I believe we have already survived two " great filters", the evolution of multi celled organisms and the development of sentience. The evidence suggests that single celled life develops readily and is easily seeded throughout space. Furthermore, dinosaurs existed for tens of millions of years, longer than mammals but failed to develop civilization. There is no evidence that evolution routinely favors the development of intelligent/sentient organism capable of technological development.
Recent scientific discoveries of extremophile life forms indicate that it is possible for single celled life to survive in the harsh environment of space and asteroid impacts can and do eject rock into space at escape velocity. It is highly likely that space is already being colonized by such events and possible that life on earth started by such single celled colonization.
Late technological filters such nanotechnology or a technological singularity involving artificial intelligence or robotic replacement would not be restricted in space and would likely colonize therefore suggesting, because we have not been subject to such colonization, that is not a likely filter.
Why can't extremophile bacteria be the Von Neumann probes? They are perfectly designed to withstand the harsh radiation drenched conditions of space. They land on a planet, evolve photosynthesize and terraform it and make it nice and inhabitable. They get launched into space via natural disasters and to nearby planets etc by their intelligent progeny.... (we've already probably taken radiodurans to Mars and beyond by accident, although it could have been there already). If conditions are suitable where the extremophile Von Neumann probes land the planet gets really transformed and the intelligent life which evolves starts radioing its presence.... this one's done, come and get it..... but it may then take a few hundred or thousand years for the extremophile creators to come and harvest their world........ that speed of light thing is a bit of a problem.... :-)
Nick Bostrom has a strange concluding sentence to his extremely interesting article. In it he seems to worry about 'the future of humanity'. But from what I know of much of his previous work he believes that humanity will have to be replaced by a superior successor intelligence of perhaps initially human design. i.e. As I understand it most people believe that if there is a Future for Intelligence outside of Earth it is not going to be 'human intelligence'. There is of course another way of looking at the 'singularity' and believing that it is going to occur here on Earth through continued development of computer technology, artificial intelligence etc. My own concern is simply a kind of retrograde one. I find what is most valuable in my world and life is other human beings. All the superior intelligences in the world going on to higher energy computing futures are for me, no substitute for poor little us.
"... the Boskops, an almost forgotten group of early humans who lived in southern Africa between 30,000 and 10,000 years ago. Judging from fossil remains, scientists say the Boskops were similar to modern humans but had small, childlike faces and huge melon heads that held brains about 30 percent larger than our own. That's what fascinates psychiatrist Gary Lynch and cognitive scientist Richard Granger. "Just as we're smarter than apes, they were probably smarter than us," they speculate. More insightful and self-reflective than modern humans, with fantastic memories and a penchant for dreaming, the Boskops may have had "an internal mental life literally beyond anything we can imagine." Lynch and Granger base their characterization on our current understanding of how the human brain works, describing in detail its physiology and structure and comparing it with the brains of other primates."
Watch a human brain morph into a "Boskop" brain http://media.newscientist.com/data/images/ns/av/mg19726462V1.mov
Compare the two brains side-by-side http://media.newscientist.com/data/images/ns/av/mg19726462V2.mpg
It is very anthropomorphic to assume that intelligent extraterrestrial beings must:
1. Be roughly comparable in size to humans;
2. Operate on a similar time scale to humans;
3. Be open to detection by current or potential human technologies;
4. Be as interested in discovering or communicating with other intelligent species as humans are;
5. Be as concerned with reproduction and self-preservation as humans are;
6. Be as willing to blunder into other species' environments and disrupt them radically as humans are;
7. Be as reluctant to admit total ignorance of any relevant facts as humans are .. no, sorry, that's the author.
You can take your mind off humans but you can't take the human out of your mind.
If one supposes that interstellar travel is possible, and therefore that a technological civilization, once it has reached that level of technology, may starting spreading outward across interstellar space, that completely destroys the kind of basically steady state assumptions that underlie most analyses of the Fermi Paradox that I have seen, e.g. those that lead to the famous Drake Equation. With spreading interstellar civilizations, one needs to take into account (1) the fact that for a star system within the sphere of influence of such a civilization the likelihood of the arisal of a new and independent civilization may be very different than for a star system outside that sphere, and (2) those spheres of influence, once they start to expand, may continue to do so indefinitely, until they start running into an interprenetrating one another, eventually filling all of space, thus creating a very different environment than that encountered by the earliest civilizations to arise on the seen. Below is an essay I wrote in 2003 where I attempt to take these considerations into account. My apologies for the various formatting glitches.
