Even so, our nascent technology has discovered planets around something like five percent of stars similar to the sun. The emerging science of abiogenesis seems to suggest that life forming from non-life is not as hard to fathom as was once thought (indeed, in the right environment, it may be almost certain to develop given some time). How likely it is that developing life will evolve intelligence is not known, but it is not implausible to think that intelligence is enough of a benefit to organisms that it would eventually be selected for in just about any niche (such convergent evolution explains why dolphins and sharks have similar streamlined shape despite coming from very different evolutionary paths - their evolutionary adaptations converged on the optimal torpedo-like body for quickest swimming). Combining this with the billions of stars in each of the billions of galaxies out there (which have been around for billions of years), it is hard to imagine life being extremely uncommon.

So assuming there are likely to be a fair number of other intelligent civilizations out there who have not annihilated themselves with something like nuclear technology or nanotech grey-goo, where and how do we look for them?

The current method mainly revolves around searching for radio or optical signals sent from other intelligent civilizations, intentionally or not. Intentional signals would be things like messages sent directly to us by a civilization that discovered our presence and technological ability to receive signals and wanted to communicate, or maybe just a random beacon sent out to say "we are here". Unintentional signals might be interception of leaked signals (like the T.V. signals Earth has been beaming into space for a few decades) or the detection of a distinctive blackbody spectrum suggesting the presence of a Dyson Sphere.

For intentional communications, radio and optical signals are considered the best thing to search for right now because they are ideal for broadcasting compared to sending through other parts of the spectrum like infrared (radio penetrates the interstellar medium very well, is cheap to produce and can be sent widely in many directions at once; optical signals are quite efficient for sending narrowly directed signals within a few thousand light-years). It is assumed that most any civilization which developed very far in science would discover a method of broadcasting radio/optical signals, understand their natural (i.e. not human-dependent) benefit over other options and therefore choose to broadcast these sorts of signals rather than infrared or otherwise. Further, there are certain narrow bands of the radio spectrum which are ideal natural frequencies at which to send a signal to make it easiest to detect, so they would likely choose to use these frequencies. Basically, we look for radio and optical signals (in the optimal parts of the spectrum) because these are the places most likely to be used by any civilization which is trying to be found.

Certainly there may be more powerful and efficient methods of interstellar communication based on scientific discoveries we have not yet made, but any civilization which was actively seeking to communicate with other civilizations (and not just limit itself to those which had come as far as themselves - i.e. developed technology built on these other scientific advances) would presumably use the simplest available technologies (radio/optical) in order for their signal to reach the most possible potential listeners. Primitive though they may be, our current methods of searching for intentional signals are actually quite reasonable.

Unintentional signals are a little more complicated. We are unlikely to stumble upon leaked radio transmissions from another civilizations because the radio spectrum is vast and right now we can only search a small band of it for short periods when looking in any given direction. Unintentional optical signals might be more likely to show up if some civilization is doing something which happens to send out strong enough pulses of light in our direction (though such a civilization would likely have to be within a few thousand light-years for the signal to not be blocked by interstellar dust). More interestingly, megascale engineering projects like Dyson Spheres (huge shells built around a sun to catch and harness nearly all of its energy output rather than just the tiny fraction that hits one side of an orbiting planet) would affect the outgoing light of the local star in a distinctive manner that likely could not occur naturally; detecting this effect would be a sure sign of intelligent and advanced life even if the civilization was not seeking to communicate with us.

Finally, we may discover the presence of an extraterrestrial civilization by finding artifacts made by them. Maybe a probe they sent for scientific exploration; maybe a copy of the Encyclopedia Galactica dropped off at some stable Lagrange point in our solar system for us to eventually find, or maybe an actual ship full of alien beings. So far no such artifacts have turned up. No probes or ships have been stumbled upon in our search of the skies, and we have already made a cursory search of the Lagrange points around our own planet to no avail (though more careful searches will certainly happen in the future). At any rate, these are the sort of things that right now we are only able to look for locally in our own solar system, and they rely on the aliens having actually sent some physical object toward us or visited very recently (during the short time we have had the technology to search the skies for unusual objects of that size, which is not even a blink of time on the cosmic scale). For the time being, we are probably much more likely to discover another civilization through radio or optical searches.

