being inflated.
This part may be only 80 feet.

274
00:16:36,715 --> 00:16:38,927
And that's enough.
That's all you need.

275
00:16:38,927 --> 00:16:42,419
You get enough free lift out of
that, and then as the balloon

276
00:16:42,419 --> 00:16:45,272
rises, the gas expands.
And then the balloon would

277
00:16:45,272 --> 00:16:48,124
completely inflate.
You see radar reflectors here.

278
00:16:48,124 --> 00:16:50,977
So that makes it easy for us to
track the balloon.

279
00:16:50,977 --> 00:16:52,781
And here, you see the
parachute.

280
00:16:52,781 --> 00:16:56,274
There is the point where I told
you we can do this operation.

281
00:16:56,274 --> 00:17:00,000
And Jeffrey was doing some
last-minute changes.

282
00:17:00,000 --> 00:17:01,000
This is very shortly before the
launch.

283
00:17:01,000 --> 00:17:10,000



284
00:17:10,000 --> 00:17:12,941
And here it goes.
This is a moment that your

285
00:17:12,941 --> 00:17:16,293
heart beats, easily goes from my
normal 60 to 140.

286
00:17:16,293 --> 00:17:19,302
This is a moment that if things
can go wrong,

287
00:17:19,302 --> 00:17:21,491
this is the moment they go
wrong.

288
00:17:21,491 --> 00:17:25,185
It's very thin material.
It's all of a sudden released.

289
00:17:25,185 --> 00:17:29,084
The helium wants to go up.
And it bounces back against the

290
00:17:29,084 --> 00:17:32,454
top of the room.
You get a mushroom effect.

291
00:17:32,454 --> 00:17:34,803
You see that.
This is the only helium there

292
00:17:34,803 --> 00:17:36,257
is.
There's no helium here.

293
00:17:36,257 --> 00:17:38,159
And then, this whole thing goes
up.

294
00:17:38,159 --> 00:17:40,788
The launch truck,
the engine is already running.

295
00:17:40,788 --> 00:17:43,696
You can see the exhaust.
The launch truck has to wait

296
00:17:43,696 --> 00:17:46,213
for the balloon to be entirely
off the ground.

297
00:17:46,213 --> 00:17:49,569
And then the trick is that the
launch [work?] has to maneuver

298
00:17:49,569 --> 00:17:51,806
itself to get straight under the
balloon.

299
00:17:51,806 --> 00:17:53,820
And you'll see why that is
necessary.

300
00:17:53,820 --> 00:17:57,064
And the wind is blowing the
bubble towards the launch truck

301
00:17:57,064 --> 00:18:00,364
and if the direction of the wind
changed during the past few

302
00:18:00,364 --> 00:18:04,000
hours of layout,
then we lose the balloon.

303
00:18:04,000 --> 00:18:07,287
And we cannot because the
launch truck cannot meet the

304
00:18:07,287 --> 00:18:10,451
location of the balloon at the
right moment in time.

305
00:18:10,451 --> 00:18:14,049
And then the balloon is ahead,
and then we abort the flight

306
00:18:14,049 --> 00:18:17,461
and you lose all your money.
But you don't you lose your

307
00:18:17,461 --> 00:18:20,563
payload, of course.
You will still hold the payload

308
00:18:20,563 --> 00:18:23,727
on the launch truck.
And so, here you see a close-up

309
00:18:23,727 --> 00:18:26,580
of that bubble.
It makes a tremendous amount of

310
00:18:26,580 --> 00:18:29,000
noise.
It's like a storm.

311
00:18:29,000 --> 00:18:32,500
Really, it feels like a storm.
You can see,

312
00:18:32,500 --> 00:18:35,333
you know, this material 15 °
thin.

313
00:18:35,333 --> 00:18:40,083
This reading of this material
is amazing that some of them

314
00:18:40,083 --> 00:18:43,833
actually survive.
And so, now it's picking up.

315
00:18:43,833 --> 00:18:48,666
So, the truck is still waiting.
Some of that pink material,

316
00:18:48,666 --> 00:18:52,333
the cover, is falling off.
You see that here?

317
00:18:52,333 --> 00:18:57,000
And higher, you see the helium.
This is only helium here.

318
00:18:57,000 --> 00:19:01,870
The rest is all empty.
The balloon is now almost off

319
00:19:01,870 --> 00:19:04,322
the ground.
But I was too close in this

320
00:19:04,322 --> 00:19:08,064
case to take more pictures.
So, now I will move back to the

321
00:19:08,064 --> 00:19:12,000
United States where we used to
make these launches from Texas.

322
00:19:12,000 --> 00:19:15,612
Palestine, TX is where the
balloon launching station was.

323
00:19:15,612 --> 00:19:18,967
So, I will continue the launch
from a different site,

324
00:19:18,967 --> 00:19:22,516
also a slightly smaller
balloon, but it's the same idea.

325
00:19:22,516 --> 00:19:24,645
So, you see the balloon is
rising.

326
00:19:24,645 --> 00:19:28,129
And the launch truck is now
definitely moving to try to

327
00:19:28,129 --> 00:19:32,000
maneuver itself straight under
the balloon.

328
00:19:32,000 --> 00:19:34,802
And the balloon,
now, you see now the truck has

329
00:19:34,802 --> 00:19:37,301
managed to get straight under
the balloon.

330
00:19:37,301 --> 00:19:40,956
If the balloon gets ahead of
the truck, and if now you commit

331
00:19:40,956 --> 00:19:43,759
yourself to a flight,
that means the payload is

332
00:19:43,759 --> 00:19:46,562
attached to the truck.
And, with radio command,

333
00:19:46,562 --> 00:19:49,365
we can snap some wires,
and then the payload is

334
00:19:49,365 --> 00:19:51,741
released.
If the balloon is ahead of the

335
00:19:51,741 --> 00:19:54,971
truck, you pendulum into the
ground, and you lose your

336
00:19:54,971 --> 00:19:57,164
payload.
If somehow the person on the

337
00:19:57,164 --> 00:20:00,516
launch truck commits too early
when the balloon is still

338
00:20:00,516 --> 00:20:04,050
behind, then of course the
pendulum pushes the payload into

339
00:20:04,050 --> 00:20:07,714
the truck.
And that can also destroy the

340
00:20:07,714 --> 00:20:09,546
payload.
So, it's very critical.

341
00:20:09,546 --> 00:20:11,497
This moment is extremely
critical.

342
00:20:11,497 --> 00:20:15,044
The balloon has to be straight
overhead within a few degrees.

343
00:20:15,044 --> 00:20:18,472
And then the person who is
responsible for this launch must

344
00:20:18,472 --> 00:20:21,842
also make sure that there is
enough tension in the line so

345
00:20:21,842 --> 00:20:25,330
that the payload will actually
go up rather than go down and

346
00:20:25,330 --> 00:20:27,458
plunge to the ground.
In other words,

347
00:20:27,458 --> 00:20:32,307
have we perhaps lost helium?
And all those decisions have to

348
00:20:32,307 --> 00:20:36,923
be made in a matter of seconds.
And then the commitment is made

349
00:20:36,923 --> 00:20:39,826
to the launch.
Then the balloon goes up.

350
00:20:39,826 --> 00:20:43,622
And you see that here.
The payload is being released

351
00:20:43,622 --> 00:20:46,898
from the truck,
and this balloon is now going

352
00:20:46,898 --> 00:20:49,875
up typically about 1,000 feet
per minute.

353
00:20:49,875 --> 00:20:53,970
It takes about 2 1/2 hours to
go to an altitude of about

354
00:20:53,970 --> 00:20:57,171
140,000 feet.
And then, it sometimes happens

355
00:20:57,171 --> 00:21:01,120
that the balloon fails.
This was a beauty.

356
00:21:01,120 --> 00:21:03,484
Actually, this was a flight
from Texas.

357
00:21:03,484 --> 00:21:06,472
You see the telescope,
and you see the parachute.

358
00:21:06,472 --> 00:21:09,832
All that balloon is empty,
and all that will fill up as

359
00:21:09,832 --> 00:21:12,944
you'll see shortly.
It will all fill with helium as

360
00:21:12,944 --> 00:21:15,495
the balloon rises.
And then, occasionally,

361
00:21:15,495 --> 00:21:17,984
you see this.
You know things went wrong.

362
00:21:17,984 --> 00:21:21,158
Some hole was torn in the
balloon during the launch,

363
00:21:21,158 --> 00:21:23,087
and that's, then,
the end of it.

364
00:21:23,087 --> 00:21:25,390
In this case,
we did not terminate the

365
00:21:25,390 --> 00:21:29,000
balloon on the flight on radio
command.

366
00:21:29,000 --> 00:21:33,156
So, we did not separate here
because we were so close to the

367
00:21:33,156 --> 00:21:36,045
ground but that was a destroyed
telescope.

368
00:21:36,045 --> 00:21:38,511
The parachute would not have
opened.

369
00:21:38,511 --> 00:21:42,104
We just let the whole thing
come down all by itself.

370
00:21:42,104 --> 00:21:44,781
So, the balloon,
in a way, acted like a

371
00:21:44,781 --> 00:21:47,529
parachute.
It just came down very slowly

372
00:21:47,529 --> 00:21:50,770
and very softly.
And there was no damage on the

373
00:21:50,770 --> 00:21:53,729
payload at all.
So, we'd fly it again a few

374
00:21:53,729 --> 00:21:56,547
days later.
And here, you see the balloon

375
00:21:56,547 --> 00:22:02,024
at an altitude of 150,000 feet.
This is the largest bulletin

376
00:22:02,024 --> 00:22:04,337
that was ever flown
successfully,

377
00:22:04,337 --> 00:22:06,867
I should say.
I bought this balloon,

378
00:22:06,867 --> 00:22:09,759
which is a 52,000,000 cubic
feet balloon.

379
00:22:09,759 --> 00:22:14,096
I paid a lot of money for that.
Today's money would be easily

380
00:22:14,096 --> 00:22:16,554
$250,000.
And [a larger?] was flown

381
00:22:16,554 --> 00:22:19,879
later, but not successfully.
It was not my own,

382
00:22:19,879 --> 00:22:22,698
by the way.
So, this is the largest ever

383
00:22:22,698 --> 00:22:25,301
flown.
It's at an altitude of 150,000

384
00:22:25,301 --> 00:22:27,686
feet.
And you see straight through

385
00:22:27,686 --> 00:22:31,099
the material.
It's that thin.

386
00:22:31,099 --> 00:22:34,033
It's 15 °.
And you see the payload here,

387
00:22:34,033 --> 00:22:36,013
even.
This is the parachute.

388
00:22:36,013 --> 00:22:39,606
And this, from here to here,
it's almost 500 feet.

389
00:22:39,606 --> 00:22:42,613
What you also see here are
these [ducts?].

390
00:22:42,613 --> 00:22:45,033
We call them ducts.
They are huge.

391
00:22:45,033 --> 00:22:48,333
There are about the size of
this lecture hall.

