The search for extraterrestrial intelligence
(SETI) is a collective term for scientific
searches for intelligent extraterrestrial
life, for example, monitoring electromagnetic
radiation for signs of transmissions from
civilizations on other planets.Scientific
investigation began shortly after the advent
of radio in the early 1900s, and focused international
efforts have been going on since the 1980s.
In 2015, Stephen Hawking and Russian billionaire
Yuri Milner announced a well-funded effort
called Breakthrough Listen.
== History ==
=== 
Early work ===
There have been many earlier searches for
extraterrestrial intelligence within the Solar
System. In 1896, Nikola Tesla suggested that
an extreme version of his wireless electrical
transmission system could be used to contact
beings on Mars. In 1899, while conducting
experiments at his Colorado Springs experimental
station, he thought he had detected a signal
from that planet since an odd repetitive static
signal seemed to cut off when Mars set in
the night sky. Analysis of Tesla's research
has ranged from suggestions that Tesla detected
nothing and that he simply misunderstood the
new technology he was working with, to claims
that Tesla may have been observing signals
from Marconi's European radio experiments
and even that he could have picked up naturally
occurring Jovian plasma torus signals. In
the early 1900s, Guglielmo Marconi, Lord Kelvin
and David Peck Todd also stated their belief
that radio could be used to contact Martians,
with Marconi stating that his stations had
also picked up potential Martian signals.On
August 21–23, 1924, Mars entered an opposition
closer to Earth than at any time in the century
before or the next 80 years. In the United
States, a "National Radio Silence Day" was
promoted during a 36-hour period from August
21–23, with all radios quiet for five minutes
on the hour, every hour. At the United States
Naval Observatory, a radio receiver was lifted
3 kilometres (1.9 miles) above the ground
in a dirigible tuned to a wavelength between
8 and 9 km, using a "radio-camera" developed
by Amherst College and Charles Francis Jenkins.
The program was led by David Peck Todd with
the military assistance of Admiral Edward
W. Eberle (Chief of Naval Operations), with
William F. Friedman (chief cryptographer of
the United States Army), assigned to translate
any potential Martian messages.A 1959 paper
by Philip Morrison and Giuseppe Cocconi first
pointed out the possibility of searching the
microwave spectrum, and proposed frequencies
and a set of initial targets.In 1960, Cornell
University astronomer Frank Drake performed
the first modern SETI experiment, named "Project
Ozma", after the Queen of Oz in L. Frank Baum's
fantasy books. Drake used a radio telescope
26 metres (85 ft) in diameter at Green Bank,
West Virginia, to examine the stars Tau Ceti
and Epsilon Eridani near the 1.420 gigahertz
marker frequency, a region of the radio spectrum
dubbed the "water hole" due to its proximity
to the hydrogen and hydroxyl radical spectral
lines. A 400 kilohertz band around the marker
frequency was scanned, using a single-channel
receiver with a bandwidth of 100 hertz. He
found nothing of interest.
Soviet scientists took a strong interest in
SETI during the 1960s and performed a number
of searches with omnidirectional antennas
in the hope of picking up powerful radio signals.
Soviet astronomer Iosif Shklovsky wrote the
pioneering book in the field, Universe, Life,
Intelligence (1962), which was expanded upon
by American astronomer Carl Sagan as the best-selling
book Intelligent Life in the Universe (1966).
In the March 1955 issue of Scientific American,
John D. Kraus described an idea to scan the
cosmos for natural radio signals using a flat-plane
radio telescope equipped with a parabolic
reflector. Within two years, his concept was
approved for construction by Ohio State University.
With a total of US$71,000 in grants from the
National Science Foundation, construction
began on an 8-hectare (20-acre) plot in Delaware,
Ohio. This Ohio State University Radio Observatory
telescope was called "Big Ear". Later, it
began the world's first continuous SETI program,
called the Ohio State University SETI program.
In 1971, NASA funded a SETI study that involved
Drake, Bernard M. Oliver of Hewlett-Packard
Corporation, and others. The resulting report
proposed the construction of an Earth-based
radio telescope array with 1,500 dishes known
as "Project Cyclops". The price tag for the
Cyclops array was US$10 billion. Cyclops was
not built, but the report formed the basis
of much SETI work that followed.
The Ohio State SETI program gained fame on
August 15, 1977, when Jerry Ehman, a project
volunteer, witnessed a startlingly strong
signal received by the telescope. He quickly
circled the indication on a printout and scribbled
the exclamation "Wow!" in the margin. Dubbed
the Wow! signal, it is considered by some
to be the best candidate for a radio signal
from an artificial, extraterrestrial source
ever discovered, but it has not been detected
again in several additional searches.
=== Sentinel, META, and BETA ===
In 1980, Carl Sagan, Bruce Murray, and Louis
Friedman founded the U.S. Planetary Society,
partly as a vehicle for SETI studies.In the
early 1980s, Harvard University physicist
Paul Horowitz took the next step and proposed
the design of a spectrum analyzer specifically
intended to search for SETI transmissions.
Traditional desktop spectrum analyzers were
of little use for this job, as they sampled
frequencies using banks of analog filters
and so were restricted in the number of channels
they could acquire. However, modern integrated-circuit
digital signal processing (DSP) technology
could be used to build autocorrelation receivers
to check far more channels. This work led
in 1981 to a portable spectrum analyzer named
"Suitcase SETI" that had a capacity of 131,000
narrow band channels. After field tests that
lasted into 1982, Suitcase SETI was put into
use in 1983 with the 26-meter (85 ft) Harvard/Smithsonian
radio telescope at Oak Ridge Observatory in
Harvard, Massachusetts. This project was named
"Sentinel" and continued into 1985.
Even 131,000 channels were not enough to search
the sky in detail at a fast rate, so Suitcase
SETI was followed in 1985 by Project "META",
for "Megachannel Extra-Terrestrial Assay".
