by Dave Moore
Dave Moore, a frequent Centauri Dreams contributor, tells me he was born and raised in New Zealand, spent time in Australia, and now makes his home in California. “As a child I was fascinated by the exploration of space and science fiction. Arthur C. Clarke, who embodied both, was one of my childhood heroes. But growing up in New Zealand in the 60s, anything to do with such things was strictly a dream. The only thing it did lead to was to getting a degree in Biology and Chemistry.” But deep space was still on Dave’s mind and continues to be, as the article below, drawing on his recent paper in the Journal of the British Interplanetary Society, attests. “While I had aspirations at one stage of being a science fiction writer,” Dave adds, “I never expected that I would emulate the other side of Arthur C. Clarke and get something published in JBIS.” But he did, and now explains the thinking behind the paper.
The words from “Science Fiction/Double Feature” in the Rocky Horror Picture Show seem particularly apt after looking into the consequences of temporal dispersion in exosolar technological civilizations.
And crawling on the planet’s face
Some insects called the human race . . .
Lost in time
And lost in space
. . . and meaning.
Hence the title of my paper in a recent issue of the Journal of the British Interplanetary Society (Vol. 63 No. 8 pp 294-302). The paper, “Lost in Space and Lost in Time: The Consequences of Temporal Dispersion for Exosolar Technological Civilizations,” grew out of my annual attendance at Contact in San Jose, an interdisciplinary convention of scientists, artists and science fiction writers. From the papers presented there, I got a general feeling for the state of play in the search for extraterrestrial civilizations but never felt inclined to make a contribution until it occurred to me to look at the results of Exosolar Technological Civilizations (ETCs) emerging at different times. It would be an exercise similar to many that have been done using the Drake equation only, but instead of looking at the consequences of the spatial dispersion, I’d be looking at the consequences of different temporal spreads.
My presentation of the results and my conclusions went over sufficiently well that it was suggested that I turn it into a paper, but not having any experience in publishing papers, I let the project drop until Paul got to see my musings and suggested JBIS as a suitable forum.
The Separation Between Civilizations
The core of the paper is a table showing the number of ETCs you would get and their average separations assuming they arose at various rates from a starting point four billion years ago.
I used an idealized galaxy which was a disk of uniform stellar density, that of our solar neighborhood, to keep things simple. (For the justification of why this is a reasonable assumption and to why it seems quite likely that potential life-bearing planets have been around for eight billion years, I’ll refer you to my paper.)
One of the first things I realized is that the median age of all civilizations is entirely independent of the frequency at which they occur. It’s always approximately one-third the age of the oldest civilization. If ETCs start emerging slowly and their frequency picks up (a more likely scenario), this skews the median age lower, but you are still looking at a period of about a billion years.
And the median age of all civilizations is also the median age of our nearest neighbor. There’s a fifty/fifty chance it will be either younger or older than that, but there’s a 90% chance it will at least be 10% of the median, which means that in all likelihood our nearest neighbor will be hundreds of millions of years older than us. And, if you want to find an ETC of approximately our own age, say within a thousand years of ours, you will on average have to pass by a million older to vastly older civilizations. As you can see from columns 5 and 6 in the table, if ETCs haven’t emerged with sufficient frequency to produce a million civilizations, then you won’t find one.
Once you realize that ETCs are not only scattered through vast regions of space but also scattered across a vast amount of time, then this casts a very different light on many common assumptions about the matter. Take the idea very prevalent in a lot of literature that the galaxy is full of approximately coequally-aged civilizations (emerging within a thousand years of each other), a scenario I will call the Star Trek universe. If you look at the bottom row of the table, you can see there are simply aren’t enough stars in our galaxy for this to work.
After discovering that when dealing with extraterrestrial civilizations, you are dealing with great age, I then began to look at the sort of effects great age would have on civilizations.
Age and Power
The first thing I did was to extrapolate our energy consumption, and I discovered that at a 2% compound growth rate our civilization would require the entire current energy output of the galaxy (reach a Kardashev III level) in less than 3000 years, which doesn’t look likely unless a cheap, convenient, FTL drive get discovered. What this does point out though is that in extraordinarily short times, geologically speaking, civilizations can theoretically grow to enormous power outputs.
The next thing I did was to review the literature on interstellar travel. Many of the interstellar propulsion scenarios have power requirements that cluster around the 100 Terawatt level. This is a million times that of a proposed 100 MW nuclear powered Mars vessel, which is considered to be within our current or near future range of capabilities. Assuming a society with a million times our current power consumption would find a 100 TW vessel similarly within its capabilities, then our first interstellar vessel would be 700 years into our future at a 2% growth rate.
Are these energy levels feasible? If Earth continues its current growth in energy consumption, we will overheat our planet through our waste heat alone in the next century, never mind global warming through CO2 emissions. So, it looks as if we remain confined to our planet, we will probably never have the ability to send out interstellar colony ships. There is, however, a way to have our civilization reach enormous energy levels while still within our solar system.
Our solar system may have as many as a trillion comets and KBOs orbiting it, ten times the mass of the Earth, all nicely broken up. (There may be more comets in our solar system than there are stars in our galaxy.) And as this is the bulk of the easily accessible material, it would be logical to assume that eventually this is where the bulk of our civilization will finish up.
A hydrogen-fusion powered civilization could spread throughout our cometary belt, and with no grand engineering schemes such as the construction of a Dyson sphere, it could, through the cumulative growth of small, individual colonies, eventually build up a civilization of immense power and size. For example, if each of a 100 billion comets were colonized with a colony that used 1000 MW of power (a small city’s worth) then the total civilizational power consumption would be in the order of 1020 Watt. Pushing it a bit, if there was a 20,000 MW colony on each of the 5 trillion comets in the Oort cloud and the postulated Hills cloud, then the total civilizational power consumption would be 1023 Watt, that of a red dwarf star.
