WALTER LEWIN: Take your seats.
Take your seats, please.
My lecture will be
about 40 minutes.
And after that, I will give
anyone the opportunity to ask
me questions for about
15 or 20 minutes.
So the whole thing
will be one hour.
And it is after that hour that
we will do book signing.
You can buy the books there
if you want to and
I'll sign them here.
We'll get back to that later.
If you have a pendulum--
this is a pendulum,
mass m, length l--
that you can derive, which I do
in class, but I won't bore
you with that.
You can derive how long it takes
for the pendulum to make
one complete oscillation.
And we call that the period
of the pendulum.
And I'd arrive in class that
that period equals 2 pi times
the square root of L over g.
You already know what L is.
If you don't know what
pi is, you might as
well leave right now.
And g is what we call the
gravitational acceleration,
which is approximately the
same everywhere on Earth.
But it is very close in
Boston to 9.80 meters
per second per second.
And you will say, well, that's
meters per second per second.
What that means is that if you
have an object and you drop it
from a very large altitude,
very high--
a few hundred meters high--
and I dropped it 0 speed.
After one second, it will
have a speed of
9.8 meters per second.
But after another second, it
will add to that 9.8 meters
per second.
So it's twice that.
And after three seconds,
it's 3 seconds.
So now you understand
why it is meters
per second per second.
And so that's the
meaning of g.
There is something weird about
this equation, something that
must go against your
intuition.
And you shouldn't feel bad
because it also goes against
my intuition.
So we have the pendulum, and
suppose I bring the pendulum
all the way out here.
We call that the amplitude
of the pendulum.
And we let is swing
back and forth.
There's a certain period.
But now, we bring it
out only this far.
It doesn't have to travel
very much at all.
Doesn't that make a difference
in the periods?
The equation says
no, it doesn't.
Because if it did make a
difference, there would be in
that equation the amplitude,
which is not.
And I'm going to demonstrate
that to you, that it is quite
remarkable that indeed that
period is independent of
amplitude if you don't
go to extreme
values of the amplitude.
There is something else in
here, which is even more
non-intuitive.
And that is doesn't it matter
whether the bob--
we call this the bob--
whether it is 1 kilogram
or 500 kilograms?
You would think, well,
that must make a
difference in the period.
But the equation says
sorry, it doesn't.
And I'm going to demonstrate
that to you, too.
We have here--
by the way, if you ever want
to test this pendulum, this
equation for yourself, put
in L equals 1 meter
and use this number.
You will get a period
of 2.0 seconds.
It's very easy to do at home.
You make one meter.
You put an apple at the bottom
and you swing it.
The one meter has
to be accurate.
You see that it's going
to be 2.0 seconds.
Our pendulum that we have
here is really the
mother of all pendulums.
Look at it.
15 and 1/2 kilograms and the
length of this pendulum is
5.21 meters.
However, you must understand
that it is very difficult to
measure that to very
high accuracy.
And so there is an uncertainty
in the measurement of the
lengths that we have to be
honest about because we are
physicists, after all.
And the uncertainty we
estimate is about 5
centimeters.
So we could be off
by 5 centimeters.
That means that is 1%, 5
centimeters is 1% out of 521.
But since L only shows up as the
square root, it means the
uncertainty in the time
that we predict
is only half a percent.
And if you don't see the reason
why the 1% becomes half
a percent, that's OK, then
just forget about it.
You take my word for it.
So now, I can make
a prediction.
So I predict using
this equation--
that's all I do.
I put in 5.21.
I put in 9.80.
I multiply by 2 pi, and I make
the prediction that the period
of that pendulum is 4.58 plus
or minus 0.02 seconds.
Why the plus or minus 0.02?
Well, that is the half a percent
uncertainty due to the
square root of L. You can
immediately see that 2 is
about half percent of 458.
So my prediction can be
no better than that.
Now comes the problem.
I have to measure the period
to convince you that it is
independent of amplitude and
to convince you that it is
independent of mass.
So the biggest problem now is
Walter Lewin himself, which is
my reaction time.
How accurately am I able to
make that measurement?
That has nothing to do with our
lack of knowledge of the
exact length.
Now, the last time that I gave
this lecture with the pendulum
is 12 years ago.
I was 63.
