The Wow! signal has a storied history in the SETI community, a one-off detection at the Ohio State ‘Big Ear’ observatory in 1977 that Jim Benford, among others, considers the most interesting candidate signal ever received. A plasma physicist and CEO of Microwave Sciences, Benford returns to Centauri Dreams today with a closer look at the signal and its striking characteristics, which admit to a variety of explanations, though only one that the author believes fits all the parameters. A second reception of the Wow! might tell us a great deal, but is such an event likely? So far all repeat observations have failed and, as Benford points out, there may be reason to assume they must. The essay below is a shorter version of the paper Jim has submitted to Astrobiology.
by James Benford
In 1977 the Big Ear radio telescope (Ohio State University Radio Telescope) recorded the famous Wow! Signal, which is the most serious contender for artificial interstellar radiation. It is called the ‘Wow!’ Signal. It has never been seen again. Its origin and nature remain a total mystery.
I offer an alternative explanation for it: The Wow! could have been leakage from an interstellar power beam. I propose that this class of radiation, which is not widely understood, can explain the observed features of the Wow! signal.
The Three Wow! Parameters
The Wow! signal has 3 prominent parameters: the power density received, the signal’s duration and its frequency .
Power Density: The Wow! signal was very strong, the strongest they ever recorded in the seven-year Ohio State SETI Survey. The shape is shown in the Figure.
Figure: The Wow! Signal. The peak is 32 times the signal to noise ratio of the observations. Courtesy of Sam Morrell.
Duration: The Big Ear was fixed in orientation, so rotated with the Earth. From Gray : “The amount of time it took the Wow! to pass through the antenna’s beam closely matches the expected transit time for celestial sources. Sources fixed amid the stars should take about 36 seconds to transit the sensitive middle half of the beam, the full-width-half-max. The Wow! signal took about 38 seconds”.
So we don’t know how long the signal lasted, just that it was on as the antenna rotated past.
Frequency: The Wow! Signal was at 1.42 GHz. The 1.4-1.427 GHz band is protected internationally, meaning, as John Kraus, designer and director at the Big Ear, says in a letter to Carl Sagan , “all emissions are prohibited” . The band was set aside to allow radio astronomy of the H 1 line, the hyperfine transition of neutral hydrogen (1.420 GHz), which is of great astronomical interest for imaging atomic hydrogen in interstellar space. Consequently, this part of the L-band is a protected radio astronomy allocation all over the world. Therefore the Wow! couldn’t be a transmission from Earth satellites or aircraft.
The Fourth Wow! Parameter
There is a fourth parameter, although it has not received attention: the Revisit Time. This is the interval until the signal is seen again. If ET were rastering their beam across the sky, the beam would be seen to repeat later. Searches have been conducted from the META array at Oak Ridge, the Green Bank National Radio Astronomy Observatory in West Virginia and the Tasmanian Mount Pleasant Radio Observatory in Australia [3, 4].
Recent extensive observations on the Allen Array by Gerry Harp, Robert Gray and colleagues, which used the 42 dishes of the ATA as an interferometer, monitored the entire 1.5 degree field of view for 100 hours. They did not see the Wow! and summarized : “As for the possibility that the Wow! signal is a repetitive transient, our observations rule out almost all periods under 40 hours, which covers many repetitive scenarios such as rotating planets or blinking beacons with periods comparable to a terrestrial day and several times longer. Our extended observations cannot rule out scenarios such as occasional targeted transmissions with repetition rates of many days or varying repetition rates.”
The Wow! observation has never recurred. I take this absence as a clue to its origin.
Power Beaming and the Wow! Signal
The most observable leakage radiation from an advanced civilization may well be from the use of power beaming to accelerate spacecraft and transfer energy. Power beams are now more credible because we’re building our own: The Starshot project plans launching probes to nearby stars in this century, making power beaming a credible source concept . And power beaming is being developed for military applications, where it is termed ‘directed energy’ . See reference 8 for a review of power beaming concept studies.
Applications suggested for power beaming are:
- launching spacecraft to orbit,
- raising satellites to a higher orbit,
- interplanetary space–to–space transfers of cargo or passengers,
- beam-driven launch of interstellar probes,
- beam-driven starships.
