Wave particle duality is one of the fundamental principles of quantum mechanics. It says that
to every wave, like electromagnetic waves or even acoustic waves, there is a particle,
a quantum of energy of the wave, associated with it and to every particle, there is a
wave that's associated with the motion of the particle. This is a text book example
of quantum mechanics and people are used to thinking of it as something that is very basic,
very fundamental and very microscopic, but today, I want to tell you something that's
happening in the borders of scientific research where we are trying to bring this fundamental
principle of quantum mechanics closer than you would ever think to some objects that
actually matter to your life.
Before I start, let's take the classic example of wave particle duality, in fact, the wave
interference phenomenon that is normally associated with things like light which is an electromagnetic
wave. So let's say you take a source of light and you have a screen with two small apertures.
This is the famous double slit experiment that you will find in many excellent videos
on the internet. Light is an electromagnetic wave that is characterised by a certain colour,
as you know, which corresponds to a certain wavelength. So the electromagnetic field associated
with the light has a maxima and minima of intensity that are separated by a certain
distance that's called the wavelength. Once the light passes through the slits, then each
slit becomes like an emitter of the light again and so the two illumination points start
to create light that interferes with itself and therefore if you look at a screen behind
these two slits, you will not see two spots of light just behind the splits, but you will
see a complicated pattern that will have a maximum of intensity in the middle and then
some weaker intensity spots to the sides. And the distance between these peaks of intensity
depends on the wavelength of the light and on the geometry of the system. So visible
light is characterised by wavelengths that are of the order between 400 and 700 nanometres.
To give you an idea of what a nanometre is, remember that one atom is about 0.3 nanometres
in diameter and the human hair is 20,000 nanometres. So if you take let's say red light, which
is about 700 nanometre wavelength, there will be 30 maxima of the electromagnetic field
of a red light within the thickness of your hair. So with these numbers, let's say you
take a millimetre here, and one metre distance between the slits and the screen, what you'll
find is that the distance between these interference fringes is given by this formula here and
so you have this amplification factor that can easily be one thousand right, if you have
a millimetre here and a metre there this is one thousand times the wavelength so these
peaks will be separated by half a millimetre almost, so it's easy to see.
Now things become a lot more profound and sophisticated when you try to do the same
experiment with matter, with objects, that classically people are used to thinking of
as particles, but in fact, they also have a quantum mechanical wave associated with
them. So let's say you repeat the same experiment where instead of a source of light, you have
a source of particles. Let's take protons for example. So let's say you have a proton
gun and again a screen with two slits and a screen at the back. So your classical mind
will tell you that this proton will be some particle of mass, M, and velocity, V, and
it can only go through one of the two slits so let's say it goes this way, you will find
the proton here. If it goes this way, you will find the proton at the bottom side of
the screen. In fact, this is not what happens. The proton is, like every particle, associated
to a wave and in fact, you will find that the positions where the protons arrives on
the screen in this experiment, reproduce this interference pattern that you see with real
waves like light. So you will find many protons amassing themselves in the middle, behind
this screen here, and then some others here and then some others here and then some others
there and so on. The same pattern as you see with light. But this has actually a very profound
meaning. What it means is that the proton has gone through both slits. In other words,
you have a particle that has been in two places at the same time. People who are used to quantum
mechanics make peace with this idea. They say, "okay it's a very small subatomic particle,
it behaves in the weird quantum mechanical way, okay." But now, people are trying to
do these experiments with much larger particles. That's where it becomes interesting.
Again, what does it take to see this interference pattern? Same story, you need to know what
is the wavelength of the wave associated with the proton. If you do the maths, you put in
the mass of a proton and you take a velocity for example, of one metre per second, very
slow velocity. The formula for the wavelength tells you the wavelength is the Planck constant,
the fundamental constant of quantum mechanics divided by the mass of the velocity. In this
case, for the proton you will get a 400 nanometre wavelength which is the same as violet light
with a velocity of 1 metre per second. With the same apparatus, you can see the quantum
interference over proton which means the proton has been in two places at the same time. But
this is actually not too hard to see, so can we push it a little bit harder?
Can we try and see the quantum interference that the wave particle duality of something
bigger. This has been done already several years ago. People have managed to see the
quantum interference with a molecule called carbon sixty or fullerene or buckyballs. It's
called a buckyball because it looks like a soccer ball, it's got six carbon atoms in
a pattern that reminds us of a soccer ball. And this is a molecule that has an atomic
mass unit of 720. So it has 720 protons and neutrons. It's a fairly large and fairly heavy
molecule, and yet, if you make this distance very small and this distance fairly large,
you can still see the interference pattern.
The current record is 430 atom molecule. It's a complicated molecule that I'm going to describe
made out of 430 atoms and 6 nanometres in diameter. Six nanometre is a small size, but
it's actually not that small. If you take the smallest virus know, it's called porcine
circovirus, it's 17 nanometres. The smallest virus which you could call the smallest living
creature, although some biologists would not agree with that, is only a factor three larger
and about a factor a thousand heavier, than the largest molecule for which people have
been able to see this quantum interference which means this large molecule has crossed
through the two slits at the same time. Now imagine if you could do that with a virus,
with a living creature, and you could show that it goes through two different places
at the same time. This is what quantum mechanics in principle tells you should be possible.
Whether we will be able to demonstrate that or not, this has not yet happened with viruses,
depends simply on the progress of the fabrication and the construction of scientific instruments
that are allowing us to get more and more insight into how the quantum mechanics of
some atomic particles can move towards our daily life.