The Fermi paradox is named after the mid-twentieth century physicist who posed a simple question: calculations based on reasonable estimates indicate that this galaxy should host a large number of extraterrestrial civilisations capable of interstellar communication or travel (which Webb shortens to ETCs), yet we have so far been unable to find any evidence for the existence of even one such civilisation. So where are they all?
The astronomer Drake later quantified the calculation like this: the number of ETCs in the galaxy (N) is determined by the rate at which stars form (R), the fraction of stars with planets (fp), the number of those planets with an environment suitable for life (ne), the fraction of those planets on which life actually develops (f1), the fraction of those which produce intelligent life (fi), the fraction of those which develop a civilisation capable of interstellar communications (fc), and finally the number of years that such a culture will devote to communication (L). The "Drake equation" therefore reads N=(R)x(fp)x(ne)x(f1)x(fi)x(fc)x(L). This looks impressively authoritative, but a moment's thought reveals that we have no means of knowing most of the factors, so figures which we enter for them are little better than guesswork. And the calculated number of ETCs will vary greatly depending on the particular guesses we make. Another point is that should any of the factors be zero, then the outcome will also be zero. Despite this, calculations of star formation rates for this galaxy result in N being a very large number even with pessimistic assumptions being made about the other factors. In other words, this galaxy should have been swarming with ETCs for millions of years, which we could hardly have failed to notice.
Having discussed this paradox, the author then briefly describes and evaluates a select fifty (there have been many more) explanations put forward to account for this interstellar silence, before revealing his own solution. As the reviewer, I will of course conclude by proposing a slightly different answer! Webb divides up the explanations into three broad categories: "They are here"; "They exist but have not yet communicated"; and "They do not exist". I'll take each of these in turn; I obviously can't do justice to a book full of ideas in a blog, so I'll just pick a few examples.
They Are Here
This group contains only eight explanations, ranging from the amusing (they exist and are meddling in human affairs), through the paranoid (we have been isolated by the galactic civilisation, or we live in a simulation), to the more serious (panspermia: we are all aliens, because life was kicked off by being seeded from outer space) and finally the religious (God created this universe only for us; any other ETCs have their own universes created for them).
Webb clearly doesn't rate any of these very highly. The most feasible, panspermia, doesn't actually solve the problem, since if our planet was seeded so presumably was every other suitable one – so where are they all?
They Exist But Have Not Yet Communicated
More than twenty explanations here, some of which argue that ETCs may for various reasons not be interested in travelling to, or even communicating with, other civilisations. Just because we are explorers doesn't mean that everyone else has to be. However, Webb points out that it only takes one with the same urges as we have to reach out to other worlds, and potentially spread throughout the galaxy.
Other explanations are therefore more practical, focusing on the difficulty of interstellar communication – let alone interstellar travel. The popular belief that ETCs in our neighbourhood would have detected us now via our routine radio and TV broadcasts is shot down; it appears that these would fade out before they could reach even the nearest star. Even a focused radio beam aimed at another star would be hard to detect; lasers are more promising, but a nearby ETC would have to exist now, and be beaming a signal directly at us, and we would have to be looking in the right place at the right time to notice it. Another idea is that they are signalling but we're not picking it up, for various reasons; or we have picked it up, but haven't properly analysed the data. Or possibly ETCs don't spend long in an active signalling phase before they upload themselves into computers or some higher non-material plane to enjoy the unlimited pleasures of virtual reality.
The difficulties of interstellar travel are well rehearsed, since there are no indications that Faster-Than-Light (FTL) spaceships will ever be possible and frozen sleep or generation ships have their own major problems. Bracewell-Von Neumann probes (which are sent to other star systems to mine their resources and then replicate themselves to send out to more systems) would be one way of spreading an inanimate presence throughout the galaxy quite quickly. The fact that we have not detected such probes is therefore a puzzle. Somebody should have got around to doing it.
They Do Not Exist
Webb lists almost as many ideas under this heading, which to me represents the most interesting part because it is more solidly based in science rather than speculations about alien psychologies or the like. These explanations look at the sequence of improbable events which has led to our civilisation and argue that this sequence may be unique.
