All right, you did well on the exam. Class average was sixty-two. I always aim for sixty-five,
so I was very happy. Eleven students scored a hundred. I believe that my exam review
was extremely fair. According to some instructors,
perhaps even too close for comfort. I did a problem with parallel resistors
and a battery. I applied Gauss's Law
for cylindrical symmetry. I spent quite a bit of time discussing
where charge occurs and where charge can not
be located on conductors and I hit the idea of capacitors
and dielectrics also quite hard. I prefer not to think about a rigid division
between pass and fail, but I'd rather tell you that all of you
who scored less than forty-seven, in my book, are sort of in
the danger zone. Now, that doesn't mean that
you're going to fail the course, nor does it mean that you will pass the course
if you scored seventy. But those people
are in the danger zone. I think you should talk
to your instructor and I would advise those people also
to make frequent use of our tutors. Two exams to go,
plus the final. Today I'm going to uncover
a whole new world for you and you will see
how 802 comes in there
in a very natural way. The Lorentz force F is the charge
times the cross product-- of the velocity of that charge-- and the B field
that the charge experiences. If I have here a positive charge plus q
and it has a velocity v in this direction and the magnetic field would be uniform
and coming out of the blackboard, there's going to be a force on this charge
according to this relationship and the force is then like so. Perpendicular to v,
perpendicular to B. In this case the charged particle
is going to go around in a circle . The Lorentz force can not change the speed,
can not change the kinetic energy, because the force is always perpendicular
to the velocity but it can change the direction
of the velocity. And so, what you're going to see is
that the charged particle will go around into a perfect circle
if the magnetic field is constant throughout. And the radius of this circle
can very easily be calculated using some of our knowledge
of 802. The force is qvB because I chose B
also perpendicular to v and so there is no sign--
the sign of the angle between them is one. And this now has to be the centripetal
force that we encountered in 801, which is mv squared divided by R,
m now being the mass of this particle. And so you'll find now that R
equals mv divided by qB. And this, by the way, I want to remind you,
is the momentum of that particle. If you look at this equation,
it's sort of pleasing. If the charge is high then the Lorentz force
is high so the radius is small. If the magnetic field is high then the Lorentz
force is high so the radius is small. If the mass of the particle is high,
there is a lot of inertia and so it is very difficult
to make it go around, so to speak, so a very high mass,
you expect a very high radius. And so that looks all
intuitively quite pleasing. Let's do a numerical example. I take a proton, p stands for proton
and I take a one MeV proton. It's the same I took
during my test review. One MeV means that the kinetic energy,
is one MeV, is the charge
times the potential difference over which this proton
was accelerated, in this case, delta V
would be one million volts. And this now equals one-half
times the mass of that proton times the velocity squared. In this case, if I have a one MeV,
so it is a million volts, you will find that this is one point six
times ten to the minus thirteen joules. I gave you there the charge of the proton,
you multiplied it by a million, and this is the energy. And so now you can
calculate the velocity because you know
the mass of the proton. I gave you that too, there. And so you will find exactly what we found
during my test review, one point four times ten to the seventh
meters per second, which is five percent
of the speed of light, comfortably low, so we don't have to make any
relativistic corrections. If this proton now enters a magnetic field B,
which is one Tesla, then by using the equation I have up there,
you know the mass of the proton, we just calculated the velocity,
you know the charge of the proton, and you know the B field. You will find that R is oh point one five
meters, which is fifteen centimeters, just a numerical example. It is more common,
or at least often done, to eliminate
out of that equation there-- the velocity and replace it
by the potential difference, capital V, over which we accelerate
these particles. And so, what you can do,
you can replace this v by using the equation I have there,
the one half mv squared, so we have that one-half mv squared
equals q times delta V, but I will write for that just a capital V
and I substitute this v now in here, and so I now longer see the velocity
but I now see this potential difference. In the case of that proton,
this V would be a million and you will find then that R
is then the square root-- of two m times that capital V
divided by qB squared. And so the two equations are of course
the same physics, but it's different representation. If you put in for V now
ten to the sixth, mass of the proton, charge of the proton
and one Tesla field, of course you find exactly
the same point one five meters. Now this is all nice and dandy,
