SETI: All About the Search for Extraterrestrial Intelligence (Infographic)
Are we the only intelligent life in the universe? Here's how we intend to find out.
Credit: By Karl Tate, Infographics Artist
The search for extraterrestrial intelligence (SETI) got a boost in July 2015, when investor Yuri Milner and physicist Stephen Hawking (left) announced a new $100 million SETI initiative called Breakthrough Listen.
The first serious, scientific attempt to listen for alien radio signals was Project Ozma in 1960, conducted by astronomer Frank Drake.
Since the invention of radio in 1900, researchers have occasionally detected unexplained signals that have led them to wonder about the possibility of life on other planets. In the 1960s, robotic probes revealed that the other planets of this solar system are not compatible with advanced civilizations. The many other planets and moons in the Milky Way galaxy and in billions of other galaxies in the universe still provide hope for the discovery of alien civilizations.
Popular culture often depicts unidentified flying objects (UFOs) as alien spacecraft and bizarre ancient artifacts as evidence that alien beings visited the Earth in the distant past. Scientists, however, have never found evidence that either of these things have ever occurred and do not take alien UFOs and ancient astronauts seriously.
Astronomer Frank Drake created an equation to estimate the number of intelligent, communicating civilizations currently living in the galaxy.
N: The total number of communicating technological civilizations in the galaxy equals
R: Formation rate of life-friendly stars, times
fp: Fraction of those stars with planets, times
ne: Average number of life-friendly planets per solar system, times
fl: Fraction of those planets where life evolves, times
fi: Fraction of those planets where intelligence evolves, times
fc: Fraction of those planets where interstellar-communication-worthy civilizations evolve, times
Lc: Length of time those civilizations remain detectable from Earth
The current estimate of the number of communicating technological civilizations in this galaxy is between 2 civilizations and 280 million civilizations.
An anomalous radio signal was detected by astronomer Jerry R. Ehman of Ohio State University at 10:16 p.m. on Aug. 15, 1977. The signal, 30 times louder than the background noise, appears to have come from a point on the sky near the giant orange star Tau Sagittarii, which is 122 light-years away.
It has not been determined if the “Wow! signal” was of extraterrestrial origin, but the signal bears hallmarks expected of a signal transmitted from deep space. The signal lasted 72 seconds, which would be the expected duration as the Earth rotated out of alignment with the source. Also, the signal was very close to the frequency of hydrogen, the most common element in the universe and an especially quiet part of the electromagnetic spectrum.
Another method of communicating with aliens would be to send an interstellar messenger spacecraft (a “Bracewell probe”), as proposed by Ronald Bracewell in 1960. The alien pyramid in Arthur C. Clarke’s story “The Sentinel” (later depicted as a black monolith in the film “2001: A Space Odyssey”) could be considered a Bracewell probe. The probe would attempt to communicate with aliens and transmit its findings back to the species that launched it.
Active SETI, or messaging to extraterrestrial intelligence (METI), has taken many forms, from encoded radio signals to artifacts attached to space probes sent out of this solar system.
Critics of METI point out that alerting intelligent aliens to our existence may not be a good idea. Stephen Hawking has noted that meetings between advanced and less advanced human populations in the past have often not worked out well for the less advanced population.
Another means of detecting advanced alien civilizations would be to spot evidence of artificially modified environments. Giant structures such as Dyson spheres (as seen in “Star Trek: The Next Generation”) or habitable rings surrounding a star (as described in Larry Niven's novel “Ringworld,” 1970) would reveal great technological power to reshape a solar system. The lights of cities on a planet surface would indicate a civilization at or above our own level of development.
Some scientists estimate that life could have evolved very quickly after the Big Bang, perhaps when the universe was only 15 million years old. Life that evolved intelligence rapidly could therefore have a civilization that is now more than 13 billion years old.
Russian astronomer Nikolai Kardashev proposed in 1964 the Kardashev scale, a means of estimating possible levels of technology and resource usage:
Type 0
The most basic technological civilization might be capable of searching radio waves for signs of intelligent transmissions. Carl Sagan wrote in “The Cosmic Connection” (1973) that Earth at that time was about 0.7 on the Kardashev scale (adjusted to 0.724 in 2012).
Type I
A civilization with the ability to utilize all the energy generated by a planet’s atmosphere, or about 1016 power watts.
Type II
A civilization capable of stellar engineering, including the building of a Dyson sphere surrounding a solar system. Energy utilization of about 1026 power watts.
Type III
A civilization capable of utilizing the energy of an entire galaxy. About 1036 power watts.
Type IV: 
Beings who utilize the resources of the entire universe.
Physicist Enrico Fermi asked in 1950, if most stars have planets, and alien civilizations could be billions of years older than humans’, and if the galaxy could be explored and colonized within only about a million Earth years (even at sub-light speed) ... where is everybody?
Some possible refutations of the Fermi Paradox:
IT’S NOT A PARADOX: The fact that we don’t see evidence of aliens in space does not mean they do not exist. Aliens could be all around us, but we don’t know how to recognize them. Perhaps radio is not the best means of interstellar communications and nobody else uses it.
ALIENS DON’T BEHAVE HOW WE EXPECT: The assumption that alien civilizations would expand continually through the galaxy, and that their colonies would persist for millions or billions of years, could be wrong. Perhaps space travel is so difficult and expensive that nobody does it.
TECHNOLOGICAL CIVILIZATIONS COMMIT SUICIDE: Carl Sagan observed that even 20th-century humans had the ability to destroy all life on Earth with nuclear weapons. Perhaps all civilizations, once they reach a certain stage of development, wipe themselves out.
STARS GO BOOM: Gamma-ray bursts could flood the galaxy with deadly radiation from time to time. This would destroy all life that had managed to evolve.
THE WORLD ISN’T WHAT WE THINK IT IS: It has been proposed that the Earth’s solar system is in a “quarantine zone,” a protected zoo set up by aliens. Or perhaps we are living in a computer simulation a la “The Matrix.”
WHACK-A-MOLE: Vicious predators could roam the galaxy, wiping out any civilization that reveals itself. That would also be why we don’t hear any radio signals: Nobody’s sending them.
NOBODY’S HOME: Perhaps advanced technological beings move from normal space-time to some safer plane of existence, or implant their consciousness into machines that roam the space between galaxies.
WE’RE No. 1: Maybe humans are the first technological species to evolve in this galaxy.

Embed: Paste the code below into your site.