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I was reading about the James Webb Telescope, which was launched into orbit last year at the L2 LaGrange point—a stable orbital position on the other side of the Moon—and began producing stunning images of the farthest stars as well as our own system’s more distant planets. The interesting thing about the JWT is that it views the universe not in “visible light” but in lower and longer wavelengths—called “infrared,” or “below the red”—which we normally feel as heat.
And that got me thinking about what we call the visible spectrum, electromagnetic pulses or photons vibrating with a wavelength between 380 and 700 nanometers (billionths of a meter, abbreviated “nm”). These are the wavelengths that virtually all animals perceive, although some—such as the pit viper—can see or sense in the infrared, and many animals and insects can see a bit into the ultraviolet—or “beyond the violet.” But 380 to 700, going from red to orange to yellow to green to blue to indigo, seems to be the animal standard. After all, most of us walking on or using four limbs on land are related by an evolution that extends back to the stump-finned fishes who first crawled out of the swamps.
But why those particular wavelengths? Why can’t we perceive the world in the longer radio waves and microwaves, or in heat like the JWT, or in the much shorter x-rays and gamma rays? It would be useful to see in x-rays, because then we could look through doors and see who was on the other side.
Then I started to think about the nature of our bodies. Our cells, the components that make up all of our different tissues, are sized between 10 and about 120 micrometers (millionths of a meter, abbreviated “µm”). Just for reference, there are a thousand nanometers in a micrometer. And the rod cells in our retinas are about 2 µm in diameter. Taking about the middle of the visible spectrum, say 500 nm, somewhere deep in the shades of green, that means the cells in our eyes that perceive light are about four times wider than the wavelengths they are encountering.
That seems about right. If the cells were much smaller than the wavelength, they would likely miss parts of it. If they were much bigger, their exposed area would be redundant, and so their resolution—the ability to pack individual light-sensing cells into a given area of the eye’s retina—would be reduced. Smaller light-receiving points equals greater ability to see details, in the same way that smaller pixels in a photograph or on a monitor screen means sharper images.
In evolutionary terms, it is an advantage to be able to see in the same wavelengths as your prey or your nearest predator. Everyone is playing in the same field, so to speak, and no one—except maybe the pit viper or the bee, who are able to see in those outside wavelengths—has a clear superiority.
But the relationship of cell size to perceived wavelength tells me something else. If we eventually meet extraterrestrials, and they are approximately our size and have about the same cellular complexity, they will likely see mostly in what to us is the visible spectrum. If they are very large or very tiny, then all bets are off. But if they are sized about like us, then they will probably see in the same way we do.
And our size has everything to do with the internal composition and radius of our home planet—that is, the acceleration of our particular gravity. Creatures from much larger, rocky, and iron-filled planets with stronger gravity will likely be much larger. Their bigger structure would stand up to the greater pull. Or, conversely, they might be much smaller, flatter, less upstanding, more like starfish than bipedal creatures. Similarly, creatures from smaller, less dense planets with weaker gravity might be much smaller. Or, conversely, they might be larger, taller, and more willowy than sturdy, more like waving grasses.
But if they are about our size, with about our cellular complexity—which in humans numbers about 37 trillion cells of various sizes, but none larger than about 130 µm in diameter, the human egg cell—then they will likely be seeing the same landscape as we do.
And if the extraterrestrials are much larger than us—say, the size of an Ultrasaurus of the early Cretaceous—or much smaller—say, with the size and organization of a swarm of gnats—then again all bets are off. They might still perceive the world in the visible spectrum, but we would likely perceive them as some kind of animal.