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Today I'd like to talk with you
about my early days at MIT

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and the research
that I did here.

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This is a long time ago.

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I got my Ph.D. in the
Netherlands, in nuclear physics,

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and I came to MIT in January
1966, almost 34 years ago.

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And the idea was

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that I was only going
to spend here one year

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on a postdoc position,

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but I liked it so much
that I never left,

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and I don't regret it.

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I joined the X-ray Astronomy
group here of Professor Rossi.

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Now, X-ray astronomy
has to be done

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from above
the Earth atmosphere

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or at least near the top
of the Earth atmosphere

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because the X rays
are absorbed by air,

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unlike optical astronomy
and radio astronomy,

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which can be done
from the ground.

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The kind of X rays
that we measure

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are not unlike those
that your dentist is using

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when he takes an X-ray picture.

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The energy range of these X rays

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is somewhere between one
and 30, 40 kilo-electron volts,

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and if you don't know
what a kilo-electron volt is,

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that's fine, too,

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but you never express the energy
of an X ray in terms of joules,

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because the number becomes
so ridiculously small.

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During World War II,
under Hitler's Germany,

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Wernher von Braun developed
the V-2 rockets

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for destructive war purposes.

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It was developed in Peenemuende.

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And after the war,

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the Americans used these V-2
rockets for scientific purposes,

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and the first rocket flights to
search for X rays from the Sun

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took place in 1948.

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And X rays were found
from the Sun.

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That was quite a surprise.

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And the power, energy per second
that the Sun puts out in X rays

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divided by the power in optical,

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which is almost
all the radiation of the Sun--

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I'll give it
the symbol of the Sun--

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is approximately
10 to the minus 7,

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so only one ten-millionth of all
the energy comes out in X rays.

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So, from an energy point
of view, it's very, very little.

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It varies a great deal, too.

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But it is really very little.

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In 1962, scientists
here in Cambridge,

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among them
Professor Bruno Rossi,

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who was a professor at MIT,

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and Riccardo Giacconi
and Herb Gursky--

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who were working
across the street

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at American Science
and Engineering--

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attempted to do an experiment

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to see whether
they could detect X rays

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from objects
outside our solar system.

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Now, the odds were very low
that they were going to succeed,

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and the reason is very simple.

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If you take the Sun and you move
it out to the nearest stars,

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which is typically ten
to a hundred light-years,

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you wouldn't stand
a chance to see X rays.

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In fact, the sensitivity
of the detectors in these days

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was too low by at least
nine orders of magnitude,

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a factor of one billion.

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To everyone's surprise--

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to everyone's, yeah,
happy surprise, I should say--

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they succeeded, and they
discovered an object

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which was later called Sco X-1.

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It's in
the constellation Scorpius,

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"X" stands for X rays,

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and "1" for the first X-ray
source in that constellation.

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The total power output
of that source

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was about 10,000 times more
than the Sun.

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That doesn't make
the source so special,

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because there are
many stars in the sky

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that radiate way more energy
than our Sun does,

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but what's so very special
about Sco X-1,

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that the X-ray power over
the optical power for Sco X-1

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was approximately 1,000.

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In other words, the X rays are
the dominant source of energy

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and the optical is sort of,
let's call it a by-product,

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whereas with the Sun, the
optical is the main thing

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and the X rays is sort
of a by-product.

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And so the $64 question
in those days was,

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what can these objects be?

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They must be very different
from the Sun,

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and that's what
I want to discuss with you.

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When I came to MIT in 1966,

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there were about six
of these X-ray sources

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known in the sky.

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Today there are thousands known,
but there were six then.

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And they were discovered
from rocket flights.

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These rockets would be launched,
typically from White Sands,

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and they would spend
about five minutes

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above the Earth atmosphere.

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And during those five minutes
they scanned the sky,

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and six sources were discovered.

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I joined here the group
of Professor George Clark,

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who is still
a professor at MIT.

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He was working on observations
to be made

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from very high-flying balloons.

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So we would build a telescope,

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and we would launch it
on a balloon,

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and go near the top
of the Earth atmosphere.

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It's not as good
as a rocket flight

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which gets completely
out of the Earth atmosphere,

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but the flights on balloons
can last

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way longer than five-minute
rocket flights.

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We could fly hours and,
if we were lucky, even days,

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but the price
we paid for that

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is that even though there was

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only very little atmosphere
left above us--

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about 0.3 percent of
the atmosphere was left--

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still that caused
an effect of the absorption,

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so we did lose X rays that
the rocket flights did not lose.

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But we had the great advantage
of many, many hours.

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To give you a rough idea
of what it took in those days--

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I worked on this
with graduate students

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and with many
undergraduate students--

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a telescope in those days,

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to build it cost typically
a million dollars,

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and it would take us
two years to build one.

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The balloons that we needed
to launch them

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were about $100
in those days,

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and the helium that we needed
to get it up was about $80 --> 
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and the weight of such
a payload, of our telescope,

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was about 1,000 kilograms.

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These balloons would go
up to 140,000 feet

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and they were huge--
they were about 500 feet across.

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I will show you pictures
of them very shortly.

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It was a risky business
in that no guarantee of success.

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You bought the balloons.

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If they worked,
so much the better.

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If they didn't work, tough luck.

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There was just no way that
you could recover the money.

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They were very thin; 

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the balloons are made
of polyethylene,

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and the thickness
of the polyethylene

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was thinner
than cigarette paper,

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so you can imagine
how easy it is to damage them,

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and if you don't damage them
during the launch,

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it's easy to damage them
on the way up,

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due to the jet stream
and the very cold layers of air

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that you have in the tropopause.

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So I would like
to show you now some slides,

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and then we will get back
to talking a little bit more

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about X-ray astronomy.

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All right, so let's see
what we have first.

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You see here two
of my undergraduate students.

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At the time they were
undergraduate students.

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Now they are both Ph.D.s,

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and some of you may think

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that science doesn't have
much romance,

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but there is a lot.

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They married and they have kids.

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This is Pat Downey; 

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she's still very important
in working with me on PIVoT,

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and this is Jim Ballantine.

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They were working
on the electronics,

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which is
an enormously tedious task,

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to do the wiring
of the electronics.

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And here you see
the plant in Texas

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where these balloons were built.

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These were extremely long halls,
as you can imagine.

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And here the gores
of the balloon,

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which are like pieces
that you see on a tangerine,

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on the surface of a tangerine,

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they were sealed together
with heat sealers.

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And only women were allowed
to do this work

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because it was well known
that men are too impatient

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and make many more mistakes.

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And so only women
were allowed there.

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It was nothing to do
with discrimination,

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but simply because women
were better at doing this work.

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Here, a balloon comes out
of a box.

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This is a picture
I took in Texas,

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where we launched balloons from.

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It's nicely covered
with this pink sheet

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and we have this huge cloth
on the grass

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because the balloon is so thin

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that if you would put it
on the grass,

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it would get damaged right away.

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I told you, the skin is about...
thinner than cigarette paper,

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so it's carefully taken out
and we inspect it closely.

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And in this case,
there was some concern

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that there might be
a hole in the balloon.

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In fact, there was at least
a hole in the pink cover,

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and so we carefully inspected

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whether the hole
propagated deeper in.

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I wasn't too worried,
because this was not my balloon,

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but nevertheless,

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it's always sad if you see
some of your colleagues

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having a balloon
that doesn't get up.

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And now I'm taking you
to the desert town

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in Australia,
central Australia--

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Alice Springs.

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I've launched many balloons
from Alice Springs.

