View source for Wow! signal

{{Short description|1977 narrowband radio signal from SETI}}
{{About|the radio signal|the racehorse|The Wow Signal}}
{{Use American English|date=March 2021}}
{{Use mdy dates|date=March 2021}}
{{Good article}}

[[File:Wow signal.jpg|thumb|upright=1.3|The Wow! signal represented as "6EQUJ5". The original printout with Ehman's handwritten exclamation is preserved by [[Ohio History Connection]].<ref>{{Cite web |last=Wood |first=Lisa |date=July 3, 2010 |title=WOW! |url=https://www.ohiohistory.org/wow/ |access-date=July 2, 2016 |website=Ohio History Blog |publisher=Ohio History Connections}}</ref>]]

The '''Wow! signal''' was a strong [[narrowband]] radio signal detected on August 15, 1977, by [[Ohio State University]]'s [[Big Ear]] radio telescope in the United States, then used to support the [[search for extraterrestrial intelligence]]. The signal appeared to come from the direction of the constellation [[Sagittarius (constellation)|Sagittarius]] and bore expected hallmarks of [[Extraterrestrial intelligence|extraterrestrial]] origin.

Astronomer Jerry R. Ehman discovered the anomaly a few days later while reviewing the recorded data. He was so impressed by the result that he circled on the computer printout the reading of the signal's intensity, "6EQUJ5", and wrote the comment "Wow!" beside it, leading to the event's widely used name.<ref name="npr">{{Cite web |last=Krulwich |first=Robert |date=May 29, 2010 |title=Aliens Found In Ohio? The 'Wow!' Signal |url=https://www.npr.org/templates/story/story.php?storyId=126510251 |access-date=July 2, 2016 |publisher=[[NPR]]}}</ref>

The entire signal sequence lasted for the full 72-second window during which Big Ear was able to observe it, but has not been detected since, despite many subsequent attempts by Ehman and others. Several hypotheses have been advanced on the origin of the emission, including natural and human-made sources.

== Background ==
In a 1959 paper, [[Cornell University]] physicists [[Philip Morrison]] and [[Giuseppe Cocconi]] had speculated that any [[extraterrestrial civilization]] attempting to communicate via radio signals might do so using a frequency of {{val|1420}} [[megahertz]] ({{val|21}}-centimeter [[spectral line]]), which is [[Hydrogen line|naturally emitted by hydrogen]], the most common element in the universe and therefore likely familiar to all technologically advanced civilizations.<ref name="Nat Geo">{{Cite web |last=Kiger |first=Patrick J. |date=June 21, 2012 |title=What is the Wow! signal? |url=http://channel.nationalgeographic.com/chasing-ufos/articles/what-is-the-wow-signal/ |url-status=dead |archive-url=https://web.archive.org/web/20150313204253/http://channel.nationalgeographic.com/chasing-ufos/articles/what-is-the-wow-signal/ |archive-date=March 13, 2015 |access-date=July 2, 2016 |website=National Geographic Channel}}</ref>

In 1973, after completing an extensive [[Ohio Sky Survey|survey of extragalactic radio sources]], Ohio State University assigned the now-defunct [[Ohio State University Radio Observatory]] (nicknamed "Big Ear") to the scientific [[search for extraterrestrial intelligence]] (SETI), in the longest-running program of this kind in history.<ref>{{Cite web |date=August 15, 2005 |title=Big Ear Entered in Guinness Book of Records |url=http://www.bigear.org/guinness.htm |access-date=May 30, 2021 |publisher=Big Ear Radio Observatory}}</ref> The radio telescope was located near the [[Perkins Observatory]] on the campus of [[Ohio Wesleyan University]] in [[Delaware, Ohio]].<ref>{{Cite book |last=Kraus |first=John Daniel |title=Big Ear Two: Listening for Other Worlds |publisher=Cygnus-Quasar |year=1995 |isbn=978-1-882484-12-6 |edition=2nd |location=Powell, OH |page=161}}</ref>

By 1977, Ehman was working at the SETI project as a volunteer; his job involved analyzing by hand large amounts of data processed by an [[IBM 1130]] computer and recorded on [[line printer]] paper. While perusing data collected on August 15 at 22:16&nbsp;[[Eastern Time Zone|EDT]] (02:16&nbsp;[[UTC]]), he spotted a series of values of signal intensity and frequency that left him and his colleagues astonished.<ref name="Nat Geo" />
The event was later documented in technical detail by the observatory's director.<ref name="Kraus">John Kraus, Director, Ohio State Radio Observatory 31, January 1994, [https://www.nrao.edu/archives/files/original/2ec6ba346ab16e10a10d09462507beda.pdf "The Tantalizing WOW! Signal"],  Copy of letter to Carl Sagan containing an unpublished paper describing the event.</ref>

