Detectability of Extraterrestrial Technological
Activities
by
Guillermo A. Lemarchand
(Visiting Fellow under ICSC World Laboratory scholarship.)
Center for Radiophysics and Space Research
Cornell University, Ithaca, New York, 14853
Address: University of Buenos Aires, C.C.8-Suc.25, 1425,
Buenos Aires, Argentina
This paper was originally presented at the Second United
Nations/European Space Agency Workshop on Basic Space Science.
Co-organized by The Planetary Society in cooperation with the
Governments of Costa Rica and Colombia, 2-13 November 1992,
San Jose, Costa Rica - Bogota, Colombia
Republished in SETIQuest, Volume 1, Number 1, pp. 3-13.
If we want to find evidence for the existence of extraterrestrial civilizations (ETC),
we must work out an observational strategy for detecting this evidence in order to
establish the various physical quantities in which it involves. This information must be
carefully analyzed so that it is neither over- interpreted nor overlooked and can be
checked by independent researchers.
The physical laws that govern the Universe are the same everywhere, so we can use our
knowledge of these laws to search for evidence that would finally lead us to an ETC. In
general, the experimentalist studies a system by imposing constraints and observing the
system's response to a controlled stimulus. The variety of these constraints and stimuli
may be extended at will, and experiments can become arbitrarily complex.
In the problem of the Search for Extraterrestrial Intelligence (SETI), as well as in
conventional astronomy, the mean distances are so huge that the "researcher" can
only observe what is received. He or she is entirely dependent on the carriers of
information that transmit to him or her all he or she may learn about the Universe.
Information carriers, however, are not infinite in variety. All information we
currently have about the Universe beyond our solar system has been transmitted to us by
means of electro-magnetic radiation (radio, infrared, optical, ultraviolet, X-rays, and
gamma rays), cosmic ray particles (electrons and atomic nuclei), and more recently by
neutrinos. There is another possible physical carrier, gravitational waves, but they are
extremely difficult to detect.
For the long future of humanity, there have also been speculations about interstellar
automatic probes that could be sent for the detection of extrasolar life forms around the
nearby stars. Another set of possibilities could be the detection of extraterrestrial
artifacts in our solar system, left here by alien intelligences that want to reveal their
visits to us.
Table 1 summarizes the possible "information
carriers" that may let us find the evidence of an extraterrestrial civilization,
according to our knowledge of the laws of physics (Lemarchand, 1992). The classification
of techniques in Table 1 is not intended to be complete in all respects. Thus, only a few
fundamental particles have been listed. No attempt has been made to include any
antiparticles. This classification, like any such scheme, is also quite arbitrary.
Groupings could be made into different "astronomies".
The methods of collecting this information as it arrives at the planet Earth make it
immediately obvious that it is impossible to gather all of it and measure all its
components. Each observation technique acts as an information filter. Only a fraction
(usually small) of the complete information can be gathered. The diversity of these
filters is considerable. They strongly depend on the available technology at the time.
In this paper a review of the advantages and disadvantages of each "physical
carrier" is examined, including the case that the possible ETCs are using them for
interstellar communication purposes, as well as the possibility of detection activities of
extraterrestrial technologies.
CLASSIFICATION OF EXTRATERRESTRIAL CIVILIZATIONS
The analysis of the use of each information carrier is deeply connected with the
assumption of the level of technology of the other civilization. Kardashev (1964)
established a general criteria regarding the types of activities of extraterrestrial
civilizations which can be detected at the present level of development. The most general
parameters of these activities are apparently ultra-powerful energy sources, harnessing of
enormous solid masses, and the transmission of large quantities of information of
different kinds through space. According to Kardashev, the first two parameters are a
prerequisite for any activity of a supercivilization. In this way, he suggested the
following classification of civilizations by energy usage:
TYPE I: A level "near" contemporary terrestrial
civilization with an energy capability
equivalent to the solar insolation on Earth,
between 10 to the 16 power and 10 to the 17 power Watts.
TYPE II: A civilization capable of utilizing and
channeling the entire radiation output of
its star. The energy utilization would then
be comparable to the luminosity of our Sun,
about 4 x 10 to the 26 power Watts.
TYPE III: A civilization with access to the power
comparable to the luminosity of the entire
Milky Way galaxy, about 4 x 10 to the 37 power Watts.
Kardashev also examined the possibilities in cosmic communication which attend the
investment of most of the available power into communication. A Type II civilization could
transmit the contents of one hundred thousand average-sized books across the galaxy, a
distance of one hundred thousand light years, in a total transmitting time of one hundred
seconds. The transmission of the same information intended for a target ten million light
years distant, a typical intergalactic distance, would take a transmission time of a few
weeks. A Type III civilization could transmit the same information over a distance of ten
billion light years, approximately the radius of the observable Universe, with a
transmission time of just three seconds.
Sagan (1973) considered that Kardashev's classification should be completed using
decimal numbers to indicate a difference of one order of magnitude in the energy
consumption. For example, a civilization Type 1.7 expends 10 to the 23 power Watts, while
a civilization Type 2.3 expends 10 to the 29 power Watts. Sagan also suggested that, in
order to be more accurate, a letter could indicate the societal information level (degree
of their knowledge). According to Sagan, a Class A civilization will have 10 to the 6
power bits of information, a Class B, 10 to the 7 power bits, a Class C, 10 to the 8 power
bits, and so on. Under this classification, our terrestrial civilization is Type 0.7H.
The level of the first extraterrestrial civilization that we can make contact with
would be between 1.5J and 1.8K. A galactic civilization would be Type III Q, while a
civilization with the capacity to control a federation of 10 to the 9 power galaxies would
be Type IV Z. Other classification alternatives were suggested by Tang and Chang (1991).
TABLE 2: Characteristics of Extraterrestrial Civilizations
TYPE I
Characteristics:
Planetary Society.
Developed Technology.
* understanding the laws of physics.
* space technology.
* nuclear technology.
* electromagnetic communications.
Initiation of spaceflight, interplanetary travel, settlement of space.