And at that time, I told the
class my reaction time is a
tenth of a second.
And it was.
But now, I'm 75.
And so my reaction time is
no longer 0.1 seconds.
What is it?
Well, I do not know.
But I have a feeling that if you
want to be kind to me for
a change, let us assume that my
reaction time at age 75 is
now 2/10 of a second.
So if you can live with that,
then every measurement that I
make, no matter how long
it is, it is uncertain
to 2/10 of a second.
Do not confuse that
with that 0.02.
That has to do with
the length.
All right.
So I'm going to first make
a period measurement at 5
degrees, and then at 10
degrees amplitude.
And I'm going to measure
10 periods, not 1.
Some of you may think, well,
isn't that a waste of time to
do 10 periods if you can
get away with 1?
You will very quickly
see why it is 10.
You will see that .
So the 10T is then going to be
some number plus or minus my
own inability, which is my 0.2
seconds reaction time.
I can't change that.
And so then I do
the 10 degrees.
And then we get again, 10T.
We get a number.
And then we get again, plus
or minus 0.2 seconds.
And I'm going to demonstrate
to you that within the
uncertainty of the measurements,
that I get the
same numbers in all three cases,
within the uncertainty.
So if you're ready for that,
you see here the timer.
All of your can see.
And here you see the pendulum.
And I have two marks
on the floor here.
If I hold the bob here,
then it is 5 degrees.
This is 5 degrees.
And when I hold the bob here,
it is 10 degrees.
Timing is not easy.
The best way to do it is to
start the timing when the
pendulum comes to a stop.
That is rather well defined.
And then you let it
swing 10 times.
And then when it comes to
a stop, you stop it.
And it would help me if you
would count how many
oscillations we have made,
because then I don't have to
look at it.
All I have to do is when I come
close to 10, I have to
watch for the moment
of stopping, and
then I will end it.
So we'll do this first
at 5 degrees.
I'm going to start it
when it comes here.
OK.
Now you count.
AUDIENCE: One, two,
three, four.
WALTER LEWIN: You're
doing very well.
AUDIENCE: Five.
WALTER LEWIN: You're going
to pass this course.
AUDIENCE: 6, 7, 8, 9, 10.
[APPLAUSE]
WALTER LEWIN: 45.7.
So that becomes that T. That
means this whole equation now
has to be divided by 10.
And now you will see why I
measured 10 oscillations.
So T is going to be 4.57 plus
or minus 0.2 divided by 10.
That is, plus or minus
0.02 seconds.
And you see comfortably
within the prediction.
So maybe my reaction time
is a little better
than 2/10 of a second.
Don't count on it because you
haven't seen the rest yet.
So now 10 degrees.
That moment is crucial.
That moment is crucial.
That's where you can lose 4/10
of a second, and then you look
like an idiot in front
of your students.
AUDIENCE: 3, 4, 5,
6, 7, 8, 9, 10.
[APPLAUSE]
WALTER LEWIN: Now comes
the hardest part.
The hardest part is that we
have to change the mass of
this object.
And the way that I'm going
to do that is prominently
demonstrated on the
cover of my book.
Yes, I'm going to hang
on that pendulum.
It is a difficult
demonstration.
First of all, it is painful.
It really is.
Second, the timing is tricky.
Because when you look at the
pendulum and when you see it
standstill, that is really
well defined plus
or minus 0.1 seconds.
When you're swinging yourself
however, then you can only do
it by sensing the moment that
you think you standstill.
And that's what I will do.
And then you will
do the counting.
And this is very unpleasant.
It is.
Oh, there's something else
I haven't told you.
If you're a good physicist, you
will say, if you're going
to sit on that bob, then
effectively you bring the mass
of the bob up.
And so the lengths of the
pendulum will shorten.
And so you get a
shorter period.
And I know that, too.
Therefore, I will have to
stretch my body so that when
it is here, that it is
almost completely
parallel to the floor.
If I don't do that, I will not
be able to convince you that
the period is independent
of the mass.
And that makes it very
difficult for me.
So I will start it
at some moment.
You will see when, and then
you do the counting.
Are you ready?
AUDIENCE: Yes.
WALTER LEWIN: OK.
You count.
AUDIENCE: One.
WALTER LEWIN: This happens
sometimes.
And in fact, nobody knows why.