== Signal measurement ==
[[File:Wow signal profile.svg|thumb|Plot of signal intensity versus time]]
The string 6EQUJ5, commonly misinterpreted as a message encoded in the radio signal, represents in fact the signal's [[Intensity (physics)|intensity]] variation over time, expressed in the particular measuring system adopted for the experiment. The signal itself appeared to be an unmodulated [[continuous wave]], although any [[modulation]] with a period of less than 10 seconds or longer than 72 seconds would not have been detectable.<ref name="Calib Wow">{{Cite web |last=Shuch |first=H. Paul |date=November 9, 1996 <!--Presented to the AMSAT Annual Meeting and Space Symposium, Tucson AZ--> |title=SETI Sensitivity: Calibrating on a Wow! Signal |url=http://www.setileague.org/articles/calibwow.htm |access-date=June 25, 2016 |publisher=SETI League}}</ref><ref>{{Cite book |last=Ehman |first=Jerry R. |url=https://books.google.com/books?id=jAk9bTm3Sj4C |title=Searching for Extraterrestrial Intelligence: SETI Past, Present, and Future |publisher=Springer |year=2011 |isbn=978-3-642-13196-7 |editor-last=Shuch |editor-first=H. Paul |page=59}}</ref>

=== Intensity ===
The signal intensity was measured as [[signal-to-noise ratio]], with the noise (or baseline) averaged over the previous few minutes. The signal was [[Sampling (signal processing)|sampled]] for 10 seconds and then processed by the computer, which took 2 seconds. The result for each frequency channel was output on the printout as a single [[alphanumeric]] character, representing the 10-second average intensity, minus the baseline, expressed as a [[dimensionless]] multiple of the signal's [[standard deviation]].<ref name="code">{{Cite web |last=Ehman |first=Jerry |title=Explanation of the Code "6EQUJ5" On the Wow! Computer Printout |url=http://www.bigear.org/6equj5.htm |access-date=July 2, 2016 |publisher=Ohio History Central}}</ref>

In this particular intensity scale, a [[Space (punctuation)|space character]] denoted an intensity between 0 and 1, that is between baseline and one standard deviation above it. The numbers 1 to 9 denoted the correspondingly numbered intensities (from 1 to 9); intensities of 10 and above were indicated by a letter: "A" corresponded to intensities between 10 and 11, "B" to 11 to 12, and so on. The Wow! signal's highest measured value was "U" (an intensity between 30 and 31), which is thirty standard deviations above background noise.<ref name="npr" /><ref name="code" />

=== Frequency ===
[[John Kraus]], the director of the observatory, gave a value of {{val|1420.3556|fmt=none|ul=MHz}} in a 1994 summary written for [[Carl Sagan]].<ref name=Kraus/> However, Ehman in 1998 gave a value of {{val|1420.4556|.005|fmt=none|u=MHz}}.<ref name="Big Ear 97">{{Cite web |last=Ehman |first=Jerry R. |date=February 3, 1998 |title=The Big Ear Wow! Signal. What We Know and Don't Know About It After 20 Years |url=http://www.bigear.org/wow20th.htm |access-date=July 2, 2016}}</ref> This is ({{val|50|5|u=kHz}}) above the [[hydrogen line]] value (with no [[Redshift|red-]] or blue-shift) of {{val|1420.4058|fmt=none|u=MHz}}. If due to blue-shift, it would correspond to the source moving about {{cvt|10|km/s}} towards Earth.

[[File:Wow signal spectrogram.svg|thumb|A [[heat map]] of the computer printout, giving a [[spectrogram]] of the beam; the Wow! signal appears as a bright spot in the lower left.]]
An explanation of the difference between Ehman's value and Kraus's can be found in Ehman's paper. The first [[local oscillator]] in the telescope's radio receiver was specified to a frequency value of {{val|1450.4056|fmt=none|u=MHz}}. However, the university's purchasing department made a [[typographical error]] in the order form, instead obtaining an oscillator with frequency {{val|1450.5056|fmt=none|u=MHz}} (i.e., {{val|0.1|u=MHz}} higher than desired). The software used in the experiment was then written to adjust for this error. When Ehman computed the frequency of the Wow! signal, he took this error into account.{{citation needed|date=August 2020}}