Early attempts at interstellar communication.
Starting to push planetary resource limits.
Information Level: I.
Energy Consumption: 10 to the 16 power to 10 to the 17 power Watts.
Manifestations: Intentional or unintentional electromagnetic emissions,
especially radio waves.
TYPE II
Characteristics:
Stellar system society.
Construction of space habitats.
"dyson sphere" as an ultimate limit.
Search for intelligent life in space.
Long societal lifetimes (10 to the 3 power to 10 to the 5 power years).
Initiation of interstellar travel/colonization.
Ultimately all radiant energy output of native star is utilized.
Information Level: L.
Energy Consumption: 10 to the 26 power to 10 to the 27 power Watts.
Manifestations:
Electromagnetic.
* radio waves.
* optical lasers.
* X-rays.
* Gamma rays.
Gravity Waves.
Mass transfer.
* probes.
* panspermia.
* stellar arks.
TYPE III
Characteristics:
Galactic Civilization.
Interstellar communication/travel.
Very long societal lifetimes (10 to the 8 power to 10 to the 9 power years).
Effectively "the immortals", for planning purposes.
Energy resources of the entire galaxy (10 to the 11 power to 10
to the 12 power stars) are commanded.
Information Level: Q (Encyclopedia Galactica).
Energy Consumption: 10 to the 37 power to 10 to the 38 power Watts.
Manifestations:
Feats of astroengineering.
Exotic Communication.
* neutrinos.
* tachyons?
* Waves "i"?
Kardashev and Zhuravlev (1992) considered that the highest level of development
corresponds to the highest level of utilization of solid space structures and the highest
level of energy consumption. For this assumption, they considered the temperature of solid
space structures in the range of 3 Kelvin to 300 Kelvin, the consumption of energy in the
range of 1 solar luminosity to 10 to the 12 power solar luminosities, structures with
sizes up to 100 kiloparsecs (kpc), and distances up to ~1000 megaparsecs (Mpc). One parsec
equals 3.26 light years.
Searching for these structures is the domain of millimeter wave astronomy. For the 300
Kelvin technology, the maximum emission occurs in the infrared region (15-20 micrometers)
and searching is accomplished with infrared observations from Earth and space. The
existing radio surveys of the sky (lambda = 6 centimeters on the ground and lambda = 3
millimeters for the Cosmic Background Explorer (COBE) satellite) place an essential limit
on the abundance of ETC 3 Kelvin technology. The analyzes of the Infrared Astronomical
Satellite (IRAS) catalog of infrared sources sets limitations on the abundance of 300
Kelvin technology.
INFORMATION CARRIERS AND THE MANIFESTATIONS OF ADVANCED
TECHNOLOGICAL CIVILIZATIONS
BOSON AND PHOTON ASTRONOMY
Electromagnetic radiation carries virtually all the information on which modern
astrophysics is built. The production of electromagnetic radiation is directly related to
the physical conditions prevailing in the emitter. The propagation of the information
carried by electromagnetic waves (photons) is affected by the conditions along its path.
The trajectories it follows depend on the local curvature of the Universe, and thus on the
local distribution of matter (gravitational lenses), extinction affecting different
wavelengths unequally, neutral hydrogen absorbing all radiation below the Lyman limit (912
Angstrom), and absorption and scattering by interstellar dust, which is more severe at
short wavelengths.
Interstellar plasma absorbs radio wavelengths of kilometers and above, while the
scintillations caused by them become a very important effect for the case of ETC radio
messages (Cordes and Lazio, 1991). The inverse Compton effect lifts low-energy photons to
high energies in collisions with relativistic electrons, while gamma and X-ray photons
lose energy by the direct Compton effect. The radiation reaching the observer thus bears
the imprint of both the source and the accidents of its passage though space.
The Universe observable with electromagnetic radiation can be characterized as a
multi-dimensional phase space. Within this cosmic search space, several dimensions --
frequency coverage plus spatial, spectral, and temporal resolutions -- should properly be
measured logarithmically with each unit corresponding to one decade (Tarter, 1984).
Another dimension is polarization, which has four possible states: Circular, linear,
elliptical, and unpolarized.
It is useful to attempt to estimate the volume of the search space which may need to be
explored to detect an ETC signal. For the case of electromagnetic waves, we have a
"Cosmic Haystack" with an eight-dimensional phase space: Three spatial
dimensions (coordinates of the source), one dimension for the frequency of emission, two
dimensions for the polarization, one temporal dimension to synchronize transmissions with
receptions, and one dimension for the sensitivity of the receiver or the transmission
power.
If we consider only the microwave region of the spectrum (300 MHz to 300 GHz), it is
easy to show that this cosmic haystack has roughly 10 to the 29 power cells, each of 0.1
Hz bandwidth, per the number of directions in the sky in which an Arecibo (305-meter)
radio telescope would need to be pointed to conduct an all- sky survey, per a sensitivity
between 10 to the -20 power and 10 to the -30 power Watts m to the -2 power, per two
polarizations. The temporal dimension (synchronization between transmission and reception)
was not considered in the calculation. The number of cells increase dramatically if we
expand our search to other regions of the electromagnetic spectrum. Until now, only a
small fraction of the whole haystack has been explored (~10 to the -15 power to 10 to the
-16 power).
RADIO WAVELENGTH RADIATION
In the last thirty years, most of the SETI projects have been developed in the radio
region of the electromagnetic spectrum. A complete description of the techniques that all
the present and near-future SETI programs are using for detecting extraterrestrial
intelligence radio beacons can be found elsewhere (e.g., Horowitz and Sagan, 1993). The
general hypothesis for this kind of search is that there are several civilizations in the
galaxy that are transmitting omnidirectional radio signals (civilization Type II), or that
these civilizations are beaming these kind of messages to Earth. In this section we will
discuss only the detectability of extraterrestrial technological manifestations in the
radio spectrum.
DOMESTIC RADIO SIGNALS
Sullivan et al. (1978) and Sullivan (1981) considered the possibility of eavesdropping
on radio emissions inadvertently "leaking" from other technical civilizations.
