Messaging to extraterrestrial intelligence (METI) is a branch of study concerned with
constructing and broadcasting a message toward habitable planets. Since the Arecibo message of 1974, the handful of METI broadcasts have increased in content and complexity, but the lack of an established protocol has produced unorganized or cryptic messages that could be difficult to 2interpret. Here we outline the development of a self-consistent protocol for messaging toextraterrestrial intelligence that provides constraints and guidelines for the construction of amessage in order to maximize the probability that the message effectively communicates. AMETI protocol considers several factors including signal encoding, message length, informationcontent, anthropocentrism, transmission method, and transmission periodicity. Once developed,the protocol will be released for testing on different human groups worldwide and across culturalboundaries. An effective message to extraterrestrials should at least be understandable byhumans, and releasing the protocol for testing will allow us to improve the protocol and developpotential messages. Through an interactive website, users across the world will be able to createand exchange messages that follow the protocol in order to discover the types of messages bettersuited for cross-cultural communication. The development of a METI protocol will serve toimprove the quality of messages to extraterrestrials, foster international collaboration, and extendastrobiology outreach to the public.
1. Introduction
The seminal publication by Cocconi and Morrison [1] suggests that if advanced extraterrestrialsocieties exist in the Milky way, then they may attempt interstellar communication across thegalaxy. This idea led to the first search for extraterrestrial intelligence (SETI) by Frank Drakeknown as Project Ozma, which used a 26 meter radio telescope to examine two neighboring Sunlikestars near the 1.420 gigahertz frequency [2]. Nearly fifty years after Project Ozma, SETI hasexpanded to include searches for both radio and optical signals by piggybacking on conventionalastronomical sky surveys. Additionally, dedicated SETI observing facilities, such as the Allen3Telescope Array, can target specific stellar systems to listen for an extraterrestrial transmission[3].Many current planet detection methods are biased toward the discovery of gas giants, butextrasolar planet detection techniques will soon be able to detect habitable terrestrial planetsthrough both ground and space based observations. The recently launched Kepler telescope willobserve over 100,000 stars in the Milky Way in search of transiting Earth-sized planets, whilemissions such as the New Worlds Observer and the Terrestrial Planet Finder are being designed(and will hopefully have financial backing in the next decade) to directly detect spectroscopicsignatures of extrasolar terrestrial planets [4]. Although the presence of life cannot be confirmedwith these methods, a planet could be deemed a likely candidate for life based on its atmosphericcomposition and orbital position. For example, the simultaneous presence of oxygen andmethane in a planet’s atmosphere could be a strong indicator of biological processes [5].Spectroscopic biosignatures can only provide evidence for the probable existence of life, butpotentially inhabited terrestrial planets are also excellent targets for communication withextraterrestrials. For practical purposes, SETI considers intelligent life to be any society capableof radio transmission so that interstellar communication is possible. (According to this definition,Earth has had intelligent life for less than 100 years.) An Earth-like planet that showsspectroscopic indications of biological processes has a higher probability of housing intelligentlife that has developed communicative technology. If (or perhaps, when) we find planets likethis, they will be the best known targets for sending a message to extraterrestrials.Communication with extraterrestrial intelligence has been widely discussed [6], mostnotably in the 1970's [7,8], but only a handful of attempts at messaging to extraterrestrials4(METI) have been undertaken. The content of past METI broadcasts has also containedinformation dependent on human culture and biology, which may not be universally understood.Many people would choose to construct a message containing sights and sounds of the humanexperience [9], but overly anthropocentric signals that implicitly rely upon certain facets ofhuman culture may go unnoticed by extraterrestrial listeners. METI may therefore increase itsprobability of success by decreasing the cultural dependence of messages.Some critics of METI argue that broadcasting our presence to extraterrestrials is asecurity risk because it would alert a malevolent extraterrestrial civilization of our presence [10].An extraterrestrial civilization that detects our transmissions will likely have superior technologyto our own, so they would almost certainly posses the capability to destroy us. However, Earthhas been emitting electromagnetic signals to space since the beginning of the radio age, mostlyas unintended leakage from television, aviation, and telecommunication [11]. An advancedcivilization within a radius of 100 light years could detect our television shows and already knowwe are here, so there is little hope in concealing our location in space. Extraterrestrials may reactto an intentional or unintentional human broadcast with war, benevolence, apathy, or suicide,depending on their ethical framework [12], and total annihilation of one civilization by the otheris not necessarily the most likely outcome. Additionally, we have not yet observed anyextraterrestrial civilizations or received any extraterrestrial broadcasts, so this conspicuousabsence remains to be explained [13,14]. Maybe complex life is rare [15], or maybe we areactually being stealthily observed [16,17]. But perhaps civilizations that rapidly expand acrossthe galaxy are also quickly forced into ecological collapse, while less expansive civilizations thatgrow within their carrying capacity may not have had enough time to colonize the galaxy5[18,19]. If this is the case, then we are unlikely to encounter an expansive extraterrestrialcivilization, and those that we do encounter may be less inclined toward violent conflict.Here we discuss the development of a protocol for METI to address the shortcomings ofpast broadcast attempts. A protocol for METI will provide guidelines for the message length,signal encoding, transmission method, and information content in order to maximize thelikelihood that the message is understood. Successful messages will minimize anyanthropocentric or culturally dependent content and avoid dependence on basic human senses.Additionally, a message that is expected to be decoded and comprehended by extraterrestrialsshould be decipherable across human cultural boundaries on Earth, so the construction of aMETI protocol will provide a means to develop and test messages before they are broadcast.2. Previous messages to extraterrestrialsThe first and most well-known message was the Arecibo message sent from the Arecibo radiotelescope in Puerto Rico via frequency modulated radio waves in 1974 [20,21]. It was targeted atthe globular star cluster M13, which is approximately 