Nowadays, we don't really have to search
too hard for a motivation to study quantum
physics.
There are plenty of experiments happening
around the world every days which exhibit
quantum effects.
But the situation, of course, was quite
different about 100 years ago or so, when
quantum physics was born.
When people couldn't rely on such
sophisticated techniques and methods and
manifestations of quantum physics we're
very, very subtle.
In this video, I'm going to discuss two
such pioneering experiments.
But before going to this part, let me tell
you a little bit about the mood of the
scientists back then on the eve of the
discovery of quantum physics, which in
fact was very pessimistic in that people
believed that there was nothing else but
classical science and Maxwell's equations.
Newton's equations and Newtonian gravity.
So to illustrate this, let me here, I here
present two quotes by very influential
scientists Albert Michelson and Lord
Kelvin.
For example, Albert Michelson in 1894, at
the dedication ceremony for a physical
laboratory at the University of Chicago
said the following, the more important
fundamental laws and facts of physical
science have all been discovered, and
these are now so firmly established that
the possibility of their ever being
supplanted In consequence of new
discoveries is exceedingly remote.
A little later, Lord Kelvin says little
later, there is nothing new to be
discovered in Physics now.
All that remains is more and more precise
measurement.
So you can see that even the most famous
and the most influential physicist didn't
believe that there was anything new there
and they didn't expect any, anything new.
An aside coming here let me mention that
Michelson actually received, in 1907 the
Nobel Prize in Physics for his
groundbreaking experiments on the
measurement of the speed of light, which
laid the foundation of the relativity
theory.
So he obviously was proven wrong by
himself, actually, but back at the end of
the nineteenth century, he and Lord Kelvin
were not the only ones thinking in this
pessimistic way.
So the reason for this was lack of obvious
experiments available at the time that
wouldn't be explained by the classical
theory.
So in some sense you may see, that the
situation was similar to what's going on.
Now, actually, with the Large Hadron
Collider at CREN where all experimental
data, including the discovery of the
god-particle or Higgs boson last year are
actually consistent with the so-called
standard model.
So hopefully, this station will change
soon and there will be some exciting new
discoveries.
Just like the change back in the, back
hundred years ago or so, when the few
elephants sort of entered the classical
room, by which I mean experimental data
that couldn't be explained by classical
theory.
So here I present the list of a few such
experiments and two of them we're going to
discuss later in this segment, but among
them is black-body radiation.
Quantization of atomic spectra which we'll
talk about later in the course.
The photoelectric in fact we showed that
under certain circumstances light can
behave as a beam of particles, and also
there was another very curious and
interesting in it's history experiment
which I refer as accident at Bell Labs
Shows electrons behave like waves which
normally goes under the name of electron
diffraction.
So I will now discuss in detail these two
experiments.
Philip Leonard was a German physicist and
the winner of the 1905 Nobel prize in
physics for his work on [unknown].
But back in the beginning of the 20th
century, he was studying how ultraviolet
light interacts with metal by knocking off
electrons from it.
This effect originally was discovered
actually by Hertz in 1887.
It's now called the photoelectric effect.
But originally it was actually called the
hearse effect.
So if you represent a schematic Lenard's
experiment copied from his original paper
published back in 1902, note as the year
here.
In this experiment he had two metal
plates.
Here is the left plate and the right
plate.
Connected by an electric circuit so we
should imagine having an electric circuit
here and one could adjust voltage across
the place.
So in the photoelectric effect light comes
in from a source and we're here.
And illuminates the metallic plate so in
certain conditions that we'll discuss in
the second, electrons are emitted from the
metal and if their energy is large enough
to overcome the potential difference here
they reach the right plate, which
effectively closes the circuit and
resolves in a measureable electric
current.
So, by adjusting the volt as you cross the
blades one can measure the energy of the
metered electrics this way.
So, what's very important is that, that if
we rely on the classical theory of light
which views it as an electromagnetic wave
of this of this [inaudible] here's and
expression for the electromagnetic wave
which we'll see actually pretty often in
this course.
