It is a well known fact that many birds migrate north when spring arrives to go through a mating cycle. It is commonly said that birds “see” magnetic fields in order to work out which way is north which they can do with extreme accuracy. It has been shown through experiment that both direction and magnitude of a magnetic field can be sensed by the birds. Of course, seeing something requires having a wave, most often an electromagnetic wave, either reflecting off or being emitted by an object. So the idea that birds see these massive arching lines heading south is not true. Now the magnetic field of the Earth is about 50 micro Tesla making a small bar magnet 200 times stronger than it. How can a organism like a bird pick up and distinguish such a faint magnetic field when we can’t even tell even when a full Tesla is going through us?
The idea that has been suggested is to do with Spin Correlated Radical pairs. These radical pairs are when a flash of light produces two electrons that have unpaired spins (aren’t bonded) but are still coupled together and have correlating spins because they originate from the same source. Quantum effects need to be introduced because otherwise classical physics predicts that a weak magnetic field would never have any effect in biochemical reactions due to their complex surroundings having a greater impact. This radical pair mechanism is the only known way for trace fields to alter a chemical reaction through the field biasing the likely spin on the electrons. These radical pairs are believed to exist inside the eye and would divert a continuous chemical loop through a sensing pathway depending on the strength of the magnetic field. By collating data from many independent bird studies and analysing the uncertainty in the bird’s flight against the uncertainty in the quantum system it is believed that this quantum model could very well explain the extraordinary orienteering demonstrated by birds.