392
00:22:48,333 --> 00:22:52,366
They are connected to the
balloon up here like the veins

393
00:22:52,366 --> 00:22:56,180
going into your heart.
And then they go to the bottom

394
00:22:56,180 --> 00:22:59,039
of the balloon,
and then they just open,

395
00:22:59,039 --> 00:23:03,000
openly connected to the
atmosphere.

396
00:23:03,000 --> 00:23:05,943
And the reason for that is the
balloon is so thin.

397
00:23:05,943 --> 00:23:07,866
It cannot stand any
overpressure.

398
00:23:07,866 --> 00:23:10,089
It would pop.
So, as the balloon keeps

399
00:23:10,089 --> 00:23:13,453
rising, and the gas expands,
there comes a point that the

400
00:23:13,453 --> 00:23:16,697
balloon is fully inflated,
and you must get rid of your

401
00:23:16,697 --> 00:23:18,740
helium.
Otherwise the balloon would

402
00:23:18,740 --> 00:23:20,482
pop.
And then, the helium just

403
00:23:20,482 --> 00:23:22,825
naturally flows out through
these ducts.

404
00:23:22,825 --> 00:23:25,348
I think there were five ducts
that we have.

405
00:23:25,348 --> 00:23:27,631
You see three here.
So, these openings,

406
00:23:27,631 --> 00:23:31,116
these five openings here near
the bottom are each about the

407
00:23:31,116 --> 00:23:36,326
size of this lecture hall.
So, you see here at the time my

408
00:23:36,326 --> 00:23:38,653
graduate student,
George Ricker,

409
00:23:38,653 --> 00:23:39,929
is now Dr.
Ricker.

410
00:23:39,929 --> 00:23:42,932
And he's still at MIT,
still very active.

411
00:23:42,932 --> 00:23:46,835
A lot of this equipment was
built by him and by other

412
00:23:46,835 --> 00:23:50,213
students, graduate students and
undergraduate.

413
00:23:50,213 --> 00:23:54,416
But also a lot of it was,
of course, owned by the balloon

414
00:23:54,416 --> 00:23:57,869
launching station.
And so, we have contact with

415
00:23:57,869 --> 00:24:01,171
the telescope.
We received the data by radio.

416
00:24:01,171 --> 00:24:05,000
We can also command the
telescope.

417
00:24:05,000 --> 00:24:07,867
And you see me here sitting on
the chase plane.

418
00:24:07,867 --> 00:24:10,672
We use a small plane,
and resolve the balloon,

419
00:24:10,672 --> 00:24:12,792
of course at very modest
altitudes.

420
00:24:12,792 --> 00:24:15,535
In the United States,
you hop from airport to

421
00:24:15,535 --> 00:24:17,529
airport.
You stay as close to the

422
00:24:17,529 --> 00:24:21,145
balloon as you possibly can.
And then you terminate it when

423
00:24:21,145 --> 00:24:24,823
you have to for reasons that I
mentioned [SOUND OFF/THEN ON]

424
00:24:24,823 --> 00:24:26,381
over traffic,
air traffic,

425
00:24:26,381 --> 00:24:29,817
or you get close to water.
In Australia,

426
00:24:29,817 --> 00:24:33,526
that is way hotter because in
Australia, you don't have many

427
00:24:33,526 --> 00:24:36,795
airports in the desert.
You only have some airstrips.

428
00:24:36,795 --> 00:24:40,567
It's very, very difficult in
Australia to follow the balloon,

429
00:24:40,567 --> 00:24:43,836
but we try, and we hop from
little airstrip to little

430
00:24:43,836 --> 00:24:45,220
airstrip.
But at night,

431
00:24:45,220 --> 00:24:48,552
you cannot do that because
there's no way you can land

432
00:24:48,552 --> 00:24:51,758
[SOUND OFF/THEN ON] in
Australia, no lights on these

433
00:24:51,758 --> 00:24:54,210
airstrips.
So, this is the kind of plane

434
00:24:54,210 --> 00:24:56,913
that we used.
We'll call it a chase plane to

435
00:24:56,913 --> 00:25:01,000
follow the balloon as it starts
drifting away.

436
00:25:01,000 --> 00:25:04,458
Here is a map of Australia,
one of my many successful

437
00:25:04,458 --> 00:25:06,454
flights.
This is Alice Springs,

438
00:25:06,454 --> 00:25:09,247
desert town.
We launched the days before we

439
00:25:09,247 --> 00:25:11,708
think we are going to have our
flight.

440
00:25:11,708 --> 00:25:15,100
We launched data balloons.
We tracked them by radar.

441
00:25:15,100 --> 00:25:18,293
They go to a pretty high
altitude, 120 5000 feet,

442
00:25:18,293 --> 00:25:22,350
which gives us a good idea the
direction that our balloon will

443
00:25:22,350 --> 00:25:24,878
start moving.
And we had all reasons to

444
00:25:24,878 --> 00:25:30,000
believe that it will probably go
somewhere in this direction.

445
00:25:30,000 --> 00:25:33,449
And so, we alerted all these
radar stations in Australia.

446
00:25:33,449 --> 00:25:36,529
They will pick up our payload
because we have radar

447
00:25:36,529 --> 00:25:40,410
reflectors, and they could tell
us, then, where the balloon was.

448
00:25:40,410 --> 00:25:43,244
And that, of course,
we had to do in Australia.

449
00:25:43,244 --> 00:25:46,878
You don't have to do it in the
United States for the reasons

450
00:25:46,878 --> 00:25:49,219
that we cannot follow my
airplane here.

451
00:25:49,219 --> 00:25:51,560
There's no way that you can
land there.

452
00:25:51,560 --> 00:25:54,332
So, we relied,
then, on radar stations to tell

453
00:25:54,332 --> 00:25:56,919
us where the balloon was.
Instead, however,

454
00:25:56,919 --> 00:26:00,000
the balloon went straight
south.

455
00:26:00,000 --> 00:26:02,175
[LAUGHTER] So,
our chase plane was perfect

456
00:26:02,175 --> 00:26:04,935
because this is an area where
Australia is reasonably

457
00:26:04,935 --> 00:26:06,951
populated.
And then, when the sun sets,

458
00:26:06,951 --> 00:26:09,605
we didn't quite know where the
balloon was anymore.

459
00:26:09,605 --> 00:26:12,842
There was no GPS in those days.
So, we didn't quite know where

460
00:26:12,842 --> 00:26:15,018
the balloon was.
But then, in the morning,

461
00:26:15,018 --> 00:26:17,512
we picked it up again.
It's gorgeous to see that

462
00:26:17,512 --> 00:26:19,475
balloon in the sky.
It's unbelievable.

463
00:26:19,475 --> 00:26:22,394
It is way more impressive than
Venus now in the morning.

464
00:26:22,394 --> 00:26:24,782
It is fabulous.
When the sun hits the balloon,

465
00:26:24,782 --> 00:26:27,701
you see something that you'll
never forget in your life.

466
00:26:27,701 --> 00:26:32,000
Hundreds of calls to police
stations about reports on UFOs.

467
00:26:32,000 --> 00:26:35,333
[LAUGHTER] Hundreds of them.
And, I can understand that.

468
00:26:35,333 --> 00:26:37,818
These people have no idea how
far is away.

469
00:26:37,818 --> 00:26:40,303
You can't estimate that it's
150,000 feet.

470
00:26:40,303 --> 00:26:43,757
Later, I talked to a pilot
which was in one of those small

471
00:26:43,757 --> 00:26:46,424
airports, and he said,
when I saw this thing,

472
00:26:46,424 --> 00:26:48,121
I'm going to it.
And he says,

473
00:26:48,121 --> 00:26:50,606
and I went higher,
and higher, and higher,

474
00:26:50,606 --> 00:26:52,787
and higher, and I couldn't get
there.

475
00:26:52,787 --> 00:26:55,696
And I said, well,
it was a little higher than you

476
00:26:55,696 --> 00:26:58,060
can fly.
And, he realized that it had to

477
00:26:58,060 --> 00:27:00,904
be that far.
But, of course,

478
00:27:00,904 --> 00:27:02,895
he had never made that
connection.

479
00:27:02,895 --> 00:27:04,825
So, for him,
it was really a UFO.

480
00:27:04,825 --> 00:27:08,445
I'm not even sure he believed
my story when I told him it was

481
00:27:08,445 --> 00:27:10,858
a balloon.
So, when we were getting close

482
00:27:10,858 --> 00:27:13,452
to Melbourne,
we had to cut it loose because

483
00:27:13,452 --> 00:27:15,925
this is an area where you cannot
fly over.

484
00:27:15,925 --> 00:27:18,700
There's a city here.
So, this is very heavy air

485
00:27:18,700 --> 00:27:20,993
traffic.
And so, you then terminate the

486
00:27:20,993 --> 00:27:23,104
flight.
And if everything goes well,

487
00:27:23,104 --> 00:27:27,661
it comes down on a parachute.
And then the balloon becomes

488
00:27:27,661 --> 00:27:30,212
brittle, and the balloon comes
down as well.

489
00:27:30,212 --> 00:27:33,654
Now, how do you recover your
payload in Australia where the

490
00:27:33,654 --> 00:27:37,333
payload comes down somewhere in
the desert may be 70 miles away

491
00:27:37,333 --> 00:27:41,071
from the nearest house and maybe
120 miles away from the nearest

492
00:27:41,071 --> 00:27:43,029
airstrip?
Well, what you do is the

493
00:27:43,029 --> 00:27:45,343
following.
You try to find a house close

494
00:27:45,343 --> 00:27:48,547
to where the payload is.
So, you know where the payload

495
00:27:48,547 --> 00:27:51,811
is because you terminated it.
You see the parachute come

496
00:27:51,811 --> 00:27:52,938
down.
You locate it.

497
00:27:52,938 --> 00:27:56,617
You put it on the map where it
is, and so you know the location

498
00:27:56,617 --> 00:28:00,000
so that you can find it again
later.

499
00:28:00,000 --> 00:28:03,024
And then, what you do is you
find the nearest house.

500
00:28:03,024 --> 00:28:05,278
In this case,
it was the house of Jack.

501
00:28:05,278 --> 00:28:07,472
It was 70 miles away from the
payload.

502
00:28:07,472 --> 00:28:10,022
And you fly over that house
obnoxiously low.

503
00:28:10,022 --> 00:28:13,284
You make a lot of noise,
and you make many dives until a

504
00:28:13,284 --> 00:28:16,842
person comes out of that house.
And they know what that means

505
00:28:16,842 --> 00:28:20,341
because they've lived there.
They know that you want to meet

506
00:28:20,341 --> 00:28:23,603
them at the nearest airstrip.
Once they come out of that

507
00:28:23,603 --> 00:28:26,627
house and they wave at you,
you just go back to your

508
00:28:26,627 --> 00:28:30,172
airstrip and you wait.
And we did.

509
00:28:30,172 --> 00:28:34,224
We waited 24 hours.
And 24 hours later,

510
00:28:34,224 --> 00:28:37,743
there was Jack.
Jack was this car.

511
00:28:37,743 --> 00:28:40,622
Jack is crazy:
always drunk,

512
00:28:40,622 --> 00:28:44,994
and his profession was to shoot
kangaroos.