For this society, interstellar colonization would be but another step.
The End of a Civilization
Ian Crawford has done some analysis of galactic colonization using a scenario in which a tenth-lightspeed colony ship plants a colony on a nearby star system. The colony then grows until it is capable of launching its own ship, and so on. This produces a 1000-2000 year cycle, with the assumptions I’ve been using, but even if you work this scenario conservatively, the galaxy is colonized in 20 million years, which is an order of magnitude less that the expected age of our nearest neighbor.
Of course, all the previous points may be moot if a civilization’s lifetime is short, so I then looked into the reasoning advanced for civilizational termination.
Various external causes have been postulated to truncate the life span of a technological civilization–Gamma Ray Bursters are a favorite. When you look at them though, you realize that anything powerful enough to completely wipe out an advanced technological civilization would also wipe out or severely impact complex life; there’s at most a 10,000 year window of vulnerability before a growing civilization spreading throughout the galaxy becomes completely immune to all these events. This is one fifty-thousandth of the 500 million years it took complex life to produce sentience. So any natural disasters frequent enough to destroy a large portion of extraterrestrial civilizations would also render them terminally rare to begin with. If extraterrestrial civilizations do come to an end, it must be by their own doing.
There’ve been numerous suggestions as to why this may happen, but these arguments are usually anthropocentric and parochial and not universal. If they don’t apply to just one civilization, that civilization can go on to colonize the galaxy. So, at most, self-extinction would represent but another fractional culling akin to the other terms in the Drake equation. There’ve also been many explanations for the lack of evidence of extraterrestrial civilizations: extraterrestrials are hiding their existence from us for some reason, they never leave their home world, our particular solar system is special in some way, etc., but these are also parochial arguments; the same reasoning applies. They also fail the test of Occam’s razor. The simplest explanation supported by the evidence is that our civilization is the only one extant in our galaxy.
Into the Fermi Question
The only evidence we have about the frequency and distribution of ETCs is that we can find no signs of them so far. This has been called the Fermi paradox, but I don’t regard this current null result as a paradox. Rather I regard it as a bounding measurement. Since the formation of the Drake equation, two major variables have governed our search for ETCs: their frequency and longevity. This leads to four possibilities for the occurrence of Exosolar civilizations.
- i) High frequency and longevity
- ii) High frequency and short life spans
- iii) Low frequency and longevity
- iv) Low frequency and short life spans
These four categories are arbitrary, in effect being hacked out of a continuum. The Fermi paradox eliminates the first one.
We can get a good idea of the limits for the second by looking at an article that Robert Zubrin did for the April 2002 issue of Analog. In it, he postulated a colonization scenario similar to Ian Crawford’s but cut the expanding civilizations off at arbitrary time limits. He then found the likelihood for Earth having drifted through the ETCs’ expanding sphere of influence in the course of our galactic orbit. The results indicated that unless all civilizations have lifetimes of under 20,000 years, we are very likely to have been visited or colonized frequently in the past. But to have every civilization last less than a specified time requires some sort of universalist explanation, which is hard to justify given the naturally expected variation in ETCs’ motivation.
Nothing that we had seen so far eliminates the third possibility however.
Implications for SETI Strategy
Finally, in the paper, I turned to reviewing our search strategies for ETCs in light of what has been learned.
Given that ETCs will most probably be very distant and have a high power throughput, then looking for the infrared excess of their waste heat looks like a good bet. Low frequency but high power also implies searching extra galactically. Take the Oort cloud civilization I postulated earlier and assume it colonizes every tenth star in a galaxy like ours. Its total power consumption would be in the order of 1030 Watt. This would show up as an infrared excess of one part in 107 to 108 of a galaxies’ output.
I found other ideas like searching for ancient artifacts and using gravitational lensing for a direct visual search seem to have some potential, but when I looked at radio searches, this turned out to be one of the least likely ways to find a civilization. The problem quickly becomes apparent after looking at Table I. Any ETCs close enough to us to make communication worthwhile will most likely be in the order of 108 to 109 years old, which gives them plenty of time to become very powerful, and therefore highly visible, and to have visited us. If civilizations occur infrequently, as in the top row of Table I, then the distances are such that the communication times are in the order of 10,000 years. If civilizational lifetimes are short but the frequency is high, then you still have enormous distances. (You can use Table I to get some idea of the figures involved. The last two columns show the distances at various frequencies for civilizations within 1000 years of our age. For ten thousand years move those figures up one row, for 100,000 years two rows, etc.) Under most cases, the signal reply time to the nearest civilization will exceed the civilizations’ lifetime–or our patience. Looking for stray radio signals under the distant but short-lived scenario does not look very hopeful either. To send a signal tens of thousands of light years, an effective isotropic radiated power of 1017 – 1020 Watts is required, and while this is within sight of our current technology, the infrastructure and power levels are far in excess of anything required for casual communication even to nearby stars.
The results of all my thinking are not so much answers but, hopefully, a framing for asking the right questions.
Considerations in SETI searches have tended to focus on the nearby and a close time period and were set when our knowledge in this field was in its infancy. There’ve been some refinements to our approach since them, but generally our thinking has been built on this base. It’s time to carefully go over all our assumptions and reexamine them in the light of our current knowledge. The Fermi paradox needs to be explained — not explained away.
The paper is Moore, “Lost in Time and Lost in Space: The Consequences of Temporal Dispersion for Exosolar Technological Civilisations,” JBIS Vol. 63, No. 8 (August 2010), pp. 294-301.