The "Wow!" signal. Credit: Wikimedia Commons
The "Wow!" signal. Credit: Wikimedia Commons

Radio Astronomy, SETI

35 Years Later, the ‘Wow!’ Signal Still Tantalizes

24 Feb , 2012 by


Since the SETI program first began searching for possible alien radio signals a few decades ago, there have been many false alarms but also instances of fleeting signals of interest which disappeared again as quickly as they had appeared. If a potential signal doesn’t repeat itself so it can be more carefully observed, then it is virtually impossible to determine whether it is of truly cosmic origin. One such signal in particular caught astronomers’ interest on August 15, 1977. The famous “Wow!” signal was detected by the Big Ear Radio Observatory at Ohio State University; it was thirty times stronger than the background noise but lasted only 72 seconds and was never heard again despite repeated subsequent searches.

In a new book titled The Elusive Wow, amateur astronomer Robert Gray chronicles the quest for the answer to this enduring puzzle.

When the signal was first seen in the data, it was so pronounced that SETI scientist Jerry Ehman circled it on the computer printouts in red ink and wrote “Wow!” next to it. It appeared to fit the criteria for an extraterrestrial radio signal, but because it wasn’t heard again, the follow-up studies required to either confirm or deny this were not possible. So what was it about the signal that made it so interesting?

First, it did appear to be an artificial radio signal, rather than a natural radio emission such as a pulsar or quasar. The Big Ear telescope used a receiver with 50 radio channels; the signal was only heard on one frequency, with no other noise on any of the other channels. A natural emission would cause static to appear on all of the frequencies, and this was not the case. The signal was narrow and focused, as would be expected from an artificial source.

The Big Ear Radio Observatory. Credit: Big Ear Radio Observatory / North American AstroPhysical Observatory / Ohio State University

The signal also “rose and fell” during the 72 seconds, as would be expected from something originating in space. When the radio telescope is pointed at the sky, any such signal will appear to increase in intensity as it first moves across the observational beam of the telescope, then peak when the telescope is pointed straight at it and then decrease as it moves away from the telescope. This also makes a mere computer glitch a less likely explanation, although not impossible.

What about satellites? This would seem to be an obvious possible explanation, but as Gray notes, a satellite would have to be moving at just the right distance and at just the right speed, to mimic an alien signal. But then why wasn’t it observed again? An orbiting satellite will broadcast its signal repeatedly. The signal was observed near the 1420 MHz frequency, a “protected spectrum” in which terrestrial transmitters are forbidden to transmit as it is reserved for astronomical purposes.

There may be a bias in thinking that any alien signals will be like ours which leak out to space continuously, ie. all of our radio and TV broadcasts. That is, “normal” radio emissions from every-day type technologies which could easily be seen on an ongoing basis. But what if they were something more like beacons, sent out intentionally but only on a periodic basis? As Gray explains, radio searches to date have tended to look at many different spots in the sky, but they will only examine any particular spot for a few minutes or so before moving on to the next. A periodic signal could easily be missed completely, or if seen, it may be a long time before it is seen again.

Of course, it is also possible that any other civilizations out there might not even use radio at all, especially if they are more advanced than us (while other intelligent life might be behind us, as well). A newer branch of SETI is now searching for artificial sources of light, like laser beams, used as beacons.

So where does this leave us? The “Wow!” signal still hasn’t been adequately explained, although various theories have been proposed over the years. Perhaps one day it will be observed again, or another one like it, and we will be able to solve the mystery. Until then, it remains a curiosity, a tantalizing hint of what a definite signal from an extraterrestrial civilization might look like.

More information is available at the Big Ear Radio Observatory website.

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By  -        
Paul Scott Anderson is a freelance space writer with a life-long passion for space exploration and astronomy and has been a long-time member of The Planetary Society. He currently writes for Universe Today and His own blog The Meridiani Journal is a chronicle of planetary exploration.

34 Responses

  1. IVAN3MAN_AT_LARGE says:

    […] The signal was observed near the 1420 MHz frequency, a “protected spectrum” in which terrestrial transmitters are forbidden to transmit as it is reserved for astronomical purposes. […] So where does this leave us? The “Wow!” signal still hasn’t been adequately explained, although various theories have been proposed over the years. […].

    It was probably caused by some radio-amateur having a laugh.

  2. Aqua4U says:

    Given the equipment at the time and the predominance of a multitude of secret military satellites, U.S., Russian and etc. on orbit at the time. There’s actually no real way to determine whether this signal came from ‘beyond’. Radio signals used by an advanced extraterrestrial civilization? Doubtful… there’s got to be a better way to communicate over the vast distances of interstellar space? Something that we haven’t imagined… yet?

    • Really20 says:

      Yes, they’re probably beyond electromagnetic waves of any sort, perhaps the Alcubierre drive if it is feasible for them.