=== Bandwidth ===
The Wow! signal had a [[Bandwidth (signal processing)|bandwidth]] of less than {{val|10|u=kHz}}.  It is considered [[narrowband]] emission in the sense that its fractional bandwidth was relatively small (~1%).  However, the {{val|10|u=kHz}} bandwidth is not small compared to the bandwidth of some [[Maser#Astrophysical masers|astrophysical masers]] (~{{val|1|u=kHz}}) or to the frequency resolution of modern narrowband SETI searches (~{{val|1|u=Hz}}).<ref>{{Cite journal |last1=Margot |first1=Jean-Luc |last2=Pinchuk |first2=Pavlo |last3=Geil |first3=Robert |last4=Alexander |first4=Stephen |last5=Arora |first5=Sparsh |last6=Biswas |first6=Swagata |last7=Cebreros |first7=Jose |last8=Desai |first8=Sanjana Prabhu |last9=Duclos |first9=Benjamin |last10=Dunne |first10=Riley |last11=Lin Fu |first11=Kristy Kwan |last12=Goel |first12=Shashwat |last13=Gonzales |first13=Julia |last14=Gonzalez |first14=Alexander |last15=Jain |first15=Rishabh |display-authors=1 |date=6 January 2021 |title=A Search for Technosignatures around 31 Sun-like Stars with the Green Bank Telescope at 1.15–1.73 GHz |journal=[[The Astronomical Journal]] |volume=161 |issue=2 |pages=55 |arxiv=2011.05265 |bibcode=2021AJ....161...55M |doi=10.3847/1538-3881/abcc77 |s2cid=226290131 |doi-access=free |last16=Lam |first16=Adrian |last17=Lewis |first17=Briley |last18=Lewis |first18=Rebecca |last19=Li |first19=Grace |last20=MacDougall |first20=Mason |last21=Makarem |first21=Christopher |last22=Manan |first22=Ivan |last23=Molina |first23=Eden |last24=Nagib |first24=Caroline |last25=Neville |first25=Kyle |last26=O’Toole |first26=Connor |last27=Rockwell |first27=Valerie |last28=Rokushima |first28=Yoichiro |last29=Romanek |first29=Griffin |last30=Schmidgall |first30=Carlyn |last31=Seth |first31=Samar |last32=Shah |first32=Rehan |last33=Shimane |first33=Yuri |last34=Singhal |first34=Myank |last35=Tokadjian |first35=Armen |last36=Villafana |first36=Lizvette |last37=Wang |first37=Zhixian |last38=Yun |first38=In |last39=Zhu |first39=Lujia |last40=Lynch |first40=Ryan S.}}</ref> The Big Ear telescope was equipped with a receiver capable of measuring fifty {{val|10|u=kHz}}-wide channels. The output from each channel was represented in the computer printout as a column of alphanumeric intensity values. The Wow! signal is essentially confined to one column.<ref name="Big Ear 97" />

=== Time variation ===
At the time of the observation, the Big Ear radio telescope was only adjustable for [[Horizontal coordinate system|altitude]] (or height above the horizon), and relied on the [[rotation of the Earth]] to scan across the sky. Given the speed of Earth's rotation and the spatial width of the telescope's observation window, the Big Ear could observe any given point for just 72 seconds.<ref name="Skeptoid">{{Skeptoid |id=4342 |number=342 |title=Was the Wow! Signal Alien? |date=December 25, 2012 |access-date=October 8, 2016}}</ref> A continuous extraterrestrial signal, therefore, would be expected to register for exactly 72 seconds, and the recorded intensity of such signal would display a gradual increase for the first 36 seconds—peaking at the center of the observation window—and then a gradual decrease as the telescope moved away from it. All these characteristics are present in the Wow! signal.<ref name="edn">{{Cite web |last=Deffree |first=Suzanne |date=August 15, 2019 |title=Big Ear receives 'Wow! Signal,' August 15, 1977 |url=http://www.edn.com/electronics-blogs/edn-moments/4394137/Big-Ear-receives--Wow--Signal--August-15--1977 |access-date=July 2, 2016 |website=EDN Moments}}</ref><ref name="space_com">{{Cite news |last=Shostak |first=Seth |date=December 5, 2002 |title=Interstellar Signal From the 70s Continues to Puzzle Researchers |url=http://www.bigear.org/shostak_wow_20021205.htm |access-date=July 2, 2016 |work=Space.com}}</ref>