25,000 light years away. The message waswritten by Frank Drake, with help from Carl Sagan and others, and consisted of 1679 binarydigits (approximately 210 bytes) transmitted at a frequency of 2380 MHz and modulated byshifting the frequency by 10 Hz, with a power of 1000 kW [22]. (By comparison, strongtelevision transmitters have a power of about 170 kW [11]. The "ones" and "zeroes" composingthe message were transmitted by frequency shifting at the rate of 10 bits per second. Themessage consisted of seven parts that encode the following:61. The numbers one through ten2. The atomic numbers of the five elements hydrogen (H), carbon (C), nitrogen (N),oxygen (O), and phosphorus (P), which make up deoxyribonucleic acid (DNA)3. The formulae for the sugars and bases in the nucleotides of DNA4. The number of nucleotides in DNA, and a graphic of the double helix structure ofDNA5. A graphic figure of a human, the physical height of an average man, and the humanpopulation of Earth6. A graphic of the Solar System7. A graphic of the Arecibo radio telescope and the physical diameter of the transmittingantenna dishBecause it will take 25,000 years for the transmission to reach its destination and at least another25,000 years to receive a reply, the Arecibo message was more of a demonstration of humantechnological achievements than a real attempt to communicate with aliens.The second realized project was called Cosmic Call [23], which consisted of twointerstellar radio messages that were sent from Evpatoria, Ukraine in 1999 and 2003 to variousnearby stars. The formatting and characters used in the messages were designed to resistalteration by noise. The third, called "Teen Age Message", was transmitted from the EvpatoriaPlanetary Radar to six nearby Sun-like stars during August-September 2001 [23]. Unlike theprevious digital-only messages, the Teen Age Message has a three-section structure containingvarious forms of information, suggested by Russian astronomer Aleksandr Zaitsev: Section 1represents a coherent sounding radio signal with slow Doppler wavelength tuning to imitate7transmission from Sun's center; Section 2 uses analog information and contains musical melodiesperformed on the theremin. This electric musical instrument produces a quasi-monochromaticsignal, which is detectable across interstellar distances; Section 3 contains binary digitalinformation including the Teen Age Message logo and bilingual Russian and English greetingsto the aliens. The message is named after the Russian teens who arranged and performed theclassical music compositions in Section 2.The fourth and latest attempt was made in 2008 called “A Message From Earth”(AMFE), which was a digital radio signal sent towards Gliese 581c, a large terrestrial extrasolarplanet orbiting the red dwarf star Gliese 581. This message was also emitted from the radartelescope at Evpatoria, Ukraine. Five hundred and one text messages, photographs and drawingssubmitted by the public were selected to be transmitted in the digital time capsule. Thesemessages consisted of various topics, such as the submitters' own lives and ambitions, claims ofworld peace, and views of the Earth. These three recent broadcasts have targeted stars between20 and 69 light years from Earth.The description of past transmissions illustrates that the complexity and theanthropocentric nature of transmitted messages has increased significantly with time. Thesophistication of human technology has enabled us to transmit more complex messages thatinclude digital imagery, digital sound, and analog sound. However, greater complexity probablymakes it more difficult for an extraterrestrial listener to decode and decipher the message. Giventhat we know very little about the nature of extraterrestrial civilizations, if they exist, we arelikely to increase the probability of us successfully communicating to them if we use a messagethat the recipient is likely to understand. Messages that relate strongly to certain specifics of8humankind or to one of our physical senses may be difficult to understand by a listener farremoved from the contexts of Earth. Messages using proprietary compression or conversionalgorithms also may be difficult for extraterrestrials to decode. The likelihood of success forMETI may thus be improved by the construction of a protocol for messaging to extraterrestrialintelligence.3. Developing a Protocol for Messaging to ExtraterrestrialsSending a message to an unknown extraterrestrial intelligence may require a loosening of theanthropocentric assumptions that have characterized previous METI attempts. Mostcommunication between humans on Earth relies on vision and sound because of the specifics ofour biology, but we cannot necessarily assume that extraterrestrials will share any of our basicsenses. The dependence on visual imagery in the Arecibo message, for example, necessitates thatthe recipients can interpret information stored as pictures. However, a subsurface life form is lesslikely to develop visual sensory perception, and not even all life on Earth is vision-oriented (e.g.,bats) or auditory (e.g., invertebrates). Likewise, while we expect that the basic physics ofcompression waves applies throughout the universe, not all life forms will necessarily develop asense of hearing. Messages that rely on the specifics of human biology or culture will be lesslikely to effectively communicate to an unfamiliar extraterrestrial listener.Additionally, the information content of METI signals has increased since the initialArecibo message. Modern technology allows for large amounts of data to be transmitted atmoderate costs, but the broadcast of massive amounts of information assumes that the recipient9extraterrestrials will be capable of comprehending a complex message. Even if advancedextraterrestrial technology can decode a massive message with ease, a society of blindextraterrestrials would be unsuccessful at retrieving the information from a visual message justas deaf recipients would be unable to understand an auditory message. Extraterrestrials mayidentify a signal containing video or music as originating from Earth-based life because itscomplexity resembles nothing else found in nature, but they may have little to no success atcomprehending the information within an overly complex message. Thus, it is imperative thatany message intending to communicate with extraterrestrials be short and simple enough that itcan be understood by the widest possible audience.In order to improve upon past METI attempts, we propose to develop a METI protocol.This protocol will provide constraints and guidelines for the construction of a message in orderto maximize the probability that the message is understood. A METI protocol will considerseveral factors, including:1. Signal encoding2. Message length3. Information content4. Anthropocentrism5. Transmission method6. Transmission periodicityA transmitted message should be encoded in a way that can be recognized and interpreted by abroad audience. Past choices for encoding have included the use of a binary signal—ones andzeros—though frequency modulation allows for the use of an arbitrary base system. Certain10wavelengths may be more common for communication than others, and repetition of a broadcastmessage toward a given target is almost certainly necessary. Complex and proprietary