The Planetary Society - 1


From: The Planetary Report, Volume XVI, No. 3, March/April 1996 Questions and Answers, page 20.

I've read that it would actually be better for an extraterrestrial civilization to send its messages by laser light instead of radio waves. For example, infrared lasers travel more easily through dust clouds and can carry large amounts of information. Why not an additional search for laser messages?

Han Starlife, Stockholm, Sweden


Response from Seth Shostak, SETI Institute:

While advanced civilizations might be tempted to use optical means such as lasers to send information between the stars, there are some good reasons that nearly all the major Search for Extraterrestrial Intelligence experiments are looking for radio waves instead. To begin with, microwave transmissions are completely oblivious to the dust clouds that clog th spaces between the stars. They pass through these gauzy barriers far better than light, even infrared light. So if extraterrestrials are far away, or are interested in truly long-distance communication, then we should take advantage of the fact that radio will get through when a laser beam won't.

There's also the matter of cost. Optical photons are considerably more energetic than their radio counterparts; they are "heavier bullets". For example, and infrared photon at 20 microns wavelength has 10,000 times as much energy as a radio photon at the 21-centimeter wavelength. To make an infrared signal as simple as a radio signal (given the noise present in the receiving system), the extraterrestrials will have to expend a lot more energy generating it. Consequently, SETI researchers are inclined to look for cheaper, longer wavelength communications. They are easier to pick out of the noise and might be more prevalent.

Finally, optical broadcasts are generally most suitable when you know exactly where your listeners are. If you mount a powerful microwave transmitter on a 10-meter telescope and beam a greeting into space, that beam will cover an amount of sky that's about the same as a dozen full moons. But if you use an infrared transmitter mounted on a 10-meter optical telescope, the beam will be a hundred million times smaller. You would have to be very careful with your aim, even to the point of knowing the orbit of the planet your are trying to signal. Obviously, unless the extraterrestrials know a lot about our solar system and are deliberately targeting us with a broadcast, there's a much greater chance that we'll be in someone's radio beam than in an optical one.

For these and other practical reasons, radio is still the wavelength of choice for SETI. Some optical observations have been made, and Stuart Kingsley in Ohio is running an optical SETI experiment now. But for the moment at least, most scientists are betting on radio. Seth Shostak, SETI Institute

Copyright (c), 1996, The Planetary Society



The following response was submitted to The Planetary Society via email on April 21, 1996 (not published):


In the most recent issue of TPR, my microwave SETI colleague, Seth Shostak, replied to a Swedish reader who inquired about the lack of an optical search for extraterrestrial intelligence program. I would like to respond to Seth's comments.

I believe that the reason that SETI on this planet has taken a predominantly microwave approach is only a historical accident, cause by the personalities of a few key individuals in the SETI community. After 1961, when Charles Townes first proposed a carbon dioxide laser SETI system operating at 10.6 microns, there was a brief interest in the subject. This wavelength can penetrate the entire width of the galaxy, and our atmosphere is relatively transparent at 10.6 microns. The 1972 Cyclops Study indicated that the laser approach was very poor. However, this erroneous conclusion was due to a very skewed comparative analysis. From that time there has been relatively little said or done about the subject.

While it is true that microwaves can penetrate the galaxy easily, and lasers are more easily absorbed by interstellar dust and gas, if we are only considering point-to-point interstellar communications over just several thousand light years, instead of communications over 30,000 light years or more, then most laser frequencies are not substantially attenuated outside the atmosphere. Indeed, over the 100 or so light years presently being targeted by Project Phoenix, interstellar extinction is negligible, even in the galactic plane.

Seth's last objection to the laser approach is that a 10-meter diameter telescope transmitting an infrared beam would be a hundred million times smaller that its microwave counterpart. But that is just the point, and contradicts his first objection that the energy cost for optical photons is much higher than for microwave photons, even if that still is an important consideration for an advanced technical civilization that perhaps corresponds to a Kardashev Type II.

If the substantial gain advantage of laser transmitters is fully realized, which the late Barney Oliver claimed was not possible even for ETIs, then the energy cost for microwave photons is actually higher than for optical photons! Before he died, Barney had set an upper limit to usable transmitter gain, be it microwave or optical, of about 94 dB. Who's to say that ETIs cannot produce visible or infrared phased transmitter arrays with usable gains in excess of 155 dB? Receiver quantum noise at optical wavelengths for monochromatic continuous wave lasers becomes a non-issue because the received signal intensity can be so much higher, and if short beacon pulses are used, the peak Effective Isotropic Radiated Power (EIRP) can easily exceed that of the ETIs' star by many orders of magnitude. Small ground-based telescopes should be capable of detecting such pulsed beacons if they exist at visible or near-infrared wavelengths.

The real issue here is whether it is possible for ETIs to transmit very narrow optical beams into nearby stars. Dan Goldin has recently charged NASA with mounting a long-term program to detect a large number of extrasolar planets and image the continents of such planets! The latter is likely to be science fiction for a long time, requiring massive space-based telescope arrays for such imaging. But one can imagine that ETIs, who by definition are likely to be millions of years ahead of us, would have long ago done this and be able to land laser beams with diameters comparable to 1 A.U. onto planets around nearby stars. Advanced, sophisticated civilizations do not broadcast crude omnidirectional or semi-directional beams that waste most of their energy in empty space. Rather, they will multiplex high intensity narrow beams from space-based stellar or nuclear-pumped lasers only to those targets they know from observations are capable of supporting life.

I would like to take the opportunity here to make a public challenge to the Microwave SETI community. Both for SETI beacon and data channel purposes, the wavelength regime used by ETIs will probably be determined by bandwidth considerations for the associated wideband data channel, which should be capable of conveying "real-time" high-definition video. The interstellar media must be relatively free of dispersion and scintillation effects to allow wideband communications. The microwave regime prevents this. If the microwave SETI community can prove that the optical regime, covering the far-infrared to the ultra-violet, cannot support information bandwidths as high as say, 1 GHz, then there may be some validity for their objections to Optical SETI.

So, notwithstanding the signal-to-noise ratio issue, lasers are probably the only way that an "electromagnetic" galactic-wide Information Superhighway or Galactic Internet could be maintained, where intelligences throughout the galaxy could add their rich cultural database to a data-stream the would be relayed between star systems.

Stuart Kingsley, The Columbus Optical SETI Observatory


British readers should note that on May 21 (1996), Dr. Kingsley is giving an evening lecture on Optical SETI at the Institution of Electrical Engineers, Savoy Place, London, England.

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