== Celestial location ==
[[File:Wow! signal source.svg|thumb|upright=1.8|The two regions of space in the constellation [[Sagittarius (constellation)|Sagittarius]] from where the Wow! signal may have originated. The ambiguity is due to how the telescope was designed. For clarity, the widths (right ascension) of the red bands have been exaggerated.]]
The precise location in the sky where the signal apparently originated is uncertain due to the design of the [[Big Ear]] telescope, which featured two [[feed horn]]s, each receiving a beam from slightly different directions, while following Earth's rotation. The Wow! signal was detected in one beam but not in the other, and the data was processed in such a way that it is impossible to determine which of the two horns received the signal.<ref name="feedhorns">{{Cite web |title=Big Ear's Twin Feed Horns |url=http://www.bigear.org/hshorns.htm |access-date=July 2, 2016}}</ref> There are, therefore, two possible [[right ascension]] (RA) values for the location of the signal (expressed below in terms of the two main [[Equatorial coordinate system|reference systems]]):<ref name="gray_2001">{{Cite journal |last1=Gray |first1=Robert |last2=Marvel |first2=Kevin |year=2001 |title=A VLA Search for the Ohio State 'Wow' |url=http://www.bigear.org/Gray-Marvel.pdf |journal=[[The Astrophysical Journal]] |volume=546 |issue=2 |pages=1171–77 |bibcode=2001ApJ...546.1171G |doi=10.1086/318272 |s2cid=17141857}}</ref>

{| class="wikitable" style="border: none;"
|-
|style="border: none; background: white;"|
! [[B1950]] equinox
! [[J2000]] equinox
|-
! RA (positive horn)
| {{nowrap|19<sup>h</sup>22<sup>m</sup>24.64<sup>s</sup> ± 5<sup>s</sup>}}
| {{nowrap|19<sup>h</sup>25<sup>m</sup>31<sup>s</sup> ± 10<sup>s</sup>}}
|-
! RA (negative horn)
| {{nowrap|19<sup>h</sup>25<sup>m</sup>17.01<sup>s</sup> ± 5<sup>s</sup>}}
| {{nowrap|19<sup>h</sup>28<sup>m</sup>22<sup>s</sup> ± 10<sup>s</sup>}}
|}

In contrast, the [[declination]] was unambiguously determined to be as follows:

{| class="wikitable" style="border: none"
|-
|style="border: none; background: white"|
! scope="col" | B1950 equinox
! scope="col" | J2000 equinox
|-
! scope="row" | Declination
| {{nowrap|−27°03′ ± 20′}}
| {{nowrap|−26°57′ ± 20′}}
|}

The [[galactic coordinates]] for the positive horn are {{math|''l''}}=11.7°, {{math|''b''}}=−18.9°, and for the negative horn {{math|''l''}}=11.9°, {{math|''b''}}=−19.5°, both being therefore about 19° toward the southeast of the galactic plane, and about 24° or 25° east of the [[Galactic Center]]. The region of the sky in question lies northwest of the [[Messier 55|globular cluster M55]], in the constellation [[Sagittarius (constellation)|Sagittarius]], roughly 2.5&nbsp;degrees south of the [[Magnitude (astronomy)|fifth-magnitude]] star group [[Chi Sagittarii]], and about 3.5&nbsp;degrees south of the plane of the [[ecliptic]]. The closest easily visible star is [[Tau Sagittarii]].<ref name="30thwow">{{Cite web |last=Ehman |first=Jerry R. |date=May 28, 2010 |title=The Big Ear Wow! Signal (30th Anniversary Report) |url=http://www.bigear.org/Wow30th/wow30th.htm |access-date=July 2, 2016 |publisher=North American AstroPhysical Observatory}}</ref>

