In the shadow of Mount Hopkins, four giant, segmented mirrors stare at the night sky. Surrounded by the scrub of the Arizona desert, they usually spend their time watching for evidence of incoming gamma-rays from deep space. Tonight, however, they are searching for something more – evidence that we are not alone.
These four telescopes make up the VERITAS array. Its acronym stands for Very Energetic Radiation Imaging Telescope Array System. Gamma rays are the most energetic form of electromagnetic radiation there is, and they tend to come from equally energetic sources in space, such as supernovae, magnetic neutron stars and accreting black holes.
The VERITAS telescopes don’t actually detect gamma rays directly. Instead, the gamma rays strike molecules within the atmosphere, smashing those molecules apart, and transferring the gamma rays’ immense energy into a shower of secondary particles that are produced in the collision.
Since light travels about 90 kilometers per second slower through air than through a vacuum, and the secondary particles are so energetic that they travel at a speed faster than the speed of light through air (but still slower than the absolute speed of light in a vacuum), they emit a flash of bluish light – called Cherenkov radiation – whenever they exceed the speed limit of light in air. Think of it as the optical equivalent of a sonic boom produced when the sound barrier is breached.
VERITAS stands sentinel, observing these nanosecond flashes of light that signal the impact of a gamma ray. But the telescopes have the potential to do so much more.
Jamie Holder is an expat Brit working as an astrophysicist at the University of Delaware, and as a member of the VERITAS collaboration. Holder is acutely aware of the telescope’s capabilities. In the early 2000s, while working as a postdoc at the University of Leeds in the UK, he realized that instead of looking for flashes of Cherenkov radiation, telescopes like VERITAS could also be put to work searching for pulses of laser light beamed towards Earth as a deliberate signal from extraterrestrial life.
“I wrote a short conference paper about it, but I got busy with other things so I didn’t take it any further,” Holder tells Supercluster.
Then, in 2015, the phenomenon of the ‘alien megastructure star’ hit the headlines.
More appropriately known as KIC 8462852, or Boyajian’s Star after the astronomer Tabetha Boyajian who led the research effort into it, this bewildering star exhibits bizarre dips in light caused by objects passing in front of, or transiting, it. We now know these objects to be dense clumps of dust. But back in 2015, when the news of its discovery was just breaking, it was seriously considered as evidence of extraterrestrial technology.
This realization promptly dawned on Holder. Looking through VERITAS’ data archives on a hunch, he found that the telescope had collected ten hours worth of data on the star, after VERITAS had coincidentally been observing gamma rays from an active galaxy in the same field of view. Writing software designed to detect pulsing points of light rather than the column-shaped flashes of Cherenkov light, Holder put his new code to work on those ten hours of data. It turned up negative, but the idea that VERITAS could look for alien laser signals had been firmly planted.
LASERS IN THE LIMELIGHT
This April, the Search for Extraterrestrial Intelligence turns 60 years old, and throughout those six decades, searches with radio telescopes have dominated. A quick look at the history of SETI shows why. In 1960, when Frank Drake turned his radio telescope at Green Bank Observatory in West Virginia towards the stars Epsilon Eridani and Tau Ceti, radio was already a mature technology, whereas 250km to the south-east the laser was only just being invented at Columbia University by Charles Townes. Radio had a huge head start, and while Townes saw the potential of lasers very quickly, it took a few decades for that potential to be fully realized as a means of communication. As such, radio SETI still dominates, even in Yuri Milner’s $100 million mega-project, Breakthrough Listen.
Radio has its strong points. It can pass through the gas and dust of the interstellar medium, whereas optical light is gradually absorbed by that same gas and dust. Our modern-day, sophisticated detectors can listen to billions of narrowband radio channels simultaneously as radio telescopes scour the sky for that elusive signal. But radio also suffers from dispersion – an effect whereby longer wavelengths are slowed by interacting with electrons in the interstellar medium, so that signals arrive at their destination separated in wavelength, garbling the message. And far more information can be encoded into a laser beam than in a radio transmission of comparable energy expenditure.
So in the last couple of decades, led by pioneers such as Stuart Kingsley, Paul Horowitz and Dan Werthimer, the search for alien laser signals, known as optical SETI, has really begun to mature. So much so that Breakthrough Listen has taken notice.
Holder’s analysis of Boyajian’s Star using VERITAS data was published in the eminent Astrophysical Journal Letters. His paper prompted scientists working on Breakthrough Listen to contact Holder and his colleague, David Williams at the University of California, Santa Cruz.
“They were holding a workshop at NASA Ames, and since I was nearby, they invited me along, and that’s what launched our connection with Breakthrough Listen,” says Williams.
Breakthrough Listen’s goal is to observe a million stars in detail as part of SETI. Its collaboration with the VERITAS group was launched in the summer of 2019. It partly involves looking through the gamma-ray telescope’s data archive, which encompasses nearly 20 percent of the sky visible from Arizona, but it is also following up on about 500 stars listed on Breakthrough Listen’s catalogue of promising targets.
At the distance of Boyajian’s Star, over 1,400 light years away, a 500 terawatt pulsed laser would shine, in nanosecond bursts, far brighter than the star itself. Even brighter is the most powerful laser that we have here on Earth, which is the 10 petawatt pulsed laser housed at the Extreme Light Infrastructure for Nuclear Physics in Romania. Meanwhile, Chinese scientists are busy designing a 100 petawatt laser that could come into operation by 2023. Such lasers tend to be used in nuclear fusion experiments, or high-energy physics, but it’s feasible that technological aliens might point their lasers towards us as a means of communication.
