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