Initially, no nearby Sun-like stars were known to lie within the antenna coordinates, although in any direction the antenna pattern would encompass about six distant Sun-like stars as estimated in 2016.<ref name="Calib Wow"/>  In 2022, a paper published in the ''[[International Journal of Astrobiology]]'' identified three likely Sun-like stars within the antenna-pointed coordinates. The better characterized star, [[2MASS 19281982-2640123]], is located 1,800 light years away, only 132 light years away from Maccone's estimation where an intelligent civilization is more likely to exist.<ref>{{Cite report |url=https://www.dia.mil/FOIA/FOIA-Electronic-Reading-Room/FileId/170023/ |title=Statistical drake equation |last=Maccone |first=Claudio |publisher=Defense Intelligence Agency |year=2009}}</ref> The other two candidates, 2MASS 19252173-2713537 and 2MASS 19282229-2702492, were insufficiently characterized but still likely to be Sun-like stars. Also, 14 other catalogued stars at the antenna coordinates may still turn out to be similar to the Sun after more data becomes available.<ref>{{Cite journal |last=Caballero |first=Alberto |year=2022 |title=An approximation to determine the source of the WOW! Signal |url=https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/an-approximation-to-determine-the-source-of-the-wow-signal/4C58B6292C73FE8BF04A06C67BAA5B1A |journal=International Journal of Astrobiology |volume=21 |issue=3 |pages=129–136 |arxiv=2011.06090 |bibcode=2022IJAsB..21..129C |doi=10.1017/S1473550422000015 |issn=1473-5504 |s2cid=226307031}}</ref><ref>{{Cite news |last=Anderson |first=Paul Scott |date=December 2, 2020 |title=Did the Wow! signal come from this star? |url=https://earthsky.org/space/source-of-wow-signal-in-1977-sunlike-star-2mass-19281982-2640123/ |access-date=May 14, 2022 |work=EarthSky}}</ref><ref>{{Cite news |last=Mann |first=Adam |date=May 24, 2022 |title=Famous 'alien' Wow! signal may have come from distant, sunlike star |url=https://www.space.com/wow-signal-origin-star |access-date=June 9, 2022 |work=Space.com}}</ref> As a response to the discovery, [[Breakthrough Listen]] conducted the first targeted search for the Wow! Signal in its first collaboration between the Green Bank Telescope and the Allen Telescope Array of the SETI Institute.<ref>{{Cite web |date=September 29, 2022 |title=1st Coordinated Green Bank Telescope/Allen Telescope Array Observes Possible Source of the WOW! Signal |url=https://www.seti.org/1st-coordinated-green-bank-telescopeallen-telescope-array-observes-possible-source-wow-signal |publisher=SETI}}</ref><ref>{{Cite news |last=Howell |first=Elizabeth |date=November 7, 2022 |title=No signs of alien life found near source of famous 'Wow!' signal |url=https://www.space.com/seti-wow-signal-search-no-life-signs |access-date=November 9, 2022 |work=Space.com}}</ref> The observations were performed on May 21, 2022, lasting 1 hour from Greenbank, 35 minutes from ATA, and 9 minutes and 40 seconds simultaneously.<ref>{{Cite journal |last1=Perez |first1=Karen I. |last2=Farah |first2=Wael |last3=Sheikh |first3=Sofia Z. |last4=Croft |first4=Steve |last5=Siemion |first5=Andrew |last6=Pollak |first6=Alexander W. |last7=Brzycki |first7=Bryan |last8=Cruz |first8=Luigi F. |last9=Czech |first9=Daniel |last10=DeBoer |first10=David |last11=Drew |first11=Jamie |last12=Gajjar |first12=Vishal |last13=Garrett |first13=Michael A. |last14=Isaacson |first14=Howard |last15=Lebofsky |first15=Matt |date=September 26, 2022 |title=Breakthrough Listen Search for the WOW! Signal* |journal=Research Notes of the AAS |volume=6 |issue=9 |pages=197 |bibcode=2022RNAAS...6..197P |doi=10.3847/2515-5172/ac9408 |issn=2515-5172 |s2cid=252540293 |doi-access=free}}</ref> No technosignature candidates were found.<ref>{{Cite web |title=Breakthrough Listen Search for the WOW! Signal |url=https://seti.berkeley.edu/wow/ |access-date=November 9, 2022 |website=Berkeley SETI Research Center |publisher=University of California}}</ref>

== Hypotheses on the signal's origin ==
Interstellar {{linktext|scintillation}} of a weaker [[continuous signal]]—similar in effect to atmospheric [[twinkling]]—could be an explanation, but that would not exclude the possibility of the signal being artificial in origin. The significantly more sensitive [[Very Large Array]] did not detect the signal, and the probability that a signal below the detection threshold of the Very Large Array could be detected by the Big Ear due to interstellar scintillation is low.<ref name="discovery" />{{fv|date=August 2024}} Other hypotheses include a rotating lighthouse-like source, a signal sweeping in frequency, or a one-time burst.<ref name="gray_2001" />

