We often think of quantum mechanics as only affecting only the smallest scales of reality, with classical reality taking over at some intermediate level.

But in his 1944 book, What is Life?, the quantum physicist Erwin Schrödinger suggested that “incredibly small groups of atoms, much too small to display exact statistical laws, do play a dominating role in the very orderly and lawful events within a living organism.” Schrodinger was a visionary - and perhaps very specifically in this case.

Because it turns out we might need all the weirdness of quantum mechanics to explain birds.

In the summer of 1943, a spy from the UK parachuted into Vienne, a French town 20 kilometers south of Lyon.

There, the spy learned critical information from the French Resistance about la Milice, a Vichy paramilitary group with orders to round up Jewish people for concentration camps and to assassinate Resistance leaders.

To deliver the information back to London securely and rapidly, the spy was sent with an unusual piece of technology: a pigeon named All Alone.

The spy attached his message to All Alone and set her free.

With unerring accuracy and speed, All Alone covered the 481 miles back to her perch in Staines, England in less than 24 hours.

For her service, she was awarded the Dickin Medal—the Victoria Cross for animals.

While All Alone’s flight was undoubtedly heroic, she was not unique in her navigational abilities.

By some estimates, carrier pigeons were 98% reliable in delivering messages during the war.

And pigeons are far from alone, and plenty of other species of migratory birds also have the uncanny ability to make their way home across hundreds, or even thousands of miles.

How do they do it and what does quantum mechanics have to do with all of this?

Over short distances and on familiar paths, birds can use familiar benchmarks for guidance.

Across long distances, birds often rely on the sun or even the stars.

But All Alone and other birds manage just fine at night and when it’s overcast, seemingly pulled to their destinations like iron filings to a magnet.

And that may not be so far from the truth.

The first proposal that birds have a “magnetoreception” that they navigate by the Earth’s magnetic field came from the Russian zoologist Alexander von Middendorf back in 1855.

A hundred years later, definitive evidence came in the 1960s when German biologists Wolfgang Wiltschko and Friedrich Merkel applied magnetic fields to enclosures with European robins, preventing them from navigating properly.

How exactly birds detect magnetic fields remains an open question.

Some theories have suggested that birds have iron structures in their beaks which help them orient; others have proposed electrically charged fluids sloshing around in the inner ear.

But today we’re going to talk about an increasingly favored hypothesis, and surely the most interesting one: the idea that proposes birds can in a sense see the Earth’s magnetic field due to quantum weirdness happening inside their eyes.

Before we get into all the cool quantum stuff, a quick review on Earth’s magnetic field is in order.

For a more thorough explanation, we have an episode on how that field sometimes flips direction - which I guess drives birds crazy.

This “geomagnetic” field is generated by the convective motion in Earth’s outer core - which is a churning liquid mass of white-hot nickel and iron.

The result is a dipole field, similar to that of a bar magnet: two poles connected by force lines forming a sort of cage around the planet.

At the poles, the force lines are roughly vertical compared to the surface; at the equator, they’re parallel.

At any point on the surface of the earth, our geomagnetic field can be described with just a few properties.

There’s their vertical orientation or “inclination”, so whether they’re parallel to the ground or point into the ground.

And their horizontal orientation, or angle relative to lines of longitude.

This “declination” points towards the magnetic poles, which are offset from the true poles defined by Earth’s rotational axis.

There’s also a directional arrow attached to the lines - as in one direction pointing “north” and the other “south”.

And finally there’s an intensity of the field, represented by how close together the field lines are.

At Earth’s surface it’s about 30 microTesla, which is about 100 times weaker than a fridge magnet.

As far as we know, birds can sense the orientation of the field lines, but NOT their polarity arrows.

So they can tell the direction to the nearest pole, but don’t know WHICH pole it is.

In principle it’s easy to come up with ways to sense a magnetic field.

Magnetic fields exert a force on a moving or rotating charged particle.

An electron, for example, can be thought of as a spinning charge, and magnetic fields can cause that spin to flip direction.

Typical bar magnets like the one found in a compass needle aure ferromagnets, and their magnetic fields come from countless electrons with aligned spins.

External magnetic fields tug on those electrons resulting in a force that can swivel the compass needle.

But you need a lot of electrons to register Earth’s extremely weak field - far more than you could fit into the microscopic structures within a bird’s eye.

And this is where quantum mechanics comes in.

The insight came back in 1978, when a German biophysicist named Klaus Schulten was studying “radical pairs.” A radical is any atom or molecule with a lone electron in an outermost or valence shell.

A radical pair is, well, a pair of radicals - connected in a very special way.

Their unpaired valence electrons are entangled.

A quick review of quantum entanglement is in order here, although we’re talked about it before..

