STAND by a payphone and wait for it to ring: that just about sums up most of our attempts to search for ET. Since the late 1950s, the prevailing logic has been that aliens might be beaming radio signals into space, so we need only tune to the right frequency to listen in.
But our attempts to date have been underfunded, covering just a few thousand stars in a galaxy of hundreds of billions. Little wonder, then, that we haven’t heard a peep.
Advertisement
The latest round starts this month, courtesy of a 10-year, $100 million project called Breakthrough Listen. Funded by tech entrepreneur Yuri Milner, it will set two of the world’s largest radio telescopes surveying the million closest stars across a broader swathe of the radio spectrum, and will cover 10 times as much sky as all previous searches combined.
Yet for all its promise, this is still mid-20th century thinking, as its architects readily admit. “In the 1950s, when the SETI field started, radio waves were the best technology we could imagine aliens using,” says Avi Loeb of Harvard University, one of Breakthrough Listen’s research leaders. But just like steam power on Earth, communicating with electromagnetic radiation could prove no more than a passing craze for a distant civilisation. Considering that extraterrestrial societies might be millions of years old, our radio search may be laughably passé.
“There are other signals we can look for,” says Loeb. “It all depends on our imagination.” From scrutinising the way aliens breathe to detecting any vast feats of cosmic engineering they might have achieved, exciting new strategies are opening up thanks to our growing ability to scan the sky and crunch data. “We’re able to cast a much wider net than before,” says , an astrophysicist at Penn State University.
And the net will need to be wide, considering that the Milky Way is probably home to at least a billion habitable, Earth-like planets, and that the universe teems with some 200 billion galaxies. Given all this, the odds that we are its only self-aware entities seem infinitesimally small.
This profusion, however, means we have little idea what to expect. “We are mostly forced to look for life as we know it,” says of Princeton University. That means we have assumed that aliens, like ourselves, will imprint themselves on their environment in increasingly profound ways as their civilisations develop.
Although humanity has so far only taken baby steps out into the solar system, our footprints could eventually grow conspicuous in the cosmic landscape. By the same token, even the most standoffish alien culture might give away telltale signs of its existence, visible to a new generation of SETI searches.
Over the following pages, we examine what signs would allow us to finally answer one of humanity’s most profound questions: is there anybody out there?
1|The air up there
You can dress it up how you like, but all life forms we know of are essentially machines for self-replicating, converting fuel into waste as they do so. Over time, the biochemistry of countless billions of individual creatures can transform a world in remarkable ways.
On Earth, for example, virtually all animals inhale oxygen and emit carbon dioxide, while conveniently plants spend much of their time doing the opposite. Certain bacteria, for their part, churn out methane and ammonia. All told, life here produces a cocktail of gases – the distinctive biosignature of our planet.
“With existing telescopes, the lights of Tokyo would be visible from Pluto”
When NASA’s James Webb Space Telescope launches in 2018, its deep scans of exoplanet atmospheres might give us our first tantalising glimpses of biosignatures from other worlds. The telescope will study planets that transit, or cross, the faces of their host stars. During these minuscule eclipses, background starlight passes through the halo of the planet’s atmosphere. It can then be picked apart, trillions of kilometres away, to uncover the “fingerprints” of molecules – a concept hardly conceivable when SETI took shape in the 1950s.
With luck, hardy biosignatures similar to our own will jump right out. In reality, though, exotic, un-Earthly geological phenomena coupled with novel alien biochemistries could muddy the signal and frustrate our efforts to decode it.
2|Poison gas
Even if an exoplanet’s atmosphere does bear clear signs of life, that won’t tell us whether it is inhabitated by mindless green slime or sentient city-builders. A more reliable approach to finding kindred spirits is to look for technosignatures – chemicals that only brainy ETs can produce.
Assuming that these intelligent aliens have a basic understanding of chemistry and that – for a while at least – they shared our affinity for generating large-scale pollution, Harvard astronomer Avi Loeb has suggested we target gases such as chlorofluorocarbons (CFCs). Once used in air conditioners and aerosol cans, these nasty, ozone-depleting agents are now being phased out. Yet some can persist in the atmosphere for tens of thousands of years, possibly offering an extended detection window.
For the James Webb Space Telescope to see a CFC signature, levels in an exo-atmosphere would need to be at least 10 times those on Earth. That might be a step too far even for the least environmentally conscious species. Then again, the denizens of a cold planet might want to load their air with CFCs, using their potent greenhouse effect to trap heat. “Aliens might surround a planet with a blanket of atmospheric pollution to warm it up enough for life,” says Loeb. That really would be a telltale signature, says astrobiologist and geochemist at NASA’s Goddard Space Flight Center.
