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Nowadays, we don't really have to search
too hard for a motivation to study quantum

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physics.
There are plenty of experiments happening

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around the world every days which exhibit
quantum effects.

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But the situation, of course, was quite
different about 100 years ago or so, when

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quantum physics was born.
When people couldn't rely on such

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sophisticated techniques and methods and
manifestations of quantum physics we're

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very, very subtle.
In this video, I'm going to discuss two

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such pioneering experiments.
But before going to this part, let me tell

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you a little bit about the mood of the
scientists back then on the eve of the

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discovery of quantum physics, which in
fact was very pessimistic in that people

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believed that there was nothing else but
classical science and Maxwell's equations.

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Newton's equations and Newtonian gravity.
So to illustrate this, let me here, I here

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present two quotes by very influential
scientists Albert Michelson and Lord

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Kelvin.
For example, Albert Michelson in 1894, at

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the dedication ceremony for a physical
laboratory at the University of Chicago

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said the following, the more important
fundamental laws and facts of physical

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science have all been discovered, and
these are now so firmly established that

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the possibility of their ever being
supplanted In consequence of new

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discoveries is exceedingly remote.
A little later, Lord Kelvin says little

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later, there is nothing new to be
discovered in Physics now.

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All that remains is more and more precise
measurement.

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So you can see that even the most famous
and the most influential physicist didn't

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believe that there was anything new there
and they didn't expect any, anything new.

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An aside coming here let me mention that
Michelson actually received, in 1907 the

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Nobel Prize in Physics for his
groundbreaking experiments on the

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measurement of the speed of light, which
laid the foundation of the relativity

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theory.
So he obviously was proven wrong by

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himself, actually, but back at the end of
the nineteenth century, he and Lord Kelvin

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were not the only ones thinking in this
pessimistic way.

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So the reason for this was lack of obvious
experiments available at the time that

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wouldn't be explained by the classical
theory.

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So in some sense you may see, that the
situation was similar to what's going on.

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Now, actually, with the Large Hadron
Collider at CREN where all experimental

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data, including the discovery of the
god-particle or Higgs boson last year are

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actually consistent with the so-called
standard model.

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So hopefully, this station will change
soon and there will be some exciting new

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discoveries.
Just like the change back in the, back

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hundred years ago or so, when the few
elephants sort of entered the classical

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room, by which I mean experimental data
that couldn't be explained by classical

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theory.
So here I present the list of a few such

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experiments and two of them we're going to
discuss later in this segment, but among

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them is black-body radiation.
Quantization of atomic spectra which we'll

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talk about later in the course.
The photoelectric in fact we showed that

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under certain circumstances light can
behave as a beam of particles, and also

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there was another very curious and
interesting in it's history experiment

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which I refer as accident at Bell Labs
Shows electrons behave like waves which

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normally goes under the name of electron
diffraction.

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So I will now discuss in detail these two
experiments.

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Philip Leonard was a German physicist and
the winner of the 1905 Nobel prize in

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physics for his work on [unknown].
But back in the beginning of the 20th

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century, he was studying how ultraviolet
light interacts with metal by knocking off

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electrons from it.
This effect originally was discovered

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actually by Hertz in 1887.
It's now called the photoelectric effect.

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But originally it was actually called the
hearse effect.

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So if you represent a schematic Lenard's
experiment copied from his original paper

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published back in 1902, note as the year
here.

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In this experiment he had two metal
plates.

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Here is the left plate and the right
plate.

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Connected by an electric circuit so we
should imagine having an electric circuit

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here and one could adjust voltage across
the place.

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So in the photoelectric effect light comes
in from a source and we're here.

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And illuminates the metallic plate so in
certain conditions that we'll discuss in

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the second, electrons are emitted from the
metal and if their energy is large enough

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to overcome the potential difference here
they reach the right plate, which

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effectively closes the circuit and
resolves in a measureable electric

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current.
So, by adjusting the volt as you cross the

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blades one can measure the energy of the
metered electrics this way.

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So, what's very important is that, that if
we rely on the classical theory of light

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which views it as an electromagnetic wave
of this of this [inaudible] here's and

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expression for the electromagnetic wave
which we'll see actually pretty often in

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this course.
So the classical theory would clearly

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predict, and this is very important, that
increasing the intensity of the light

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should lead to more energetic
photoelectrons.

