E12 Physical quantities available in the medium

Topic

The physical environment surrounding a perceiver contains multiple distinct physical quantities capable of carrying information about that environment—the very same information that defines other models in this set: electromagnetic fields (such as light, radio waves, or infrared radiation), mechanical waves (such as sound, vibration, or seismic tremors), thermal fields (such as heat flow or temperature gradients), chemical concentration gradients, the gravitational field, and the Earth's own magnetic field. The fact that this physical environment presents an additional quantity among these available options means there is, consequently, an additional source of information about the environment for any receiver capable of detecting it.

Among the physical quantities present in this environment, the inverse-square law is one of the most universal: it applies to electromagnetic radiation and sound propagating through free space, as well as to an odor dispersing without convective currents; it thereby determines the effective range of any sensor attempting to detect these signals. If the distance between the sensor and the signal source doubles, the inverse-square law dictates that the intensity of the signal reaching the sensor is reduced to just one-quarter of its original value. Consequently, an organism that needs to detect a signal already weakened by this attenuation has evolved highly sensitive receptors—matching the sensitivity found in the sensory channels defining other models in this set—precisely to compensate for that attenuation. The mechanical waves in this environment transport equal amounts of kinetic and potential energy throughout their propagation; in water—which has high density and high elastic stiffness—sound travels much faster and attenuates far less than it does in air, explaining why fish and cetaceans possess superior underwater hearing compared to equivalent land animals. The fact that the propagation medium is denser and more rigid—as water is compared to air—causes the sound to travel faster and undergo significantly less attenuation along its path.

In contrast, chemical concentration gradients in the environment disperse via diffusion, following Fick’s solution for a point source; this relationship implies that a chemical signal takes longer to arrive the further it must travel from its source, and that the signal becomes diluted in proportion to the square of the distance traveled. Ultimately, of all the physical quantities defined by this model, chemical concentration gradients are the only ones that a chemoreceptor—whether a nose, a taste organ, or an antenna—can detect directly. If the source of the chemical signal is located further away from the chemoreceptor, Fick’s solution dictates that the signal will arrive later and at a much lower concentration. Consequently, this model—characterized by the inverse-square law and the chemical concentration gradients mentioned at the beginning—enables the Earth's magnetic field to serve as an absolute navigational reference for numerous species, such as migratory birds, turtles, and even bacteria. The spatial variation of this geomagnetic field is very gradual over long distances, making it highly stable and acting as a sort of biological compass; however, it is of little use for precise positioning over short distances. The fact that a species relies on detecting this geomagnetic field for orientation necessitates the development of a biological sensor with extremely high resolution—one capable of distinguishing variations far smaller than the gradual spatial variation described earlier.

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