In celebrating the half century of SETI, we should not forget that the laser, too, is fifty years old this year. It is somewhat fitting that the technology of lasers has since converged with the search for extraterrestrial intelligence. Fifteen years ago radio was considered far and away the best medium by which to pick up ET, but today the odds are considered even that the green-skinned ones are broadcasting not with radio waves, but on super-rapid laser pulses that shine brighter than any star.

Although lasers have been a trope of science fiction for many a year, the idea of using lasers for interstellar communication is not wishful thinking. No less than Charles Townes – Nobel Laureate and one of the pivotal scientists behind the invention of the laser and its microwave equivalent, the maser – was the first to suggest that SETI astronomers should be searching for the light of alien lasers, a project that today we call ‘optical SETI’, or OSETI for short. According to Dr Dan Werthimer of the University of California, Berkeley, Townes was way ahead of his time.

“It was Charlie Townes who first suggested optical SETI, very soon after he invented the laser,” recalls Werthimer. “His office is right next to mine and we had been looking for radio waves for 35 years, and he would keep saying, ‘Dan, look at these lasers’.”

A few weeks after Townes and Arthur Schawlow published a revolutionary paper on lasers, Theodore Maiman constructed the first such device called the Ruby laser, which was no more powerful than modern day laser pointers that operate at a few milliwatts. Fine for playing your CDs, but not much use for communicating to the stars.

“We didn’t see what Charlie saw, which was that lasers could be very powerful,” continues Werthimer. “Now of course we have lasers that outshine the Sun, but only for a nanosecond. When lasers got bigger that made us rethink Charlie’s arguments.”

The efficiency of a laser is far greater than a radio signal; with lasers it is possible to focus all your power into a much narrower beam than radio waves, concentrating your efforts onto a smaller patch of sky. Modest efforts to search for alien laser beams had been conducted by British amateur Stuart Kingsley, who was living in the United States at the time, but it was not until 19 October 1998 that the first major professional OSETI searches began in earnest at both the Harvard–Smithsonian Center for Astrophysics, under the watchful eye of Paul Horowitz, and at Berkeley by Werthimer’s team. Today, there are several OSETI searches, including one at Lick Observatory, another at UC Berkeley’s Leuschner Observatory and an all-sky survey that has been developed by the Harvard team. What kind of laser beam, exactly, are they looking for?

Laser logistics
Consider the world’s most powerful laser, housed in the National Ignition Facility at the Lawrence Livermore National Laboratory in California. It is capable of producing laser blasts with an energy totalling a petawatt (10¹⁵, or a quadrillion, watts) and more advanced lasers planned for the next decade will approach an exawatt (10¹⁸ watts). Such powerful bursts can only be achieved in short pulses, but for a beacon signalling thousands if not millions of worlds in a regular cycle, short pulses are all that are needed. Modern photon-counting detectors (photomultipliers) are more than capable of detecting nanosecond flashes.

But laser beams are pretty narrow; would it not be impossible for ET to target a laser so that it lands directly on our telescope? Not on your nelly. Lasers disperse. Even shining a laser pointer at a wall ten metres away shows this – the spot of laser light is slightly wider than the lens of the laser because the laser is not perfectly collimated when it is emitted. Travelling over hundreds or thousands of light years, a laser beam could easily be made to widen so that it encapsulates a target ten astronomical units across – in our Solar System that would include everything within the orbit of Jupiter. That probably gives the sender an astronomical unit or two leeway (an astronomical unit is the distance between Earth and the Sun, 149.6 million kilometres). At a range of 1,000 light years it corresponds to an accuracy of 3 milliarcseconds, and if they were closer, say 100 light years, they would only need 0.03 arcsecond accuracy. Both are doable with the technology we have here on Earth today. Plus, the aliens would have to compensate for the time delay in the pulse reaching its destination, for in that time the target’s proper motion would have moved it through space, so the pulse would require to be aimed at where its target solar system is going to be, rather than where it is when the pulse sets out. At 1,000 light years this might amount to 15 microarcseconds per year. If the aliens are unsure about their targeting accuracy, they can widen the beam, but that would result in a decrease in power, resulting in a trade-off that depends upon how confident an ET civilisation is in their targeting ability. If extraterrestrials really are thousands or millions of years more advanced than us, it should be a piece of cake for them.