Have to start all over.
[AUDIENCE CHATTERING]
WALTER LEWIN: I did
not stop it.
I really didn't.
Oh, it's still counting?
OK, I have enough energy to give
it one more attempt, but
not to give it two
more attempts.
OK.
You're ready?
AUDIENCE: Yeah.
WALTER LEWIN: OK.
AUDIENCE: 1, 2, 3--
WALTER LEWIN: Ah, this
really hurts.
AUDIENCE: --4, 5--
WALTER LEWIN: Can't you
count a little faster?
AUDIENCE: --6, 7, 8, 9, 10.
[APPLAUSE]
WALTER LEWIN: 10T with
Walter Lewin.
What is it?
45.9 plus or minus 0.2.
Period is 4.59 plus
or minus the 0.02.
I told you, physics works.
[APPLAUSE]
WALTER LEWIN: If I have a tennis
ball in my hand and I
dropped the tennis ball from a
certain height, give it no
speed, and it will bounce back,
then it can never bounce
higher than where
it started from.
If it did, then we physicists
would say that is a gross
violation of the conservation
of energy.
And that is the holiest or
all laws in physics.
It cannot go higher.
Suppose the object has
a mass m and is a
distance h from the floor.
Here's the floor.
Here's the objects, h.
We associate with the position
of that object an energy that
has a name.
We call that potential energy.
And that potential
energy is m g h.
You already know what g is.
Well, h is this distance
in meters.
So when the object is here
on the floor, h is 0.
So there is no potential
energy.
As the object goes down, it
picks up speed and we
associate with speed energy,
which we call kinetic energy.
And the kinetic energy
of an object with
mass m is 1/2 mv squared.
m is the mass and v is the
speed of that object.
If the object goes down and it
hits the floor, then the
potential energy is 0.
And all that energy is now
converted to kinetic energy.
Because energy, we believe,
is conserved.
Now, when it hits the floor,
some of that kinetic energy
may be converted into heat
because of the compression.
If it were a tennis ball, that
would certainly happen.
So in other words, when it
bounces back, the total energy
is no longer the full energy h,
but is a little less, and
so it won't bounce as high.
But there's no way that it could
come up higher than h.
Suppose it could come after
one bounce up here.
Well, that would solve the
world's energy problem.
Because you simply sit down and
you watch the ball game.
And there goes the ball higher,
and the second one it
goes again higher.
But if it goes the first time
higher, it will do that the
second time.
And so after an hour, that thing
is about a few thousands
meters in the sky.
And when it comes down on the
floor, it has an enormous
speed, great amount
of kinetic energy.
And so you got energy
out of nothing.
But there is no such thing in
physics as a free lunch, so
that's not going to happen.
So the object can never
go any higher than h.
And with a tennis ball, there
is also what we call the
dissipation of heat when
it hits the floor.
Now, the situation is difficult
with a pendulum
because a pendulum doesn't
hit the floor.
And so there is no heat
loss because it
doesn't hit the floor.
So if you bring a pendulum at
a certain distance above the
ground like this, and you let
it swing, when it comes back
here, it is almost exactly
at that same height.
It cannot be higher.
That would be a violation of
the conservation of energy.
But since the air drag
is so small,
there's almost no damping.
And in fact, when you saw the
demonstration I just did, you
may have noticed that you
really didn't see this.
It really kept going
and it kept going.
As long as you realize that if
I release it from a certain
location with 0 speed, it can
never, when it comes back, be
higher than that location.
This whole idea is behind
demolishing buildings.
With a building demolishing,
you take a huge mass.
You lift it up over
a distance h.
And then, you put your target,
which is your house, or
whatever it is, right at the
bottom when all these
potential energy is being
converted to kinetic energy.
And so this object is hit with
an enormous amount of energy,
at high speed, and you demolish,
thereby, the wall.
Here, we have a glass plate.
You better go out of the way
because this is a dangerous
experiment.
This is a glass wall.
So if I bring this object
exactly at that glass wall--
and if I'm clever enough to
let it go with 0 speed, it
could not break that glass.
But if my hands shake a little,
and if I gave it a
little push, then of course,
it can come back.
And it may want to go
higher than this.
And that would mean--
[LAUGHTER]
[APPLAUSE]
WALTER LEWIN: I know
you, guys.
