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31 March 2015 by
Sarah Scoles
Mysterious radio wave flashes from far outside the galaxy are
proving tough for astronomers to explain. Is it pulsars? A spy
satellite? Or an alien message?
BURSTS of radio waves flashing across the sky seem
to follow a mathematical pattern. If the pattern is real, either some
strange celestial physics is going on, or the bursts are artificial,
produced by human – or alien – technology.
Telescopes have been picking up so-called fast radio bursts
(FRBs) since 2001. They last just a few milliseconds and erupt with
about as much energy as the sun releases in a month. Ten have been
detected so far, most recently in 2014, when the Parkes Telescope in New
South Wales, Australia, caught a burst in action for the first time.
The others were found by sifting through data after the bursts had
arrived at Earth. No one knows what causes them, but the brevity of the
bursts means their source has to be small – hundreds of kilometres
across at most – so they can't be from ordinary stars. And they seem to
come from far outside the galaxy.
The weird part is that they all fit a pattern that doesn't match what we know about cosmic physics.
To calculate how far the bursts have come,
astronomers use a concept called the dispersion measure. Each burst
covers a range of radio frequencies, as if the whole FM band were
playing the same song. But electrons in space scatter and delay the
radiation, so that higher frequency waves make it across space faster
than lower frequency waves. The more space the signal crosses, the
bigger the difference, or dispersion measure, between the arrival time
of high and low frequencies – and the further the signal has travelled.
Michael Hippke of the Institute for Data
Analysis in Neukirchen-Vluyn, Germany, and John Learned at the
University of Hawaii in Manoa found that all 10 bursts' dispersion
measures are multiples of a single number: 187.5 .
This neat line-up, if taken at face value, would imply five sources for
the bursts all at regularly spaced distances from Earth, billions of light-years away.
A more likely explanation, Hippke and Lerned say, is that the FRBs all
come from somewhere much closer to home, from a group of objects within
the Milky Way that naturally emit shorter-frequency radio waves after
higher-frequency ones, with a delay that is a multiple of 187.5 (arxiv.org/abs/1503.05245).
They claim there is a 5 in 10,000
probability that the line-up is coincidence. "If the pattern is real,"
says Learned, "it is very, very hard to explain."
Cosmic objects might, by some natural but unknown process, produce dispersions in regular steps. Small, dense remnant stars called pulsars
are known to emit bursts of radio waves, though not in regular
arrangements or with as much power as FRBs. But maybe superdense stars
are mathematical oddities because of underlying physics we don't
understand.
It's also possible that the telescopes are picking up evidence of human technology, like an unmapped spy satellite, masquerading as signals from deep space.
The most tantalising possibility is that the source of the bursts might be a who, not a what.
If none of the natural explanations pan out, their paper concludes, "An
artificial source (human or non-human) must be considered."
"Beacon from extraterrestrials" has always been on the list of weird possible origins for these bursts.
"These have been intriguing as an engineered signal, or evidence of
extraterrestrial technology, since the first was discovered," says Jill
Tarter, former director of the SETI Institute in California. "I'm
intrigued. Stay tuned."
Astronomers have long speculated
that a mathematically clever message – broadcasts encoded with pi, or
flashes that count out prime numbers, as sent by aliens in the film Contact – could give away aliens' existence. Perhaps extraterrestrial civilisations are flagging us down with basic multiplication.
Power source
But a fast radio burst is definitely not the
easiest message aliens could send. As Maura McLaughlin of West Virginia
University, who was part of the first FRB discovery points out, it takes
a lot of energy to make a signal that spreads across lots of
frequencies, instead of just a narrow one like a radio station. And if
the bursts come from outside the galaxy, they would have to be
incredibly energetic to get this far.
If the bursts actually come from inside
the Milky Way, they need not be so energetic (just like a nearby
flashlight can light up the ground but a distant light does not). Either
way, though, it would require a lot of power. In fact, the aliens would
have to be from what SETI scientists call a Kardashev Type II civilisation .
But maybe there's no pattern at all, let alone one that aliens embedded.
There are only 10 bursts, and they fit into just five groups. "It's
very easy to find patterns when you have small-number statistics," says
McLaughlin. "On the other hand, I don't think you can argue with the
statistics, so it is odd."
The pattern might disappear as more FRBs
are detected. Hippke and Learned plan to check their finding against new
discoveries, and perhaps learn something about the universe. "Science
is the best game around," says Learned. "You don't know what the rules
are, or if you can win. This is science in action."
If the result holds up, says Hippke,
"there is something really interesting we need to understand. This will
either be new physics, like a new kind of pulsar, or, in the end, if we
can exclude everything else, an ET."
Hippke is cautious, but notes that remote
possibilities are still possibilities. "When you set out to search for
something new," he says, "you might find something unexpected."
This article appeared in print under the headline "Cosmic radio plays an alien tune"