Detection, even with such a widely dispersed beam, should be equally straightforward for us. A ten-metre telescope looking at a Sun-like star would collect ten million optical light photons per second; broken down into even smaller incremental units, that’s one photon per 100 nanoseconds. Then suppose that an extraterrestrial civilisation on a planet orbiting that star had fired a laser pulse towards us. Suddenly, the photomultipliers attached to the telescope would experience an influx of perhaps ten photons per nanosecond, equivalent to just one photon per square metre – a statistically unlikely phenomenon. An intensity of one photon per square metre spread across ten astronomical units would require 1.5 x 10²⁴ photons in total, with a total pulse energy of 3 x 10⁵ joules, and 2 x 10^-19 joules per photon. A one-metre telescope would need a slightly more intense beam, 2 x 10^-18 joules per photon. So long as we are looking in the right direction, detection should be dead simple.

One pulse, however, tells us nothing. In December 2007 SETI scientist Dr Ragbir Bhathal of the University of Western Sydney detected a spurious pulse from a distant star using the 0.4-metre telescope that he operates. The trouble is, it flagrantly ignored the golden rule of SETI – for a signal to be real, it has to be seen to repeat. As such, Bhathal doesn’t know if it was a real signal, an astronomical phenomena, or just a glitch in his telescopic hardware.

So, how often may we expect to see a pulse to confirm that it’s real? This will depend on the power available to the aliens and how often they can re-aim. Popular (and, in the eyes of some, optimistic) estimates of the number of civilisations sharing the Galaxy with us range from 10,000 to a million, and with around 250 billion stars in the Milky Way that means ET will have to sweep across, on average, a couple of million stars just to find one civilisation such as ourselves. Assuming they attribute 10,000 joules per pulse, and are willing to spend one gigawatt in total every second, they can hit on the order of 100,000 stars per second with separate beams. So even if they wanted to reach half the stars in the Galaxy they could do so in less than an hour, and perhaps they’ll need a day to re-aim. So realistically a pulsed laser beacon would a daily occurrence, which suggests that whatever Bhathal’s pulse was, it wasn’t from ET.

It’s easy to be tricked. Radioactive decay in the photomultipliers and cosmic ray flashes have the potential to mimic ET’s hailing signals. During the first 27 months of the original Harvard survey 191 events were detected from 160 stars, all of which turned out to be nothing but interference in the detection equipment. That’s why any good OSETI system will make use of a beamsplitter to direct the incoming light to several photomultipliers, and only events detected by all the photomultipliers will deserve investigation. Any ‘dark current’ seen in just one or two can be written off as radioactive decay or the like.

Content management
A lot of information can be contained within a series of pulses. On Earth we use pulsed communication where data is encoded into either the number of pulses or their timing. Alternatively, the pulse could literally be a lighthouse, merely a signal that something is out there, and that by tuning in at other wavelengths we would pick up the real, lower power, message behind it. Conversely, a continuous laser beam could transmit a huge amount of information. “You could send the whole library of Congress in a minute or two,” says Werthimer (see accompanying table).

Another tricky issue is what wavelengths of light to focus on. For half a century radio SETI has revolved around the ‘water hole’ of hydrogen emission at 1420MHz. Is there a water hole equivalent for OSETI? Sadly not. Some people have suggested conducting spectroscopic surveys of unique absorption lines, like the calcium H or K lines, or iron lines, while Townes has pointed to infrared wavelengths. We don’t currently have the resources to conduct multi-wavelength optical surveys, but as our technology improves so do our prospects. The SETI Institute’s Seth Shostak often speaks of SETI following Moore’s Law of computer processing power, doubling in capability every 18–24 months. Laser power and pulse energy have grown just as dramatically over the last decade, doubling approximately every two years.

“What we’d really like to do is go deeper into the infrared,” says Werthimer, echoing Townes’ idea. This is because infrared light can pass straight through the obscuring dust that runs through the Galaxy’s spiral arms and would otherwise block visible light lasers. “If you want to look for signals from civilisations [in the plane of the Galaxy] then infrared is a good wavelength and we’re not doing that right now. We’d like to but that would take some money and that’s hard to come by.”

So the eternal battle for funding in SETI continues, and while it may slow the pace of the search, it never stops it completely. It may be still a long way short of the capabilities and resources of radio SETI, but OSETI is a technology still in its infancy. Perhaps we’ve got this SETI lark all wrong; rather than listening to the stars, maybe we should be looking to the stars instead to light the way.

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Stay tuned throughout April for more SETI articles.