|Figure 1 from the article.|
A favorite subject of Science Fiction is how we would deal (and be dealt) with by ET's, aliens from outer space. The idea that ETs are real and have, could, or are trying to contact earth, is irresistable. NASA panders to it widely to justify spending (wasting?) gobs of money on sending people to find life on Mars. Even they are at least restrained enough not to claim that there are Little Green Men there. Still, so distinguished a scientist as DNA co-discoverer Francis Crick suggested that life here was seeded, perhaps by trash if not intent, by alien spacecraft passing by earth (yes, he did!).
Nobody expects to be able to spot a SpaceBus with our usual visual telescopes, but with the advent of radio and other non-optical telescopes the idea that 'intelligent' life may--or must--exist out there has been taken seriously, including that the ETs would know about elecromagnetic means of information transfer (radio, for example). This doesn't mean they know about us or want to communicate specifically with us (why would they, if they knew about what kind of beasts we are?). If so, and if we can intercept signals by al Qaeda, why not from ETs as well?
We don't know what kind of signals they might send, but they should be different from the remorselessly mechanical signals of the broiling, expanding universe. So if we at least listen, perhaps we can filter out the mechanical to detect the intelligent communications buried within it? A huge project called SETI (Search for ExtraTerrestrial Intelligence) is one such effort, and has involved co-opting thousands of volunteers' computers to screen incoming electromagnetic radiation. Wikipedia has an informative page about SETI. Perhaps sadly, but not surprisingly, the result so far can be summarized as: [nothing].
Besides having to guess at what kind of signal would be wafting around in space, detecting ETI requires defining what's mechanical, to see what's left. But there is much in the order and chaos of space, reflecting many different things: the behavior of galaxies, exploding or coalescing stars, black holes, overall expansion, remnant signals from the Big Bang, and more. This of course doesn't include such things as anti-matter and so on.
These phenomena vary from object to object or quadrant to quadrant because in the splat! of spatial history each star, galaxy, etc. is behaving differently. The first task is to be able to identify that, from all directions. Doing that is not easy since we have no prior theory (if, indeed, we have any theory that everybody agrees on) for what is going on. Much of what we believe we understand comes from interpreting the signals. There is a danger of interpreting something as ETI that is really another phenomenon we're misunderstanding.
So for the moment we need some prior thinking to decide why or whether we should make the effort. The basic reasoning, besides just human interest and curiosity is a kind of infinity argument that goes something like this:
- If there are essentially infinitely many objects in space, and if any non-zero fraction of them contain conditions on which life could exist, there simply must be life 'out there'. Our existence proves by itself that such conditions can, and do, exist
- If that is so, there must be all possible kinds of life. No probability is so small that in an infinite distribution of objects, it will never occur;
- Indeed, for the same reason, it must occur an infinite number of times, and with every possible variation!
- If life can exist, and therefore must exist, it must exist in infinitely many places
- If life can exist, it can evolve--our human existence proves that, too
- If it can evolve, it can evolve intelligence--ditto here as well
- If it can evolve intelligence, intelligent life can travel
- If it can communicate, it can also--indeed must also--communicate
- If we understand the physics of the world, we know that communication must (at least in some forms) involve electromagnetic means
If we have sensitive enough instruments, we should be able to detect the emanations. For them to be intelligent, they must have systematic rather than random structure, and must be different from the other electromagnetic 'noise' in space. Why? Because otherwise Spaceship X could not communicate with its home base.
None of this implies when a given level of intelligent life will arise, or where. But if the universe is effectively infinite, there must be some signals that will reach us at any given time (such as today). If the signal comes from very far away, all we know is what the ETs were like back at the time the signal was sent--not what they're like now. So while detection doesn't help direct space travel plans, or booking of exotic vacations, it would at least answer the question: Are We Alone in the Universe?
There have been many arguments, some of which we've given above, why we must not be alone, and thus must be able to detect the life that does exist elsewhere. On the other hand, there are various things that must be true for a given planet to have life that can communicate in the way we've discussed. The planet must be suitable for life and old enough that life there has already originated. It must have had life long enough for it to evolve intelligence. The intelligent beings must have been their and their culture evolved to the stage of sending electromagnetic signals. And long enough ago for the signal to get here. And these things must be compatible with the age of the universe, and the age of the Earth.
So, for there to be ETI there are issues of habitability, evolution, time, and coincidence. The latter is perhaps the most vital, because there must be an intersection between the ETI's stage (at the time of signal sending) and our stage today. Planets alive but only with moss or bacteria won't help. Planets that have ETI but too recently for their signals to reach Earth won't help either. Emanations blocked on their way here by other objects or gravitation, or that got sucked into a black hole, will never get here. Likewise for planets with ET's that haven't discovered radio, or who are isolationists and don't care to venture forth, or who were too far away from us when they got their smarts for the signal to have got here yet.
Still if space is really infinite there must be at least some such coincidences. Indeed, there must be infinitely many of them! Emanations we can detect, scanning in any direction, must literally be loaded with their messages. And that means such messages must be coming at us from all directions right now!
But there are more issues to discuss, that can make you depressed if you're into SETI, or happy if you wish to be left alone. We'll discuss some of these in our next post.