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And now you get
a pretty good idea

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of what the launch
is going to be like.

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Here is the launch truck,

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and the telescope is
on the launch truck.

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And all of this
is empty balloon,

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and here is what we call
the roller arm--

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it holds the balloon down,

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and only this part
of the balloon

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is going to be inflated.

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Here is the helium truck,

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and here are the inflation tubes
through which we inflate.

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The inflation takes place
almost always early morning

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or near sunset in the evening,

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because that's when
the wind is very calm,

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and we need
very calm conditions.

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It is this part of the balloon
that is going to be inflated,

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and so there's going to be a
huge force, the buoyant force--

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I hope you all remember
Archimedes' Law--

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of several thousands
of kilograms' force up.

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And so this vehicle here
weighs about four or five tons

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to hold it down.

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And when we launch the balloon,
we actually release this arm;

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we flip this arm up.

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And so you will see, then,

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how the balloon will make it
on the way up.

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So this is Alice Springs,
Australia, again.

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The Sun is rising,
and we started the inflation.

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You see here
the inflation tubes,

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and we put the helium
in the balloon.

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This is the very critical phase
of the launch,

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because if there's
a little bit of side wind

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and the balloon touches
the ground, then it's all over.

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Then it just gets damaged,

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and you abort--
you don't even continue.

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This is a little later, when
the... we call this the bubble.

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This is the roller arm here,

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and this is all this empty
balloon in your direction,

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and this part is
only some 80 feet high or so.

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And here you see these gores
that I told you about earlier.

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This is all this tedious work
that is done by these women,

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who seal these gores together,

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and they all come
together here at the apex,

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in a big aluminum plate.

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All right, now you see
the situation

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from the launch truck site.

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Here you see the radar
reflectors,

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so we can follow
the flight by radar;

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you see the telescope here--

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I won't go
into too many details.

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This is a ballast box,

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which contains
about 500 pounds of lead shot,

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which we can drop
on radio command.

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I believe this
was Jeffrey McClintock,

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who was one of my graduate
students at the time.

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You see here the parachute.

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There's a connection

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between the parachute
and the bottom of the balloon

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and we can cut that
on radio command,

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and then the telescope, we hope,
parachutes safely back to Earth.

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And here's all empty balloon,
and here is the bubble,

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that part
that was just inflated.

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They're still inflating here,

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but they are already tying off
this inflation tube,

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so we are actually getting
very close to a launch.

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And here is the moment
of launch.

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This is a moment
that no one will ever forget

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who watches a balloon launch.

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You feel ants in your pants
and butterflies in your stomach.

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It is a very tense moment.

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If things go wrong,

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this, in general, is the moment
that things go wrong,

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because when the roller arm
is here released,

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then this enormous amount
of free lift, the buoyant force,

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drives this thing up
and you get a bouncing effect.

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You get an oscillation of
the helium in this upper part--

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we call this the mushroom.

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And that can already destroy
the balloon right away.

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The layout is such
that we always lay out

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so that the wind will drive
the balloon

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towards the launch truck.

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And you will see later on
why that has to be done.

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So the launch truck
can maneuver itself

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straight under the balloon,

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and make sure that the balloon
is carrying the telescope

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before we release the payload.

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The payload is now tied
to the launch truck.

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We have to wait for the whole
balloon to be off the ground

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before the launch truck

285
00:13:52 --> 00:13:58
can actually maneuver itself
under the balloon,

286
00:13:55 --> 00:14:01
but you already see that the...
by the exhaust

287
00:13:57 --> 00:14:03
that the engines
are already running.

288
00:14:00 --> 00:14:06
You see a close-up
here of this mushroom,

289
00:14:04 --> 00:14:10
and it makes
a tremendous amount of noise.

290
00:14:07 --> 00:14:13
It's really scary.

291
00:14:08 --> 00:14:14
I'm always surprised,
when I see a launch like this,

292
00:14:11 --> 00:14:17
that the balloon, so thin,

293
00:14:12 --> 00:14:18
can actually survive
this tormenting launch.

294
00:14:18 --> 00:14:24
So here it goes higher, comes
in this direction, as you see,

295
00:14:23 --> 00:14:29
getting closer and closer
to the launch truck,

296
00:14:28 --> 00:14:34
rises higher in the sky

297
00:14:29 --> 00:14:35
and, in this case
in Alice Springs,

298
00:14:31 --> 00:14:37
I was so close to the launch

299
00:14:33 --> 00:14:39
that I couldn't follow
the bubble going up

300
00:14:37 --> 00:14:43
much further with my camera,

301
00:14:39 --> 00:14:45
so the next picture
that you're going to see

302
00:14:41 --> 00:14:47
I took from Palestine, Texas,

303
00:14:43 --> 00:14:49
from which I've flown
many balloons.

304
00:14:46 --> 00:14:52
So it's a different launch,

305
00:14:47 --> 00:14:53
but it is sort of the same stage
in the launch that you see here.

306
00:14:51 --> 00:14:57
Alice Springs was beautiful,
by the way,

307
00:14:53 --> 00:14:59
always very nice, clear skies; 

308
00:14:55 --> 00:15:01
it's a very fantastic
desert there.

309
00:14:57 --> 00:15:03
It's a wonderful desert town.

310
00:15:00 --> 00:15:06
It's really in the middle
of nowhere.

311
00:15:03 --> 00:15:09
Okay, so now we're in Texas and
you see here the launch truck.

312
00:15:08 --> 00:15:14
This is the telescope,
it was a different telescope.

313
00:15:10 --> 00:15:16
You see the parachute,

314
00:15:12 --> 00:15:18
and now the trick is for the
truck to get under the balloon.

315
00:15:17 --> 00:15:23
The amount of helium
that is in the balloon

316
00:15:19 --> 00:15:25
is only a small fraction
of the total volume.

317
00:15:22 --> 00:15:28
It's just enough for us
to get the free lift.

318
00:15:25 --> 00:15:31
We want to go up at about
a thousand feet per minute,

319
00:15:27 --> 00:15:33
and then, since the atmospheric
pressure goes down

320
00:15:30 --> 00:15:36
when the balloon goes up,

321
00:15:31 --> 00:15:37
the helium expands and finally
fills the entire volume,

322
00:15:35 --> 00:15:41
as you will see.

323
00:15:38 --> 00:15:44
So now is the moment,
this is a crucial moment

324
00:15:41 --> 00:15:47
that the launch truck
has actually maneuvered itself

325
00:15:44 --> 00:15:50
straight under the balloon.

326
00:15:46 --> 00:15:52
And now the person on
the launch truck makes sure

327
00:15:49 --> 00:15:55
that there is enough tension
in these lines

328
00:15:51 --> 00:15:57
to pick up the telescope.

329
00:15:54 --> 00:16:00
If the balloon were a little bit
too far ahead or too far behind

330
00:15:58 --> 00:16:04
and you released the telescope,

331
00:16:00 --> 00:16:06
then, of course, it would
pendulum into the ground

332
00:16:02 --> 00:16:08
and you would lose it,
so it's very important

333
00:16:04 --> 00:16:10
that it be done when the
balloon is straight overhead.

334
00:16:07 --> 00:16:13
But that alone is not enough;

335
00:16:09 --> 00:16:15
there must also be
enough pull on it

336
00:16:11 --> 00:16:17
so that the telescope doesn't
crash to the ground vertically.

