IT WAS Jerry Ehman鈥檚 turn to check on the 鈥淏ig Ear鈥� radio telescope at Ohio
State Radio Observatory on 15 August 1977. The 79-metre dish had been involved
in the search for extraterrestrial intelligence since 1973. Its receiver was
tuned close to the 1420-megahertz frequency at which the hydrogen atoms of
interstellar space broadcast radio waves like miniature radio stations.
The 1970s were the Stone Age for computers鈥攖he one at the Ohio State
Radio Observatory had a paltry 32K of RAM and a megabyte hard disc. This meant
that the fluctuating voltages picked up by the telescope could not be stored
electronically but had to be printed out and examined by the SETI volunteers at
the observatory every few days. It was this task that fell to Ehman, a professor
at Ohio State University, that day in August.
When Ehman worked his way through the reams of paper spewed out by the
observatory鈥檚 printer, he got the shock of his life. At one point, the voltages
had jumped clear through the roof. A powerful radio signal had been intercepted
by the telescope. It had come from the direction of Sagittarius and had lasted
37 seconds. Ehman scrawled 鈥淲ow!鈥� in the margin of the print-out. To this day,
the anomalous burst of radio noise picked up by the Ohio State Big Ear is
universally remembered as the 鈥淲ow!鈥� signal. Its source remains the $64
000 question.
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All that Ehman and John Krauss, the director of the observatory, were able to
find out was that it did not come from the Earth or from an artificial
satellite. 鈥淭here were insufficient data, so any questions about the source of
the emission must remain open,鈥� says Ehman, who has only just returned to the
fold as a SETI volunteer at Big Ear after two decades away teaching mathematics.
Arthur C. Clarke, the science-fiction writer agrees. 鈥淕od knows what it was,鈥� he
says.
Others are more willing to entertain the idea that the 鈥淲ow!鈥� signal came
from an advanced extraterrestrial civilisation. That it was an 鈥渦nmodulated鈥�
blast of radio noise, containing no message, is not seen as a problem. 鈥淓ach
channel of the receiver spanned 10 kilohertz,鈥� says Paul Shuch of the SETI
League, an organisation spearheading the amateur search for ET signals. 鈥淭hat
could easily have encompassed a carrier wave plus its modulation sidebands. We鈥檇
never be able to distinguish between the two.鈥�
According to Shuch, the nearest star in the direction that the Big Ear was
pointing is 220 light years away. He calculates that if the transmitting dish
was as big as the world鈥檚 biggest radio telescope鈥攖he 300-metre Arecibo
dish in Puerto Rico鈥攖he 鈥淲ow!鈥� signal would have required a 2.2 gigawatt
transmitter. 鈥淭hat鈥檚 daunting,鈥� he says, 鈥渂ut not altogether impossible.鈥�
Another possibility which is now receiving serious attention in the SETI
community is gravitational lensing. 鈥淎 much weaker signal, much farther away
could have produced the `Wow!鈥� result if the gravity of an intervening star
focused the signal our way,鈥� says Shuch.
Since 1977, astronomers at several observatories have performed more than 100
studies of the same region of sky. But the signal has never repeated. This is no
surprise. A telescope like the Big Ear sees only about a millionth of the sky at
a time and a similar dish on a similar planet would transmit into only a
millionth of the sky. So the chances of an alignment between the transmitting
and receiving 鈥渂eams鈥� is one in a million million. 鈥淔or this reason, we do not
expect to encounter another `Wow!鈥� signal,鈥� says Shuch. 鈥淗owever, there may be
countless other signals, similarly strong and intermittent, falling on our heads
this very moment.鈥�
If Shuch is right, then the omens for amateur SETI are good. Thanks to the
revolution in satellite TV, the kind of sensitive equipment that the
professionals were using 20 years ago is now available off-the-shelf. 鈥淚f the
`Wow!鈥� signal is typical, then amateurs armed with satellite dishes and home
computers will easily be able to pick up any ET signal,鈥� says Shuch
(鈥淭he alien spotters鈥�, 快猫短视频, 19 April, p 28).
Shuch remains upbeat about the 鈥淲ow!鈥� signal. 鈥淎fter 20 years of follow-on
analysis, we are left with two possibilities,鈥� he says. 鈥淓ither the `Wow!鈥�
signal was intercepted radiation from another civilisation, or it鈥檚 a previously
undiscovered astrophysical phenomenon. Either possibility is mind-boggling.鈥