Many people, including professional scientists, believe that there are most likely many other technologically advanced societies in the universe, and within our own galaxy. Enrico Fermi pointed out the fact that if this is so, one would expect that we would have seen some clearcut evidence of that fact, yet we have not - this is the so-called "Fermi paradox".
Just how paradoxical this negative result is depends on what assumptions one makes about the likely behavior of technologically advanced societies. One set of assumptions leads to the famous Drake's Equation; this assumes a steady state situation, with societies appearing and disappearing at the same rate. In effect the Drake's Equation requires each society to remain confined to its original system, since once a society begins to spread across many stars, the steady state hypothesis becomes untenable. Personally, I don't find this plausible. Interstellar travel is a clearly a big challenge, mainly because of the enormous energy required, but there are no obstacles which suggest that it is impossible. The sun pumps out many times more energy than we need; all we need to do is harness enough of it, and schemes have already benn put forward to do this. I would be surprised if we don't launch our first interstellar probe before the middle of this century.
Assuming that interstellar travel is feasible, I would expect that at least some significant fraction of technologically advanced societies would begin spreading across the stars, and once a society is spread across many stars, I would expect that it would become effectively immortal, since even a worldwide disaster on one planet would not endanger the society as a whole. So under this scenario, technological societies would pop up here and there in the universe, develop interstellar travel, and then begin to expand outward like bubbles, eventually colliding with one another. Now, just how fast would these bubbles expand? I suggest that the frontiers would probably advance at some significant fraction of the speed of light, that being the only fundamental limit that applies. I further suggest that we are right now almost at the point - within a few centuries - of beginning such an expansion ourselves.
Next, I will introduce the assumption that these societies are not invisible and intangible - if you are within their sphere of influence, and you look around with modern telescopes and other detectors, before too long you would find unmistakable signs of their presence. And I don't mean oofoes. Thus since we've not found such signs, we are not yet within any such sphere of influence.
Now consider what this implies wrt the spatial and temporal distribution of such societies in our galaxy and our universe. Visualize a space-time diagram with a random distribution of points - random in both space and time - where each point is the tip of a cone spreading outward in space as me move along the time axis. Eventually these cones ("bubbles", "spheres of influence") meet, merge, and overlap one another, until they fill all of space. (To be complete, as Kerry Williams pointed out, we also need to take into account the relative velocites of the points, including cosmological expansion, but "locally", e.g. within the Milky Way galaxy, I think those effects can be neglected, if we assume relativistic expansion rates.) Now consider how it might happen that we would find ourselves in the position of not being inside such any of these cones - we have to be far enough, spatially, from all cones that began expansion earlier, and ahead, in time, of all those that will eventually begin expanding in our local.vicinity. In brief, under this scenario, in order to find ourselves alone, as we do, we had to be the "first in our light cone" (or, to be more precise, our expansion rate cone) to (almost) reach the technological level where interstellar travel becomes possible. The spatiotemporal distribution of the hypothesized interstellar- expanding technological societies can be (in a loose sense) factored into (a) the spatial distribution of the planets (or other local environments) that will eventually give rise to such a society (or would, if left undisturbed) and (b) the temporal distribution of when those civilizations begin their expansions. The spatial and temporal effects may not be strictly independent (hence the "loosely") if there are large-scale effects, e.g. clusters of supernovae, that affect the distribution of beginning-of-expansion (b.o.e.) times on scales greater than or comparable to the distances between planets that would eventually give rise to technological societies. For the purposes of this discussion, however, I shall assume that the spatial and temporal distributions can be regarded as being essentially independent. Or, to be more precise, I shall assume that the temporal distributions of b.o.e. times *for undisturbed systems* is independent of the spatial distribution; once the sphere of influence for one society spreads to engulf planets which would otherwise eventually have given rise to other technological societies, all bets are off.
It seems reasonable to suppose that the spatial distribution might more or less follow the distribution of more-or-less sun-like stars with heavy elements present from earlier supernovae. That doesn't narrow it down a lot, especially in our immediate neighborhood. Next, consider the temporal distribution of unperturbed b.o.e. times. It seems to me it should be almost perfectly Gaussian, because I would think there would be many essentially purely additive terms - anything that adds a random delay. But what are the mean and standard deviation? The mean is the trickier one, so I'll defer it for now. To guesstimate the standard deviation, consider all the major steps in the transformation from particles falling toward a protosun into the hopefully soon-to-be interstellar civilization we are now, and ask yourself how much more or less time each step might have taken. IMO, most of the room for variation will be in long periods of relatively slow change (relative to what came later, that is) e.g. the hundreds of millions of years (IIRC) it took to from cells without nuclei to cells with. Or the many millions of years when dinosaurs ruled the earth, while early mammals hid in their burrows. I don't see how the standard deviation of b.o.e. times can be less than tens of millions of years, and it could easily be hundreds of millions of years.
