Fermi's Paradox And The Second Law Of Thermodynamics
The second law may be formulated by the observation that the entropy of isolated systems left to spontaneous evolution cannot decrease, as they always arrive at a state of thermodynamic equilibrium where the entropy is highest at the given internal energy. An increase in the combined entropy of system and surroundings accounts for the irreversibility of natural processes, often referred to in the concept of the arrow of time.
According to Wikipedia,Entropy is a scientific concept as well as a measurable physical property that is most commonly associated with a state of disorder, randomness, or uncertainty.
It's interesting that the definition of the Second Law of Thermodynamics above contains the word "evolution", since skeptics of neo-Darwinism often challenge the theory with reference to the Second Law. Somewhat restated, this will say that if you take a pot of water and cover it up, no matter how long you leave it, it won't spontaneously generate life. Even if you drop in some amino acids, if you just leave the pot alone, you won't get lfe, because that would assume the order in the pot would increase; i.e., the amino acids would somehow become more complex and regenerate themselves.Instead, the Second Law would say that without any other input, the amino acids and any other minerals you found in the pot will instead deteriorate, to the point that even the pot itself will eventually fall apart and any remaining water would spill out, except it would have evaporated long before then.
The whole idea of life on other planets hinges on the possibility -- or indeed, the probability -- that given a certain set of preconditions on a planet, life will somehow emerge. A second question is if life emerges at all, presumably as some form of primitive pond algae, how it can become more complex or more ordered over time and "evolve" into an advanced civilization capable of telecommunications or space travel. After all, a planet or a solar system can be considered an isolated system, and the assumption is that all of its components will become more disordered over time.
This is certainly borne out when we look at Mars or Venus. We might assume that Mars at one time had a more complete atmosphere, or that at one time it had surface water, but following the Second Law, Mars has become more disordered over time. There's no evidence of life there now, at least so far, and the best we can do is speculate that at one point it could have emerged, but under the Second Law this is unlikely, and it must in any case have succumbed to the increasing disorder we would expect on the planet as an isolated system.
The idea that there is some high statistical probability of alien life, and indeed not just life, but intelligent alien civilizations, emerged in the late 1950s. Again according to Wikipedia,
In September 1959, physicists Giuseppe Cocconi and Philip Morrison published an article in the journal Nature with the provocative title "Searching for Interstellar Communications". Cocconi and Morrison argued that radio telescopes had become sensitive enough to pick up transmissions that might be broadcast into space by civilizations orbiting other stars. Such messages, they suggested, might be transmitted at a wavelength of 21 cm (1,420.4 MHz). This is the wavelength of radio emission by neutral hydrogen, the most common element in the universe, and they reasoned that other intelligences might see this as a logical landmark in the radio spectrum.
Two months later, Harvard University astronomy professor Harlow Shapley speculated on the number of inhabited planets in the universe, saying "The universe has 10 million, million, million suns (10 followed by 18 zeros) similar to our own. One in a million has planets around it. Only one in a million million has the right combination of chemicals, temperature, water, days and nights to support planetary life as we know it. This calculation arrives at the estimated figure of 100 million worlds where life has been forged by evolution."
There have been numerous attempts to detect alien transmissions following the criteria set by Cocconi and Morrison, but in the 60-odd years since, no such signals have been detected. One basic problem is the Second Law: even if you have a planet with "the right combination of chemicals, temperature, water, days and nights to support planetary life as we know it", things on that planet as an isolated system can only get more disordered over time; over billions of years, that planet, like Mars, can lose its atmosphere, lose its surface water, lose its chemicals, or whatever else even before lfe has a chance to develop, much less evolve into a civilization capable of radio transmissions.