but this works as long as the speed is much smaller than
the speed of light. If that's no longer the case,
then we have to apply special relativity and that is not part
of this course but I would like to briefly
touch upon that today. I can show you
how things go sour because suppose we have a
five hundred kilo electron volt. So that means that in this equation here,
the V is five hundred thousand, the q is the charge of the electron,
m is now the mass of the electron and if I apply that equation-- I find that V is four point two times ten
to the eighth meters per second and that is larger than the speed of light,
so that's clearly not possible. The actual speed,
if you make relativistic corrections, is two point six times ten to the eighth
meters per second. And although I don't expect you to be able
to make those relativistic corrections, I will make them today
and you will see why I have to and I want to show you that in fact
this is not all that difficult even though I will not hold you responsible
for these equations. So what I have here is now kinetic energy,
is again qV, that's not changing, but is no longer one-half mv squared
but it is gamma minus one times mc squared, and gamma is defined there--
it's called the Lorenz Factor and so if you know now
for the electron that capital V is
five hundred thousand, you can calculate what gamma is
from the first equation and then you go to the second equation
and you find what the speed is, and you will see then
that you never find a speed larger than the speed of light. And so we now have to make
a correction also for the radii and those corrections become
again relatively easy. This now requires a factor gamma and you see that
on the upper blackboard there and this too now has to be replaced
by gamma plus one and then everything is OK. So I don't expect you to know this,
but I don't want you to think that all these relativistic corrections
come out of the blue, nor do I want you think
that it is very difficult. It really isn't. The equations are
extremely straightforward. So I want to show you now the--
some of the results that we just discussed. The one MeV proton and the five hundred keV
electron, this is on the Web. You can click on Lecture Supplements
and you can make yourself a hard copy. So here you see the kinetic energy,
one MeV proton. Notice the speed that we calculated there
is non-relativistic, gamma is very close to one. You don't have to make a correction. And in a one Tesla field you get a radius
of fifteen centimeters, which we just calculated. If you go to a fifty MeV proton,
it's sort of in the borderline between relativistic
and non-relativistic. It's still non-relativistic enough,
and if it is non-relativistic-- you can clearly see here that the radius
goes with the square root of capital V. And for a fifty MeV,
capital V is fifty million, and for one MeV,
capital V is one million. And since it goes
with the square root of V, you expect roughly the radius to be
the square root of fifty times larger which is seven
and indeed, you see that. So you see, from fifteen centimeters
the radius goes to about one meter. Um, here is our five hundred keV electron
and notice that I did the calculation correctly. This is relativistically corrected now. You get your two point six times ten to the
eighth meters per second by applying the formalism that you see there. I will leave this here throughout this lecture
because I will return to this several times. I want to show you a--
a cute demonstration. I have an, er, electron gun here
and the electron gun comes like so. This is the velocity of the electrons. I put a minus sign there
to remind you that they are electrons. If electrons go in this direction,
the current goes in that direction. And so if now I have
a magnetic field which, let's assume the magnetic field is
in the blackboard. This is B. Then I cross B is
the direction of the force. I is in this direction,
B is in the blackboard. So if I'm not mistaken,
I think the force is in this direction and so you will see that it
starts to bend in this direction. If you change the direction
of the magnetic field, the magnetic field is coming
out of the blackboard, then the electron will go
in this direction, and I can show you that here. It is not too different from
the distortion experiment that I did when I had the television program there
and I had the strong magnet and we distorted the image, but this of course
is a little bit more controlled. So, we're going to see
the image there and we want to make it
quite dark in the room. Mmm. And turn on the electron gun. So you see the electron gun,
it strikes a fluorescent screen and that's how you can see it, and I have here a bar magnet
and if I hold the bar magnet behind it then I can create more or less
situations like this. I can flip over the magnet and then
the direction of bending should change, so here I come with the magnet
and you see, curve up the electrons. I turn the magnet over and I come in again
and they curve down. Very straightforward,
very simple. OK. There is a fantastic way in physics
that we can separate isotopes from one and the same element. If we, for instance, take uranium,