So the classical theory would clearly
predict, and this is very important, that
increasing the intensity of the light
should lead to more energetic
photoelectrons.
So the more intense the light, the more,
the higher the voltage that the electrons
would be able to overcome.
Also, the classical theory would be that
the, that intense light of any frequency
should be able to keep some electrons off
of the metal plate.
So these are very clear cut predictions
and there was no way around them, and the,
The framework of classical physics.
However this was not at all what Lenard
observed in his experiment.
As a matter of fact his experiment
observed exactly the opposite.
So first of all, the energy of the emitted
electrons didn't depend at all on the
intensity of the light which it wasn't
share our contradiction with the classical
physics.
And also very importantly, no
photoelectrons were produced if the
frequency was smaller than the certain
critical value, if the frequency of the
light was smaller than some, I mean, the
critical, there was nothing, no effect was
observed.
The electrons would didn't reach the right
plate.
And so this was a major mystery at the
time and got actually Albert Einstein
thinking about it.
So it's probably not very surprising that
this mystery attracted Einstein's
attention as he was obviously thinking
about the properties of light at the time,
as we know.
So, three years after Lenard's paper in
the year of 1905 Which is often called the
miracle year, because in 1905 Einstein
wrote 4 amazing papers that have
completely redefined the foundations of
physics, and one of these papers was is
this paper that we're now discussing,
concerning an Heuristic Point of View
Toward the Emission and Transformation of
Light.
So in this paper Einstein basically
introduced the notion of the photon which
resolved the mystery of Linear's
experiment.
So here I have a long quote from
Einstein's paper, these are Einstein's
words in the paper.
So let me read it, the usual conception
that the energy of light is continuously
distributed over the space through which
it propagates Encounters very specific,
serious difficulties when one attempts to
explain the photoelectric phenomena, as
has been pointed out in [inaudible]
Lenard's pioneering paper, the one we
discussed in the previous slide.
And then he goes o nto the main concept of
the photons, so according to the concept
that the incident light consists of energy
quanta of however one can conceive of the
ejection of the electrons [unknown]
energy, quanta [unknown] to the surface
layer of the body in their energy is
transformed at least in part into kinetic
energy of electrons and ec etera.
Notice that Einstein didn't really call
his proposal a theory but rather a recent
point of view, which it was [unknown]
picture but the very important one.
Because it introduced the notion of
photon, a particle of light carrying a
quantized energy.
And so here is, I show little animation
which illustrates sort of Einstein's view
of what might be happening in the
photoelectric effect.
And in this picture, we have this
particles, so once again, we have these
particles, which essentially represent
light.
And each, each particle of light, each
photon interacts individually with
electrons.
And therefore only if the energy of a
single photon exceeds a certain threshold,
a photoelectric effect would occur.
And also this feature explains why the
effect was not dependent on the intensity
of light or, in other words, on the number
of photons hitting this surface per a unit
of time.
So another important element of the theory
was the assumption that the frequency of
the light [inaudible] here, was
proportional to the energy and the
coefficient of proportionality between the
energy and omega is we now know is the
Planck constant.
So there's actually going to be two
notations for Planck constant.
We use H and H bar, so H bar we're going
to use a little more often.
And the relation between them is just this
numerical factor of 2 pi.
And in any case, so in this picture
basically it was clear why the frequency
of the light was the key.
So the frequency of the light was related
one to one.
The energy of these photons and the
electrons would either be able to overcome
the voltage here or not, depending on
whether or not the frequency was high
enough.
And this essentially on resolve the
mystery of, behind the photo electric
effect.
So an interesting comment here is that
Einstein received his 1921 a Nobel Prize
in physics mostly for, formal at least,
for his work in the photoelectric effect,
here is actually citation for the Nobel
Prize.
Actually, I think it's fair to say for as
important as this insight turned out to
be, the photoelectric effect.
The theory of photoelectric effect, his
other achievements in developing special
and general relativity are even more
impressive.
So finally let me mention here, that
ironically Einstein, being one of the
pioneers of quantum theory, remained
skeptical of quantum mechanics throughout
his life, and never fully accepted