Students love it when
the glass breaks.
That's why they pay such
a high tuition at MIT.
[APPLAUSE]
[LAUGHTER]
WALTER LEWIN: That's OK.
Just take that off.
Now comes an experiment, which
is emotionally the most
difficult for me of this
whole evening.
I'm going to put my life on the
line to show you that I am
really a believer of the
conservation of energy.
And you will see how I'm
going to do that.
I'm going to take the
place of the glass.
And I'm going to hold this
object at my chin.
And I cannot move any
further back, so
there's no cheat here.
I'm going to release it right
from my chin here.
You realize, as you have just
seen, that the slightest push
and this will be my
last lecture.
And no book signing
afterwards.
So I need your collaboration.
When I count down from 3 to
0, no noise, no coughing.
And I would even appreciate it,
if for those 3 seconds,
you would not even breathe.
And I have to tell
you something.
I couldn't sleep all night.
I'm going to close my eyes.
I don't want to see it.
And I'm going to count
down from 3 to 0.
3, 2, 1, 0.
[APPLAUSE]
WALTER LEWIN: And normally,
after this demonstration, tell
the class physics works,
and I'm still alive.
And when an article was written
about me in The New
York Times a few years ago, on
the second page of The New
York Times is the wisdom
of the day.
And the wisdom of the day
was physics works,
and I'm still alive.
[APPLAUSE]
WALTER LEWIN: White light, like
sunlight, is composed of
all the colors that you
see in the rainbow.
If I scatter white light off
very small particles, then the
blue light is scattered more
than the red light.
And we give that a name in
physics, we call that Rayleigh
scattering.
Rayleigh scattering only happens
when the particles of
which the white light scatters
is smaller than
a tenth of a micron.
That means a thousand times
smaller than the
thickness of your hair.
So it has to be a very,
very small.
If the particles are as large
as half a micron, then there
is no longer Rayleigh
scattering.
There is no [? preferred ?]
scattering for the blue light.
All colors scatter equally, and
so white light scattered
off particles at a
half a micron or
larger remains white.
The dependence of the power
of scattering--
so I'll give that
P, the power--
is proportional when we have
Rayleigh scattering.
This is the only equation that
may bother you, to 1 over
lambda to the fourth,
and lambda is the
wavelength of light.
And I will not bother you to
tell you what the wavelengths
of light is.
That may confuse you.
But I will tell you that blue
light has a wavelength which
is about 1.5 times lower
than red light.
And so if you take 1.5
to the power 4--
trust me.
Yeah, 1.5 to the power
4, you get 5.
And that means, in Rayleigh
scattering, blue light has a
five times higher probability
to scatter than red light.
And I'm going to demonstrate
that to you in two complete
different ways.
The first way that I'm going
to do that is to make it
completely dark in the lecture
hall and have light going
straight up here.
Then, I will light a cigarette,
and the smoke of a
cigarette has particles
that are smaller
than a 10th of a micron.
And so the light that you will
see that is scattered off the
smoke will be blue.
So you have seen, in front
of you own eyes, Rayleigh
scattering.
Because the red lights, more
or less, goes through.
It is really the blue that
dominates it, that has the
highest probability.
So we're first going to do that
demonstration to show you
Rayleigh scattering of
cigarette smoke.
And then I have a surprise for
you to also show you Mie
scattering.
But let's first do the Rayleigh
scattering with
cigarette smoke.
This is also not a pleasant
demonstration.
For those of you who think that
lecturing is easy, no.
[LAUGHTER]
[APPLAUSE]
WALTER LEWIN: OK.
I'm going to make it completely
dark, and then I'm
going to hold it in there.
All lights off.
All off.
All off.
So we all agree that this
is white light,
which is coming up.
And you don't see the light here
because there is nothing
that scatters it in
your direction.
So you don't see light here.
But now look.
Those of you who see
blue say yeah.
AUDIENCE: Yeah.
WALTER LEWIN: Those of you who
do not see blue, say no.
AUDIENCE: No.
WALTER LEWIN: You better
see an eye doctor.
Now comes the hardest part.
If I inhale the smoke and I
leave it in my lungs for a
minute, there is water
vapor in my lungs.
And this water vapor will
precipitate onto these very
small smoke particles.
