LW - how birds sense magnetic fields by bhauth

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Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: how birds sense magnetic fields, published by bhauth on June 28, 2024 on LessWrong.
introduction
It is known that many birds are able to sense the direction of Earth's magnetic field. Here's a wikipedia page on that general phenomenon. There have been 2 main theories of how that works.
One theory is that birds have magnets in their beak that act like a compass. We know this is the correct theory because:
Small magnetite crystals have been found in bird beaks.
Anaesthesia of bird beaks seems to affect their magnetic sense, sometimes.
The other theory is that birds have some sensing mechanism in their eyes that uses magneto-optical effects. We know this is the correct theory because:
Birds can't sense magnetic field direction in red light.
Covering the right eye of birds prevents them from sensing field direction.
We also know those theories probably aren't both correct because:
Most animals don't have a magnetic field sense. It's implausible that birds developed two separate and redundant systems for sensing magnetic fields when other animals didn't develop one.
organic magneto-optics
It's possible for magnetic fields to affect the optical properties of molecules; here's an example, a fluorescent protein strongly affected by a small magnet. However, known examples of this require much stronger (~1000x) fields than the Earth's magnetic field.
Let's suppose birds sense magnetic fields using some proteins in their eyes that directly interact with fields. The energy density of a magnetic field is proportional to the field strength^2. The energy of interaction of a magnet with a field is proportional to the product of the field strengths. The earth has a field of 25 to 65 μT.
If we consider the energy of a strongly magnetic protein interacting with the Earth's magnetic field, that's not enough energy to directly cause a cellular signalling effect.
So, magnetic fields must act to control some energy-transferring process, and the only logical possibilities are light absorption/emission and transfer of excited states between molecules. Birds can sense the direction of magnetic fields, more so than field strength, so the effect of magnetic fields must be relative to the orientation of something.
Molecules are randomly oriented, but absorption/emission of a photon is relative to molecule orientation, so magnetic fields can create differences in absorption/emission of light at different angles. (That's the basis of a spectroscopy technique I previously proposed.)
For excited states of molecules to interact with a magnetic field, they must have a magnetic field. The excited states with the strongest fields would logically be triplet states, where the spin of an electron is reversed, creating a net spin difference of 2. (The magnetism of iron comes from the spin of its one unpaired electron, so triplet states are more magnetic than iron atoms.)
Molecules absorb/emit photons only of specific wavelengths: as energy and momentum are conserved, molecules must have a vibrational mode that matches the photon. Magnetic fields can shift what wavelengths are absorbed. Considering the energy density of the Earth's magnetic field and the magnetic field of triplet states, shifting the affected wavelengths of visible light by 1nm seems feasible.
A blue sky doesn't seem to have sharp enough spectral lines. Can one be made artificially? It's not normally possible to absorb a wide spectrum of light and emit a narrow spectral line: thermodynamically, a more narrow spectrum has a higher "temperature". The spectral width of emission is typically about the same as the width of absorption. (This is why early laser types are so inefficient: they only absorb a small fraction of the light used to pump them.
Systems using diode lasers are more efficient.) Thus, we need to absorb only a narrow spectral lin...