Given the current limitations on time, width of sky and frequency in these searches, where are the best places to search? Some suggest a narrow search in which we concentrate our investigation on, say, stars we know to have planets. However, this number is still very small, and it suffers from the inherent limitation that our current methods can only detect planets around stars that are relatively nearby (through the astrometric technique, which directly measures the side-to-side 'tug' on a star by an orbiting planet when the system is seen face-on) or else planets which are very large (not Earth-like) and very close to their parent star (via the Doppler technique which measures the wavelength shift in a star's light caused by the planet's 'tug' when the system is seen edge-on). This skews the results of our search and limits it to a very small range which might not be wide enough to give us a good chance of finding other intelligent life even if it is out there broadcasting its presence.

Given this, some instead suggest opting for a wider search in which we basically look at random stars in every direction (though still concentrating on relatively narrow frequency ranges). Of course, we can likely narrow even this search down somewhat by concentrating on those types of stars which have a mass similar to or less than our own sun's (because stars which are much larger burn very hot and die off quickly before any life that appears in the system is likely to have much chance to evolve). Even so, there is really no preferential direction to scan, since the universe appears to be homogenous and isotropic on large scales (that is, clusters of stars are distributed roughly equally in every direction).

There is one other interesting search factor to take into account, and that is distance away from us. Signal strength decreases proportionally to the square of the distance traveled, so signals weaken over very large distances and anything sent from, say, another galaxy would require a huge amount of energy to produce. Now, this might be no problem for a very advanced civilization which was able to harness the full power of its sun (through a Dyson Sphere, e.g.), but that is not the only problem with sending signals over such vast distances.

The nearest major galaxy, Andromeda, is over two million light-years away. This means that a signal sent at the speed of light (about 300,000 kilometers per second) from a star system in the Andromeda galaxy would take at least two million years to reach us, and any reply we sent would take another two million years to return. Thus, any sort of two-way communication to even the closest galaxy would be prohibitively slow; by the time a reply came, four million years would have passed and things would certainly have changed on the planet that sent the initial signal (indeed, the civilization may no longer exist, having destroyed itself with technology, or had its only populated planet destroyed by an impact, or having its sun die before they could master interstellar travel). In the long run, it is certainly still worth looking for signals sent from other galaxies (a positive signal would prove that we are not the only life in the universe, even if we could not easily reply and establish two-way communication), but given the hundred billion or so stars in our own galaxy are so much closer, it appears wisest to concentrate our search to the Milky Way right now.

Even within our galaxy, though, the same problem exists: the Milky Way has a diameter of around 100,000 light-years, which means a signal sent from one side to the other would take 100,000 years to arrive. We could restrict our search to stars within something like fifty light-years, but that is still a one-hundred year wait between each reply; and of course the number of stars that close is pretty small and thus even optimistic estimates on the commonality of life developing and evolving give slim odds on finding any neighbors that close. So it appears right now that signals between intelligent life in other star systems are going to be limited to one-way communication unless we find some way around the cosmic speed limit (the speed of light, faster than which no information or matter can be transmitted according to current science). We will not be carrying on conversations with far away aliens any time soon.

Yet things are not entirely gloomy. Just because we cannot talk to them on the scale of a human lifetime does not mean that our contact will be limited to simply detecting their existence. If we discover a signal from another civilization, it very well might contain a huge repository of information - perhaps a scientific primer sent by a benevolent civilization - which would be very useful to us. Someone might point out that we have not been around broadcasting our presence for more than a few decades, so there has not really been enough time for a civilization to detect our presence and find any reason to send such complicated signals directly to us. However, they might have the resources to broadcast a large signal widely and not have to be aiming for us specifically. Or they may have used spectroscopy to detect the oxygen-rich atmosphere (biogenic conditions) around our planet and decided to send signals to our solar system and other ones with similar planets just in case.

What if we want more than just one-way transmissions, however full of useful information they may be? Interstellar travel is one interesting possibility. If we detected another civilization, even one many light-years away (still within our own galaxy, of course), we could send a manned ship in their direction as a sort of ambassador or representative for our species; of course, since the ship would go slower than the speed of light, it would take even longer than a transmission, and a return flight to pass along information would be even less useful than sending signals that take ages to arrive. The trip then would be one-way as well, but at least some humans would be able to come into direct contact with the alien civilization, which is an amazing thing in itself. This assumes the aliens are still around on arrival, but it might be worth going even if there is a chance they are no longer present; after all, if we have mastered interstellar travel, some sort of colonization process to other systems would be a plausible goal, and what better place to send ships than places that at the very least have had civilizations at one point in time even if they are not around any more (we could certainly learn much from the artifacts left behind).