encoding(such as compressed audio) should be avoided, and an appropriately encoded message should beof a limited length so that the amount of information in the message is not overwhelming. Arestricted message length will also be more cost effective in terms of power requirements. PastMETI attempts have tended to increase in content, but without a proper cultural context much ofthis information may not be understood by an extraterrestrial listener. Limiting the extent towhich the information depends on human-like traits will help guide the construction of amessage. This may also require that the message attempts to represent Earth as a whole insteadof focusing exclusively on humanity.The construction of a consistent METI protocol (or several protocols) is a daunting taskthat requires a wide representation spanning a range of disciplines and extending further into thepublic sphere. Creation of an initial protocol would likely benefit from the formation of a METIcommittee that thoroughly investigates the factors involved in messaging and comes to aprotocol by consensus.4. Transmission Strategy.Since it is almost impossible to decode a message without a knowledge of the language at theother end, we should rely on a simple physical or mathematical language to communicate boththe encoding scheme and the content [24]. It is also important to regularly repeat thetransmission because there are many ways for a detector to generate a false signal. Repetition ofa broadcast allows the receiver to be certain that they have detected a genuine signal instead ofmachine noise. Periodic transmission will increase the probability of a signal being received11because the receiving antenna may not always be tuned to the particular frequency and directionwhen the signal arrives. It may also be wise to repeat the fundamentals of the encoding language(e.g., the definition of addition) at a greater frequency than the more complicated parts of themessage. The length of the message should be optimized to the number of targets selected andthe periodicity of transmission.There are several aspects that need to be taken into account when choosing a reliabletransmission method, including wavelength of transmission, the type of modulation andpolarization at which the signal will be transmitted. The range of wavelengths that can be used intransmitting the message is controlled by the following relationship:Wavelength of transmission = (transmitter power × gain of transmitting antenna × gain ofreceiving antenna) / noise temperature of the receiving antennaBecause of the absence of information on the receiving antenna, one could transmit over a broadrange of wavelengths depending on the parameters chosen in the above equation. Mostconservative estimates restrict the wavelength range from 1 - 20 cm, which will make receptionmore likely for a civilization with modest technical capabilities. A civilization with moreadvanced capabilities has no reason not to detect a signal in this range. This gives us a reasonableconstraint on the range of wavelengths that can be used for transmission, keeping thetransmission system economical both technically and financially. This range can be increasedwith the availability of resources in order to broadcast over a broader frequency spectrum. Sincethe bandwidth of radio signals is quite large, it is highly improbable that the receiving antennawill be tuned at the particular frequency at which the message had been sent. One approach tothis problem takes advantage of the common frequencies observed in nature within the radio12range. A popular choice is the 21 cm emission line of interstellar neutral hydrogen [23, 25].Using this frequency for communication in the galactic plane is noisy due to the background ofinterstellar 21 cm hydrogen; however, narrow-band signals near the 21 cm line that rapidly varyin brightness could still stand out due to the Doppler spread of hydrogen in the galaxy. Anotherclose frequency based on universal constants is 21cm/π = 6.72 cm. Other combinations offundamental constants for frequencies at which to transmit await further exploration [26].Another important parameter is the type of modulation to be used in transmission. Thereare several modulation options available but frequency modulation (FM) is the simplest and mostwidely used one. Pulse modulation is another option and one that can be detected easier. A thirdoption would take advantage of polarization modulation. Natural signals are typically randomlypolarized, so any polarization with a specific pattern can convey the artificial nature of ourmessage. (Natural signals that are polarized do not vary rapidly or periodically, and so theyshould be distinguishable from an artificially polarized signal.)A METI protocol should include a set of transmission techniques and guidelines formaking a broadcast. Ideally, a dedicated beacon would be established for conducting regularbroadcasts, but this is a much longer-term ambition that will require significant internationalinvestment and cooperation. For the time being, there are a few antennas that have the capabilityto transmit the message at planetary distances at any location within our galaxy [23]:Name of the telescope (disk diameter, antenna power, wavelength transmitted)1. Arecibo, Puerto Rico (300 m; 1000 kW; 12.5 cm)2. Solar System Planetary Radar in Goldstone, California (70 m; 480 kW; 3.5 cm)133. Planetary Radar near Evpatoria, Crimea (70 m; 150 kW; 6.0 cm)LASER transmission is yet another very powerful means of communication. A powerfulLASER, because of its narrow beaming angle, can outshine a star at a distance of a few lightyears. This makes it easy to be identified as an artificial signal from the normal background. Anarrow beam LASER broadcast must be aimed at a particular target, though, while anomnidirectional fan beam radio transmitter can broadcast across the entire sky.
constructing and broadcasting a message toward habitable planets. Since the Arecibo message of 1974, the handful of METI broadcasts have increased in content and complexity, but the lack of an established protocol has produced unorganized or cryptic messages that could be difficult to 2interpret. Here we outline the development of a self-consistent protocol for messaging toextraterrestrial intelligence that provides constraints and guidelines for the construction of amessage in order to maximize the probability that the message effectively communicates. AMETI protocol considers several factors including signal encoding, message length, informationcontent, anthropocentrism, transmission method, and transmission periodicity. Once developed,the protocol will be released for testing on different human groups worldwide and across culturalboundaries. An effective message to extraterrestrials should at least be understandable byhumans, and releasing the protocol for testing will allow us to improve the protocol and developpotential messages. Through an interactive website, users across the world will be able to createand exchange messages that follow the protocol in order to discover the types of messages bettersuited for cross-cultural communication. The development of a METI protocol will serve toimprove the quality of messages to extraterrestrials, foster international collaboration, and extendastrobiology outreach to the public.