And so the smoke particles
will grow.
They will become small
water drops, larger
than all 0.5 microns.
And that means if I hold it
one minute in my lungs and
puff it out, you will not see
blue light, but you will see
white light.
Because you're now in the Mie
scattering domain, all colors
scatters equally.
I will tell you that just before
I puff it out and you
will see the white smoke, I will
just before I do that, I
will remind you of the color
that you see now.
I will only do that
for a few seconds.
Then I will remove it and
I will empty my lungs.
Terrible demo.
[APPLAUSE]
WALTER LEWIN: Who saw
the white light?
Just say yes.
AUDIENCE: Yes.
WALTER LEWIN: If any one of you
has the courage to say so
no, who did not see
white light?
AUDIENCE: No.
WALTER LEWIN: Thank you.
[APPLAUSE]
WALTER LEWIN: And now I'm going
to explain to you-- in
fact, you could probably guess
that why the sky is blue and
why clouds are white.
Clouds consist of very small
water drops, surely larger
than half a micron, which
is Mie scattering.
So the white light of the sun
scattered off the cloud--
white remains white.
So you now, for the first time
in your life, may have an
explanation why clouds
are white.
And you should, or may,
also understand now
why the sky is blue.
Here is the ground
and you are here.
And here is, say, roughly the
top of the atmosphere.
And the sunlight comes
in like this.
Sun is infinitely far away, so
the sun comes in like this.
The atmosphere is full of very
small dust particles, smaller
than a 10th of a micron.
And even the density
fluctuations of the air
molecules themselves are clearly
smaller than the 10th
of a micron.
And so you get ideal Rayleigh
scattering.
So white light comes in,
you're standing here.
But what is the light
that comes to you?
Predominantly blue.
So the sky is blue.
The light that is scattered
here, comes to you is
predominantly blue.
So that's why the sky is blue.
And so the reason is simply that
it is Rayleigh scattering
of the dust particles
in the atmosphere.
If the sun is high in the sky,
the total amount of sunlight
that is scattered in your
direction is only 1%.
So it's very little.
If the sun is 5 degrees above
the horizon, then the sunlight
has to travel through a
lot more atmosphere.
And so I think here a situation
which is extreme,
when we have sunrise
or sunset.
So the sun is there and the
light comes from this side and
you are standing here.
This is not to scale.
This layer of atmosphere is now
so enormously large that
more than 99% of all the
sunlight on the way to you is
scattered away.
So what is scattered away?
The blue is gone.
But if you look at 1 over lambda
to the fourth, the
green is gone.
All colors are gone.
There's only one color which has
the largest wavelengths,
which by the way, is
650 nanometers.
I wasn't supposed to tell
you, but I decided.
So the only light that makes
it through you is red.
And so that is the reason
why the sun looks red.
And there is a cloud here in the
sky, and that cloud sees
light where all the small
wavelengths have been
scattered out.
And so this side of the
cloud is also red.
You can now understand that the
more pollution there is in
the air, the more beautiful
sunsets are.
And it is well known that after
volcanic eruptions, the
sunsets and the sunrises
are truly fantastic.
It's also the moon that is
red when it comes up.
And even the stars
and the planets.
You may never have noticed
it because it's not an
overwhelming thing.
It is the sun that is the
overwhelming thing that makes
the entire sky red.
And so I have decided that I'm
going to create in 26-100, a
blue sky for you and a red
sunset, killing two
birds with one stone.
And for the physicists in my
audience, I'm going to kill
three birds with one stone.
But the third bird comes
a little later.
I have here a bucket which
is filled with sodium
thiosulfate--
in this bucket.
And when I turn the light on,
you will not even see any
light from that bucket.
Nothing is scattered
in your direction.
I think of that as being
the sun, by the way.
Now I'm going to add a little
bit of sulfuric acid.
And when I do that, very small
sulfur particles, smaller than
the 10th of a micron, will
precipitate in that solution.
Rayleigh scattering.
And so the light that will
come to you is blue.
And you will see blue light,
just like with the smoke.
But now, as time goes on, we
will get more and more and
more and more of those
0.1 micron particles.
And so the light that
comes out here has
no blue in it anymore.
It doesn't have any green
in it anymore.
It's all scattered in your
direction, just like here with
the sunset.
