The Halos of Vanished Civilizations -- Revised, Updated, and Expanded
It has become a commonplace of contemporary thought that, if there are other civilizations in our universe, these civilizations are likely very old—millions of years old or even billions of years old. Carl Sagan often wrote of million year old civilizations in the universe. In chapter 12 of his Cosmos Sagan wrote:
“What does it mean for a civilization to be a million years old? We have had radio telescopes and spaceships for a few decades; our technical civilization is a few hundred years old, scientific ideas of a modern cast a few thousand, civilization in general a few tens of thousands of years; human beings evolved on this planet only a few million years ago. At anything like our present rate of technical progress, an advanced civilization millions of years old is as much beyond us as we are beyond a bush baby or a macaque. Would we even recognize its presence? Would a society a million years in advance of us be interested in colonization or interstellar spaceflight? People have a finite lifespan for a reason. Enormous progress in the biological and medical sciences might uncover that reason and lead to suitable remedies. Could it be that we are so interested in spaceflight because it is a way of perpetuating ourselves beyond our own lifetimes? Might a civilization composed of essentially immortal beings consider interstellar exploration fundamentally childish? It may be that we have not been visited because the stars are strewn abundantly in the expanse of space, so that before a nearby civilization arrives, it has altered its exploratory motivations or evolved into forms indetectable to us.”
Ray P. Norris sought to quantify Sagan’s speculations in a paper titled, “How old is ET?” in which he explicitly argued that any extraterrestrial civilization we might discover through SETI would be millions or billions of years old. Norris concluded his paper:
“Conventional models imply that supernovae and gamma-ray-bursters will extinguish life on planets at intervals of about 200 Myr. Since this has not happened on Earth, either these conventional models are wrong, or else life on Earth is probably unique in the Galaxy. The first case predicts a median age of ET as being of the order of 1 billion years. The second case predicts that we will never detect ET. Thus, if we detect ET, the median age is of order 1 billion years. Note that, in this case, the probability of ET being less than one million years older than us is less than 1 part in 1000. Therefore, any successful SETI detection will have detected a civilisation almost certainly at least a million years older than ours, and more probably of order a billion years older.”
This estimate may have to be revised, however. Recent research published on a particular kind of gamma ray burst, the long duration gamma ray burst or LGRB, published in the paper, “The Relative Rate of LGRB (long duration gamma bursts) Formation as a Function of Metallicity” by J. F. Graham and A. S. Fruchter (a paper brought to my attention by Paul Carr), suggests that, as the authors put it, LGRBs “are preferentially formed in low-metallicity environments,” which implies that, as the universe ages and metallicity gradually increases through stellar nucleosynthesis and supernovae, LGRBs decrease in frequency. This in turn implies that as LGRBs decline over time with the rise of metallicity, fewer “sterilization” events occur in the history of the universe, thus shifting the likelihood of life, and its expansion through the universe, to later in the history of the universe. In other words, the conditions for life and for civilization may improve as the universe ages, which may require a recalculation of the assumptions made in Ray Norris’ paper on the longevity of ET.
Of course, the astrophysics of the early universe is an area of active scientific research, and other recent discoveries are also relevant here. Even if gamma ray bursts are tied to metallicity, metallicity may increase more rapidly than previously thought. A recent paper, “A Uniform Contribution of Core-Collapse and Type Ia Supernovae to the Chemical Enrichment Pattern in the Outskirts of the Virgo Cluster,” by Simionescu, Werner, Urban, Allen, Ichinohe, and Zhuravleva suggests that this may be the case (also cf. the pre-print, and the popular exposition by Bruce Dorminey, “Cosmos Was Ready For Life Almost From The Start, Japanese X-Ray Telescope Confirms”). There is hardly a day that passes that some new scientific discovery does not force us to reconsider our conception of the universe. Suffice it to say, for the moment, that our views must be subject to revision.