337
00:16:15 --> 00:16:21
And when there is any chance
that things go wrong,

338
00:16:17 --> 00:16:23
we just abort the flight,

339
00:16:19 --> 00:16:25
because the telescope is so much
more expensive than the balloon,

340
00:16:22 --> 00:16:28
even though the balloon
and the helium together

341
00:16:24 --> 00:16:30
is close
to a quarter-million dollars.

342
00:16:29 --> 00:16:35
And here you see it
after release:

343
00:16:32 --> 00:16:38
payload, parachute,

344
00:16:36 --> 00:16:42
empty balloon and here,
the helium.

345
00:16:38 --> 00:16:44
And this, from here to here,
oh, is about two-thirds

346
00:16:40 --> 00:16:46
of the height
of the Empire State Building.

347
00:16:43 --> 00:16:49
These are huge, huge balloons.

348
00:16:48 --> 00:16:54
And here you see it
at an altitude of 150,000 feet.

349
00:16:54 --> 00:17:00
This is the largest balloon that
was ever flown successfully.

350
00:16:57 --> 00:17:03
It's still the world record.

351
00:16:59 --> 00:17:05
It was a balloon with a volume
of 52 million cubic feet.

352
00:17:03 --> 00:17:09
This picture was taken
through a telescope.

353
00:17:06 --> 00:17:12
And you see here the telescope,

354
00:17:07 --> 00:17:13
and you can look
straight through the balloon;

355
00:17:09 --> 00:17:15
it's that thin.

356
00:17:11 --> 00:17:17
From here to here is
about 500 feet, almost 600 feet.

357
00:17:14 --> 00:17:20


358
00:17:19 --> 00:17:25
All right, here you see
my ex-graduate student.

359
00:17:22 --> 00:17:28
He's now Dr. Ricker,
George Ricker.

360
00:17:24 --> 00:17:30
He's still a staff member
at MIT.

361
00:17:26 --> 00:17:32
This is in Australia.

362
00:17:29 --> 00:17:35
A lot of this equipment
was built by undergraduates

363
00:17:32 --> 00:17:38
and graduate students of mine;

364
00:17:34 --> 00:17:40
a lot of it we borrowed from
the balloon launching stations.

365
00:17:38 --> 00:17:44
And the radio data come in here,

366
00:17:41 --> 00:17:47
and we can command
the telescope from here.

367
00:17:43 --> 00:17:49
We can orient the telescope,
we can draw ballast

368
00:17:46 --> 00:17:52
and, very important,
we can terminate the flight.

369
00:17:49 --> 00:17:55
So that we rescue,
that we save the telescope

370
00:17:52 --> 00:17:58
when it starts drifting
out over the ocean.

371
00:17:54 --> 00:18:00
Because the balloon
starts going with the winds

372
00:17:56 --> 00:18:02
at the altitude of 150,000 feet,

373
00:17:59 --> 00:18:05
and those winds can vary

374
00:18:00 --> 00:18:06
anywhere from 20, 30 miles per
hour up to 100 miles per hour.

375
00:18:05 --> 00:18:11
We try to fly only during
the days that the wind is low,

376
00:18:10 --> 00:18:16
and that's the case
in the spring,

377
00:18:12 --> 00:18:18
and in the fall,
we call that the turnaround.

378
00:18:15 --> 00:18:21
The winds at these altitudes
change twice per year.

379
00:18:20 --> 00:18:26
They change from about
100 miles per hour to the west,

380
00:18:22 --> 00:18:28
to 100 miles per hour
to the east.

381
00:18:24 --> 00:18:30
It happens in the spring
and in the fall,

382
00:18:26 --> 00:18:32
and that's when we try to fly,
when the winds are very low,

383
00:18:30 --> 00:18:36
when they are in the process
of turning around.

384
00:18:34 --> 00:18:40
We follow the balloon at low
altitude-- 5 -->  6,000 feet.

385
00:18:37 --> 00:18:43
It's a small airplane.

386
00:18:39 --> 00:18:45
I'm sitting here
on the airplane.

387
00:18:41 --> 00:18:47
This is a typical airplane

388
00:18:43 --> 00:18:49
that we use to hop
from airport to airport

389
00:18:47 --> 00:18:53
and stay as close
to the balloon as we can,

390
00:18:50 --> 00:18:56
so that we can give
the "terminate" command.

391
00:18:52 --> 00:18:58
And later we can recover
the payload,

392
00:18:55 --> 00:19:01
which is an adventure
all by itself.

393
00:18:57 --> 00:19:03
In Australia,

394
00:18:59 --> 00:19:05
that is much harder
than in the United States.

395
00:19:01 --> 00:19:07
This was El Paso,

396
00:19:02 --> 00:19:08
but in Australia, there are
no airports in the desert,

397
00:19:06 --> 00:19:12
and so that is way harder
to hop from place to place.

398
00:19:12 --> 00:19:18
I'm taking you to Australia now,
here is Alice Springs.

399
00:19:15 --> 00:19:21
And when we launched
this flight, the days before,

400
00:19:19 --> 00:19:25
we had balloons,
weather balloons, test balloons,

401
00:19:24 --> 00:19:30
which we drove
up to 140 -->  150,000 feet,

402
00:19:27 --> 00:19:33
and then we probed the winds
at that altitude.

403
00:19:30 --> 00:19:36
And the people who did that
give us good reasons to believe

404
00:19:33 --> 00:19:39
that the balloon would either
go sort of in this direction

405
00:19:36 --> 00:19:42
or maybe here, but in any case,
it would go north-northwest.

406
00:19:40 --> 00:19:46
And so we alerted all these
radar stations in Australia

407
00:19:42 --> 00:19:48
to look out for the balloon--
that we have radar reflectors,

408
00:19:45 --> 00:19:51
so they could give us
an early warning,

409
00:19:47 --> 00:19:53
because between here and here,
there are really no airports.

410
00:19:51 --> 00:19:57
So if we follow this
by airplane,

411
00:19:53 --> 00:19:59
you can land during the day
at some air strips,

412
00:19:57 --> 00:20:03
but really,
there are really no runways,

413
00:20:00 --> 00:20:06
so that's pretty dangerous.

414
00:20:01 --> 00:20:07
At night,
you couldn't land here.

415
00:20:03 --> 00:20:09
And so we were hoping
that these radar stations

416
00:20:05 --> 00:20:11
would give us an early alert and
tell us where the balloon is.

417
00:20:08 --> 00:20:14
What happened, however,
the balloon went straight down.

418
00:20:11 --> 00:20:17
And then it was sunset,

419
00:20:13 --> 00:20:19
so we don't know exactly
where the balloon is at sunset.

420
00:20:16 --> 00:20:22
We can't see it, but we have
radio contact with it

421
00:20:20 --> 00:20:26
and so we were flying
close to it.

422
00:20:22 --> 00:20:28
And then at sunrise we picked
it up, we could see it,

423
00:20:25 --> 00:20:31
and then here,

424
00:20:26 --> 00:20:32
when it was getting close
to forbidden zone--

425
00:20:28 --> 00:20:34
because there is
commercial flights here--

426
00:20:31 --> 00:20:37
we cut it down,
so we gave the radio command

427
00:20:34 --> 00:20:40
which separates the parachute
from the balloon.

428
00:20:37 --> 00:20:43
The balloon is very brittle--
it's very cold there.

429
00:20:40 --> 00:20:46
The balloon shatters,
breaks in pieces, comes down

430
00:20:43 --> 00:20:49
and, if everything goes well,

431
00:20:45 --> 00:20:51
then the parachute brings the
telescope safely back to Earth.

432
00:20:51 --> 00:20:57
This is the person that we
contacted during that flight.

