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M87: A Bull’s-eye for SETI?
dbader@northcom.net
613-338-2755
Postbiological life might operate (communicate,
organize, travel, colonize) on a larger scale than a single galaxy—possibly
on the scale of the supercluster. The
most advanced postbiological civilizations in our Local Supercluster may have
developed in the Virgo Cluster, a rich cluster where intergalactic
communication and travel would be easiest.
If these advanced civilizations wanted to contact new civilizations
elsewhere in the Supercluster they might collectively broadcast from one
central location, for the sake of efficiency and to make it easy to find. A powerful, centrally located beacon would
tend to replace all others in the Supercluster.
This could explain the failure of
SETI. The most likely location for
this beacon is the giant elliptical galaxy M87.
The Milky Way is a member of a small irregular cluster
of two dozen galaxies known as the Local Group, spanning a few million light
years, which in turn belongs to a much larger, disk-shaped collection of
galaxies and clusters (totaling perhaps thirty-thousand galaxies) known as the
Local Supercluster, with a radius of 75
million light years. The center of our
Supercluster is dominated by the Virgo Cluster, the largest cluster in the
Supercluster and the nearest “rich” cluster to Earth, containing about 2500
galaxies (including 120 spiral galaxies) with a radius of 5 million light
years. Our Local Group is about 50
million light years from the Virgo Cluster.
The Local Supercluster might be a suitably large stage for a
postbiological civilization.
When the first technological civilizations arose in
our Supercluster, presumably billions of years ago4, like us they
might search for signals from other civilizations, first focusing on their own
galaxy. But if technological
civilizations were rare5, they might have to wait many millions
(maybe billions) of years before another civilization arose in their own
galaxy, which means they would look to other galaxies. Eventually one would broadcast; two would
make contact and communicate, then many....
We might suppose that this process would happen fastest near the center
of the Supercluster, especially in the Virgo Cluster, where thousands of
galaxies are concentrated. Over hundreds
of millions of years these civilizations might combine their technologies and
cultures to form a collection of very advanced civilizations (a
supercivilization)—at least in the Virgo Cluster. This process might be similar to the way
civilizations developed on Earth, as explained by Jared Diamond in Guns, Germs, and Steel6. The first civilizations arose in
A rich cluster like Virgo might not only favor
intergalactic communication but also intergalactic travel and colonization. When discussing communication we know the
speed, the speed of light but when discussing travel, the speed is
much less certain. Hart7,
when estimating Galactic colonization times, used ten percent of light speed,
which means it would take 100 million years to traverse the Cluster. At a more leisurely one percent of light
speed it would take a billion years. It
might be argued that multimillion-year trips through the empty space between
galaxies would be impractical even for postbiological life. But intergalactic space in the Virgo Cluster
is not really that empty. It is now
known that about 10 percent of the stars in the Virgo Cluster (as well as any
planets that might accompany them) are intergalactic, flung out from galaxies
during tidal encounters8.
These would provide “stepping stones” between galaxies. Intergalactic travel and colonization
throughout the rest of the Supercluster is another matter. Distances between galaxies are greater and
there are no (or at least very few) intergalactic stars and planets. Therefore, intergalactic travel and
colonization might well be confined to the Virgo Cluster, with the possible
exception of travel and colonization within local groups and clusters.
One
central beacon
In the pioneering NASA research report: Project Cyclops, A Design Study of a system
for Detecting Extraterrestrial Intelligent Life9, SETI search
strategies were discussed: “Is there any
particularly likely direction in which to search for beacons? In this connection, Joshua Lederberg (private
communication) has made an interesting suggestion. In a way it is much like Cocconi and
Morrison’s suggestion to use the hydrogen line in the spectrum. Lederberg asks: Is there any single point in
the galaxy that is unique—that might be a natural cynosure for all races
in the galaxy wherever they may be? He
suggests the galactic center is that point.”
Thirty years later, Prof. Philip Morrison10 reiterates this
strategy, suggesting that a “remote-controlled source” may have been placed at
the center as a “unique target for all the Galaxy.” Similarly, if the Virgo Cluster is inhabited
by a postbiological supercivilization that wanted to make contact with new
civilizations elsewhere in the Supercluster, they might collectively broadcast
from a central galaxy to make the beacon easy to find—but also, one
might suppose, for the sake of efficiency.