513
00:28:44,994 --> 00:28:51,072
There is no windshield in this
car, and this is a piece of

514
00:28:51,072 --> 00:28:54,804
wood.
And he would drive 60 mph on a

515
00:28:54,804 --> 00:29:00,348
desert, and would have his gun
on that piece of wood,

516
00:29:00,348 --> 00:29:06,000
and he would shoot that way
kangaroos.

517
00:29:06,000 --> 00:29:08,568
We are in contact with the
plane.

518
00:29:08,568 --> 00:29:11,458
The plane [talks us?] to the
payload.

519
00:29:11,458 --> 00:29:14,989
So, we are in contact [with
this?] telephone.

520
00:29:14,989 --> 00:29:17,157
Jack had a dog.
And he said,

521
00:29:17,157 --> 00:29:21,250
I'll show you something.
This is a very special dog.

522
00:29:21,250 --> 00:29:26,147
And he put the dog on the roof.
He started driving 60 miles an

523
00:29:26,147 --> 00:29:30,000
hour, and you would slam the
brakes.

524
00:29:30,000 --> 00:29:33,729
And if the dog would catapult
through the air.

525
00:29:33,729 --> 00:29:36,546
I mean, I felt sick in the
stomach.

526
00:29:36,546 --> 00:29:40,690
And then he would say,
you can teach an old dog any

527
00:29:40,690 --> 00:29:43,756
new tricks.
You must have done it many

528
00:29:43,756 --> 00:29:46,408
times with his dog.
He was crazy,

529
00:29:46,408 --> 00:29:48,977
and I think also a little
cruel.

530
00:29:48,977 --> 00:29:52,458
But, we got to the payload,
in other words,

531
00:29:52,458 --> 00:29:56,436
through the telephone with the
airplane overhead.

532
00:29:56,436 --> 00:30:01,317
He managed to get it there.
And, here you see,

533
00:30:01,317 --> 00:30:03,365
oh, there's a kangaroo.
Yeah.

534
00:30:03,365 --> 00:30:07,317
By the way, on our way to the
payload, he shot this one

535
00:30:07,317 --> 00:30:10,390
because the moment he said,
he said, sorry,

536
00:30:10,390 --> 00:30:14,048
you've got to wait.
And then, he changed direction,

537
00:30:14,048 --> 00:30:16,609
and there he went.
And he chased it,

538
00:30:16,609 --> 00:30:19,609
and he's amazing,
killed it with one shot.

539
00:30:19,609 --> 00:30:24,146
I felt sick in the stomach more
than once during that recovery.

540
00:30:24,146 --> 00:30:27,585
And, we also encountered
somewhere a koala bear.

541
00:30:27,585 --> 00:30:31,968
Isn't that nice?
Wonderful koala bear in a

542
00:30:31,968 --> 00:30:35,276
eucalyptus tree.
Ah, and then we got to the

543
00:30:35,276 --> 00:30:37,403
payload.
And what did I see?

544
00:30:37,403 --> 00:30:40,868
A 6-foot iguana 5 feet away
from the payload,

545
00:30:40,868 --> 00:30:44,097
maybe 10 feet.
But it was so scarily close

546
00:30:44,097 --> 00:30:48,350
that my stomach was turning.
And I was with my graduate

547
00:30:48,350 --> 00:30:52,367
student, Jeffrey McClintock,
and I said to Jeffrey,,

548
00:30:52,367 --> 00:30:56,305
not that I believed in myself,
but I said, Jeffrey,

549
00:30:56,305 --> 00:30:58,983
this animal is absolutely
harmless.

550
00:30:58,983 --> 00:31:03,000
Why don't you go first?
[LAUGHTER]

551
00:31:03,000 --> 00:31:05,801
And Jeffrey did go first.
And not only that,

552
00:31:05,801 --> 00:31:09,710
but these animals don't move
because they think if they don't

553
00:31:09,710 --> 00:31:13,553
move that you don't see them.
During the seven hours that it

554
00:31:13,553 --> 00:31:17,201
took us to take the payload
apart and to put it on Jack's

555
00:31:17,201 --> 00:31:19,612
truck, this animal didn't move
at all.

556
00:31:19,612 --> 00:31:23,000
So, I had plenty of time to
photograph it.

557
00:31:23,000 --> 00:31:30,000



558
00:31:30,000 --> 00:31:33,175
And here, you see the payload.
This was Jack's life.

559
00:31:33,175 --> 00:31:36,785
And, this portion here came
with me from the United States.

560
00:31:36,785 --> 00:31:39,587
He was from the Palestine
relaunching station,

561
00:31:39,587 --> 00:31:41,828
very competent electronic
technician.

562
00:31:41,828 --> 00:31:44,879
You may think that the payload
is heavily damaged.

563
00:31:44,879 --> 00:31:48,428
But that's not really true.
He protected with a crash pad.

564
00:31:48,428 --> 00:31:51,603
And so, when it hits the
ground, it may tumble over.

565
00:31:51,603 --> 00:31:55,214
But the crash pad in general
absorption most of the shocks.

566
00:31:55,214 --> 00:31:58,451
So, to his absolutely no
serious damage in this case.

567
00:31:58,451 --> 00:32:02,000
And then, you go back to Alice
Springs.

568
00:32:02,000 --> 00:32:06,332
You must understand that Alice
Springs is a hole in the ground.

569
00:32:06,332 --> 00:32:10,245
Nothing happens there ever.
And so, there's a big article

570
00:32:10,245 --> 00:32:13,110
in the newspaper.
It says: perfect balloon

571
00:32:13,110 --> 00:32:15,626
launch.
And it says balloon professor

572
00:32:15,626 --> 00:32:18,351
back in town.
They called me the balloon

573
00:32:18,351 --> 00:32:21,077
professor.
I gave talks for high schools

574
00:32:21,077 --> 00:32:24,291
and for the [Lion Club?],
your local celebrity.

575
00:32:24,291 --> 00:32:28,065
And then when you read this
article your stomach turns.

576
00:32:28,065 --> 00:32:32,902
This reporter had no idea.
I explained to him the whole

577
00:32:32,902 --> 00:32:36,368
idea about the absorption of the
Earth's atmosphere.

578
00:32:36,368 --> 00:32:40,174
So we had to get very close to
the top of the atmosphere.

579
00:32:40,174 --> 00:32:43,165
And all he could think of in
his article was,

580
00:32:43,165 --> 00:32:47,378
the reason why we fly balloons:
because that makes the distance

581
00:32:47,378 --> 00:32:50,165
between us and the stars a
little smaller.

582
00:32:50,165 --> 00:32:52,000
[LAUGHTER] Yeah.


583
00:32:52,000 --> 00:32:57,000


584
00:32:57,000 --> 00:32:59,000
All right, that's enough for
now.

585
00:32:59,000 --> 00:33:05,000



586
00:33:05,000 --> 00:33:08,748
I had about 20 successful
flights in the period 1966 to

587
00:33:08,748 --> 00:33:10,969
1980.
And at 1980I stopped flying

588
00:33:10,969 --> 00:33:14,508
balloons because then the
satellites were completely

589
00:33:14,508 --> 00:33:17,354
taking over.
I had flights from the United

590
00:33:17,354 --> 00:33:20,408
States, from Canada,
and many from Australia.

591
00:33:20,408 --> 00:33:24,156
Australia has the vintage that
you can see the Southern

592
00:33:24,156 --> 00:33:28,321
Hemisphere, which is a totally
different part of the sky from

593
00:33:28,321 --> 00:33:32,000
the United States.
I had two freefalls.

594
00:33:32,000 --> 00:33:35,291
I already mentioned to you,
that happens when there is a

595
00:33:35,291 --> 00:33:37,984
balloon burst.
And then, the parachute doesn't

596
00:33:37,984 --> 00:33:39,720
open.
And, twice did I lose my

597
00:33:39,720 --> 00:33:40,916
payload.
I was lucky,

598
00:33:40,916 --> 00:33:43,909
though, and I made several
interesting discoveries.

599
00:33:43,909 --> 00:33:46,602
We discovered early on five new
x-ray sources,

600
00:33:46,602 --> 00:33:49,115
which had never been seen from
the rockets.

601
00:33:49,115 --> 00:33:51,330
And that is perhaps not so
surprising.

602
00:33:51,330 --> 00:33:54,621
I mentioned the rockets have
only five minutes up there.

603
00:33:54,621 --> 00:33:57,254
And in five minutes,
they scan the whole sky.

604
00:33:57,254 --> 00:34:00,546
We have flights 20 hours.
The largest and longest flight

605
00:34:00,546 --> 00:34:05,154
I had was 26 hours.
And we are very sensitive above

606
00:34:05,154 --> 00:34:08,838
20 kilo electron volts where the
spectrum is very low,

607
00:34:08,838 --> 00:34:11,688
very low flux.
So, the rockets would never

608
00:34:11,688 --> 00:34:14,260
observe that,
would never detect that,

609
00:34:14,260 --> 00:34:17,526
not enough time.
But, we were able to find whole

610
00:34:17,526 --> 00:34:21,836
new sources, a whole new class
of sources which had a very high

611
00:34:21,836 --> 00:34:24,269
energy spectrum.
And now, of course,

612
00:34:24,269 --> 00:34:27,188
we know many of those.
So that was an early

613
00:34:27,188 --> 00:34:29,829
conclusion.
And, we also discovered the

614
00:34:29,829 --> 00:34:34,000
variability in x-rays as we were
observing.

615
00:34:34,000 --> 00:34:37,905
So, we were looking at Sco X-1.
I mentioned Sco X-1 earlier.

616
00:34:37,905 --> 00:34:41,215
And, as we were observing with
our balloon payload,

617
00:34:41,215 --> 00:34:44,193
we noticed that in ten minutes
time, the flux,

618
00:34:44,193 --> 00:34:48,099
the x-ray, the number of x-ray
counts per second increase by

619
00:34:48,099 --> 00:34:51,276
about a factor of three.
And that was a bombshell

620
00:34:51,276 --> 00:34:55,049
discovery at the time because
clearly, how on Earth can an

621
00:34:55,049 --> 00:34:59,021
astronomical object change its
intensity by a factor of three

622
00:34:59,021 --> 00:35:03,679
on a time scale of ten minutes.
Imagine that the sun would do

623
00:35:03,679 --> 00:35:05,127
that.
You look at the sun,

624
00:35:05,127 --> 00:35:08,254
and all of a sudden ten minutes
later, it's three times

625
00:35:08,254 --> 00:35:10,397
brighter.
That doesn't make any sense.

626
00:35:10,397 --> 00:35:13,119
But, in any case,
we now know that x-ray sources

627
00:35:13,119 --> 00:35:15,378
do that.
And, that was discovered in one

628
00:35:15,378 --> 00:35:18,563
of my early balloon flights.
We also discovered a source

629
00:35:18,563 --> 00:35:20,532
that we gave a name,
GX1 plus four.

