Summary: The drive to place humanity at the center of the universe has led to a stream of assumptions that, as facts have been collected, are shown to be ill founded. The Ptolemaic Earth centered view was replaced by Copernican Sun centered view which in its time was also replaced. The assumption that we are alone in the universe is also under threat of replacement. One of the more interesting aspects of our apparent aloneness was pointed out by Enrico Fermi and is know as Fermi's Paradox (1).
When facts are few, speculations are most likely to represent individual psychology.
- Carl Jung
When you have eliminated the impossible, what is left, no matter how unlikely, is the truth.
- Sherlock Holmes/Sir Arthur Conan Doyle
There is nothing so big nor so crazy that one out of a million technological societies may not feel itself driven to do, provided it is physically possible.
- Freeman J. Dyson
1. Are we alone in the universe?
The drive to place humanity at the center of the universe has led to a stream of assumptions that, as facts have been collected, are shown to be ill founded. The Ptolemaic Earth centered view was replaced by Copernican Sun centered view which in its time was also replaced. The assumption that we are alone in the universe is also under threat of replacement. One of the more interesting aspects of our apparent aloneness was pointed out by Enrico Fermi and is know as Fermi's Paradox (1).
Fermi's Paradox may be succinctly stated as: Extraterrestrials should have colonized Earth long ago, but they have not. So where are they? Whether you agree with the point that Earth was colonized or not the fact remains that no concrete evidence has ever been found indicating a visitation of Earth by extraterrestrials. This is in itself is a very significant point whose ramifications I intend to explore here.
A very conservative estimate for the amount of time required for a technologically advanced civilization (a civilization on par with our own) to colonize the Milky Way galaxy using sub-light travel speeds is on the order of 50 million years (2). While the value of 50 million years is a long time in comparison to a human life span, it is short in comparison to the life span of main sequence stars. When this is combined with the point noted above, that there is no real evidence of visitations, we are left with Fermi's Paradox. The possible answers to this problem can be broadly split into two categories.
We are alone in the universe, or at least this galaxy.
1. We are not alone in the universe.
2. We can further reduce the categories as follows:
1.1 We are unique or life is rare.
This item derives its main support from a lack of evidence contradicting it. The lack of contradiction for any argument makes it a 'weak' argument, while on the other hand, a 'strong' argument is one that is based on existing evidence. Thus when the argument states: "We have no evidence that we are not alone in the universe." we are left with no absolute way to refute it except by producing evidence of other life than what can be found in Earth's biosphere. This ignores a great deal of circumstantial evidence. Circumstantial evidence that parallels evidence that we accept in the more mundane aspects of our lives, such as the fact that atoms exist. No one has ever seen an atom, but the evidence of their existence is considered overwhelming.
An argument used against the existence, or at least for a low probability for life elsewhere is one of shear number of random events that lead to life on Earth. The implication here is that this long sequence of events leading to life is delicately balanced and any significant variation would be sufficient to break it. This is misleading in that it implies that the sequence of events leading to life, and more particularly to intelligent tool using life, is sparse and narrowly defined. A similar argument about the chain of events that leading to a specific human individual can be made, yet there are billions of us on earth. Like the events leading to each unique human, the events leading to an advanced culture are not sparse but bushy and probably quite robust.
One of the ways that theoreticians have estimated the likelihood of other life, as we know it, is by using an equation which postulates the combinatorial likelihood of the right conditions arising (i.e. conditions for life as we know it here on Earth). This equation (4)(5) can be summed up as the product of the following conditions:
f(hab) - fraction of star's with habitable planets
f(sun) - fraction of sun-like stars
f(I) - fraction of Population I stars (i.e. have heavy elements)
f(p) - fraction of Population I stars with planets
f(ec) - fraction of planets that orbit within the 'ecological' zone
f(ter) - fraction of f(ec) planets that are Earth like or terrestrial
f(ax) - fraction of planets with viable axial tilt
f(rot) - fraction of planets with viable rotation rates
f(hab) = f(sun)f(I)f(p)f(ec)f(ter)f(ax)f(rot)
Baugher (4) assigns a value of 0.002 to f(hab). This value when multiplied by the number of suns in the Milky Way (200 billion) produces a significant 400 million current candidates. For f(hab) to be brought to zero or very close to zero one (or more) of the fraction components must be made very small or zero. The factors f(sun) and f(I) can be found by observation of the stars in our galaxy and those in other galaxies. The exact value for f(p) is currently considered to be significant, probably approaching one. In addition, planets that form within what is called the "inner system" and more specifically the "temperate" zone, will not be gas giants (17). The rest, f(ec), f(ter), f(ax) and f(rot) are all statistical variations on f(p) and thus won't be very close to zero.