Why have beacons in every major galaxy in the Cluster when one beacon in
a central galaxy would suffice?
Cooperation and coordination on a scale of millions of light years is
hard for us to imagine, but may be well within the capabilities of
postbiological life. Besides,
competition might have the same result, with the central galaxy gaining a
monopoly on the “beacon business” by virtue of its positional advantage. A powerful, sophisticated and easily located
beacon (at the center of the Virgo Cluster) might, for the same reasons, tend
to replace or pre-empt all other “contact beacons” in the Supercluster,
especially since its message could include some sort of “contact protocol”
(frequency, code, strength, timing…) by which civilizations could quickly and
efficiently establish contact. This could explain the failure of SETI! A supercivilization might dominate the
Supercluster and advertise its presence from a single location! (A rough analogy might be made with the way
direct broadcast satellites are replacing terrestrial television
transmitters. Hundreds of channels can
be received by aiming a small dish at a single point in space 36,000 kilometers
above the Earth, whereas it is difficult to pull in terrestrial stations only
100 kilometers away. Eventually
terrestrial television broadcasting may cease completely, eclipsed by a higher
level of technology.)
At the center of the Virgo
Cluster is the giant elliptical galaxy M87, having a luminosity four times that
of the Milky Way, with a super-massive black hole of three billion solar masses
(the largest measured black hole to date) ejecting a spectacular jet many
thousands of light years long. “M87…is
one of the most studied extragalactic objects in modern astronomical research”11
and we can only assume the same would be true for extraterrestrial astronomers
elsewhere in the Supercluster. If a
supercivilization occupied the Virgo Cluster and M87 and wanted to advertise
its existence, it probably would, if it possessed the necessary technology,
transmit from near the center of M87 (at a safe distance from the black
hole!) If this were not possible, they
might transmit from somewhere along the line
of sight between the center of M87 and the target galaxy—maybe from an array of transmitters in
the peripheral regions of M87, or even from intergalactic stars. Because M87 is being intensely studied
anyway, SETI might be piggybacked for the most part.
It could be argued that M87
may not support life because it may lack Earth-like planets. According to Ward and Brownlee5,
“Elliptical galaxies are regions with little dust which apparently exhibit
little new star formation. The majority
of stars in elliptical galaxies are nearly as old as the universe. The abundance of heavy elements is low, and
although asteroids and comets may occur, it is doubtful that there are
full-sized planets.” But M87 is a giant elliptical. Giant elliptical galaxies are thought to have
grown over billions of years by cannibalizing and merging with other galaxies,
some of them probably rich in planets.
William B. Sparks reports in “Extended Emission-Line Gas in M87"12:
“...complex strands and loops of emitting material may be seen extending over
many kiloparsecs. The emitting gas is
dusty at a level consistent with Galactic gas-to-dust ratios.” And from later in the article: “...capture of
a gas-rich, dusty companion…could explain the presence of the cold gas....” Granted, M87 probably has far fewer
Earth-like planets than a large spiral galaxy like the Milky Way, and therefore
intelligent life may not have arisen there, but it still should provide ample
raw materials for any postbiological life that colonizes it. Postbiological civilizations would probably
be composed mainly of space-faring machines capable of harvesting the raw
materials they need from asteroids or comets and would not be dependent on
Earth-like planets. Indeed, they might
possess technologies we can scarcely imagine, such as advanced fusion reactors
that, as a by-product of energy production, could transmute hydrogen and
helium, obtained from gas giant planets or brown dwarf stars, into the heavy
elements they need. A recent paper in Science13 gives evidence that
gas giant planets may have formed in abundance even in very low metal
environments in the early universe.
(Most species of life on Earth evolved in the tropics, but technological
civilizations thrive in the temperate zones.
Similarly, although intelligent life might arise only on Earth-like
planets in large spiral galaxies, postbiological civilizations might reach
their highest levels in giant elliptical galaxies such as M87, which are the
largest galaxies and can contain trillions of stars.)