Ehman said in 1994: "We should have seen it again when we looked for it 50 times. Something suggests it was an Earth-sourced signal that simply got reflected off a piece of [[space debris]]."<ref name="cleveland_plain_dealer">{{Cite news |last=Kawa |first=Barry |date=September 18, 1994 |title=The Wow! signal |url=http://www.bigear.org/wow.htm |access-date=July 2, 2016 |work=Cleveland Plain Dealer}}</ref> He later somewhat recanted his skepticism, after further research showed the unrealistic requirements that a space-borne reflector would need to have to produce the observed signal.<ref name="Big Ear 97" />
The signal's frequency of {{nowrap|1420 MHz}} is also part of a [[L band#Astronomy|protected spectrum]]:<ref>{{Cite web |title=Significant Radio Astronomy Frequencies |url=http://www.setileague.org/articles/protectd.htm |access-date=2016-07-02 |publisher=SETI League}}</ref><ref name="CRAF">{{Cite book |url=http://www.astrosmo.unam.mx/~luisfr/CRAFHandbook3.pdf |title=Committee on Radio Astronomy Frequencies Handbook for Radio Astronomy |publisher=European Science Foundation |year=2005 |edition=3rd |page=101 |access-date=November 29, 2007 |archive-url=https://web.archive.org/web/20160603233854/http://www.astrosmo.unam.mx/~luisfr/CRAFHandbook3.pdf |archive-date=June 3, 2016 |url-status=dead}}</ref> a frequency range reserved for astronomical research in which terrestrial transmissions are forbidden, although a 2010 study documented several instances of terrestrial sources either interfering from adjacent frequency bands or illegally transmitting within the spectrum.<ref>{{Cite web |date=October 6, 2010 |title=SMOS Water mission winning battle with interference |url=https://www.esa.int/Applications/Observing_the_Earth/SMOS/SMOS_water_mission_winning_battle_with_interference |access-date=August 24, 2021 |publisher=European Space Agency}}</ref> In a 1997 paper, Ehman resists "drawing vast conclusions from [[wikt:half-assed|half-vast]] data"—acknowledging the possibility that the source may have been military or otherwise a product of Earth-bound sources.<ref>{{Cite news |last=Frank |first=Adam |date=July 10, 2012 |title=Talking To Aliens From Outer Space |url=https://www.npr.org/sections/13.7/2012/07/10/156540615/talking-to-aliens-from-outer-space |access-date=July 2, 2016 |publisher=NPR}}</ref> In a 2019 interview with [[John Michael Godier]], Ehman stated: "I'm convinced that the Wow! signal certainly has the potential of being the first signal from extraterrestrial intelligence."<ref>{{Cite interview |last=Ehman |first=Jerry |title=The Wow! Signal with Discoverer Dr. Jerry Ehman |url=https://www.youtube.com/watch?v=x67K-Vq1KWk&t=0s |access-date=June 11, 2022 |work=Event Horizon |date=December 20, 2019 |interviewer-first=John Michael |interviewer-last=Godier}}</ref>

[[METI (Messaging Extraterrestrial Intelligence)|METI]] president [[Douglas Vakoch]] told {{Lang|de|[[Die Welt]]}} that any putative SETI signal detections must be replicated for confirmation, and the lack of such replication for the Wow! signal means it has little credibility.<ref>{{Cite news |last=Marsiske |first=Hans-Arthur |date=September 12, 2007 |title=Welche Sprache sprechen Außerirdische? |url=https://www.welt.de/wissenschaft/article1439767/Welche-Sprache-sprechen-Ausserirdische.html |work=Die Welt |language=de}}</ref>

In August 2024, the [[Planetary Habitability Laboratory]] published a preprint reporting observations made in 2020 at the [[Arecibo Observatory]] in Puerto Rico—where they conclude that the Wow! signal was likely caused by a rare astrophysical event, in which stellar emissions energizing a cold [[hydrogen cloud]] caused it to suddenly surge in brightness.<ref>{{Cite web |date=August 16, 2024 |title=Wow! Signal Likely Caused by Rare Astrophysical Event |url=https://phl.upr.edu/wow |archive-url=https://web.archive.org/web/20240817123828/https://phl.upr.edu/wow |archive-date=August 17, 2024 |website=[[Planetary Habitability Laboratory]] |publisher=[[University of Puerto Rico at Arecibo]]}}</ref><ref>{{Cite arXiv |eprint=2408.08513 |first1=Abel |last1=Méndez |first2=Kevin Ortiz |last2=Ceballos |title=Arecibo Wow! I: An Astrophysical Explanation for the Wow! Signal |date=August 16, 2024 |mode=cs1 |last3=Zuluaga |first3=Jorge I.|class=astro-ph.HE }}</ref>