Perhaps as a countervailing influence to the commonplace of million-year-old supercivilizations inhabiting the cosmos, such civilizations being part of a Encyclopedia Galactica to which we, too, will be able to gain access once we have the technology to join the galactic club of advanced civilizations, is the equally commonplace idea that advanced civilizations destroy themselves not long after having attained technological maturity, which gives them the technological wherewithal to preemptively bring an end to themselves. This also was an idea that Carl Sagan mentioned on many occasions, which seems to prove the famous line from F. Scott Fitzgerald that, “The test of a first-rate intelligence is the ability to hold two opposing ideas in mind at the same time and still retain the ability to function.”
The idea of self-destructive advanced civilizations has been given an interesting twist in a recent paper by Adam Stevens, Duncan Forgan, and Jack O’Malley James, titled “Observational Signatures of Self Destructive Civilisations,” which seeks to take up the last term of the Drake equation – L, or the longevity of technical civilizations – by specifically looking for signatures of civilizations that have destroyed themselves. What kind of traces would vanished industrial-technological civilizations leave in the universe? I find this to be a very innovative approach to the question of seeking signatures for ETI.
The authors of this latter paper consider four scenarios that could result in the complete extinction of an advanced civilization:
i) complete nuclear, mutually assured destruction ii) a biological or chemical agent designed to kill either only the intelligent species, all multi-cellular species in a given biosphere, all eukaryotes, or all living things without exception iii) a technological disaster such as the “grey goo” scenario, or iv) excessive pollution of the star, planet or interplanetary environment
Taking up each of these scenarios in turn, the authors inquire as to how gamma ray detection, transit spectroscopy, photometry, asteroseismology, stellar abundance studies, and disk debris imaging could reveal signatures of these civilization-ending events.
In addition to traces of destructive events noted in this paper, the end of an advanced technical civilization would leave other relics, and one of these relics would be the particular structure of the electromagnetic signals generated by the civilization, which would begin with those signals most easily produced by early radio technologies, move on to signals of greater complexity and sophistication, and then the signals would cease. I have called such structures of EM signatures The Halos of Vanished Civilizations, and it is possible that if technical civilizations are common, or relatively common, but also short-lived, the signals that SETI researchers are seeking would have the structure not of some beacon aimed at us, but of a number of overlapping bubbles, and we could ourselves be inside a bubble (or halo) that we cannot detect at present, since we would be inhabiting the void at the center of such a halo.
An industrial-technological civilization that masters electromagnetic spectrum communications — i.e., ordinary radio and television signals at first, followed by more sophisticated technologies later, such as microwave radar — generates an expanding globe of EM spectrum signals as long as the civilization in question is transmitting these signals. If an industrial-technological civilization that has been transmitting EM signals comes to an end, these signals will cease to be generated, and the expanding globe of EM signals will taper off to silence at the interior of this globe, which means that there will be an expanding sphere of weakening EM signals—an expanding bubble of EM radiation, hollow at the center, like a three dimensional halo. The universe might contain these ghostly structures as a sequence of overlapping bubbles of EM radiation that describe the past structure of industrial-technological civilization in the universe.
While such signals would be very faint, and largely lost in the background radio noise of the universe, we cannot discount the possibility that advanced detection technology of the future might reveal such EM structures. It is important to appreciate that our scientific knowledge advances not only as a result of new and improved scientific instruments of greater precision and sensitivity, but also more sophisticated methods of research, and new ways of teasing out a narrative from extant data. We cannot exclude the possibility that innovative methods of scientific investigation may reveal to us faint traces of EM spectrum signatures that we cannot now detect.
The ability to travel in interstellar space and to sample EM spectrum signals throughout a large volume of space would allow us to reconstruct the halos of vanished civilizations. As our technologies exponentially improve, I can imagine a time in the future when fleets of drones with the ability to sense very subtle EM signatures depart from our solar system in all directions in an attempt to map any halos that might exist, like a ghostly remnant of civilizations long past.