630
00:35:20,532 --> 00:35:23,196
One plus four makes a
connection to where it is

631
00:35:23,196 --> 00:35:25,976
located in the sky.
And there was a clear hint of

632
00:35:25,976 --> 00:35:28,061
periodicity.
We saw the x-ray signal,

633
00:35:28,061 --> 00:35:30,204
the periodic.
The period was about 2.3

634
00:35:30,204 --> 00:35:34,137
minutes.
And we didn't have a clue what

635
00:35:34,137 --> 00:35:35,919
that meant.
But now we do.

636
00:35:35,919 --> 00:35:39,197
And you will know shortly also
what that means.

637
00:35:39,197 --> 00:35:42,048
So, the big problem is,
and then remains,

638
00:35:42,048 --> 00:35:45,967
what are these x-ray sources?
And I think this is a good

639
00:35:45,967 --> 00:35:49,673
moment to have a break.
You can think about what they

640
00:35:49,673 --> 00:35:54,020
may be, and then after the break
I will tell you what they are

641
00:35:54,020 --> 00:35:57,583
and a little bit more.
So, we'll have a four-minute

642
00:35:57,583 --> 00:35:59,365
break.
So, enjoy yourself.

643
00:35:59,365 --> 00:36:03,000
Walk around and stretch your
legs.

644
00:36:03,000 --> 00:38:00,000



645
00:38:00,000 --> 00:38:01,000
Two minutes left in the break.


646
00:38:01,000 --> 00:39:04,000


647
00:39:04,000 --> 00:39:09,229
OK, then, the last time you
will hear the whistle.

648
00:39:09,229 --> 00:39:14,351
What are these sources?
These are binary systems.

649
00:39:14,351 --> 00:39:18,727
Here is a star.
It's a normal star burning

650
00:39:18,727 --> 00:39:23,422
nuclear fuel.
And it is in orbit with another

651
00:39:23,422 --> 00:39:31,000
star, which is either a neutron
star, or which is a black hole.

652
00:39:31,000 --> 00:39:34,672
And it's a binary system.
Go around each other like this.

653
00:39:34,672 --> 00:39:38,672
And there is somewhere a point
between these two stars why the

654
00:39:38,672 --> 00:39:42,147
gravitational pull in one
direction is the same as the

655
00:39:42,147 --> 00:39:44,836
gravitational pull in the other
direction.

656
00:39:44,836 --> 00:39:47,983
We call that the [inner/inter?]
Lagrangian point.

657
00:39:47,983 --> 00:39:51,721
There is also such a point
between the Earth and the moon.

658
00:39:51,721 --> 00:39:55,327
It's very close to the moon.
But, there is such a point.

659
00:39:55,327 --> 00:39:58,606
And, if that point is inside
that star, so when the

660
00:39:58,606 --> 00:40:02,147
gravitational pull in this
direction is the same as the

661
00:40:02,147 --> 00:40:05,754
gravitational pull in the
direction of the neutron star,

662
00:40:05,754 --> 00:40:09,032
if it's inside that star,
then the matter will flow

663
00:40:09,032 --> 00:40:12,770
towards the neutron star because
that's energetically more

664
00:40:12,770 --> 00:40:17,340
favorable.
And so, since the whole thing

665
00:40:17,340 --> 00:40:20,487
is rotating, it cannot radially
do that.

666
00:40:20,487 --> 00:40:24,280
And so, it spirals in through
an accretion disk.

667
00:40:24,280 --> 00:40:28,396
And then, it ultimately ends up
on the neutron star.

668
00:40:28,396 --> 00:40:32,592
So, this star is called the
donor the name speaks for

669
00:40:32,592 --> 00:40:35,094
itself.
And this disc is called,

670
00:40:35,094 --> 00:40:39,987
then, the accretion disk.
And this object is called the

671
00:40:39,987 --> 00:40:43,139
accretor [SP?].
Now, let us assume that this is

672
00:40:43,139 --> 00:40:45,743
a neutron star.
It can be a black hole.

673
00:40:45,743 --> 00:40:49,649
But let's assume it's a neutron
star, and that this is the

674
00:40:49,649 --> 00:40:52,322
neutron star.
And the neutron star has a

675
00:40:52,322 --> 00:40:54,241
mass, M, and has a radius,
R.

676
00:40:54,241 --> 00:40:56,571
And, I take a little bit of
matter.

677
00:40:56,571 --> 00:40:59,449
It's a test particle from a
large distance,

678
00:40:59,449 --> 00:41:02,670
a little test particle.
And I dump that onto the

679
00:41:02,670 --> 00:41:05,000
neutron star.
And

680
00:41:05,000 --> 00:41:08,474
And I want to know,
what is the speed with which it

681
00:41:08,474 --> 00:41:11,184
reaches the surface of the
neutron star?

682
00:41:11,184 --> 00:41:14,658
All of you should be able to do
that in 30 seconds.

683
00:41:14,658 --> 00:41:17,229
Well, not all of you,
but most of you.

684
00:41:17,229 --> 00:41:20,009
Right, Emma?
All of you who have had 8.01

685
00:41:20,009 --> 00:41:23,552
can obviously do this.
It is a matter of conversion,

686
00:41:23,552 --> 00:41:28,000
of gravitational potential
energy to kinetic energy.

687
00:41:28,000 --> 00:41:32,383
And so, I can write that down
blindly even if you [wake up?] 3

688
00:41:32,383 --> 00:41:35,043
o'clock at night,
one half MV squared,

689
00:41:35,043 --> 00:41:38,349
M being this mass,
V being the speed as it hits

690
00:41:38,349 --> 00:41:41,223
the neutron star is then MG
divided by R,

691
00:41:41,223 --> 00:41:42,661
not R squared,
but R.

692
00:41:42,661 --> 00:41:46,829
This is gravitational potential
energy converted to kinetic

693
00:41:46,829 --> 00:41:49,201
energy.
And, the little mass never

694
00:41:49,201 --> 00:41:53,225
enters into the equation.
If you pick for the mass of the

695
00:41:53,225 --> 00:41:56,388
neutron star the mass of the
sun, in reality,

696
00:41:56,388 --> 00:41:58,759
they are probably a little
larger.

697
00:41:58,759 --> 00:42:03,000
But, take the mass of the
neutron star.

698
00:42:03,000 --> 00:42:06,721
And you take for the radius of
the neutron star 10 km,

699
00:42:06,721 --> 00:42:10,582
which is enormously small.
That is 100,000 times smaller

700
00:42:10,582 --> 00:42:13,531
than the radius of the sun.
If you do that,

701
00:42:13,531 --> 00:42:17,534
you get an enormously high
speed of about one third of the

702
00:42:17,534 --> 00:42:20,272
speed of light.
So, that matter hits the

703
00:42:20,272 --> 00:42:23,782
neutron star is about one third
the speed of light,

704
00:42:23,782 --> 00:42:27,504
100,000 commoners per second.
And, if enough matter is

705
00:42:27,504 --> 00:42:30,874
transferred onto the surface of
the neutron star,

706
00:42:30,874 --> 00:42:34,385
this kinetic energy,
of course, is all converted to

707
00:42:34,385 --> 00:42:37,668
heat.
You get an enormously high

708
00:42:37,668 --> 00:42:40,556
temperature: ten million,
100 million degrees,

709
00:42:40,556 --> 00:42:44,278
and at that high temperature,
the radiation that comes out,

710
00:42:44,278 --> 00:42:47,422
electromagnetic radiation is
almost all in x-rays.

711
00:42:47,422 --> 00:42:50,181
And that is what really is the
x-ray source.

712
00:42:50,181 --> 00:42:53,582
If you take a marshmallow,
and you throw a marshmallow

713
00:42:53,582 --> 00:42:56,085
from a large distance on a
neutron star,

714
00:42:56,085 --> 00:42:59,486
when it hits the surface,
the energy that is released,

715
00:42:59,486 --> 00:43:03,016
that means the explosion that
is caused by the impact is

716
00:43:03,016 --> 00:43:06,673
comparable to the energy that
was released when the atomic

717
00:43:06,673 --> 00:43:12,000
bomb was thrown on Hiroshima at
the end of the Second World War.

718
00:43:12,000 --> 00:43:15,889
So the x-ray source that we saw
from Sco X-1 is really the

719
00:43:15,889 --> 00:43:18,687
neutron star.
That's where the x-rays come

720
00:43:18,687 --> 00:43:21,076
from.
And the optical emission comes

721
00:43:21,076 --> 00:43:24,215
from the donor.
And also, some of it comes from

722
00:43:24,215 --> 00:43:27,763
the accretion disk itself.
And so, when you take this

723
00:43:27,763 --> 00:43:31,721
ratio the way I did it for this
system, this is really what

724
00:43:31,721 --> 00:43:35,543
comes from one of the two
objects, and this is what comes

725
00:43:35,543 --> 00:43:40,272
from the other object.
So that's why you get such an

726
00:43:40,272 --> 00:43:43,329
absurd ratio.
It is not just one object.

727
00:43:43,329 --> 00:43:47,796
But, it's a binary system.
The neutron stars are formed in

728
00:43:47,796 --> 00:43:51,793
a supernova explosion.
When a star burns its nuclear

729
00:43:51,793 --> 00:43:55,319
fuel like our sun,
then there is a heat source

730
00:43:55,319 --> 00:43:59,708
which is the nuclear furnace
inside, burning hydrogen and

731
00:43:59,708 --> 00:44:03,000
burning helium,
nuclear fusion.

732
00:44:03,000 --> 00:44:05,290
So you have heat production
here.

733
00:44:05,290 --> 00:44:08,798
And that obviously will make
the star grow bigger,

734
00:44:08,798 --> 00:44:11,303
expand the star and the heat
source.

735
00:44:11,303 --> 00:44:15,383
And then, there is of course
gravity, which holds the star

736
00:44:15,383 --> 00:44:17,746
together.
And there is always that

737
00:44:17,746 --> 00:44:21,969
equilibrium between gravity and
the pressure due to the heat

738
00:44:21,969 --> 00:44:25,047
that determines,
then, the final size of the

739
00:44:25,047 --> 00:44:27,338
star as an equilibrium
situation.

740
00:44:27,338 --> 00:44:30,845
But, when all the nuclear fuel
has been exhausted,

741
00:44:30,845 --> 00:44:36,000
there comes a time that the
nuclear fuel is just gone.

742
00:44:36,000 --> 00:44:39,457
Then you get an implosion
because gravity takes over.

743
00:44:39,457 --> 00:44:42,849
So the red goes away,
but the green always is there.

744
00:44:42,849 --> 00:44:46,439
Gravity is very patient,
and ultimately gravity takes a

745
00:44:46,439 --> 00:44:49,432
chance, and it pulls the whole
thing together.

746
00:44:49,432 --> 00:44:51,560
And that's a supernova
explosion.

747
00:44:51,560 --> 00:44:55,283
We have about one of those
every hundred years in our own

748
00:44:55,283 --> 00:44:57,345
galaxy.
They are extremely rare.

749
00:44:57,345 --> 00:45:02,000
Our sun will actually not
undergo a supernova explosion.