Complex organic molecules, such as alcohols, ketones, amides (6) and possibly amino acids exist in interstellar clouds and are found in meteorites. If they can form in space, and on our planet, then they will form on most terrestrial size planets. Fossil evidence of life on Earth can be dated back to 3.8 billion years ago (7) which corresponds to the time that major impacts ceased in the inner solar system (Mercury through Mars). Conditions on Earth at this time were harsh and very different from what we think of as terrestrial today. It would therefor seem that life can establish a foothold even under extreme conditions. While this by no means proves the existence of life anywhere but Earth, it presents a range of potential candidates that is to large to ignore. It is probable that any planet that even supplies a modicum of hospitality produces life fairly quickly.
Baugher (8) extends the equation as follows for the probability of an advanced civilization to:
f(life) - fraction of planets developing life
f(int) - fraction of life developing intelligence
f(tech) - fraction of intelligence developing tool use
f(civ) - fraction of tool users to develop advanced civilizations
f(civ) = f(hap)f(life)f(int)f(tech)
Baugher (8) derives a value of 0.001 for f(civ). This equates to 200 million advanced civilizations in our galaxy, assuming they all reached this point at the same time. Sagan (9) derives an estimate between 50 thousand and one million advanced civilizations currently existence in the Milky Way today. An even more important value is the estimated rate that advanced civilizations occur in the galaxy. Baugher (13) uses the following equation.
R(*) - average Rate of star formation in the galaxy
R(civ) - Rate of advanced civilization formation
R(civ) = R(*)f(civ)
Baugher (13) derives the value for R(civ) to be 0.015 advanced civilizations come into existence per year, or one about every 70 years. Rounding this number off to one about every 100 years results in the following table:
Event | Years Before Now | Adv. Civilizations
| | Occurring
Life on Earth | 3.8 billion | 38 million
Life on Land | 400 million | 4 million
Rise of Dinosaurs | 200 million | 2 million
Rise of Mammals | 60 million | 600 thousand
Rise of Man | 5 million | 50 thousand
Rise of Homo Sapiens | 300 thousand | 3 thousand
When presented with general scientific thought the belief that humans are uniquely intelligent and tools using is ultimately based around a belief that we are somehow special. This is understandably an emotional issue and presents an assault upon closely held beliefs about the origin and nature of man.
1.2 We are the first.
Like the other points, this is possible, but highly unlikely. As noted above evidence indicates that life arose on this planet as early as conditions would permit, within a 1.2 billion years of condensation out of the solar nebula and within 200 million years of the cessation of major bombardment of Earth by planetoids, such as the one that formed the Earth-Moon system (11). Our star system is a Population I stellar system, i.e. it is made up elements heavier than Lithium. Population I stars are necessary for the existence of life as we know it. Without the heavier elements no terrestrial planets could form, let alone the complex organic molecules that life is constructed of. Population I stars are formed from the remnants of nova and more importantly supernova. They are necessary processes to create and disperse the heavier elements that we are made up of. Stars that explode as supernova are massive and short lived, less than a million years. The amount of time required for a star to form out of an interstellar nebula is less than a million years (12). Given that the age of our galaxy is estimated to be 12 to 15 billion years it is unreasonable to assume that terrestrial type planets capable of supporting life would not have formed within first several billion years of the birth of the galaxy. This would give a span of time of 10 to 13 BILLION years that no intelligent life capable of sustaining an advanced civilization occurred in our galaxy. This seems highly unlikely.