=== Discredited hypotheses ===
In 2017, Antonio Paris, Assistant Professor of Astronomy and Astrophysics at St. Petersburg College, Florida,<ref>{{Cite web |title=Prof. Antonio Paris |url=https://web.spcollege.edu/instructors/id/paris.antonio/BIO/ |publisher=St. Petersburg College}}</ref> proposed that the hydrogen cloud surrounding two [[comet]]s, [[266P/Christensen]] and [[335P/Gibbs]], now known to have been in the same region of the sky, could have been the source of the Wow! signal.<ref name="Paris-1">{{Cite journal |last=Paris |first=Antonio |date=January 1, 2016 |title=Hydrogen Clouds from Comets 266/P Christensen and P/2008 Y2 (Gibbs) are Candidates for the Source of the 1977 "WOW" Signal |url=http://planetary-science.org/hydrogen-clouds-from-comets-266p-christensen-and-p2008-y2-gibbs-are-candidates-for-the-source-of-the-1977-wow-signal/ |url-status=dead |journal=Journal of the Washington Academy of Sciences |arxiv=1706.04642 |bibcode=2017arXiv170604642P |archive-url=https://web.archive.org/web/20170615182709/http://planetary-science.org/hydrogen-clouds-from-comets-266p-christensen-and-p2008-y2-gibbs-are-candidates-for-the-source-of-the-1977-wow-signal/ |archive-date=June 15, 2017 |access-date=June 13, 2017}}</ref><ref name="Paris-2">{{Cite journal |last=Paris |first=Antonio |date=1 April 2017 |title=Hydrogen Line Observations of Cometary Spectra at 1420 MHZ |url=http://planetary-science.org/research/the-wow-signal/ |url-status=dead |journal=Journal of the Washington Academy of Sciences |volume=103 |issue=2 |archive-url=https://web.archive.org/web/20220509154206/http://planetary-science.org/research/the-wow-signal/ |archive-date=May 9, 2022 |access-date=13 June 2017}}</ref><ref>{{Cite arXiv |eprint=1706.04642 |class=astro-ph.EP |first1=Antonio |last1=Paris |first2=Evan |last2=Davies |title=Hydrogen Clouds from Comets 266P Christensen and P2008 Y2 (Gibbs) are Candidates for the Source of the 1977 WOW!Signal |year=2017}}</ref> This hypothesis was dismissed by astronomers, including members of the original Big Ear research team, as the cited comets were not in the beam at the correct time. Furthermore, comets do not emit strongly at the frequencies involved, and there is no explanation for why a comet would be observed in one beam but not in the other.<ref name="NAAPO-1">{{Cite web |last=Dixon |first=Robert S. |title=Rebuttal of the claim that the "WOW!" signal was caused by a comet |url=http://naapo.org/WOWCometRebuttal.html |access-date=June 13, 2017 |website=NAAPO |publisher=North American Astrophysical Observatory}}</ref><ref name="NS-1">{{Cite magazine |last=Emspak |first=Jesse |date=11 January 2016 |title=Famous Wow! signal might have been from comets, not aliens |url=https://www.newscientist.com/article/dn28747-famous-wow-signal-might-have-been-from-comets-not-aliens/ |access-date=13 June 2017 |magazine=[[New Scientist]]}}</ref><ref>{{Cite news |last=Mack |first=Eric |date=14 June 2017 |title=Aliens could still explain the 'Wow signal,' scientists say |url=https://www.cnet.com/news/aliens-wow-signal-comets-antonio-paris-seti/ |access-date=2021-05-31 |publisher=CNET}}</ref>

== Searches for recurrence of the signal ==
Several attempts were made by Ehman and other astronomers to recover and identify the signal. The signal was expected to occur three minutes apart in each of the telescope's feed horns, but that did not happen.<ref name="space_com" /> Ehman unsuccessfully searched for recurrences using Big Ear in the months after the detection.<ref name="discovery">{{Cite news |title=The 'Wow!' Signal |url=http://news.discovery.com/space/alien-life-exoplanets/the-wow-signal-130524.htm |url-status=dead |archive-url=https://web.archive.org/web/20160507072801/http://news.discovery.com/space/alien-life-exoplanets/the-wow-signal-130524.htm |archive-date=May 7, 2016 |access-date=2016-07-02 |publisher=[[Discovery Channel]]}}</ref>

In 1987 and 1989, [[Robert H. Gray]] searched for the event using the META array at [[Oak Ridge Observatory]], but did not detect it.<ref name="discovery" /><ref name="gray_2012">{{Cite book |last=Gray |first=Robert H |title=The Elusive WOW: Searching for Extraterrestrial Intelligence |date=2012 |publisher=Palmer Square Press |isbn=978-0-9839584-4-4 |location=Chicago}}</ref>{{page needed|date=August 2020}} In a July 1995 test of signal detection software to be used in its upcoming [[Project Argus (SETI)|Project Argus]], SETI League executive director [[H. Paul Shuch]] made several drift-scan observations of the Wow! signal's coordinates with a 12-meter radio telescope at the [[National Radio Astronomy Observatory]] in [[Green Bank, West Virginia]], also achieving a [[null result]].

