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Firstly, is there intelligent life out there? Well, it seems to me that the Universe is so vast it is actually quite improbable that we are completely alone in it. There are estimated to be about 100,000,000,000 stars in our galaxy, and about 100,000,000,000 galaxies in the observed Universe. That's 10²² stars: an awful lot of stars. It is also known from observations of nearby systems that planets are quite common, and at least for life as we know it, planets are pretty important. I am not going to discuss life as we don't know it.

Let's try and calculate how common it would be (this has to be just hand-waving since the true figures are unknown of course; but I'm trying to come up with reasonably likely figures). What follows is based upon what scientists call the Drake Equation. Suppose that 1 in a million planets is suitable for life; on 1 in a million of those, intelligent life develops; on 1 in 10 of those, a technological civilization develops interstellar space travel. In our galaxy, that would be:

100,000,000,000 stars in our galaxy / 1,000,000 planets = 100,000

100,000 / 1,000,000 with intelligent life = 0.1

0.1 / 10 developing interstellar travel = 0.01

In other words, a technological civilization might develop in about 1 in 100 galaxies, so there might be about 1,000,000,000 such civilizations in the entire Universe. Notice how even though a lot of civilizations may develop, they are still not all that common: 1 in 100 galaxies means that the odds are that there are none around here. Furtheremore, it doesn't say when they may develop, or how long they may last.

This is another problem: timing. The Universe seems to be about 12 billion years old. In that time, stars have been born and died, and indeed our Sun is a second-generation star. A whole generation of stars has already been and gone! Now, I am going to presume that planets were rare around first generation stars, since the planets we have now are mostly made from minerals that were generated by the first generation stars in the first place. These minerals (essentially, everything except hydrogen and helium) were quite rare until those first generation stars started exploding and scattering the heavier elements throughout space.

So, sticking to second generation stars, intelligent life has had a good six billion years in which it could have developed. Our Earth is about 4.5 billion years old, and it seems life developed on it as quickly as it could (pretty much as soon as the planet was cool enough). Intelligent life seems to have taken a lot longer though, I think for a number of reasons: first, environmental disasters kept killing creatures off. There have been several mass extinctions, which could have set the development of intelligent life back several times. Secondly, who needs intelligence anyway? Even stupid animals survive quite well, after all, and Nature does not seem to plan ahead, saying "We need to develop some intelligence here." No, Nature develops only what works in the here and now. So the evolutionary impetus to develop intelligence does not seem to be all that strong: possibly it requires special circumstances such as an environment that is changing fairly rapidly, but not too fast, as is believed to have happened in Africa, driving certain great apes to change their habits and their physiology about as fast as they could. Had they been unable to keep up with the changes, we would not be here. Had the pressure to change not been enough, we also would not be here. Unless aliens genetically altered our ancestors, of course.

The upshot of all this is that intelligent life could apparently have developed on Earth just about any time in the last 250,000,000 years or so, at least. The fact that reasonably intelligent humans have been around for some 1,000,000 years is just the way it happened. Our specific species is even younger: a mere 40,000 years or so. Our civilization a mere 10,000 years or so. We are a very new species. Aliens could have developed millions, or even hundreds of millions of years ago. Given what we have developed in the last 10,000 years, what could a species develop in 10,500 years? Or 20,000 years? Or 20,000,000 years? And that is why I am glad that they are nowhere to be seen around here - but more of that later.

Just to give you an idea of what I mean, consider the technology we are developing at the moment that will probably become reality within the next hundred years. Genetic engineering plus the fact that medical knowledge is doubling every 3.5 years provides a good chance that all diseases will be curable or preventable, possibly within as little as 25 years. What genetic engineering and other medical skills can't achieve, nanotechnology might: with microscopic molecular machines, we can expect to be able to fix any genetic "defect" or enhance our bodies, say with diamond fibre reinforcement, within maybe 50 years. Oh - and we should be able to prevent "natural" death, and most accidental deaths too, with the same technology. When you divide accidental deaths between the population at large, you find that the mean time between accidental deaths is about 1600 years at present, so even without the prevention of accidental deaths by physiological reinforcement and rapid repair work, your expected life span would be 1600 years. In practice however, we would probably be able to live as long as we liked, in 25 year-old bodies, always perfectly fit. Nanomachines can also be used for manufacturing: place a vat of chemicals down, and the microscopic machines will "grow" the product - any product - as you watch. And it has been calculated that computers with the processing power of a PC can be miniaturised with nanotechnology so that 100 of them could fit inside a single human cell, and still leave room for the cell to operate normally. Think about the brain enhancements that might be possible with that technology! Or these computers can sit in your body, watching for damage or invading germs, and would be able to repair any problems as soon as they happen: you would probably even be bullet-proof once the technology has really matured. All that is coming, I think easily within 50 years. The first nano-supercomputer is scheduled to be tested in 2011.  If you want to read more about these things, look up the writings of Eric Drexler, check out Nanotechnology Magazine, or speak to the Nanogirl. Even the possibility of warp technology for space travel has been mooted as not impossible by some scientists lately.