2. We are not alone in the universe.
2.1 It is technologically extremely difficult or impossible for a civilization to colonize the galaxy.
The technology to support manned colonization of a galactic scale is extant on Earth today. The easiest interstellar craft to manufacture right now would be one based on nuclear propulsion (14). This idea dates back to the early 1960's and probably would not require a large advance upon 1960's technology. Simply put, a nuclear starship would propel itself by detonating nuclear bombs against a pusher plate-radiation shield and shock absorber array. While it would require centuries to reach another star, it is very feasible. More advanced forms of propulsion, such as the interstellar ramjet (15) are technically not to advanced over what we are currently capable of building. A dedicated effort by the U.S. government, much like the Apollo program, could put a probe in orbit around Alpha Centauri within a century. The majority of this time would be required to make the transit instead of develop and launch the probe (16).
The time requirements for a colonization wave spreading from one edge of the Milky Way to the other (i.e. "Worst Case Fast Transit") can be derived from the diameter of the Milky Way (160 thousand Light Years) and the method of propulsion used (which determines the maximum theoretical velocity achievable by that propulsion method) (10). Refueling time requirements for fission, fusion, and antimatter have been ignored as well as the time requirements for setting up of stationary laser bases.
Propulsion | Max. Theoretical Velocity | Worst Case Fast Transit
Fission | .06 the speed of light | 2.66 million years
Fusion | .15 the speed of light | 1 million years
Laser | .98 the speed of light | 160 thousand years
Antimatter | .999 the speed of light | 160 thousand years
Ramjet | arbitrarily close to c | 160 thousand years
In addition to the above time requirements a "latency" value can be added. The latency for colonization takes into account the rate at which stops are required and the amount of time each stop. A slow colonization wave (2) produces a latency 2000 years per 3 LY of expansion. A distribution of 7 LY between colonizable star systems and latency of 50 years for an aggressive expansion rate produces a latency of 1.14 million years, or:
Propulsion | Time Required to Colonize the Galaxy
Fission | 3.8 million years
Fusion | 2.14 million years
Laser | 1.3 million years
Antimatter | 1.3 million years
Ramjet | 1.3 million years
If the civilization which started the colonization wave started closer to the center of the galaxy, then the time would be halved. Many other factors can be suggested that would significantly effect the amount of time required for galactic colonization.
It has been suggested that galactic colonization might be to large of a resource investment for a civilization or that it might present to much of a strain on a society over the time period required to achieve it. But not all colonization efforts need to be "manned". The alternative is von Neumann or self reproducing automated systems colonization (18). This form of colonization be based upon a ship (or ships) capable of autonomously reproducing itself in the target system and launching copies of itself at the next set of target systems. The great advantage of his system is that it propagation, once launched, does not require resources or any other input for that matter, from the civilization that launched it. A von Neumann colonization wave would even survive the death of its parent civilization and would be difficult to stop (22).
A conservative (i.e. slow expansion rate) estimate of the amount of time required to colonize the galaxy is on the order of millions of years. While this seems like a very long time indeed, it is a very short period when considered on the astronomical scale. Remember that it has been four billion years or so since the appearance of life on Earth. In that four billion years all it would take is one civilization to decide that the thing that it should do before all else is to make sure that all possible niches in the galaxy should be filled with its offspring. And that would mean that they would now occupy Earth, not us.
2.2 We are already the result of a colonization effort.
If we are the results of a colonization effort, it occurred during the early, and thus "simple" period of life on Earth, long before multicellular life began one billion years ago. This form of colonization must have been of the form of a "seed" and forget as there is no evidence of any long term interference with Earth's bio-system by an advanced culture. In fact there is a great deal of evidence that we have not been "managed". All higher life forms on Earth share a genetic heritage that is complex and ancient. If another civilization had been manipulating evolution and genetics on Earth, why would they have done it in such a way that has left the shear number of "hacks" and "kludges" as there are here (19). Even if they just manipulated homo sapiens, as suggested by von Daniken et. el., there is again a large list of poor design decisions, such as hemorrhoids, acne, and "swallowing down the wrong pipe" (20). Regardless of the dramatic suggestions of Arthur C. Clarke in "2001", the evidence supports an independent and "natural" evolution for humans and other life forms on Earth.