To better understand the information which might be derived from radio leakage, the case
of our planet Earth was analyzed. As an example, they showed that the United States Naval
Space Surveillance
TABLE 3: Characteristics of the Electromagnetic Spectrum
Spectrum Frequency Wavelength Minimum Energy
Region Region [Hz] Region [m] per photon [eV]
_____________________________________________________________
Radio 3x10(6-10) 100-0.01 10(-8) to 10(-6)
Millimeter 3x10(10-12) 0.01-10(-4) 10(-6) to 10(-4)
Infrared 3x10(12-14) 10(-4)-10(-6) 10(-4) to 10(-2)
Optical* 3x10(14-15) 10(-6)-3x10(-7) 10(-2) to 5
Ultraviolet 10(15)-3x10(16) 3x10(-7)-10(-8) 5 to 10(2)
X-rays 3x10(16-19) 10(-8)-10(-11) 10(2-5)
Gamma rays > 3x10(19) <10(-11)> 10(5)
* Visible - SAK definition
System (Breetz, 1968) has an effective radiated power of 1.4x10 to the 10 power Watts
into a bandwidth of only 0.1 Hz. Its beam is such that any eavesdropper in the declination
range of zero to 33 degrees (28 percent of the sky) will be illuminated daily for a period
of roughly seven seconds. This radar has a detectability range of leaking terrestrial
signals to sixty light years for an Arecibo-type (305-meter) antenna at the receiving end,
or six hundred light years for a full-up Cyclops array (one thousand dishes of 100-meter
size each).
Recently, Billingham and Tarter (1992) estimated the maximum range at which radar
signals from Earth could be detected by a search similar to the NASA High Resolution
Microwave Survey (HRMS) (now the SETI Institute's "Project Phoenix") assumed to
be operating somewhere in the Milky Way galaxy. They examined the transmission of the
planetary radar of Arecibo and the ballistic missile early warning systems (BMEWS). For
the calculation of maximum range R, the standard range equation is:
R=(EIRP/(4 pi phi min))1/2
Where phi(min) is the sensitivity of the search system in [W m-2]. For the former NASA
HRMS Target Search from Arecibo, phi(min) = 10 to the -27 power and the NASA HRMS Sky
Survey phi(min) = ~10 to the -23 power (f)1/2, where f is the frequency in GHz. Table 4
shows the distances where the Arecibo and BMEWS transmissions could be detected by a
similar NASA HRMS spectrometer.
All these calculations assumed that the transmitting civilization is at the same level
of technological evolution as ours on Earth. Von Hoerner (1961) classified the possible
nature of the ETC signals into three general possibilities: Local communication on the
other planet, interstellar communication with certain distinct partners, and a desire to
attract the attention of unknown future partners. Thus he named them as local broadcast,
long-distance calls, and contacting signals (beacons). In most of the past sixty SETI
radio projects, the strategy was planned with the hypothesis that there are several
civilizations transmitting omnidirectional beacon signals. Unfortunately, no one has been
able to show any positive evidence of this kind of beacon signal.
Another possibility is the radio detection of interstellar communications between an
ETC planet and possible space vehicles. Vallee and Simard-Normandin (1985) carried out a
search for these kind of signals near the galactic center. Because one of the
characteristics of artificial transmitters (television, radar, etc.) is the highly
polarized signal (Sullivan et al., 1978), these researchers made seven observing runs of
roughly three days each in a program to scan for strongly polarized radio signals at the
wavelength of lambda = 2.82 centimeters.
RADAR WARNING SIGNALS
Assuming that there is a certain number N of civilizations in the galaxy at or beyond
our own level of technical facility, and considering that each civilization is on or near
a planet of a Main Sequence star where the planetoid and comet impact hazards are
considered as serious as here, Lemarchand and Sagan (1993) considered the possibility for
detecting some of the "intelligent activities" developed to warn of these
potentially dangerous impacts.
Because line-of-sight radar astrometric measurements have much finer intrinsic
fractional precision than their optical counterparts, they are potentially valuable for
refining the knowledge of planetoid and comet orbits. Radar is an essential astrometric
tool, yielding both a direct range to a nearby object and the radial velocity (with
respect to the observer) from the Doppler shifted echo (Yeomans et al., 1987, Ostro et
al., 1991, Yeomans et al., 1992, and Ostro, 1993).
Since in our solar system, most of Earth's nearby planetoids are discovered as a result
of their rapid motion across the sky, radar observations are therefore often immediately
possible and appropriate. A single radar detection yields astronomy with a fractional
precision that is several hundred times better than that of optical astrometry. The
inclusion of radar with the optical data in the orbit solution can quickly and
dramatically reduce future ephemeris uncertainty. It provides both impact parameter and
impact ellipse estimates. This kind of radar research gives a clearer picture of the
object to be intercepted and the orientation of asymmetric bodies prior to interception.
This is particularly important for eccentric or multiple objects.
Radar is also the unique tool capable for making a survey of such small objects at all
angles with respect to the central star. It can also measure reflectivity and polarization
to obtain physical characteristics and composition.
For this case, we can assume that each of the extraterrestrial civilizations in the
galaxy maintains as good a radar planetoid and/or comet detection and analysis facility as
is needed, either on the surface of their planet, in orbit, or on one of their possible
moons.
The threshold for the Equivalent Isotropic Radiated Power (EIRP) of the radar signal
could be roughly estimated by the size of the object (D) that they want to detect
(according to the impact hazard) and the distance to the inhabited planet (R), in order to
have enough time to avoid the collision.
One of the most important issues for the success of SETI observations on Earth is the
ability of an observer to detect an ETC signal. This factor is proportional to the
received spectral flux density of the radiation. That is, the power per unit area per unit
frequency interval. The flux density will be proportional to the EIRP divided by the
spectral bandwidth of the transmitting radar signals B (expressed in units of Hertz).
The EIRP is defined as the product of the transmitted power and directive antenna gain
in the direction of the receiver as EIRP = P(T).G, where P(T) is the transmitting power
and G the antenna gain. This quantity has units of [W].
