IT is manifestly ridiculous to think that strange beings (perhaps with tentacles, three eyes, or infrared vision) are now blazing through the expanses of space in massive, nuclear powered spaceships. But of course, many ideas that sounded absurd at first have actually turned out to be true (e.g. black holes, dark matter and quantum mechanics). Let’s put aside our immediate gut reactions and dispassionately examine the evidence for the existence of alien life.
The Formation of Life
The theory of evolution tells us that given the right conditions (a sufficiently powerful source of energy, the right kinds of atoms, etc.) life can form through a natural and essentially automatic process. Scientists do not understand the precise details of how this occurred on our planet (which shouldn’t come as a surprise, given that the event happened billions of years ago on a microscopic scale during a time when earth had a very different chemical composition then it does today) but they do have reasonable conjectures. What seems very likely is that prior to life there existed what we might call pre-life: simple, self-replicating molecules, the first of which was formed by a lucky collision of atoms. These replicators would occasionally make errors while copying themselves, which most of the time would be disastrous to the reproductive capabilities of their offspring (destroying the possibility of further replication), but which every so often would lead to slightly more efficient replicators. Of course, copying requires the use of chemical materials, so the replicators would be in competition with each other for molecular resources. Over time, through the processes of natural selection, those replicators that were most efficient at surviving and copying themselves in the prevailing environment would spread wider and faster than the others. Over many generations, the lineages of replicators that survived would get progressively better suited for their environment, very slowly becoming what we would recognize as life.
If this process of life generation can spontaneously begin on earth, then it can occur on any other planet, assuming that the planet has sufficiently favorable conditions for a sufficiently long period of time. In particular, since the laws of physics on earth appear to be precisely the same as those that govern the rest of the universe, and since the universe looks to be approximately the same in every direction, we should anticipate seeing other earth-like bodies distributed throughout the universe (in terms of size, position within their solar systems, chemical composition, etc), and we should expect that these planets have a probability of developing life that is similar to the chance earth had. It may be possible that life could take forms that are difficult for us even to imagine, occurring on astronomical bodies that are not very much like earth at all, but since life occurred once on a earth-like planet, earth-like planets are our best guess for where we can expect to find alien life.
It is estimated that there are far more than a trillion planets†† in the universe, providing a staggering number of opportunities for life to begin. Furthermore, the universe is more than 13 billion years old†, so there has been an enormous amount of time for life creating processes to take place. Given enough planets, it is inevitable that at least one will be sufficiently like our early earth to house life. Given enough time, it is inevitable that life will eventually form on such a planet. Hence, the spontaneous formation of life on earth indicates that life very likely exists elsewhere in the universe, though estimating the actual probabilities involved is tricky. The question of exactly how much life has existed in our universe can be reduced to a question that can be studied empirically through scientific means. Essentially, one needs to calculate how many planets (or other astronomical bodies) in our universe today are likely to have undergone long periods during which conditions were suitable for life to begin. Unfortunately however, even with the best telescopes our ability to peer through the vast expanses of space is severely limited, and our understanding of planet formation and evolution is far from complete, so this question is unlikely to be adequately answered for a long time. Ultimately though, the question of the existence of aliens is a scientific one, and therefore we should expect scientists rather than philosophers to eventually have the greatest insight into its answer.
