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Screen shot of the screensaver for
SETI@home, a
distributed computing project in which volunteers donate idle computer power to analyze radio signals for signs of extraterrestrial intelligence.
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 led to a range of explanations
including: Tesla simply misunderstood the new technology he was working
with, that he may have been observing signals from Marconi's European radio experiments, and even speculation that he could have picked up naturally occurring radio noise caused by a moon of Jupiter (Io) moving through the magnetosphere of Jupiter. 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).
The Wow! SignalCredit: The Ohio State University Radio Observatory and the North American AstroPhysical Observatory (NAAPO).
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, Barney Oliver of Hewlett-Packard laboratories,
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
Sensitivity
vs range for SETI radio searches. The diagonal lines show transmitters
of different effective powers. The x-axis is the sensitivity of the
search. The y-axis on the right is the range in
light-years,
and on the left is the number of Sun-like stars within this range. The
vertical line labeled SS is the typical sensitivity achieved by a full
sky search, such as BETA above. The vertical line labeled TS is the
typical sensitivity achieved by a targeted search such as 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.
Arecibo Telescope
in Puerto Rico with its 300 m (980 ft) dish was one of the world's
largest filled-aperture (i.e. full dish) radio telescope and conducted
some SETI searches.
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
Microwave window as seen by a ground based system. From NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence
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
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 a beamforming 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 to 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, Berkeley 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 Telescope in June 2009. The digital back-end instrument was an FPGA-based
128 million-channel digital spectrometer covering 200 MHz of bandwidth.
It took data commensally with the seven-beam Arecibo L-band Feed Array (ALFA). The program 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.
In October 2019, Breakthrough Listen started a collaboration with scientists from the TESS team (Transiting Exoplanet Survey Satellite) to look for signs of advanced extraterrestrial life. Thousands of new planets found by TESS will be scanned for technosignatures
by Breakthrough Listen partner facilities across the globe. Data from
TESS monitoring of stars will also be searched for anomalies.
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.
UCLA
Since 2016, UCLA undergraduate and graduate students have been participating in radio searches for technosignatures with the Green Bank Telescope. Targets include the Kepler field, TRAPPIST-1, and solar-type stars.
The search is sensitive to Arecibo-class transmitters located within
420 ly of Earth and to transmitters that are 1000 times more powerful
than Arecibo located within 13,000 ly of Earth.
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.
On March 31, 2020, the project stopped sending out new work to
SETI@home users, bringing this particular SETI effort to an indefinite
hiatus.
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 was initially 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 Tabby's Star (designated KIC 8462852 in the Kepler Input Catalog),
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.
Quantum communications
In a 2021 preprint, an astronomer described for the first time how one could search for quantum communication transmissions sent by ETI
using existing telescope and receiver technology. He also provides
arguments for why future searches of SETI should also target
interstellar quantum communication networks.
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 with us, could not travel across
interstellar distances, or destroy themselves before they master the
technology of either interstellar travel or communication.
The German astrophysicist and radio astronomer Sebastian von Hoerner suggested
that the average duration of civilization was 6,500 years. After this
time, according to him, it disappears for external reasons (the
destruction of life on the planet, the destruction of only rational
beings) or internal causes (mental or physical degeneration). According
to his calculations, on a habitable planet (one in 3 million stars)
there is a sequence of technological species over a time distance of
hundreds of millions of years, and each of them "produces" an average of
4 technological species. With these assumptions, the average distance
between civilizations in the Milky Way is 1,000 light years.
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".
In 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 humankind's history of treating its own
kind harshly in meetings of civilizations with a significant technology
gap, e.g., the extermination of Tasmanian aborigines. 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. Mark Buchanan
argued – in the context of potentially detected extraterrestrial
activity on Earth – that humanity needs to figure out whether it would
be safe or wise to attempt to communicate with extraterrestrials and
work on ways to handle such attempts in an organized manner.
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."
Critics claim that 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".
Ufology
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." The Galileo Project headed by Harvard astronomer Avi Loeb is one of the few scientific efforts to study UFOs or UAPs.
Loeb criticized that the study of UAP is often dismissed and not
sufficiently studied by scientists and should shift from "occupying the
talking points of national security administrators and politicians" to
the realm of science. The Galileo Projects finds that after the publication of the UFO Report by the U.S. Intelligence the scientific community
needs to "systematically, scientifically and transparently look for
potential evidence of extraterrestrial technological equipment".