Even if we are the results of an early seeding colonization effort, there has been plenty of time since then for a more aggressive wave of colonization to arrive at Earth.
The form that colonization takes can be divided into two major categories, aggressive (21) and non-aggressive. Non-aggressive colonization is essentially one where only non-life bearing planets (and other resources) are used, or at least those that only have very primitive life forms. This might include seeding as noted above, terraforming of non-terrestrial and other non-life bearing terrestrial planets or non-planetary based colonization, i.e. deep space colonies. Aggressive colonization is one of "might makes right" and may be subdivided additionally as:
Scrub & Repopulate
Smash & Grab
Interstellar imperialism colonization would be similar to the practice of imperialism practiced by Europe during the 18th and 19th centuries here on Earth. Resource exploitation would be similar to way the West uses the seas or exploits mineral resources, taking what is wanted with little regard for indigenous life. Subversion, either by biological or nanotechnological means, would involve the conversion of a planets ecosystem to compatibility with the colonizers native ecosystem. Scrub and repopulate would be the elimination of all native life forms and then terraform the planet. And finally the smash and grab scenario would involve the destruction of target planet or stellar system and conversion into something that the civilization found important, such as space based colonies, ships, Dyson spheres or Ring worlds (24).
The only form of aggressive colonization that we might be currently experiencing is a subtle form of resource exploitation which is not really a direct form of colonization and still begs the question of why a wave of more aggressive colonization has not reached us.
2.3 No civilization has ever wanted to colonize the galaxy.
It is hard to believe that all advanced civilizations have decided not to attempt to colonize the galaxy. As noted in 2.1, all that is required for one civilization to start a von Neumann colonization wave in the last four billion years for Earth to have been colonized.
2.4 We have been overlooked.
The possibility that we have been overlooked is quite small when considering a von Neumann colonization wave. By shear force of numbers self replicating colonizers would eventually discover the existence of Earth. Alternatively space based optical telescopes, like the Hubble (25), are capable of discerning planets orbiting around other stars. It would be easy for a space faring civilization to construct optical telescopes several orders of magnitude larger than the Hubble for surveying purposes.
2.5 We are not worth bothering with.
Earth or our stellar system has value based, at the very minimum, on the fact that we present someplace to live. One could also make the argument, like Greg Bear does (21), that Earth civilization presents a long term threat if we decide at some point to launch a von Neumann colonization effort. There are many other potential reasons why we might be of interest, from cultural or artistic to purely biological. All it takes is one advanced civilization out looking around for that particular thing to visit us.
2.6 The Toolmaker Koan (3).
It would be foolish to assume that all civilizations survive their childhood and reach a stage where they are capable of colonizing the galaxy. In addition to childhood diseases there are threats to mature civilizations as well and those that can occur to any civilization, young or old. A partial litany of these threats are:
War - War can result in the destruction of a civilization before or after star faring capabilities are developed. The threat is more acute when a civilization is confined to a single planet. Sagan's Nuclear Winter scenario is just one possibility.
Over Population (28)
Resource Crunch - Resources may become so scarce that a civilization is trapped in a position where all of its resources are expended on maintenance or to prevent to drastic a fall in the level of the technology.
Bio/Nano Disaster - Some biological or nanotech agent might escape the control of the developers and destroy a civilization.
Inward Turning - The development of some technological wonder (such as Virtual Reality) or philosophical movement might permanently remove any interest in anything outside of the bounds of a civilization (29).
Cosmic Disasters - An asteroid might strike the planet or a nearby star might go nova thus destroying the civilization (30).
Delicate Balance Disruption - A civilization may be so dependent upon technology that is delicately balanced that a small failure could lead to the destruction of the civilization (31).
