Richard Dawkins - The Magic of Reality

If the gravitational pull were too strong, as it would be on a neutron star, there could be no life at all. A neutron star is a kind of collapsed star. As we learned in Chapter 4, matter normally consists almost entirely of empty space. The distance between atomic nuclei is vast, compared with the size of the nuclei themselves. But in a neutron star the ‘collapsing’ means that all that empty space has gone. A neutron star can have as much mass as the sun yet be only the size of a city, so its gravitational pull is shatteringly strong. If you were plonked down on a neutron star, you would weigh a hundred billion times what you weigh on Earth. You’d be flattened. You couldn’t move. A planet would only need to have a tiny fraction of the gravitational pull of a neutron star to put it outside the Goldilocks zone – not just for life as we know it, but for life as we could possibly imagine it.

If there are living creatures on other planets, what might they look like? There’s a widespread feeling that it’s a bit lazy for science fiction authors to make them look like humans, with just a few things changed – bigger heads or extra eyes, or maybe wings. Even when they are not humanoid, most fictional aliens are pretty clearly just modified versions of familiar creatures, such as spiders, octopuses or mushrooms. But perhaps it is not just lazy, not just a lack of imagination. Perhaps there really is good reason to suppose that aliens, if there are any (and I think there probably are), might not look too unfamiliar to us. Fictional aliens are proverbially described as bug-eyed monsters, so I’ll take eyes as my example. I could have taken legs or wings or ears (or even wondered why animals don’t have wheels!). But I’ll stick to eyes and try to show that it isn’t really lazy to think that aliens, if there are any, might very well have eyes.

Eyes are pretty good things to have, and that is going to be true on most planets. Light travels, for practical purposes, in straight lines. Wherever light is available, such as in the vicinity of a star, it is technically easy to use light rays to find your way around, to navigate, to locate objects. Any planet that has life is pretty much bound to be in the vicinity of a star, because a star is the obvious source of the energy that all life needs. So the chances are good that light will be available wherever life is present; and where light is present it is very likely that eyes will evolve because they are so useful. It is no surprise that eyes have evolved on our planet dozens of times independently.

There are only so many ways to make an eye, and I think every one of them has evolved somewhere in our animal kingdom. There’s the camera eye, which, like the camera itself, is a darkened chamber with a small hole at the front letting in light, through a lens, which focuses an upside-down image on a screen – the ‘retina’ – at the back. Even a lens is not essential. A simple hole will do the job if it is small enough, but that means that very little light gets through, so the image is very dim – unless the planet happens to get a lot more light from its star than we get from the sun. This is of course possible, in which case the aliens could indeed have pinhole eyes. Human eyes have a lens, to increase the amount of light that is focused on the retina. The retina at the back is carpeted with cells that are sensitive to light and tell the brain about it via nerves. All vertebrates have this kind of eye, and the camera eye has been independently evolved by lots of other kinds of animals, including octopuses. And invented by human designers too, of course.

Jumping spiders have a weird kind of scanning eye. It is sort of like a camera eye except that the retina, instead of being a broad carpet of light-sensitive cells, is a narrow strip. The strip retina is attached to muscles which move it about so that it ‘scans’ the scene in front of the spider. Interestingly, that is a bit like what a television camera does too, since it has only a single channel to send a whole image along. It scans across and down in lines, but does it so fast that the picture we receive looks like a single image. Jumping spider eyes don’t scan so fast, and they tend to concentrate on ‘interesting’ parts of the scene such as flies, but the principle is the same.

Then there’s the compound eye, which is found in insects, shrimps and various other animal groups. A compound eye consists of hundreds of tubes, radiating out from the centre of a hemisphere, each tube looking in a slightly different direction. Each tube is capped by a little lens, so you could think of it as a miniature eye. But the lens doesn’t form a usable image: it just concentrates the light in the tube. Since each tube accepts light from a different direction, the brain can combine the information from them all to reconstruct an image: rather a crude image, but good enough to let dragonflies, for instance, catch moving prey on the wing.

Our largest telescopes use a curved mirror rather than a lens, and this principle too is used in animal eyes, specifically in scallops. The scallop eye uses a curved mirror to focus an image on a retina, which is in front of the mirror. This inevitably gets in the way of some of the light, as the equivalent does in reflecting telescopes, but it doesn’t matter too much as most of the light gets through to the mirror.

That list pretty much exhausts the ways of making an eye that scientists can imagine, and all of them have evolved in animals on this planet, most of them more than once. It is a good bet that, if there are creatures on other planets that can see, they will be using eyes of a kind that we would find familiar.

Let’s exercise our imaginations a bit more. On the planet of our hypothetical aliens, the radiant energy from their star will probably range from radio waves at the long end to X-rays at the short. Why should the aliens limit themselves to the narrow band of frequencies that we call ‘light’? Maybe they have radio eyes? Or X-ray eyes?

A good image relies on high resolution . What does that mean? The higher the resolution, the closer two points can be to each other while still being distinguished from each other. Not surprisingly, long wavelengths don’t make for good resolution. Light wavelengths are measured in minute fractions of a millimetre and give excellent resolution, but radio wavelengths are measured in metres. So radio waves would be lousy for forming images, although they are very good for communication purposes because they can be modulated . Modulated means changed, extremely rapidly, in a controlled way. So far as is known, no living creature on our planet has evolved a natural system for transmitting, modulating or receiving radio waves: that had to wait for human technology. But perhaps there are aliens on other planets that have evolved radio communication naturally.

What about waves shorter than light waves – X-rays, for example? X-rays are difficult to focus, which is why our X-ray machines form shadows rather than true images, but it is not impossible that some life forms on other planets have X-ray vision.

Vision of any kind depends on rays travelling in straight, or at least predictable, lines. It is no good if they are scattered every which way, as light rays are in fog. A planet that is permanently shrouded in thick fog would not encourage the evolution of eyes. Instead, it might foster the use of some kind of echo ranging system like the ‘sonar’ used by bats, dolphins and man-made submarines. River dolphins are extremely good at using sonar, because their water is full of dirt, which is the watery equivalent of fog. Sonar has evolved at least four times in animals on our planet (in bats, whales, and two separate kinds of cave-dwelling birds). It would not be surprising to find sonar evolving on an alien planet, especially one that is permanently shrouded in fog.