According to the kind of object that the ETC wants to detect (nearby planetoids,
comets, spacecraft, etc.), the distance from the radar system and the selected wavelength,
a galactic civilization that wants to finish a full-sky survey in only one year will arise
from a modest "Type 0" (flux density ~10 to the 13 power W/Hz, R ~0.4 A.U., D
~5000 m, and lambda ~1 m) to the transition from "Type I" to "Type II"
(flux density ~2x10 to the 24 power W/Hz, R ~0.4 A.U., D ~10 m, lambda ~1 mm).
Lemarchand and Sagan (1993) also presented a detailed description of the expected
signal characteristics, as well as the most favorable positions in the sky to find one of
these signals. They also have compared the capability of detection of these transmissions
by each present and near future SETI projects.
INFRARED RADIATION
There have been some proposals to search in the infrared region for beacon signals
beamed at us (Lawton, 1971, and Townes, 1983). Basically, the higher gain available from
antennas at shorter wavelengths (up to 10 to the 14 power Hz) compensates for the higher
quantum noise in the receiver and wider noise bandwidth at higher frequencies. One
concludes that for the same transmitter powers and directed transmission which takes
advantage of the high gain, the detectable signal-to-noise ratio is comparable at 10 mu
and 21 centimeters. Since non- thermal carbon dioxide (CO2) emissions have been detected
in the atmospheres of both Venus and Mars (Demming and Mumma, 1983), Rather (1991)
suggested the possibility that an advanced society could construct transmitters of
enormous power by orbiting large mirrors to create a high-gain maser from the natural
amplification provided by the inverted atmospheric lines.
An observation program around three hundred nearby solar-type stars has just begun
(Tarter, 1992) by principal investigators Albert Betz (University of Colorado) and Charles
Townes(University of California at Berkeley). These observations are currently being made
on one of the two 1.7-meter elements of an IR interferometer at Mount Wilson observatory.
On average, 21 hours of observing time per month is available for searching for evidence
of technological signals.
Dyson (1959, 1966) proposed the search for huge artificial
biospheres created around a star by an intelligent species as part of its technological
growth and expansion within a planetary system. This giant structure would most likely be
formed by a swarm of artificial habitats and mini-planets capable of intercepting
essentially all the radiant energy from the parent star.
According to Dyson (1966), the mass of a planet like Jupiter could be used to construct
an immense shell which could surround the central star, having a radius of one
Astronomical Unit (A.U.). The volume of such a sphere would be 4 pi r(squared) S, where r
is the radius of the sphere (1 A.U.) and S the thickness. He imagined a shell or layer of
rigidly built objects D ~10 to the 6 power kilometers in diameter arranged to move in
orbits around the star. The minimum number of objects required to form a complete
spherical shell is about N = 4 pi r(squared)/D(squared)~10 to the 5 power objects.
This kind of object [2], known as a "Dyson Sphere", would be a very powerful
source of infrared radiation. Dyson predicted the peak of the radiation at ten
micrometers.
FIGURE 1: In 1959, Freeman Dyson suggested that very advanced civilizations,
bound only by the presently known laws of physics, may surround their parent star with
spherical shells made from dismantled planets. A representation of this idea applied to
our solar system, using the mass of Jupiter to form a sphere at one astronomical unit (AU)
from the Sun, is shown in this figure. The Dyson Sphere has a radius of 1.5x10 to the 8
power kilometers and is 3 meters thick.
The Dyson Sphere is certainly a grand, far-reaching concept. There have been some
investigations that tried to find them in the IRAS database (V. I. Slysh, 1985; Jugaku and
Nishimura, 1991; and Kardashev and Zhuravlev, 1992).
OPTICAL (VISIBLE) RADIATION
In the optical (visible) domain, there have been several proposals to use the
visible region of the spectrum for interstellar communications.
2 The concept of this extraterrestrial construction was first described in the science
fiction novel STAR MAKER by Olaf Stapledon in 1937.
TABLE 4: HRMS Sensitivity for Earth's Most Powerful
Transmissions (Billingham and Tarter, 1992)
ARECIBO PLANETARY RADAR
(1) TARGETED SEARCH MAXIMUM RANGE (light years)
Unswitched
With CW detector 4217
With pulse detector 2371
Switched
With CW detector 94
With pulse detector 290
(2) SKY SURVEY
Unswitched
With CW detector 77
Switched
With CW detector 9
BMEWS
(1) TARGETED SEARCH
Pulse transmit CW detector 6
Pulse transmit pulse detector 19
(2) SKY SURVEY
Pulse transmit CW detector 0.7
Since the first proposal by Schwartz and Townes (1961),
intensive research has been performed on the possible use of lasers for interstellar
communication. Ross (1979) examined the great advantages of using short pulses in the
nanosecond regime at high energy per pulse at very low duty cycle. This proposal was
experimentally explored by Shvartsman (1987) and Beskin et al. (1993), using a
Multi-channel Analyzer of Nanosecond Intensity Alterations (MANIA), from the six-meter
telescope in Russia. This equipment allows photon arrival times to be determined with an
accuracy of 5x10 to the -8 power seconds, the dead time being 3x10 to the -7 power seconds
and the maximum intensity of the incoming photon flux is 2x10 to the 4 power
counts/seconds.
Other interesting proposals and analysis of the advantages of lasers for interstellar
communications have been performed by Betz (1986), Kingsley (1993), Ross (1980), and
Rather (1991).
The first international SETI in the Optical Spectrum (OSETI) Conference was organized
by Stuart Kingsley, under the sponsorship of The International Society for Optical
Engineering, at Los Angeles, California, in January of 1993.