The META spectrum analyzer had a capacity
of 8.4 million channels and a channel resolution
of 0.05 hertz. An important feature of META
was its use of frequency Doppler shift to
distinguish between signals of terrestrial
and extraterrestrial origin. The project was
led by Horowitz with the help of the Planetary
Society, and was partly funded by movie maker
Steven Spielberg. A second such effort, META
II, was begun in Argentina in 1990, to search
the southern sky. META II is still in operation,
after an equipment upgrade in 1996.
The follow-on to META was named "BETA", for
"Billion-channel Extraterrestrial Assay",
and it commenced observation on October 30,
1995. The heart of BETA's processing capability
consisted of 63 dedicated fast Fourier transform
(FFT) engines, each capable of performing
a 222-point complex FFTs in two seconds, and
21 general-purpose personal computers equipped
with custom digital signal processing boards.
This allowed BETA to receive 250 million simultaneous
channels with a resolution of 0.5 hertz per
channel. It scanned through the microwave
spectrum from 1.400 to 1.720 gigahertz in
eight hops, with two seconds of observation
per hop. An important capability of the BETA
search was rapid and automatic re-observation
of candidate signals, achieved by observing
the sky with two adjacent beams, one slightly
to the east and the other slightly to the
west. A successful candidate signal would
first transit the east beam, and then the
west beam and do so with a speed consistent
with Earth's sidereal rotation rate. A third
receiver observed the horizon to veto signals
of obvious terrestrial origin. On March 23,
1999, the 26-meter radio telescope on which
Sentinel, META and BETA were based was blown
over by strong winds and seriously damaged.
This forced the BETA project to cease operation.
=== MOP and Project Phoenix ===
In 1978, the NASA SETI program had been heavily
criticized by Senator William Proxmire, and
funding for SETI research was removed from
the NASA budget by Congress in 1981; however,
funding was restored in 1982, after Carl Sagan
talked with Proxmire and convinced him of
the program's value. In 1992, the U.S. government
funded an operational SETI program, in the
form of the NASA Microwave Observing Program
(MOP). MOP was planned as a long-term effort
to conduct a general survey of the sky and
also carry out targeted searches of 800 specific
nearby stars. MOP was to be performed by radio
antennas associated with the NASA Deep Space
Network, as well as the 140-foot (43 m) radio
telescope of the National Radio Astronomy
Observatory at Green Bank, West Virginia and
the 1,000-foot (300 m) radio telescope at
the Arecibo Observatory in Puerto Rico. The
signals were to be analyzed by spectrum analyzers,
each with a capacity of 15 million channels.
These spectrum analyzers could be grouped
together to obtain greater capacity. Those
used in the targeted search had a bandwidth
of 1 hertz per channel, while those used in
the sky survey had a bandwidth of 30 hertz
per channel.
MOP drew the attention of the United States
Congress, where the program was ridiculed
and canceled one year after its start. SETI
advocates continued without government funding,
and in 1995 the nonprofit SETI Institute of
Mountain View, California resurrected the
MOP program under the name of Project "Phoenix",
backed by private sources of funding. Project
Phoenix, under the direction of Jill Tarter,
is a continuation of the targeted search program
from MOP and studies roughly 1,000 nearby
Sun-like stars. From 1995 through March 2004,
Phoenix conducted observations at the 64-meter
(210 ft) Parkes radio telescope in Australia,
the 140-foot (43 m) radio telescope of the
National Radio Astronomy Observatory in Green
Bank, West Virginia, and the 1,000-foot (300
m) radio telescope at the Arecibo Observatory
in Puerto Rico. The project observed the equivalent
of 800 stars over the available channels in
the frequency range from 1200 to 3000 MHz.
The search was sensitive enough to pick up
transmitters with 1 GW EIRP to a distance
of about 200 light-years. According to Prof.
Tarter, in 2012 it costs around "$2 million
per year to keep SETI research going at the
SETI Institute" and approximately 10 times
that to support "all kinds of SETI activity
around the world".
== Ongoing radio searches ==
Many radio frequencies penetrate Earth's atmosphere
quite well, and this led to radio telescopes
that investigate the cosmos using large radio
antennas. Furthermore, human endeavors emit
considerable electromagnetic radiation as
a byproduct of communications such as television
and radio. These signals would be easy to
recognize as artificial due to their repetitive
nature and narrow bandwidths. If this is typical,
one way of discovering an extraterrestrial
civilization might be to detect artificial
radio emissions from a location outside the
Solar System.
Many international radio telescopes are currently
being used for radio SETI searches, including
the Low Frequency Array (LOFAR) in Europe,
the Murchison Widefield Array (MWA) in Australia,
and the Lovell Telescope in the United Kingdom.
=== Allen Telescope Array ===
The SETI Institute collaborated with the Radio
Astronomy Laboratory at the Berkeley SETI
Research Center to develop a specialized radio
telescope array for SETI studies, something
like a mini-cyclops array. Formerly known
as the One Hectare Telescope (1HT), the concept
was renamed the "Allen Telescope Array" (ATA)
after the project's benefactor Paul Allen.
Its sensitivity would be equivalent to a single
large dish more than 100 meters in diameter
if completed. Presently, the array under construction
has 42 dishes at the Hat Creek Radio Observatory
in rural northern California.The full array
(ATA-350) is planned to consist of 350 or
more offset-Gregorian radio dishes, each 6.1
meters (20 feet) in diameter. These dishes
are the largest producible with commercially
available satellite television dish technology.
The ATA was planned for a 2007 completion
date, at a very modest cost of US$25 million.