750
00:45:02,000 --> 00:45:04,786
So, our sun will not become a
neutron star.

751
00:45:04,786 --> 00:45:08,369
Stars have to be at least 10
times the mass of the sun,

752
00:45:08,369 --> 00:45:12,218
10 to 20 times the mass of the
sun for a neutron star to be

753
00:45:12,218 --> 00:45:14,805
formed or a black hole,
for that matter.

754
00:45:14,805 --> 00:45:18,786
But our sun will also die at
some point when the nuclear fuel

755
00:45:18,786 --> 00:45:21,905
has been used up.
That's going to happen about 5

756
00:45:21,905 --> 00:45:25,488
billion years from now,
so you don't have to worry yet.

757
00:45:25,488 --> 00:45:28,540
In 5 billion years,
the sun will become a white

758
00:45:28,540 --> 00:45:32,036
dwarf.
It will become the size of the

759
00:45:32,036 --> 00:45:34,140
Earth, very small.
And for that,

760
00:45:34,140 --> 00:45:36,855
it will actually first become
very large.

761
00:45:36,855 --> 00:45:39,027
And it will swallow up the
Earth.

762
00:45:39,027 --> 00:45:42,149
It will swallow Venus,
and all kinds of misery.

763
00:45:42,149 --> 00:45:45,135
So you don't have to worry
about that either.

764
00:45:45,135 --> 00:45:49,208
When a neutron star is formed,
the magnetic field of the star

765
00:45:49,208 --> 00:45:53,348
before it became a neutron star.
It may have been very modest,

766
00:45:53,348 --> 00:45:56,063
but in the formation of the
neutron star,

767
00:45:56,063 --> 00:46:00,000
that magnetic field goes up
staggeringly.

768
00:46:00,000 --> 00:46:03,875
And that has to do with 8.02.
If you make a very na°ve

769
00:46:03,875 --> 00:46:07,966
calculation, you get the right
answer may be for the wrong

770
00:46:07,966 --> 00:46:09,976
reasons.
And the very na�ve

771
00:46:09,976 --> 00:46:12,344
calculation is done the
following.

772
00:46:12,344 --> 00:46:15,645
If you have a star,
which has a magnetic dipole

773
00:46:15,645 --> 00:46:19,952
field, and you shrink the star,
say, by a factor of a million

774
00:46:19,952 --> 00:46:23,540
in terms of radius,
then the surface area goes down

775
00:46:23,540 --> 00:46:27,775
by a factor of 10 to the 12th.
But the magnetic dipole field

776
00:46:27,775 --> 00:46:32,079
doesn't change.
And so, at the surface of that

777
00:46:32,079 --> 00:46:36,239
new star, you get the magnetic
dipole field which is about 12

778
00:46:36,239 --> 00:46:40,607
orders of magnitude higher than
what you had originally when the

779
00:46:40,607 --> 00:46:44,004
star was this big.
So that very na°ve back on the

780
00:46:44,004 --> 00:46:48,163
envelope calculation tells you,
then, that the magnetic field

781
00:46:48,163 --> 00:46:52,531
would increase by a factor of 10
to 12 x 12 orders of magnitude.

782
00:46:52,531 --> 00:46:54,957
So, you have strong magnetic
fields.

783
00:46:54,957 --> 00:46:57,107
That's 8.02.
Then, we have 8.01,

784
00:46:57,107 --> 00:47:00,226
which says, yeah,
but you cannot shrink a star

785
00:47:00,226 --> 00:47:04,282
without spitting it up.
There is this idea of

786
00:47:04,282 --> 00:47:06,235
conservation of angular
momentum.

787
00:47:06,235 --> 00:47:09,349
I omega is conserved,
I being the moment of inertia.

788
00:47:09,349 --> 00:47:13,134
And so, if you make the radius
of a star 100,000 times smaller,

789
00:47:13,134 --> 00:47:16,187
then the moment of inertia
which goes always with R

790
00:47:16,187 --> 00:47:18,629
squared, right?
I hope you remember that.

791
00:47:18,629 --> 00:47:20,949
Yeah, you do.
OK, 100,000 times smaller

792
00:47:20,949 --> 00:47:24,795
radius means that the moment of
inertia goes down by a factor of

793
00:47:24,795 --> 00:47:27,543
10 to the 10th.
And that means that omega goes

794
00:47:27,543 --> 00:47:32,000
up by ten to the tenth because I
omega remains constant.

795
00:47:32,000 --> 00:47:35,510
So, the star spins up by a
factor of 10 billion.

796
00:47:35,510 --> 00:47:38,125
So, we have a strong magnetic
field.

797
00:47:38,125 --> 00:47:41,038
The neutron star is spinning
like crazy.

798
00:47:41,038 --> 00:47:43,503
And then, there is something
else.

799
00:47:43,503 --> 00:47:46,566
And that is,
if you have here that neutron

800
00:47:46,566 --> 00:47:49,853
star, and here you have this
magnetic field,,

801
00:47:49,853 --> 00:47:53,587
this magnetic dipole field it
is enormously strong.

802
00:47:53,587 --> 00:47:57,621
Then, there is matter that
comes from the side from the

803
00:47:57,621 --> 00:48:03,000
accretor, and that wants to fall
onto this neutron star.

804
00:48:03,000 --> 00:48:06,879
That matter is highly ionized
because it's hot.

805
00:48:06,879 --> 00:48:11,855
But, ionized matter cannot just
go through like this because

806
00:48:11,855 --> 00:48:15,397
there's 8.02.
And, 8.02 says that the force

807
00:48:15,397 --> 00:48:19,530
on a charged particle,
Q, is Q times V cross [B?].

808
00:48:19,530 --> 00:48:22,313
This is the famous Lawrence
force.

809
00:48:22,313 --> 00:48:25,855
And so, you cannot cross these
field lines.

810
00:48:25,855 --> 00:48:31,000
It is a cross product between
the velocity and B.

811
00:48:31,000 --> 00:48:33,980
And so, there is a problem.
And what happens now,

812
00:48:33,980 --> 00:48:37,085
the same thing happens,
by the way with the Earth's

813
00:48:37,085 --> 00:48:40,625
magnetosphere that as the matter
gets close enough to this

814
00:48:40,625 --> 00:48:43,233
neutron star,
it begins to spiral around to

815
00:48:43,233 --> 00:48:46,524
field lines that it can do,
and then it ends up on the

816
00:48:46,524 --> 00:48:49,132
magnetic pulse.
And so, most of the matter,

817
00:48:49,132 --> 00:48:51,368
then, ends up here and ends up
there.

818
00:48:51,368 --> 00:48:54,473
And so, you get onto the
neutron star two hotspots.

819
00:48:54,473 --> 00:48:58,012
They may be no larger than
maybe a few kilometers in size,

820
00:48:58,012 --> 00:49:01,524
not even that.
They may be as small as a

821
00:49:01,524 --> 00:49:03,963
soccer field.
So, that means you have two

822
00:49:03,963 --> 00:49:06,402
hotspots.
Now, imagine that I'm a neutron

823
00:49:06,402 --> 00:49:09,756
star, and that here is one hot
spot and there is another

824
00:49:09,756 --> 00:49:11,768
hotspot.
So, these are my magnetic

825
00:49:11,768 --> 00:49:13,963
poles.
But this happens to be my axis

826
00:49:13,963 --> 00:49:16,524
of rotation.
So we rotate like this and you

827
00:49:16,524 --> 00:49:19,451
will see a strong x-ray signal.
Now you see less,

828
00:49:19,451 --> 00:49:22,073
and now you see another strong
x-ray signal.

829
00:49:22,073 --> 00:49:24,756
And now, you see another strong
x-ray signal.

830
00:49:24,756 --> 00:49:26,646
So now, you will see
pulsations.

831
00:49:26,646 --> 00:49:30,121
So, now you expect that the
x-ray intensity varies because

832
00:49:30,121 --> 00:49:35,000
you were looking at these
hotspots, these magnetic poles.

833
00:49:35,000 --> 00:49:37,884
And then, there is,
of course, the possibility in

834
00:49:37,884 --> 00:49:40,528
these binary systems.
As seen from the Earth,

835
00:49:40,528 --> 00:49:43,894
it may be that when they go
around each other is that the

836
00:49:43,894 --> 00:49:46,298
neutron star hides behind the
donor star.

837
00:49:46,298 --> 00:49:49,242
And when that happens,
the x-rays from the neutron

838
00:49:49,242 --> 00:49:52,728
star are completely absorbed
because the donor star absorbs

839
00:49:52,728 --> 00:49:55,192
the x-rays.
So, then you don't see x-rays.

840
00:49:55,192 --> 00:49:58,497
That's called an x-ray eclipse.
And all of that has been

841
00:49:58,497 --> 00:50:01,849
observed.
And I will show you the

842
00:50:01,849 --> 00:50:06,353
evidence for that so that you
can fully appreciate what's

843
00:50:06,353 --> 00:50:09,812
going on here.
So let me then go back to the

844
00:50:09,812 --> 00:50:13,914
slides, make it dark.
And, I'll show you the idea of

845
00:50:13,914 --> 00:50:16,166
a binary system.
So, this is,

846
00:50:16,166 --> 00:50:19,222
of course, just an artist's
conception.

847
00:50:19,222 --> 00:50:21,957
You see on the left side the
donor.

848
00:50:21,957 --> 00:50:26,300
And then you see here the
location of the neutron star,

849
00:50:26,300 --> 00:50:30,000
or in some cases,
a black hole.

850
00:50:30,000 --> 00:50:34,545
And then the inner Lagrangian
point is apparently inside this

851
00:50:34,545 --> 00:50:37,121
star.
And so, the matter will flow,

852
00:50:37,121 --> 00:50:41,136
makes an accretion disk,,
and gradually finds its way.

853
00:50:41,136 --> 00:50:45,378
And then the energy is released
the way that we discussed

854
00:50:45,378 --> 00:50:50,000
gravitational potential energy
is converted to kinetic energy.

855
00:50:50,000 --> 00:50:54,318
This data is from one of the
early x-ray satellites called

856
00:50:54,318 --> 00:50:57,121
[UNINTELLIGIBLE].
It's data from 1971.

857
00:50:57,121 --> 00:51:00,000
Horizontally,
it's time.

858
00:51:00,000 --> 00:51:03,704
This is 1 1/4 seconds.
So, that gives you an idea of

859
00:51:03,704 --> 00:51:06,319
the timescale.
And, this is the x-ray

860
00:51:06,319 --> 00:51:10,242
intensity, how many x-rays per
unit time were observed.

861
00:51:10,242 --> 00:51:14,382
And, the actual data are the
very faint lines in the back.

862
00:51:14,382 --> 00:51:17,433
And they immediately notice the
pulsations.