The question then is how manageable interstellar travel is. At first, given the distances involved between even relatively close stars, it appears that the trip would take longer than a human lifetime. So it seems that unless some sort of cryogenic freezing and thawing process is perfected, no human could set out from Earth and arrive at the destination. More likely, the flight would have to be multigenerational and the ship self-sustaining for a very long trip; it would be the children or grandchildren or great-great-great grandchildren who arrived rather than the astronaut population that set out initially. Finding the energy to propel such a ship (which would have to be fairly large to support livable conditions for so many people) would be tough, but it is conceivable given plausible technological advances.

A more interesting possibility for interstellar travel exists thanks to the discoveries of Einstein in the early twentieth century and developments since then. We now understand that when a ship (or particle or anything else) approaches very close to the speed of light (say, .9999c where c is the speed of light), time 'passes slower' for the traveling object than it does for objects not traveling that fast. What this means is that a ship traveling to the nearby star Sirius - measured at ~8.6 light-years away - at near the speed of light would be observed by watchers on Earth to take a little over 8.6 years to arrive; but the clocks inside the ship itself would show less than a year as having passed (and the people on board would only be a year older).

While it seems paradoxical that a ship traveling slower than the speed of light could cover X light-years distance in less than X years, the apparent paradox is resolved when one realizes that distance measurements are defined based upon time (speed * time = distance traveled), so if time passes differently depending on the reference frame (basically sitting still on Earth versus traveling near the speed of light in the ship), then the distance will be measured differently depending on the reference frame. So the measurement of Sirius as being about 8.6 light-years away is accurate only in the Earth's reference frame; in the reference frame of the ship when it is going its fastest, the distance to Sirius actually decreases. These strange relativistic effects (called this because they are predicted by Einstein's theory of relativity) are why astronomers now think of our universe in terms of one space-time fabric rather than the previous notions of three-dimensional space being separate from some objective, separate 'dimension' of time.

Given this understanding of the effects of relativity at near-light speeds, we could theoretically take advantage of this to travel huge distances in a human lifetime (as measured on board the ship, of course; Earth time would still pass at a normal rate and thus the astronaut's children would age and die long before the astronauts themselves). Unfortunately, there are some major limitations on near-light speed travel which might make it all but impossible for us, short of some major and as-yet-unimagined technological advances.

For one, the energy required to get a ship up to sufficient speed is way beyond anything we can produce on Earth today (even adding up all of the energy we have used as long as our civilization has been around). Perhaps if we found a way to create and contain antimatter we could harness some sort of matter-antimatter drive to power the ship (this is the most efficient type of reaction, where every bit of matter is transferred into energy in the process). Even then there may not be room to store enough fuel to get up to relativistic speeds (and maintain them despite drag caused by interstellar dust). On top of that, traveling at such high speeds would mean that running into even tiny dust particles would hit with the force of nuclear explosions. Perhaps a streamlined needle-shaped ship with a strong shield in front (repaired constantly by self-replicating nano-scale robots) could penetrate the interstellar medium safely, but this technology is still far beyond us. Thus while travel at near light speed would allow humans to travel immense distances while aging very little, it does not appear to be the ideal option for meeting up with alien civilizations at the moment.

It is plausible that other more advanced civilizations have indeed conquered all of the problems of interstellar travel, so perhaps they could show up on our doorstep one day, not requiring us to do the traveling. However, this seems rather unlikely for anything but the most casually ubiquitous civilization since we have only been around broadcasting our presence (leaking radio and TV signals into space) for a few decades, which means anything past fifty or so light-years could not have discovered us yet. At best, Earth would only appear a biogenic planet to someone who was looking in this direction, and these hypothetical interstellar voyagers probably have more interesting places to visit than an unassuming planet with potential for life (or possibly an intelligence that has not been around very long); surely there are some other more advanced alien worlds out there worth visiting instead.

So for now it seems like our best bet is to continue searching the skies, listening for signals sent from afar (intentionally or not) in order to confirm once and for all that we are not alone (and perhaps learn some amazing stuff if aliens can afford to set beacons broadcasting extensive messages). More direct, two-way communication with alien intelligence appears to be out of reach for the time being, and interstellar travel is still far off for an adolescent civilization like our own (which is still in danger of self-destruction). We do, however, have a few billion years before our sun begins to expand and die, which seems like plenty of time to develop in ways we cannot even imagine (transhuman artificial evolution, e.g., and megascale engineering); and if and when we master interstellar travel ourselves, we may be able to finally meet up with our fellow cosmic inhabitants as we travel around the galaxy, a nomad civilization offshoot from our progenitors back on Terra Firma.