1. Introduction
The seminal publication by Cocconi and Morrison [1] suggests that if advanced extraterrestrialsocieties exist in the Milky way, then they may attempt interstellar communication across thegalaxy. This idea led to the first search for extraterrestrial intelligence (SETI) by Frank Drakeknown as Project Ozma, which used a 26 meter radio telescope to examine two neighboring Sunlikestars near the 1.420 gigahertz frequency [2]. Nearly fifty years after Project Ozma, SETI hasexpanded to include searches for both radio and optical signals by piggybacking on conventionalastronomical sky surveys. Additionally, dedicated SETI observing facilities, such as the Allen3Telescope Array, can target specific stellar systems to listen for an extraterrestrial transmission[3].Many current planet detection methods are biased toward the discovery of gas giants, butextrasolar planet detection techniques will soon be able to detect habitable terrestrial planetsthrough both ground and space based observations. The recently launched Kepler telescope willobserve over 100,000 stars in the Milky Way in search of transiting Earth-sized planets, whilemissions such as the New Worlds Observer and the Terrestrial Planet Finder are being designed(and will hopefully have financial backing in the next decade) to directly detect spectroscopicsignatures of extrasolar terrestrial planets [4]. Although the presence of life cannot be confirmedwith these methods, a planet could be deemed a likely candidate for life based on its atmosphericcomposition and orbital position. For example, the simultaneous presence of oxygen andmethane in a planet’s atmosphere could be a strong indicator of biological processes [5].Spectroscopic biosignatures can only provide evidence for the probable existence of life, butpotentially inhabited terrestrial planets are also excellent targets for communication withextraterrestrials. For practical purposes, SETI considers intelligent life to be any society capableof radio transmission so that interstellar communication is possible. (According to this definition,Earth has had intelligent life for less than 100 years.) An Earth-like planet that showsspectroscopic indications of biological processes has a higher probability of housing intelligentlife that has developed communicative technology. If (or perhaps, when) we find planets likethis, they will be the best known targets for sending a message to extraterrestrials.Communication with extraterrestrial intelligence has been widely discussed [6], mostnotably in the 1970's [7,8], but only a handful of attempts at messaging to extraterrestrials4(METI) have been undertaken. The content of past METI broadcasts has also containedinformation dependent on human culture and biology, which may not be universally understood.Many people would choose to construct a message containing sights and sounds of the humanexperience [9], but overly anthropocentric signals that implicitly rely upon certain facets ofhuman culture may go unnoticed by extraterrestrial listeners. METI may therefore increase itsprobability of success by decreasing the cultural dependence of messages.Some critics of METI argue that broadcasting our presence to extraterrestrials is asecurity risk because it would alert a malevolent extraterrestrial civilization of our presence [10].An extraterrestrial civilization that detects our transmissions will likely have superior technologyto our own, so they would almost certainly posses the capability to destroy us. However, Earthhas been emitting electromagnetic signals to space since the beginning of the radio age, mostlyas unintended leakage from television, aviation, and telecommunication [11]. An advancedcivilization within a radius of 100 light years could detect our television shows and already knowwe are here, so there is little hope in concealing our location in space. Extraterrestrials may reactto an intentional or unintentional human broadcast with war, benevolence, apathy, or suicide,depending on their ethical framework [12], and total annihilation of one civilization by the otheris not necessarily the most likely outcome. Additionally, we have not yet observed anyextraterrestrial civilizations or received any extraterrestrial broadcasts, so this conspicuousabsence remains to be explained [13,14]. Maybe complex life is rare [15], or maybe we areactually being stealthily observed [16,17]. But perhaps civilizations that rapidly expand acrossthe galaxy are also quickly forced into ecological collapse, while less expansive civilizations thatgrow within their carrying capacity may not have had enough time to colonize the galaxy5[18,19]. If this is the case, then we are unlikely to encounter an expansive extraterrestrialcivilization, and those that we do encounter may be less inclined toward violent conflict.Here we discuss the development of a protocol for METI to address the shortcomings ofpast broadcast attempts. A protocol for METI will provide guidelines for the message length,signal encoding, transmission method, and information content in order to maximize thelikelihood that the message is understood. Successful messages will minimize anyanthropocentric or culturally dependent content and avoid dependence on basic human senses.Additionally, a message that is expected to be decoded and comprehended by extraterrestrialsshould be decipherable across human cultural boundaries on Earth, so the construction of aMETI protocol will provide a means to develop and test messages before they are broadcast.2. Previous messages to extraterrestrialsThe first and most well-known message was the Arecibo message sent from the Arecibo radiotelescope in Puerto Rico via frequency modulated radio waves in 1974 [20,21]. It was targeted atthe globular star cluster M13, which is approximately 25,000 light years away. The message waswritten by Frank Drake, with help from Carl Sagan and others, and consisted of 1679 binarydigits (approximately 210 bytes) transmitted at a frequency of 2380 MHz and modulated byshifting the frequency by 10 Hz, with a power of 1000 kW [22]. (By comparison, strongtelevision transmitters have a power of about 170 kW [11]. The "ones" and "zeroes" composingthe message were transmitted by frequency shifting at the rate of 10 bits per second. Themessage consisted of seven parts that encode the following:61. The numbers one through ten2. The atomic numbers of the five elements hydrogen (H), carbon (C), nitrogen (N),oxygen (O), and phosphorus (P), which make up deoxyribonucleic acid (DNA)3. The formulae for the sugars and bases in the nucleotides of DNA4. The number of nucleotides in DNA, and a graphic of the double helix structure ofDNA5. A graphic figure of a human, the physical height of an average man, and the humanpopulation of Earth6. A graphic of the Solar System7. A graphic of the Arecibo radio telescope and the physical diameter of the transmittingantenna dishBecause it will take 25,000 years for the transmission to reach its destination and at least another25,000 years to receive a reply, the Arecibo message was more of a demonstration of humantechnological achievements than a real attempt to communicate with aliens.The second realized project was called Cosmic Call [23], which consisted of twointerstellar radio messages that were sent from Evpatoria, Ukraine in 1999 and 2003 to variousnearby stars. The formatting and characters used in the messages were designed to resistalteration by noise. The third, called "Teen Age Message", was transmitted from the EvpatoriaPlanetary Radar to six nearby Sun-like stars during August-September 2001 [23]. Unlike theprevious digital-only messages, the Teen Age Message has a three-section structure containingvarious forms of information, suggested by Russian astronomer Aleksandr Zaitsev: Section 1represents a