There have also been independent suggestions by Drake and Shklovskii (Sagan and
Shklovskii, 1966) that the presence of a technical civilization could be announced by the
dumping of a short-lived isotope, one which would not ordinarily be expected in the local
stellar spectrum, into the atmosphere of a star. Drake suggested an atom with a strong,
resonant absorption line, which may scatter about 10 to the 8 power photons sec to the -1
power in the stellar radiation field. A photon at optical frequencies has an energy of
about 10 to the -12 power erg or 0.6 eV, so each atom will scatter about 10 to the -4
power erg sec to the -1 power in the resonance line. If we consider that the typical
spectral line width might be about 1 Angstrom and if we assume that a ten percent
absorption will be detectable, then this "artificial smog" will scatter about (1
Angstrom/5000 Angstrom)x10 to the -1 power = 2x10 to the -5 power of the total stellar
flux.
Sagan and Shklovskii (1966) considered that if the central star has a typical solar
flux of 4x10 to the 33 power erg sec to the -1 power, it must scatter about 8x10 to the 28
power erg sec to the -1 power for the line to be detected. Thus, the ETC would need (8x10
to the 28 power)/10 to the -4 power = 8x10 to the 32 power atoms. The mass of the hydrogen
atom (mH) is 1.66x10 to the -24 power g, so the mass of an atom of atomic weight (mu) is
approximately mu.mH grams.
Drake proposed the used of Technetium (Tc) for this purpose. This element is not found
on Earth and its presence is observed very weakly in the Sun, in part because it is
short-lived. Tc's most stable form decays radioactively within an average of twenty
thousand years. Thus, for the case of Tc, we need to distribute some 1.3x10 to the 11
power grams, or 1.3x10 to the 5 power tons, of this element into the stellar photosphere.
However, technetium lines have not been found in stars of solar spectral type, but rather
only in peculiar ones known as S stars. We must know more than we do about both normal and
peculiar stellar spectra before we can reasonably conclude that the presence of an unusual
atom in an stellar spectrum is a sign of extraterrestrial intelligence.
Whitmire and Wright (1980) considered the possible observational consequences of
galactic civilizations which utilize their local star as a repository for radioactive
fissile waste material. If a relatively small fraction of the nuclear sources present in
the crust of a terrestrial-type planet were processed via breeder reactors, the resulting
stellar spectrum would be selectively modified over geological time periods, provided that
the star has a sufficiently shallow outer convective zone. They have estimated that the
abundance anomalies resulting from the slow neutron fission of plutonium-239 and
uranium-233 could be duplicated (compared with the natural nucleosynthesis processes), if
this process takes place.
Since there are no known natural nucleosynthesis mechanisms that can qualitatively
duplicate the asymtotic fission abundances, the predicted observational characteristics
(if observed) could not easily be interpreted as a natural phenomenon. They have suggested
making a survey of A5-F2 stars for (1) an anomalous overabundance of the elements of
praseodymium and neodymium, (2) the presence, at any level, of technetium or plutonium,
and (3) an anomalously high ratio of barium to zirconium. Of course, if a candidate star
is identified, a more detailed spectral analysis could be performed and compared with the
predicted ratios.
Following the same kind of ideas, Philip Morrison discussed (Sullivan, 1964) converting
one's sun into a signaling light by placing a cloud of particles in orbit around it. The
cloud would cut enough light to make the sun appear to be flashing when seen from a
distance, so long as the viewer was close to the plane of the cloud orbit. Particles about
one micron in size, he thought, would be comparatively resistant to disruption. The mass
of the cloud would be comparable to that of a comet covering an area of the sky five
degrees wide, as seen from the sun.
FIGURE 2: Concept of an "artificial" blue straggler star according to
Reeves (1985). In this figure, a series of hydrogen bombs or powerful laser beams are
aimed at the surface of a star, creating a "hot point" and rejuvenating the
unused hydrogen, thus keeping the star on the Main Sequence for a longer period of time
than would be natural.
Every few months, the cloud would be shifted to constitute a slow
form of signaling, the changes perhaps designed to represent algebraic equations. Reeves
(1985) speculated on the origin of mysterious stars called blue stragglers. This class of
star was first identified by Sandage (1952). Since that time, no clear consensus upon
their origins has emerged. This is not, however, due to a paucity of theoretical models
being devised. Indeed, a wealth of explanations have been presented to explain the origins
of this star class. The essential characteristic of the blue stragglers is that they lie
on, or near, the Main Sequence, but at surface temperatures and luminosities higher than
those stars which define the cluster turnoff. A review of current thinking about these
stars in the light of recent visible and ultraviolet Hubble Space Telescope observations
assigns an explanation to stellar mergers occurring in the dense stellar environment of
globular clusters (Bailyn, 1994).
Reeves (1985) suggested the intervention of the inhabitants that depend on these stars
for light and heat. According to Reeves, these inhabitants could have found a way of
keeping the stellar cores well-mixed with hydrogen, thus delaying the Main Sequence
turn-off and the ultimately destructive, red giant phase.
Beech (1990) made a more detailed analysis of Reeves' hypothesis and suggested an
interesting list of mechanisms for mixing envelope material into the core of the star.
Some of them are as follows:
* Creating a "hot spot" between the stellar core and surface through the
detonation of a series of hydrogen bombs. This process may alternately be achieved by
aiming "a powerful, extremely concentrated laser beam" at the stellar surface.
* Enhanced stellar rotation and/or enhanced magnetic fields. Abt (1985) suggested from
his studies of blue stragglers that meridional mixing in rapidly rotating stars may
enhance their Main Sequence lifetime.
If some of these processes can be achieved, the Main Sequence lifetime may be greatly
extended by factors of ten or more. It is far too early to establish, however, whether all
the blue stragglers are the result of astroengineering activities.
ULTRAVIOLET RADIATION
Our planet Earth's atmosphere is opaque to radiation in the ultraviolet waveband due to
ozone and molecular absorption. As a result, astronomy in these wavelengths has to be
carried out from above the atmosphere, preferably with artificial satellites.
The band divides rather naturally into two regions. The region 300 > lambda > 120
nanometers can be studied using techniques similar to those used in optical astronomy. At
shorter wavelengths, however, it is difficult to find materials which reflect radiation at
normal incidence. Rather, the incident radiation is simply absorbed by the mirror material
with little or no reflection.
