Mention extra-terrestrial intelligence and most people will perk up and say "E.T.!", or maybe more likely in Bollywood-obsessed India, "Krrish!" (Forgive me, but what a painful movie). How firmly that ugly-but-cute puppet, or a perfectly muscled Hrithik Roshan, has been etched into our brains. So when we hear that there are people actually looking for signs from outer space of an intelligence other than ours -- assuming, for the time being, that we are intelligent -- it's almost amusing. What are they looking for, little green men? The Man in the Moon? Hrithik leaping from planet to planet?
Ha ha, but actually, none of those. The search for extra-terrestrial intelligence (SETI) is a serious scientific endeavour that many dedicated researchers have pursued diligently for years. It's a pity to think of it in terms of images from films.
But when you do give SETI some serious thought, a fundamental question comes up right away: How do you go about doing it? Answering it turns out to be extraordinarily difficult.
A major problem is the unimaginably vast distances in space. Our known means of travel are far too slow for humans to bridge them. Even our speediest spacecrafts would take 40,000 years to reach Alpha Centauri, the nearest star to us on Earth. And we don't even know if there is any life, let alone intelligent life, in the vicinity of Alpha Centauri.
Some scientists have suggested that we find other means to power such a spacecraft than chemical fuels. They imagine one that will slowly accelerate to close to the speed of light. At that speed, it would reach Alpha Centauri in a little over 4 years -- which is how long it takes light to reach us from there. But the energy required for such a trip would be enough to supply India's electricity needs for at least 100,000 years. Who has that kind of money to spend on a spaceship?
Clearly, manned travel is a thoroughly impractical way to carry out SETI. What about sending out unmanned probes? A good idea, you might think, but where would we send them? In just our own corner of our own medium-sized galaxy, we have thousands of stars, and we have no idea which of them, if any, might harbour an ETI. So sending probes to each of those stars would be an enormously expensive project. Yes, who has that kind of money?
What about transmitting radio signals on our own? Clearly a better option than travel, there are people who have tried this. But it will be a long time before we get any kind of reply -- again, from Alpha Centauri alone we'd have to wait over 8 years for one, if it comes at all. Also, we have to listen constantly for that reply, hoping that when it comes, we will be able to filter it out of the random radio noise that fills space anyway.
For all these reasons, scientists have decided that listening for evidence of an ETI is a better strategy than sending out either spacecraft or signals.
But even just listening presents hard, fundamental problems to address. For one, where do we turn our ears?
We can start by aiming radio telescopes at nearby sun-like stars. There are about 1000 of these within 100 light years from us. We can search them carefully for even weak signals, then work outwards to stars that are more distant. Another possibility is to scan the entire sky slowly, looking only for strong signals. The proper SETI strategy is likely a mix of these two methods.
Still, these are just the mechanics of carrying out SETI. Once we start listening systematically, we're up against even more fascinating challenges: What exactly are we listening for? What kind of signal would we recognize as the transmission of intelligent beings? If we get one, how do we interpret it? Once we do, should we respond? How?
A candidate ETI signal would be obviously artificial, to distinguish it from radio noise. Perhaps it would be a pattern of pulses and spaces broadcast over a period long enough to leave us in no doubt that it was produced by intelligent beings.
What would it mean, though?
Speculation is easy, sure. But there are some things we might be able to deduce from such a signal. A good first guess would be to treat each pulse as a one and each space as a zero (or vice versa). That would give us a message coded in binary, the simplest number system we know. But what do we do with this stream of ones and zeros? All kinds of things, really. We might look for patterns in the stream, then see if we can hit upon a code that explains them. Or we might arrange the digits in a rectangle instead of a line. If this is what the ETI had intended for us to do, and if we can hit upon the right dimensions for the rectangle, the numbers might form a picture, or some other information, that tells us more about these beings and where they live.
This is a plausible enough way to proceed that we ourselves have sent out just such pictures. One went out from the Arecibo radio telescope in Puerto Rico (an awe-inspiring machine by itself, filling an entire valley) when it was inaugurated in 1974. We aimed that transmission at a cluster of stars in Hercules, where it will arrive in about 27,000 years. That is, let's not worry too much about getting a reply.
Several SETI efforts have used these ideas for years now, but they have found no candidate signals yet. Is that discouraging? Well, they have only searched a little over 0.01% of the sky. There's plenty of reason to keep hoping.
But assuming we receive an ETI message one day, what should we do then? To me, it seems entirely possible that political, religious or economic compulsions will keep us silent. We might even be scared of replying. But if we do find the imagination to reply, we will probably send back a similarly coded message on the same radio band, and then wait for another message from the ETI. With the exchange of a few such messages, we will work out a logical way to send information back and forth.
Of course, given the distances involved, this "conversation" will be extremely slow. Again, with an ETI near Alpha Centauri it would be eight years before we got a reply to our message. With further stars, those who hear the first ETI signal on Earth might be dead by the time the next one arrives.
The constraints of space and time are a serious barrier to direct contact with any intelligent galactic neighbours we might have. Talking to them will be a slow, tedious affair; meeting them will remain a dream.
So you might wonder: why do SETI at all? Ah, but that has to do with our perpetual wonder about the unknown. All through history we have set off to explore dark corners of the world. Today, when there are pretty much no dark corners left on this planet, we turn to space. How can we not stop to wonder: is there anybody out there? Are we unique? Are we alone?
Answering those questions poses totally new challenges that need totally new thinking. How can we travel faster than our painfully slow spacecrafts now manage? Can we find a better medium than radio waves to carry our messages? How do we send out a signal that will not seem threatening to an ETI?
What's fascinating about all this is that it makes us consider ourselves more closely. Do we have the patience, the vision, the courage, to sustain a long and frustrating SETI? If we can't resolve our own petty quarrels, how will we seem intelligent and friendly to an ETI? If we ignore lessons our own history teaches us, what will we learn from an ETI?
For me, this is the most compelling thing about SETI: that a search for something entirely outside our home planet, even our imaginations, eventually makes us look at ourselves anew. In the end, there's the greatest reason to do it at all.