coherent sounding radio signal with slow Doppler wavelength tuning to imitate7transmission from Sun's center; Section 2 uses analog information and contains musical melodiesperformed on the theremin. This electric musical instrument produces a quasi-monochromaticsignal, which is detectable across interstellar distances; Section 3 contains binary digitalinformation including the Teen Age Message logo and bilingual Russian and English greetingsto the aliens. The message is named after the Russian teens who arranged and performed theclassical music compositions in Section 2.The fourth and latest attempt was made in 2008 called “A Message From Earth”(AMFE), which was a digital radio signal sent towards Gliese 581c, a large terrestrial extrasolarplanet orbiting the red dwarf star Gliese 581. This message was also emitted from the radartelescope at Evpatoria, Ukraine. Five hundred and one text messages, photographs and drawingssubmitted by the public were selected to be transmitted in the digital time capsule. Thesemessages consisted of various topics, such as the submitters' own lives and ambitions, claims ofworld peace, and views of the Earth. These three recent broadcasts have targeted stars between20 and 69 light years from Earth.The description of past transmissions illustrates that the complexity and theanthropocentric nature of transmitted messages has increased significantly with time. Thesophistication of human technology has enabled us to transmit more complex messages thatinclude digital imagery, digital sound, and analog sound. However, greater complexity probablymakes it more difficult for an extraterrestrial listener to decode and decipher the message. Giventhat we know very little about the nature of extraterrestrial civilizations, if they exist, we arelikely to increase the probability of us successfully communicating to them if we use a messagethat the recipient is likely to understand. Messages that relate strongly to certain specifics of8humankind or to one of our physical senses may be difficult to understand by a listener farremoved from the contexts of Earth. Messages using proprietary compression or conversionalgorithms also may be difficult for extraterrestrials to decode. The likelihood of success forMETI may thus be improved by the construction of a protocol for messaging to extraterrestrialintelligence.3. Developing a Protocol for Messaging to ExtraterrestrialsSending a message to an unknown extraterrestrial intelligence may require a loosening of theanthropocentric assumptions that have characterized previous METI attempts. Mostcommunication between humans on Earth relies on vision and sound because of the specifics ofour biology, but we cannot necessarily assume that extraterrestrials will share any of our basicsenses. The dependence on visual imagery in the Arecibo message, for example, necessitates thatthe recipients can interpret information stored as pictures. However, a subsurface life form is lesslikely to develop visual sensory perception, and not even all life on Earth is vision-oriented (e.g.,bats) or auditory (e.g., invertebrates). Likewise, while we expect that the basic physics ofcompression waves applies throughout the universe, not all life forms will necessarily develop asense of hearing. Messages that rely on the specifics of human biology or culture will be lesslikely to effectively communicate to an unfamiliar extraterrestrial listener.Additionally, the information content of METI signals has increased since the initialArecibo message. Modern technology allows for large amounts of data to be transmitted atmoderate costs, but the broadcast of massive amounts of information assumes that the recipient9extraterrestrials will be capable of comprehending a complex message. Even if advancedextraterrestrial technology can decode a massive message with ease, a society of blindextraterrestrials would be unsuccessful at retrieving the information from a visual message justas deaf recipients would be unable to understand an auditory message. Extraterrestrials mayidentify a signal containing video or music as originating from Earth-based life because itscomplexity resembles nothing else found in nature, but they may have little to no success atcomprehending the information within an overly complex message. Thus, it is imperative thatany message intending to communicate with extraterrestrials be short and simple enough that itcan be understood by the widest possible audience.In order to improve upon past METI attempts, we propose to develop a METI protocol.This protocol will provide constraints and guidelines for the construction of a message in orderto maximize the probability that the message is understood. A METI protocol will considerseveral factors, including:1. Signal encoding2. Message length3. Information content4. Anthropocentrism5. Transmission method6. Transmission periodicityA transmitted message should be encoded in a way that can be recognized and interpreted by abroad audience. Past choices for encoding have included the use of a binary signal—ones andzeros—though frequency modulation allows for the use of an arbitrary base system. Certain10wavelengths may be more common for communication than others, and repetition of a broadcastmessage toward a given target is almost certainly necessary. Complex and proprietary encoding(such as compressed audio) should be avoided, and an appropriately encoded message should beof a limited length so that the amount of information in the message is not overwhelming. Arestricted message length will also be more cost effective in terms of power requirements. PastMETI attempts have tended to increase in content, but without a proper cultural context much ofthis information may not be understood by an extraterrestrial listener. Limiting the extent towhich the information depends on human-like traits will help guide the construction of amessage. This may also require that the message attempts to represent Earth as a whole insteadof focusing exclusively on humanity.The construction of a consistent METI protocol (or several protocols) is a daunting taskthat requires a wide representation spanning a range of disciplines and extending further into thepublic sphere. Creation of an initial protocol would likely benefit from the formation of a METIcommittee that thoroughly investigates the factors involved in messaging and comes to aprotocol by consensus.4. Transmission Strategy.Since it is almost impossible to decode a message without a knowledge of the language at theother end, we should rely on a simple physical or mathematical language to communicate boththe encoding scheme and the content [24]. It is also important to regularly repeat thetransmission because there are many ways for a detector to generate a false signal. Repetition ofa broadcast allows the receiver to be certain that they have detected a genuine signal instead ofmachine noise. Periodic transmission will increase the probability of a signal being received11because the receiving antenna may not always be tuned to the particular frequency and directionwhen the signal arrives. It may also be wise to repeat the fundamentals of the encoding language(e.g., the definition of addition) at a greater frequency than the more complicated parts of themessage. The length of the message should be optimized to the number of targets selected andthe periodicity of transmission.There are several aspects that need to be taken into account when choosing a reliabletransmission method, including wavelength of transmission, the type of modulation andpolarization at which the signal will be transmitted. The range of wavelengths that can be used intransmitting the message is controlled by the following relationship:Wavelength of transmission = (transmitter power × gain of transmitting antenna × gain