The SETI Institute provided money for building
the ATA while University of California, Berkeley
designed the telescope and provided operational
funding. The first portion of the array (ATA-42)
became operational in October 2007 with 42
antennas. The DSP system planned for ATA-350
is extremely ambitious. Completion of the
full 350 element array will depend on funding
and the technical results from ATA-42.
ATA-42 (ATA) is designed to allow multiple
observers simultaneous access to the interferometer
output at the same time. Typically, the ATA
snapshot imager (used for astronomical surveys
and SETI) is run in parallel with the beam
forming system (used primarily for SETI).
ATA also supports observations in multiple
synthesized pencil beams at once, through
a technique known as "multibeaming". Multibeaming
provides an effective filter for identifying
false positives in SETI, since a very distant
transmitter must appear at only one point
on the sky.SETI Institute's Center for SETI
Research (CSR) uses ATA in the search for
extraterrestrial intelligence, observing 12
hours a day, 7 days a week. From 2007-2015,
ATA has identified hundreds of millions of
technological signals. So far, all these signals
have been assigned the status of noise or
radio frequency interference because a) they
appear to be generated by satellites or Earth-based
transmitters, or b) they disappeared before
the threshold time limit of ~1 hour. Researchers
in CSR are presently working on ways to reduce
the threshold time limit, and to expand ATA's
capabilities for detection of signals that
may have embedded messages.Berkeley astronomers
used the ATA to pursue several science topics,
some of which might have turned up transient
SETI signals, until 2011, when the collaboration
between the University of California and the
SETI Institute was terminated.
CNET published an article and pictures about
the Allen Telescope Array (ATA) on December
12, 2008.In April 2011, the ATA was forced
to enter an 8-month "hibernation" due to funding
shortfalls. Regular operation of the ATA was
resumed on December 5, 2011.In 2012, new life
was breathed into the ATA thanks to a $3.6M
philanthropic donation by Franklin Antonio,
Co-Founder and Chief Scientist of QUALCOMM
Incorporated. This gift supports upgrades
of all the receivers on the ATA dishes to
have dramatically (2x - 10x from 1–8 GHz)
greater sensitivity than before and supporting
sensitive observations over a wider frequency
range from 1–18 GHz, though initially the
radio frequency electronics go to only 12
GHz. As of July, 2013 the first of these receivers
was installed and proven. Full installation
on all 42 antennas is expected in June, 2014.
ATA is especially well suited to the search
for extraterrestrial intelligence SETI and
to discovery of astronomical radio sources,
such as heretofore unexplained non-repeating,
possibly extragalactic, pulses known as fast
radio bursts or FRBs.
=== SERENDIP ===
SERENDIP (Search for Extraterrestrial Radio
Emissions from Nearby Developed Intelligent
Populations) is a SETI program launched in
1979 by the Berkeley SETI Research Center.
SERENDIP takes advantage of ongoing "mainstream"
radio telescope observations as a "piggy-back"
or "commensal" program, using large radio
telescopes including the NRAO 90m telescope
at Green Bank and the Arecibo 305m telescope.
Rather than having its own observation program,
SERENDIP analyzes deep space radio telescope
data that it obtains while other astronomers
are using the telescopes.
The most recently deployed SERENDIP spectrometer,
SERENDIP V.v, was installed at the Arecibo
Observatory in June 2009 and is currently
operational. The digital back-end instrument
is an FPGA-based 128 million-channel digital
spectrometer covering 200 MHz of bandwidth.
It takes data commensally with the seven-beam
Arecibo L-band Feed Array (ALFA). The program
has found around 400 suspicious signals, but
there is not enough data to prove that they
belong to extraterrestrial intelligence.
=== Breakthrough Listen ===
Breakthrough Listen is a ten-year initiative
with $100 million funding begun in July 2015
to actively search for intelligent extraterrestrial
communications in the universe, in a substantially
expanded way, using resources that had not
previously been extensively used for the purpose.
It has been described as the most comprehensive
search for alien communications to date. The
science program for Breakthrough Listen is
based at Berkeley SETI Research Center, located
in the Astronomy Department at the University
of California, Berkeley.
Announced in July 2015, the project is observing
for thousands of hours every year on two major
radio telescopes, the Green Bank Observatory
in West Virginia and the Parkes Observatory
in Australia. Previously, only about 24 to
36 hours of telescope per year were used in
the search for alien life. Furthermore, the
Automated Planet Finder at Lick Observatory
is searching for optical signals coming from
laser transmissions. The massive data rates
from the radio telescopes (24 GB/s at Green
Bank) necessitated the construction of dedicated
hardware at the telescopes to perform the
bulk of the analysis. Some of the data are
also analyzed by volunteers in the SETI@home
distributed computing network. Founder of
modern SETI Frank Drake is one of the scientists
on the project's advisory committee.
=== FAST ===
China's 500 meter Aperture Spherical Telescope
(FAST) lists detecting interstellar communication
signals as part of its science mission. It
is funded by the National Development and
Reform Commission (NDRC) and managed by the
National Astronomical observatories (NAOC)
of the Chinese Academy of Sciences (CAS).
FAST is the first radio observatory built
with SETI as a core scientific goal. FAST
consists of a fixed 500 m (1,600 ft) diameter
spherical dish constructed in a natural depression
sinkhole caused by karst processes in the
region. It is the world's largest filled-aperture
radio telescope.
According to its website, FAST could search
out to 28 light-years, and would be able to
reach 1400 stars. If the transmitter's radiated
power is increased to 1000,000 MW, FAST would
be able to reach one million stars. This is
compared to the Arecibo 305 meter telescope
detection distance of 18 light-years.
== Community SETI projects ==
=== SETI@home ===
The SETI@home project uses distributed computing
to analyze signals acquired by the SERENDIP
project.