then uranium, when you find it, is for ninety-nine point three percent
uranium two thirty-eight. That means it has ninety-two protons,
otherwise it wouldn't be uranium and it has hundred and forty-six neutrons,
ninety-nine point three percent. Oh point seven percent
is uranium two thirty-five. Again, ninety-two protons,
otherwise it wouldn't be uranium, but only a hundred and forty-three
neutrons and that you'll find in nature
for oh point seven percent. So you go to a chemist and you give a chemist
a little bit of uranium and you say would you please separate
these two isotopes for me and he of course would laugh at you
and he would say, "Go fly a kite!" because the chemical properties are
exactly the same for the two because uranium is uranium. Neutral uranium here
has ninety-two electrons and neutral uranium here
has ninety-two electrons, so there's no way
that they could separate those. And I will show you now
how they can be separated with what we call
a mass spectrometer. You heat the uranium
so that it ionizes. Let's assume it's ionized once
so it loses one electron, so it's positively charged
with one unit charge, one of those charges that
you see here. And we now accelerate it
over certain potential difference so these uranium atoms, two thirty-five
and the two thirty-eight get a certain speed
and they come in here with this speed v, so they're positively charged
and let's assume that we have a magnetic field that is uniform and that is in this direction,
comes out of the blackboard. So what will happen is that these charged
particles which are positively charged now, one unit charge, are going to go
around a circle and hit here. This is a radius. But if you look here
at these equations-- so you will see that the radius
is proportional with the square root
of the mass of the particle. And the mass of two thirty-eight.
is one point two percent larger-- than the mass of two thirty-five. And so with one point two percent larger,
since we have the square root there, we see here the square root, we accelerate them over
the same potential difference, so this one doesn't change. This is the only thing
that changes. So then you expect an oh point
six percent change in radius and so the two thirty-eight
will end up here. I exaggerate that very highly. And the two thirty-five
will end up here. The two thirty-eight has a larger radius
because it has a larger mass and you see that here. There's no change in B, there's no change in q
and there's no change in capital V. We accelerate them over the same
potential difference. And so if the radius, for instance,
were one meter of this mass spectrometer then the difference here,
remember this is two R, the difference would come out to be
about one point two centimeters, and so you have a collector here where you
collect your two thirty-eight nuclei atoms and here you collect your two thirty-five
and that is the idea behind a mass spectrometer. Why did I choose this particular example? Well, this example changed our world
and it made history. Uranium two thirty-five was needed by the
Americans to build an atomic bomb to end the Second World War. This is- this was done under the famous
Manhattan Project. And Ernest Lawrence of Berkeley built
mass spectrometers which were able to separate uranium
two thirty-five from two thirty-eight. In the beginning, it went very slowly,
about one hundred micrograms per day. But a few kilograms was required
for an atomic bomb. They finally managed to get up to
one gram per day and in combination with other
separation techniques such as the gas
diffusion techniques which I will not discuss here now they managed to get a few kilograms
and they dropped a bomb on Hiroshima on August 6th 1945 and three days later
a bomb was dropped on Nagasaki. The Japanese surrendered
and it was the end of World War Two. It's a good thing that there are many peaceful
applications nowadays of mass spectrometers, particularly in the medical area. People sometimes require radiation and they need radiation
from a particular radioactive isotopes, but you don't want the other isotopes
from the same element and so you separate them then
with a mass spectrometer. It's a whole industry,
very important industry. And I would like to address the issue
how you accelerate protons to extremely high speeds,
almost approaching the speed of light. And that is also something for which
Ernest Lawrence is credited. In the early days it was done in a cyclotron,
which I will describe to you now. The cyclotron consists of a chamber
which is called a D. This is one D
and here's another D. These are conducting chambers. If you look from the side
it would look like so. This is the left chamber
and this is the right chamber and all of this is in vacuum and let's assume that we have
a magnetic field coming out of the board
like so. Let's revisit our one MeV proton. Suppose I release in this chamber here
a one MeV proton-- and I know the speed
with which it comes out, because the one MeV proton
had a speed-- Oh, you still see it there, one point four times
ten to the seven meters per second. We also know that in a one Tesla field,
let's make this one Tesla, that the radius is going to be
fifteen centimeters. You see it up there. So what is this proton going to do?