It has been said that astronomy is a form of time travel, and the farther we look from Earth, the farther back we see in time. (This is called “look back time.”) Thus we can think of astronomy as a kind of luminous archaeology. Another way to think of this is that the sky reveals a kind of luminous stratigraphy, with layers of light of differing ages reaching us from the various parts of the cosmos. The EM halos of vanished civilizations would also admit of a certain stratigraphy, since these halos would possess a definite structure.
This internal structure of an EM halo would reveal essential properties of the now-vanished civilization. The thickness of this three-dimensional halo in light years will correspond to the age in years of the now-vanished industrial-technological civilization. If precise measurements of the EM halo were possible, and its exact curvature could be determined, it would be possible to extrapolate the original source of the signal. Once the curvature of the halo has been determined, and therefore also the source, the measurement of the distance from the source to the inner boundary of the halo to the source in light years will yield the number of years that have elapsed since the end of the industrial-technological civilization in question. In the case of a civilization brought to a sudden end by a nuclear war, the inner boundary of the halo would terminate with an enormous electromagnetic pulse (EMP), punctuating the moment of destruction, followed by unbroken silence.
The outermost stratigraphic layer of an EM halo would likely consist of the simplest kind of high energy radio signals without any kind of subtle modulation of the signal — like Morse code transmitted by radio, rather than vocal modulation. This would be followed, deeper within the EM halo, by analog radio modulation corresponding to spoken language. Next within the EM halo would be analogue television signals, and then digital television signals and data signals of the sort that would be transmitted by a radio link for the internet. This, at least, is the approximate structure of Earth’s expanding EM halo, and if our civilization destroys itself (or is destroyed) in the near future, our EM halo would be approximately 100 light years thick. The longer we last, the thicker our EM halo. A million year old supercivilization as imagined by Carl Sagan would possess an EM halo with a million light years’ radius, and if such a civilization had been extinct for a million years, there would be a hollow center of a million light years’ radius, while the halo itself would extend over four million light years’ diameter—a structure that is sufficiently large to easily overlap both the Milky Way and Andromeda galaxies, and almost large enough to contain both galaxies within its hollow and silent center.
An EM halo may drop off in the intensity of its radiation as an industrial-technological civilization makes the transition from openly radiated EM signals to the pervasive use of fiber optic cables, but if that civilization begins to expand within its planetary system, and possesses numerous settlements in contact with each other through EM spectrum transmissions (as I described in Cyberspace and Outer Space), then the halo will reflect these developments as well — this would be a further historical structure layered into the EM stratigraphy of the halo.
Given that the structure of a large EM halo would consist mostly of space empty of intelligent EM signals, much of the structure of these halos would be void. It is entirely possible that Earth at present lies within the void of an EM halo or halos that both began and ceased to transmit prior to our ability to detect such signals, and perhaps prior to the existence of ourselves.
In the event of human exploration of the cosmos, as we move outward within a possible void within a halo, it is possible that our first contact with a xenomorphic exocivilization will take the form of encountering the inner boundary of an EM halo, which, as we pass through it, it will reveal in reverse order the history of that civilization, beginning with its destruction and ending with its emergence. If we encounter an EM halo from the outer boundary, it will reveal in chronological order the development of that civilization, from its earliest and most primitive signals to its last and most sophisticated transmissions, perhaps even including hopeful SETI beacons or broadcasts.
Even if exocivilizations are rare, and humanity never comes into direct contact with another civilization, the possibility remains that we may eventually formulate a science of exocivilizations that allows us to investigate and to achieve at least a partial understanding of very distant or long vanished civilizations; for such a science of civilization, the EM stratigraphy of the halos of vanished civilizations would yield a trove of empirical data about such civilizations, even if the signals themselves defied interpretation or resisted translation.
