The quantum attraction
I will talk about today has nothing to do with people being attracted to the
to each other because of some quantum phenomena although
it will enter in some living creatures
life which is the Gecko sticking up glass walls it's an incredible feature
that the Gecko has
and is based upon a true attractive force which is a quantum-mechanical
origin it's called the Van Der Waals
force and I will try to explain to you in brief where it comes from
let me get back to the really simple basics of
attractive forces everybody knows that if you take a
positively charged particle and a negatively charged particle
they attract each other, opposite charges attract
is called the Coulomb force everyone knows that the next level of
sophistication
is to take dipoles, a dipole is
two charges of opposite sign
held at some distance from each other so
you can imagine that if you have two dipoles they will
attract each other if they are oriented in the right direction so in this case
there will be a
attractive force between the positive and negative here
that is stronger than the attractive force between these ones because they're
for their way through the two dipoles would like to click together
along these axis a real-life example
of an electric dipole is water
water is H2O. It's made of an oxygen
atom and two hydrogen atoms. The thing that keeps the water molecule together
is that covalent chemical bond where electrons are shared between the
hydrogen and the oxygen
but in fact there is a little bit more electron density
on the oxygen side than there is on the hydrogen side
so the water is like a negative charge on the side of the oxygen and a positive
charge
on the side of the hydrogen so the reason why water is liquid at room
temperature is because
water molecules like to be together they attract each other through this force
and when you go below zero degrees centigrade actually freezes and forms a
solid
because of these attractive forces between dipoles.
Now let's go to yet the next level let's try to find out what is the force
between neutral particles
and the simplest example of that is a hydrogen atom, a hydrogen atom is one
positive charge a proton
and the negative charge an electron
orbiting around the proton in fact it doesn't quite orbit around the proton
like the moon around the earth but is
spread out in a wave of probability
around the proton and this wave of probability is spherically
symmetric is the same in every direction now let's imagine you take another
hydrogen atom
nearby and you ask yourself
is there any force between these two hydrogen atoms
well you would be tempted to say that there isn't
because this atom is overall charged neutral there's a positive and a
negative charge canceling each other the same for this one
you would say well but this proton attracts to that electron yes but it
also repels equally strongly
with this other proton so if this was the end of the story there would be no
force between
two neutral hydrogen atoms in fact things in a bit more subtle than that
this wave of probability of the electron around the proton
has quantum fluctuations so the actual position
of the electron around the proton fluctuates
in time so you can well imagine that at some point in time
the first proton could have
an electron cloud around it
that is a little bit skewed towards the right
and if the same happens at the same time
with the hydrogen
on the right hand side then what you have
is two dipoles right, because this is essentially
like a dipole that is positive on the left a negative on the right and the
same on this one
so these two dipoles now attract each other
but these fluctuations of the electron cloud are random
so on average the electron is around the middle so
there could be some time where these two
hydrogen atoms attract each other but then that could be another time when the
cloud of the electron on the right hand side
hydrogen is on skewed to the left and so they actually repel so on average you
would think
they still don't have any force between each other
the interesting thing is that when you put them really close together
you get some form of correlation between the quantum fluctuations of the two
electron charges
so in other words if you calculate the average position let's say along the
X axis the horizontal axis of these electron
the average value is zero
and the same for this one
but if you calculate the product
of the position of electron number one and the position of electron number two
so x1 times x2
on average is non-zero
when the fluctuations are correlated meaning that
the fluctuation of one electon cloud
essentially induces the fluctuation of the other electron cloud so they go
together
and when they do so there is a net attractive force
between these atoms that are otherwise charged neutral
This is a very important very fundamental microscopic force
of pure quantum mechanical origin and the amazing thing is that there are
large
living creatures that use it. The geckos are capable of climbing
on glass walls. They stick to the glass with an incredible
force and what they use is precisely a microscopic version
of this quantum mechanical force and now a day there are people who are trying
to build robots
that artificially exploit this force
to climb for example on glass walls