Perhaps the main problem for using this waveband as a detector for extraterrestrial
intelligence is that at wavelengths shorter than 912 Angstrom, the Lyman limit for
hydrogen, interstellar gas becomes opaque, probably due to photoelectric absorption by
neutral hydrogen in the Lyman continuum. This effect is particularly important for objects
localized inside our Milky Way galaxy.
In spite of all these limitations, in 1974, H. Wischnia used the facilities of the
Orbiting Astronomical Satellite (OAO, also called COPERNICUS) to search for ultraviolet
laser pulses around the nearby stars Epsilon Eridani, Tau Ceti, and Epsilon Indi (Tarter,
1991). The observations were carried out in ultraviolet wavelengths below 0.28
micrometers.
Of particular interest in this waveband are the Lyman series of absorption lines with
wavelengths of 1215 Angstrom, 1025 Angstrom, 972 Angstrom, 949 Angstrom, and 937 Angstrom,
corresponding to the various fully excited states of the hydrogen atom. The predominant
line is the so-called "Lyman Alpha" of 1215 Angstrom. It is quite possible that
an extraterrestrial civilization could choose to generate a signal which may lie close to,
or preferably right within, the absorption region. The background radiation of our Sun
(and presumably other solar-type stars) is down by a factor of 10 to the 6 power at Lyman
Alpha wavelengths when compared with the output of visible light. Such a reduction offers
a considerable improvement in the signal-to-noise ratio.
X-RAY RADIATION
Elliot (1973) pointed out that X-rays are not appropriate as a means of transmitting a
continuous stream of information because of their high quantum noise. As a means of
sending and receiving the "first beacon signal", however, he considered that
X-rays could have certain advantages. Elliot analyzed the X-ray emissions of the
terrestrial nuclear explosions carried out in the early 1960s. When a nuclear weapon
explodes, about seventy percent of the energy released is in the form of kilovolt X-rays.
This X-ray pulse is formed in less than one microsecond. If the explosion occurs above
eighty kilometers, the X-rays are not absorbed by the atmosphere and are free to propagate
into space.
Other advantages of using the X-ray pulse generated by a high-altitude nuclear
explosion are: (a) The pulse is short and will not be broadened or appreciably attenuated
by propagation through the interstellar medium; (b) there are no stringent frequency
requirements on the receiver, since the pulse covers a broad X-ray spectrum; and (c) the
X-ray flux involved is much larger than that of a solar-type star, a natural source of
X-rays.
Elliot estimated the distance at which the United States "Starfish" nuclear
test could be detected by our present technology of X-ray detectors. Assuming that the
energy of the explosion is equivalent to 1.4 megatons and that the X-ray pulse was equally
intense in all directions, he found that this explosion should be detected from a distance
of ~400 Astronomical Units, about ten times the radius of Pluto's solar orbit.
Supposing that all the terrestrial nuclear powers [3] pooled their nuclear weapons
stockpiles to produce a single explosion in space (E~2x10 to the 4 power megatons).
Considering that the X-ray pulse could be concentrated into a conical beam of about thirty
degrees in angle with no loss of radiation, a typical terrestrial X-ray detector should be
able to detect a signal from a distance of ~190 light years.
3 - In 1989 the United States and the Soviet Union had almost 55,000 nuclear warheads
with a combined destructive power of 15,500 megatons (Source: Bulletin of Atomic
Scientist, 1990).
PHOTO 1: The neutrino telescope of Raymond Davis, Jr. (University of
Pennsylvania), deep inside the Homestake Mine, South Dakota. The large vessel contains
100,00 gallons of a chlorine compound. Once in a great while, a neutrino from the Sun hits
a chlorine nucleus and converts it to argon, a gas detectable by its radioactivity.
Fabian (1977) suggested that a supercivilization (in the
transition from Kardashev's Type I to Type II) could -- in the same sense of the proposals
by Drake and Shklovskii -- drop material onto a neutron star. This material could, from
gravitational acceleration, reach the surface at about one-third of the velocity of light.
About ten percent of the rest mass energy of the matter involved in such an impact will
emerge as radiation, predominantly as X-rays if the mass flow rate exceeds ~10 to the 16
power f g m sec to the -1 power, where f is the fraction of the surface onto which matter
falls. Thus, to produce an X-ray flash as energetic as that suggested by Elliot (1973),
ten tons of matter of any composition would have to be dropped onto a neutron star.
According to Fabian (1977), such a flash will be broadband and almost omnidirectional,
although some beaming may occur if the neutron star possesses a strong magnetic field. The
above arguments show that an object of about one kilometer in size and 10 to the 16 power
gm dropped onto a neutron star could produce an X-ray pulse (E~10 to the 36 power ergs)
strong enough to be easily detectable throughout the Milky Way galaxy. Harwit and Salpeter
(1973) argue that the soft gamma-ray burst observed by Klebesadel et al. (1973) may be due
to comets falling onto the surface of a neutron star. If an advanced civilization could
cause individual objects that might be easily found in the neutron star's neighborhood to
fall onto the star in a pre-arranged pattern, this could be detected across the galaxy.
GAMMA-RAY RADIATION Viewing et al. (1977) pointed out that an alternative method for SETI
could be the detection of alien artifacts. Because most of the different models for
interstellar propulsion (Mallove and Matloff, 1989; Mauldin, 1992) produce copious
quantities of gamma rays, they proposed the search for this kind of radiation from the
antimatter annihilation process. Harris (1986) showed that interstellar spacecraft as
gamma-ray sources will be most easily recognized by their proper motions. Their
velocities, being substantial fractions of the velocity of light, will be at least one
hundred times the highest velocities characteristic of normal astronomical objects. No
steady gamma-ray source is known with a proper motion as large as one degree per year. For
spacecraft of sufficient size, the poor angular resolution of current gamma-ray telescopes
imposes a limit on identification rather than detection. Of the Compton Gamma-Ray
Observatory (GRO) instruments, EGRET has an angular resolution of 1.6 degrees at 100 MeV.