Why am I talking about these things? To point out that even if an alien civilization developed a mere 500 years earlier than ours, and developed at the same pace (or faster), their technology would be vastly superior to ours. Would we really like to meet such beings? Look at what a few hundred years of technological advantage did to the third world and the Americas when the Europeans arrived. But more of that later, again!

So. Let's suppose that the aliens are out there somewhere, and that they are vastly superior to us in their technology. With several million civilizations out there, shouldn't we be inundated with visitors? Well, maybe the space containing a 100 galaxies or so is larger than we can imagine easily, so visiting us isn't all that easy, even if they know we're here (which they probably wouldn't - we're a bit too young to have made much of an impact yet: our radio signals have only stretched 100 light-years or so, and it could be millions of years before they reach another civilization). Perhaps warp drive or advanced technology would make it easy for them to get here, or teleport here, or whatever they do. So why aren't they here?

Perhaps they are. A lot of people claim to have seen something. There are strange incidents such as the Roswell crash (probably lies about weather balloons, in my view). Surely this is evidence enough? What about all the "reliable" witnesses, such as airline pilots? Don't these people know what they're seeing?

The problem with this sort of "evidence" is that it is not proper evidence at all. Consider the "reliable" witnesses. In a court of law, the testimony of an aircraft pilot about what he has seen would have to be taken seriously, but only in the absence of proper evidence. Proper evidence would be something like a fingerprint, or a murder weapon: something tangible and incontrovertible. It is the same in science. Mere hearsay is just not good enough. Sometimes, it is all we have to go on, but from a scientific viewpoint, it just doesn't cut it. That is why most scientists don't believe that we have been visited by aliens: there is no incontrovertible evidence. Lots of hearsay, yes, but nothing tangible. Maybe something has happened, but if so, why is there no hard evidence?

Some people say that most civilizations have legends of beings from the sky who, if we interpret the legends liberally, interbred with or genetically engineered our ancestors to cause us to develop as we are today. Well, the evidence for this is lacking too, and the evidence for the normal scientific picture is adequete to explain the facts. What else there is is all circumstantial. Yes, such stories are pretty common, but humans are pretty common too: maybe, having the same type of minds exploring the same type of world, they come up with the same type of explanations. Especially when the humans appear to have all come from common ancestors anyway. That seems much more likely to me, and it doesn't require any outlandish theories to back it up.

Consider also that if civilizations were all that common out there, or simply nearby, wouldn't there be some sign of them? For example, a technological civilization would use up resources, yet sky surveys show no areas of depleted resources. There are clouds of gas and rubble all over the place, all apparently untouched. This is good evidence that spacefaring civilizations are not nearby, even if they are out there somewhere. Also, it seems likely that many technological civilizations (and it is these I am talking about) would eventually build Dyson spheres around various stars to capture the maximum energy and to provide the maximum living space. A Dyson sphere is simply a sphere that completely surrounds a star; inhabitants would live on the inside surface of it, or within it. Now, there are some dark galaxies out there: these are galaxies whose stars radiate mainly infra-red rather than visible light, and this is what I would expect if the stars were surrounded by Dyson spheres: the spheres would block the light, but waste heat would still have to be radiated outwards. There are whole galaxies whose stars do just that. However, I think that advanced civilizations are not the answer in this case, as these galaxies are all of a specific type: large ones, roughly twice the size of an average galaxy. The size of a galaxy would not be relevant to the development of a civilization, I think (although perhaps it matters for the development of a civilization that can build a Dyson sphere - but I doubt it: it is a difficult task for sure, but not that difficult or that expensive that you would need a double-size galaxy to provide the resources for it). Incidentally, our galaxy is a large galaxy too - but it doesn't have these infra-red stars. So maybe there is something in it after all...

Another argument against the existence of nearby aliens is the Search for Extra-Terrestrial Intelligence (SETI) radio survey, which has found nothing, though surely something would have been detected fairly quickly if there were many civilizations out there - although maybe they don't use radio much (it may be that such technology will be superceded by something better before long). Incidentally, if you want to help with the SETI project, you can: go here and download their screen saver, which analyses radio data from the Arecibo radio telescope while your computer is idle.

Another argument against alien visitors is a physiological one: their biology would probably be incompatible with life on Earth! Think about it. As I understand it, our DNA uses an arbitrary four out of 20 or so possible amino acids. So there are thousands of possibilities there. Plus, aliens may not use DNA to replicate anyway. They might use RNA, or any of the many other chemicals that I expect can do the same job. In us, and life on Earth, DNA instructs the production of proteins, which make up our bodies. There is a near infinite number of possible proteins, and our bodies use only a small subset of them. Aliens, at best, would use a different subset. So what? So, life on Earth would almost certainly be poisonous to them. Our bacteria, our plants, our animals, would be lethal. The aliens would probably be lethal to us too. Certainly, if they came here, they would not be able to breathe, eat or drink here, or they would die. Unless their advanced technology could cope with that, which perhaps it could: quick-thinking nanodigesters could perhaps sort it all out for them.