It's going to do this. But when it gets there, a potential difference
is introduced between these two D's, so that this is low pot- high potential
and this is the low potential. And so you're going to get an electric field now
in this gap in this direction and so this proton is being accelerated. And let's suppose that the difference
in potential is twenty kilovolts. Then this proton will gain in electric--
in kinetic energy, it will gain kinetic energy,
twenty kilo electron volts. That's the way electron volt
is defined. And so you start off with one MeV,
so when it has crossed this gap it is now one point oh two MeV. Twenty keV more. The radius, now, is larger. If capital V is two percent higher
and I go to this equation, then the radius
is one percent higher and so when it comes out here
and it makes a circle, the radius now is one percent higher
than fifteen centimeters. But when it gets to this
part of the D, this potential difference
is reversed and so the electric field
is again in this direction, in the direction of the proton
and so it is accelerated again by twenty kilo electron
volts. Now the radius, of course,
is even larger and so very gradually every time that it
reaches the gap the potential difference is changed
in direction to accelerate the proton and so it gradually spirals out, then,
to the largest radius that you have. So during one full rotation
it gains twenty kilo electron volts once, and twenty kilo electron volts twice,
so it gains forty kilo electron volts. And so the electric fields
are doing the work. They accelerate the particles. Magnetic fields can not accelerate. Magnetic fields can change the direction
but they can do no work on the particles. So the magnetic fields
confine the particles. So let's assume we go twelve hundred
and twenty-five full rotations. During each rotation the kinetic energy
increased by forty KeV. And so if you multiply the two then you see
now that the kinetic energy of this proton increased by forty-nine million
electron volts, because it went twelve-hundred and
twenty-five times all the way around, and so now you have forty-nine MeV
plus the one MeV that you started with, so now you have
a fifty MeV proton. You see the second line there? There we have that fifty MeV proton
that I discussed with you earlier. In a one Tesla field
now the radius is one meter, so if this unit had a radius of one meter
that would be fine. By that time it would be all the way near
the circumference of this unit. What is remarkable and not intuitive,
that the time to go around as long as we don't have to make
relativistic corrections, that the time for a proton
to go around is independent
of its speed. Not so intuitive, and you can see
that very easily because the time to go around
is two pi R divided by its speed. You see, the radius
is proportional to V. And so the time itself
is independent of V, because R itself is linearly proportional
with the speed and so that cancels and so you'll find now
that the time to go around is simply two pi times the mass of that particle
divided by qB. And if you correct relativistically,
then you have to multiply by gamma, but if you stay non-relativistic, then,
it's independent of the speed of the protons. So if we stick to this particular case
of our one MeV proton that became a fifty MeV proton going around twelve hundred and
twenty-five times, this time to go around once
is only sixty-six nanoseconds, so this is six point six times
ten to the minus eight seconds. Give you some feeling
of how fast all this is going. So if you go around twelve hundred
and twenty-five times, that would take
only eighty microseconds so in eighty microseconds
does all of this occur and that means you have to
switch this field twice per full rotation, make sure that
the E-field is in this direction, but when the proton comes here
the e-field has to be in that direction. And so the switching frequency
which easily be calculated becomes about thirty million times per second,
about thirty megahertz. And all of that takes place in eighty microseconds
and you create one MeV protons, you turn them into fifty MeV protons. A mind-boggling concept,
but it works. Quite remarkable. Now because of the relativistic corrections
that you see here with gamma, if you go to very high energies then the time
is not constant for a full rotation, so you have to adjust now the frequency
with which you switch the potential between these gaps. So if the time increases then this
switching frequency has to go down and we call those instruments
synchrotrons or synchrocyclotrons. They have names. So you synchronize now
and correct for relativistic effects. Modern accelerators
have constant radii. They are rings. And so if you have a ring
with constant radius, the only way that you can keep the particles
in the ring when they have a low energy and when they have the high energy
is by gradually increasing the magnetic field. So you start off with a weak magnetic field,