ofreceiving antenna) / noise temperature of the receiving antennaBecause of the absence of information on the receiving antenna, one could transmit over a broadrange of wavelengths depending on the parameters chosen in the above equation. Mostconservative estimates restrict the wavelength range from 1 - 20 cm, which will make receptionmore likely for a civilization with modest technical capabilities. A civilization with moreadvanced capabilities has no reason not to detect a signal in this range. This gives us a reasonableconstraint on the range of wavelengths that can be used for transmission, keeping thetransmission system economical both technically and financially. This range can be increasedwith the availability of resources in order to broadcast over a broader frequency spectrum. Sincethe bandwidth of radio signals is quite large, it is highly improbable that the receiving antennawill be tuned at the particular frequency at which the message had been sent. One approach tothis problem takes advantage of the common frequencies observed in nature within the radio12range. A popular choice is the 21 cm emission line of interstellar neutral hydrogen [23, 25].Using this frequency for communication in the galactic plane is noisy due to the background ofinterstellar 21 cm hydrogen; however, narrow-band signals near the 21 cm line that rapidly varyin brightness could still stand out due to the Doppler spread of hydrogen in the galaxy. Anotherclose frequency based on universal constants is 21cm/π = 6.72 cm. Other combinations offundamental constants for frequencies at which to transmit await further exploration [26].Another important parameter is the type of modulation to be used in transmission. Thereare several modulation options available but frequency modulation (FM) is the simplest and mostwidely used one. Pulse modulation is another option and one that can be detected easier. A thirdoption would take advantage of polarization modulation. Natural signals are typically randomlypolarized, so any polarization with a specific pattern can convey the artificial nature of ourmessage. (Natural signals that are polarized do not vary rapidly or periodically, and so theyshould be distinguishable from an artificially polarized signal.)A METI protocol should include a set of transmission techniques and guidelines formaking a broadcast. Ideally, a dedicated beacon would be established for conducting regularbroadcasts, but this is a much longer-term ambition that will require significant internationalinvestment and cooperation. For the time being, there are a few antennas that have the capabilityto transmit the message at planetary distances at any location within our galaxy [23]:Name of the telescope (disk diameter, antenna power, wavelength transmitted)1. Arecibo, Puerto Rico (300 m; 1000 kW; 12.5 cm)2. Solar System Planetary Radar in Goldstone, California (70 m; 480 kW; 3.5 cm)133. Planetary Radar near Evpatoria, Crimea (70 m; 150 kW; 6.0 cm)LASER transmission is yet another very powerful means of communication. A powerfulLASER, because of its narrow beaming angle, can outshine a star at a distance of a few lightyears. This makes it easy to be identified as an artificial signal from the normal background. Anarrow beam LASER broadcast must be aimed at a particular target, though, while anomnidirectional fan beam radio transmitter can broadcast across the entire sky.
5. Public Evaluation of the Protocol.
Once a METI protocol has been developed, we suggest to use the protocol as a framework by
which to test communication across human cultural boundaries and engage the public in thinking
about METI. Although the idea of constructing a message free of anthropocentrism and testing it on humans sounds paradoxical, it is the most reasonable method of testing given the absence of any other species capable of building radio telescopes (which SETI defines as the lower limit of intelligence). Even if the METI protocol is never consistently used for messaging
extraterrestrials, it will still provide a unique educational tool for science outreach to students
and the public.
Took From
which to test communication across human cultural boundaries and engage the public in thinking
about METI. Although the idea of constructing a message free of anthropocentrism and testing it on humans sounds paradoxical, it is the most reasonable method of testing given the absence of any other species capable of building radio telescopes (which SETI defines as the lower limit of intelligence). Even if the METI protocol is never consistently used for messaging
extraterrestrials, it will still provide a unique educational tool for science outreach to students
and the public.
Through an interactive website, we can allow users to create their own messages that
conform to the protocol. Whatever the constraints of the protocol may be, there will nonetheless
be some degree of freedom by which a user can decide what message they think is important.
Messages submitted through the website can then be retrieved by other users, who then attempt to decrypt the message to see if the communication attempt was successful. Using a tool such as 14this, we will be able to simultaneously 1) test the METI protocol and see where improvement is needed, and 2) collect the messages to extraterrestrials that at least some portion of the publicwould like to send. On this second objective, even if the messages are never sent, they will almost certainly provide novel information about the values of our culture.
The effectiveness of the protocol can be tested through international educational efforts
by sending messages across cultural boundaries. For example, students in the United States may construct a set of messages according to the METI protocol, which are then exchanged with students in China. The differences in cultures between the respective student groups will likely be reflected in the message composition, so not all messages may be successfully communicated.
conform to the protocol. Whatever the constraints of the protocol may be, there will nonetheless
be some degree of freedom by which a user can decide what message they think is important.
Messages submitted through the website can then be retrieved by other users, who then attempt to decrypt the message to see if the communication attempt was successful. Using a tool such as 14this, we will be able to simultaneously 1) test the METI protocol and see where improvement is needed, and 2) collect the messages to extraterrestrials that at least some portion of the publicwould like to send. On this second objective, even if the messages are never sent, they will almost certainly provide novel information about the values of our culture.
The effectiveness of the protocol can be tested through international educational efforts
by sending messages across cultural boundaries. For example, students in the United States may construct a set of messages according to the METI protocol, which are then exchanged with students in China. The differences in cultures between the respective student groups will likely be reflected in the message composition, so not all messages may be successfully communicated.
In this case, students in the United States may discover that their particular ideologies that they
believed to be obvious were somewhat lost in translation when deciphered by their international
colleagues. Extension of these efforts to a diverse sample of human populations will help refine
the protocol to be more universally decipherable. Evaluating the protocol across cultural
boundaries on Earth will help refine the protocol while also fostering educational collaboration
between human groups on Earth. Although this process will not remove all anthropocentric bias,
it will help identify some cultural biases in the protocol.