433
00:20:55 --> 00:21:01
You try to draw the attention
of local people,

434
00:20:59 --> 00:21:05
and you do that by flying
with your airplane

435
00:21:01 --> 00:21:07
low over their house.

436
00:21:02 --> 00:21:08
This person lived in the desert,

437
00:21:04 --> 00:21:10
and his nearest neighbor was
from 70 miles away from him.

438
00:21:06 --> 00:21:12
He was crazy,
he was always drunk...

439
00:21:09 --> 00:21:15
(students laugh )

440
00:21:10 --> 00:21:16
he was shooting kangaroos
in the desert.

441
00:21:13 --> 00:21:19
There is no windshield here.

442
00:21:15 --> 00:21:21
He would drive 60 miles an hour,

443
00:21:17 --> 00:21:23
and then he would chase
these kangaroos and shoot them.

444
00:21:21 --> 00:21:27
And he had a crazy game
which I didn't like at all.

445
00:21:24 --> 00:21:30
He would put the dog
on the roof--

446
00:21:26 --> 00:21:32
and he gave me
a demonstration once;

447
00:21:28 --> 00:21:34
I was with him in this truck--

448
00:21:30 --> 00:21:36
and he would drive 60 miles
per hour, would slam the brakes,

449
00:21:33 --> 00:21:39
and the dog would
catapult through the air,

450
00:21:36 --> 00:21:42
and then he would say,

451
00:21:37 --> 00:21:43
"You can't teach
an old dog any new tricks."

452
00:21:42 --> 00:21:48
We encountered wonderful animals
during recovery: koala bear,

453
00:21:48 --> 00:21:54
quiet, peaceful, very lazy,
unlike most 8.01 students.

454
00:21:54 --> 00:22:00
(students laugh )

455
00:21:56 --> 00:22:02
And then, when we got closer
to payload--

456
00:21:58 --> 00:22:04
it took us a day and a half
to get to the payload--

457
00:22:01 --> 00:22:07
there was this nasty iguana,
he was about six feet long.

458
00:22:07 --> 00:22:13
And let me tell you,
I was really scared.

459
00:22:09 --> 00:22:15
It scared the hell out of me.

460
00:22:11 --> 00:22:17
But of course I didn't want
to show that,

461
00:22:13 --> 00:22:19
so I said
to my graduate student,

462
00:22:14 --> 00:22:20
"There's no problem,
these animals are harmless; 

463
00:22:17 --> 00:22:23
you go first."

464
00:22:18 --> 00:22:24
(laughter )

465
00:22:20 --> 00:22:26
And he did, and it turns out
theyare harmless,

466
00:22:23 --> 00:22:29
and during the entire four hours
that it took us

467
00:22:26 --> 00:22:32
to recover the payload
and get it on Jack's truck,

468
00:22:28 --> 00:22:34
this animal was just sitting
still, didn't move at all.

469
00:22:31 --> 00:22:37
That is his way of thinking
that we don't see him, then.

470
00:22:36 --> 00:22:42
Beautiful animals,
these iguanas.

471
00:22:38 --> 00:22:44
The aborigines eat them, it's
very precious food, by the way.

472
00:22:42 --> 00:22:48
So here you see Don Brooks,

473
00:22:43 --> 00:22:49
who was an American
who came with me.

474
00:22:45 --> 00:22:51
He was an electronic expert,

475
00:22:46 --> 00:22:52
and this is Alice,
which was the wife of Jack.

476
00:22:49 --> 00:22:55
You see the payload here,
tumbled over,

477
00:22:51 --> 00:22:57
but it's in good condition.

478
00:22:53 --> 00:22:59
The crash pad is there purposely
to protect against the impact,

479
00:22:58 --> 00:23:04
to get a lower deceleration,

480
00:22:59 --> 00:23:05
and, of course, it's okay

481
00:23:01 --> 00:23:07
that that cardboard crash pad
is destroyed--

482
00:23:04 --> 00:23:10
that's the whole idea.

483
00:23:07 --> 00:23:13
And then when you come back a
few days later in Alice Springs,

484
00:23:10 --> 00:23:16
it's... nothing happens,
ever, in Alice Springs.

485
00:23:12 --> 00:23:18
I mean, it's completely
a hole in the ground.

486
00:23:15 --> 00:23:21
So this front-page news,
"Perfect Balloon Launch"

487
00:23:18 --> 00:23:24
and "1,000 watch the start
of a space probe"--

488
00:23:21 --> 00:23:27
they called it a space probe.

489
00:23:23 --> 00:23:29
I had a long interview
with this news reporter,

490
00:23:28 --> 00:23:34
and I told him that the reason
why we have to go high

491
00:23:31 --> 00:23:37
is because of the absorption
of the X rays

492
00:23:33 --> 00:23:39
in the Earth atmosphere,

493
00:23:35 --> 00:23:41
but the article
didn't get that across.

494
00:23:37 --> 00:23:43
They said, "They fly balloons

495
00:23:38 --> 00:23:44
because then they're
closer to the stars."

496
00:23:40 --> 00:23:46
Well, I suppose
that is close enough,

497
00:23:42 --> 00:23:48
but it really missed the issue
of the absorption of the X rays,

498
00:23:46 --> 00:23:52
which of course... that's
the reason why we have to go up,

499
00:23:49 --> 00:23:55
not because we want
to get closer to the stars.

500
00:23:53 --> 00:23:59
Okay, so now I'll go back
to the blackboard,

501
00:23:58 --> 00:24:04
if I can find my way.

502
00:24:00 --> 00:24:06


503
00:24:19 --> 00:24:25
So between 1966
and roughly late '70s,

504
00:24:25 --> 00:24:31
I had about
20 successful flights

505
00:24:28 --> 00:24:34
from the United States, from
Canada, and many from Australia.

506
00:24:33 --> 00:24:39
Now, we also had some problems,
we had some bad luck.

507
00:24:37 --> 00:24:43
Twice during my flights,
the balloons popped.

508
00:24:40 --> 00:24:46
70,000 feet,
there is the tropopause; 

509
00:24:42 --> 00:24:48
it's very cold, -70 degrees.

510
00:24:45 --> 00:24:51
There are jet winds
and they beat on the balloon,

511
00:24:48 --> 00:24:54
and then the balloon can burst.

512
00:24:49 --> 00:24:55
And when that happens,

513
00:24:50 --> 00:24:56
we don't have enough time
to terminate the flight--

514
00:24:53 --> 00:24:59
we can't separate the parachute
from the balloon; 

515
00:24:55 --> 00:25:01
it happens all of a sudden--
and then, in general,

516
00:24:58 --> 00:25:04
the parachute gets entangled
and then you get a free fall.

517
00:25:02 --> 00:25:08
So the payload
is entirely destroyed,

518
00:25:04 --> 00:25:10
and that happened twice.

519
00:25:06 --> 00:25:12
But we were lucky enough
that at several occasions

520
00:25:09 --> 00:25:15
we made some
interesting discoveries.

521
00:25:13 --> 00:25:19
During the early years
of X-ray astronomy,

522
00:25:15 --> 00:25:21
we discovered
five new X-ray sources,

523
00:25:18 --> 00:25:24
so we doubled
the number of sources

524
00:25:20 --> 00:25:26
that were known
from rocket flights before us.

525
00:25:24 --> 00:25:30
And some of these sources
that we saw from balloons

526
00:25:27 --> 00:25:33
were highly variable.