Traveling Through the Universe
Some people believe that intelligent life is unlikely to exist outside of our planet, because if it did, we likely would have encountered it by now. This belief reflects an ignorance of the incredible size and spaciousness of the universe. To give an illuminating example, suppose that humans were interested in traveling to the star (other than the sun) that is the closest to our earth. This star is Proxima Centauri (a red dwarf, it turns out), at a distance of about 4.2 light years from earth. That means that if a space shuttle traveled to Proxima and back at the speed of light, 8.4 years would have elapsed on earth by the time they returned. In practice though, the equations of Einstein’s theory of relativity tell us that it would take an infinite amount of energy to get a massive object to move at light speed. We therefore might employ the more realistic assumption that the space shuttle traveled at about 25% of the speed of light (or, as a physicist would say, 0.25 C). This would still be an incredible scientific achievement, considering that it is about 37,000 times the speed of the fastest plane. To get a space shuttle (weighing, say, 24,000 kg like some space shuttles do today) going at this speed would require a minimum (assuming perfect efficiency in converting potential energy to kinetic energy) of about 67 million terra joules (actually, slightly more, taking into account relativistic effects), which is approximately 280 times the energy released by the most powerful nuclear weapon ever created. Of course, naively setting off 280 nuclear weapons of this kind would be far more likely to kill all life on earth than to get an object close to the size of a space shuttle traveling at the desired speed. As a matter of fact, the process would have to progress rather slowly. Since the human body is limited in the amount of acceleration it can withstand (before blood pools heavily in the legs, and worse), it would take a minimum of 15 days of accelerating to get up to this enormous velocity (unless drastic means were taken, such as keeping the astronauts submerged in fluid during acceleration). In the end, even with the stupendous speed of 25% of the speed of light, a journey to Proxima and back would take over 33 years from the point of view of someone on earth, more than a third of a human life span, and long enough to ruin all of one’s personal relationships. And let us not forget, all this would get us is a roundtrip voyage to our second closest star! The universe is an extremely spacious place, and even for an alien civilization far more advanced than ours, it would take an enormous amount of time to explore even a single galaxy, let alone the more than a hundred billion galaxies that are thought to exist. Our own milky way galaxy is thought to contain approximately one hundred billion stars, and from the point of view of someone on earth, a traveler moving at the speed of light would take approximately 26,000 years to reach earth from our galaxy’s center. In particular, this means that aliens located at our galaxy’s center could not possibly be aware of the first human radio broadcasts for many thousands of more years. Finding alien civilizations, and exploring a galaxy is long, hard work!
There is an important point to be made about the way in which Einstein’s theory of relativity relates to travel at speeds approaching that of light. Because of an effect known as time dilation, time literally moves slower for a traveler that accelerates than it does for the people (or should I say creatures?) on the traveler’s home planet. At speeds like 25% the speed of light, the effect is a rather minor one, but at 99% the speed of light or faster it is very pronounced. In a sense, this provides a space travel loophole, because an alien could theoretically travel extremely long distances at close to the speed of light without aging very much (though, the rest of the universe would continue to age normally of course, so their loved ones would probably die off before they returned from their journey). For example, while a trip from our galactic center to earth would take about 26,000 years for a traveler moving at the speed of light from the point of view of someone still on earth, from the point of view of the traveler himself the journey would feel almost instantaneous. The trouble with using this method for space travel, however, is that the closer to the speed of light one wants to travel, the more energy is required to do so (with, as we have said, an infinite amount of energy being needed to travel at light speed exactly). Hence, time dilation comes at the price of dramatically increased energy requirements. It is also probably worth mentioning that a ship traveling at these sort of speeds would be insanely difficult to navigate. There would be very little time to change course to avoid any interstellar objects, and at the speed of even only 25% the speed of light, a rather small piece of rock or ice carries an enormously destructive amount of momentum. All this being sent, with phenomenal technology, an enormously plentiful energy source, lots of time for acceleration, incredibly good navigation systems, no desire ever to return home again, and a lot of luck, aliens might be able to get from planet to planet quickly without aging very much in the process. The amount of aging they would undergo on such journeys depends on how close to the speed of light they can get without destroying themselves or using up their energy source. A hypothetical method that has been proposed for achieving speeds close to that of light is the Bussard ramjet, which involves using powerful electromagnetic fields to harvest free floating space particles during flight, and converting them to energy with a fusion powered rocket.
What about traveling at speeds beyond that of light? Unfortunately, as far as physicists can tell, no object can ever go faster than light. Of course, we can never rule out the possibility that science not yet discovered will one day allow that possibility, but the extremely well validated theory of relativity currently denies it. That being said, there are at least a couple of ways that the light speed constraint can theoretically be circumvented without actually breaking the ultimate speed limit, but they may not be of any practical use.
Quantum theory predicts the existence of “wormholes”, which are essentially open passages that connect two points in spacetime. While wormholes have as of yet never been observed directly, they may exist at an extremely small scale, popping into existence for miniscule amounts of time and bridging together very short spacetime distances. Expanding or creating large wormholes, getting them to stabilize for reasonably long periods, and controlling which parts of spacetime they connect are currently very far beyond the scope of modern science. It could easily turn out that worm holes are essentially useless from a space traveler’s perspective, no matter how advanced technology has become. Some initial calculations done by physicists of what it would take to create a useful wormhole make demands that are probably beyond the scope of ANY civilization. Some physicists have argued that large, stable wormholes could already exist strewn throughout the universe, a consequence of the inflation of the universe after the big bang. While wormholes like these could potentially be useful for interstellar travel, locating them and actually getting to them could itself be extremely difficult, and since their pathways would already be determined they would not provide much flexibility to a traveler. It is worth mentioned that these strange tunnels can cut through time as well as through space, transporting a traveler into the past while at the same time altering his position (though a wormhole could not take you back to a time prior to when it was created).