So what color do you think
the sun is going to get?
AUDIENCE: Red.
WALTER LEWIN: It's
going to be red.
That's why I said I'm going to
kill two birds with one stone.
So I will add the
sulfuric acid.
The difficulty with this
experiment is always if you
put too much sulfuric acid
in it, the whole
process goes too fast.
And if you put too little
in it, then
you will become impatient.
At least, MIT students would.
So I'm going to put
this in and stir.
And then, make it immediately
dark.
And I want you to look at
the sky, which is--
here is the sky.
If you sit all the way there,
you don't see it so well.
But look, how much did
you pay for this?
These people have
a better view.
So just keep looking.
For me, it's already beginning
to turn a little bluish.
We'll just give it a little
bit more time.
The sun looks just white
light as it was before.
I always have a backup
you see.
If this takes too long, then
what I do, I add another
teeny, weeny little bit of
sulfuric acid to speed up the
process a little.
I see blue light.
And when I look at the
sun, it looks a
little reddish already.
For the physicists among you,
light that scatters over an
angle of 90 degrees, this light
that scatters in this
direction--
the people who paid the most
tonight, who are sitting right
here, the light is also
linearly polarized.
That was also the case with the
smoke experiment, but I
didn't mention that.
But for those of you who are
sitting here, I can show you
with my polarimeter, when I
rotate my polarimeter, that I
can-- the blue sky
completely dark.
And the blue sky completely
bright again.
The people who are sitting
there, the angle of scattering
is not 90 degrees.
So they won't see it so well.
But you people see it very
well, don't you?
AUDIENCE: Yes.
WALTER LEWIN: 100% polarized.
[APPLAUSE]
WALTER LEWIN: Look
at that sun.
Let's face it, isn't this
incredibly romantic?
In 26-100, at the center of MIT,
you are seeing, in the
lecture hall, a red sunset.
And in fact, the sun is so red
now, that I think the sunset
is very close.
[APPLAUSE]
WALTER LEWIN: I have given,
in this lecture
hall, about 800 lectures.
And it is wonderful to be back
here, but it really hurts to
know that this is my last
lecture in 26-100.
I have, therefore, decided
that I want to
leave you in style.
And the way I will do that is
to leave 26-100 in my own
private rocket.
[APPLAUSE]
[APPLAUSE]
WALTER LEWIN: Thank you.
[APPLAUSE]
WALTER LEWIN: Thank you.
So now we have about 15 minutes
left for questions.
And if you have a question,
raise your hand.
Then Claire will come to you
with the microphone.
And then, I hope we can
communicate that way.
You phrase the question and I
will try to give the answer.
So who wants to go first?
There's a person there.
Yes, we see your hand here.
You'll come next.
AUDIENCE: Thank you for
a beautiful lecture.
I am tempted to ask
you what pi is.
WALTER LEWIN: It's not so easy
for me to understand you.
AUDIENCE: Thank you for
a beautiful lecture.
I'm tempted to ask
you what pi is.
But I think I'd better
ask instead--
WALTER LEWIN: Have you
ever gone to a
Thanksgivings dinner?
AUDIENCE: OK.
WALTER LEWIN: That
is what pie is.
AUDIENCE: Are you familiar with
the alleged phenomenon of
a green flash at sunset.
WALTER LEWIN: And the
answer is yes.
AUDIENCE: Can you explain
that, please?
WALTER LEWIN: I've seen
it many times.
The explanation is not as
simple as you may think.
I would suggest--
and I mean that seriously.
Since a short answer is not
possible, that you look it up
on the web.
It's well described.
I have seen it many times in
Austria in the mountains.
That, indeed, the last fraction
of setting of the
sun, that you may, but not
always, see a green flash.
And if you ever want
to see it, you have
to be with two people.
One person has to be standing
next to you and you should not
look at the sun.
Because if you look at the
sun, even though there's
almost no sunlight left, your
retina is still too
overexposed.
And so the person next to you,
when you look like this,
should say, look now.
And then you look, and
that's the way it is.
[APPLAUSE]
WALTER LEWIN: There's a
question right here.
Why don't we take that first?
And then, there's a
woman over there.
And this gentleman also.
AUDIENCE: Well, I just wanted to
thank you for the lecture.