Now if that guesstimate is in the right ballpark, that gives us a starting point from which we can make inferences about how few and far between technological societis are. To begin, suppose we fall roughly in the middle of the distribution for unpertuerbed b.o.e. times; in that case, roughly half of all planets that would eventually give rise to interstellar-expanding technological societies would have already done so, millions of years ago. If there were even one such society anywhere in our galaxy, they'd already be here by now, as per Fermi. If there were any such societies in nearby galaxies (the local group), they may or may not be here by now - the distances, and hence the travel times, are a bit more an order of magnitude greater. But in any event, we are left with a spatial distribution of less than or approximately one technological society per galaxy.
Or maybe not. What if, instead of us being right in the middle of the temporal distribution, we are instead way out on the leading edge, three, four, even five standard deviations ahead of the mean. Yes, I know, at first blush that seems to violate the usual assumption of "ordinariness", but bear with me. Under that assumption, the maximum plausible spatial frequency goes up by the inverse of the rarity of our b.o.e. time in the temporal distribution - suddenly we are right back to the possibility of having lots of neighbors, eventually, but they are likely all behind us by quite a bit - millions or tens of millions of years.
But why should we give any credence to such a seemingly unlikely scenario? Because it may not be so unlikely after all. When the sphere of influence for one technological society reaches a planet that otherwise would probably eventually have given rise to another technological society, the odds of that happening, and the probable time scale, might change dramatically. For example, depending on how the new arrivals choose to use the planet, they might routinely exterminate all native life. Or, more subtly, they might simply fill the "intelligent toolmaker" niche that otherwise would have been left open. Or, simply as a side effect of their activities the native life might go through one continual mass extinction event from the time of their arrival onward - much like what's happening on earth now, because of us. In such a scenario the evolutionary selection criteria change drastically, favoring life forms that fit into the interstices of the dominant civilization - like crows, cockroaches, and coyotes today - and which don't cause enough trouble to be worth exterminating. Suppose it were the case that once a planet falls within the sphere of influence of one technological society, the likelihood that a native technological society will arise drops dramatically, let's say to effectively zero. In that case, only those societies that arise before there neighbors can spread to reach them ever get a chance to arise *at all*. This would have the effect of truncating the original, *unperturbed*, b.o.e. distribution, where the point of truncation would depend upon how the travel times required to cross the spatial separations compare with the spread of the temporal b.o.e. distribution. At one limit, when the spatial separations are great enough, there is little or no effect - only the trailing edge of the b.o.e. distribution is affected. But at the other limit, when the planets that would eventually give rise to life are close enough together, only those societies that would have been at the extreme leading edge of the unperturbed b.o.e. distribution - three, four, five sigma out front - ever come into existence at all. Thus it is possible that such a society, e.g. us, could find themselves in a position which seems absurdly improbable with respect to *unperturbed* b.o.e. distribution, requiring all kinds of improbable events, like perfectly timed mass extinction events, and yet this would turn out to be perfectly ordinary, as compared to the actual, *perturbed* b.o.e. distribution observed in the societies that actually come into existence.
It's very nice to ponder about the future of Humanity, but the sad fact is that for all we here the "Great Filter" is just a few years ahead of us. We will all die and become extinct individuals, with no business at all in interstellar travel and whatnot.
One remarkable aspect of life on Earth is that it arose but once. Every living thing, from extremeophiles to human beings, has the same essential biochemistry--we're all tied ineluctably to DNA or its simpler cousin, RNA. Perhaps RNA/DNA outcompeted some other form of life, but there doesn't seem any indirect support for this in the fossil record. Not only did it take hundreds of millions of years for life to appear on this planet, the only one we know is conducive to life--but even here, it only happened once.
Massachusetts Institute of Technology
irreverent
4
colonization is already occuring
Recent scientific discoveries of extremophile life forms indicate that it is possible for single celled life to survive in the harsh environment of space and asteroid impacts can and do eject rock into space at escape velocity. It is highly likely that space is already being colonized by such events and possible that life on earth started by such single celled colonization.
Late technological filters such nanotechnology or a technological singularity involving artificial intelligence or robotic replacement would not be restricted in space and would likely colonize therefore suggesting, because we have not been subject to such colonization, that is not a likely filter.