Leakage of the beam around the sides of the vehicle being accelerated would be observable at great range because of the highly directed high power. The spilled energy can be reduced to less than half, but that is not the cost optimum. Spillage losses of more than half are typical of Starshot calculations, for reasons of economics and performance that will apply to other civilizations .
For power beam missions involving changing orbits of spacecraft, the first three applications on the above list, an important quantity is the slew rate, the rate at which the beam propelling the spacecraft sweeps to direct it toward its orbit . A review of representative parameters for applications of power beaming shows that, for orbit–related missions, the slew rates are high, so the observation time of the beam leakage is too short, ≲ 1 second. So the orbit–related applications cannot explain the Wow! But interstellar probes and starships have small slew rate, so are the best candidates.
Power Beaming Examples
In beaming of power, the power density S at range R is determined by W, the effective isotropic radiated power (EIRP), which is the product of radiated peak power P and aperture gain G . Since we know the power density of the Wow! received and its frequency, we can calculate both the range to the source of Wow! and the power needed at a given range.
For example, if the Wow! source had the EIRP of Arecibo, it would have to be at range < 2 light-years, so it must be far more powerful.
Bob Forward’s ultralight microwave-propelled Starwisp, a 1-kg sail reaching 0.1 c, would be at a range of ~ a million light-years, so from extra-galactic distances .
An interstellar precursor probe described by Benford and Matloff for 100 km/sec = 0.03% c would be at range 116 light-years, a nearby star .
We can also assume a distance, then calculate the EIRP that would be required. First, choose 2000 light-years for the distance to the Wow! source. (Maccone estimates that the mean distance to a communicative civilization is ~2,000 light-years away ). Then choose an aperture diameter of 3 km. (Starshot envisions a ~3-km diameter laser array to drive its interstellar probe.) The required beam power is 11 GW. This is similar to Starshot, with ~ 11 GW.
From these examples, it is credible that the Wow! signal was leakage from launch of an interstellar probe.
Will we see it again? Probably not
Probably not if it was due to power beaming. With a launch driven by an intense beam, to arrive years later at a neighboring stellar system, the starship would be launched toward where the stellar system will be when the starship arrives. The ratio of the distance the star would move to the beam spot size is given by vs/(vss Δθ), where vs is the average velocity of the star relative to stars on our stellar neighborhood, typically 20 km/sec, and vss is the starship velocity, Δθ the angular beamwidth. For the starship concepts proposed, that ratio varies from 104 to 107 .
The angle of the radiated beam with respect to the light path between the two stars is larger than the width of the beam. Thus, the beam is generally not observable from the target planetary system. If the Wow! was driving a probe to a star, that star was at that time far from the direction of the beam. Earth could accidentally receive the leakage from the beam, since stars move relative to each other. So leakage radiation from star probe launches using the Wow! beam will not be seen again from Earth. This fits the non-observations to date.
Comparison of Explanations for the Wow! Signal
There are three suggested explanations for the Wow!: either spurious emissions from Earth, an interstellar communication or leakage from a power beam. Here is a brief summary of the evidence for and against each explanation:
Arguments for power beaming leakage as a cause:
- The power beaming explanation for the Wow! accounts for all four of the Wow! parameters: the power density received, the duration of the signal, its frequency, and the reason why the Wow! has not occurred again. The Wow! power beam leakage hypothesis gets stronger the longer that listening for the Wow! to recur doesn’t observe it repeat.
- Power beams are now more credible because we’re building our own: the Starshot project plans launching probes to nearby stars in this century. The technology required for the Beamers for such interstellar probe launches are within our grasp.
Arguments against ET communication as a cause:
- The theory that the Wow! Signal was an interstellar communication predicts that it will recur. It fits within the overall SETI strategy, which looks for deliberate beaming of messages to us from ETI. But the long series of the subsequent non-observations of the Wow! shows that the SETI messaging hypothesis is gradually being falsified by being tested.
