Bird navigation- The Quantum Around You
You may have heard about bird navigation.
There are some species of birds that in some seasons of the year, migrate, for example
from north to south or from south to north, to follow the seasons and the climate that
they prefer.
And for a long time, people have been wondering how do these birds navigate?
How do they know which way to fly?
There is a very fascinating hypothesis in science, in biology, and in quantum mechanics
nowadays, that certain types of birds use quantum effect to find what is the direction
of the Earth's magnetic field going from north to south.
So to understand how this works, I'm going to start from a very, very basic principle
of physics and of quantum mechanics, which is the Pauli Exclusion Principle.
Let's take for example a helium atom.
A helium atom has two positive charges in the nucleus and two electrons orbiting around
the nucleus.
Now, you shouldn't think of these two electrons as moons orbiting around a planet, they are
actually waves of probability around the nucleus and so in fact, they occupy the same space,
the same cloud around the nucleus and because of that, they need to obey what's called the
Pauli Exclusion Principle which tells that their magnetic dipole needs to be anti-aligned.
Electrons not only have a negative charge, but they also have a magnetic moment that's
called the spin, although nothing is spinning in this case and the Pauli Exclusion Principle
tells you that the spins must be pointing in the opposite direction.
Now let me clarify here, that when I draw one spin pointing up and one spin pointing
down, this is not a proper representation of the state that the electrons are in.
These states, these electrons, are in what is called an entangled quantum state.
Entanglement is a very special form of correlation that exists only in quantum mechanics where
two particles are correlated with each other but they lose their individuality.
So in this case, when I draw a spin pointing up and a spin pointing down, this is not the
true entangled state.
The true entangled state is one where the two spins are opposite to each other but they
don't have a state of their own.
It's like taking two people and saying they have the opposite gender but they don't have
a gender of their own.
Their gender is only defined in the correlation between each other, but not in the individual.
This is in fact, the kind of quantum state that two electrons sharing an orbit will have
to be into.
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Now imagine you can separate these two electrons by some means, then these electrons can move
individually and flop around and precess and move and so after some time, you might find
yourself in a situation where the two electrons are now pointing parallel.
�Now, if that were the case, these two electrons could no longer go back together onto the
helium atom.
�The Pauli Exclusion Principle excludes that.
�The interesting thing is that there is a hypothesis, and I might stress, this is
just a hypothesis, it's not easy to prove it although there is some good evidence, that
certain birds, in particular the European Robins, use these quantum mechanical correlation
to find the direction of the Earth magnetic field.
And the way it's presumed to work is this: the receptor is the eye of the bird.
Imagine, this is the eye of the bird.
There is a retina in the eye of the bird.
This retina contains some complicated molecules that are connected to the optical nerve of
the bird and these molecules may have some pairs of electrons that share the same orbit.
So imagine there's two electrons that are bound together and share the same orbit.
Because of the Pauli Exclusion Principle, they have to have opposite spin, which I'm
drawing like this, but remember this drawing is not the real thing because I cannot classically
draw an entangled state.
The next thing that happens is that if a quantum of light, a photo, hits the retina of the
bird, the energy of that quantum of light can split the two electrons apart.
So now I have these two electrons that are pointing in opposite directions.
But now on Earth, we have a magnetic field.
It's the Earth's magnetic field so I can draw (I draw it as an arrow like this) magnetic
field is normally indicated with the letter B and in this magnetic field, these spins
will start to precess.
They will wobble around like a gyroscope.
It's difficult to explain it in a drawing and it is very likely to involve more spins
than just these two ones, but the hypothesis is that depending on the orientation of the
bird with respect to the Earth's magnetic field, these two spins may or may not remain
in this quantum correlated quantum entangled state.
So now you have two possibilities.
If they remain in the entangled state where they have the opposite spin, they can recombine
together in their molecular orbital and when they do so, they release back the original
energy that was given by the photon.
So in doing so, in recombining, they give out a stimulus to the optical nerve of the
bird.
Whereas, if the orientation is wrong, these two spins end up parallel and they can no
longer recombine so the optical nerve of the bird does not receive the stimulus.
The reason why this is so fascinating is not only because it is a possible quantum phenomenon
that takes place in a living creature, in a bird, but also, because it makes you think
that there is an intelligent living creature that can see the quantum entanglement.
I will keep talking about the quantum entanglement over and over in this series of videos, but
every time I will try to do so, I will struggle to visualise it for you because we cannot
see the quantum entanglement.
I cannot draw on the board a pair of entangled particles.
Our eyes don't see it.
But maybe, the eyes of these birds can see it.
You see, our eyes can only see light photons of the colours of the rainbow from red to
violet.
Whereas this hypothesis is that these birds can see more than we can see and through the
sensitivity to the quantum correlations between the electrons in their retina, they can actually
see with their eyes whether they're going west or they're going north.