In 1995 and 1996, Gray again searched for the signal using the [[Very Large Array]], which is significantly more sensitive than Big Ear.<ref name="discovery" /><ref name="gray_2012" />{{page needed|date=August 2020}} Gray and Simon Ellingsen later searched for recurrences of the event in 1999 using the 26-meter radio telescope at the [[University of Tasmania]]'s [[Mount Pleasant Radio Observatory]].<ref name="gray_2002">{{Cite journal |last1=Gray |first1=Robert |last2=Ellingsen |first2=S. |year=2002 |title=A Search for Periodic Emissions at the Wow Locale |journal=The Astrophysical Journal |volume=578 |issue=2 |pages=967–71 |bibcode=2002ApJ...578..967G |doi=10.1086/342646 |doi-access=free}}</ref> Six 14-hour observations were made at positions in the vicinity, but nothing like the Wow! signal was detected.<ref name="space_com" /><ref name="gray_2012" />{{page needed|date=August 2020}}

== Response ==
In 2012, on the 35th anniversary of the Wow! signal, [[Arecibo Observatory]] beamed a digital stream towards Hipparcos 34511, 33277, and 43587.<ref>{{Cite news |last=Noyes |first=Katherine |date=November 22, 2012 |title=Earth Replies to Space Signal After 35-Year Delay |url=https://www.technewsworld.com/story/76694.html |url-status=dead |archive-url=https://web.archive.org/web/20201130201504/https://www.technewsworld.com/story/76694.html |archive-date=November 30, 2020 |access-date=November 30, 2020 |work=TechNewsWorld}}</ref> The transmission consisted of approximately 10,000 Twitter messages solicited for the purpose by the [[National Geographic Channel]], bearing the [[hashtag]] "#ChasingUFOs" (a promotion for one of the channel's TV series).<ref name="wolchover" /> The sponsor also included a series of video vignettes featuring verbal messages from various celebrities.<ref name="spacedotcom">{{Cite news |last=Staff |date=August 12, 2012 |title=Humanity Responds to 'Alien' Wow Signal, 35 Years Later |url=http://www.space.com/17151-alien-wow-signal-response.html |work=Space.com}}</ref>

To increase the probability that any extraterrestrial recipients would recognize the signal as an intentional communication from another intelligent life form, Arecibo scientists attached a repeating-sequence [[Header (computing)|header]] to each individual message, and beamed the transmission at roughly 20 times the power of the most powerful commercial radio transmitter.<ref name="wolchover">{{Cite news |last=Wolchover |first=Natalie |date=2012-06-27 |title=Possible Alien Message to Get Reply from Humanity |url=https://www.livescience.com/21171-wow-signal-alien-message-reply.html |publisher=Discovery News}}</ref>

== In popular culture ==
The signal was featured in the 2024 television series [[3 Body Problem (TV series)|''3 Body Problem'']], in which the signal is also detected in [[Inner Mongolia]].<ref>{{Cite news |last=Spry |first=Jeff |date=March 16, 2024 |title='3 Body Problem:' How Netflix's sci-fi saga employs the famous Wow! SETI signal |url=https://www.space.com/3-body-problem-seti-wow-signal |access-date=April 18, 2024 |work=[[Space.com]]}}</ref> The signal was also referenced in the 1994 season 2 premier of [[The X Files]], "Little Green Men".

== See also ==
* [[List of interstellar radio messages]]
* {{annotated link|Arecibo message}}
* [[BLC1]] (Breakthrough Listen Candidate 1), signal detected in 2019
* {{annotated link|Fast radio burst}}
* {{annotated link|Quiet and loud aliens}}
* {{annotated link|Radio signal from HD 164595}}
* [[SHGb02+14a|Radio source SHGb02+14a]]
* [[Oh-My-God particle]]

== References ==
{{Reflist|30em}}

== External links ==
{{Commons category}}
* [http://www.google.com/sky/#latitude=-27.002855190521245&longitude=111.8023681640625&zoom=11&Spitzer=0.00&ChandraXO=0.00&Galex=0.00&IRAS=0.00&WMAP=0.00&Cassini=0.00&slide=1&mI=-1&oI=-1 Location] on [[Google Sky]]
* [http://www.fourmilab.ch/cgi-bin/Yourtel?lat=26.9500&ns=South&lon=291.250&fov=14.142&date=0&utc=2009-06-09+21%3A04%3A25&jd=2454992.37807&deepm=6.5&consto=on&limag=99.0&starn=on&starnm=99.0&starb=on&starbm=99.0&showmb=-1.5&showmd=6.0&imgsize=800&scheme=1&elements= Location] on [http://www.fourmilab.ch/yoursky/ YourSky]

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