Wild Speculations Time: Optical SETI & Interstellar Travel (revis ed version)

Clements, Robert (Robert.Clements@dva.gov.au)
Wed, 15 Sep 1999 14:56:03 +1000

The following is a revised version of a post i made here some months ago.
The broad strokes of the argument remain the same; but some details on the
proposal have been rethought. I've also tried to tone down my taste for
neobaroque punction....

All the best,
Robert Clements <Robert.Clements@dva.gov.au>

> -----Original Message-----
> Notwithstanding the X-Files and despite 40 years of effort, SETI (the
> Search for Extra-Terrestrial Intelligence) research has been unsuccessful
> in obtaining clear any unambiguous evidence for its subject.
> The crucial issue here is probably range: given our current limitations, a
> target technology would need to be extraordinarily close by galactic
> standards - almost certainly less than 100 lightyears distant - for a
> direct signal to be detectable; and an incidental signal would need to be
> even closer. For budgetary reasons, we are also unable to monitor suitable
> systems over an extended period of time; so the welcoming signal from Tau
> Ceti could easily be missed because our telescopes were pointing in the
> wrong direction at the time.
> Despite a movement towards designated - & self-financed - SETI
> observatories, the increasing technological development in this field is
> locked in a race with growing bandwidth noise for range, capacity and
> clarity. So much so that - if we do not pick up a SETI signal in the next
> decade or so - serious SETI research in the field will have to wait the
> transfer of our investigative technology to quieter space-based locations
> such as the Lunar farside... something which is unlikely to occur within
> the first half of the 21st century.
> The following method is designed to detect incidental noise - ie, not a
> designated SETI signal - over significant galactic distances; & is
> intended to be relatively unaffected by environmental noise. It can used
> as a parasitical research program on existing optical astronomy programs;
> but broad - and surprisingly inexpensive - designated searches can also be
> conducted.
> The program makes one key assumption: that the ETI we're looking for is
> capable of interstellar travel - whether in the form of robotic probes or
> organic travel is irrelevant in this context - & that no trans-Einsteinian
> shortcuts being used. Any assumptions in a field like this can be
> dangerous, of course; but you have to start somewhere.
> Based on our current understanding of physics, the simplest (& most fuel
> efficient) way to travel interstellar distances in something resembling
> reasonable timeframes is to use beamed energy sails. This technology -
> closely related to the better known solar sail concept - uses a beam has
> been concentrated in some form... either a mirror or lens will do - from
> energy obtained directly from the system's star/s.
> Unfortunately, sails like this are basically one way thrusters. Given the
> entire impetus provided by the beam, the target star unlikely to be able
> to completely decelerate the beam driven vehicle; unless it is:
> a) inherently brighter; or
> b) the sail can unfurl further on arrival.
> Both options are entirely feasible, of course; but place distinct
> limitations on the missions which can be flown); so regular two way
> interstellar flight seems to call for other technologies like advanced
> antimatter drives....
> ... however: if you were able to build a second concentrating unit in the
> target system, you would have created a beam of energy between the two
> systems; which could be used to accelerate &
> decelerate two way traffic. In effect, you would have an interstellar
> river; along which starships can literally sail.
> Very appealing (romantically; as well as technically); & once the capital
> cost is met, the system could basically run automatically... only
> maintenance - and the inevitable debt servicing - would be required. The
> slow accumulating thrust to near-Relativistic velocity would be perfect
> for robotic flight, of course; & if the concentrated energy beam could be
> made strong enough, organic lifeforms could sail to and from the stars as
> well... an idea well worth exploring if you're an ambitious, exploratory
> ETI....
> That's the basic assumption behind this proposal.
> Now assume that my reasoning is valid; and that these beams of energy
> actually exist... what would they look like from the outside? The beam/s
> would be relatively bright but highly concentrated in space, so we may not
> be able to see the beams directly; but the spectrum of any star from which
> a beam flows may very well be distorted by the beam's presence: probably
> as a pronounced spike in the direction of transmission. This effect is
> likely to be most noticeable at some particular - & quite conceivably,
> physically unexpected - wavelength if the beam has been synchronised to
> maximise sail efficiency.
> If the beam was not pointed towards the Earth, detection would be more
> difficult; but it could reveal itself as an anomalous light intensity
> extending substantially beyond - and probably discrete from - the visible
> surface in the star; with further analysis revealing that intensity was
> flowing in the direction of a nearby star. The beam would also be
> paralleled by a second energy anomaly extending from the apparent target
> star back to the home star; although this would only be the case if two
> way travel was taking place.
> Since the beam driven acceleration to a suitable interstellar travel
> velocity is likely to take a year or more, these energy spikes are likely
> to be long lived and quite possibly even permanent. They would also be
> highly directional, obviously; and this direction is likely to vary only
> as the angle of transmission between the two stars shifts in relationship
> to Earth. It should be noted, though, that a single could conceivably be
> serving flights to more than one target star by shifting periodically
> between an each... an interesting visual prediction to test in itself.
> Most importantly of: the limit to detection of this kind of optical SETI
> signal could effectively be a far as a light intensity curve and a
> spectrographic analysis can be successfully.
> It should be noted that no useful information - beyond an apparently
> remarkable technical capacity - about the civilisation which created this
> beaming highway could be obtained from such a detection. It would not even
> be clear from such a detection whether the civilisation which created it
> was still extant; as there's no good reason why a system such as this
> couldn't be completely automatic & therefore capable of surviving its
> erstwhile creators.
> The strength of this optical SETI concept has the major advantage of not
> requiring immediate additional designated technology to begin a search: I
> suspect a review of existing light intensity and spectra data would be
> enough to determine whether the idea is literally off the beam; or worth
> exploring further.
> One could, however, imagine a designated search experiment being conducted
> which used any modern optical telescope capable of doing light intensity
> and spectral analysis across multiple targets simultaneously... just about
> all of them nowadays. By targeting promising sector/s of the sky over an
> extended period of time - much in the manner of the proposed Kepler space
> telescope - a statistically useful sample of space could be scanned
> quickly and effectively for anomalous beams of energy.
> Even if this kind search is completely parasitical - that is to say: it
> has to be done in any direction the telescope happens to be pointing at
> the time - the potential for interesting supporting science based on this
> search seems significant enough for a trial to be justified.
> All the best,
> Robert Clements <Robert.Clements@dva.gov.au>
> ------
> reference urls (this choice is uncompromisingly eclectic; & offered as a
> guide only):
> SETI Institute: http://www.seti-inst.edu/
> SETI League: http://www.setileague.org/
> "Optical SETI at the University of Berkeley, California":
> http://sag-www.ssl.berkeley.edu/opticalseti/
> "Optical SETI at Harvard/Smithsonian":
> http://mc.harvard.edu/oseti/index.html
> "The Array of [SETI] Search Strategies" by Allen Tough:
> http://members.aol.com/AllenTough/strategies.html
> Kepler mission homepage: http://www.kepler.arc.nasa.gov/
> "Setting Sail for the Stars::
> http://science.nasa.gov/newhome/headlines/prop08apr99_1.htm
> "Advanced Solar- and Laser-pushed Lightsail Conceps - Final Report" by
> Geoffrey A Landis:
> http://niac.usra.edu/studies/9801/9801Final/LandisFinal.pdf
[Geoffrey Landis's homepage: http://www.sff.net/people/geoffrey.landis/]
> "Beam Power to Space": http://powerweb.lerc.nasa.gov/psi/DOC/beams.html
> Solar Sails: http://www.ugcs.caltech.edu/~diedrich/solarsails/

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