There are several elements to this: first that the galaxy is a dangerous place, regularly blasted by intense bursts of gamma radiation from supernovae which would affect life for thirty light years around. The outer galactic zone in which the Solar System sits may be in the "Galactic Habitable Zone" (GHZ), a less vulnerable position than the more crowded central zone. The mysterious Gamma Ray Bursters (GRBs) are even more devastating; they could affect an entire galaxy and reset the evolution clock each time (for obvious reasons, they have so far only been observed in other galaxies). It is estimated that a GRB could happen in a galaxy like ours about every hundred million years, which would approximately match the frequency of mass extinction events on Earth. So maybe we are among the first to achieve a technological civilisation since the last GRB wiped out any previous ones.
The next point is that planetary systems are inherently dangerous. Catastrophic events such as asteroid strikes, supervolcanoes like Toba or other causes of wild fluctuations in the global climate may have led to many mass extinctions even without the help of GRBs or supernovae. Life may also require very particular circumstances in which to develop intelligence: obviously, any life like ours needs liquid water to be available for hundreds of millions of years, which means that the planet must be in exactly the right circular orbit (the continuously habitable zone, or CHZ) to achieve this even through various fluctuations in the sun's output. Finally, the tidal effects of one large moon plus the constant crustal renewal of plate tectonics may also be important elements in the conditions which led to us, although that is more speculative.
Then we come on to the biological improbabilities. A key one identified by Webb is the development of multi-cellular eukaryotic life, compared with much simpler prokaryotic life such as bacteria. This was a remarkable event which took billions of years to happen – possibly, it's uncommon. So might be the development of intelligence at our level. Perhaps most significantly, of all of Earth life, we are the only one to develop the sophisticated language without which our civilisation could never have arisen, so this may be a very rare feat. And we cannot assume that every intelligent civilisation will be a technological one.
The Author's Solution
Webb makes clear at the beginning that in assessing the probabilities of ETCs developing, he is looking only at life "as we know it, Jim": based on carbon and liquid water. He acknowledges that there may be other forms of life, but since we know nothing about this, there is no basis even for speculating what it might be capable of. Anyway, that doesn't affect the basic problem that we have detected no indications of any forms of life.
Webb's conclusion, based on the arguments raised in the last section, is that we can't detect any ETCs because there aren't any – at least in our galaxy. (There are estimated to be hundreds of billions of galaxies in the universe, but the difficulties of communication and travel escalate by orders of magnitude if we try to include them; our own galaxy is big enough to grapple with!)
His view is that our complete failure to identify any signs of life elsewhere, when all the logic of Fermi's paradox suggests that there should be countless ETCs out there, probably with successive waves of expansion affecting the Earth, has only one feasible explanation – that we are alone. He works through several steps to justify this. First, he estimates that star systems in the galactic habitable zone make up only about 20% of those in the galaxy. Next, stars like our sun are needed to develop life as we know it; they make up only about 5% of the total. So we are down to only 1% of stars being suitable. Thirdly, a terrestrial planet needs to remain in an orbit within the continuously habitable zone for billions of years. He guesstimates that applies to perhaps only 0.1% of all planets (assuming 10 planets per star, that's 1% of the suitable stars). We are now down to about ten million such planets in our galaxy.
Now we switch from the potential for life to its actuality. How many of these ten million will support life? Webb guesstimates maybe half a million, of which 20% might suffer catastrophic extinctions; now we have 400,000. Factor in the number on which life progresses to the complex multicellular eukaryotic stage – he suggests one in forty – and we're down to 10,000. Then apply factors for tool use, high-level intelligence and complex language – and Webb believes we're left with just one; us.
Your Reviewer's Conclusion
Webb puts forward a well-reasoned case to explain why we might have the only technological civilisation in the galaxy. However, I still find his conclusion improbable. Obviously, this is purely a matter of subjective opinion – emotional prejudice, if you wish – as there is no hard evidence one way or the other. It is just that faced with the early development of life on Earth and its tenacity in colonising every possible environmental niche and developing a myriad forms of increasing complexity, I find it impossible to accept that, among the billions of star systems, we might be in the only one to have produced a technological civilisation.