863
00:51:17,433 --> 00:51:21,864
And so, then they did a fit to
the data, and they came up with

864
00:51:21,864 --> 00:51:24,624
this shape.
And they unfortunately made

865
00:51:24,624 --> 00:51:27,893
that very bold.
And so, you can hardly see the

866
00:51:27,893 --> 00:51:31,085
data anymore.
But in any case,

867
00:51:31,085 --> 00:51:35,089
it is very clear that you have
here a signal of the rotating

868
00:51:35,089 --> 00:51:37,940
neutron star.
And, the neutron star in this

869
00:51:37,940 --> 00:51:41,061
case goes around,
spins around in 1.24 seconds,

870
00:51:41,061 --> 00:51:43,640
so from here to here it's 1.24
seconds.

871
00:51:43,640 --> 00:51:45,880
This object is called Hercules
X1.

872
00:51:45,880 --> 00:51:48,866
Spinning like crazy,
you think about the sun.

873
00:51:48,866 --> 00:51:51,920
The sun rotates about its own
axis in 25 days.

874
00:51:51,920 --> 00:51:55,449
You can see that very well
because there's nothing to

875
00:51:55,449 --> 00:51:58,978
recognize on the sun.
But, it goes around in 25 days.

876
00:51:58,978 --> 00:52:04,000
This one goes around this
neutron star in 1.24 seconds.

877
00:52:04,000 --> 00:52:07,975
Here we see data from the same
source, but on a very different

878
00:52:07,975 --> 00:52:10,582
timescale.
From here to here is 1.7 days.

879
00:52:10,582 --> 00:52:14,492
And I show you this so that you
can see the eclipse is that I

880
00:52:14,492 --> 00:52:17,815
mentioned, the x-ray eclipses.
It's a very different

881
00:52:17,815 --> 00:52:20,292
timescale.
You can't see the pulsations

882
00:52:20,292 --> 00:52:22,703
anymore.
And so, we see that every 1.7

883
00:52:22,703 --> 00:52:25,635
days, the neutron star hides
behind the donor,

884
00:52:25,635 --> 00:52:29,350
and is invisible until it
emerges again on the other side.

885
00:52:29,350 --> 00:52:33,000
And then, the x-rays come back
up again.

886
00:52:33,000 --> 00:52:38,636
And, this is not a set of data.
So, this is many,

887
00:52:38,636 --> 00:52:43,685
many days of data.
And so, there you see the

888
00:52:43,685 --> 00:52:50,731
overpowering evidence for these
objects being binary systems.

889
00:52:50,731 --> 00:52:54,958
The eclipses,
you see the pulsations,

890
00:52:54,958 --> 00:53:01,534
and what more do you want?
I am no longer flying balloons

891
00:53:01,534 --> 00:53:06,524
these days.
Clearly, satellites have taken

892
00:53:06,524 --> 00:53:09,300
over.
I have flown observing every

893
00:53:09,300 --> 00:53:13,592
conceivable x-ray observatory
that you can think of.

894
00:53:13,592 --> 00:53:17,042
[UNINTELLIGIBLE] was a European
satellite.

895
00:53:17,042 --> 00:53:20,071
[UNINTELLIGIBLE] Japanese
satellites.

896
00:53:20,071 --> 00:53:23,941
[UNINTELLIGIBLE] was also a
Japanese satellite,

897
00:53:23,941 --> 00:53:30,000
was the x-ray timing explorer,
which is still up and running.

898
00:53:30,000 --> 00:53:32,913
[Chandra?] is the big thing in
town nowadays.

899
00:53:32,913 --> 00:53:35,033
I make observations with
Chandra.

900
00:53:35,033 --> 00:53:38,145
And, XMM Newton,
very important observatory also

901
00:53:38,145 --> 00:53:42,251
in orbit: European observatory.
So, I have been doing satellite

902
00:53:42,251 --> 00:53:44,635
work for most of my time here at
MIT.

903
00:53:44,635 --> 00:53:47,814
And, in the period,
1975 to 79, we at MIT had our

904
00:53:47,814 --> 00:53:50,066
own small little x-ray
observatory.

905
00:53:50,066 --> 00:53:53,245
It was called SAS-3,
Small Astronomical Satellite

906
00:53:53,245 --> 00:53:56,158
number three.
And, we were running in from my

907
00:53:56,158 --> 00:54:00,000
building, the Center for Space
Research.

908
00:54:00,000 --> 00:54:03,532
George Clarke was the principal
investigator.

909
00:54:03,532 --> 00:54:07,064
24 hours a day:
data come in 365 days a year.

910
00:54:07,064 --> 00:54:10,676
Compare that with a five-minute
rocket flight,

911
00:54:10,676 --> 00:54:13,325
and with a 20 hour balloon
flight.

912
00:54:13,325 --> 00:54:17,339
It was at that time that an
astronomical satellite,

913
00:54:17,339 --> 00:54:20,791
which was, I think it was a
Dutch satellite.

914
00:54:20,791 --> 00:54:24,965
It was called the astronomical
Netherlands satellite.

915
00:54:24,965 --> 00:54:29,300
It was a collaboration with the
United States that they

916
00:54:29,300 --> 00:54:35,000
discovered a phenomenon which we
now call x-ray bursts.

917
00:54:35,000 --> 00:54:38,094
The person who discovered it
was Josh Grinkey [SP?],

918
00:54:38,094 --> 00:54:40,885
who was at Harvard.
And, John [Heisey?] at that

919
00:54:40,885 --> 00:54:43,555
time was in [UNINTELLIGIBLE] the
Netherlands.

920
00:54:43,555 --> 00:54:47,135
What I noticed is that when
they look at some of these x-ray

921
00:54:47,135 --> 00:54:50,291
sources, one in particular,
they all of a sudden in a

922
00:54:50,291 --> 00:54:53,507
matter of a few seconds,
seeing the x-ray intensity go

923
00:54:53,507 --> 00:54:56,298
up by a factor of 10 or 20.
It was very bright,

924
00:54:56,298 --> 00:55:00,000
and then a few years later it's
back to normal.

925
00:55:00,000 --> 00:55:03,455
And that's referred to nowadays
as x-ray bursts.

926
00:55:03,455 --> 00:55:06,029
And with SAS-3,
with our spacecraft,

927
00:55:06,029 --> 00:55:10,000
we were ideally suited to
search for these x-ray bursts

928
00:55:10,000 --> 00:55:13,529
even much more so than the
Netherlands satellite.

929
00:55:13,529 --> 00:55:17,352
Our system was perfect.
We never thought of that when

930
00:55:17,352 --> 00:55:20,514
we designed it,
but it just turned out to be

931
00:55:20,514 --> 00:55:23,088
perfect.
And in the first two years,

932
00:55:23,088 --> 00:55:25,514
we discovered a new burst
sources.

933
00:55:25,514 --> 00:55:28,676
And it is largely due to those
observations,

934
00:55:28,676 --> 00:55:33,014
and also due to the theoretical
work by Professor Paul Joss,

935
00:55:33,014 --> 00:55:36,544
who is still at MIT,
that we now know what causes

936
00:55:36,544 --> 00:55:41,922
these crazy x-ray bursts.
These x-ray bursts are huge

937
00:55:41,922 --> 00:55:46,051
nuclear bomb explosions on the
surface of the neutron star.

938
00:55:46,051 --> 00:55:50,038
What happens is the following.
You get accretion onto the

939
00:55:50,038 --> 00:55:53,100
neutron star.
And the matter that comes from

940
00:55:53,100 --> 00:55:57,300
the donor is largely hydrogen,
maybe a little bit of helium,

941
00:55:57,300 --> 00:56:02,000
largely hydrogen and maybe some
helium just like our sun.

942
00:56:02,000 --> 00:56:05,099
And so, that matter falls onto
the neutron star,

943
00:56:05,099 --> 00:56:09,122
and it releases than the x-rays
due to what we just discussed,

944
00:56:09,122 --> 00:56:12,816
the gravitational potential
energies converted to kinetic

945
00:56:12,816 --> 00:56:16,114
energy, but nevertheless,
it's still hydrogen as it

946
00:56:16,114 --> 00:56:18,884
reaches here.
And so, it forms a layer onto

947
00:56:18,884 --> 00:56:21,786
the neutron star.
And the densities are high.

948
00:56:21,786 --> 00:56:25,479
And the temperature is high,
and there is a thermonuclear

949
00:56:25,479 --> 00:56:28,579
reaction, and it turns the
hydrogen into helium.

950
00:56:28,579 --> 00:56:32,008
And then, in the temperatures
are even higher and the

951
00:56:32,008 --> 00:56:35,372
density's just right,
the helium starts to go into a

952
00:56:35,372 --> 00:56:39,000
nuclear reaction informs carbon
12.

953
00:56:39,000 --> 00:56:41,769
And that is a very peculiar
reaction.

954
00:56:41,769 --> 00:56:45,923
You need three helium nuclei,
helium 4, and they merge,

955
00:56:45,923 --> 00:56:48,846
then, to carbon 12.
And you get energy.

956
00:56:48,846 --> 00:56:52,230
This reaction rate is
enormously sensitive to

957
00:56:52,230 --> 00:56:55,538
temperature.
I forgot the exact number but I

958
00:56:55,538 --> 00:56:59,307
recall something like,
that is proportional to the

959
00:56:59,307 --> 00:57:03,692
temperature to the power 30,
temperature in degrees Kelvin

960
00:57:03,692 --> 00:57:07,690
to the power 30.
So, when this reaction starts,

961
00:57:07,690 --> 00:57:10,281
energy is released.
So the temperature goes up.

962
00:57:10,281 --> 00:57:13,380
When the temperature goes up,
the reaction rate goes up.

963
00:57:13,380 --> 00:57:16,704
And when the reaction rate goes
up, the temperature goes up.

964
00:57:16,704 --> 00:57:18,619
And that's what you call a
runaway.

965
00:57:18,619 --> 00:57:21,042
You get a runaway process,
and we call that,

966
00:57:21,042 --> 00:57:23,971
then, a thermonuclear flash.
So the whole system gets

967
00:57:23,971 --> 00:57:27,014
completely out of hand and you
get a thermonuclear bomb

968
00:57:27,014 --> 00:57:29,492
explosion on the surface of the
neutron star,

969
00:57:29,492 --> 00:57:31,746
which gives the immediate
increase, then,

970
00:57:31,746 --> 00:57:35,825
in your x-ray signal.
If you wait a few hours,

971
00:57:35,825 --> 00:57:40,236
then you build up a new layer
of hydrogen, and that goes to

972
00:57:40,236 --> 00:57:43,202
helium, and you get another
x-ray burst.

973
00:57:43,202 --> 00:57:47,309
So, when you look at the
sources, you may see one every

974
00:57:47,309 --> 00:57:51,340
hour, one every two hours,
or sometimes a few per day.

975
00:57:51,340 --> 00:57:55,752
These bomb explosions are about
18 orders of magnitude more

976
00:57:55,752 --> 00:58:01,000
powerful than the hydrogen bomb
that we can make on Earth.

977
00:58:01,000 --> 00:58:05,509
So, the optical counterparts of
these x-ray binaries are rather

978
00:58:05,509 --> 00:58:07,254
feint.
You see the donor.

979
00:58:07,254 --> 00:58:11,181
I mentioned that to you in the
optical from the ground.

980
00:58:11,181 --> 00:58:13,436
Then you see the accretion
disk.