527
00:25:28 --> 00:25:34
We noticed an X-ray flare;

528
00:25:31 --> 00:25:37
the X-ray intensity went up
by a factor of three or four

529
00:25:35 --> 00:25:41
on as little time
as ten minutes.

530
00:25:37 --> 00:25:43
And that was completely new
in those days,

531
00:25:38 --> 00:25:44
and that could not have been
discovered from rockets,

532
00:25:41 --> 00:25:47
because the rockets themselves

533
00:25:43 --> 00:25:49
are only five minutes
above the Earth atmosphere,

534
00:25:45 --> 00:25:51
and they're not looking
at one source all the time.

535
00:25:47 --> 00:25:53
They are scanning the sky,
because their objective

536
00:25:50 --> 00:25:56
was to find as many X-ray
sources as they could.

537
00:25:53 --> 00:25:59
But we were up
sometimes 26 hours,

538
00:25:55 --> 00:26:01
so we had plenty of time to look
at one portion of the sky

539
00:25:58 --> 00:26:04
for a long time, for hours on,
and so it was not an accident

540
00:26:02 --> 00:26:08
that we discovered
these flaring events

541
00:26:05 --> 00:26:11
which lasted up
to ten minutes and longer.

542
00:26:09 --> 00:26:15
We also discovered an object
which we called "gx1+4"--

543
00:26:13 --> 00:26:19
the number has to do with
where it is in the sky--

544
00:26:16 --> 00:26:22
and we noticed,
much to our surprise,

545
00:26:18 --> 00:26:24
that the X rays seemed
to fluctuate

546
00:26:20 --> 00:26:26
in a periodic fashion,
2.3 minutes.

547
00:26:23 --> 00:26:29
At the time, we had no clue
what that meant,

548
00:26:26 --> 00:26:32
but later,
as you will see very shortly,

549
00:26:28 --> 00:26:34
it became clear

550
00:26:30 --> 00:26:36
that that was the rotation
period of a neutron star.

551
00:26:35 --> 00:26:41
So the big question was,
in the early days:

552
00:26:37 --> 00:26:43
What are these objects?

553
00:26:39 --> 00:26:45
And this is something that
we have discussed in 8.01

554
00:26:43 --> 00:26:49
and I will go over it
very briefly again,

555
00:26:46 --> 00:26:52
but we discussed it

556
00:26:47 --> 00:26:53
and you even had
some homework problems on it.

557
00:26:50 --> 00:26:56
These objects
are X-ray binaries,

558
00:26:54 --> 00:27:00
whereby one object
is very compact--

559
00:26:57 --> 00:27:03
which could be a neutron star,

560
00:26:59 --> 00:27:05
or in some cases
even a black hole--

561
00:27:02 --> 00:27:08
and the other object,
the other star,

562
00:27:05 --> 00:27:11
is a normal nuclear burning
star, something like our Sun.

563
00:27:09 --> 00:27:15
And they are
very close together.

564
00:27:11 --> 00:27:17
They are so close together
that the matter, which is here,

565
00:27:16 --> 00:27:22
is attracted by the neutron star

566
00:27:18 --> 00:27:24
stronger than it is attracted
by the star itself,

567
00:27:22 --> 00:27:28
and so it starts to find its way
to the neutron star.

568
00:27:25 --> 00:27:31
This is a binary system,
so they go around each other.

569
00:27:28 --> 00:27:34
This matter cannot
just go in radially,

570
00:27:31 --> 00:27:37
but it would spiral in slowly

571
00:27:34 --> 00:27:40
and find its way
to the neutron star.

572
00:27:36 --> 00:27:42
Strangely enough

573
00:27:37 --> 00:27:43
that we still don't understand
how it makes it,

574
00:27:40 --> 00:27:46
but it does make it,
ultimately, to the neutron star,

575
00:27:42 --> 00:27:48
and this is, then,
what we call the accretion disk.

576
00:27:47 --> 00:27:53
This is the accretor
and this is the donor.

577
00:27:51 --> 00:27:57
The donor provides the fuel

578
00:27:54 --> 00:28:00
that finds its way
to the neutron star.

579
00:27:58 --> 00:28:04
And if you take
a little bit, mass m,

580
00:28:02 --> 00:28:08
and you drop that
on a neutron star--

581
00:28:04 --> 00:28:10
and a neutron star has

582
00:28:06 --> 00:28:12
mass capital M, say,
and radius capital R--

583
00:28:09 --> 00:28:15
then the kinetic energy
that is released at impact

584
00:28:12 --> 00:28:18
is something that all
of you should be able

585
00:28:14 --> 00:28:20
to do next Monday.

586
00:28:15 --> 00:28:21
That is the following:

587
00:28:18 --> 00:28:24
mMG divided by R
equals one-half mV squared.

588
00:28:26 --> 00:28:32
This is the gravitational
potential energy

589
00:28:28 --> 00:28:34
that becomes available

590
00:28:29 --> 00:28:35
if an object of mass little m
crashes onto the star.

591
00:28:34 --> 00:28:40
The surface of the star
has a radius capital R; 

592
00:28:36 --> 00:28:42
the mass of the star
is capital M.

593
00:28:38 --> 00:28:44
And that is converted
to kinetic energy,

594
00:28:40 --> 00:28:46
which is one-half mV squared,
so this is the speed at impact.

595
00:28:44 --> 00:28:50
Of course, it's always
independent of little m,

596
00:28:46 --> 00:28:52
and so you can calculate
that speed

597
00:28:48 --> 00:28:54
and that speed is horrendous
for a neutron star,

598
00:28:51 --> 00:28:57
the reason being that
the radius of the neutron star

599
00:28:54 --> 00:29:00
is so absurdly small;
it's only ten kilometers.

600
00:28:57 --> 00:29:03
It is roughly
100,000 times smaller

601
00:29:01 --> 00:29:07
than the radius of our Sun.

602
00:29:02 --> 00:29:08
The mass of the neutron star is
comparable to that of our Sun--

603
00:29:06 --> 00:29:12
a little larger,
but it's comparable.

604
00:29:08 --> 00:29:14
But it is the radius
which is so small,

605
00:29:10 --> 00:29:16
and that's why
you get a speed at impact

606
00:29:13 --> 00:29:19
which is about one-third
of the speed of light.

607
00:29:16 --> 00:29:22
And this kinetic energy
is converted to heat--

608
00:29:19 --> 00:29:25
for the same reason
that when we drop something

609
00:29:22 --> 00:29:28
here on the floor,

610
00:29:23 --> 00:29:29
that the kinetic energy
ultimately goes into heat--

611
00:29:25 --> 00:29:31
and so it heats up the surface
layers of the neutron star,

612
00:29:29 --> 00:29:35
and the temperature becomes
horrendously high,

613
00:29:32 --> 00:29:38
ten to the seven,
ten to the eighth degrees--

614
00:29:34 --> 00:29:40
10 million,
100 million degrees--

615
00:29:36 --> 00:29:42
and at that
very high temperature,

616
00:29:38 --> 00:29:44
almost all the energy,

617
00:29:39 --> 00:29:45
almost all electromagnetic
radiation comes out

618
00:29:42 --> 00:29:48
in the form of X rays.

619
00:29:44 --> 00:29:50
The Sun has a temperature
of only 6,000 degrees;

620
00:29:47 --> 00:29:53
most of it comes out
in the form of optical light,

621
00:29:50 --> 00:29:56
but when you go
to 10 million degrees,

622
00:29:52 --> 00:29:58
that's no longer the case.

623
00:29:53 --> 00:29:59
The spectrum shifts
in favor of the X rays.