The best possible resolution of BATSE is 0.5 degrees (Kurfess, 1985, Morris, 1985). In the
most favorable circumstances -- purely tangential velocity close to c; observations made
over the ten-year lifetime of GRO -- these resolutions will be able to detect the motion
of spacecraft at distances of ~100 parsecs and ~300 parsecs, respectively. The distinctive
line emission from these objects is readily detectable if the spacecraft is sufficiently
massive. A consumption of ~(1 to 10)R(squared) tons per second of antimatter is necessary
for detection by GRO of a spacecraft at a distance R (in parsec) (Harris, 1991b).
Harris (1991a) reported the negative results of a search for the linear alignments of
burst along a potential spacecraft trajectory, using burst locations and spectra observed
by the 1978-1980 interplanetary network of gamma-ray detectors.
Improvements in the measurement of gamma-ray source positions may be expected from two
directions. For steady sources and very bright transients, larger detectors will achieve
improved signal-to-noise ratios and hence better source positions for a given angular
resolution. However, the positions of most of the gamma-ray bursts are measured more
accurately from the event delay times across interplanetary distances (Harris, 1991b).
Progress in this field is dependent on future space mission launch schedules.
PARTICLE ASTRONOMY
ATOMIC MICROSCOPIC PARTICLES
Another technique of astronomical investigation relies on the
observation of microscopic particles which stream toward Earth in huge numbers from
different directions. They include elementary particles such as electrons, protons, and
heavier and complex atomic particles. The more energetic of these particles are frequently
referred to as cosmic rays. Another fundamentalnuclear particle that could be use as an
"information carrier" is the neutrino. NEUTRINOS
A neutrino is a weakly interacting particle that travels at essentially the speed of
the light and has an intrinsic angular momentum of 1/2 (h/4pi). Neutrinos are
produced on Earth by natural radioactivity, nuclear reactors, and high-energy
accelerators. There are six types of neutrinos and three flavors of neutrinos and
anti-neutrinos. Each are associated with a massive lepton that experiences weak,
electromagnetic, and gravitational forces, but not strong interactions. The known leptons
are electrons, muons, and taus (in increasing order of their rest masses).
Because neutrinos interact only weakly with matter, they can reach us from otherwise
inaccessible regions where photons are trapped. Hence, with neutrinos, we can look inside
stars and examine directly energetic physical processes that occur only in stellar
interiors. Large detectors, consisting typically of hundreds or thousands of tons of
material, are required to observe "astronomical neutrinos".
The neutrino burst detected from Supernova 1987A confirmed the basic theory of core
collapse and refined our views of neutrino properties. Twenty neutrinos in the energy
range from 6 to 39 MeV were detected over ~12 seconds (Chevalier, 1992). Given the
distance to the Large Magellanic Cloud galaxy and the sensitivity of the instruments, the
results were consistent with an emission of about 10 to the 58 power neutrinos in the
supernova. To have a real idea about the difficulties in the detection of this particle,
it was estimated that during those ~12 seconds, more than 10 to the 11 power neutrinos
passed through each square centimeter of Earth. Our technology was only capable of
detecting twenty of them.
There have been several proposals to use collimated neutrino beams in the energy range
from 1 to 100 GeV as a potential means for telecommunication over global distances of 10
to the 3 and 4 power kilometers (Saenz et al. 1977, Ueberall et al. 1979). Similar
proposals for telecommunication for not only global but also interstellar and
intergalactic distances were investigated by Subotowicz (1979) and Pasachoff and Kutner
(1979).
The main difficulty of all these ideas is the low cross-section of interaction of the
neutrino and the very low detection efficiency. Subotowicz (1979) suggested that some
advanced civilizations may deliberately shut out emergent civilizations such as ours from
the conversation. Generation and detection of neutrinos is so difficult that an advanced
civilization may purposely choose such a system in order to find and communicate only with
their own level of development.
ELECTRONS, PROTONS, AND ATOMIC NUCLEI (COSMIC RAYS)
Highly energetic protons, neutrons, electrons, positrons, and a wide variety of heavier
nucleons constitute the cloud of galactic cosmic ray particles. Anti-nucleons consisting
of antimatter have been sought: During the 1980s, anti-protons were found. Unstable
sub-nuclear particles are not expected to figure significantly as carriers of cosmic
information. Their short lifetimes preclude travel over long distances in the Universe,
unless the most extreme energies are assumed.
The energy range of galactic, or possible extragalactic, cosmic ray particles that we
can hope to use as carriers of information lies between 10 to the 11 power eV and 10 to
the 21 power eV. However, most of the stable cosmic rays are atomic nuclei and are highly
charged, therefore studies at high angular resolution, high spectral (energy) resolution,
or high time resolution are not likely to yield a great deal of "information".
Due to the electromagnetic (Lorentz force) interaction of such highly charged moving
particles with the Galactic Magnetic fields, all "memory" of where and how the
particles originated and the time of origin is lost, including the initial energy of the
particle.
This kind of information carrier presents several disadvantages for interstellar
communication. Due to the electric charge of electrons and protons and their interaction
with the magnetic interstellar fields (~5x10 to the -6 power Gauss), a Lorentz force will
act on both particles, but with opposite direction. The relevant relativistic equations
are V/c = ((1-(M to the 2 power c to the 4 power))/E to the 2 power)1/2 and R = (MV/QB(1-V
to the 2 power/c to the 2 power)1/2)sin alpha, where c is the speed of light, V is the
particle velocity, M is the particle mass, Q is charge, B is the magnetic flux density, E
is the particle energy, R is the curvature radius, and alpha is the angle between the
galactic field and the direction of the particle beam.
Assuming the beam is aimed properly, it will curve around and just barely reach the
target if R is one-half the distance thereto (Freitas, 1977). If we send a message via
high energy protons across ten light years, E must be at least 1.4x10 to the 16 power eV
if alpha equals ninety degrees. Of course this assumes a nice uniform B field all the way,
a dubious proposition at best. In view of the galactic deflection problem, it is difficult
to see how charged particle beams could represent a communication mode for advanced
extraterrestrial civilizations.