Arguments against radio frequency interference (RFI) as a cause:
- The Wow! Signal was at 1.42 GHz. The band from 1.4 to 1.427 GHz is protected internationally, meaning, all emissions are prohibited . Therefore it is very doubtful that the Wow! Was a transmission from Earth satellites or aircraft because they are forbidden to transmit in this band. Secret satellites would avoid it because they would be detected by radio astronomers. (Emissions in this band are sometimes detected, but at very low levels. These are likely due to intermodulation products, which are nonlinear effects in electronics.)
- Aircraft would be unlikely to remain static in the sky. Spacecraft would pass through the beam much faster. To match that lack of angular motion, an Earth satellite would have to be millions of kilometers distant, out far beyond the Moon.
- Ohio had good RFI rejection because what was recorded was the difference between two offset beams, so a local signal appearing in both horns simultaneously would cancel. This was frequently verified.
- The possibility that the signal was a harmonic or sub-harmonic of a local signal is countered by Ohio State having monitored the 21 cm band for many years, would have noticed a local interfering signal.
- A deliberate hoax? This lacks credibility, as hoaxes are a practical joke, which succeed if they are later revealed. Then why keep it secret for decades?
Conclusion and Implications
I’ve looked at the various power beaming applications and found that credible ones are low mass interstellar probes such as Starwisp and Starshot. This does not mean that the orbit–changing power-beaming applications cannot be seen.
The power beaming explanation for the Wow! accounts for all four of the Wow! parameters. This includes the absence of any later observation, for it has not been seen since, despite the several attempts to repeat observing it. This allows a prediction: the Wow! signal will not be seen again.
If one accepts the possibility that Wow! was power beam leakage, several lines of action should be followed for the future of SETI:
- Such interstellar power beams would be visible over large interstellar distances. All-sky surveys in both the microwave and laser could detect more power beam leakages. Instruments with large instantaneous field of view could detect infrequent transitory leakage signals. Ultimately, we should have a full-sky capability for both hemispheres.
- Because ET would understand that its beam leakage could be observed, there may well be modulations on the beam to communicate to any inadvertent listener. This would add little additional energy or cost for ET. Therefore all-sky surveys should have sufficient electronics to capture messaging embedded on the beam. Extraterrestrial intelligences would know their power beams could be observed. That message may use optimized power-efficient designs such as spread spectrum and energy minimization [12, 13].
On the other hand, if one thinks that the Wow! signal was an attempt to communicate, one should follow up on a possibility that is not been explored: If with Wow! we inadvertently intercepted a radio link between one star and another, we should look in the opposite direction to see if signals are transmitting toward the Wow! direction. To my knowledge this has not been explored.
1. R. H. Gray, The Elusive Wow!, Palmer Square Press, 2012.
2. J. D. Kraus, “The Tantalizing “Wow!” Signal”, letter to Carl Sagan, NRAO Archives, accessed December 3, 2020, https://www.nrao.edu/archives/items/show/3684, 1994.
3. R. H. Gray, “Intermittent Signals and Planetary Days in SETI”, Int. J. Astrobiology, https://doi.org/10.1017/S1473550420000038
4. R. H. Gray, A Search For Periodic Emissions At The Wow! Locale”, Astrophysical Journal, 578:967–971, 2002.
5. G. Harp et al., “An ATA Search for a Repetition of the Wow! Signal”, Astrophysical Journal 160:162, 2020.
6. K. Parkin, “The Breakthrough Starshot System Model”, Acta Astronautica 152, 370, 2018.
7. J. Benford, J. Swegle and E. Schamiloglu, High Power Microwaves, 3rd Ed., Taylor & Francis, Boca Raton, FL 2016.
8. J. Benford and D. Benford, “Power Beaming Leakage Radiation as a SETI Observable”, Astrophysical Journal 101 825, 2016.
9. R. L. Forward, “Starwisp: An Ultralight Interstellar Probe,” J. Spacecraft and Rockets, 22 345-350 1985.
10. J. Benford and G. Matloff, Intermediate Beamers for Starshot”, JBIS 72, 51, 2019.
11. C. Maccone, “The Statistical Drake Equation”, Acta Astronautica 67, 1366, 2010.
12. D. Messerschmitt, “The case for spread spectrum”, Acta Astronautica, 81, 227, 2012.
13. D. Messerschmitt, 2015, ‘Design for minimum energy in interstellar communication”, Acta Astronautica, 107, 20-39, 2015.