Breakthrough Discuss: SETI in the Optical

At the Yuri’s Night party on Yuri Milner’s Palo Alto estate, I found myself thinking about a novel, Allen Steele’s Arkwright. Like Breakthrough Starshot, initial work on the starship in the book is funded by a wealthy man who wants to see a human future among the stars. The propulsion method is a beam-driven sail, though at that point the comparisons get strained, for Steele is assuming a much larger craft and a mission of colonization rather than a flyby. Even so, there was enough similarity to make a book I had been reading on the airplane seem prescient.

The music on Yuri’s Night was loud, the gathering crowd relaxed, and I had imagined it would be an entirely social event, but after a time we were called in to watch a short video about Starshot and then participated in a question-and-answer session with some of the project’s principal players, including Milner himself. It was a good chance to hear some of the challenges the project faces examined, and tomorrow we’ll look at many of these. We also have to dig back into the gravitational focus mission and its role in all this.

But I kept thinking about Steele’s target planet, around Gliese 667C, and pondering interstellar distances. Alpha Centauri is 40,000,000,000,000 kilometers out, a whopping 271,000 AU. I seemed to be feeling the distance as an almost tangible thing, suddenly realizing that I was completely frazzled from the day’s sessions. I’m compulsive about note-taking (I would wind up with 52 single-spaced pages on my MacBook), and that feeling of distance fatigue was actually a very physical weariness. I made an early night of it (and thank you, Jill

Tarter and Jack Welch, for your kind offer of a ride back to the hotel). I slept that night like I haven’t slept in years.

Time to Target

That morning, in the opening session on optical SETI, I had been wondering whether key papers, like the Cocconi and Morrison paper that launched serious SETI study in 1959, or the Townes and Schwartz paper that brought optical SETI to our attention, had begun a gradual cultural widening of our horizons. If many people I talk to in everyday life still don’t have a grasp of just how far away even the nearest stars are, it’s still true that SETI has brought the topic of other civilizations to the fore, as reflected in countless science fiction novels and films.

And now that we’re thinking about an actual interstellar mission, it falls to us, as Jill Tarter had reminded the conference that morning, to think about the SETI consequences of pushing sails between planets and stars. Let me quote her on the point:

“A fiducial for optical seti has been a petawatt transmitter focused by a ten meter telescope. And even though we don’t have signs of that kind of technology in our future for interstellar communications, Breakthrough Starshot demonstrates there is reason to believe there may be transient sources of emission as byproducts of other plausible activities.”

Plausible activities include, as we have seen yesterday, a beaming infrastructure within a stellar system that could drive spacecraft on missions of exploration and supply. But we don’t even have to assume that another species is necessarily exploring the cosmos. For surely, like us, they would be aware of the danger of rogue asteroids or comets within their own system, and possibly using beamed lasers to deflect their trajectories. The lesson: If we can imagine it, so can someone else. And if they were to build it, we just may be able to see it.

Eliot Gillum’s talk on optical SETI reinforced these ideas. The director of optical SETI at the SETI Institute, Gillum noted that the older search paradigm had been to look at small patches of the sky for just a few minutes. The assumption here is a continuous or rapidly repeating signal, but beamed power calls upon us to change strategies. ‘All sky all the time’ is Gillum’s solution, given our understanding that beaming presents huge problems of geometry. Are we lined up by chance to see a beam, and would we be able to recognize it when we did?

The beamed SETI observable is a bright transient, and we have more than a few of these in the catalog already, though we don’t know what they are. But Gillum pointed to a program called AllSat as an indication of how we might proceed. It’s a highly automated program whose prototype is now under construction, using multiple cameras to observe fields of view of 120 degrees. AllSat is about signal detection, not detailed characterization, but its six primary observatories would be a full-sky search allowing follow-up at higher apertures.

Image: Equipment for an all-sky optical SETI survey, as shown on one of Eliot Gillum’s slides.

Fast, sporadic, bright targets we would miss with older optical SETI methods should become observable in a five-year survey of the entire sky. Harvard’s Paul Horowitz discussed new photon-counting detectors and showed a notional design of a telescope that could be built around them. Horowitz and team began searching for intense laser pulses in 1998, understanding that the beam produced by a high-intensity pulsed laser teamed with a moderate-sized telescope — something that could be built with current technology — would appear during its pulse a thousand times brighter than the Sun.

But it was Philip Lubin (UC-Santa Barbara) who made the most explicit link between optical SETI and what Breakthrough Starshot is doing. Lubin told me during a break that the genesis of the current beamed sail concept as conceived by Starshot grew out of his recent “Roadmap to Interstellar Flight,” a lengthy paper submitted to the Journal of the British Interplanetary Society and available here. I should also mention in relation to SETI that a new paper, “The Search for Directed Intelligence” has just become available on the arXiv site.

