How to find aliens

How can we find aliens? Some people have noted that Earth radiates various radio signals leading to the hope that other intelligent life may radiate radio signals. Life implies a much more general profile than "radio signals" which we might hope to detect.

The definition of life has always been rather problematic. The interesting definition which we will work from here is "An object is alive if it decreases it's own entropy." This fits most recognizable forms of life (humans, animals, plants, etc...) and some non-recognized forms of life (computers).

How alive is the Earth today by this standard? Humanity is certainly causing more heat than they did 100 years ago, but quantity is still small on the scale of daily sunlight. Still, the amount of power being consumed by humanity is gradually increasing. Extrapolating on this trend, we might expect that someday the quantity of power in use will exceed the daily input of energy from the sun. The collection of power on such a scale would, of course, be nontrivial but still not violate any known laws of physics. A large solar array could beam power back to the planet using microwaves (for example).

Here are some statements which are best guesses of current science:

Given these statements, and a projected increase in power requirements, an alive system may evolve to the following state: an airless planet covered with radiators radiating heat away while a solar array beams power down onto the planet. We expect a planet because we expect that life evolved on a planet and the energy requirements necessary to deform a planet are very high. The planet is airless because an atmosphere slows the radiation of heat. The radiators and solar array are simply optimizations upon the basic form.

The profile of an alien is a planet which is hotter than it should be based upon radiation from a nearby star.

How close are we to being able to detect such objects? The keck telescope recently achieved a .02 arcsecond resolution imaging Titan (a moon of Saturn). This implies that the Keck telescope can image objects with size/distance proportional to 1.5*10^(-8). If we assume that an interesting planet the size of earth is 10 light years ~ 10^14 kilometers then we can hope to detect an object of size ~1.5*10^6 kilometers. This is a little to large perhaps - Earth has a diameter of just ~1.5*10^4 km. Since resolution grows linearly with the size of the viewing instrument and the Keck telescope is 10 meters across we need a telescope that is a kilometer across to image an Earth sized planet in a nearby solar system. Larger distances would imply that larger telescopes are necessary.


There is also the question of what to do when an alien planet is discovered. Because interstellar travel is so difficult, it is unlikely that any kind of physical trade could be profitable which implies that the only trade worthwhile is trade in information. The "right" choice of communication instrument is probably a laser.