SETI@home was conceived by David Gedye along
with Craig Kasnoff and is a popular volunteer
distributed computing project that was launched
by the Berkeley SETI Research Center at the
University of California, Berkeley, in May
1999. It was originally funded by The Planetary
Society and Paramount Pictures, and later
by the state of California. The project is
run by director David P. Anderson and chief
scientist Dan Werthimer. Any individual can
become involved with SETI research by downloading
the Berkeley Open Infrastructure for Network
Computing (BOINC) software program, attaching
to the SETI@home project, and allowing the
program to run as a background process that
uses idle computer power. The SETI@home program
itself runs signal analysis on a "work unit"
of data recorded from the central 2.5 MHz
wide band of the SERENDIP IV instrument. After
computation on the work unit is complete,
the results are then automatically reported
back to SETI@home servers at University of
California, Berkeley. By June 28, 2009, the
SETI@home project had over 180,000 active
participants volunteering a total of over
290,000 computers. These computers give SETI@home
an average computational power of 617 teraFLOPS.
In 2004 radio source SHGb02+14a set off speculation
in the media that a signal had been detected
but researchers noted the frequency drifted
rapidly and the detection on three SETI@home
computers fell within random chance.As of
2010, after 10 years of data collection, SETI@home
has listened to that one frequency at every
point of over 67 percent of the sky observable
from Arecibo with at least three scans (out
of the goal of nine scans), which covers about
20 percent of the full celestial sphere.
=== SETI Net ===
SETI Network is the only operational private
search system.
The SETI Net station consists of off-the-shelf,
consumer-grade electronics to minimize cost
and to allow this design to be replicated
as simply as possible. It has a 3-meter parabolic
antenna that can be directed in azimuth and
elevation, an LNA that covers the 1420 MHz
spectrum, a receiver to reproduce the wideband
audio, and a standard personal computer as
the control device and for deploying the detection
algorithms.
The antenna can be pointed and locked to one
sky location, enabling the system to integrate
on it for long periods. Currently the Wow!
signal area is being monitored when it is
above the horizon. All search data are collected
and made available on the Internet archive.
SETI Net started operation in the early 1980s
as a way to learn about the science of the
search, and has developed several software
packages for the amateur SETI community. It
has provided an astronomical clock, a file
manager to keep track of SETI data files,
a spectrum analyzer optimized for amateur
SETI, remote control of the station from the
Internet, and other packages.
It can be reached at https://www.seti.net
=== The SETI League and Project Argus ===
Founded in 1994 in response to the United
States Congress cancellation of the NASA SETI
program, The SETI League, Inc. is a membership-supported
nonprofit organization with 1,500 members
in 62 countries. This grass-roots alliance
of amateur and professional radio astronomers
is headed by executive director emeritus H.
Paul Shuch, the engineer credited with developing
the world's first commercial home satellite
TV receiver. Many SETI League members are
licensed radio amateurs and microwave experimenters.
Others are digital signal processing experts
and computer enthusiasts.
The SETI League pioneered the conversion of
backyard satellite TV dishes 3 to 5 m (10–16
ft) in diameter into research-grade radio
telescopes of modest sensitivity. The organization
concentrates on coordinating a global network
of small, amateur-built radio telescopes under
Project Argus, an all-sky survey seeking to
achieve real-time coverage of the entire sky.
Project Argus was conceived as a continuation
of the all-sky survey component of the late
NASA SETI program (the targeted search having
been continued by the SETI Institute's Project
Phoenix). There are currently 143 Project
Argus radio telescopes operating in 27 countries.
Project Argus instruments typically exhibit
sensitivity on the order of 10−23 Watts/square
metre, or roughly equivalent to that achieved
by the Ohio State University Big Ear radio
telescope in 1977, when it detected the landmark
"Wow!" candidate signal.The name "Argus" derives
from the mythical Greek guard-beast who had
100 eyes, and could see in all directions
at once. In the SETI context, the name has
been used for radio telescopes in fiction
(Arthur C. Clarke, "Imperial Earth"; Carl
Sagan, "Contact"), was the name initially
used for the NASA study ultimately known as
"Cyclops," and is the name given to an omnidirectional
radio telescope design being developed at
the Ohio State University.
== Optical experiments ==
While most SETI sky searches have studied
the radio spectrum, some SETI researchers
have considered the possibility that alien
civilizations might be using powerful lasers
for interstellar communications at optical
wavelengths. The idea was first suggested
by R. N. Schwartz and Charles Hard Townes
in a 1961 paper published in the journal Nature
titled "Interstellar and Interplanetary Communication
by Optical Masers". However, the 1971 Cyclops
study discounted the possibility of optical
SETI, reasoning that construction of a laser
system that could outshine the bright central
star of a remote star system would be too
difficult. In 1983, Townes published a detailed
study of the idea in the United States journal
Proceedings of the National Academy of Sciences,
which was met with widespread agreement by
the SETI community.There are two problems
with optical SETI. The first problem is that
lasers are highly "monochromatic", that is,
they emit light only on one frequency, making
it troublesome to figure out what frequency
to look for. However, emitting light in narrow
pulses results in a broad spectrum of emission;
the spread in frequency becomes higher as
the pulse width becomes narrower, making it
easier to detect an emission.
The other problem is that while radio transmissions
can be broadcast in all directions, lasers
are highly directional. Interstellar gas and
dust is almost transparent to near infrared,
so these signals can be seen from greater
distances, but the extraterrestrial laser
signals would need to be transmitted in the
direction of Earth in order to be detected.Optical
SETI supporters have conducted paper studies
of the effectiveness of using contemporary
high-energy lasers and a ten-meter diameter
mirror as an interstellar beacon. The analysis
shows that an infrared pulse from a laser,
focused into a narrow beam by such a mirror,
would appear thousands of times brighter than
the Sun to a distant civilization in the beam's
line of fire. The Cyclops study proved incorrect
in suggesting a laser beam would be inherently
hard to see.
