Jeffrey Heninger, 14 February 2023
Epistemic status: Written for engagement. More sober analysis coming soon.
Bird navigation is surprisingly cruxy for the future of AI.– Zach Stein-Perlman
This seems pretty wrong.– Richard Korzekwa
Birds are astonishingly good at navigating, even over thousands of miles. The longest migration routes, of the arctic term, are only limited by the size of the globe. Homing pigeons can return home after being released 1800 km (1100 mi) away. White-crowned sparrows have been able to migrate to their wintering grounds after being displaced 3700 km (2300 mi) shortly before they began migration.
How they do this is not entirely understood. There seem to be multiple cues they respond to, which combine to give them an accurate ‘map’ and ‘compass’. Which cues are most important might be different for different species. Some of these cues include watching the stars & sun, low frequency sounds, long-range smells, and detecting the earth’s magnetic field. This last one is the most interesting. Birds can detect magnetic fields, and there is increasing consensus that the detection mechanism involves quantum mechanics (See Appendix for details).
The result is a precise detector of the magnetic field. It is located in the retina and transferred up the optical nerve to the brain, so birds can ‘see’ magnetic fields. Leaving aside questions like “What is it like to be a [Bird]?”, this result has implications for the difficulty of Whole Bird Emulation (WBE).
WBE is important for understanding the future development of artificial intelligence. If we can put an upper bound on the difficulty of WBE, we have an upper bound on the difficulty of making AI that can do everything a bird can do. And birds can do lots of cool things: they know how to fly, they sing pretty songs, and they even drop nuts in front of cars !
In order to put bounds on WBE, we need to determine how much resolution is needed in order to emulate everything a bird can do. Is it good enough to model a bird at the cellular level? Or at the protein level? Or do you need an even finer resolution?
In order to model the navigational ability of a bird, you need a quantum mechanical description of the spin state of a pair of electrons. This is extremely high resolution.
A few caveats:
- Not all parts of a bird require quantum mechanics to describe their macroscopic behavior. You can likely get away with coarse-graining most of the bird at a much higher level.
- This is a simple quantum system, so it’s not hard to figure out the wave function over the singlet and triplet states.
- What you need to know to determine the behavior of the bird is the concentration of the two final products as a function of the external magnetic field. Once this (quantum mechanical) calculation is done, you likely don’t need to model the subsequent evolution of the bird using quantum mechanics.
On the other hand:
- Birds are extremely complicated things, so it is always somewhat surprising when we understand anything in detail about them.
- If quantum mechanics is necessary to understand the macroscopic behavior of some part of a bird, we should think that it is more likely that quantum mechanics is necessary to understand the macroscopic behavior of other parts of a bird too.
- If there are other parts of a bird which depend on quantum mechanics in a more complicated way, or if the macroscopic response cannot be well modeled using classical probabilities, we almost certainly would not have discovered it. Getting good empirical evidence for even simple models of biological systems is hard. Getting good empirical evidence for complex models of biological systems is much harder.
WBE requires a quantum mechanical calculation in order to describe at least one macroscopic behavior of birds. This dramatically increases the resolution needed for at least parts of WBE and the overall expected difficulty of WBE. If your understanding of artificial intelligence would have predicted that Whole Bird Emulation would be much simpler than this, you should update accordingly.
Unless, of course, Birds Aren’t Real.
- Lambert et al. Quantum Biology. Nature Physics 9. (2013) https://quantum.ch.ntu.edu.tw/ycclab/wp-content/uploads/2015/01/Nat-Phys-2013-Lambert.pdf.
- Holland. True navigation in birds: from quantum physics to global migration. Journal of Zoology 293. (2014) https://zslpublications.onlinelibrary.wiley.com/doi/pdfdirect/10.1111/jzo.12107.
- Ritz. Quantum effects in biology: Bird navigation. Procedia Chemistry 3. (2011) https://www.sciencedirect.com/science/article/pii/S1876619611000738.
Here is a brief description of how a bird’s magnetic sense seems to work:
A bird’s retina contains some pigments called cryptochromes. When blue or green light (<570 nm) is absorbed by the pigment, an electron is transferred from one molecule to another. This electron had previously been paired with a different electron, so after the transfer, there is now an excited radical pair. Initially, the spins of the two electrons are anti-parallel (they initially are in the singlet state). An external magnetic magnetic field can cause one of the electrons to flip so they become parallel (they transition to a triplet state). Transitions can also occur due to interactions with the nuclear spins, so it is better to think of the external magnetic field as changing the rate at which transitions happen instead of introducing entirely new behavior. The excited singlet state decays back to the original state of the cryptochrome, while the excited triplet state decays into a different product. Neurons in the retina can detect the change in the relative concentration of these two products, providing a measurement of the magnetic field.
This model has made several successful predictions. (1) Cryptochromes were originally known from elsewhere in biology. This theory predicted that they, or another pigment which produces radical pairs, would be found in birds’ eyes. (2) Low amplitude oscillating magnetic fields with a frequency of between 1-100 MHz should also affect the transition between the singlet and triplet states. Exposing birds to these fields disrupts their ability to navigate.