Wasn't it Joseph Lazio who wrote:

"DH" == Dale Hurliman <hurliman@no.spam.please.sunlink.net> writes:

 Even if there were a natural source of primes, it would take an
intelligence to build a device for transmiting the output of that
source.

DH> Think pulsars. A natural source, a rotating neutron star,
DH> transmits very precise timing signals.  Nature is sometimes
DH> subtle.

Yes and no.  While the first pulsars were considered as possible alien
beacons, it was recognized quickly that they were slowing down.  One
of the ways that we can time pulsars so accurately is because we can
take into account the slowing of their rotation.  Any pulsar can be
observed to be slowing down and recognized as natural.

It wasn't the slowing down of the pulsars that wiped out Hewish and
Bell's idea that the pulsar signals might be alien beacons. They were
completely convinced that the signals were natural when they made the
announcement of the discovery of pulsars (Feb 1968). Later that year,
other teams discovered the fast pulsar in the Crab nebula and noticed
that it was slowing down. The first pulsars discovered by Bell were
slowing down too gradually for their equipment to be able to measure it.

The following considerations led to Bell dismissing the Little Green Men
hypothesis. None of them are showstoppers on their own, but together she
reckoned that they made it very unlikely that they were artificial.

* The signals were around a frequency that was already cluttered with
lots of natural signals. That would make it a bad choice for an
artificial interstellar transmission. 

* Pilkington managed to measure the approximate distance to the first
pulsar. Because the interstellar medium isn't a perfect vacuum, there's
a tiny dispersion effect. The radio waves travel slightly slower than c
(the speed of light in a perfect vacuum) and the lower frequency radio
waves travel slightly slower than the higher frequency waves. Pilkington
was able to measure how much later the pulses arrived when observed with
lower frequencies, and determined the pulsar to be about a thousand
light years away. That meant that the source had to be considerably more
powerful than would seem practical for a broad band artificial source.

* There was no Doppler effect that would have been expected from a
source transmitting from the surface of a rotating planet or in orbit
around one.

* A second pulsar was found transmitting on the same unlikely frequency
from a completely different part of the sky. 

Another consideration that came slightly later, but before the discovery
of pulsar deceleration, was the fact that one of the non-artificial
pulsar theories predicted that one might expect to find pulsars in the
centres of supernova remnants. That's exactly where the Vela pulsar, and
later the decelerating Crab pulsar, were found.

-- 
Mike Williams
Gentleman of Leisure