Such a system could be made to automatically
steer itself through a target list, sending
a pulse to each target at a constant rate.
This would allow targeting of all Sun-like
stars within a distance of 100 light-years.
The studies have also described an automatic
laser pulse detector system with a low-cost,
two-meter mirror made of carbon composite
materials, focusing on an array of light detectors.
This automatic detector system could perform
sky surveys to detect laser flashes from civilizations
attempting contact.
Several optical SETI experiments are now in
progress. A Harvard-Smithsonian group that
includes Paul Horowitz designed a laser detector
and mounted it on Harvard's 155 centimeters
(61 inches) optical telescope. This telescope
is currently being used for a more conventional
star survey, and the optical SETI survey is
"piggybacking" on that effort. Between October
1998 and November 1999, the survey inspected
about 2,500 stars. Nothing that resembled
an intentional laser signal was detected,
but efforts continue. The Harvard-Smithsonian
group is now working with Princeton University
to mount a similar detector system on Princeton's
91-centimeter (36-inch) telescope. The Harvard
and Princeton telescopes will be "ganged"
to track the same targets at the same time,
with the intent being to detect the same signal
in both locations as a means of reducing errors
from detector noise.
The Harvard-Smithsonian SETI group led by
Professor Paul Horowitz built a dedicated
all-sky optical survey system along the lines
of that described above, featuring a 1.8-meter
(72-inch) telescope. The new optical SETI
survey telescope is being set up at the Oak
Ridge Observatory in Harvard, Massachusetts.
The University of California, Berkeley, home
of SERENDIP and SETI@home, is also conducting
optical SETI searches and collaborates with
the NIROSETI program. The optical SETI program
at Breakthrough Listen is being directed by
Geoffrey Marcy, an extrasolar planet hunter,
and it involves examination of records of
spectra taken during extrasolar planet hunts
for a continuous, rather than pulsed, laser
signal. This survey uses the Automated Planet
Finder 2.4-m telescope at the Lick Observatory,
situated on the summit of Mount Hamilton,
east of San Jose, California, USA. The other
Berkeley optical SETI effort is being pursued
by the Harvard-Smithsonian group and is being
directed by Dan Werthimer of Berkeley, who
built the laser detector for the Harvard-Smithsonian
group. This survey uses a 76-centimeter (30-inch)
automated telescope at Leuschner Observatory
and an older laser detector built by Werthimer.
In May 2017, astronomers reported studies
related to laser light emissions from stars,
as a way of detecting technology-related signals
from an alien civilization. The reported studies
included KIC 8462852, an oddly dimming star
in which its unusual starlight fluctuations
may be the result of interference by an artificial
megastructure, such as a Dyson swarm, made
by such a civilization. No evidence was found
for technology-related signals from KIC 8462852
in the studies.
== Gamma-ray bursts ==
Gamma-ray bursts (GRBs) are candidates for
extraterrestrial communication. These high-energy
bursts are observed about once per day and
originate throughout the observable universe.
SETI currently omits gamma ray frequencies
in their monitoring and analysis because they
are absorbed by the Earth's atmosphere and
difficult to detect with ground-based receivers.
In addition, the wide burst bandwidths pose
a serious analysis challenge for modern digital
signal processing systems. Still, the continued
mysteries surrounding gamma-ray bursts have
encouraged hypotheses invoking extraterrestrials.
John A. Ball from the MIT Haystack Observatory
suggests that an advanced civilization that
has reached a technological singularity would
be capable of transmitting a two-millisecond
pulse encoding 1×1018 bits of information.
This is "comparable to the estimated total
information content of Earth's biosystem—genes
and memes and including all libraries and
computer media".
== Search for extraterrestrial artifacts ==
The possibility of using interstellar messenger
probes in the search for extraterrestrial
intelligence was first suggested by Ronald
N. Bracewell in 1960 (see Bracewell probe),
and the technical feasibility of this approach
was demonstrated by the British Interplanetary
Society's starship study Project Daedalus
in 1978. Starting in 1979, Robert Freitas
advanced arguments for the proposition that
physical space-probes are a superior mode
of interstellar communication to radio signals.
See Voyager Golden Record.
In recognition that any sufficiently advanced
interstellar probe in the vicinity of Earth
could easily monitor the terrestrial Internet,
Invitation to ETI was established by Prof.
Allen Tough in 1996, as a Web-based SETI experiment
inviting such spacefaring probes to establish
contact with humanity. The project's 100 Signatories
includes prominent physical, biological, and
social scientists, as well as artists, educators,
entertainers, philosophers and futurists.
Prof. H. Paul Shuch, executive director emeritus
of The SETI League, serves as the project's
Principal Investigator.
Inscribing a message in matter and transporting
it to an interstellar destination can be enormously
more energy efficient than communication using
electromagnetic waves if delays larger than
light transit time can be tolerated. That
said, for simple messages such as "hello,"
radio SETI could be far more efficient. If
energy requirement is used as a proxy for
technical difficulty, then a solarcentric
Search for Extraterrestrial Artifacts (SETA)
may be a useful supplement to traditional
radio or optical searches.Much like the "preferred
frequency" concept in SETI radio beacon theory,
the Earth-Moon or Sun-Earth libration orbits
might therefore constitute the most universally
convenient parking places for automated extraterrestrial
spacecraft exploring arbitrary stellar systems.
A viable long-term SETI program may be founded
upon a search for these objects.
