Quantum Sailors of the Air

Nature's Navigators

Starting in September millions of Monarch butterflies (Danaus plexippus) migrate from Eastern Canada to a very few mountain peaks in Mexico to pass the winter. Every year the migration takes them to the same few mountain peaks amidst hundreds of others. As the lifetime of a Monarch is shorter than the time it takes to complete this journey it is in fact the grandchildren of those that set out that return the following year. Successful navigation across thousands of miles is achieved by a keen sense - located in the antennae - to detect the Earth’s magnetic field. This magnetoreception sense is not unique to Monarch butterflies either.

Russian scientist Aleksandr von Middendorf (1815-1894) studied the migration routes of several bird species. After discovering their tendency to converge towards the magnetic north pole and suggesting that they orient themselves using the Earth’s magnetic field he quite beautifully described them as ‘sailors of the air’.

Quite how these animals use the Earth’s magnetic field for navigation has been in contention for many years. Initial studies on Homing Pigeons suggested that magnetite found in the beak could sense an external field. However, further inquiry discovered that the magnetite was part of iron rich cells called microphages, which are actually used in defence against pathogens and not in navigation at all. One of the problems with detecting the Earth’s magnetic field is that it is incredibly weak, indicating that any magnetoreceptor sense must be incredibly sensitive.

The European robins (Erithacus rubecula) annual migration takes them between northern Europe and the Mediterranean and was one of the first species in which the mechanism for magnetoreception was discovered. Of particular note is that when you blindfold a robin it loses this magnificent ability. So, what is it like for a robin that ‘sees’ the Earth’s magnetic field?

Entangled States of Mind

The most likely mechanism for navigation by magnetic field was actually proposed in 1976 by Klaus Schulten, a German chemist. However, his argument was so weird for the time that it was largely dismissed by the eminent scientists of the era – some of whom were even researching paranormal activity. His argument was that photons of lights could generate quantum entangled electrons whose chemical properties were sensitive to a weak magnetic field. What..?

European Robin (Erithacus rubecula) singing a chirpy little song (photo credit: Tim Sträter)

To understand this a little better (but maybe not by much), a brief background in quantum mechanics is needed. Quantum entanglement is the quite real phenomenon of a subatomic particle to have an instantaneous impact on the state of another ‘entangled’ subatomic particle. For example, one electron in an entangled pair would instantaneously influence the state of the other electron in the pair. What’s more, once entangled the distance between the pair does not impede this phenomenon. It will even occur if the electrons are separated by the vastness of the whole universe.

So what ‘state’ do these electrons have that is being so remotely altered? Among others, electrons occupy what is known as spin states, which are quantised as either ‘up’ or ‘down’. In the quantum world this spin is not the same as what we know as spin in the macro world. Electrons are not balls of charge that physically spin – despite what we may learn in A-level physics. The term spin is just a means to explain some characteristic of the electron – which is definitely not its ability to rotate. Electrons spin in a superposition of both spin-up and spin-down states at the same time. This may sound counterintuitive but in the quantum world this is totally viable, and not some theoretical model to make the mathematics neater. It really does happen…apparently. What’s perhaps even more counterintuitive is that when electrons are paired and have the same energy they must have opposite spins. However, we cannot know which spin state each electron occupies – due to them spinning in a superposition of both states – until it is measured. This will immediately collapse the superposition which means that the measured electron must occupy a single spin state.

The ‘up’ or ‘down’ quantisation of spin are given mathematical values of ½ or -½ respectively. When a spin up and a spin down electron are paired their spins cancel each other out (1/2 – 1/2 = 0) and the electrons occupy what is called a spin singlet state. This is what we normally see in atoms and molecules as electrons are often paired with the same energy. However, a pair of electrons with different energies have the option to spin in opposite directions (they are not confined by the rule that if they have the same energy they must have opposite spins) and thus can occupy a spin triplet state (1/2 + 1/2 = 1). The ‘triplet’ refers to the three directions that the spin can point in: up, down or to the side.

This quantum compass may well have been helping animals navigate for over 500 million years

In both the spin singlet and spin triplet states the electrons maintain the superposition of spin up and spin down simultaneously until one of the electrons of the pair is measured. A consequence of quantum entanglement is that a measurement on one electron in an entangled pair will influence the state of the other electron, as it too must occupy a single spin state (either ‘up’ or ‘down’). For example, in a spin singlet state when one of the electrons is measured to be spin up the other must be spin down, as the pair have opposite spins. And in a spin triplet state when one is measured to be spin up the other must also occupy a spin up state. Once entangled an electron pair can be separated by any distance and the rules of the entanglement will still apply. A measurement on one will result in the other simultaneously occupying whichever state is most appropriate.

In atoms and molecules electrons are more often than not paired with the same energy, thus occupying a spin singlet state. If one of the paired electrons then jumps to another nearby atom or molecule the electrons can still maintain their entanglement – as the separation is not a problem. Now that the two electrons are separated there is the possibility that the one that jumped will change its spin direction. However, this is not a definite. Due to the quirky nature of quantum mechanics the electron pair are now in a superposition of a spin singlet and a spin triplet state at the same time. They are spinning in the opposite direction and the same direction simultaneously!

"If you think you understand quantum mechanics, you do not understand quantum mechanics.” Richard Feynman

An Eye for Detail

One of the ways that molecules can form is through the sharing of electrons in covalent bonds. The electrons in these bonds are an entangled pair that more often than not will occupy a spin singlet state – as they are likely to have the same energy. When the bond between the atoms is broken the electrons may still maintain their entanglement. If they do they will be in a superposition of spin singlet and spin triplet states. Interestingly, the probabilities of the pair occupying these two states are not equal. The really interesting bit, however, is that these probabilities can be influenced by a weak magnetic field. In fact, the crux of the issue is that these probabilities are heavily influenced by the orientation of an external magnetic field with respect to the entangled electron pair. In other words, the probabilities of a separated entangled pair to be in a spin singlet or spin triplet state is largely dependent on the orientation of an external magnetic field.

Once a chemical bond is split the products are often very reactive and the products will often readily recombine. The exact chemical properties of the product will then depend on the spin singlet/triplet outcome, which as we know is influenced by a magnetic field. So what we have is a chemical reaction whose products have slight chemical differences and the ratio of these products is influenced by the angle of an external magnetic field.

The controversial mechanism for magnetoreception proposed by Klaus Schulten in 1976 is as follows. A photon of blue light enters the eye of an animal and is subsequently absorbed by a pigment which is sensitive to blue light, FAD (flavin adenine dinucleotide). The energy of the photon knocks an electron from the FAD molecule resulting in an electron vacancy. The cryptocrome protein which surrounds the FAD molecule contains an amino acid called tryptophan which is crucial in magnetoreception due to its ability to produce entangled electron pairs. One of the electrons in the entangled pair within the tryptophan is donated to the FAD molecule to fill its vacancy. As the entangled electron pair have now been separated it is possible for them to be in a superposition of singlet and triplet states. This balance between singlet and triplet states is incredibly sensitive to the angle of a magnetic field, and will produce different chemical products depending on which way this delicately poised system is found (singlet or triplet). The direction in which the bird flies therefore alters the chemical products of this reaction and a signal (in a mechanism not yet understood) is sent to the birds brain informing it thusly.

This quantum compass may well have been helping animals navigate for over 500 million years. It has shaped the behaviours and life histories of many animals; from sharks, dolphins and fin whales to birds, bees and microbes. But quantum compasses are just the tip of the iceberg. There are a host of other quantum processed which have huge implications for all life on Earth…

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