My conclusion goes part-way with Webb, in that I think that while life may be very common, complex animal life may be very much less so; beings intelligent enough to develop technology far less still; and the actual development of a technological civilisation extremely rare. Just look at the history of our planet; simple monocellular life seems to have occurred quite early, perhaps less than a billion years after Earth's formation. But the oldest evidence for complex animals comes almost three billion years later. These rapidly developed to dinosaur levels of complexity, but then stagnated for hundreds of millions of years. Finally, through sheer luck, humanity evolved, but the earliest hominims were around for several million years before modern humans arrived about 200,000 years ago; and for 95% of those 200,000 years, our ancestors did nothing but live in hunter-gatherer packs, like clever animals. Our technological civilisation is the result of a long series of improbable accidents.
As a result of studying Webb's arguments, I am more pessimistic than I used to be about the chances of other ETCs developing. However, given that there are calculated to be 100 billion stars in our galaxy (that's 100,000,000,000), even if our planet was literally "one in a million" in producing a technological civilisation, that still works out as 100,000 ETCs. So where are they? The answer I favour is "not here now". Two different timescales need to be borne in mind: the age of the galaxy, and the probable lifespan of an ETC. Our own star is around 4.5 billion years old, compared with the average for our galaxy of 6.5 billion years (the oldest star being over 13 billion). So if we assume that it takes 4.5 billion years after star formation to produce a technological civilisation (the only example we've got), that means that other stars average a two billion year advantage over us – lots of time to produce a huge range of ETCs. But how long can these ETCs be expected to last?
Just consider our situation again. We achieved the theoretical capability to communicate with other star systems only within the last century. Only half a century after that, we came dangerously close to wiping out our civilisation in a global thermonuclear war. Many scientists fear that over the next century or two we will have devastated our global environment to such a degree that our civilisation will collapse, giving us only a few centuries of possessing advanced technology. By definition, any civilisation with the technology capable of communicating with ETCs will develop the potential to destroy itself, one way or another. So perhaps ETCs just don't last very long. Suppose that the average is 1,000 years; multiply that by the nominal 100,000 ETCs mentioned above, and you get a total of 100 million "ETC years". Compare that with the 2 billion year average time advantage the galaxy's stars have over our sun, and you will see that an ETC will have been in existence for only about five percent of the last two billion years. So at any given moment there may be only a one-in-twenty chance of a single ETC existing anywhere in this galaxy. And no ETC would have the time to spread very far even if it wanted to; possibly none would ever manage to establish itself on another star system.
This is, of course, speculation built on speculation, but with a grand total to date of just one known example of a life-bearing planet to go on, that is bound to be the case. My vision is this: imagine if a camera could have been sited over our galaxy, filming continuously for the last few billion years, and recording each ETC as a bright flash. Then replay the film in quick time. I think we would see a huge number of ETCs sparkling all over the galaxy, from two billion years ago to the present. But slow the film down, and we may see only one flash at a time, with long pauses between them. Occasionally we might see two or more flashes occurring simultaneously, but on average they would be so far apart that communication between them would be highly improbable.
Webb didn't mention the Gamma Ray Burster problem in his conclusion, but if our galaxy is blasted by one every hundred million years or so, clearly many of the above calculations become rather academic. That could explain the silence all by itself.
And another thing…
A further point may limit the number of ETCs likely to be in existence at any one time. If an ETC is established on a planet and fails, for any of the reasons mentioned above, it may prove to be the one and only chance that planet ever has to establish an ETC. To understand why, just imagine the outcome if our present civilisation collapsed, leaving what would inevitably be a relatively small number of survivors existing at a subsistence level. Unless the environment had become irrevocably hostile to humanity, it is reasonable to suppose that some kind of recovery could be made, based on utilising organic resources such as wood to make carts, ploughs etc. The problem would arise with the switch to the mineral-based economy (metal processing and fuel) which, as far as we are aware, is needed to achieve an ETC – because the easily accessible mineral deposits have mostly been exhausted. Even if our unfortunate successors knew where the remaining oil or metal ore deposits could be found, they would be unable to reach them without the advanced technology we deploy. It would be a classic Catch-22; they couldn't develop a technological civilisation without advanced technology! Perhaps they would find a different, non-mineral, route to a more sophisticated level of civilisation, but it seems highly unlikely that this would result in the technology needed to communicate with ETCs, let alone travel to them.
SF is full of beautiful dreams about humanity spreading through the galaxy and meeting other technological civilisations (or nightmares if they turn out to be hostile). Sadly, these are looking increasingly like fantasy rather than SF. I hope this is wrong, and that SETI will discover proof of ETCs, but I'm more pessimistic than I used to be.