Robert Gray was something of an outsider in the community of SETI scientists, spending most of his career in the world of big data, calculating mortgage lending patterns and examining issues in urban planning from his office in Chicago. As an independent consultant specializing in data analysis, his talents were widely deployed. But SETI was a passion more than a hobby for Gray, and he became highly regarded by scientists he worked with, many of whom were both surprised to hear of his death on December 6, 2021. It was Jim Benford who gave me the news just recently, and it humbles me to think that a Centauri Dreams post I worked with Gray to publish (How Far Can Civilization Go?) appeared just months before he died.

Gray’s independent status accounts for the lack of publicity about his death in our community, but I’m still startled that I’m only now learning about it. His name certainly has resonance on this site, particularly his book The Elusive Wow: Searching for Extraterrestrial Intelligence (Palmer Square Press, 2011), which should be on the bookshelf of anyone with a serious interest in SETI. The eponymous 1977 signal, received at Ohio State’s ‘Big Ear’ observatory, with Jerry Ehman’s enthusiastic ‘Wow!’ penciled in next to the printout, remains an enigma.

So obsessed was Gray with the signal – and with SETI investigation in general – that he built a radio telescope with a 12-foot dish in his backyard and pursued the work at professional installations like the Harvard/Smithsonian META radio telescope at Harvard’s Oak Ridge Observatory as well as the Very Large Array in New Mexico. He included Mount Pleasant Radio Observatory in Hobart, Tasmania in his world-spanning list of hunting grounds. His papers appeared in the likes of The Astrophysical Journal and Icarus; other essays went to such venues as Sky & Telescope, The Planetary Report, and Scientific American.

SETI has historically dealt in short ‘dwell’ times, meaning the observing instrument looks at a star and moves on so as to widen the search to as many stars as possible. Gray became interested in what would happen if we attempted long, fixed stares at high-interest stars, thus searching for signals that are intermittent as opposed to continuous. An interstellar beacon, whether targeted at specific stars or not, might send a signal that appeared and then disappeared for an arbitrary amount of time.

When I talked to Jim Benford about Gray’s work, he reminded me of his own prediction that the Wow Signal would never repeat. Interstellar applications of power beaming of the sort even now being investigated by Breakthrough Starshot would produce a small slew rate that would be consistent with a signal like the Wow as it moved across our sky. See Was the Wow Signal Due to Power Beaming Leakage for more, where Jim notes that:

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.

Jim discussed power beaming as an option to explain the signal with Gray, who evidently found it one of several feasible explanations. When I first discussed it with Gray a few years back, he told me that a terrestrial explanation couldn’t be ruled out, but he found it hard to come up with a plausible one. On the phone with Jim Benford last night, I learned that Gray felt the signal was deeply enigmatic. The idea that we might never know what it was, as per Jim’s idea, would have deepened the mystery.

As to a terrestrial origin for the enigmatic reception, we would have to explain a signal that appeared at 1.42 GHz, while the band from 1.4 to 1.427 GHz is protected internationally – no emissions allowed. Aircraft can be ruled out because they would not remain static in the sky; moreover, the Ohio State observatory had excellent RFI rejection. You can see why the Wow! Signal retains its interest after all these years (see The Elusive Wow for much more). If power beaming is indeed in use in the galaxy, we may find SETI happening upon many such one-off signals as they sweep past, without serious hope of ever pinning them down to a definite source.

Power beaming aside, Gray was quite interested in intermittency as an antidote to the idea that we should look primarily for continuous isotropic broadcast signals. In a 2020 paper on the matter, he wrote:

…reducing the duty cycle to 1% could provide a 100-fold reduction in average power required, perhaps radiating for 1 s out of every 100 s. Searches observing targets for a matter of minutes might detect such signals, such as the Ohio State and META transit surveys which observed objects for 72 s and 120 s respectively, or Breakthrough Listen observing targets for three five minute periods…, or a targeted search such as Phoenix observing objects for 1,000 s in each of several spectral windows…, or the ATA observing for 30 minutes… Reducing duty cycle further yields further savings—for example a 10-4 duty cycle with a 104 reduction in average power might result in a 1 s signal every three hours, but most searches to date would be likely to miss such signals. Assuming longer signal duration does not help much; a 1-hour signal present every 100 or 10,000 hours would be very unlikely to be found by most current search strategies unless the population of such signals is large.

Gray had an excellent reputation among radio astronomers for the quality of his work; he was independent, disliked self-promotion (and particularly social media), and remained dogged in the pursuit of high-quality data. David Kipping (Columbia University), who worked with Gray on a recent paper in Monthly Notices of the Royal Astronomical Society, remembered him this way in an email this morning:

“I didn’t know Robert well, we only interacted virtually. I can say that I found his passion and persistence of the Wow signal to be an inspiration and reminded me that often the scientific investigative journey itself is the most valuable product of our endeavors. He’s an inspiration to amateur astronomers everywhere, a man who taught himself everything there was to know about that signal and more, and ended up using some of the largest radio telescopes in the world in pursuit of his dream. He was a gentleman to collaborate with and I only wish we’d had more time. It gives me a certain sense of satisfaction that his last paper with me demonstrated that for the Wow signal, hope still persists that it could repeat, not a lot of hope, but a little. And I think that’s a wonderful bookend to his publishing career.”

Beyond his work in SETI observations, Gray also looked at extending the familiar Kardashev scale, which ranks civilizations based on the energy they are able to use. In How Far Can Civilization Go?, which ran in these pages in 2021 and referenced an Astronomical Journal paper he had written on the subject, he noted that choosing what to look for in a SETI search depends on what we assume about the future course of a civilization and its power capabilities:

Whether any interstellar signals exist is unknown, and the question of how far civilization can go is critical in deciding what sort of signals to look for. If we think that civilizations can’t go hundreds or thousands of times further than our energy resources, then searches for broadcasts in all directions all of the time like many in progress might not succeed. But civilizations of roughly our level have plenty of power to signal by pointing a big antenna or telescope our way, although they might not revisit us very often, so we might need to find ways to listen to more of the sky more of the time.

I really admired the self-actualizing nature of Robert Gray’s scientific career. The man was a highly disciplined polymath whose academic background involved urban planning and policy, not physics, but without academic affiliation he made himself into a force in a highly specialized field and won the admiration of astronomers wherever he traveled. His papers are shrewd, thoughtful and provocative. I like what Jill Tarter told the Chicago Tribune shortly after his death. Calling Gray “larger than life,” Tarter added: “Bob was a constant. He really was intrigued and wanting to help, and he did — he actually followed through and was very forceful.”

Of the Gray papers I’ve mentioned above, his paper on SETI and intermittent reception is “Intermittent Signals and Planetary Days in SETI,” International Journal of Astrobiology 4 April 2020 (abstract). His paper on Kardashev is “The Extended Kardashev Scale,” Astronomical Journal 159, 228-232 (2020). Abstract. See also Kipping and Gray, “Could the ‘Wow’ signal have originated from a stochastic repeating beacon?” Monthly Notices of the Royal Astronomical Society Vol. 515, Issue 1 (September 2022), 1122-1129 (abstract).