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So the more intense the light, the more,
the higher the voltage that the electrons

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would be able to overcome.
Also, the classical theory would be that

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the, that intense light of any frequency
should be able to keep some electrons off

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of the metal plate.
So these are very clear cut predictions

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and there was no way around them, and the,
The framework of classical physics.

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However this was not at all what Lenard
observed in his experiment.

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As a matter of fact his experiment
observed exactly the opposite.

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So first of all, the energy of the emitted
electrons didn't depend at all on the

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intensity of the light which it wasn't
share our contradiction with the classical

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physics.
And also very importantly, no

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photoelectrons were produced if the
frequency was smaller than the certain

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critical value, if the frequency of the
light was smaller than some, I mean, the

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critical, there was nothing, no effect was
observed.

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The electrons would didn't reach the right
plate.

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And so this was a major mystery at the
time and got actually Albert Einstein

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thinking about it.
So it's probably not very surprising that

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this mystery attracted Einstein's
attention as he was obviously thinking

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about the properties of light at the time,
as we know.

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So, three years after Lenard's paper in
the year of 1905 Which is often called the

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miracle year, because in 1905 Einstein
wrote 4 amazing papers that have

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completely redefined the foundations of
physics, and one of these papers was is

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this paper that we're now discussing,
concerning an Heuristic Point of View

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Toward the Emission and Transformation of
Light.

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So in this paper Einstein basically
introduced the notion of the photon which

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resolved the mystery of Linear's
experiment.

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So here I have a long quote from
Einstein's paper, these are Einstein's

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words in the paper.
So let me read it, the usual conception

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that the energy of light is continuously
distributed over the space through which

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it propagates Encounters very specific,
serious difficulties when one attempts to

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explain the photoelectric phenomena, as
has been pointed out in [inaudible]

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Lenard's pioneering paper, the one we
discussed in the previous slide.

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And then he goes o nto the main concept of
the photons, so according to the concept

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that the incident light consists of energy
quanta of however one can conceive of the

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ejection of the electrons [unknown]
energy, quanta [unknown] to the surface

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layer of the body in their energy is
transformed at least in part into kinetic

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energy of electrons and ec etera.
Notice that Einstein didn't really call

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his proposal a theory but rather a recent
point of view, which it was [unknown]

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picture but the very important one.
Because it introduced the notion of

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photon, a particle of light carrying a
quantized energy.

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And so here is, I show little animation
which illustrates sort of Einstein's view

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of what might be happening in the
photoelectric effect.

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And in this picture, we have this
particles, so once again, we have these

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particles, which essentially represent
light.

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And each, each particle of light, each
photon interacts individually with

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electrons.
And therefore only if the energy of a

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single photon exceeds a certain threshold,
a photoelectric effect would occur.

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And also this feature explains why the
effect was not dependent on the intensity

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of light or, in other words, on the number
of photons hitting this surface per a unit

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of time.
So another important element of the theory

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was the assumption that the frequency of
the light [inaudible] here, was

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proportional to the energy and the
coefficient of proportionality between the

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energy and omega is we now know is the
Planck constant.

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So there's actually going to be two
notations for Planck constant.

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We use H and H bar, so H bar we're going
to use a little more often.

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And the relation between them is just this
numerical factor of 2 pi.

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And in any case, so in this picture
basically it was clear why the frequency

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of the light was the key.
So the frequency of the light was related

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one to one.
The energy of these photons and the

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electrons would either be able to overcome
the voltage here or not, depending on

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whether or not the frequency was high
enough.

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And this essentially on resolve the
mystery of, behind the photo electric

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effect.
So an interesting comment here is that

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Einstein received his 1921 a Nobel Prize
in physics mostly for, formal at least,

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for his work in the photoelectric effect,
here is actually citation for the Nobel

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Prize.
Actually, I think it's fair to say for as

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important as this insight turned out to
be, the photoelectric effect.

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The theory of photoelectric effect, his
other achievements in developing special

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and general relativity are even more
impressive.

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So finally let me mention here, that
ironically Einstein, being one of the

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pioneers of quantum theory, remained
skeptical of quantum mechanics throughout

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his life, and never fully accepted