981
00:58:13,436 --> 00:58:16,636
And x-rays, then,
come from the neutron star.

982
00:58:16,636 --> 00:58:20,781
But you can see from the ground
with an optical telescope,

983
00:58:20,781 --> 00:58:23,327
you can see the optical
counterpart.

984
00:58:23,327 --> 00:58:27,254
And we had reasons to believe
that if there is an x-ray

985
00:58:27,254 --> 00:58:33,000
bursts, that somehow it should
be followed by an optical burst.

986
00:58:33,000 --> 00:58:39,862
And I can explain that to you
best by showing you a

987
00:58:39,862 --> 00:58:45,490
transparency.
When the x-ray burst occurs,

988
00:58:45,490 --> 00:58:50,294
the x-rays go off in all
directions.

989
00:58:50,294 --> 00:58:54,823
So, here you see the neutron
star.

990
00:58:54,823 --> 00:59:01,000
And you see there the accretion
disk.

991
00:59:01,000 --> 00:59:04,070
And when the x-ray [bond?]
takes place, you see the

992
00:59:04,070 --> 00:59:07,385
direction to the Earth.
This is the direction to Earth.

993
00:59:07,385 --> 00:59:10,701
That's where you happen to be.
So you see the immediate

994
00:59:10,701 --> 00:59:13,710
increase in x-rays,
but the x-rays also go in this

995
00:59:13,710 --> 00:59:16,228
direction.
And then they hit the accretion

996
00:59:16,228 --> 00:59:18,254
disk.
And they get absorbed by the

997
00:59:18,254 --> 00:59:21,017
accretion disk.
And they heat up the accretion

998
00:59:21,017 --> 00:59:22,982
disk.
They can heat the accretion

999
00:59:22,982 --> 00:59:25,868
disk locally up to maybe
30,000°, a much higher

1000
00:59:25,868 --> 00:59:29,000
temperature than it normally
had.

1001
00:59:29,000 --> 00:59:31,945
And so the accretion disk,
now, begins to radiate an

1002
00:59:31,945 --> 00:59:34,545
optical light,
at least more light than it did

1003
00:59:34,545 --> 00:59:36,624
before.
And then, when this becomes a

1004
00:59:36,624 --> 00:59:39,512
source of optical light,
it goes in all directions.

1005
00:59:39,512 --> 00:59:42,054
The optical light,
and some of it goes in the

1006
00:59:42,054 --> 00:59:44,942
direction of the Earth.
And so, then that result is

1007
00:59:44,942 --> 00:59:48,407
that you first see the x-ray
source, and then a little later,

1008
00:59:48,407 --> 00:59:50,833
the x-ray bursts.
And then, a little later,

1009
00:59:50,833 --> 00:59:54,126
you'll see the optical burst
because there is extra travel

1010
00:59:54,126 --> 00:59:56,725
time from here to there.
It may take a second,

1011
00:59:56,725 --> 1:00:01,000
and then there is extra travel
time from here to here.

1012
1:00:01,000 --> 1:00:04,790
It may take another second.
And so, don't be surprised that

1013
1:00:04,790 --> 1:00:07,142
the optical flash,
the optical burst,

1014
1:00:07,142 --> 1:00:10,605
then comes a little later.
But if you can measure that

1015
1:00:10,605 --> 1:00:14,592
delay, then you have succeeded
in getting a reasonable idea of

1016
1:00:14,592 --> 1:00:18,447
the size of these accretion
disks because if the delay is 10

1017
1:00:18,447 --> 1:00:22,368
seconds, then you would know
roughly that the radius of these

1018
1:00:22,368 --> 1:00:25,635
accretion disks might be five to
six light seconds.

1019
1:00:25,635 --> 1:00:29,033
But if the delay is only two
seconds, that tells you,

1020
1:00:29,033 --> 1:00:34,000
then, that this distance is
probably only one light seconds.

1021
1:00:34,000 --> 1:00:37,955
And you have to add,
of course, this extra distance.

1022
1:00:37,955 --> 1:00:41,213
So, this was,
for us, the reason to start a

1023
1:00:41,213 --> 1:00:44,315
worldwide campaign in the summer
of 1977.

1024
1:00:44,315 --> 1:00:49,047
We committed SAS-3 to serving
one and only one of these stars,

1025
1:00:49,047 --> 1:00:52,227
binary systems,
and we ask everyone in the

1026
1:00:52,227 --> 1:00:55,639
world to have a telescope,
optical telescope,

1027
1:00:55,639 --> 1:00:58,354
radio telescope,
infrared telescope,

1028
1:00:58,354 --> 1:01:02,000
to watch the optical
counterpart.

1029
1:01:02,000 --> 1:01:06,285
And then we would tell them
later when the x-ray bursts were

1030
1:01:06,285 --> 1:01:09,191
observed.
And they would look at the data

1031
1:01:09,191 --> 1:01:13,549
to see whether there was any
change in the optical light from

1032
1:01:13,549 --> 1:01:15,365
that star.
And I remember,

1033
1:01:15,365 --> 1:01:18,779
the summer in 1977,
there were 17 countries that

1034
1:01:18,779 --> 1:01:21,757
contributed.
44 observatories contributed.

1035
1:01:21,757 --> 1:01:24,590
We saw 110 x-ray bursts from
the source.

1036
1:01:24,590 --> 1:01:28,731
And not a single observatory
saw an optical increase or in

1037
1:01:28,731 --> 1:01:34,266
the radio or in the infrared.
So, we decided we needed larger

1038
1:01:34,266 --> 1:01:37,702
telescopes because we were not
going to give up.

1039
1:01:37,702 --> 1:01:41,503
So, we tried it again in 1978.
And then we succeeded.

1040
1:01:41,503 --> 1:01:45,378
It was a collaboration with
Josh Grinkey from Harvard,

1041
1:01:45,378 --> 1:01:48,887
and also Jeff McClintock,
my ex-graduate student,

1042
1:01:48,887 --> 1:01:51,446
who was also at Harvard at the
time.

1043
1:01:51,446 --> 1:01:54,224
We succeeded.
We saw an optical burst a

1044
1:01:54,224 --> 1:01:57,002
little bit after we saw an extra
burst.

1045
1:01:57,002 --> 1:02:01,958
It was a smashing success.
And we made it to the cover

1046
1:02:01,958 --> 1:02:05,005
page of Nature,
which is a very prestigious

1047
1:02:05,005 --> 1:02:08,560
scientific journal.
It was covered in the New York

1048
1:02:08,560 --> 1:02:11,172
Times, the usual thing,
Boston Globe,

1049
1:02:11,172 --> 1:02:14,147
and even more.
I want to show you an x-ray

1050
1:02:14,147 --> 1:02:18,064
burst and an optical burst,
but not the data from 1978.

1051
1:02:18,064 --> 1:02:21,909
I have data of much higher
quality, which we got to in

1052
1:02:21,909 --> 1:02:23,360
1979.
In other words,

1053
1:02:23,360 --> 1:02:26,045
we pursued this research very
heavily.

1054
1:02:26,045 --> 1:02:29,527
We worked with Holga Patterson,
who was in Chile,

1055
1:02:29,527 --> 1:02:33,154
the European Southern
Observatory, and with Michael

1056
1:02:33,154 --> 1:02:39,628
[Laver Beyon?] from [Paradise?].
We often went there and we made

1057
1:02:39,628 --> 1:02:42,851
the optical observations.
By that time,

1058
1:02:42,851 --> 1:02:45,480
SAS-3 was no longer
functioning.

1059
1:02:45,480 --> 1:02:49,042
So, we had the Japanese
satellite [Hocho?],

1060
1:02:49,042 --> 1:02:53,706
looking at the particular
object that we were interested

1061
1:02:53,706 --> 1:02:56,420
in.
And, I want to show you first

1062
1:02:56,420 --> 1:03:01,000
the x-ray burst that was
observed by Hocho.

1063
1:03:01,000 --> 1:03:04,095
Here is the x-ray burst.
You see the timescale.

1064
1:03:04,095 --> 1:03:06,988
The [rise?] of this burst is
unusually slow.

1065
1:03:06,988 --> 1:03:10,218
It's just the way it is.
It happens occasionally.

1066
1:03:10,218 --> 1:03:13,044
See, this rises.
It takes about five or six

1067
1:03:13,044 --> 1:03:15,533
seconds.
Normally it goes up in one or

1068
1:03:15,533 --> 1:03:18,494
two seconds, but nothing I can
do about that.

1069
1:03:18,494 --> 1:03:21,320
So, you see here the data
before the burst.

1070
1:03:21,320 --> 1:03:24,819
So that's the accretion.
That is the x-ray it becomes

1071
1:03:24,819 --> 1:03:30,000
available because of the metal
falling onto the neutron star.

1072
1:03:30,000 --> 1:03:32,947
And then you see the
thermonuclear bomb explosion,

1073
1:03:32,947 --> 1:03:36,255
and then afterwards here,
there is still a little bit of

1074
1:03:36,255 --> 1:03:38,661
afterglow.
But it settles very quickly to

1075
1:03:38,661 --> 1:03:41,548
the pre-burst level.
And then, Holga Patterson in

1076
1:03:41,548 --> 1:03:43,774
Chile came up with this optical
burst.

1077
1:03:43,774 --> 1:03:46,300
You'd think it's almost like a
carbon copy.

1078
1:03:46,300 --> 1:03:50,030
And, I plotted it for you in a
way that the peak is at the same

1079
1:03:50,030 --> 1:03:52,255
height.
That is just an artificial way

1080
1:03:52,255 --> 1:03:54,180
that I can scale that,
of course.

1081
1:03:54,180 --> 1:03:56,706
And, if now,
I put the x-ray data on top of

1082
1:03:56,706 --> 1:03:59,233
the optical data,
then what do you see that

1083
1:03:59,233 --> 1:04:04,097
indeed there is a delay?
The optical data are a little

1084
1:04:04,097 --> 1:04:08,508
bit later than the x-ray data.
And all you actually have to do

1085
1:04:08,508 --> 1:04:12,920
is just slide it by two seconds,
and they almost become carbon

1086
1:04:12,920 --> 1:04:16,029
copies of each other.
And so, indeed we have

1087
1:04:16,029 --> 1:04:18,995
succeeded.
And we are the first to measure

1088
1:04:18,995 --> 1:04:22,827
the size, the rough size,
of the accretion disk at the

1089
1:04:22,827 --> 1:04:27,022
point where the x-rays were
absorbed, and then converted to

1090
1:04:27,022 --> 1:04:31,000
optical, which then was seen by
the Earth.

1091
1:04:31,000 --> 1:04:34,441
And we concluded that the
accretion disk was something

1092
1:04:34,441 --> 1:04:37,753
like one light seconds in
radius, which is about the

1093
1:04:37,753 --> 1:04:41,000
distance from Earth to moon,
one light second.

1094
1:04:41,000 --> 1:04:49,000



1095
1:04:49,000 --> 1:04:55,199
Oh my goodness,
I recognize you.

1096
1:04:55,199 --> 1:05:05,000
What are you coming here for?
I'm getting flowers?