      • Torbjörn Larsson says:

        You can’t transmit signals (or anything else) with Alcubierre bubbles, even if you could make them (you can’t). They presuppose ftl to place something into the ftl bubble to be “driven” somewhere.

        It is the same type of classical paradox that lies at the heart of all GR ftl loopholes, which basically says you need to have causality to do something in the first place.

        • Really20 says:

          Then just create a cosmic train system, with relay stations laid by STL spacecraft while future Alcubierre-drive craft do not communicate with the outside until they reach such a waypoint.

  3. Tony Power says:

    Could it have been a signal from much further away being amplified thanks to the lensing effect of a bubble of dark matter?

  4. Frank Blackstone says:

    Frank Blackstone, The Wow Signal has alwayse interested me in the fact it Did not repeat itself , Could have been a Burst signal Might have been something un intential , But whatever it was it wasnt space noise . The Location and distance frome the ploted signals origine is light years away So . Time certainly plays a Huge problem of sending and receiving anything Especialy assuming it came from a round object as a planet in rotation itself There was a Novel written by Mitchner ‘Space” In it a politician asks the astronomer why they have not heard anything , he responds in the same maner , Distance time and Length of exestance and planet rotation along with our own ability to send radio waves into space …So the posibilitys are endless and , Interesting , Hope one day they might make contact , Maby Carl will be watching …..

  5. Martin_Lambert says:


    WOW’s signal 1420 MHz frequency is ~emission frequency of hydrogen.

  6. I looked at the comments and didn’t see one quite like the one I have a theory on for the signal.
    My little theory stems from our use of nuclear weapons and the signals they produce when set off. This could be the very reason why it never showed up again. It could have been a weapon test or massive planetary accident of some type out there by some other race, possibly. This could then show the reason it was never heard again. Either they used the weapon or the accident destroyed them. One reason I think it might be a weapon’s test of some type is the fact that our own nuclear weapons testing echo is still traveling in space from our planet from our first explosions of the A-bomb even.
    Even if it was a piece of noise only, it should have been investigated a lot more.
    Yes, other races out there might use other forms of communications, but think for a moment, lasers need line of sight (that even a stray particle can throw off unless a massive beam), other forms need a lot more power than even we might be able to generate too. Radio or wavelength transmissions are still just being touched upon even with our technology really. We have a lot further to go and yet people dismiss the simple as non-technologically advanced for some reason. You would have to be very advanced to send any signal light years away from the source and might even use carrier waves of planets and stars to boost and disguise your signals. Not everyone wants to be found possibly either. One reason we might often think about regulating our own transmissions from this planet. Our own history alone should have taught us this lesson.

  7. Starcastle2011 says:

    SETI is getting desperate resurrecting the WOW signal. Since it never reappeared there isn’t much you can say about it other than it was anomalous. In addition, the article failed to mention a study by JPL that noted radio and television signals do no remain coherant for more than a couple of light years. Despite the movie “Contact” nobody out there is watching “I Love Lucy”. What can you say about a program that has produced no tangible results in 51 years, but instead piles one myth upon another. You know, for decades we’ve been told that receipt of any kind of signal, regardless of the distance from which it comes, or our inability to decipher it will somehow change humankind forever. Based on what I’ve seen in my life, I think 4K television transmission will have more impact.

    • Skipdallas1 says:

      So, in your opinion the cut-off date for listening for signs of another civilization would be exactly how many years? The sky is a VERY big place to monitor you know. After 51 years, you would give up? Why?

      • David Krauss says:

        Because maybe we’re barking up the wrong tree.

        On one hand, we might as well pursue the best known strategy.

        On the other, contrary to the popular belief, we aren’t broadcasting anything that looks like transmission of information. If I understand Starcastle, the interstellar medium disperses narrow-band transmissions like TV transmissions into garbage after a couple light-years. So the strategy we employ wouldn’t work *on us*, even if ET were looking directly at Earth, blocking solar interference.

    • David Krauss says:

      Not sure I agree that the dispersion is relevant. Since the same channel is reused among different geographical areas, the signals are a superimposed mess in the first place. ET should still be able to see an array of narrowband signals, anyway. That’s a telltale sign of civilization even if they can’t be decoded.

      Likely that when we really become sufficiently advanced for interstellar communication, we’ll put up a proper beacon, far from the Sun and transmitting a nice, slow signal that won’t break down within the galaxy.