When two particles are entangled, it means one or more of their quantum properties are correlated.

This can lead to all sorts of weird effects, including an apparent faster-than-light influence - measure the property of one particle and you instantaneously influence the entangled partner.

Today we’re interested in a different aspect of the weirdness of entanglement.

The entangled properties are the quantum spins of the two valence electrons in two separate radical molecules.

There are four possible combinations for the spins: the first is the so-called singlet state, where the spins are pointing in opposite directions - we'll call them up and down.

We don't know which electron is up or down, and this is undefinedness is part of the whole entanglement deal.

Simply “opposite each other” becomes this single state - hence “singlet”.

The other three states are when the electrons have the same spin direction - either both up, both down, or a quantum superposition of both at the same time.

These three are energetically equivalent, so together they make up a triplet state.

If you have just one radical, its valence electron spin tends to stay fixed until disturbed by its environment.

And Earth's magnetic field isn't strong enough to influence spin in that time.

But in a radical pair spin states will oscillate between the singlet and triplet states.

They do that evenly in the absence of a magnetic field - 75% of the time in the triplet state and 25% in the singlet.

But even a weak magnetic field like the Earth's can affect the amount of time the radical pair spends in these states.

If that field has the correct orientation, the system will spend more time in the triplet state - with both electron spins aligned in the same direction, and less in the singlet state where they have opposite alignments.

OK, so we have a mechanism to influence two tiny electrons - but a few questions remain: how is the radical pair produced, how long does the entanglement need to last in order to be influenced by Earth's magnetic field, and how does the simple slipping of electron spins go on to give the bird magnetovision?

Klaus Schulten and colleagues proposed the mechanism that has remained mostly unchanged to this day.

It goes like this: birds have some protein in their eyes.

When light hits the protein, it knocks an electron off an attached molecule that goes onto an adjacent molecule.

The two molecules now share a pair of entangled electrons—they become a radical pair for a short period of time.

Then they quickly react to produce some chemical byproducts.

But there’s the key - those byproducts are sensitive to the spin state of the valence electrons at the time of the reaction.

So during the short lifespan of the radical pair, its valence spin state can be modified if the bird changes the orientation of its head relative to the Earth’s magnetic field.

That leads to a changing yield of different possible byproducts across the bird’s eyes.

That could lead to a true visual sense of magnetic field orientation.

We won’t delve too far into the biology or chemistry here, but there is a protein that does all of this - it’s called cryptochrome.

Experiments since Schulten’s proposal have shown that it’s possible to affect cryptochromes with a weak magnetic field and get that characteristic change of rate of chemical reaction.

Notably, fruit flies without the gene for cryptochromes are unable to navigate.

Although birds do have cryptochromes, the mechanism itself has not been directly observed in a bird, and it remains only the most likely explanation.

This “avian compass” presents a tantalizing possibility of quantum biology.

It’s strange to think of quantum effects being relevant in living organisms.

Normally, to observe the strange behavior of the quantum world we need to perform incredibly careful experiments in highly controlled environments - ideally isolated systems of very few particles, perhaps in a vacuum or near absolute zero temperature.

Not in the warm, wet, and messily macroscopic environment of a living organism.

Quantum entanglement is very quickly destroyed in such environments - but birds may have found a workaround.

The radical pairs only need to stay entangled for a microsecond for this mechanism to do its job - because after the entanglement is destroyed the subsequent chemical reactions remember the quantum state, and so remember the magnetic field.

So is this true quantum biology?

Earlier this year, Peter Hore, a physical chemist at Oxford co-authored a paper that seems to answer that.

The team’s calculations showed that only a full quantum description of the process could produce the required sensitivity to magnetic fields.

If, for example, the valence electrons were just interacting due to their magnetic fields - so-called spin-spin interactions - rather than true entangled states - their spin state wouldn’t be sensitive enough to detect Earth’s field.

So Erwin Schrodinger’s ideas about quantum mechanics influencing living organisms may be right.

And quantum magnetoreception in birds isn’t the only example of what we sometimes call “quantum biology”.

We know for sure that it happens in some cases - like the quantum tunneling that drives enzyme catalysis.

There are other contentious, but intriguing cases - like the idea that long-range quantum coherence may drive photosynthesis.

And there are some highly contentious ideas - like quantum entanglement in the brain’s microtubule proteins as a key ingredient in human consciousness.

The quantum magnetoreception of the avian compass sits somewhere in the middle - not yet proved, but more and more favoured.

Klaus Schulten’s microtubules must have been working overtime to hit on such a brilliant insight.

He may still be proved wrong, but it’s a beautiful idea: pigeons and geese and albatrosses, swallows - european especially - birds of many a feather using quantum physics to flock together to navigate the hidden lines of a geomagnetic space time.