you go around, huge circle, very large radius, and you gradually increase the magnetic field
as you keep accelerating them and by making the magnetic field go up
just in the right way, maybe all the way up to two Tesla, you can keep them in that ring. The first slide that I'd like to show you
is the slide of an ancient cyclotron it is actually a synchrocyclotron,
was built by Lawrence in Berkeley and this was capable of accelerating protons
to seven hundred thirty MeV. You see here a person to give you feeling
for the size of this instrument. Lawrence received the Nobel Prize for Physics
in 1939 for his invention of the cyclotron. The next slide is Fermilab
near Chicago. This is one of these modern accelerators
they're also called sometimes colliders and this has a diameter of two point two kilometers,
and this instrument, this year, plans to accelerate protons up to thousand GeV, G stands for giga,
giga is the same as billion. A thousand giga electron volts would be
ten to the twelfth electron volts. The beams of high energy protons
are made to collide with other nuclei to uncover the inner workings
of nuclear physics. The higher the energy
of the protons, the larger is the impact
when the protons collide and the more one expects to learn. By using ever-increasing energies of the protons,
which are nuclear bullets, one explores unknown territory. In the news, these colliders are often called
atom smashers. That is a flashier name
which appeals more to the general public who pays for all this
with their tax money. This research is a
multibillion dollar industry. The words atom smasher
are actually a misnomer. The colliders smash nuclei which are
ten thousand times smaller than atoms. And the next slide shows you the tunnel
of the largest ring in the world, which is in Geneva, at CERN,
which is a European collaboration. This tunnel which already exists
for many years has a confluence
of seventeen miles, has a radius of four point three
kilometers, and in here-- are these protons
being accelerated [inaudible] on the high vacuum. And with very modern techniques
of superconducting magnets they can even go up now
to about five Tesla. And in this tunnel right now
a whole new experiment is under development
which is called the Large Hadron Collider which is considered the Holy
Grail for particle physicists and it's hoped that that will go on the air
in the year 2007 or so and it will accelerate protons
to an unprecedented energy that will give them kinetic energies
of seven thousand GeV seven times ten to the twelfth
electron volts. Do you recognize me? Yes, I am Walter Lewin. And it is now 2013. A few months ago, the LHC made one of the most important
discoveries in particle physics of all times. One of the main objectives of the LHC
was to prove or disprove the existance of the Higgsboson which was already hypothesized
about 45 years ago. The Higgsboson is part of what's called the
Standard Model of particle physics. which is a set of rules
that lays out our understanding of the fundamental building blocks
of the universe. And the LHC has discovered
the Higgsboson. It has a mass of about 125 GeV And there is little doubt in my mind
that the Nobel Prize for this major discovery will be awarded very soon. Let's now go back to 2002. So I would like to return to my overhead there,
so that you can see some of that what we just discussed right there,
thank you Tom. So here we have Fermilab. You see a radius
of one point one kilometers. I showed you a picture from the air
and so they went up to one point five Tesla, so that's the maximum
magnetic field strength. Get very close to the speed of light
by the way [chuckles] and you see
five hundred GeV protons. And here you see the holy grail,
the Large Hadron Collider, European collaboration
in Geneva at CERN, whereby you have the circumference
of seventeen miles and the magnetic fields
that they hope to achieve going up to five
and a half Tesla using modern techniques
of superconductors. So if you want to go around in these tunnels
by the way, you need a motorcycle, you need to go seventeen miles around. The goal of all this physics,
of all these experiments, is to enter new territory, to learn about these
mysterious nuclear forces and to see what is inti--
inside protons and to see what is
inside neutrons. And with these experiments
many nuclear particles were discovered whose existence was
completely unknown previously. Now comes the issue
how can you see the results of these collisions of these particles
with very high energies. Well, you can make the tracks
of these particles visible. In fact, today you will see them
with your own eyes. And in the old days this was done
with cloud chambers and that's the demonstration
I will do today for you. But nowadays they do them
with bubble chambers. Let's first understand
the principle. If you had a charged particle,
whether it is an electron or a proton or an alpha particle-- alpha particle
is the nucleus of helium, it's two protons, two neutrons. If it goes through the air it makes ions