Another, totally different way that the speed constraint of relativity is sometimes circumvented is through the phenomenon known as quantum tunneling, where tiny particles jump locations approximately instantaneously. Unfortunately though, this is inherently an unpredictable process, and therefore not useful for controlled traveling. What’s more, like worm holes, this phenomenon is only known to happen at miniature scales, far smaller than even a single celled organism. In the macroscopic world of massive objects like people and spaceships, quantum tunneling is never observed.
As we have said, if aliens managed to achieve speeds nearing that of light, they would be able to explore the planets of their galaxy without aging a great deal. However, from the point of view of planet earth the process would still be extremely slow (the aliens would experience time dilation due to their enormous acceleration, causing time to run more slowly for them than usual, but we would age at our normal pace). If, for example, there were aliens at the center of our galaxy traveling at almost the speed of light directly towards earth, they might find earth in what to them is a few years, but what for us is more than 500 generations. Hence, even if aliens knew where we were located, had a desire to get to us, and didn’t care about ever returning home (after all, their own civilization might be extinct by the time they returned) we would be waiting an enormous amount of time before they got here. By the time they arrived so much time would elapsed that our planet might be destroyed, or our technology might have become as good as their own (making us a potential threat). Unless an alien civilization was lucky (or unlucky) enough to have evolved on a planet very nearby us (by which I mean, within a few dozen light years), even discovering where we are located would be highly problematic. While it is true that the radio waves that we transmit travel at the speed of light, and might potentially be detected by another civilization, these signals grow dramatically weaker as they propagate away from earth, and by the time they are a hundred light years out may be extremely hard to detect (especially when mingled with many natural interstellar sources of noise).
It is probably worth mentioning that there are some reasons to think that aliens might not travel all that much around their galaxies even after they are able achieve speeds near that of light. For one thing, humans already have the technology to destroy all life on earth, and we aren’t yet anywhere close to having ships that can quickly carry humans through interstellar space. It may well be the case that most alien civilizations that exist destroy themselves long before gaining these capabilities. If, as seems to be the case, creating massively devastating weapons is much easier than near light speed travel, civilizations might generally not last long enough to travel. On the other hand, if near light speed travel is sufficiently difficult to achieve (requiring, say hundreds of thousands of years of research by modern humans), then species might tend to be destroyed by natural disasters (such as supernova) before acquiring the technology. What is more, it is interesting to consider the role that virtual reality would likely play in an extremely technologically advanced civilization. If essentially any experience can be had virtually, the need felt to travel outside of one’s own planet (let alone, to ever leave the virtually constructed world) may become limited. Once a civilization comes to construct and accept a virtual world that accurately simulates experiences, but in which one can create or do whatever one one wishes (perhaps even invoke pleasurable feelings upon command), how much time would creatures really choose to spend in the real world?
Aliens Here On Earth?