Beyond the evidence of individual civilizations that could be derived from an EM halo such as I have described above, the overarching structure of EM halos in the universe—if there are any, and if we are able to detect them—would reveal to us the overall structure of technically mature civilizations in the universe. If we were to discover a large number very thin halos -- perhaps like our halo, about 100 light years thick -- we could conclude the many civilizations come to technological maturity and then rapidly annihilate themselves. If we were to discover one or a small number of very thick halos, we could conclude that few civilizations achieve technological maturity, but, when they do, they endure for a significant period of time. And if we were to discover a mixture of both, we would conclude that the universe gives rise to many civilizations that fail, and a few that survive. In such a study we would have to recognize that thin halos will be more difficult to detect than thick halos, much as small planets are more difficult detect than large planets, which is one reason that “hot Jupiters” were the first exoplanets to be confirmed.
As our science of civilizations and exocivilizations develops and matures, we will learn much from the subtle details of EM halos that can now only be guessed, but that there is much to be learned I do not doubt. Moreover, what we learn we can reflexively apply to the study of our own civilization, so that in looking outward to understand the world, we will at the same time be looking inward to better understand ourselves.
Note Added Later the Same Day: The Milky Way galaxy is about 100,000 to 180,000 light years in diameter; the Andromeda galaxy (M31), larger than the Milky Way, is about 220,000 light years in diameter; the Triangulum galaxy (M33), smaller than the Milky Way, is about 60,000 light years in diameter. Any technologically mature civilization that has left an EM halo will have left its parent galaxy within the hollow center of its halo in less than a quarter million years, which is cosmological terms is a rather short period of time. (For geological perspective, this is about a tenth as long as, i.e., an order of magnitude less than, the current Quaternary glaciation.) What this means is that once a technologically mature civilization goes extinct, its halo rapidly passes beyond its parent galaxy. For the greater part of the life of any given galaxy, the EM halos of the civilizations to which that galaxy is parent leave the parent galaxy in the silent void within the halo. We would expect that the EM halos of civilizations from other galaxies would wash over the Milky Way at temporal orders of magnitude more suitable to cosmology, though if these civilizations are short lived it would be a remarkable coincidence if we happened to detect them. Also, such signals would be exceedingly weak.
Note Added Monday 23 November 2015: In the very long term, the best place to search for the halos of vanished civilizations may be the large voids between galactic clusters, where the least amount of interference with EM signals would be expected. If technologically mature civilizations are very rare in the history of the universe (which I expect to be the case), the discovery of other civilizations (and a fuller knowledge of the overall role of civilization in the cosmos) may have to wait until a civilization achieves routine intergalactic travel, as it is only at such a technological threshold that a civilization could send a large number of probes into the great voids of the universe to search for the EM halos of other civilizations (this would truly be SETI on a cosmological scale) and would also possess some technological work-around for relativistic effects that would allow the observations made in this way to be scientifically useful.
My original blog post:
The Halos of Vanished Civilizations
The recent spoken word version of the same, of which the above is an expanded version:
Burst 9 - The Halos of Vanished Civilizations
Other blog posts of mine cited in the above:
Who Will Read the Encyclopedia Galactica?
Cyberspace and Outer Space
The papers cited in the above text:
“How old is ET?” by Ray P. Norris
“The Relative Rate of LGRB (long duration gamma bursts) Formation as a Function of Metallicity” by J. F. Graham and A. S. Fruchter
“A Uniform Contribution of Core-Collapse and Type Ia Supernovae to the Chemical Enrichment Pattern in the Outskirts of the Virgo Cluster,” by Simionescu, Werner, Urban, Allen, Ichinohe, and Zhuravleva
“Cosmos Was Ready For Life Almost From The Start, Japanese X-Ray Telescope Confirms” by Bruce Dorminey
“Observational Signatures of Self Destructive Civilisations,” by Adam Stevens, Duncan Forgan, and Jack O’Malley James
