Stuart Kingsley's Rebuttal to Barney Oliver
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Chairman's Prerogative!
In Dr. Bernard M. Oliver's conference paper "Fundamental
Factors Affecting The Optimum Frequency Range For SETI" (1867-08), a number of
conclusions are made. Each of these conclusions will be examined in turn, with the
chairman's rebuttal.
But first, there was a statement in Barney's paper that
challenged us "to build 1,000 transmitters to interrogate the stars out to 100
light years? And if that fails we will make a million transmitters to cover the stars to
1,000 light years? Very soon we are filling space with our beams and the advantage over
omnidirectional broadcasting begins to disappear.".
One must say that the last sentence is a gross
distortion of the true facts. Even at a range of 1,000 light years, the space between the
million solar-type stars on the celestial sphere is still large and empty. If there are 106
beams for 1 million stars, and the antenna gains are limited to 25 x 106, there
is still a huge ratio of the potential gain of an optical antenna to that of Barney's
directional microwave antenna, i.e., a ratio of 1015 to 25 x 106 or
40 million. On a transmitter watt for watt (equal power) basis, this doesn't come anywhere
near wiping out the advantage, in terms of EIRP and SNR, of highly directional optical
beams over their semi-omnidirectional microwave counterparts.
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Barney's Conclusions:
1.It is not possible to take advantage of the high
potential directivity of optical transmitters without a costly program to determine the
peculiar proper motion of all the targeted stars searched, which may be many millions.
Such a program would have few other benefits and would have to be supported from the SETI
budget.
SAK's Response
In the next several decades the need will no doubt arise
for far more detailed astrometric measurements of nearby stars to determine their peculiar
proper motions. Even if the technology to determine these peculiar proper motions is not
developed through other astrophysical endeavors, surely mankind would develop large
space-based optical antennas to make such measurements possible on receipt of an ETI
optical signal. The SETI budget would then have become a CETI budget and grown enormously!
This is no different to what has been proposed by Carl Sagan for the "Machine"
in his novel Contact. Developing the "Optical Machine" to respond to the
ETI signal would become an international effort. Note that Sagan's "Machine"
cost two trillion dollars!
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2. The maximum usable transmitter directivity is
available from existing microwave antennas.
SAK's Response
This presupposes that "we know" what is the
maximum usable transmitter directivity. This is a time and technology dependent issue. I
would suggest that it is much higher than obtainable with even the largest present-day
terrene microwave antennas.
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3. The usable receiving antenna area is orders of
magnitude greater and less costly at microwave than at optical frequencies.
SAK's Response
It is readily admitted that very large microwave
antennas, with there increased receiving area, are no more expensive to late 20th century
man than relatively small optical antennas. However, the relative economics may not be so
apparent to ETIs with their very mature photonics technology. Again, the high directivity
and correspondingly high EIRPs of relatively small optical uplinks can more than make up
for the small collection areas of Optical Earth Receiving Stations!
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4. The power needed for optical SETI is orders of
magnitude greater than at microwave because of (a) the higher energy per photon, (b) the
smaller collecting area, and (c) the impracticality of transmitter antenna gain large
enough to afford (a) and (b).
SAK's Response
Transmitter antenna gains in excess of 150 dB will not
be impractical to ETIs! Thus, the power requirements of optical SETI need be no more than
for microwave SETI. Indeed, they will probably be a lot less. How many more times do we
need to say this?
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5. Any attempt to reduce transmitter power through
beaming leads either to an enormous number of transmitters or to low duty cycle problems.
SAK's Response
Omnidirectional beaming is not the way to go in any
frequency regime if one wants to supports high bandwidth interstellar communications.
Again, ETIs will have access to far more power than we have at out present stage of
technological development, so would be capable of spatially and time-multiplexing laser
beams to many stars, should they so desire.
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6. Omnidirectional radiation, practical only at
microwaves, leads to very favorable growth law that will encourage successful SETI.
SAK's Response
For beacons yes, but not for high bandwidth
communications. As already indicated, for wideband interstellar communications over many
targeted stars, there is no way to avoid having to have a large number of high-power
beams. An Advanced Technical Civilization (ATC) could afford to do this if it so
desired.
There is an assumption implicit here that ETIs or more
precisely, signaling and listening ETIs, are very rare and that we will have to extend our
target range to considerable distances before we make First Contact. Time will tell
whether this is actually the case. We might find to our astonishment, that there is a
complex interstellar laser network in our galaxy, relaying the encyclopedia galactica
between inhabited star systems, to which we are presently completely oblivious.
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7. Nothing has happened in the last 20 years to cause
increased interest in optical SETI except possibly the loss of SDI contract support.
SAK's Response
In many ways it is true that nothing has happened in the
last 20 years to warrant the increased interest in optical SETI, which makes it even
stranger that the optical approach was not more strongly supported 20 or even 32 years
ago! However, what we have seen developing in terrene technology clearly gives us reason
to believe that the "wave of the future" is photonics. It was true then, and it
is even more true today.
On the issue of those in the laser community who have
suddenly "discovered" optical SETI. This comment probably arises from my
rhetorical conference preview defense conversion statement in the printed technical media
about "Can there be anything more ennobling than to turn SDI laser swords into
SETI/CETI plowshares?". A poll of the conference participants should shed some
"light" as to their major motivations. It may well be that future employment
prospects is not the primary issue here - it is not with me!
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One Additional Point
In Dr. Oliver's conclusions, one aspect of
communications was not mentioned, and this is the bandwidth question. While SETI has
mainly concentrated on aspects of detecting so-called "beacon signals", whether
they be continuous wave or pulse, the main aim of an ETI communications must surely be one
in which a "rich stream of information" is exchanged between their world and
ours.
What limits the bandwidth of such communications? As we
know, there are two main limitations, (a) signal-to-noise ratio (SNR) or bit-error-rate
(BER), and (b) dispersion. We have spent a lot of time discussing which approach to
SETI/CETI can maintain the greatest SNR or smallest BER. But what about dispersion?
Many years ago, Barney Oliver showed that dispersion
would prevent fast pulsed modulated signals being used in the microwave regime, thus
proving that it was not sensible for the former Soviet SETI researchers to be looking for
wideband pulsed signals in the microwave spectrum. This is not believed to be the case at
optical frequencies. In the microwave regime, dispersion due to electrons in space is
proportional to the signal bandwidth and inversely proportional to the cube of the carrier
frequency. It is thought that interstellar dispersion at optical frequencies is
essentially zero! This reason alone, may be the deciding factor for the use of lasers in
interstellar communications - for us and for them!
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