Are we Alone In The Universe?

If intelligent aliens were common, shouldn’t they have visited us already? Some people, of course, claim that aliens have indeed visited us. But the evidence for UFOs is no better than that for ghosts, and I'm personally quite unconvinced. Some astronomers cite this as evidence that aliens are rare. They note that some stars are billions of years older than our Sun, and point out that, if life were common, its emergence should have had a 'head start' on planets around these ancient stars.

But the fact that we haven't been visited doesn't, in my view, imply that aliens don't exist -- the question remains open. It would be far harder to traverse the mind-boggling distances of interstellar space than to send a radio signal. That's perhaps how aliens would reveal themselves first. The nearest stars are so far away that signals would take many years in transit. For this reason alone it makes sense to 'listen' rather than transmit --- if a signal were detected, there would be time to send a measured response, but no scope for quick repartee! (Aliens equipped with large radio antennae could in any case pick up the combined output of all our TV transmitters -- if they could decode them, it's hard to think what they might conclude about 'intelligent' life on Earth!).

Seth Shostak ET please phone Earth - SETI listens for radio signals from alien civilisations at the Arecibo Radio Telescope

Attempts to search for such signals have had a hard time getting public funding (even at the level of the tax revenues from a single science fiction movie) because the topic is encumbered by 'flakey' associations with UFOs, and so forth. But there’s a serious effort in California, backed by hefty donations from some silicon-valley millionaires.

We have no idea what intelligent aliens would look like -- it would depend on the habitat that their 'home planet' offered. They could be balloon-like creatures floating in dense atmospheres; they could be the size of insects, on a big planet where gravity pulled strongly. Or they may be freely-floating is space. They could even, as some science fiction reminds us, be super-intelligent computers, created by a race of alien beings that had already died out.

Even if intelligent aliens existed, they may not be transmitting any signals; and their brains and senses may be so different from ours that we couldn't recognise them. There may be a lot more life out there than we could ever detect -- absence of evidence wouldn't be evidence of absence. There are heavy odds against such searches succeeding.