MACROSCOPIC OBJECTS
The planet Earth is incessantly bombarded by matter whose information content is far
from negligible. For example, the stream of extraterrestrial matter gives direct
information about the abundance of the elements in the sites where the matter was
produced, which mainly involves the history of the solar system. Meteorites are one such
group of objects which can be handled and subjected to laboratory tests and analyzes.
Meteoric dust brought to Earth by rain or snow can be studied in the same way. The
difference here is that these particles or objects can be examined and re-examined in a
leisurely way. They are not so transient as individual photons or fermions.
There have been some speculations that a simple biological system carrying a message
and capable of self-replication in suitable environments may be one possible channel for
interstellar communication (Yokoo and Oshima, 1979, and Nakamura, 1986). These kinds of
ideas have several and severe objections. For example, the impossibility of predicting the
environment of the target star in order to favor the self-replication of the molecular
structure, the impossibility of avoiding the destruction of the content of the message by
molecular mutations, and the impossibility for us to distinguish between a
"natural" organism and a real biological interstellar message.
DIRECT TECHNIQUES
With the development of the Space Age, the way has been opened for a more direct
engineered approach to many phases of extraterrestrial exploration using space probes.
Such probes can transport sensing devices for direct measurements at distant locations and
telemeter their findings back to Earth. The human being's capacity for observation and
interpretation of diverse and unexpected phenomena make manned exploration the ultimate
goal.
In the past thirty years, there have been several proposals of rapid space probes (v
> 0.1 c) for interstellar travel purposes, including pulsed fusion and
antimatter-powered rockets, laser-pushed light sails, and interstellar ramjets (Mallove
and Matloff, 1989; Mauldin, 1992). The scale of the undertaking, from both a technological
and economic perspective, is such that they are unlikely to be realized for at least a
century (Crawford, 1990).
The most ignored factor in discussing interstellar flight is the kinetic energy that
must be invested in the ship to make its tons of matter move at a substantial fraction of
the speed of light (Oliver, 1981; Purcell, 1961; von Hoerner, 1962). This turns out to be
the dominant energy requirement and is thus a useful lower bound on the total. If, in
assessing the cost of interstellar travel, we find this lower bound too big, all the other
costs will be negligible.
Bracewell (1960, 1975) and Freitas (1980a) have discussed the possible superiority of
interstellar probes in missions of galactic exploration. Freitas (1980b) and Valdes and
Freitas (1980) have raised the issue of self-organized machines in related context. The
non-detectability of the proposed probes has been used as an argument to establish that we
are the only technological civilization in the entire Milky Way galaxy (Tipler, 1980).
Under the assumption that an extraterrestrial probe will be interested in life in our
solar system, a near-Earth search was carried out for these kinds of artificial objects by
Freitas and Valdes (1980, 1983, 1985). Other proposals to search for extraterrestrial
artifacts in our neighborhood were proposed by Papagiannis (1978, 1985).
CONCLUDING REMARKS
At present the human race is limited in its experience of sentient life to the beings
of a single planet in one solar system out of potentially billions in the galaxy. We must
therefore be aware and open to the numerous ways in which advanced civilizations across
the Milky Way and beyond may make themselves known to others in the galaxy, both directly
and as the extraneous results of their technological activities. As we have just seen,
there is a wide range of photon and particle phenomena potentially available for direct or
indirect communication among technological civilizations.
Direct interstellar contact methods may range from radio and optical beacons or other
transmissions to interactions with robot or crewed star vessels. Indirect (extraneous)
means of contact may consist of similar methods, such as audiovisual broadcasts
"leaking" from a planet into space, much as Earth has been doing for most of
this century. We may also come upon the artificial "noise" from cultures
conducting extensive activities in their solar systems. This activity may be anything from
initial planetary exploration and colonization to the reconfiguring of entire star systems
to suit the needs of their inhabitants.
Alien civilizations could be as varied as the means of interstellar communication.
Humanity is just emerging into the galaxy, having barely begun to utilize the resources of
our solar system and the incredible potentials of space travel. Other civilizations at a
more advanced stage of technology may have turned their entire planetary system into an
immense Dyson Sphere around their sun to capture every photon of solar energy. Even more
advanced life forms may control a whole galaxy of star systems or groups of galaxies using
technologies almost beyond our current comprehension.
It is quite conceivable that there are types of communication between the stars for
which we are completely unaware of. Extra-terrestrial beings could be signaling us --
still using the fundamental forms of radiation we have summarized here -- with encoding
and symbols which we can neither understand nor respond to. We could be witnessing
activities and messages which we do not recognize as artificial due to our limited
experience and knowledge.
Our ignorance of what dwells in the Universe should compel the human race to make even
more extensive celestial explorations with as many techniques at our disposal as possible.
If there are civilizations amongst the stars of the Milky Way, then it may be only a
matter of time before we find them, if we have the patience and skill to search.
FIGURE 3: Nakamura (1986) examined the DNA structure
of the SV40 virus. In (a) is shown a part of the genetic structure that the author
considered to be a star map. In (b) is a representation of the map of the constellation
Eridani.
FIGURE 4: Scheme for von Neumann's machines. The
`universal constructor' A; when given a program In for constructing other machine N, A
will read through that program, compute upon it, piece together components from its
environment, and construct a copy of the machine N, as shown in the top left of the
figure. In the bottom row is shown the self-reproducing automaton sequence, in which A is
augmented by two new subsystems, B and C.
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Note from SAK:
In this paper, Dr. Lemarchand uses the old definition of the word "optical" as a
synonym with the word "visible". In several places, I have added the word
"visible" in brackets to remind readers that this web site and author use the
modern definition of the word as a superset to cover the far-infrared to the ultraviolet
region of the electromagnetic spectrum. There is only one type of laser-based SETI
and that is "Optical SETI". No distinction is made between ultraviolet
lasers, visible lasers or infrared lasers. See modern
definition of "Optical SETI". Just as the "l" in lasers
stands for "light", no distinction is made between the wavelength of light with
respect to the word "optical". Today, the world of photonics or
optoelectronics encompasses these entire bands of electromagnetic radiation.
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