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Hi, in this video, I will introduce to you, or at least attempt to introduce to you, the origin story of quantum mechanics
let me begin by saying welcome to the 20th century because
unless if you've done special relativity, this is probably your first
view of physics discovery that are done in the 20th century
now
this is an origin story, it is a story so we won't be super
rigorous and technical and everything but I want to show you that
quantum mechanics is not something that a bunch of physicists sat around a room and
somehow, with really wild imagination and made this thing together to torment students for eternity
what it is a story of: really interesting and weird observations; fairly hard intellectual struggle;
some ad hoc explanations; ultimately leading to a more coherent framework as we know as quantum mechanics
I will begin by conceding to the great
Richard Feynman
he said that ...
I will refer you to the actual video where I got this from, if you follow this link here
he does a very good job setting you up for
how to view this quantum theory as a whole and approach it on the outset
I suggest doing 20 to 35 minutes, but he's quite captivating so
I won't blame you if you go all the way, but it is an hour long
I'll let you do that now
I hope that was fairly entertaining and insightful for you guys. Our story back here now begins in the 1880's
This was probably the days of perhaps your grandfather's grandfather or your grandmother's grandmother of course
and surprisingly, it doesn't seem so far back, but the world was a very different place back then
there were no widespread automobile, as the first manufactured automobile was in 1886 by Benz
telephones were just invented, there was telegraphs but not telephones
widespread electric generations start coming in 1882
and people if they didn't think you can fly
in any controlled manner until 1903, so obviously a very different world
so not only do everyday life look very different because there's none of these everyday necessities
that we think of nowadays
but
the idea of the electron and the nucleus
hasn't been discovered yet. Can you imagine a world where people don't know about electrons and nucleus?
In any case,
that's going to all change in te next 50 years
we have from 1880s to 1930s, and this next 50 years a lot of exciting
things happened in the world of physics and completely changed the face of it
so first, the coming about of quantum mechanics
is the interplay between the electron and light
or so called the photon as we come to know them.
On the right hand side, here I have summarized some of the major steps that establishes
different discoveries about the electron, you have
first of all, discovering the electron, and then
Planck goes on to explain blackbody radiaion saying
electrons can only have certain energy levels
thus starting the "quantization" that we will get to later, and the Rutherford discovers the nucleus, leading to the Bohr model of
the atom
and we're confirming that electrons can have specific energies level only
once they are in the atom. So that's the electron side, the photon side, we have the photoelectric effect
which then Einstein combines with Planck's idea to explain, thus coming up with
how electromagnetic radiation
their energy is also quantized, similar to how electric charge is quantized, in that you can't have less than one electron worth of charge
you can't have less than one of these steps worth of EM energy as well and ultimately
Compton through Compton scathering
establishes that
the light is actually a particle and that eventual the term "photon" was coined
and from that
De Broglie came up with this idea of matter waves
and Schrodinger [and Heisenberg] and Dirac come up with a good framework with which we build our understanding of
quantum mechanics today
So despite everything I have said up to now, 1880s isn't exactly the dark ages of science, at this point
physicists actually know quite a few things: we have Newton's laws of motion,
we have Maxwell's electromagnetism and we have thermodynamics
and statistical mechanics is just coming online around this time
these, all together, do a very good job at explaining
macroscopic phenomenon, things we can see: motion of a car
lighting up a light bulb, how things heat up, steam engine and such and such
All those are very well explained and we still use them today
as a testament of the fact that you've spent all these years learning about all these stuff
but this is about the turning point of when we can start looking at more microscopic phenomena
and there the observations are really quite weird, they can't be explained by these theories
and thus starting the whole "revolution"
at this point, the view of matter and lights are very separate. You have matter which is made up of particles
particles in the sense that they are atoms and molecules because the electron hasn't been quite discovered yet
but
they are countable, in the sense that you can't
have an infinitely small chuck of matter, at some point you can't cut up matter anymore
and an atom is the basic unit of that, you can't have half an atom, say
whereas light is thought of as an EM wave
which we have dealt with earlier in the course
and this wave, because it is a wave, has a large spatial extent, unlike a particle
which is more or less localized, the wave stretches out
for big vast of space, and you can make
the energy smaller, you can have a smaller [amplitude] wave, and then smaller, you can have a smaller wave
and so on and so forth and the thought was you can make the wave as small as you want
everything is nice and continuous
In 1900, that view suddenly get brought into question