The frontier of scientific imagination can be an elusive horizon. People used to believe that the heavens were made of different stuff from the base matter on Earth - the sun, moon and stars were supposed to be fashioned from special celestial material. The ancients projected a zodiac of creatures and deities across the night sky. Up there, it seemed, normal rules did not apply.
But as far as science can tell, the universe out there is made of the same boring old gas and dust as we have down here. This seems to hold no matter how far we look out into space: we find the usual brew of atoms, light beams and X-rays.
The cosmos looks suspiciously samey.
Yet it takes a certain kind of imagination to believe that there is a limiting order to the universe, as it does to believe in a science-fiction universe in which anything goes.
In other words, it may take as much imagination to believe that the universe just goes on and on - a dark vast nothingness scattered with yet more dust and stars - as it does to believe that there is an outlandish frontier somewhere, beyond which our 'normal' universe ends, and an unimaginable new one begins.
Reaching such a frontier would be as if a dweller of the interior of a vast continent, who had not guessed that the world was made of anything other than land, suddenly stumbled upon the shore of a great ocean. It would seem like the end of the earth, with a new world of water everywhere stretching to the horizon. This frontier challenges the imagination: does the sea go all the way out from here? And so it goes for us peering into space: is it space all the way out?
It is perhaps natural to believe that stuff just goes on and on indefinitely. But when we look out into spacetime, we seem to perceive the universe as a finite whole unfolding from the Big Bang. This gives us reason to suspect that this is all there is. At least, we have no way of knowing what else might be 'outside' this defined universe. There could yet be a giant celestial turtle holding everything 'up' - but science has no way of perceiving this turtle, or whatever else might be 'out there'.
We may find it hard to imagine an 'outside' to our universe, or whether it's held up by a giant turtle, or forms part of the spout of a giant teapot. But if there is a frontier to discover, we can get a feel for the sensation in store by looking down the telescope from the other end.
Imagine an alien observer adrift in the cosmos - an itinerant whose 'continent' is the endless void of space. Its worldview is pretty much black and white. There is no atmosphere; no sun ever 'goes away'. The stars barely twinkle.
There are only ever a few chemical elements knocking around, locked in rather unappetising combinations. Nothing much moves. All is silent.
After aeons of nothing but this sort of normality, the alien finds itself approaching a curious new dot on the horizon. As it grows closer, a kind of blue-green swirly ice cream reveals itself as an extraordinary new planet. The alien observer finds a surprising new world, bursting with diversity: an exceptional cocktail of substances and landscapes, from iceberg mountains to turquoise lagoons to sandy deserts, which would each make a handsome sample for any single planet. And on top of all that, a rich layer of life: from spiders' webs to suspension bridges, indoor ski slopes and pachinko parlours, sushi conveyors, turtles and teapots.
Our diverse planet gives us a wealth of material for our imagination to work with, as we project from our planetary oasis out over a desert of dust and darkness.
We can easily extrapolate from observed phenomena on Earth, to conjure up molten planets that rain iron, or planets of solid hydrogen, or seas of plasma - quite apart from imagining celestial bestiaries of bug-eyed inhabitants, crustacean-shaped nebulae, macaroni-shaped wormholes or teapot-shaped constellations.
In contrast, in its homeland of the interstellar void, the alien observer has an effort imagining anything other than constellation-shaped constellations. It would really need a science-fiction imagination to grasp what passes for our reality.
In reaching Earth, the alien has burst out of the 'normal' universe into an unimaginable new one. From this perspective, the frontier to the outlandish beyond is real enough - but (after all) just a few kilometres away, directly above us.
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