“If you took the biggest laser on the Earth now, and fired it at a star 1,000 light years away, the pulses would shine a few thousand times brighter than our Sun would appear,” says Holder. Admittedly, the nanosecond bursts of laser light would be imperceivable to the unaided eye, but they’d be obvious to a sensitive photometer and fast electronics. “We already have the technology to send a detectable signal, so it’s not much of a stretch to think that someone else could too.”
It’s less of an issue these days, with modern radio telescope’s back-end hardware able to record billions of narrow radio channels synchronously, but in the early days of SETI only one or two channels could be observed at any one time. Given the huge range of possible wavelengths on which to listen, SETI astronomers picked a handful of ‘magic wavelengths’ at which they thought that aliens might choose to transmit. These included the 21cm line, typically emitted by neutral hydrogen gas, and the ‘water hole’, between wavelengths of 21cm and 18cm (the latter emitted by hydroxyl molecules – add a hydrogen atom to hydroxyl and you get a water molecule, the elixir of life as we know it).
Holder wonders whether the VERITAS experiment could hold a clue as to a magic wavelength for optical SETI.
“On Earth, we’ve been looking at Cherenkov radiation since about 1965, and any civilization that lives on a planet with a transparent atmosphere would likely do the same,” he says. His conjecture is that aliens might realize that we have telescopes designed to look for the blue flashes of Cherenkov radiation, and so might choose to beam their lasers at those wavelengths to increase their chances of being detected. The absorption by interstellar dust would be pretty severe at those wavelengths however, so it would only work for stars that at most are a few hundred light years apart.
Still, there may be other magic wavelengths to which the effects of dust absorption are more benign. Flying the flag for SETI in Japan is Dr Shin-ya Narusawa, of the Nishi-Harima Astronomical Observatory at the University of Hyogo.
Following the retirement of some of his colleagues, he believes he is currently the sole scientist in Japan conducting SETI research.
“I think Japanese astronomers regard SETI as a study that has no success,” he says, adding that unlike Milner’s millions in the United States, there’s a distinct lack of funding for SETI in Japan.
Still, that’s not stopped Narusawa. Writing in the journal New Astronomy, he highlights three narrow optical bands – at 393.8nm near the calcium-K line, 656.5nm near the hydrogen-alpha line, and 589.1nm, which is the wavelength of the neutral sodium doublet. Solar observers will be familiar with the first two lines, since they are narrowband wavelengths that the Sun can be observed in, and all three are commonly observed interstellar emission lines, and that’s the key: Narusawa has identified these optical wavelengths as being frequently observed by astronomers, and ET might also realize that and transmit at these wavelengths. By eschewing the photometric method of just counting photons, and instead pursuing alien lasers spectroscopically, communication at these wavelengths could be identified.
THE LIGHT BUCKETS
For now though, the brute-strength method of photometry by telescopic ‘light buckets’ – as Holder and Williams refer to VERITAS’s four giant 12m diameter telescopes – will have to do. Alas, nothing lasts forever. VERITAS’ replacement, the Cherenkov Telescope Array (CTA) that will be based in two locations, on La Palma in the Canary Islands and in Chile, will come online in the next few years, and VERITAS’ funding is set to run out in 2022. It’s fate isn’t cast in stone just yet – the CTA might not be ready in time, or VERITAS could be applied to different kinds of astronomy instead – but when it happens, Holder and Williams are already putting plans in place to continue SETI on the new observatory.
“CTA will be much more capable,” says Williams. For one thing, the La Palma site will feature 19 telescopes and the observatory in Chile will consist of 99 telescopes, with the largest spanning 23 meters in aperture.
“If the CTA sees a flash that appears as a point source in exactly the same location in each of the cameras of all these telescopes, then we’d be sure beyond sure that something was there,” says Holder. It might not necessarily be alien in nature – potentially natural astrophysical phenomena could mimic a regular pulsing signal, like pulsars do in radio wavelengths – but if a pattern could be detected in the pulses, then it would be a sure sign of artificiality.
There’s other optical SETI projects out there. Shelley Wright, of the University of California, San Diego, runs the NIROSETI project, which is an instrument on the Nickel Telescope at Lick Observatory that searches for laser pulses in near-infrared light, which can pass through much more interstellar dust than shorter, bluer wavelengths. Recent results from NIROSETI failed to find any laser pulses from 1,280 celestial objects, including individual stars, star clusters and even galaxies. Now Wright is spearheading the next great optical SETI project, called PANOSETI, which is a panoramic, all-sky instrument capable of detecting laser pulses across the optical and near-infrared spectrum. Already several telescopes have been installed at Lick Observatory as part of the project. Although PANOSETI won’t be able to see laser pulses as faint as VERITAS or the CTA, its ability to observe the entire sky synchronously will help make sure that we don’t miss anything, and that if a signal is coming from an unexpected direction, PANOSETI would see it.
“There’s different approaches in terms of the strengths and capabilities of the different instruments,” says Williams. “And it’s a field that’s really developing now.”
VERITAS and NIROSETI observations, and the upcoming experiments with the CTA and PANOSETI, mean that optical SETI is finally getting into gear — becoming an essential part in the quest to show we are not alone in the Universe.