3|City lights
An advanced planetary civilisation with night vision as limited as ours might festoon its built environment with artificial lighting. Could we detect alien cities shining on their planet’s dark side?
“Tokyo can be seen all the way to edge of the solar system, roughly where Pluto is, with existing telescopes,” says Avi Loeb of Harvard University, who has studied the challenge in collaboration with Edwin Turner of Princeton University. However, seeing city lights on planets around even our nearest stars would require a space telescope with a mirror 200 metres across. That’s around 40 times the size of the one to be fitted in the James Webb Space Telescope, and is unlikely to be on the cards until next century.
But maybe Tokyo is small beer next to some alien cities. “If you have something like [the planet-wide cityscape of] Coruscant from Star Wars, then you might have enough light for a detection,” says NASA’s Shawn Domagal-Goldman, and with a space telescope we could hope to build in the near future.
Of course, this assumes alien lighting has a distinctive spectrum, like that from our fluorescent bulbs and LEDs. If ETs choose illumination with the same spectrum as their star, it would be tough to pick out. Our attempts might also be thwarted by humongous blooms of bioluminescent exo-algae, which could mimic gleaming cityscapes.
Like radio communications, cities with artificial lighting might only be a passing phase. We’d have to get very lucky to catch an alien species at just the right moment.
4|Aliens on the go
Before too long, our future selves might be setting up shop elsewhere in the solar system for commercial purposes – hello, asteroid mining – or to establish outposts. We might even embark on interstellar missions of exploration.
Alien civilisations have probably already beaten us to the punch. Setting aside physics-bending hyperdrives and wormholes – whose signals would be hard to predict anyway – what exotic forms of transport should we be looking out for?
If our own cutting-edge technology is any guide, spacefaring aliens might be travelling in modified sailboats, using pressure generated by incoming light. So far, our efforts in this direction have been puny, but scaling up to a football-pitch-sized sail to reap the winds of high-powered lasers or radio beams is feasible. And the leakage of such bright light should be easy to spot with today’s telescopes. The Breakthrough Listen project (see “Life signs“) will be complementing its search for alien radio signals with the biggest scan yet for optical lasers being beamed in our direction, indirectly helping us to spot aliens surfing the cosmos.
Less promisingly for us, aliens might have got as far as building craft powered by nuclear fission and fusion. Their minimal light output would be hard to pick up, unless the star-trekking ETs happened to be right under our noses. “There might be loads of spacecraft cruising our galaxy,” says Loeb, “but it’s possible these signals are too weak to detect right now.”
A better bet would be aliens capable of exploiting physics as we know it to its limits. They might be using a matter-antimatter annihilating drive, which gives the most bang for your buck in terms of fuel conversion.
Building one seems technically daunting, given that antimatter is the most volatile stuff imaginable in a universe of matter, but a few centuries of innovation might be all that stands in the way of its realisation. Such a vessel would spew intense plumes of light, which Robert Zubrin of the R&D firm Pioneer Astronautics in Lakewood, Colorado, thinks would be detectable hundreds of light years away with an instrument like the Hubble Space Telescope.
Alas, Hubble has seen nothing of the sort. But tomorrow’s gargantuan telescopes might reveal an interstellar rush hour to which we’ve hitherto been blind.
5|Building big
Last October, the internet was suddenly awash with rumours that an alien civilisation had been spotted. The Kepler space telescope had seen the light output of a star called KIC 8462 fluctuating by as much as 22 per cent. Normally this would be put down to the eclipsing effect of an exoplanet, but there was a snag. By comparison, Jupiter would block only 1 per cent of our sun’s light, and as KIC 8462 is more than half as big again as our sun, whatever was obscuring it had to be colossal. Had we stumbled upon a vast extraterrestrial construction project?
That idea would have sat nicely with what’s known as Dysonian SETI, after the physicist and engineer Freeman Dyson, who laid its foundations in 1960. He suggested we look for evidence of alien macro-engineering: just as the pyramids of Egypt have outlasted their creators, megastructures in space would offer ET-hunters an enduring target.
A classic example is the Dyson sphere, solar-energy collectors encircling a star in a discrete swarm or rigid shell. It would harvest a star’s energy, ideal for projects such as supercomputers powerful enough to model the entire past and future of the universe.
A complete sphere would eclipse its star entirely, but we could still hope to spy waste heat from its construction, or perhaps the funky flickerings of a work in progress, which might look to us like large-scale exoplanet transits.
“Ruins of advanced civilisations may litter the cosmos”
So is this what we saw last October? “It’s the most interesting SETI target to date,” says Jason Wright of Penn State University, “but that doesn’t mean I think aliens are a likely possibility.” A far more likely explanation is a flotilla of broken-up comets, coupled with a star whose rapid spinning makes it wider, cooler, and thus darker at its equator than its poles. That would also explain why listening for narrow radio emissions and laser pulses from KIC 8462 has so far picked up nothing.
