Incredibly,it's been only a bit more than a century since Oliver Heaviside consolidatedthe work of several 19th century physicists into the four compact mathematicalformulations known as Maxwell's Equations. You may gleefully recall them fromsophomore physics.
Aside fromtheir display by the rabidly nerdy on pretentious t-shirts, the formulae have asplendid utility: they describe all electromagnetic radiation — in particular,light and radio. In the short time since their discovery, we have been able tomilk these elegant equations to build crude spark transmitters, and eventuallyto develop the diminutive cell phones that allow you to blithely ring up yourpals while comfortably seated in restaurants and movie theaters. We haveexploited Maxwell's Equations like an old-growth forest, and many technicaltypes aver that we know all there is to know about them.
Not true. Andthe fact that it's untrue may affect our thinking about SETI.
Today'sSETI experiments generally look for what are politely termed "narrow-bandsignals." In other words, the receivers at the back ends of ourradio telescopes search wide swaths of the spectrum looking for a signalthat'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 canensure that their signal will stand out like Yao Ming at a Munchkin picnic.
That makessense — at least if the aliens want only to help us find their signal. But theymight have other priorities. In particular, the history of earthlycommunication suggests that there is an inexorable pressure to increase the bitrate of any transmission channel. A half-decade ago, most readers accessed thisweb site with a simple dial-up phone line. Today, you're more likely to havesome sort of wide-band service, which is to say, you're inhaling Internet bitsat least ten times quicker than before.
Moregenerally, in 150 years, we've gone from telegraph wires, capable of a few bitsper second, to optical fibers that are billions of times speedier. The idea of "morebandwidth" is so compelling, the phrase has entered the lexicon ofeveryday speech — even among those who couldn't tell a hertz from a hub nut. Communicationtechnology is always driven to send more bits — more information — per second.
Nowconsider the plight of aliens wishing to get in touch. Because theseparation between one civilization and another is likely to be at leasthundreds — and maybe thousands — of light-years, any interstellar pinging iseffectively one-way. Back and forth conversations will take too long. Soperhaps the aliens will opt to send, not the easiest-to-find signal, but asignal that says it all — a signal bristling with information. If you're goingto stuff a message into a bottle, why not use onion-skin paper and write small?
Thestraightforward way to get more information down a radio channel is, aseveryone knows, by using greater bandwidth. Nearly once a week someone sends mean e-mail pointing this out, saying that SETI should be looking for wide-bandsignals, not narrow-band ones. But there's a problem here. While sending awide-band, information-rich signal between nearby stars is perfectly practical(assuming you're willing to pay the power bill), once the distance exceeds athousand light-years or so the billowing hot gas that permeates interstellarspace begins to wreak havoc and destruction on the transmission. A process of "dispersion"occurs, which works to slow the broadcast — but it slows different frequenciesby different amounts. The result is to distort a wide-bandwidth signal in muchthe way that a highly reverberant hall would distort the music from anorchestra. A narrow-band signal (the acoustical analog is a simple flute note)would not be adversely affected.
So it seemsthat there may be difficulties in sending certain kinds of complex radiosignals over significant distances in the Galaxy. Interstellar correspondencecould be restricted to mere postcards, which would be a disappointment toaliens interested in heavy-duty data distribution.
However,some Swedish physicists are pointing out a possible scheme for beating thisrap. In careful analyses of some of the subtle properties of Maxwell'sEquations, Bo Thide and Jan Bergman at the Swedish Institute of Space Physicsin Uppsala have explored a property of radio waves called orbital angularmomentum. You can think of this orbital momentum as a twisting of the wave'selectric and magnetic fields — a twisting that would show up if you weremeasuring the wave with an array of antennas. The technical details areintricate, but suffice it to say that the Swedish scientists are noting anotherway to send information in a radio signal — even a narrow-band radio signal —by encoding it in the orbital angular momentum.
It's as ifthey've found "subspace channels," a là Star Trek. Hiddenhighways down which additional bits can be moved. And there's reason to thinkthat these momentum channels might be impervious to the interstellar jumblingthat afflicts the usual types of wide-band signals when sent over greatdistances.
So it maybe that our search for narrow-band signals is actually a very good SETIstrategy, and not just an obvious one. While such monotonic messages may seemto be elementary and devoid of much information, they could be laden withadditional, hidden complexity.
Theinvestigation of new transmission modes by Thide and Bergman hints that if wedo find a signal from ET, we may wish to reconfigure our radiotelescopes to look for encoding of the message via such subtle effects asorbital angular momentum. A simple signal may only be a cipher for a morecomplex message, and there may be more things in heaven and earth than evenMaxwell had dreamt of …
- VIDEO: Figure the Odds of E.T.!
- All About SETI
- GALLERY: Scenes from SETI@Arecibo