What is a person to make of the innumerable tales of alien abductions, alien autopsies, crop circles, and spaceship sightings that have been floating around since the 1940’s? While it is impossible to examine every single one of these cases (there are far too many), upon close examination every case that I know of that was thoroughly investigated did not provide solid evidence for alien life on earth. Most of the alien abduction stories come with no corroborating evidence whatsoever (only witness testimony), and most of them occur just after (or during) sleep. Hence, it is impossible to rule out the very mundane explanations of lucid dreaming, sleep paralysis, deliberate fraud, misperception, drug use, illness, and psychosis as explanations for these alleged experiences. Once the idea of aliens became popularized back in the 1950’s alien creatures began to appear in great numbers in people’s dreams and hallucinations. As far as crop circles go, those have essentially been debunked as the work of mischievous (and creative) humans. A number of UFO sightings have turned out to be either unusual aircraft (such as air balloons), normal aircraft flying in formation, normal aircraft with unusual lighting, meteorological events, or tricks of the nighttime sky coupled with human perception errors. While many UFO sightings have gone unexplained, a lack of explanation is a far cry from proving that aliens are here on earth as there are dozens of possible explanations for a person claiming to see moving lights in the sky. In order to prove a point, professional skeptic James Randi once claimed on a radio show to have just seen a UFO. People began calling into the show, claiming also to have seen the same UFO that evenining, unaware that he had made the sighting up. Unfortunately, even photos of UFO’s do not provide good evidence as they can easily be forged (in fact, many of them are known to have been), or are ambiguous as to their interpretation. As far as alien autopsy videos go, those I have seen are obvious or known fakes (like the alleged Roswell autopsy video). Essentially, there doesn’t seem to be any strong evidence behind the alien sighting phenomenon, and it is clear that there is an enormous amount of human psychology involved. Even if aliens are on earth, the vast majority of alleged alien sightings are undoubtedly not genuine. Until actual alien technology is found (that has a function or material composition unachievable with human technology), an alien is actually detained, or video of aliens is taken that could not have been forged using computer generated imaging, it is wise to remain skeptical of stories about aliens.
An important point to keep in mind is that the fastest way that aliens have to discover that intelligent life exists on earth is to detect the radio or microwave signals that we produce. However, if the alien’s home planet is hundreds of light years away then our earliest signals would not even have reached them by now! In particular, since the invention of radio is less than 150 years old, the only possible civilizations that could have discovered that there is intelligent life on earth must be within 150 light years of us. Even worse, if aliens travel at a maximum speed of 25% of the speed of light, the only way that they could have both detected our signals and traveled to earth is if they were within 30 light years of earth when we first started broadcasting, which is a very tiny sphere compared to the size of our galaxy.
While it is very unlikely that aliens have arrived here already, if humans continue to exist for another one hundred thousand years (admittedly a somewhat dubious prospect), it would not be terribly surprising if contact with aliens occurred at some point, most likely through electromagnetic signals rather than through direct face to face contact. Astronomers have now broadcasted messages into space, designed specifically to be detected and interpreted by technologically advanced aliens. Because of their design, these signals are more likely to be detected and properly interpreted than ordinary radio or television broadcasts. Unfortunately, contact with aliens may well be a very bad idea from the point of view of human survival. While aliens could potentially be friendly, there is reason to expect the opposite. Evolution, wherever it occurs, is a highly competitive process, requiring creatures to kill each other for resources. As essentially all creatures on earth (including humans) have many selfish and exploitative tendencies, it seems likely that aliens will as well. Under natural circumstances, creature that are too altruistic tend to be taken advantage of by more selfish individuals, and hence weeded out of the gene pool.
If humans are detected by an alien civilization in the next hundred years, the aliens will almost certainly have far superior technology to our own (by the very fact that they are able to detect us at all), and therefore could likely annihilate us with ease if they felt we might one day be a threat. With sufficiently powerful technology, for example, they could launch a weapon at us at near light speed. By the time we could even detect such an object (if indeed we would be able to detect it at all), there would be little time for us to understand it or react to prevent our destruction. The sad truth is that much of the time when groups of humans have encountered those who are much less technologically advanced than themselves (but who, nonetheless, are in control of useful resources), the weaker group has been suppressed, enslaved, or annihilated. Just look, for example, at the colonization of North America and Africa by Europeans. Unfortunately, it is not terribly unlikely that aliens would be similar to us in this regard. Even if there were only a 30% that the aliens would decide to kill us all, would that really be worth the risk? One thing to keep in mind is that if aliens do ever receive our signals and choose to respond to them in kind, the wise thing for them to do would be to obscure the origin of their messages, perhaps by reflecting them off of astronomical objects (to avoid the danger that we one day develop technologically strong enough to harm them). Hence, it is unlikely that contact with advanced aliens will actually allow us to identify their location.
While technologically advanced alien life is quite likely to exist elsewhere in the universe, it seems that it would be very difficult for these creatures to travel to earth, and perhaps even detect our existence in the first place. Moreover, if they did try to travel to us, it would almost certainly take an extremely long time (from our point of view) for them to make the journey. Finally, if aliens do visit us, there are reasons to think that the visit may be hostile. Unfortunately (or, perhaps quite fortunately) our only interactions with aliens in the next hundred years will almost certainly be only in our imaginations.