624
00:29:58 --> 00:30:04
The amount of energy that
is released is horrendous.

625
00:30:01 --> 00:30:07
To give you some feeling for
that, if you take a marshmallow

626
00:30:04 --> 00:30:10
and you throw a marshmallow
from a large distance

627
00:30:07 --> 00:30:13
onto a neutron star,

628
00:30:08 --> 00:30:14
then the energy that is
released, which is this energy,

629
00:30:13 --> 00:30:19
is comparable to the energy
that was released

630
00:30:16 --> 00:30:22
of the atomic bomb
that was thrown

631
00:30:18 --> 00:30:24
on Hiroshima and Nagasaki.

632
00:30:21 --> 00:30:27
So that tells you something

633
00:30:22 --> 00:30:28
about the enormous
gravitational forces

634
00:30:25 --> 00:30:31
that are at work on the surface
of a neutron star.

635
00:30:31 --> 00:30:37
We know now
what these systems are; 

636
00:30:34 --> 00:30:40
the evidence is overwhelming.

637
00:30:37 --> 00:30:43
We have observed the rotation
of the neutron stars.

638
00:30:41 --> 00:30:47
The 2.3 minutes that we found,

639
00:30:43 --> 00:30:49
we now know is the rotation
of the neutron star.

640
00:30:46 --> 00:30:52
These neutron stars have
a strong magnetic field,

641
00:30:49 --> 00:30:55
and the matter that accretes
onto the neutron star

642
00:30:52 --> 00:30:58
reaches the magnetic poles.

643
00:30:54 --> 00:31:00
In 8.02 you will see...
you will learn why this plasma,

644
00:30:58 --> 00:31:04
which is highly ionized,

645
00:31:00 --> 00:31:06
why that cannot just reach
the neutron star anywhere,

646
00:31:03 --> 00:31:09
but it is forced to only enter
the neutron star

647
00:31:06 --> 00:31:12
near the magnetic poles,

648
00:31:08 --> 00:31:14
and if the neutron star rotates,

649
00:31:10 --> 00:31:16
then the magnetic poles
can rotate like this.

650
00:31:12 --> 00:31:18
And when you are on Earth,

651
00:31:13 --> 00:31:19
you see X rays, X rays,
X rays, no X rays, X rays

652
00:31:16 --> 00:31:22
and so you see pulsations.

653
00:31:18 --> 00:31:24
And so these pulsations
have been seen

654
00:31:20 --> 00:31:26
from many neutron stars now,

655
00:31:22 --> 00:31:28
from many
of these binary systems.

656
00:31:23 --> 00:31:29
It's very clear
that it's a binary system.

657
00:31:27 --> 00:31:33
If you are in the plane
or near the plane

658
00:31:30 --> 00:31:36
of the orbits of the two stars,

659
00:31:32 --> 00:31:38
then the neutron star can go
behind the donor,

660
00:31:35 --> 00:31:41
and then you don't see
any X rays,

661
00:31:37 --> 00:31:43
because the X rays are
then absorbed by the donor.

662
00:31:39 --> 00:31:45
And then you see an X-ray
eclipse, so the X rays vanish.

663
00:31:43 --> 00:31:49
So you would see the pulsations,
strong X-ray signal,

664
00:31:47 --> 00:31:53
and all of a sudden, boom--
it's gone.

665
00:31:49 --> 00:31:55
And then a few hours later,
it starts up again

666
00:31:51 --> 00:31:57
when the neutron star reappears,
reemerges from the donor star.

667
00:31:55 --> 00:32:01
So that picture
is all very clear,

668
00:31:59 --> 00:32:05
but I do want to show you
at least a sketch

669
00:32:03 --> 00:32:09
of what we think such a system
would look like,

670
00:32:06 --> 00:32:12
which is just the next slide,

671
00:32:08 --> 00:32:14
and maybe the person
in the booth...

672
00:32:10 --> 00:32:16
Oh, I can do it from here.

673
00:32:11 --> 00:32:17


674
00:32:14 --> 00:32:20
So this is
what it sort of looks like.

675
00:32:17 --> 00:32:23
You see the donor
there on the left,

676
00:32:19 --> 00:32:25
and you see here
the neutron star,

677
00:32:23 --> 00:32:29
or it could be a black hole--

678
00:32:25 --> 00:32:31
that's sort of the same idea,
you would not be able to tell--

679
00:32:27 --> 00:32:33
and you see how
the matter swirls in.

680
00:32:30 --> 00:32:36
Of course
this is a not a real picture,

681
00:32:32 --> 00:32:38
this is a sketch made
by an illustrator.

682
00:32:37 --> 00:32:43
We know hundreds of these
systems in our own galaxy,

683
00:32:40 --> 00:32:46
and, of course, there are many
in other galaxies as well.

684
00:32:44 --> 00:32:50


685
00:32:52 --> 00:32:58
I discussed with you in 8.01

686
00:32:55 --> 00:33:01
that if you measure the
Doppler shifts of these stars--

687
00:32:57 --> 00:33:03
and if you are lucky,
you get the Doppler shift

688
00:32:59 --> 00:33:05
both from the pulsations
of the neutron star

689
00:33:02 --> 00:33:08
and from the optical lines
in the donor--

690
00:33:04 --> 00:33:10
that you can even find the mass

691
00:33:07 --> 00:33:13
of both the star here
and the star there.

692
00:33:11 --> 00:33:17
And if the mass
becomes horrendously high,

693
00:33:13 --> 00:33:19
as in some cases,
then you have to conclude

694
00:33:16 --> 00:33:22
that you are dealing
with a black hole.

695
00:33:18 --> 00:33:24
And you had a problem
on section...

696
00:33:20 --> 00:33:26
in one of
your homework assignments.

697
00:33:24 --> 00:33:30
I'm no longer flying balloons,

698
00:33:25 --> 00:33:31
because all the work
that I do nowadays

699
00:33:27 --> 00:33:33
is done,
of course, from satellites.

700
00:33:29 --> 00:33:35
You get 365 days per year data,

701
00:33:32 --> 00:33:38
you are always
above the Earth atmosphere,

702
00:33:34 --> 00:33:40
so that's clearly the way to go.

703
00:33:36 --> 00:33:42
And I have used European
satellites, Japanese satellites,

704
00:33:40 --> 00:33:46
and nowadays I am using
the Rossi X-ray Timing Explorer,

705
00:33:43 --> 00:33:49
which is an American satellite,

706
00:33:45 --> 00:33:51
and Chandra, which was
launched early this year,

707
00:33:47 --> 00:33:53
which is
the biggest thing in town.

708
00:33:51 --> 00:33:57
In 1975, here at MIT we had
our own satellite, called SAS-3.

709
00:33:58 --> 00:34:04
And we operated SAS-3 from
the Center for Space Research,

710
00:34:01 --> 00:34:07
which is Building 37,
where my office is,

711
00:34:03 --> 00:34:09
365 days per year,
24 hours per day.

712
00:34:08 --> 00:34:14
And in '75, Josh Grindley,
from Harvard,

713
00:34:11 --> 00:34:17
and John Heise
in Utrecht, the Netherlands,

714

715
00:34:15 --> 00:34:21
discovered something
which we call an X-ray burst.

716
00:34:17 --> 00:34:23
And an X-ray burst
is a phenomenon

717
00:34:19 --> 00:34:25
that you see the X-ray signal
become very strong

718
00:34:22 --> 00:34:28
all of a sudden.