and as it goes through the air and it makes ions
it slowly loses its kinetic energy and it finally comes to a halt. If we take a ten MeV electron
and one atmosphere air it could go forty meters. If you take a proton of ten MeV
it would only go one meter because the density of ions is higher
because it has a higher mass and if you take an alpha particle
which has a higher mass than a proton--
and it has a double charge of a proton, then it would only go ten centimeters. It's a very high density track that you
would get from an alpha particle. And so one way you can see these tracks
is using cloud chambers, and a cloud chamber works
in principle as follows. You can just have a chamber in air,
one atmosphere air, which you put liquid alcohol in there, that's the way we will do it
and you cool the bottom. You see one there, which you
will see a little later and you cool the bottom
with solid CO2 and then you get inside this chamber,
you get a temperature gradient and there's a layer there where the alcohol
should really condense into little drops because it's that cold, but for reasons that are complicated
it doesn't do it quite yet. We call that undercooled alcohol. Even rain can be undercooled. Just below freezing point,
still liquid. When the moment it hits the ground it will
immediately become solid, by the way. That's also undercooled liquid. Now, here we deal
with an undercooled vapor and so when these ions are made
by these charged particles, these ions act as seeds for the drops,
in this case the alcohol drops, and you can literally with your eyes,
visually see these drops being formed. I'd like to go through one
numerical example and I want to go to a
five hundred keV electron which you see there. Um, notice that I have corrected
the speed relativistically, otherwise you would get this ridiculous number
that we calculated earlier which is larger than
the speed of light. And suppose we have a
one-tenths Tesla field. Then the radius would be
two point nine centimeters. But after a while this electron
will lose its energy and then there comes a time that it
has only hundred kilo electron volts left. By that time, the radius in a one-tenths Tesla
field would now be one point one centimeters and so when you look at cloud chambers
at the tracks of electrons and you have magnets there,
you will see the tracks being curled up which of course is the result of the fact
that the radius gets smaller in time and since the magnetic field is constant
you can see then, large radius here. And as the kinetic energy
slowly decreases the radius gets smaller
and smaller and smaller. So let's look at
a few more slides. In 1932, Anderson noticed
a track in a cloud chamber which had the appearance
of an electron. It had the right mass,
it had the right charge, but the curve-
the direction of curvature was wrong. And so he concluded that it was an electron
which was positively charged, which is now called a positron. And these positrons had been predicted
on purely theoretical grounds by Dirac, and Anderson received a Nobel Pri--
[break in file] discovery in 1936, only four years after
he discovered the positron and Dirac had already received his Nobel Prize
in 1933 for his theoretical work. The bubble chamber is an advanced
form of the cloud chamber. In the bubble chamber, liquid hydrogen is
used and if now the ions go through... The ions become the seeds now
for little gas bubbles. So you have liquid which really should
have been gas but, uh, not quite and so now it forms gas bubbles. So in a cloud chamber,
you're going to see the drops. In a bubble chamber, you see gas bubbles
but the idea is the same, and Glaser who invented these chambers,
is also from Berkeley by the way, he got a Nobel Prize
for that in 1960. So let's look at the discovery
by Anderson. Here you see a positron
coming from above, and this positron has 63 MeV--
kinetic energy and Anderson put some
half a centimeter of lead in there which was very clever,
think about that. When it comes out,
the energy now is less, because in the lead
it produces a lot of ions and so it loses a lot of kinetic energy
and it comes out with roughly 23 MeV. And why did he do that? Because now he knows for sure
that this particle came from above, because when it loses energy
the radius is smaller. That's why he was sure that it was curved
in the wrong direction. If he didn't have the lead, you never know
whether the electron came this way, in which case the curvature
would be perfect. But now he knows it comes from above
and if this had been an electron it would've curved this way. So this is one of the early discovery, cloud
chamber photographed by Anderson. And the next slide is a bubble chamber and
you see here both a positron and an electron in a constant magnetic field
and it speaks for itself, notice that the curvatures are
exactly in opposite directions and you see this spiral structure
that I discussed with you as the electrons lose their energy and
