The SETI crowd concentrate on scanning the sky for alien radio signals. To a certain extent this is a case of looking for your lost contact lens under the street-lamp, rather than in the shadows where you dropped it. This is because radio (and visible) electromagnetic radiation is what makes it all the way down through the atmosphere to us. What's more, the Earth radiates in the radio spectrum like a small star. Everybeing with a receiver in a sphere 200 light-years in diameter (and that's about 60,000 stars) will know that we are here, and will be uncomprehendingly trying to deduce the relationship between The Sopranos and Bugs Bunny.
If we were looking at us using radio, we would see us. But we would not be seen because we are trying to communicate beyond the Earth, but because of leakage from our inefficient broadcasting system. We can imagine that, with technical advances such as the one on which you are reading this, in but a few decades our skies will become silent again even as our communications become ever more feverish.
We have also sent a few deliberate transmissions, with clever clues as to how to decode them into pictures (the lengths of the sides were prime numbers). Of course, any intelligent listeners may have no concept of a picture. But - much more importantly - they will have to be listening in the right direction at the right time. Radio signals flash past at the speed of light. Blink, and you've missed them.
For a moment consider the matter from the perspective of the Things doing the transmitting. Like our prime-product-picture senders, they have something to say. What characteristics would their ideal transmission medium have? I contend that they are these:
- The medium should selectively seek out places where life, and hence receiving intelligence, is most likely;
- It should travel at the speed of light.
- It should last a very long time when it arrives, avoiding the blink-and-miss problem.
In short, and in twenty-first century human terms, what a transmitting civilization needs to do is to drop - at the speed of light - a DVD of Wikipedia and a few Bach partitas down on every habitable planet; a DVD that will remain playable for hundreds of millions of years. The trouble is, of course, that it requires infinite energy to accelerate a DVD to the speed of light, and they don't last more than a few decades, let alone hundreds of millions of years.
But what about a bacterial spore with the message encoded in its junk DNA?
Again, it needs infinite energy to get it to the speed of light. But - with a mass of 10-15 Kg - it only requires 7x10-15 joules to get it to 99% of the speed of light. And that will convey it to its destination almost as quickly as a radio signal. It would have to be coated in something to protect it - probably a microscopic metal labyrinth from which it could escape, but which shielded it from every direction. This is because, when you move at 99% of the speed of light, ordinary starshine pours down your throat as hard gamma rays; it is Doppler shifted. The protection would raise the mass a bit, and hence the launch energy. And, of course, most spores would never make it.
However, those that found a convivial home would start to reproduce, spreading the message and copying it down the aeons. The message would become a little corrupted with time, and the bacteria would change through evolution. But by comparing the DNA from many of its descendants it ought to be possible to reconstruct the message.
Instead of looking to the skies, we should be examining junk bacterial DNA for the first hundred digits of the base-4 expansion of pi.
It's possible, given long enough, that the message itself might evolve into a species capable of understanding it. So maybe we should be looking at our own junk DNA too...