Friday, July 23, 2010


Listening for space aliens

New Scientist reports on a study that addresses whether we’re using the right frequencies to listen for extraterrestrial communications:

A new study suggests that cost-effective galactic radio transmissions would be at higher frequencies than SETI projects traditionally monitor, and ET’s attempts to make contact would be only few and far between.

If ET was building cost-effective beacons, would our searches have detected them? The answer turns out to be no, says James Benford, president of the company Microwave Sciences in Lafayette, California.

Aliens wishing to communicate would probably broadcast at frequencies between 1 and 10 gigahertz, where there is less astronomical background noise than in other wavebands. Most SETI projects tune in to the cosmic water hole waveband between 1.42 and 1.72 gigahertz. The reasoning goes that alien astronomers might expect earthly scientists to be looking there anyway as this is the frequency of radiation emitted by interstellar hydrogen and hydroxyl clouds.

But this fails to consider the cost to aliens. Societies are always constrained by their resources, Benford points out. Why did cathedrals take centuries to build? Partly because they had only so many artisans, but also their capital was limited.

Benford’s analysis of the economics of extraterrestrial beacons with his brother Gregory at the University of California, Irvine, and son Dominic at NASA’s Goddard Space Flight Center in Maryland suggests that aliens would choose to transmit at nearer to 10 gigahertz, as this makes it easier and cheaper to create a powerful beam.

Short pulses rather than a continuous signal would also enable frugal aliens to use small and cheap transmitters. Small transmitters can beam out powerful radiation using high voltages — but only if they broadcast brief pulses that don’t give the electric fields time to discharge.

It’s interesting to look at how we might listen better. But is that enough?

Back at the end of 2007, I wrote in these pages about a presentation by and conversations with Jill Tarter, who heads research at the SETI Institute. Over lunch during Dr Tarter’s visit, she said that they only listen for transmissions from elsewhere in the universe. SETI researchers don’t send their own transmissions, attempting to actively communicate, because there would then be too many questions to deal with, Earth-based logistics and politics. What do we transmit, and who (which country, what officials) gets to control that? There was a great deal of work put into what to send on the Pioneer and Voyager probes, and criticism remains about the choices that were made. This, a constant set of messages actively sent, would be more difficult.

But, I asked Dr Tarter, if we’re not able to get around the socio-political issues and actively transmit, why should we assume that other intelligent societies can? What if there are millions of SETI researchers throughout the universe, all listening... and no one is sending anything to be listened to?

That’s an excellent point, she replied, adding that we just have to hope that someone, somewhere has gotten past those issues.


Brent said...

I don't get it...just ask Lt. Uhura to "scan all frequencies" - then you are done!

Barry Leiba said...

Ah, for sub-space radio.

Jim Fenton said...

I went to a talk several years ago by Dr. Kent Cullers, a well-known SETI scientist who was the model for one of the characters in the movie Contact.

I noted the fact that we don't transmit signals (very frequently, at least) that are intended for reception by other societies' SETI equivalents, and therefore we might expect them to do the same. I asked him at what range, using our then-current receiving technology, would we be able to detect Earth's background radiation (broadcast stations, radars, etc.). The answer, as I recall, was about 7 light-years. That would lead me to conclude that we should be focusing on star systems that are relatively close to us (apparently only 4 are <7 light-years away; there are quite a few more <12 light-years distant but I don't know which, if any, might be able to support viable planetary systems), or that we need to add a new factor to Drake's Equation to account for the postulated probability that another society is trying to be heard.

Even so, I don't think SETI is a waste of time. They're doing good science, have made significant contributions in signal processing and distributed computation technology, and they stand a good chance of making an important and totally unexpected discovery.