AUDIENCE: As a youngster, I read
a book which told me to
stare at the sun, then
I can change things
by staring at things.
Now what I did was I used to
stare at sun at sunset.
WALTER LEWIN: You stare
at the sun?
AUDIENCE: At sunset.
WALTER LEWIN: At sunset.
Not a very good idea.
You have to be very careful.
AUDIENCE: I found out
the hard way.
WALTER LEWIN: And so you damaged
your eyes and you
asked for it.
AUDIENCE: But the key thing
was that after that, I was
supposed to stare at a white
wall, which I did.
But what I saw a red--
WALTER LEWIN: I know exactly
what you saw.
AUDIENCE: A red spot moving
around wherever--
WALTER LEWIN: Probably the spot
was not red, but green.
It's a well-known phenomenon.
So you have, indeed, done
something to your retina and
the message that is sent to you
brains then tell you the
green aftereffect.
It's very well-known effect.
You don't even have to
look to the sun.
You can even do it with a light
like that and stare in
that light for some time.
And then, all of a sudden look
at the white wall and you see
a different color.
It's a very interesting thing.
And physics cannot
explain that.
But you see, this
is neurology.
And so it's not our
responsibility to explain it.
[APPLAUSE]
AUDIENCE: So this is, I guess,
a more personal--
WALTER LEWIN: Speak as loud as
you can because I have a
hearing disability.
AUDIENCE: This is a more
personal question.
I was just wondering, what
inspired you to become such a
great professor?
WALTER LEWIN: Why is what?
AUDIENCE: What inspired you to
become a professor, and a
great one at that?
WALTER LEWIN: Can
you translate?
I can't hear.
AUDIENCE: What inspired you?
WALTER LEWIN: Why is what?
AUDIENCE: What inspired you
to become a professor?
WALTER LEWIN: Oh,
that was luck.
I had my training in
nuclear physics.
And then, I had some offers for
one year post-doc to the
United States.
And for reasons that are not so
clear, I picked MIT because
a whole new field was born
here in Cambridge,
Massachusetts, initiated
by Professor Bruno
Rossi who was at MIT.
Executed beautifully by
Riccardo Giacconi who
received, not too long ago,
the Nobel Prize for that.
That was the discovery
of x-ray astronomy.
And even though I knew nothing
about astronomy at the time, I
decided it was time
to change fields.
So I accepted the
offer at MIT.
And then, for reasons that are
still unclear to me, but
George Clarke, who is in the
audience, knows probably why
they offered me a
professorship.
And I never left.
Does that answer
your question?
Yeah, you need the
microphone here.
We have 10 more minutes,
so we can
handle quite a few questions.
AUDIENCE: So I sat in on your
lectures, here I think about
20 years ago.
And I had forgotten that one
thing that I learned from you
was how to draw dotted
lines on chalkboards.
[APPLAUSE]
AUDIENCE: Which I actually
used myself when I was a
professor for some
number of years.
So very useful skill.
But here's what I'd
like to ask you.
I'd like to ask you, when you
started teaching physics, and
how your lectures evolved
over time?
WALTER LEWIN: What was
the last question?
AUDIENCE: How did your lectures
evolve over time?
WALTER LEWIN: Didn't get it.
AUDIENCE: How did your lectures
evolve over time?
WALTER LEWIN: Yeah.
I think I was always
eccentric.
It's true.
And so from day one,
my lectures were
different from the mean.
But of course, they evolved in a
way that grew substantially.
And that is not because of the
dotting of the line, because I
could already do that
in high school.
Today, there are hundreds of my
lectures that can be viewed
on the web.
Two complete courses--
the first course for freshman,
the second course Electricity
and Magnetism.
And the first course
for sophomores,
Vibrations and Waves.
They are now being viewed daily,
on average, by 6,000
people all over the world, which
is 2 million per year.
And so every morning when I wake
up and during the day,
about two dozen questions
come to me by email from
all over the world.
Many ask questions and I answer
every single email.
But it is amazing that many
physics professors want to
know how I make those
dotted lines.
[APPLAUSE]
WALTER LEWIN: There is a
two-minute videotape, which
someone made.
Someone looked at all the dotted
lines that I ever drew
in 801 and put that
in one videotape.
It's a riot.