In 1979, Freitas and Valdes conducted a photographic
search of the vicinity of the Earth-Moon triangular
libration points L4 and L5, and of the solar-synchronized
positions in the associated halo orbits, seeking
possible orbiting extraterrestrial interstellar
probes, but found nothing to a detection limit
of about 14th magnitude. The authors conducted
a second, more comprehensive photographic
search for probes in 1982 that examined the
five Earth-Moon Lagrangian positions and included
the solar-synchronized positions in the stable
L4/L5 libration orbits, the potentially stable
nonplanar orbits near L1/L2, Earth-Moon L3,
and also L2 in the Sun-Earth system. Again
no extraterrestrial probes were found to limiting
magnitudes of 17–19th magnitude near L3/L4/L5,
10–18th magnitude for L1/L2, and 14–16th
magnitude for Sun-Earth L2.
In June 1983, Valdes and Freitas used the
26 m radiotelescope at Hat Creek Radio Observatory
to search for the tritium hyperfine line at
1516 MHz from 108 assorted astronomical objects,
with emphasis on 53 nearby stars including
all visible stars within a 20 light-year radius.
The tritium frequency was deemed highly attractive
for SETI work because (1) the isotope is cosmically
rare, (2) the tritium hyperfine line is centered
in the SETI waterhole region of the terrestrial
microwave window, and (3) in addition to beacon
signals, tritium hyperfine emission may occur
as a byproduct of extensive nuclear fusion
energy production by extraterrestrial civilizations.
The wideband- and narrowband-channel observations
achieved sensitivities of 5–14 x 10−21
W/m²/channel and 0.7-2 x 10−24 W/m²/channel,
respectively, but no detections were made.
== Technosignatures ==
Technosignatures, including all signs of technology,
are a recent avenue in the search for extraterrestrial
intelligence. Technosignatures may originate
from various sources, from megastructures
such as Dyson spheres and space mirrors or
space shaders to the atmospheric contamination
created by an industrial civilization, or
city lights on extrasolar planets, and may
be detectable in the future with large hypertelescopes.Technosignatures
can be divided into three broad categories:
astroengineering projects, signals of planetary
origin, and spacecraft within and outside
the Solar System.
An astroengineering installation such as a
Dyson sphere, designed to convert all of the
incident radiation of its host star into energy,
could be detected through the observation
of an infrared excess from a solar analog
star, or by the star's apparent disappearance
in the visible spectrum over several years.
After examining some 100,000 nearby large
galaxies, a team of researchers has concluded
that none of them display any obvious signs
of highly advanced technological civilizations.Another
hypothetical form of astroengineering, the
Shkadov thruster, moves its host star by reflecting
some of the star's light back on itself, and
would be detected by observing if its transits
across the star abruptly end with the thruster
in front. Asteroid mining within the Solar
System is also a detectable technosignature
of the first kind.Individual extrasolar planets
can be analyzed for signs of technology. Avi
Loeb of the Harvard-Smithsonian Center for
Astrophysics has proposed that persistent
light signals on the night side of an exoplanet
can be an indication of the presence of cities
and an advanced civilization. In addition,
the excess infrared radiation and chemicals
produced by various industrial processes or
terraforming efforts may point to intelligence.
Light and heat detected from planets need
to be distinguished from natural sources to
conclusively prove the existence of civilization
on a planet. However, as argued by the Colossus
team,
a civilization heat signature should be within
a "comfortable" temperature range, like terrestrial
urban heat islands, i.e. only a few degrees
warmer than the planet itself. In contrast,
such natural sources as wild fires, volcanoes,
etc. are significantly hotter, so they will
be well distinguished by their maximum flux
at a different wavelength.
Extraterrestrial craft are another target
in the search for technosignatures. Magnetic
sail interstellar spacecraft should be detectable
over thousands of light-years of distance
through the synchrotron radiation they would
produce through interaction with the interstellar
medium; other interstellar spacecraft designs
may be detectable at more modest distances.
In addition, robotic probes within the Solar
System are also being sought out with optical
and radio searches.For a sufficiently advanced
civilization, hyper energetic neutrinos from
Planck scale accelerators should be detectable
at a distance of many Mpc.
== Fermi paradox ==
Italian physicist Enrico Fermi suggested in
the 1950s that if technologically advanced
civilizations are common in the universe,
then they should be detectable in one way
or another. (According to those who were there,
Fermi either asked "Where are they?" or "Where
is everybody?")
The Fermi paradox is commonly understood as
asking why extraterrestrials have not visited
Earth, but the same reasoning applies to the
question of why signals from extraterrestrials
have not been heard. The SETI version of the
question is sometimes referred to as "the
Great Silence".
The Fermi paradox can be stated more completely
as follows:
The size and age of the universe incline us
to believe that many technologically advanced
civilizations must exist. However, this belief
seems logically inconsistent with our lack
of observational evidence to support it. Either
(1) the initial assumption is incorrect and
technologically advanced intelligent life
is much rarer than we believe, or (2) our
current observations are incomplete and we
simply have not detected them yet, or (3)
our search methodologies are flawed and we
are not searching for the correct indicators,
or (4) it is the nature of intelligent life
to destroy itself.
There are multiple explanations proposed for
the Fermi paradox, ranging from analyses suggesting
that intelligent life is rare (the "Rare Earth
hypothesis"), to analyses suggesting that
although extraterrestrial civilizations may
be common, they would not communicate, or
would not travel across interstellar distances.
Science writer Timothy Ferris has posited
that since galactic societies are most likely
only transitory, an obvious solution is an
interstellar communications network, or a
type of library consisting mostly of automated
systems. They would store the cumulative knowledge
of vanished civilizations and communicate
that knowledge through the galaxy. Ferris
calls this the "Interstellar Internet", with
the various automated systems acting as network
"servers". If such an Interstellar Internet
exists, the hypothesis states, communications
between servers are mostly through narrow-band,
highly directional radio or laser links. Intercepting
such signals is, as discussed earlier, very
difficult. However, the network could maintain
some broadcast nodes in hopes of making contact
with new civilizations.
