Galactic Wi-fi?Jan. 31, 2008
by Seth Shostak - Senior Astronomer
Incredibly, it's been only a bit more than a century since Oliver Heaviside consolidated the work of several 19th century physicists into the four compact mathematical formulations known as Maxwell's Equations. You may gleefully recall them from sophomore physics.
Aside from their display by the rabidly nerdy on pretentious t-shirts, the formulae have a splendid utility: they describe all electromagnetic radiation — in particular, light and radio. In the short time since their discovery, we have been able to milk these elegant equations to build crude spark transmitters, and eventually to develop the diminutive cell phones that allow you to blithely ring up your pals while comfortably seated in restaurants and movie theaters. We have exploited Maxwell's Equations like an old-growth forest, and many technical types aver that we know all there is to know about them.
Not true. And the fact that it's untrue may affect our thinking about SETI.
Today's SETI experiments generally look for what are politely termed "narrow-band signals." In other words, the receivers at the back ends of our radio telescopes search wide swaths of the spectrum looking for a signal that's at one spot on the dial — a signal that's very constrained in frequency. By putting all the transmitted power into this small bandwidth, the aliens can ensure that their signal will stand out like Yao Ming at a Munchkin picnic.
That makes sense — at least if the aliens want only to help us find their signal. But they might have other priorities. In particular, the history of earthly communication suggests that there is an inexorable pressure to increase the bit rate of any transmission channel. A half-decade ago, most readers accessed this web site with a simple dial-up phone line. Today, you're more likely to have some sort of wide-band service, which is to say, you're inhaling Internet bits at least ten times quicker than before.
More generally, in 150 years, we've gone from telegraph wires, capable of a few bits per second, to optical fibers that are billions of times speedier. The idea of "more bandwidth" is so compelling, the phrase has entered the lexicon of everyday speech — even among those who couldn't tell a hertz from a hub nut. Communication technology is always driven to send more bits — more information — per second.
Now consider the plight of aliens wishing to get in touch. Because the separation between one civilization and another is likely to be at least hundreds — and maybe thousands — of light-years, any interstellar pinging is effectively one-way. Back and forth conversations will take too long. So perhaps the aliens will opt to send, not the easiest-to-find signal, but a signal that says it all — a signal bristling with information. If you're going to stuff a message into a bottle, why not use onion-skin paper and write small?
The straightforward way to get more information down a radio channel is, as everyone knows, by using greater bandwidth. Nearly once a week someone sends me an e-mail pointing this out, saying that SETI should be looking for wide-band signals, not narrow-band ones. But there's a problem here. While sending a wide-band, information-rich signal between nearby stars is perfectly practical (assuming you're willing to pay the power bill), once the distance exceeds a thousand light-years or so the billowing hot gas that permeates interstellar space begins to wreak havoc and destruction on the transmission. A process of "dispersion" occurs, which works to slow the broadcast — but it slows different frequencies by different amounts. The result is to distort a wide-bandwidth signal in much the way that a highly reverberant hall would distort the music from an orchestra. A narrow-band signal (the acoustical analog is a simple flute note) would not be adversely affected.
So it seems that there may be difficulties in sending certain kinds of complex radio signals over significant distances in the Galaxy. Interstellar correspondence could be restricted to mere postcards, which would be a disappointment to aliens interested in heavy-duty data distribution.
However, some Swedish physicists are pointing out a possible scheme for beating this rap. In careful analyses of some of the subtle properties of Maxwell's Equations, Bo Thide and Jan Bergman at the Swedish Institute of Space Physics in Uppsala have explored a property of radio waves called orbital angular momentum. You can think of this orbital momentum as a twisting of the wave's electric and magnetic fields — a twisting that would show up if you were measuring the wave with an array of antennas. The technical details are intricate, but suffice it to say that the Swedish scientists are noting another way to send information in a radio signal — even a narrow-band radio signal — by encoding it in the orbital angular momentum.
It's as if they've found "subspace channels," a là Star Trek. Hidden highways down which additional bits can be moved. And there's reason to think that these momentum channels might be impervious to the interstellar jumbling that afflicts the usual types of wide-band signals when sent over great distances.
So it may be that our search for narrow-band signals is actually a very good SETI strategy, and not just an obvious one. While such monotonic messages may seem to be elementary and devoid of much information, they could be laden with additional, hidden complexity.
The investigation of new transmission modes by Thide and Bergman hints that if we do find a signal from ET, we may wish to reconfigure our radio telescopes to look for encoding of the message via such subtle effects as orbital angular momentum. A simple signal may only be a cipher for a more complex message, and there may be more things in heaven and earth than even Maxwell had dreamt of …