Given the fascination with aliens in Hollywood science fiction movies, one would think that the public and the government would consider SETI (the Search for ExtraTerrestrial Intelligence) to be an exciting and worthwhile area of research. This is a research field that costs little, but has faced active opposition. The federally funded SETI program was started in 1992 and just a year later, Senator Richard Bryan of Nevada argued that funding should be shut down: "...As of today, millions have been spent and we have yet to bag a single little green fellow. Not a single Martian has said 'take me to your leader,' and not a single flying saucer has applied for FAA approval."
Cleverly argued, but is the issue really fiscal conservatism? Scientists have been funded to search for illusive dark matter particles for more than 40 years, but not a single one has yet been found. Searches for black holes went on for decades before the first candidates were found. The hunt for exoplanets dates back a few centuries, but the first exoplanet was not detected around a Sun-like star until 1995. Transformational science requires that we push the boundaries.
The issue cannot simply be that little green men (LGM) have not yet been found. It seems likely that there are other reasons motivating Richard Bryan and others who have wanted to stop SETI. It is possible that religious or cultural beliefs lends a privileged (if incorrect) view on this issue. Perhaps they believe that humans are the pinnacle of existence and the reason for the universe; therefore, they cannot imagine that anything else exists. Or perhaps people are afraid - if LGM really exist, some people might just prefer to be oblivious about this issue. However, individuals who are curious about the universe want to know. The range of possibilities is staggering. On the one hand, we may be alone. On the other... if life turns out to exist on even one-in-a-million Earth-like planets, then our galaxy could be teaming with life. Either case has profound implications. SETI is a scientific exploration and a search for knowledge. Ultimately, being informed and knowledgeable is an advantage, no matter what the truth. SETI is no longer funded by the federal government. Instead, private donors now fund this enterprise.
In 2007, Jill Tarter wrote:
"If we can find technosignatures - evidence of some technology that modifies its environment in ways that are detectable - then we will be permitted to infer the existence, at least at some time, of intelligent technologists."
In the late 1950's radio astronomers were able to send or receive radio signals over large distances in our galaxy and realized that they could use this technology to search for technological, communicating civilizations that might either be leaking or transmitting signals. SETI began in 1960, when Frank Drake used the Green Bank radio telescope to observe the nearby stars Tau Ceti and Epsilon Eridani in a project that he dubbed "Project Ozma." To minimize confusion from background sources, the radio search concentrated on a very narrow wavelength band near 1420 Megahertz. Drake observed the stars for four months, always with a negative result until he found a surprising signal on April 8, 1960. An extraordinarily careful scientist, Drake followed up and ultimately discovered that this was a false signal from a high-flying aircraft. But, the realization of what it would feel like to detect another civilization started him on a life long search for technological signatures (coined "technosignatures" by Jill Tarter).
Narrow band radio searches were the first way that astronomers searched for technological signals. The low energy radio waves pass uninterrupted through the dust in our galaxy. Hydrogen, which is the most common element in the universe, forms H2 molecules in cold interstellar space and radiates at the 1420 MHz wavelength that Frank Drake was observing. Alien astronomers would likewise understand the importance of this wavelength - they would know that astute astronomers elsewhere would be studying the galaxy at the 1420 MHz wavelength. If the alien astronomers wanted their signals to be found, what better way than to broadcast their message at a wavelength where others would be looking?
In addition to searches for radio signals, there is an optical SETI program that scans the sky to search for brief flashes of laser light from known planetary systems. If humans set up a research station on Mars, one efficient way of communicating with Earth would be to send pulsed laser signals - a sort of Morse code. The high frequency of optical light makes it possible to send vast amounts of information. Lasers are highly coherent beams, but there would still be some scattered spill-over light that might be detectable in directions that are approximately aligned at that moment with Earth and Mars.
Another possible technosignature would be the observation of a structure built by aliens, perhaps to efficiently harvest energy from their host star. This idea was popularized by Freeman Dyson in his 1960 paper "Search for Artificial Sources of Infrared Radiation." Dyson thought that advanced civilizations might build build artificial megastructures in response to increasing energy demands. He envisioned the megastructure as a swarm of satellites orbiting the star and beaming energy back to their home planet. Starlight would be dimmed and perhaps reddened if the swarm was dense enough and this effect might be observable in the stellar spectrum.
KIC 8462852: Tabby's star
In 2016, a postdoctoral fellow at Yale University, Tabby Boyajian, published a paper on a strange light curve measured by Kepler. The signal was strong but irregular and unlike anything that had been seen before. The paper sat on her desk for two years as the Planet Hunter team tried to reason out an explanation - the best idea was that this was a disintegrating asteroid or irregular dust cloud. Failing any certain ideas for the source of the signal, the paper was published in the hopes that someone else would have a better explanation. Almost immediately, Jason Wright at Penn State University suggested an idea: perhaps this was an alien megastructure, like the one suggested by Freeman Dyson. Of course, this idea was picked up in the popular press. While the answer is still unknown, there is still an ongoing research program to better understand this star.
While there is no evidence for life elsewhere today, in an interview with Wired magazine, Jill Tarter posits that we will figure out if life exists beyond Earth in this century. Even though SETI searches have been carried out since 1960, Tarter compares the vastness of search space to the oceans and the relative size of space that has been searched to a bathtub. If we find even a single example of life elsewhere, we will know that life is statistically ubiquitous.