Although somewhat dated in terms of "information
culture" arguments, not to mention the obvious
technological problems of a system that could
work effectively for billions of years and
requires multiple lifeforms agreeing on certain
basics of communications technologies, this
hypothesis is actually testable (see below).
=== Difficulty of Detection ===
A significant problem is the vastness of space.
Despite piggybacking on the world's most sensitive
radio telescope, Charles Stuart Bowyer said,
the instrument could not detect random radio
noise emanating from a civilization like ours,
which has been leaking radio and TV signals
for less than 100 years. For SERENDIP and
most other SETI projects to detect a signal
from an extraterrestrial civilization, the
civilization would have to be beaming a powerful
signal directly at us. It also means that
Earth civilization will only be detectable
within a distance of 100 light-years.
== Post detection disclosure protocol ==
The International Academy of Astronautics
(IAA) has a long-standing SETI Permanent Study
Group (SPSG, formerly called the IAA SETI
Committee), which addresses matters of SETI
science, technology, and international policy.
The SPSG meets in conjunction with the International
Astronautical Congress (IAC) held annually
at different locations around the world, and
sponsors two SETI Symposia at each IAC. In
2005, the IAA established the SETI: Post-Detection
Science and Technology Taskgroup (Chairman,
Professor Paul Davies) "to act as a Standing
Committee to be available to be called on
at any time to advise and consult on questions
stemming from the discovery of a putative
signal of extraterrestrial intelligent (ETI)
origin."
However, the protocols mentioned apply only
to radio SETI rather than for METI (Active
SETI). The intention for METI is covered under
the SETI charter "Declaration of Principles
Concerning Sending Communications with Extraterrestrial
Intelligence".
On October 2000 astronomers Iván Almár and
Jill Tarter presented a paper to The SETI
Permanent Study Group in Rio de Janeiro, Brazil
which proposed a scale (modelled after the
Torino scale) which is an ordinal scale between
zero and ten that quantifies the impact of
any public announcement regarding evidence
of extraterrestrial intelligence; the Rio
scale has since inspired the 2005 San Marino
Scale (in regard to the risks of transmissions
from Earth) and the 2010 London Scale (in
regard to the detection of extraterrestrial
life) The Rio Scale itself was revised in
2018.The SETI Institute does not officially
recognize the Wow! signal as of extraterrestrial
origin (as it was unable to be verified).
The SETI Institute has also publicly denied
that the candidate signal Radio source SHGb02+14a
is of extraterrestrial origin. Although other
volunteering projects such as Zooniverse credit
users for discoveries, there is currently
no crediting or early notification by SETI@Home
following the discovery of a signal.
Some people, including Steven M. Greer, have
expressed cynicism that the general public
might not be informed in the event of a genuine
discovery of extraterrestrial intelligence
due to significant vested interests. Some,
such as Bruce Jakosky have also argued that
the official disclosure of extraterrestrial
life may have far reaching and as yet undetermined
implications for society, particularly for
the world's religions.
== Active SETI ==
Active SETI, also known as messaging to extraterrestrial
intelligence (METI), consists of sending signals
into space in the hope that they will be picked
up by an alien intelligence.
=== Realized interstellar radio message projects
===
In November 1974, a largely symbolic attempt
was made at the Arecibo Observatory to send
a message to other worlds. Known as the Arecibo
Message, it was sent towards the globular
cluster M13, which is 25,000 light-years from
Earth. Further IRMs Cosmic Call, Teen Age
Message, Cosmic Call 2, and A Message From
Earth were transmitted in 1999, 2001, 2003
and 2008 from the Evpatoria Planetary Radar.
=== Debate ===
Physicist Stephen Hawking, in his book A Brief
History of Time, suggests that "alerting"
extraterrestrial intelligences to our existence
is foolhardy, citing mankind's history of
treating his fellow man harshly in meetings
of civilizations with a significant technology
gap. He suggests, in view of this history,
that we "lay low". In one response to Hawking,
in September 2016, astronomer Seth Shostak,
allays such concerns. Astronomer Jill Tarter
also disagrees with Hawking, arguing that
aliens developed and long-lived enough to
communicate and travel across interstellar
distances would have evolved a cooperative
and less violent intelligence. She does think
it is too soon for humans to attempt active
SETI and that humans should be more advanced
technologically first but keep listening in
the meantime.The concern over METI was raised
by the science journal Nature in an editorial
in October 2006, which commented on a recent
meeting of the International Academy of Astronautics
SETI study group. The editor said, "It is
not obvious that all extraterrestrial civilizations
will be benign, or that contact with even
a benign one would not have serious repercussions"
(Nature Vol 443 12 October 06 p 606). Astronomer
and science fiction author David Brin has
expressed similar concerns.Richard Carrigan,
a particle physicist at the Fermi National
Accelerator Laboratory near Chicago, Illinois,
suggested that passive SETI could also be
dangerous and that a signal released onto
the Internet could act as a computer virus.
Computer security expert Bruce Schneier dismissed
this possibility as a "bizarre movie-plot
threat".To lend a quantitative basis to discussions
of the risks of transmitting deliberate messages
from Earth, the SETI Permanent Study Group
of the International Academy of Astronautics
adopted in 2007 a new analytical tool, the
San Marino Scale. Developed by Prof. Ivan
Almar and Prof. H. Paul Shuch, the scale evaluates
the significance of transmissions from Earth
as a function of signal intensity and information
content. Its adoption suggests that not all
such transmissions are equal, and each must
be evaluated separately before establishing
blanket international policy regarding active
SETI.