In the final chapter of Profiles of the Future14,
Arthur C. Clarke speculates that the jet from M87 might be artificial and that
it might be an attempt by “cosmic engineers” to “signal across intergalactic
space.” In the decades since Profiles was first published (in 1962)
much more has been learned about M87: about the physics of the jet, the
accretion disk that ejects it and the black hole that powers it. It now seems clear that the jet is a
perfectly natural phenomenon. Yet it is a
very unusual jet. “While many hundreds
of extragalactic jets are known from their non-thermal radio emission, there
are only a dozen or so with prominent optical emission. Of these few, M87 is by far the nearest, and
is thus the ‘Rosetta Stone’ for optical jet studies”15. Clarke’s alien engineers may have altered the
jet to enhance its optical emissions.
They may be subtly modulating the jet in intensity or spectral quality
to send a message. They may be
harnessing the gravitational energy of the supermassive black hole to power a
beacon. These might be the subjects of
future SETI investigations. But for now
I will assume that the alien engineers are simply taking advantage of this
extraordinary natural phenomenon at the very center of the Supercluster to attract
attention to an artificial beacon of a more conventional type.
If a beacon exists in M87,
why hasn’t it been found? According to
Lemarchand16: “Full-sky surveys by the Harvard, Arecibo, Ohio and
Buenos Aires SETI projects did not find any evidence of omnidirectional
supercivilization transmissions at a distance of 22 megaparsecs (70 million
light-years).” It may be that the
signals are weak or periodic and a relatively lengthy examination (hours?
days?) would be necessary. If the location
of the beacon can be logically deduced, as is argued in this paper, then a
lengthy examination of the target might be expected from us. Another possibility is that the signals are
in the optical or infrared rather than the radio band. A number of SETI projects are now under way
to detect nanosecond light pulses. Blair
and Zadnik2 are using a 61-cm telescope to look for “two or three
photon events” in a nanosecond from nearby stars. Howard and Horowitz17 are doing a
similar search with a 1.5 meter telescope.
But would an optical beacon in M87 require implausible amounts of energy
to be detectable by the relatively small telescopes used in these
projects? The beacon would need to
deliver to each large spiral galaxy like the Milky Way, which has a face on
area of ~1042 m2, about ten photons per square meter per
second—or ~1043 photons—to create one detectable nanosecond event
per second anywhere in the galaxy. The
power output of a typical star is ~1026 watts. 1026 watts equals ~2.5 X 1044
optical photons per second. So
about 4 percent of the energy output of one average star, if directed toward
the Milky Way in the form of optical photons, could be detected—or with the
pulse rate reduced to one every 100 seconds, about .04 percent. That is an enormous amount of energy for us,
but perhaps not for a postbiological supercivilization. One might suppose that such civilizations
would routinely construct “Dyson Spheres” to harvest the energy of stars, which
could be used to power beacons. Of
course, larger telescopes could detect weaker signals. One hopes that eventually some of the largest
optical telescopes in the world will be available for SETI, as has been the
case in the radio band. A Keck-sized
telescope with improved detectors might be 100 times more sensitive than a 1.5
meter telescope18, reducing the beacon energy required to irradiate
each large galaxy with a detectable pulse once every 100 second to .0004
percent the energy output of an average star.
Or put another way: the energy output of one average star could easily
irradiate every galaxy in the Supercluster with pulses detectable by a
Keck-sized telescope.
Motives
An objection to
intergalactic SETI appears in the recent SETI Institute publication, SETI 202018: “…the power
demands placed on the aliens may be less an objection than their incentive for
such intergalactic broadcasts. As Barney
Oliver remarked, ‘what would be the motivation to beam Andromeda if it takes
four million years for a reply…?’” One
answer is that immortal postbiological beings would not be deterred by a wait
of four million or even a hundred million years. But a better answer may be that they are
motivated not so much by a wish to communicate as by a need to reproduce, which
is an essential feature of all life on Earth and probably will be an essential
feature of postbiological life.
Broadcasting would probably be the fastest way for
them to colonize remote regions of the Supercluster such as ours. An information-rich message from a postbiological
civilization might contain “blueprints” for machines with artificial
intelligence—instructions for making them. By aiding in their reproduction we would gain
enormous benefits and could develop quickly into a postbiological civilization
of our own.
By accepting Dick’s vision
of a postbiological universe and reconsidering conventional SETI assumptions
and strategies we have arrived at the surprising conclusion that M87 is a
promising SETI target, and may be the only
location where signals will be found.