It is certainly possible that messages collected through an interactive website may be
suitable for broadcast, and there even may be several user submitted messages that withstand a wide array of cross-cultural scrutiny. Nevertheless, it seems more likely that, at least prior to
contact, our messages to extraterrestrials will be based on observable features of the physical
universe and contain mostly mathematics and science [24]. The call for public input may help
improve the effectiveness of the protocol itself, but it will also actively engage the public in
15 thinking about METI. After all, the first inclination of many users will be to communicate
something deeply religious or humanistic, but they may find that their encoded message fails to
effectively communicate to a different human culture. Involving the public in message
construction in this way will increase awareness of cultural assumptions that are implicit in every
society. This activity will also increase public understanding of Earth and humans in the context
of astronomical spatial and temporal scales, which will hopefully inspire people to reduce
catastrophic risk [27] and prepare for the future.
believed to be obvious were somewhat lost in translation when deciphered by their international
colleagues. Extension of these efforts to a diverse sample of human populations will help refine
the protocol to be more universally decipherable. Evaluating the protocol across cultural
boundaries on Earth will help refine the protocol while also fostering educational collaboration
between human groups on Earth. Although this process will not remove all anthropocentric bias,
it will help identify some cultural biases in the protocol.
It is certainly possible that messages collected through an interactive website may be
suitable for broadcast, and there even may be several user submitted messages that withstand a wide array of cross-cultural scrutiny. Nevertheless, it seems more likely that, at least prior to
contact, our messages to extraterrestrials will be based on observable features of the physical
universe and contain mostly mathematics and science [24]. The call for public input may help
improve the effectiveness of the protocol itself, but it will also actively engage the public in
15 thinking about METI. After all, the first inclination of many users will be to communicate
something deeply religious or humanistic, but they may find that their encoded message fails to
effectively communicate to a different human culture. Involving the public in message
construction in this way will increase awareness of cultural assumptions that are implicit in every
society. This activity will also increase public understanding of Earth and humans in the context
of astronomical spatial and temporal scales, which will hopefully inspire people to reduce
catastrophic risk [27] and prepare for the future.
6. Conclusion.
A METI protocol is needed in order for a unified and international effort to be made in
messaging extraterrestrials. By carefully constructing a framework by which to write and send
messages, we will optimize the quality of messages as they are broadcast and increase the
probability that we are understood. Additionally, the release of a protocol on an interactive
public website will allow users ranging from scientists to students to test the protocol and
construct sample messages. These encoded messages can then be tested across human cultural
boundaries, which will provide a unique outreach opportunity to engage the public in thinking
about METI. It is often said that SETI is a search for ourselves, and as we develop a message
that we would send to unknown listeners, we will come to an even deeper appreciation of our
diversity as humans.
16
Acknowledgments
We thank Carl Devito, Michael Busch, and Seth Baum for insightful comments that helped to
improve this paper. Lingling Wu contributed to an earlier draft of this paper that was developed
at the 2009 Astrobiology Graduate Conference (AbGradCon) Research Focus Group in Seattle,
Washington.
References
[1] Cocconi, G. and Morrison, P. 1959. Searching for interstellar communications. Nature;
184:844-846.
[2] Drake, F. D. 1961. Project Ozma. Physics Today; 14:40-46.
[3] Tarter, J. 2001. The search for extraterrestrial intelligence (SETI). Annual Rev Astron Astr;
39:511-548.
[4] National Research Council, Committee for a Decadal Survey of Astronomy and Astrophysics
2010. Report in Brief: New Worlds, New Horizons in Astronomy and Astrophysics. Washington
D.C.: The National Academies Press
[5] Des Marais, D. J., M. O. Harwit, K. W. Jucks, J. F. Kasting, D. N. C. Lin, J. I. Lunine, J.
Schneider, S. Seager, W. A. Traub and N. J. Woolf. 2002. Remote sensing of planetary
properties and biosignatures on extrasolar terrestrial planets. Astrobiology; 2:153-181.
17
[6] Vakoch, D. A., 1998. Signs of life beyond Earth: A semiotic analysis of interstellar messages.
Leonardo; 31:313-319.
[7] Sagan, C. 1973. Communication With Extraterrestrial Intelligence (CETI). Cambridge, MA:
MIT Press.
[8] Kaplan, S. A. 1971. Extraterrestrial Civilizations: Problems of Interstellar Communication.
Jerusalem: Israel Program for Scientific Translations.
[9] Lomberg, J. 2004. A portrait of humanity, Contact in Context 2: v2i1/portrait.pdf
[10] Shuch, H. P. and I. Almar. 2007. Shouting in the jungle: The SETI transmission debate.
JBIS-J Brit Interpla 60:142-146.
[11] Scheffer, L. 2004. Aliens can watch 'I Love Lucy'. Contact in Context 2: v2i1/lucy.pdf.
[12] Baum, S. D. 2010. Universalist ethics in extraterrestrial encounter. Acta Astronaut; 66:617-
623.
[13] Hart, M. H. 1975. An explanation for the absence of extraterrestrials on Earth. Q J Roy
Astron Soc; 16:128-135.
[14] Webb, S. 2002. If the Universe is Teeming with Aliens--where is Everybody?: Fifty
Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life. New York: Springer.
[15] Ward, P. D. and Brownlee, D. 2000. Rare Earth: Why Complex Life is Uncommon in the
Universe. New York: Springer.
18
[16] Ball, J. A. 1973. The zoo hypothesis. Icarus 19:347-349.
[17] Baxter, S. 2001. The planetarium hypothesis: A resolution of the Fermi paradox. JBIS-J Brit
Interpla; 54:210-216.
[18] von Hoerner, S. 1975. Population explosion and interstellar expansion. JBIS-J Brit Interpla;
28:691-712.
[19]. Haqq-Misra, J. D. and S. D. Baum. 2009. The sustainability solution to the Fermi paradox.
JBIS-J Brit Interpla; 62:47-51.