However, some scientists consider these fears
about the dangers of METI as panic and irrational
superstition; see, for example, Alexander
L. Zaitsev's papers. Biologist João Pedro
de Magalhães also proposed in 2015 transmitting
an invitation message to any extraterrestrial
intelligences watching us already in the context
of the Zoo Hypothesis and inviting them to
respond, arguing this would not put us in
any more danger than we are already if the
Zoo Hypothesis is correct.On 13 February 2015,
scientists (including Geoffrey Marcy, Seth
Shostak, Frank Drake, Elon Musk and David
Brin) at a convention of the American Association
for the Advancement of Science, discussed
Active SETI and whether transmitting a message
to possible intelligent extraterrestrials
in the Cosmos was a good idea; one result
was a statement, signed by many, that a "worldwide
scientific, political and humanitarian discussion
must occur before any message is sent". On
28 March 2015, a related essay was written
by Seth Shostak and published in The New York
Times.
=== Breakthrough Message ===
The Breakthrough Message program is an open
competition announced in July 2015 to design
a digital message that could be transmitted
from Earth to an extraterrestrial civilization,
with a US$1,000,000 prize pool. The message
should be "representative of humanity and
planet Earth". The program pledges "not to
transmit any message until there has been
a wide-ranging debate at high levels of science
and politics on the risks and rewards of contacting
advanced civilizations".
== Criticism ==
As various SETI projects have progressed,
some have criticized early claims by researchers
as being too "euphoric". For example, Peter
Schenkel, while remaining a supporter of SETI
projects, wrote in 2006 that
"[i]n light of new findings and insights,
it seems appropriate to put excessive euphoria
to rest and to take a more down-to-earth view
... We should quietly admit that the early
estimates—that there may be a million, a
hundred thousand, or ten thousand advanced
extraterrestrial civilizations in our galaxy—may
no longer be tenable."SETI has also occasionally
been the target of criticism by those who
suggest that it is a form of pseudoscience.
In particular, critics allege that no observed
phenomena suggest the existence of extraterrestrial
intelligence, and furthermore that the assertion
of the existence of extraterrestrial intelligence
has no good Popperian criteria for falsifiability,
as explained in a 2009 editorial in Nature,
which said:
"Seti ... has always sat at the edge of mainstream
astronomy. This is partly because, no matter
how scientifically rigorous its practitioners
try to be, SETI can't escape an association
with UFO believers and other such crackpots.
But it is also because SETI is arguably not
a falsifiable experiment. Regardless of how
exhaustively the Galaxy is searched, the null
result of radio silence doesn't rule out the
existence of alien civilizations. It means
only that those civilizations might not be
using radio to communicate."Nature added that
SETI was "marked by a hope, bordering on faith"
that aliens were aiming signals at us, that
a hypothetical alien SETI project looking
at Earth with "similar faith" would be "sorely
disappointed" (despite our many untargeted
radar and TV signals, and our few targeted
Active SETI radio signals denounced by those
fearing aliens), and that it had difficulties
attracting even sympathetic working scientists
and Government funding because it was "an
effort so likely to turn up nothing".However
Nature also added that "Nonetheless, a small
SETI effort is well worth supporting, especially
given the enormous implications if it did
succeed" and that "happily, a handful of wealthy
technologists and other private donors have
proved willing to provide that support".Supporters
of the Rare Earth Hypothesis argue that advanced
lifeforms are likely to be very rare, and
that, if that is so, then SETI efforts will
be futile. However the Rare Earth Hypothesis
itself faces many criticisms.In 1993 Roy Mash
claimed that "Arguments favoring the existence
of extraterrestrial intelligence nearly always
contain an overt appeal to big numbers, often
combined with a covert reliance on generalization
from a single instance" and concluded that
"the dispute between believers and skeptics
is seen to boil down to a conflict of intuitions
which can barely be engaged, let alone resolved,
given our present state of knowledge". In
2012 Milan M. Ćirković (who was then research
professor at the Astronomical Observatory
of Belgrade and a research associate of the
Future of Humanity Institute at the University
of Oxford) claimed that Mash was unrealistically
over-reliant on excessive abstraction that
ignored the empirical information available
to modern SETI researchers.George Basalla,
Emeritus Professor of History at the University
of Delaware, is a critic of SETI who argued
in 2006 that "extraterrestrials discussed
by scientists are as imaginary as the spirits
and gods of religion or myth", and has in
turn been criticized by Milan M. Ćirković
for, among other things, being unable to distinguish
between "SETI believers" and "scientists engaged
in SETI", who are often sceptical (especially
about quick detection), such as Freeman Dyson
(and, at least in their later years, Iosif
Shklovsky and Sebastian von Hoerner), and
for ignoring the difference between the knowledge
underlying the arguments of modern scientists
and those of ancient Greek thinkers.Massimo
Pigliucci, Professor of Philosophy at CUNY-City
College, asked in 2010 whether SETI is "uncomfortably
close to the status of pseudoscience" due
to the lack of any clear point at which negative
results cause the hypothesis of Extraterrestrial
Intelligence to be abandoned, before eventually
concluding that SETI is "almost-science",
which is described by Milan M. Ćirković
as Pigliucci putting SETI in "the illustrious
company of string theory, interpretations
of quantum mechanics, evolutionary psychology
and history (of the 'synthetic' kind done
recently by Jared Diamond)", while adding
that his justification for doing so with SETI
"is weak, outdated, and reflecting particular
philosophical prejudices similar to the ones
described above in Mash and Basalla". Ufologist
Stanton Friedman has often criticized SETI
researchers for, among other reasons, what
he sees as their unscientific criticisms of
Ufology, but, unlike SETI, Ufology has generally
not been embraced by academia as a scientific
field of study, and it is usually characterized
as a partial or total pseudoscience. In a
2016 interview, Jill Tarter pointed out that
it is still a misconception that SETI and
UFOs are related. She says that, "SETI uses
the tools of the astronomer to attempt to
find evidence of somebody else’s technology
coming from a great distance. If we ever claim
detection of a signal, we will provide evidence
and data that can be independently confirmed.
UFOs—none of the above."
== See also