The space for life to flourish in the universe is all but boundless, Lubin said, showing a one square degree image from the Hubble Ultra Deep Field and noting that 1018 planets could be in the view.

“We can stare at one square area of the sky and be looking at trillions of planets all the way out to a modest redshift,” Lubin said. “As to detection probability, consider that we are about to enable a class 4 civilization with Starshot. The beam — a phased array beam small enough to hit individual stars — would be detectable out to enormous distances with a modest telescope… Phased array telescopes grow out of technology like Starshot. We can take a picture, look along the line of sight, and know that vast numbers of planets are in that field of view.”

I should mention that Lubin’s civilization classification is not Kardashev’s; he’s drawing on a scale he discusses in his most recent paper, one in which Kardashev Level 1 is a Class 11. But whatever the classification, it’s clear that a beamed laser infrastructure raises questions about our own planet’s visibility, which invariably opens the METI (Messaging to Extraterrestrial Intelligence) debate. It was not a subject the conference spent a lot of time on, though it was clear from remarks I heard that opinion was sharply divided between those who found restrictions on METI pointless and those who called for consensus among a variety of disciplines before actively trying to raise the visibility level of our own civilization.

Image: Looking into the Hubble Ultra Deep Field. Credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI).

Extracting a SETI Signal from our Data

Amy Reines (NOAO) would follow Lubin with an analysis of narrowband SETI searching using radial velocity data. In radial velocity work, we are looking for the tiny Doppler shift an orbiting planet can produce in a star’s spectrum. A laser beam would need to outshine the star’s stellar photon noise, and Reines calculated that most lasers on Earth are already approaching a value that would allow such detections. This assumes a continuous laser beam rather than a pulse, the latter demanding higher power, and it must be directed at us to be detected.

Reines’ computer code could search spectra for possible laser emission lines, comparing a reference spectrum for each star to all other spectra of the star while eliminating random noise fluctuations. Most candidates for a laser turned out to be cosmic rays that hit the detector. One possible hit emerged, a spectral feature seen in three observations of the same star at the same wavelength, but the excitement was short-lived, as the ‘hit’ turned out to be an artifact in the CCD detector. But the methods can be applied to many optical SETI data mining studies.

We can also extend optical pulse SETI into the near-infrared, as Shelley Wright (UC-San Diego) explained, citing new near-infrared photomultiplier tubes (PMT) and avalanche photodiodes (APDs) — these exploit the photoelectric effect to convert light into electricity. We are moving, she said, to higher gain, lower noise and a larger dynamic range in the available instrumentation. The NIROSETI (Near-infrared Optical SETI) instrument built by Wright’s team at Lick Observatory exploits infrared’s advantages in the first near-infrared SETI experiment.

Of course, we might consider not sending or receiving optical or radio data, if Chris Rose is right. Arguing that we should ‘write, not radiate’ our data, Rose (Brown University) wrote a paper with Gregory Wright in 2004 that calculated the energy needed to communicate a given payload of bits. Maybe we should consider sending an artifact packed with information rather than an electromagnetic signal of any kind. If you’ve been reading Centauri Dreams for a long time, you’ll recognize the argument I wrote about in ET in a Grain of Sand.

Rose believes that it can be many orders of magnitude better (from the perspective of energy use) to write a message onto a medium and actually deliver it physically to the recipient, than to send it by radio or laser signal. Clearly he’s not talking about ongoing communications but archival purposes, such as a civilization wanting to send out a record of its great works (here the Voyager golden records come to mind). And he makes the case that for these latter purposes, a truck — or spacecraft — filled with storage media is a very reliable high bit-rate channel.

Of course, it has latency issues — it takes, on the interstellar scale, a long time to get where it’s going. It was a lively presentation punctuated by the question of how we might detect such ‘packages’ of data if they have already been sent to our system from another star. An even bigger question is what Rose described as ‘incursion or evolution.’ Are we ourselves the product of another civilization’s outreach, a ‘seeding’ of the cosmos in our stellar neighborhood? For that matter, should we consider seeding the universe ourselves as well as communicating?

Image: A panel discussion on SETI followed the talks. From left: Jill Tarter, Jim Benford, James Cordes (Cornell), Andrew Howard (University of Hawaii at Manoa), Andrew Siemion (UC-Berkeley), and Dan Werthimer (UC-Berkeley).

Tomorrow I want to draw on the Yuri’s Night panel on Breakthrough Starshot as well as Pete Worden’s talk on the second day of the conference. We need to run through some of the bigger Starshot challenges, and also discuss how the project might evolve over time. I had hoped to get into the gravitational lens dimension of this initiative, but rather than skimping on the presentation, I think I’ll target that for a concluding Breakthrough Discuss post on Monday.