You see [MAKING SOUNDS].
[APPLAUSE]
OK, we have time for a
few more questions.
Yep, Ana.
Ana, please use the microphone,
although I'm so
close to you that I can probably
understand you.
Ana is my Facebook friend.
I have an art quiz on Facebook
twice a week, and then my
Facebook friends who can answer
the question are always
mentioned every day that they
have the answer right.
Ana is at or near the top.
She almost always
has it right.
And she sometimes writes me
email and it just says,
Walter, I've worked eight
hours on this one.
I really have to give up.
I don't have more time.
Go ahead.
AUDIENCE: What is your favorite
lecture of all the
lectures that you've given?
WALTER LEWIN: I don't
have a favorite one.
No.
I really don't.
I love all the subjects--
801, 802, 803.
I really cannot say
I have a favorite.
If you ask me my favorite
artist, we
can talk about that.
But we do that on Facebook.
There's a question here
and there's one there.
We have still five minutes
for questions.
Clair, who has the microphone?
AUDIENCE: Hello?
Do I need to turn this on?
AUDIENCE: It's on.
AUDIENCE: It's on?
I'm wondering how rainbows
relate to ray scattering.
Do they?
WALTER LEWIN: How
rainbows what?
AUDIENCE: Do rainbows have
anything to do with ray
scattering?
And also, why are they round?
WALTER LEWIN: Buy my book.
It's explained in my book.
[LAUGHTER]
AUDIENCE: Hi.
How do you prepare
your lectures?
How many time do you
take to prepare?
WALTER LEWIN: It's a
very good question.
And if I tell you how I
prepare them, then my
colleagues at MIT are
going to hate me.
On average, the preparation
for one lecture is
about 40 to 60 hours.
I dry run the lecture in
an empty classroom with
everything on the blackboard
that I am going to write on
the blackboard, pretending that
there is a full class in
front of me, but there
is no one.
And I talk to them as
if there is one.
I do that two weeks before
the lecture.
In general, the lecture is then
a little bit too long.
So I have to do some surgery.
Then one week before the
lecture, I dry run again.
Then I'm very close
to the time.
And then 5:00 AM of the morning
of the lecture--
you can ask my wife--
I am at MIT and I give the same
lecture dry run for an
empty classroom at 5:30 in the
morning when the lecture is at
10:00 and the same lecture
is at 11:00.
That's the way I prepare.
Don't tell my colleagues,
they will hate me.
[APPLAUSE]
WALTER LEWIN: By the
way, I made an
exception for this lecture.
I dry run this lecture
six times.
Yeah?
AUDIENCE: Hi.
What advice do you have
for a student
wanting to become a physicist?
WALTER LEWIN: What
do you have what?
AUDIENCE: What advice do you
have for students who want to
be a physicist?
WALTER LEWIN: You
have to love it.
And if you don't love
it, don't touch it.
And if you hate it,
it is because you
had a very bad teacher.
I make every student--
and not only at MIT, but
all over the world--
I make them love physics you.
You can read that in my book.
You should buy it.
[APPLAUSE]
WALTER LEWIN: One
more question.
OK, one more question.
Yeah, this gentleman here.
AUDIENCE: What do you do for
fun, besides give lectures?
WALTER LEWIN: Arts.
Art history.
It's my love.
Physics is my life, art
history is my love.
Thank you.
[APPLAUSE]
WALTER LEWIN: So the book
signing will start.
You can buy books in the hall
and you can come down.
Now, I have one request.
Put in the book a piece of paper
and write down with very
clear letters to whom
I address that book.
So if it is for your daughter
and her daughter is Emily, you
write "For Emily." Even though
your name will be Peter, I
will write in the book
"For Emily" and
signed Walter Lewin.
If you don't put in a
piece of paper, it's
going to take so long.
A few days ago, I had a book
signing and I didn't ask them
for a piece of paper.
So here comes someone from
Turkey to me and I said, what
is your name?
He said, my name is
[? Atittta. ?]
And then I said, how
do you spell that?
He says, oh, that's very easy.
It's [? A-T-I-T-T-T-A. ?]
And then I said, well, it would
be better wouldn't it,
if you write it on
a piece of paper.
And I couldn't even read it.
So please, write very carefully
the name to whom I
should address the book.
We need a table.