Let's suppose that all these arguments can be overcome, and the aliens arrive. What would they look like? Most aliens that people claim to see look basically like humans with bigger eyes or various minor distortions of body shape. Indeed, they look more like humans than dogs and birds and other animals that have evolved on the same planet alongside us! In fact, it is highly improbable that they would look much like us at all, with two arms, two legs, a head with two eyes, a nose with two nostrils and most ridiculous of all, one mouth which serves for both breathing and eating: a design defect if ever there was one (unless you like choking on your food). We are only designed like that because some lungfish ancestor had that basic design. Other creatures around at that time were different, and died out in one of those mass extinctions I mentioned earlier.

So, what could we expect them to look like? Well, they would probably be symmetrical, as we are, since any creature which moves from place to place will find that a symmetrical design facilitates locomotion. Similarly, they will probably have a head of some sort in which various senses are concentrated, since a moving creature needs to be able to perceive where it is going, so the head will be at the front; humans have the head on top, which is probably a bit unusual - but our ancestors had their heads at the front. We developed an upright posture and it freed two of our limbs for technological work, but other creatures may not need to do this: they will most likely have more limbs: even on Earth, most creatures have either six or eight or more limbs. The mammals, birds and reptiles don't by an accident of evolution; I don't think there is any particular necessity for us to have such few limbs. There is an argument that with larger creatures, energy conservation will favour fewer limbs: why waste energy supporting six limbs if four will do the job? So four-limbed creatures may well be among the most common, but don't be surprised if some aliens turn out to have more. Another thing: there seems to be no particular reason why they should be descended from brachiators (tree-climbers) as we are. Plus, trees may not be common either, of course. I have been ignoring tails here, but the odds are that an alien would need one for balance, the same as most Earth creatures of any size need one.

It is most likely that the aliens would have eyesight and hearing of some sort, since these have developed by numerous different evolutionary paths on Earth: they are easily developed so will be common. They could have any number of eyes, of any design, however, and hearing may work in different ways too.

They would probably not have an exoskeleton, as these seem to be impractical for larger creatures. Their skin could be scaly or feathery or hairy rather than naked. Again, naked skin is unusual, although it may have been promoted by our technology: the wearing of clothing would have rendered hair unnecessary and our sexual preferences would then have eliminated it over the course of a few ages. The same goes for big teeth: our teeth and claws are so pathetic most likely because we have been using other weapons and cooking our food for such a long time. This may well apply to aliens too, but as that is a cultural thing, we will have to wait until we find some to find out.

On planets without frequent mass extinctions, creatures would tend towards larger and larger sizes because big animals have clobbering power: look at the dinosaurs on our own planet, and even some of the giant mammals of a few tens of millions of  years ago. So aliens from these planets could well be massive, very strong creatures. They would also be strong with thick legs if they came from planets with higher gravity than ours. It is not likely that they would come from planets much smaller than ours as a stable atmosphere would not be present for long enough for them to develop: consider Mars, for example, which has lost most of its atmosphere to space, and which, I am certain, has never been home to native intelligence - the Cydonia face on Mars is just a freak of nature: if you look at any large pile of rubble long enough you're bound to find coincidental alignments, and something that looks like a face - ever been in a forest at night? What do the trees look like?.

What would the aliens be like psychologically? Well, in some ways they would probably be different, and in some ways the same. How would they be different? Firstly, consider how difficult it is for us to comprehend foreign cultures here on Earth. Aliens would be much more difficult to understand, I am sure! Indeed, by human standards, they may well seem quite mad: schizophrenic, psychotic, totally different family structures, more sexes, different. And yet, in other ways, they may well be chillingly similar.

How would they be the same? Well, they will have developed in the same viciously competitive, dog-eat-dog Universe as we have. So, like us, they will probably be vicious and competitive, especially if they have developed a technological civilization. The reason it requires such creatures to build a civilization can be seen from looking at our own history.

Our tribal, nomadic ancestors lived happy lives, at one with Nature, understanding the local medicinal plants, murdering their neighbours, and so on, for hundreds of thousands of years. (We now know that chimpanzees go to war and rape and murder as well, and dolphins have been witnessed raping other dolphins, and virtually all species fight each other, although not usually to the death.) Eventually, however, some of the nomads developed farming and towns, probably in response to climate changes causing a shortage of game and wild plants to eat, and probably as a development of showing off the monuments they could build to impress their neighbors to prevent them from attacking (if you seem strong, you are less likely to be attacked). The big thing about settled life is that a town, with farming, can support a much larger population and a much larger army. The nomads in the end were simply overrun by the greater numbers of the townspeople, even though life in the towns was not as healthy or as happy.

Aliens will presumably have developed from similar ancestors: creatures that fight, creatures that overwhelmed their neighbours by sheer numbers and brute force. They will not necessarily be angelic, benevolent, loving demi-gods. More likely, they, like us, will be the descendants of the best murderers on their home world. So do you really want to meet them?

I have a bit more to say about how unlikely a civilization may be on the following page, which looks at some improbable-seeming events in Earth's history, in Area 52:

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