In 2007, astronomer Duncan Lorimer was searching for pulsars in nine-year-old data when he found something he didn’t expect and couldn’t explain: a burst of radio waves appearing to come from outside our galaxy, lasting just 5 milliseconds but possessing as much energy as the sun releases in 30 days.
Pulsars, Lorimer’s original objects of affection, are strange enough. They’re as big as cities and as dense as an atom’s nucleus, and each time they spin around (which can be hundreds of times per second), they send a lighthouse-like beam of radio waves in our direction. But the single burst that Lorimer found was even weirder, and for years astronomers couldn’t even decide whether they thought it was real.
The burst belongs to a class of phenomena known as “fast radio transients” – objects and events that emit radio waves on ultra-short timescales. They could include stars’ flares, collisions between black holes, lightning on other planets, and RRATs – Rotating RAdio Transients, pulsars that only fire up when they feel like it. More speculatively, some scientists believe extraterrestrial civilizations could be flashing fast radio beacons into space.
Astronomers’ interest in fast radio transients is just beginning, as computers chop data into ever tinier pockets of time. Scientists call this kind of analysis “time domain astronomy.” Rather than focusing just on what wavelengths of light an object emits or how bright it is, time domain astronomy investigates how those properties change as the seconds, or milliseconds, tick by.
In non-time-domain astronomy, astronomers essentially leave the telescope’s shutter open for a while, as you would if you were using a camera at night. With such a long exposure, even if a radio burst is strong, it could easily disappear into the background. But with quick sampling – in essence, snapping picture after picture, like a space stop-motion film – it’s easier to see things that flash on and then disappear.
“The awareness of these short signals has long existed,” said Andrew Siemion, who searches the time domain for signs of extraterrestrial intelligence. “But it’s only the past decade or so that we’ve had the computational capacity to look for them.”
Siemion believes the field’s rapid growth will revolutionize astronomy as a whole, not just his own search for ET. “History has shown us that any time we sort of look into parameter space in astronomy — any time we develop a capability to look in a new way — we find something,” he said. “I think that our exploration of the time domain is just getting started. We’re going to find things we never expected.”
Lorimer’s discovery, which took his name, is one such unexpected find. The search for its mysterious origin initially energized the field. “Everyone was really jazzed,” said his research partner Maura McLaughlin, who first discovered RRATs. “We thought the Lorimer burst could be a new kind of source. Some theorists suggest that it was a primordial black hole evaporating or a ‘spark’ from a superconducting cosmic string.”
Exotic possibilities, to be sure. But some scientists didn’t think it was real. For a long time, Lorimer was the only one who’d found a seemingly extragalactic burst, and doubts crept in. “All we had were a few milliseconds of data from the late 1990s,” said McLaughlin. “I was even at a conference where somebody stood up and said, ‘How many people here believe the Lorimer burst? Raise your hands.’ But Dunc always believed in his bursts.”
And then, in 2010, another group saw something similar. These blasts, however, appeared to come from earthly lightning. Many in the community lumped the Lorimer burst in with these terrestrial ones, rejecting Lorimer’s evidence that his burst had traveled a long way before it arrived on our planet.
Finally, the Lorimer burst received validation in July of this year, when a team from the University of Manchester saw four bursts with similar properties. Their evidence strongly suggests the radio waves traveled from galaxies far, far away.
But another team, from the Harvard-Smithsonian Center for Astrophysics, suggested in December that the blasts were “mundane” (fighting words!) flares from stars in our own galaxy.
Thus, while it’s now very likely Lorimer bursts come from space, the arguments continue about where in space (after all, it’s a big place). The size of one “pixel” in a radio telescope’s image is much larger than a pixel in an optical image. It can contain thousands of nearby stars superimposed on hundreds of thousands of distant galaxies, and astronomers’ unenviable task is to match bursts with one of those many sources. That’s a particularly hard task when the signals pop up without warning and then disappear, like cosmic whack-a-moles.
ET Phones Fast
Time domain astronomy told us about Lorimer bursts, and someday, with any luck, it will tell us what they actually are. But even stranger stuff may await. “The Lorimer bursts are just a hint of the incredible diversity of phenomena that we’re likely to find when we look for fast transients,” said Siemion.
And that diversity might someday include an extraterrestrial organism.
Hunting for smart aliens – an endeavor known as the Search for Extraterrestrial Intelligence, or SETI – is not a new pursuit. But hunting for clap-on-clap-off messages is.
“SETI has historically focused on signals that are continuous in time but narrow in frequency,” said Siemion. The earthly equivalent is radio stations: they broadcast all the time but occupy just one spot on the FM dial.
However “in the last decade or so, there’s been a realization that something very narrow either in frequency or in time is interesting from a SETI perspective,” Siemion said. In other words, any radio signal that looks engineered and purposeful could actually be engineered and purposeful.
Observatories like the Allen Telescope Array, run by the SETI Institute, and the Low-Frequency Array for Radio Astronomy (LOFAR) in the Netherlands are planning to stare at wide swaths of sky, watching for the blips that could be from either ET or spinning, crashing objects in space. Other observatories, like the Very Large Array in New Mexico, are slapping new, transient-sensitive instruments on their antennas, letting the telescopes do time domain astronomy while they continue their regular work.
All types of time domain astronomers work together – and not just to ward off the loneliness that comes from studying things that are by definition elusive. The diverse groups benefit from partnering on engineering projects, improving the machinery and software to peer into the universe’s fast-shifting parts. “At some point, SETI scientists and pulsar scientists started working together to build our gadgets in a similar fashion,” said Siemion.
Both groups hope their collaboration will pay off, revealing the nature of cataclysmic bursts or, potentially, cosmic companions. No matter what, thinking of the cosmos as a place that changes every millisecond is sure to alter our understanding of it. “As soon as we come up with a way of looking at the universe in a different way, we see new phenomena,” said Siemion. “The best is yet to come.”
Image by Denise Coleman via Flickr