So let's recap. We've had a look at the
theory of quantum mechanics as evidence
through the two-slit experiment and
we've had look at the history of quantum mechanics.
And to recap the theory;
everything's a wave. An electron is a wave,
a nucleus is a wave, a molecule's
a wave, an atom's a wave, you're a wave,
I'm a wave, the room is a wave. And
waves aren't exactly what we thought
they were, they do slightly strange
things when you measure them, and this
theory although it does sound a little
strange to begin with turns out to
explain to a dozen significant figures
or more every property of everything
we've ever measured, and has no
known bugs. Now we saw, especially through
the history, how it explained the
existence of atoms and how they work, we
saw how it explained absorption and
emission the photoelectric effect
interference and so forth, but our
history only went through about the
1920s where the modern form of quantum
mechanics finally emerged. So what
happened after that? So after that, the
next major advance happened when we
tried to marry the theories of
relativity with quantum mechanics.
Now not only did that help us get the
details right so that we could get a
dozen digits of significant figure
agreement between theory and experiment,
it also actually put a lot of
restrictions on our quantum mechanical
theories. In order for our wave evolution
to obey the principle special relativity,
there are only certain theories that
would work and turns out that some of
those theories actually predicted
particles that we'd never seen before
and then we went and looked for those
particles they were there
and this is the first time that
completely new things have been
discovered from theory rather than from
experiment first. The normal role of
science was that things would be
observed and then theories would be
developed to explain those things, and
then those theories would be tested by
further experiments. In this case that
principles behind the theory suggested a
slightly different version of the theory,
and that version of the theory made
predictions that turned out to be
correct and we saw completely new
phenomenon in other words entirely new
particles in nature that we hadn't seen
before, and it didn't stop there.
Quantum field theory, which is what this
theory is called when you marry
relativity with quantum mechanics, has
predicted many new particles which have
been observed, and also new forces and
relationships between those forces all
of which have been verified to the best
of our ability. And our best model of all
these particles and forces is called the
standard model and you can learn more
about the standard model in an upcoming
module. The next thing that improved over
the last hundred years since quantum
mechanics was first invented, was our
understanding the correspondence
principle. The idea here is that the
quantum world where everything is a wave
somehow has to correspond to the world
we're used to for very large things.
So even if you start to accept the
evidence that an electron is a wave that can go
through two slits at the same time, the
idea that we are made of electrons and
things that can go through two slits at
the same time, the logical conclusion of
that is that we're a wave and that we
could go through say two doors at the
same time. We kind of reject that
idea because we've never experienced
anything like that, and we now have a far
better idea of exactly how quantum
mechanical behavior can lead to things
that look like classical behavior at
large scales. In the early days of
quantum mechanics, board took the rather
controversial view that the quantum
world and the classical world were
distinct.
He viewed that quantum mechanics described
the world of microscopic things so atoms
and subatomic particles and things like
that, but the classical world where you
have cats and trees and flowers, is a
very different beast and was described
differently. In other words he didn't
think that actually there was a
continuous picture that you could go
from one to the other. A lot of people
had trouble with that because it seemed
like an artificial boundary. If you
describe one atom with quantum mechanics,
and 10 to the 23 as classical mechanics,
what do you do with a hundred thousand
or a million or a billion? There
must be some kind of transition, and if
our model changes so dramatically from
one kind of description to the other,
then that gives us a kind of problem at
the boundary, and this is more of a
problem these days because we're very
deliberately engineering large quantum
states. We're building quantum systems
that have large numbers of particles or
large spatial extent, and what that means
is we really need to know how to
describe them. And thus far quantum
mechanics has always worked, and so we've
tried to resolve this conundrum of
having two different pictures for small
things and big things by trying to see
how the description for small things can
extend to the big things and look like
the world that we're used to. This is
particularly important when looking at
the most confusing of the postulates of
quantum mechanics which is this
mysterious "wavefunction collapse", which
is where the behavior of particles seems
to depend on whether you're looking at
them or not.
It's where the wave is traveling along
as a wave, it can go through multiple
slits or whatever else it does, and then
when you look at it it suddenly jumps
and the wavefunction changes. And even
over the last couple of decades we've
had an improved understanding of exactly
how a quantum system interacting with the
things around it leads to this apparent
wavefunction collapse. And the final big story of
recent developments in quantum mechanics
is that we're starting to really use the
quantum properties of things in
engineering. We're trying to build
quantum devices that really use this
wave-like nature to do things that you
can't do with classical devices.
So for many decades we've needed to understand
quantum mechanics in order to understand
the properties of materials and things
like that, but now we're using the stranger
properties of quantum mechanics as
fundamental parts of our design.
Hello, I'm still Kyle Hardman and we're in the quantum sensors lab at ANU, and in here
is still the coldest place in Canberra,
possibly Australia.
Hi Kyle, hello Joe, hi.
It seems like there's a lot of effort
going here to make these atoms really
cold why do you do that? Well we go
through all that effort so that the
wavelength of the atoms become very very
large. We can then take the atoms and
make them go through two separate paths
and interfere them at the bottom.
The interference pattern is strongly
correlated to the acceleration of
gravity and because of that we can make
a precision measurement of the acceleration
of gravity. Right, so using quantum
mechanics for good. Using quantum
mechanics for good. Right.
