A New Project for SETI

The Cepheid Galactic Internet

J.G. Learned, R.P. Kudritzki, S. Pakvasa, and A. Zee

arXiv:0809.0339v2

URL: http://arxiv.org/abs/0809.0339

This paper puts forward an interesting proposition for intergalactic communication: frequency modulation of cepheid variable stars.

Cepheids are 1,000 to 10,000 times brighter than our sun and have regular, detectable variations in brightness with a period of 1-50 days. The frequency and luminosity are strongly correlated, so one can measure the period of the stars, deduce the luminosity, compare with the observed brightness, and determine how far away the star is. Parallax is used to calibrate the distance scale on nearby cepheids.

The authors of this paper propose that an advanced race might intentionally modify the period of these stars to send information throughout the galaxy: FM at very low frequencies!

The mechanism by which the brightness varies is similar to the charging and discharging of a capacitor. According to the authors, as the star consumes hydrogen through nuclear fusion, ionized helium builds up on the surface of the star. This decreases the luminosity and increases the temperature of the star. This is followed by "violent expansion and deionization," after which the cycle repeats.

It is interesting that an avalanche process like this leads to a regular cycle of variations in the brightness. The same thing happens in circuits with a capacitor (if I remember my physics lab correctly). A build up of charge is followed by dielectric breakdown (a spark) after which the process repeats. The spark frequency is constant. What makes these systems different from, say, a pile of sand? The avalanches in a sand pile have a power law distribution in both size and frequency. What makes the two systems so different? I assume the circuit and the cepheid have nice, normal, linear equations of motion while the sand pile is governed by nonlinear equations. But what parameter differentiates the two? Is there some control parameter in the circuit that would lead to a crossover between periodic sparks and a frequency distribution more like the avalanches in the sand pile?

I digress. The reason I described the process that gives rise to the periodic variations in a cepheid is that the authors claim it can be influenced by external agents. They suggest that the deionization could be triggered early with a sufficient burst of power. The variation in the period could be used to transmit messages. To make the star fire early, the authors suggest that a neutrino beam focused on the star's core might do the trick.

The authors suggest the neutrino beam could be generated by a large space station orbiting the star running on solar power. We've got a long way to go from our handheld calculators to star-altering neutrino beams! For a society that could build such a device, it would seem a simple matter to build the beacon and leave it in place.

So are aliens broadcasting over the cepheid Internet? The authors say it wouldn't be too hard to find out. Normally astronomers measure the average period of these variable stars. Averaging the data would, of course, wash out any signal. Rather than determining the average, one would simply have to bin the data and look for a splitting of the signal around the fundamental frequency.

The authors don't mention it, but one might also perform an entropy analysis of the time series data. A periodic source would have zero entropy, while a modulated source would appear more ergodic. This would be a secondary analysis. If a splitting of the fundamental peak in the frequency spectrum were observed, one would then analyze the time series data to see if the splitting is also periodic, or if it has more structure to it.

Calling this system a galactic Internet is a bit misleading. It's more like the galactic telegraph. An early flash corresponds to a 1, an on-time flash corresponds to 0. Only one bit can be sent per period, so the maximum transmission rate is on the order of 1 bit per month. It would take forever to download a page from Encyclopedia Galactica!

I wonder what types of message would be transmitted. There seem to be two schools of thought about aliens: the Star Trek school, and the Hitchhiker school. The former envision alien civilizations as refined, peaceful, rational, and technologically advanced. The latter imagine alien civilizations to be more like our own society, with profit-driven pleasure-seekers, petty bickering, advertising, and bureaucracy. Each would certainly use the galactic beacon to transmit different kinds of messages.

The Star Trek variety of civilization might use the beacon as a lighthouse, a simple welcome message for young civilizations pointing them to a source of information with more bandwidth, or a galactic emergency broadcast system. If the cepheids were run by Hitchhikers, we might instead find messages like "Tresspassers will be shot on sight" or "Eat at Joe's." Imagine, after years of trying to crack the code, we discovered the message to be

"At the next pulse, the current time will be ..."