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The physical environment contains multiple magnitudes that carry information: electromagnetic fields (light, radio, infrared), mechanical waves (sound, vibration, seismic), thermal fields (heat flow, temperature gradients), chemical concentration gradients, gravitational field and Earth's magnetic field. Each type of signal propagates with its own physical laws and offers different information about the environment.

The inverse square law I(r) = P/(4r²) is one of the most universal: it applies to electromagnetic radiation, sound in free space, and odors in the absence of convection. Determines the effective range of any sensor: doubling the distance reduces the intensity by a quarter. Organisms that detect weak signals have evolved highly sensitive receptors to compensate for this attenuation.

Mechanical waves transport kinetic and potential energy in equal proportions: u_mec = ½(/t)² + ½K(/x)². In water ( 1000 kg/m³, K 2.2 GPa), sound travels at 1480 m/s and is attenuated much less than in air, which explains the supremacy of underwater hearing in fish and cetaceans.

Chemical concentration gradients disperse by diffusion following Fick's solution for point source: C (Dt)^(-3/2)·exp(-r²/4Dt). This dependence implies that the chemical signal arrives later at a greater distance and is diluted with r². Concentration gradients are the only physical quantity that chemoreceptors (nose, taste, antennae) can detect directly.

The Earth's magnetic field B_T 2565 T is used by numerous species (migratory birds, turtles, bacteria) as an absolute orientation reference. Its spatial variation is slow (100 km scale), which makes it very stable as a compass but not very useful for fine location. Biological sensing requires sensors with nT resolution.

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