6|Feeling the heat
We may think of ourselves as advanced. But a ranking system devised by Soviet astronomer Nikolai Kardashev puts humanity well and truly in its place. In the 1960s, he proposed to rate civilisations based on the energy they were capable of harnessing. We now think of one capable of extracting all the available energy from its home planet as having attained K1 status. A civilisation milking a tame star with a Dyson sphere–like megastructure (see “Building big”, left) would be a K2, and one able to hoover up all the energy in its home galaxy is a godlike K3. On the Kardashev scale, we humans come in at a lowly K0.73.
As impressive as K2 and K3 might be, basic physics suggests their schemes would still cast off prodigious waste heat, probably in the form of infrared light. The most ambitious search for this to date, the Glimpsing Heat from Alien Technologies (G-HAT) survey, has been hunting for potential K3s using NASA’s Wide-field Infrared Survey Explorer (WISE). It was able to whittle down nearly 100 million galaxies to 100,000 infrared-rich eyebrow-raisers. However, further analysis noted that this extra infrared was entirely consistent with vigorous star formation in particularly dusty galaxies.
Even so, sub-K3 demigods could be lurking unseen. “A galaxy would have to be filled with an alien civilisation that was outshining its stars in infrared light for us to have noticed it,” says Jason Wright at Penn State University, who conceived G-HAT. “There’s a lot of lesser civilisations that would have escaped our notice so far.”
Alongside star-enveloping solar farms, ET could go in for big science. Alien particle physicists, if any there be, might laugh at our feted Large Hadron Collider (LHC), capable of colllision energies of 13 teraelectronvolts. The real meat of physics occurs at scales millions of billions times higher than this, energies so elevated that they may at last persuade gravity to unite with the other fundamental forces of nature. A particle accelerator to probe this realm would be unmissable: literally the size of a galaxy, giving off copious waste heat and radiation.
The most likely particles to be streaming away from these XXLHCs would be neutrinos, tiny bits of nuclear shrapnel that barely interact with normal matter. Yet these neutrinos would be so vanishingly rare in the total cosmic census that today’s detectors, like the cubic-kilometre IceCube in Antarctica, wouldn’t have a prayer of pinning one down.
Instead, we’d need a detection network of billions of sensors arrayed in Earth’s oceans. “You need to be looking across the entire planet to be able to detect even a single one of these neutrinos,” said Brian Lacki, who proposed this strategy while at the Institute of Advanced Study at Princeton University. An investment on that scale might be slightly beyond the scope of any one billionaire philanthropist.
7|The Ozymandias Effect
“Look on my works, ye mighty, and despair!” The poet Shelley’s reflection on the shattered remnants of vanquished civilisations may have resonance in space too. The ruins of advanced civilisations doomed by their own technology may litter the cosmos – a disturbing prospect, given the narrow path we seem to walk between the spectres of climate change and nuclear annihilation.
Astronomer and colleagues at the University of St Andrews, UK, have dug into several such “exopocalypse” scenarios. “We thought a lot about the ways we might destroy ourselves,” Forgan says.
Exploding nuclear bombs would release flashes of gamma rays, rendering the atmosphere opaque with the irradiated dust of a nuclear winter. But from an observability standpoint the flashes would be fleeting, and the dust more easily attributable to an asteroid strike. Total planetary annihilation would similarly be hard to chalk up unambiguously to a dastardly alien intelligence, the blast of light and lingering smithereens being more likely to indicate a collision with another planet.
But we might be in luck, so to speak, if a hapless alien civilisation fell victim to some biological agent gone rogue. Our telescopes might then register “necrosignatures” – visible as spikes in compounds such as ethane and methane – released by armies of bacteria decomposing billions of corpses. The spikes might be short-lived, but if we saw them we’d know something had gone very, very wrong.
Another apocalyptic possibility involves self-replicating, autonomous nanomachines. Designed to make endless copies of themselves, nanobots could accidentally convert all of a planet’s resources into a sort of nano-sand, says Forgan. Colloquially known as the “grey goo scenario”, it might be detected through the distinctive shadows that grains of a uniform size cast on a planet’s surface.
Forgan’s team also considered the scenario of a CFC-clogged atmosphere, but reasoned this might not be a sign of civilisational suicide. ET might instead be warmly nestled on a once-frozen planet, now made liveable by the fire of intelligence.
(Images: Detlev van Ravenswaay/SPL, Colin Monteath/Hedgehog House/Getty, NASA/SPL, NASA, NASA/JPL-Caltech, AWFranco Brambilla)
This article appeared in print under the headline “Life signs”