719
00:34:23 --> 00:34:29
In about one second,
it becomes ten times stronger,

720
00:34:26 --> 00:34:32
maybe 20 times stronger
than it was before the burst,

721
00:34:29 --> 00:34:35
and it peters out on a time
scale of about a minute or so.

722
00:34:34 --> 00:34:40
And we were very lucky
at the time, 1976, with SAS-3,

723
00:34:37 --> 00:34:43
that we could do research
on these X-ray bursts

724
00:34:40 --> 00:34:46
and within a year or two,

725
00:34:42 --> 00:34:48
we discovered eight more
of these burst sources.

726
00:34:45 --> 00:34:51
And it is largely
through that work--

727
00:34:47 --> 00:34:53
that observational work
that took place--

728
00:34:49 --> 00:34:55
and through the work,
theoretical work

729
00:34:52 --> 00:34:58
by Professor Paul Joss,
who is still at MIT--

730
00:34:55 --> 00:35:01
that we now know
what causes these X-ray bursts.

731
00:34:58 --> 00:35:04
They are nuclear bomb explosions
on the surface of neutron stars.

732
00:35:03 --> 00:35:09
What happens is that on
the surface of the neutron star,

733
00:35:08 --> 00:35:14
the matter that accretes--

734
00:35:10 --> 00:35:16
which is largely hydrogen
and helium from the donor--

735
00:35:14 --> 00:35:20
becomes very hot,
it becomes very dense,

736
00:35:17 --> 00:35:23
and at that high temperature
and at very high densities,

737
00:35:20 --> 00:35:26
you get thermonuclear fusion.

738
00:35:23 --> 00:35:29
And a reaction
that can take place

739
00:35:26 --> 00:35:32
is that three helium nuclei--
helium-4--

740
00:35:32 --> 00:35:38
fuse to form carbon-12,

741
00:35:35 --> 00:35:41
and when that happens,
energy is released,

742
00:35:37 --> 00:35:43
thermonuclear energy
is released.

743
00:35:39 --> 00:35:45
This reaction is extremely
sensitive to temperature.

744
00:35:43 --> 00:35:49
When energy is released,
the temperature goes up.

745
00:35:47 --> 00:35:53
When the temperature goes up,

746
00:35:48 --> 00:35:54
the reaction rate goes up,
then the temperature goes up,

747
00:35:51 --> 00:35:57
then the reaction rate
goes up even further,

748
00:35:53 --> 00:35:59
and the whole thing
gets out of hand

749
00:35:55 --> 00:36:01
and that's why it is
a thermonuclear explosion.

750
00:35:57 --> 00:36:03
We call it
a thermonuclear flash,

751
00:35:59 --> 00:36:05
so it is an uncontrolled,
runaway process.

752
00:36:03 --> 00:36:09
And the bomb explosion
that occurs

753
00:36:05 --> 00:36:11
on the surface
of the neutron star

754
00:36:07 --> 00:36:13
would be a billion
times a billion--

755
00:36:10 --> 00:36:16
a billion times a billion,

756
00:36:11 --> 00:36:17
ten to the 18 times
more powerful

757
00:36:14 --> 00:36:20
than hydrogen bombs
that we can make here on Earth.

758
00:36:21 --> 00:36:27
We speculated early on

759
00:36:24 --> 00:36:30
that when you see
an X-ray burst in the sky,

760
00:36:26 --> 00:36:32
that you may be able to see also
an optical flash in the sky.

761
00:36:31 --> 00:36:37
The donor stars
and the accretion disk

762
00:36:36 --> 00:36:42
emit optical light.

763
00:36:38 --> 00:36:44
It's very faint,
there are very faint sources,

764
00:36:41 --> 00:36:47
but you can see them
from the ground

765
00:36:42 --> 00:36:48
with your optical telescopes.

766
00:36:44 --> 00:36:50
And we had reasons to believe

767
00:36:46 --> 00:36:52
that we might see
an optical flash

768
00:36:48 --> 00:36:54
when an X-ray burst occurs,

769
00:36:51 --> 00:36:57
and I'll tell you why we
believed that was the case.

770
00:36:56 --> 00:37:02
If you have here
a neutron star

771
00:36:59 --> 00:37:05
and here you have
the accretion disk,

772
00:37:02 --> 00:37:08
and if the bomb
explosion occurs--

773
00:37:04 --> 00:37:10
these red wiggles
are the X rays--

774
00:37:08 --> 00:37:14
then the stuff that goes
straight to the Earth

775
00:37:10 --> 00:37:16
you will see.

776
00:37:12 --> 00:37:18
But there are other X rays
which go in this direction,

777
00:37:15 --> 00:37:21
and the heat of the disk...
we call it X-ray heating.

778
00:37:18 --> 00:37:24
And the disk locally
would get a temperature

779
00:37:20 --> 00:37:26
of maybe 30 -->  40,000 degrees
and brightens in optical.

780
00:37:24 --> 00:37:30
So we expected
that we should see that effect,

781
00:37:26 --> 00:37:32
that effect of X-ray heating.

782
00:37:28 --> 00:37:34
But our goal was
even more ambitious.

783
00:37:31 --> 00:37:37
You see, the optical light
that comes from here

784
00:37:34 --> 00:37:40
must be delayed than the X rays
that go straight to the Earth,

785
00:37:38 --> 00:37:44
because first the X rays travel
in this direction,

786
00:37:41 --> 00:37:47
and then the optical light goes
in this direction.

787
00:37:43 --> 00:37:49
So if I put my pencil here,
this is the extra path

788
00:37:48 --> 00:37:54
that the electromagnetic
radiation is going,

789
00:37:50 --> 00:37:56
and that takes time.

790
00:37:52 --> 00:37:58
And if that takes one second,

791
00:37:54 --> 00:38:00
then that means this distance
is one light-second.

792
00:37:57 --> 00:38:03
If it takes 20 seconds, this
distance is 20 light-seconds.

793
00:38:00 --> 00:38:06
So our goal was to actually
measure, for the first time,

794
00:38:04 --> 00:38:10
the dimensions of the inner part
in the accretion disk.

795
00:38:08 --> 00:38:14
And so we organized
a worldwide campaign in 1977.

796
00:38:12 --> 00:38:18
17 countries were contributing,
44 observatories,

797
00:38:17 --> 00:38:23
and we told them

798
00:38:18 --> 00:38:24
that we were going to look
at a particular star in the sky,

799
00:38:21 --> 00:38:27
a very faint star, which
was this X-ray binary system.

800
00:38:24 --> 00:38:30
We would record with SAS-3
the X-ray burst,

801
00:38:27 --> 00:38:33
and we wanted them to record--
in the radio, in the infrared,

802
00:38:30 --> 00:38:36
wherever possible,
in the optical--

803
00:38:32 --> 00:38:38
whether they would see
a change in the appearance,

804
00:38:36 --> 00:38:42
in the optical
or in the radio appearance.

805
00:38:39 --> 00:38:45
In the summer of 1977,
we saw 110 bursts

806
00:38:43 --> 00:38:49
from a particular burst source.

807
00:38:45 --> 00:38:51
None of them were seen

808
00:38:46 --> 00:38:52
in the optical or in the radio--
zero results.

809
00:38:50 --> 00:38:56
We did it again in 1978
and then we succeeded.

810
00:38:55 --> 00:39:01
This was in collaboration
with Josh Grindley from Harvard,

811
00:38:58 --> 00:39:04
Jeffrey McClintock, who was
my ex-graduate student--

812
00:39:02 --> 00:39:08
he is now also at Harvard--

813
00:39:04 --> 00:39:10
and my good friend
Jan van Paradis

814
00:39:07 --> 00:39:13
from the University
in Amsterdam,

815
00:39:09 --> 00:39:15
who worked with me
at the time here at MIT.