1097
1:05:05,000 --> 1:05:13,599
Oh, my goodness,
what have I done to deserve

1098
1:05:13,599 --> 1:05:18,000
them?
[APPLAUSE]

1099
1:05:18,000 --> 1:05:23,000
Wow, I recognize some of you.
This is fantastic.

1100
1:05:23,000 --> 1:05:30,000



1101
1:05:30,000 --> 1:09:22,000
[SINGING STARTS]
Thank you very much. [APPLAUSE]

1102
1:09:22,000 --> 1:09:24,748
I'm very impressed.
Thank you very much.

1103
1:09:24,748 --> 1:09:28,765
And I gather that the text,
which I couldn't always follow

1104
1:09:28,765 --> 1:09:32,288
was very much connected to my
[OVERLAPPING VOICES].

1105
1:09:32,288 --> 1:09:35,177
So, in that sense,
I appreciate it double.

1106
1:09:35,177 --> 1:09:37,785
Thank you very much.
Let me thank you,

1107
1:09:37,785 --> 1:09:41,731
because I recall you from,
you were in my class last year

1108
1:09:41,731 --> 1:09:43,000
right?


1109
1:09:43,000 --> 1:09:50,000


1110
1:09:50,000 --> 1:09:54,919
Well, I don't know whether
those much time left for you now

1111
1:09:54,919 --> 1:09:59,583
to fill out the evaluation
forms, but that's the problem

1112
1:09:59,583 --> 1:10:03,230
that you have.
The orbital motion of neutron

1113
1:10:03,230 --> 1:10:07,725
stars and the donor stars give
rise to Doppler shifts,

1114
1:10:07,725 --> 1:10:11,287
which is something that we
covered in 8.03.

1115
1:10:11,287 --> 1:10:15,613
So, we have a rotating neutron
star with pulsations.

1116
1:10:15,613 --> 1:10:20,362
[It's a clock?] in the case of
Hercules X1, 1.24 seconds.

1117
1:10:20,362 --> 1:10:25,112
And, when that clock moves to
you, the period is smaller,

1118
1:10:25,112 --> 1:10:28,335
blue shift.
And when it moves away from

1119
1:10:28,335 --> 1:10:33,000
you, the period is longer,
red shift.

1120
1:10:33,000 --> 1:10:36,217
And you can measure that.
You can actually see as the

1121
1:10:36,217 --> 1:10:39,373
neutron star goes around,
and even comes towards the

1122
1:10:39,373 --> 1:10:43,086
Earth, that you see a different
period then when it goes away

1123
1:10:43,086 --> 1:10:44,881
from you.
And then, of course,

1124
1:10:44,881 --> 1:10:47,975
you have the donor itself.
The donor has an optical

1125
1:10:47,975 --> 1:10:51,935
spectrum, and you see absorption
lines in the spectrum which come

1126
1:10:51,935 --> 1:10:53,915
from the atmosphere of the
donor.

1127
1:10:53,915 --> 1:10:58,000
And then, you see the shift of
the absorption lines.

1128
1:10:58,000 --> 1:11:02,085
And you worked on a particular
case of [sickness?] X1.

1129
1:11:02,085 --> 1:11:05,400
You perhaps remember that,
your problem set,

1130
1:11:05,400 --> 1:11:09,795
whereby we use the Doppler
shift of the donor to calculate

1131
1:11:09,795 --> 1:11:13,803
the mass of the accretor.
And that turned out to be a

1132
1:11:13,803 --> 1:11:17,041
black hole.
That was the first x-ray binary

1133
1:11:17,041 --> 1:11:21,513
that was discovered in 1972 by
[Mervyn Vesper at Webster?],

1134
1:11:21,513 --> 1:11:24,519
and by Bolton,
a 5.6 day orbital period.

1135
1:11:24,519 --> 1:11:28,682
And they derived from the
Doppler shifts that the black

1136
1:11:28,682 --> 1:11:33,000
hole was something like 15 solar
masses.

1137
1:11:33,000 --> 1:11:38,066
So, this has led to a whole new
industry of mass determinations

1138
1:11:38,066 --> 1:11:40,681
of neutron stars and black
holes.

1139
1:11:40,681 --> 1:11:45,258
And then there is Fourier
analysis, which we covered very

1140
1:11:45,258 --> 1:11:47,954
heavily, or at least
sufficiently,

1141
1:11:47,954 --> 1:11:51,632
I hope, in 8.03,
which plays a central role in

1142
1:11:51,632 --> 1:11:54,737
uncovering these periods,
radio people,

1143
1:11:54,737 --> 1:11:57,025
x-ray people,
optical people.

1144
1:11:57,025 --> 1:11:59,886
They all for you analyze their
data.

1145
1:11:59,886 --> 1:12:04,380
The fastest rotating neutron
star that is a single radio

1146
1:12:04,380 --> 1:12:07,240
pulsar, it's not in a binary
system.

1147
1:12:07,240 --> 1:12:13,512
It has a period of 1.5 ms.
Imagine, it goes around more

1148
1:12:13,512 --> 1:12:18,358
than 650 times per second.
Think about if you're at the

1149
1:12:18,358 --> 1:12:23,294
equator of the neutron star.
You go around more than 650

1150
1:12:23,294 --> 1:12:27,064
times per second.
You would have a speed of

1151
1:12:27,064 --> 1:12:32,000
40,000 km per second,
14% of the speed of light.

1152
1:12:32,000 --> 1:12:36,312
And, it just so happens that
only a few days ago or four days

1153
1:12:36,312 --> 1:12:40,696
ago, an announcement was made
that an x-ray pulsar in a binary

1154
1:12:40,696 --> 1:12:44,937
system was found with a spin.
Of the neutron star of 1.7 ms.

1155
1:12:44,937 --> 1:12:49,321
All of this could not have been
done without Fourier analysis.

1156
1:12:49,321 --> 1:12:53,418
This was done in a binary
system with an orbital period of

1157
1:12:53,418 --> 1:12:55,000
2 1/2 hours.


1158
1:12:55,000 --> 1:13:01,000


1159
1:13:01,000 --> 1:13:06,129
We've now come to the end of
8.03.

1160
1:13:06,129 --> 1:13:14,523
Needless to say that it has
swallowed me up completely.

1161
1:13:14,523 --> 1:13:21,984
I lost 10 pounds,
and it has also taken a lot out

1162
1:13:21,984 --> 1:13:28,202
of Marcos, whose dedication was
fabulous.

1163
1:13:28,202 --> 1:13:33,652
[APPLAUSE]
And you should feel sorry for

1164
1:13:33,652 --> 1:13:38,867
my graduate students and my
postdocs who didn't get to see

1165
1:13:38,867 --> 1:13:42,618
much of me.
I'm not saying that you should

1166
1:13:42,618 --> 1:13:45,820
feel guilty, but perhaps you
should.

1167
1:13:45,820 --> 1:13:49,662
[LAUGHTER] you are on my mind
all the time,

1168
1:13:49,662 --> 1:13:54,145
and not only at MIT but also at
home, living room,

1169
1:13:54,145 --> 1:13:56,798
in the kitchen,
in the shower,

1170
1:13:56,798 --> 1:14:02,678
and even in my dreams.
It was a very difficult time

1171
1:14:02,678 --> 1:14:06,886
for my significant other,
Susan, who is here.

1172
1:14:06,886 --> 1:14:11,286
And Susan at times said to me
it was pure hell.

1173
1:14:11,286 --> 1:14:13,295
Did you forget,
Susan?

1174
1:14:13,295 --> 1:14:16,356
Good.
So, life will become normal

1175
1:14:16,356 --> 1:14:19,608
again, but I think I will miss
you.

1176
1:14:19,608 --> 1:14:23,339
And of course in it myself no
illusions.

1177
1:14:23,339 --> 1:14:28,600
You will quickly forget most of
what we covered in 8.03:

1178
1:14:28,600 --> 1:14:32,234
Maxwell's equations,
Fresnel equations,

1179
1:14:32,234 --> 1:14:36,443
[railey?] scattering,
the [Limar?] equations,

1180
1:14:36,443 --> 1:14:41,608
and even the resonance
frequencies of sound in a closed

1181
1:14:41,608 --> 1:14:46,409
box.
All that will fade away in a

1182
1:14:46,409 --> 1:14:50,195
matter of months.
I just hope for you that it

1183
1:14:50,195 --> 1:14:53,121
doesn't fade away before next
week.

1184
1:14:53,121 --> 1:14:57,166
But surely, from now on,
when you see a rainbow,

1185
1:14:57,166 --> 1:15:00,436
you will check that the red is
outside.

1186
1:15:00,436 --> 1:15:04,824
You will look for the primary,
you will look for the

1187
1:15:04,824 --> 1:15:10,332
secondary, you will look for the
bright light inside the primary.

1188
1:15:10,332 --> 1:15:14,327
You can't help it.
It's a disease.

1189
1:15:14,327 --> 1:15:18,221
It is a disease that no one can
cure anymore.

1190
1:15:18,221 --> 1:15:22,203
It will be with you for the
rest of your life.

1191
1:15:22,203 --> 1:15:25,477
And I take full responsibility
for it.

1192
1:15:25,477 --> 1:15:30,787
You cannot resist when you see
a rainbow to make those checks

1193
1:15:30,787 --> 1:15:34,749
even if you try.
I will be very proud of you

1194
1:15:34,749 --> 1:15:38,118
when you use your personal
polarizers, which I'm sure

1195
1:15:38,118 --> 1:15:41,488
you'll all carry with you
because you have one now to

1196
1:15:41,488 --> 1:15:44,079
check that the bow,
indeed, is polarized.

1197
1:15:44,079 --> 1:15:47,125
And occasionally,
don't forget the blue sky 90°

1198
1:15:47,125 --> 1:15:49,911
away from the sun.
It's also 100% polarized.

1199
1:15:49,911 --> 1:15:53,410
And those who do not have your
knowledge will enjoy the

1200
1:15:53,410 --> 1:15:55,742
rainbows.
Of course they will see the

1201
1:15:55,742 --> 1:15:58,464
pretty colors.
But you will now see so much

1202
1:15:58,464 --> 1:16:00,537
more then just the pretty
colors.

1203
1:16:00,537 --> 1:12:18,221
It's like with art.
Knowledge only adds.

1204
1:12:18,221 --> 1:02:22,559
Knowledge never subtracts.
And if that is all you will

1205
1:02:22,559 --> 00:52:38,135
ever remember of 8.03,
I will have achieved something

1206
00:52:38,135 --> 00:43:04,950
that has enriched your life.
And you'll remember me.

1207
00:43:04,950 --> 00:34:50,438
And I hope those memories will
be happy ones.

1208
00:34:50,438 --> 00:28:28,315
I have immensely enjoyed this
term.

1209
00:28:28,315 --> 00:20:25,041
And thank you for being such
great students.

1210
00:20:25,041 --> 00:09:33,184
And thank you for coming to my
lectures even when we had no

1211
00:09:33,184 --> 00:05:03,450
many quizzes like today.
Thank you.