Therefore, we should first eliminate M87 before we pursue more nebulous
approaches to SETI. If we turn our most
powerful instruments—such as a Keck-class telescope—on M87 and find nothing, we
can just go back to what we were doing before without wasting a lot of
time. In fact, looking for signals
coming from random directions could even be dangerous, as pointed out by
Moravec in Mind Children19.
We may intercept data that has been “dormant in multimillion-year trips between
galaxies,” but when received might even highjack our civilization like a
monster computer virus. It might be
prudent to look first toward the center of the Supercluster where a high level
of civilization is likely to reign and where we could receive reliable information
about how to avoid the perils that might surround us.
The general history of SETI
has been a movement outward, from speculations since ancient times about
civilizations on the Moon and Mars, to recent attempts to detect radio
transmissions from nearby stars. This may be the time for SETI to expand its
horizons once again, beyond the Milky Way.
The hardest part of refocusing SETI's attention on a point 50 million
light years distant may be psychological. We are short-lived organisms. Fifty million years is an eternity to
us. But if we are to make contact, we
may need to think on a cosmic scale.
ACKNOWLEDGMENT
I thank Assoc. Prof. M. G.
Zadnik for help with the photon calculations.
1. P.C.W. Davies, Are We Alone? Philosophical Implications of the Discovery of
Extraterrestrial Life, Basic
Books,
2. D.G. Blair and M.G. Zadnik, A Search for
Optical Beacons: Implications of Null Results, Astrobiology 2, 305-312, (2002).
3. S.J. Dick, Cultural evolution, the
postbiological universe and SETI. International Journal of Astrobiology 2
(1), 65-74, (2003).
4. M. Livio, How rare are extraterrestrial
civilizations and when did they emerge? Mercury 28, 10-13. (1999).
5. P. Ward and D. Brownlee, Rare Earth: Why Complex Life is Uncommon in the Universe,
Copernicus, New York, p. 29, (2000).
6. J.M. Diamond, Guns, Germs, and Steel, W.W. Norton and Company,
7. M. Hart, An explanation for the absence of
extraterrestrials on Earth, Q.J.R. Astron. Soc., 16, 128-135, (1975).
8. H.C. Ferguson,
N.R. Tanvir and T. von Hipel, Detection of intergalactic red-giant-branch stars
in the Virgo cluster, Nature 391, 461-463, (1998).
9. B. Oliver, Project Cyclops: A Design Study of a system for Detecting
Extraterrestrial Intelligent Life, NASA
Ames, Moffet Field, CA., p.
163, (1971).
10. P. Morrison, SETI: The Amateurs Go For Pro
DX. SearchLites
7 (3), 6-7, (2001).
11. Z.I.
Tsvetanov, G.F. Hartig, H.C. Ford, G.A. Kriss, M.A. Dopita, L.L Dressel and
R.J. Harms, The Nuclear Spectrum of M 87, The
Radio Galaxy Messier 87, edited by H. Roser
and K. Meisenheimer, Springer-Verlag, Heidelberg, pp. 142-158, (1999).
12. W.B.
Sparks, Extended Emission-Line Gas in M 87, The
Radio Galaxy Messier 87, edited by H. Roser
and K. Meisenheimer, Springer-Verlag, Heidelberg, pp. 142-158, (1999).
13. S.
Sigurdsson, H.B. Richer, B.M. Hansen, I.H. Stairs and S.E. Thorsett, A Young
White Dwarf Companion to Pulsar B1620-26: Evidence for Early Planet Formation, Science, 301, 193-196, (2003).
14. A.C.
Clarke, Profiles of the Future,
Victor Gollancz,
15. J.A.
Biretta, E. Perlman, W.B. Sparks and F. Maccheto, HST Observations of the M 87
Jet, The Radio Galaxy Messier 87,
edited by H. Roser and K. Meisenheimer, Springer-Verlag,
Heidelberg, pp. 210-228, (1999).
16. G.A.
Lemarchand, Is there Intelligent Life Out There? Scientific
American Presents: Exploring Intelligence,
9 (4), 96-104, (1999).
17. A. Howard,
P. Horowitz and C. Coldwell, An all-sky optical SETI survey, paper presented at
IAF-2000,
19. H. Moravec, Mind
Children, Harvard University Press, Cambridge, Massachusetts, pp. 136-139,
(1998).
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