[20] Goldsmith D. and Owen T. C. 2001. The Search for Life in the Universe. Herndon, VA:
University Science Books.
[21] Grinspoon D. 2003. Lonely Planets: the Natural Philosophy of Alien Life. New York:
HarperCollins Publishers Inc.
[22] Sless, D. 1981. Learning and visual communication, New York: Halsted Press.
[23] Zaitsev A. 2008. Sending and searching for interstellar messages. Acta Astronautica 63:614-
617.
[24] Devito, C. L. and R. T. Oehrle. 1990. A language based on the fundamental facts of science.
JBIS-J Brit Interpla; 43:561-568.
[25] Drake, F. D. and C. Sagan. 1973. Interstellar radio communication and the frequency
selection problem. Nature 245:257-258.
19
[26] Benford, J., G. Benford and D. Benford. 2010. Messaging with cost-optimized interstellar
beacons. Astrobiology 10:475-490.
[27] Bostrom, N. and M. M. Cirkovic 2008. Global Catastrophic Risks, Oxford: Oxford
University Press.
A METI protocol is needed in order for a unified and international effort to be made in
messaging extraterrestrials. By carefully constructing a framework by which to write and send
messages, we will optimize the quality of messages as they are broadcast and increase the
probability that we are understood. Additionally, the release of a protocol on an interactive
public website will allow users ranging from scientists to students to test the protocol and
construct sample messages. These encoded messages can then be tested across human cultural
boundaries, which will provide a unique outreach opportunity to engage the public in thinking
about METI. It is often said that SETI is a search for ourselves, and as we develop a message
that we would send to unknown listeners, we will come to an even deeper appreciation of our
diversity as humans.
16
Acknowledgments
We thank Carl Devito, Michael Busch, and Seth Baum for insightful comments that helped to
improve this paper. Lingling Wu contributed to an earlier draft of this paper that was developed
at the 2009 Astrobiology Graduate Conference (AbGradCon) Research Focus Group in Seattle,
Washington.
References
[1] Cocconi, G. and Morrison, P. 1959. Searching for interstellar communications. Nature;
184:844-846.
[2] Drake, F. D. 1961. Project Ozma. Physics Today; 14:40-46.
[3] Tarter, J. 2001. The search for extraterrestrial intelligence (SETI). Annual Rev Astron Astr;
39:511-548.
[4] National Research Council, Committee for a Decadal Survey of Astronomy and Astrophysics
2010. Report in Brief: New Worlds, New Horizons in Astronomy and Astrophysics. Washington
D.C.: The National Academies Press
[5] Des Marais, D. J., M. O. Harwit, K. W. Jucks, J. F. Kasting, D. N. C. Lin, J. I. Lunine, J.
Schneider, S. Seager, W. A. Traub and N. J. Woolf. 2002. Remote sensing of planetary
properties and biosignatures on extrasolar terrestrial planets. Astrobiology; 2:153-181.
17
[6] Vakoch, D. A., 1998. Signs of life beyond Earth: A semiotic analysis of interstellar messages.
Leonardo; 31:313-319.
[7] Sagan, C. 1973. Communication With Extraterrestrial Intelligence (CETI). Cambridge, MA:
MIT Press.
[8] Kaplan, S. A. 1971. Extraterrestrial Civilizations: Problems of Interstellar Communication.
Jerusalem: Israel Program for Scientific Translations.
[9] Lomberg, J. 2004. A portrait of humanity, Contact in Context 2: v2i1/portrait.pdf
[10] Shuch, H. P. and I. Almar. 2007. Shouting in the jungle: The SETI transmission debate.
JBIS-J Brit Interpla 60:142-146.
[11] Scheffer, L. 2004. Aliens can watch 'I Love Lucy'. Contact in Context 2: v2i1/lucy.pdf.
[12] Baum, S. D. 2010. Universalist ethics in extraterrestrial encounter. Acta Astronaut; 66:617-
623.
[13] Hart, M. H. 1975. An explanation for the absence of extraterrestrials on Earth. Q J Roy
Astron Soc; 16:128-135.
[14] Webb, S. 2002. If the Universe is Teeming with Aliens--where is Everybody?: Fifty
Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life. New York: Springer.
[15] Ward, P. D. and Brownlee, D. 2000. Rare Earth: Why Complex Life is Uncommon in the
Universe. New York: Springer.
18
[16] Ball, J. A. 1973. The zoo hypothesis. Icarus 19:347-349.
[17] Baxter, S. 2001. The planetarium hypothesis: A resolution of the Fermi paradox. JBIS-J Brit
Interpla; 54:210-216.
[18] von Hoerner, S. 1975. Population explosion and interstellar expansion. JBIS-J Brit Interpla;
28:691-712.
[19]. Haqq-Misra, J. D. and S. D. Baum. 2009. The sustainability solution to the Fermi paradox.
JBIS-J Brit Interpla; 62:47-51.
[20] Goldsmith D. and Owen T. C. 2001. The Search for Life in the Universe. Herndon, VA:
University Science Books.
[21] Grinspoon D. 2003. Lonely Planets: the Natural Philosophy of Alien Life. New York:
HarperCollins Publishers Inc.
[22] Sless, D. 1981. Learning and visual communication, New York: Halsted Press.
[23] Zaitsev A. 2008. Sending and searching for interstellar messages. Acta Astronautica 63:614-
617.
[24] Devito, C. L. and R. T. Oehrle. 1990. A language based on the fundamental facts of science.
JBIS-J Brit Interpla; 43:561-568.
[25] Drake, F. D. and C. Sagan. 1973. Interstellar radio communication and the frequency
selection problem. Nature 245:257-258.
19
[26] Benford, J., G. Benford and D. Benford. 2010. Messaging with cost-optimized interstellar
beacons. Astrobiology 10:475-490.
[27] Bostrom, N. and M. M. Cirkovic 2008. Global Catastrophic Risks, Oxford: Oxford
University Press.
Took From
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