816
00:39:13 --> 00:39:19
This was a splashing result--

817
00:39:14 --> 00:39:20
simultaneous observation
of an optical flash

818
00:39:18 --> 00:39:24
with an X-ray flash--

819
00:39:19 --> 00:39:25
and it was on the cover sheet
ofNature,

820
00:39:21 --> 00:39:27
which is a very prestigious
journal in which people publish,

821
00:39:25 --> 00:39:31
so we were extremely happy.

822
00:39:27 --> 00:39:33
I do want to show you
the simultaneous events

823
00:39:32 --> 00:39:38
but not the 1978 event,

824
00:39:34 --> 00:39:40
but I'll show you one
that is more impressive

825
00:39:37 --> 00:39:43
that my colleague
Holger Pedersen observed

826
00:39:40 --> 00:39:46
a year later, when he did
the optical work from Chile.

827
00:39:46 --> 00:39:52
This is that observatory
that I mentioned to you earlier.

828
00:39:48 --> 00:39:54
I have been there many times
myself.

829
00:39:50 --> 00:39:56
It's in La Silla,
2,400 meters above the ground.

830
00:39:55 --> 00:40:01
This is where
the atmospheric pressure

831
00:39:57 --> 00:40:03
is only three-quarters
of an atmosphere,

832
00:39:59 --> 00:40:05
and where you can't boil
a soft-boiled egg

833
00:40:01 --> 00:40:07
because the temperature
of boiling water

834
00:40:03 --> 00:40:09
is only 92 degrees.

835
00:40:05 --> 00:40:11
Okay, here is the optical flash
that Holger observed,

836
00:40:09 --> 00:40:15
and we were looking
with the Japanese satellite,

837
00:40:11 --> 00:40:17
which was called Hakucho.

838
00:40:13 --> 00:40:19
SAS-3 I think was no longer
operating at the time.

839
00:40:16 --> 00:40:22
You see here the time
of the optical signal,

840
00:40:19 --> 00:40:25
so this is the strength
of the system in quiescence

841
00:40:22 --> 00:40:28
and then the X-ray burst occurs,

842
00:40:24 --> 00:40:30
and we see here clearly
an optical flash,

843
00:40:29 --> 00:40:35
and here you see the X rays
from Hakucho.

844
00:40:31 --> 00:40:37
They look very similar, but
now comes the interesting part.

845
00:40:34 --> 00:40:40
Of course, I have scaled them up
here on the view-graph

846
00:40:37 --> 00:40:43
so that they have
the same height.

847
00:40:38 --> 00:40:44
That's just artificial,
of course.

848
00:40:40 --> 00:40:46
But now comes
the interesting part.

849
00:40:42 --> 00:40:48
If I overlay them,

850
00:40:45 --> 00:40:51
then look at the blue line,
which is the optical--

851
00:40:47 --> 00:40:53
it's clearly delayed
relative to the X rays,

852
00:40:50 --> 00:40:56
and that was our goal.

853
00:40:51 --> 00:40:57
And this was really
a very clean observation,

854
00:40:54 --> 00:41:00
cleaner than
our 1978 observations,

855
00:40:56 --> 00:41:02
and if you move
the optical back--

856
00:41:01 --> 00:41:07
or the X-ray forward,
whichever you want--

857
00:41:03 --> 00:41:09
by about two seconds, then
they almost exactly overlap.

858
00:41:08 --> 00:41:14
And so this was
conclusive evidence

859
00:41:11 --> 00:41:17
that the dimensions of the disk,

860
00:41:13 --> 00:41:19
of the accretion disk
around these neutron stars,

861
00:41:17 --> 00:41:23
had typical dimension
of about two light-seconds,

862
00:41:20 --> 00:41:26
of that order.

863
00:41:22 --> 00:41:28
We suspected that,
for other reasons,

864
00:41:24 --> 00:41:30
but nevertheless, this was
the conclusive evidence.

865
00:41:29 --> 00:41:35


866
00:41:33 --> 00:41:39
The bad news is
that during my past term

867
00:41:38 --> 00:41:44
that I was lecturing 8.01,

868
00:41:40 --> 00:41:46
I have been able to do nothing
but 8.01-- no research at all.

869
00:41:47 --> 00:41:53
And I think you have all the
right to feel guilty about this.

870
00:41:51 --> 00:41:57
(class laughs )

871
00:41:53 --> 00:41:59
Very guilty.

872
00:41:55 --> 00:42:01
Now, the good news is
that I enjoyed it,

873
00:41:58 --> 00:42:04
and whether you like it or not,

874
00:42:00 --> 00:42:06
you were on my mind almost
all the time, day and night.

875
00:42:06 --> 00:42:12
I had even nightmares about it,

876
00:42:09 --> 00:42:15
and a typical nightmare that
I would have is the following.

877
00:42:14 --> 00:42:20
I would come into 26.100,

878
00:42:17 --> 00:42:23
but I lost my lecture notes
and I couldn't find them

879
00:42:19 --> 00:42:25
and I was completely
ill-prepared,

880
00:42:21 --> 00:42:27
but I would start
my lectures anyhow,

881
00:42:24 --> 00:42:30
and you would start
laughing at me

882
00:42:27 --> 00:42:33
and I would wake up in sweat.

883
00:42:29 --> 00:42:35
I don't think we need Freud
to explain that dream.

884
00:42:33 --> 00:42:39
Now, I have enormously enjoyed
lecturing 8.01,

885
00:42:37 --> 00:42:43
and in a way,
you have touched my life

886
00:42:40 --> 00:42:46
and I trust that I, too,
have touched your life.

887
00:42:43 --> 00:42:49
Now, I make myself no illusions.

888
00:42:46 --> 00:42:52
I am sure that you will very
soon forget Kepler's Third Law,

889
00:42:50 --> 00:42:56
although I hope it won't be
before Monday,

890
00:42:53 --> 00:42:59
when we have the final.

891
00:42:55 --> 00:43:01
(class laughs )

892
00:42:56 --> 00:43:02
And you will probably also
forget how to properly apply

893
00:42:59 --> 00:43:05
the conservation
of angular momentum.

894
00:43:02 --> 00:43:08
But perhaps you will always
remember from my lectures

895
00:43:05 --> 00:43:11
that physics can be
very exciting and beautiful

896
00:43:08 --> 00:43:14
and it's everywhere around us,
all the time,

897
00:43:12 --> 00:43:18
if only you have learned to see
it and appreciate the beauty.

898
00:43:17 --> 00:43:23
And surely, when you go
on your first monkey hunt,

899
00:43:20 --> 00:43:26
wearing your own safari hat,

900
00:43:23 --> 00:43:29
or when you will orbit the Earth

901
00:43:25 --> 00:43:31
and you want to throw a ham
sandwich to your friend,

902
00:43:28 --> 00:43:34
you may be thinking of me.

903
00:43:30 --> 00:43:36
And I hope those will be
happy memories.

904
00:43:35 --> 00:43:41
I wish all of you the very best,

905
00:43:37 --> 00:43:43
and I thank you
for attending my lectures.

906
00:43:40 --> 00:43:46
(class applauds )

907
00:43:48 --> 00:43:54
LEWIN:
Thank you.

908
00:43:50 --> 00:43:56


909
00:43:53 --> 00:43:59