  8. CallanTFC says:

    I always scoff at the suggestion that extraterrestrials would simply not use radio signals to communicate because such technology is too rudimentary for an intelligent race to use. Why do we by default assume that any other intelligent beings are so sufficiently advanced technologically as to use methods of communication beyond our comprehension or detection? Suppose for a moment that intelligent life does exist, and that the alien civilization decided to announce their presence to the cosmos. Is it not possible that at some stage in their evolution, they DID use radio signals, as we do today? Humankind may discover more advanced means of communication some years down the road, but the radio waves we emitted during the period of our development where there was no other means of communication yet uncovered will still be moving through space at the speed of light. It is entirely possible that we may find evidence of radio waves coming from a distant solar system, and it is equally possible that those signals would be hundreds or thousands of years old by the time we hear them. As always and with everything, it is a case of looking in the right place at the right time. And although the mere detection of an identifiably artificial signal may be the smoking gun that finally puts to bed whether or not we are alone in the universe, it by no means predicates an ability to make contact. I think the effectiveness of the work SETI does cannot be measured after only 51 years, especially if you reflect on how much our own science and technology has evolved in that short time. We’re looking for a needle in a haystack. Can’t just stop after what is essentially a blink of the eye in cosmic time simply because we haven’t found anything conclusive yet.

    • johnb123 says:

      As we have moved to cable tv and fiber optics, less of out signals are going out into space. The same could have happened with any other civilization.

      • David Krauss says:

        Broadcast TV is still alive and well. Fiber optics replaced copper cables, not radio transmission. Furthermore, other technologies have moved onto the airwaves. Implausible that we’re radiating any less now than then.

        Either way, our signals would be drowned out by the Sun at ET’s distance, except to a detector far beyond our current technology.

    • Uncle_Fred says:

      Our TV and radio signals degenerate into noise at about 1-2 light years. It is possible for a radio signal to reach another star system, but the effort would require a focused transmission and a gargantuan power source + transmitter.

  9. Rudolf Usselmann says:

    I wonder how many signals have been found so far that have been classified by the US government and we have never heard about ….

  10. johnb123 says:

    Voyager Discovers Possible Sea of Huge, Turbulent, Magnetic Bubbles at Solar System Edge

    I wonder if this may be one reason we’re not hearing anything? Whatever signals they are using, may be blocked.

  11. johnb123 says:

    Read “The Eerie Silence: Renewing Our Search for Alien Intelligence” by Paul Davies.

  12. johnb123 says:

    47 year old television signals bouncing back to Earth

  13. lcrowell says:

    It is unlikely that our 1950’s TV or radio programs are out there, or that ETI on Gliese 581 d are watching broadcasts of the Gulf War of 1991. This is even if they happened to be out there. Our electromagnetic signals are attenuated by two factors. The first is just 1/r^2. So a megawatt signal near a transmitter distributed on a 1km spherical surface, area = 4?km^2 out to 10 light years is distributed on a sphere 4?x10^{14}km^2. Hence given any antenna with a certain spatial extent the signal drops by a factor of 10^{-14}. So an area of 1m^2 1km from the antenna has a flux of radiation with power ~ 1watt. 10 light years out the power is 10^{-28}watts. A typical antenna has about 10^{-12}watts/m^2 detectability. This would require a VLA type of array with aperature radius ~ 10km to detect. Compound the issue if the signal is sent from thousands of light years away.

    There is another attenuating factor, which is the Debye length. The existence of interstellar plasma screens electric fields out to a distance of 10m. This adds impedance to the impedance of free space. This will attenuate the signal by an exponential factor. I don’t have the numbers immediately available, but I think this would be an appreciable factor across galactic distances ~ 10^4 light years. It is to EM radiation what dust scattering is to optical radiation.

    As a result our weak signals sent out into space are probably reduced in power below the noise background from sources such as the CMB. Any ETI within a 50 ly radius would have to work hard to find our signals. Beyond that distance in the future it is not likely any surviving information could be picked out.

    The SETI program is maybe worth setting some lower bound on the existence of ETI that could be detected. I do pretty seriously doubt we will get some “contact.”


  14. Dion Jones says:

    News releases for proposed Square Kilometer Array in the Southern Hemisphere claim it will be able to detect an airport radar 50 light years away. While more advanced civilizations might have more advanced means of communications it should be noted that airports and coastal areas still use lighthouses and light beacons 60 years after RADAR came into common use and ten years after GPS became commonplace. An advanced civilization may use advanced radar for warning beacons or who knows what. Radio signals could be emitted as a by product of advanced rockets, power generation, weapons, or accidents as suggested by other post here.

  15. Torbjörn Larsson says:

    Seeing how it was so early, was detected by primitive means in the first place, and has never been repeated, it is safe to conclude it was an instrument error. You don’t need much to get a sensitive detector to oscillate spontaneously, say by a negative feedback phase problem.

    I don’t know if SETI does it yet, but accelerator groups have been forced to go to “black box” methods due to similar searches for signals in many places. They develop an analysis method on a subset of the signal, then “open up the box” and do the search once post collection. Also, there are fake positives inserted from outside the analysis team to verify analysis quality.

    If you don’t do this, you will have problems with interpreting significance.

    That was surely not done then, since they had a “wow” spontaneous analysis instead.

  16. Alex Lunix says:

    Aliens speak English?

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