since this is a bubble chamber, which has an enormous density
a thousand times higher density, say, than air, these electrons don't travel forty meters
in these chambers. In air they would, but in this case
it is substantially less and so you can
roll them up nicely. And you can study them,
their momentum and their charge. Using accelerators and cloud chambers
and bubble chambers, a whole new world
of nuclear physics emerged. Wow. And between 1958 and 1968,
thirty new nuclear particles were discovered. And MIT has always been on the forefront
in this research. Professor Sam King, who is still at MIT,
got the Nobel Prize in 1976. Steven Weinberg, a theoretician who did his
work while he was at MIT got his Nobel Prize in 1979. Jerry Friedman, still at MIT and Henry Kendall,
got the Nobel Prize for their work in 1990. And Clifford Shull got his Nobel Prize
in 1994. If I summarize the basic idea behind it,
which is very relevant to 802, you accelerate these particles
using electric fields. That's the only way
you can accelerate them. Magnetic fields can only be used
to confine them. It can not change their kinetic energy but kinect-- electr--
magnetic fields are crucial, because that allows you as you gradually
increase their speed to confine them, either by a ring,
which is done nowadays, or in the old days in these chambers
of the cyclotrons and the synchrotrons. And then we have the bubble chambers,
in the old days the cloud chambers, whereby you use
magnetic fields to get information on the radius
of these particles as they are being detected. And out of all this emerged a whole new way
of looking at our world and completely new ideas
about what makes the world tick. This is nothing short
of a revolution. And so now I want to enjoy with you
the last five minutes of this lecture by looking at a cloud chamber
and by looking at some of these tracks. You're going to see a lot of electrons
in there. The walls of the cloud chamber are radioactive
just like you are radioactive. Your bones are radioactive,
your windows are radioactive. They emit electrons. No protons,
but they certainly emit electrons. And we have in there
a radioactive isotope, a rod, which has thorium in it
which produces alpha particles. And so you're going to see electrons which
make beautiful spider web structures. Please don't clean up yet,
we have plenty of time. To be precise, we have five minutes
and eighteen seconds left. So these electrons, you will see them going
like spiders through this chamber and sometimes they change abrupt
direction because they can collide, particularly when they have low energy
and then they [break in file] so to speak and then occasionally we may see
an alpha particle coming from our radioactive
thorium and that makes
a very thick track. And so let's try this. We have an expert here which is Marcos,
who has not only borrowed these instruments and if you like it we may actually buy it,
it's not cheap, we may buy it, but he borrowed it specially for you,
for which I'm very grateful Marcos and he also did quite a bit of work
to get the light just right. It's not too easy
to see those tracks. So Marcos, you will get a chance to adjust
the lights if you want that. And, we're going to make it very dark
and let's enjoy then, this wonderful world, invisible world
of nuclear physics. So here you see this um,
the rod with thorium so Marcos feel free to adjust the light
if you feel the need. I will go into the audience
as well and see whether
we can identify the electrons. Oh, there was
an alpha particle. So this is this rod and so the bottom of this
chamber is cooled with solid CO2. Ah, there was an electron,
very nice. As I said, you know, they almost look
like spider webs. There was an electron here. Also keep an eye on this rod and occasionally
you will see a very dense track which then indicates-- there was one,
that's an alpha particle. There's a beautiful
alpha particle. Just enjoy this. You know, you're looking at a world
which is completely new. Think about it. You're looking at the world
of nuclear physics. You're seeing individual electrons
and occasionally you see alpha particles. Here, there's one coming out. And think about the physics
what's going on here, this alcohol which refuses
to become drops and then these ions say,
"We force you to become..." uh -- here's an alpha particle,
I don't know what it's doing there. So these, these, um, these ions force these,
um, these, this, this alcohol vapor to become drops. That's an incredible, complex picture
that you're looking at. It is amazing every time
I see this. It's absolutely fabulous. And all through these very simple rules,
now think about it, we have the Lorentz force,
that makes these particles go around, electric fields, that you can use
to accelerate them and then this subtle way that you can
actually make them visible, individual particles,
visible. It is a new world. And the goal of my lecture was
to make you peek into this world which has revolutionized
our whole way of thinking. Thank you.