Unknown (Physics) Disaster - Some high energy physics experiment might go awry or have unexpected results, such as the accidental creation of a black hole (27), that could cause the destruction of a planet.
But is it true that all civilizations would fail to survive their childhood or would be destroyed before they could launch a colonization effort? Very unlikely.
2.7 All civilizations refuse to colonize worlds with life already on them or just aren't interested in colonization.
Is it likely that all advanced civilizations would refuse to colonize? To assume that a technologically advanced civilization is similarly morally or ethically advanced is simply that, an assumption. Why should an alien civilization's morals and ethics be even similar to our own? It is not a law of physics that the correct moral or ethical thing to do is to respect or honor the existence of other civilizations, let alone "lesser" life forms. It is certainly within the capacity of humans to treat other intelligent beings as if they were simply property.
Civilizations that achieve a monolithic culture early in their history (i.e. before the development of space travel) might very well be severely self centered. Conversely civilizations that are fragmented (such as we are now) and achieve space-colonization capabilities will probably experience a diaspora and thus have an even wider spread in philosophies and viewpoints.
Once again it is an issue of some civilization somewhere, sometime deciding that it is the only valid civilization and having the means and the willpower to back it up.
2.8 Civilizations provide sufficient buffer to prevent expanding aggressive colonization from passing through them.
It is unlikely that an advanced civilization could stop the expansion of an aggressive and more advanced civilization. The problem of stopping colonizers in any particular star system under the control of a civilization would be fairly straight forward, if it could be done at all. Stopping colonizers from just sneaking through interstellar space is more problematic and the closer the tech level between the sneaker and the sneakee the more likely it is that something would get through.
Even if a civilization was capable of stopping a colonization wave in the volume under its control, a sufficiently aggressive colonization wave would simply go around. The galaxy is a very large place and when you take into account intergalactic space it is orders of magnitude larger. A patient form colonization could use the space above or below the galactic plane to drift colonizers around a blocking civilization.
2.9 A galactic community exists with the capability of enforcing some viewpoint concerning life bearing worlds.
The final possibility is one where a long term stable galactic community of civilizations exists. At some point during the 10 to 13 billion years that advanced civilizations have had the opportunity to arise in either a single civilization or a cooperative group of civilizations would almost certainly have spanned the galaxy. Once such a pattern of galactic community takes hold, it is self reinforcing as it has an inherent survival value (32). Cooperative pooling of resources generally produces a more efficient use of the resources. It can be seen in Earths ecosystem, both biologically and sociologically, that cooperative behavior is reinforced.
The "Anti-Social" civilizations that arise would have a harder time surviving in a galactic environment where pre-existing civilizations could bring their more advanced resources into play against such upstart civilizations. Likelihood of the survival of an "Anti-Social" civilization is low, either initially or in the long run.
It would appear that this galactic community is opposed to the haphazard colonization of life bearing worlds. The reasons for this are legion. Current attitudes in Western civilization here on Earth towards Third World cultures give some possible reasons. A short list of possibilities are:
Interest in unspoiled Native Cultures
Indigenous creativity may produce previously unknown items
Code of Non-interference
As noted in 2.8 it is still possible that a particularly lucky or aggressive civilization might launch a von Neumann wave of colonizers. Once such a "virus" is loose in the galactic corpus it might be very hard to eradicate and could be a sufficient threat to resources that it might bring about a collapse of the galactic community.
What could then produce a galactic community with sufficient capability to deal with the anti-social behavior of von Neumann colonizers? The one resource that allow such a community to deal with aggressive sub-light colonizers would be either a FTL communication system or better yet, a FTL transportation system. Given the existence of FTL travel it would become feasible for a galactic civilization to guard life bearing star systems and respond quickly enough to prevent them from being colonized.
A final point should be made concerning what physics says about Faster Than Light travel, and that is that FTL is not ruled out. Physics only says that mass cannot travel at the speed of light. It is an open question as to whether or not mass can effectively travel faster than the speed of the light.