Interstellar communication using microbial data storage: implications for SETI (part 1)
by Robert Zubrin
|Given this failure, it is appropriate to revisit the assumptions behind the Morrison-Cocconi hypothesis suggesting interstellar communication via S-band.|
While the universe is vast in space, it also is so in time, so that a spacecraft traveling at a velocity of 0.0001c (the speed of the Earth in its travel around the Sun) could in 4.5 million years (0.1 percent the age of the Earth) travel 450 light-years, a radius encompassing approximately 1 million stars, with surely enough candidates for many additional origins for life and civilization. Despite these favorable odds, no such extraterrestrials have been detected, a mystery leading the physicist Enrico Fermi to ask his famous paradoxical question at a 1950 Los Alamos lunchtime meeting: “So then, where are they?”
One possible reply to the Fermi Paradox is that they are out there, but that if you want to find them you need to look for them in the right way. In 1959, Cornell physicists Phil Morrison and Giuseppe Cocconi (Cocconi and Morrison, 1959) proposed that extraterrestrials might be communicating across space using 1.42-gigahertz radio, as the emissions of hydrogen gas at that frequency make it the most listened-to band in radio astronomy. Moreover, it is approximately the same frequency as the L-band and S-band radio systems that were becoming state of the art for spacecraft communication at that time, a fact that added credence to the Morrison-Cocconi hypothesis, and made it readily testable as well. Accordingly, shortly thereafter astronomer Frank Drake attempted to detect such signals from Tau Ceti, without success. Undeterred, Drake, his co-workers, competitors, and successors continued with many such search searches, from that time down to the present, where the SETI Institute, among others, is continuing the effort on a greatly expanded basis with vastly improved instrumentation, but no better results.
Given this failure, it is appropriate to revisit the assumptions behind the Morrison-Cocconi hypothesis suggesting interstellar communication via S-band. Certain of its supports have already been falsified by technological progress, in that, a mere 60 years later, S-band is already obsolete. Instead, our spacecraft now communicate at higher frequencies, such as X-band (10 gigahertz) and Ka-band (30 gigahertz), as these higher frequencies allow for higher bandwidths for spacecraft communication systems of a given size and power.
But while the Morrison-Cocconi hypothesis, and resulting search, can be adjusted to take into account such improvements, there are deeper problems. The first of these is that communicating effectively across interstellar distances via radio is incredibly hard and inefficient. To see this, let us consider what it would take to design such a system.
The Mars Reconnaissance Orbiter (MRO), launched in 2005, has a modern 100-watt X-band communication system. Equipped with a two-meter diameter dish, it can transmit at a rate of 6 megabits per second to a 70-meter receiving system on Earth at a distance of 100 million kilometers. A modest range for effective interstellar communication would be 10 light years, or 100 trillion kilometers. At this million-fold greater distance, the MRO communication system would have its data rate reduced by a factor of a trillion, from 6 megabits per second to 6 microbits per second, or 200 bits per year. This would appear to be too slow for practical purposes, so let’s upgrade the transmitter power to one gigawatt and its dish size to 70 meters. Taken together, these upgrades would increase the data rate by a factor of 10 billion, to 60 kilobits per second, which would be fine. The upgraded system would have a capability of 600 bits per second at 100 light-years, which is still sufficient to be useful, as those who can remember working with computer modems in the 1980s can readily attest.
Assuming that the transmitter was X-band, the diameter of the beam at 10 light-years would be about 2500 astronomical units, so it would encompass the whole solar system, and then some, but the closest neighbor solar system would be 100 times further away that the width of the beam. So ET’s one-gigawatt transmitter could only be used to signal one solar system at a time. How would they know whom to signal?
A good place to start would be to only send signals to planets manifesting a biosphere. These could be detected using astronomical techniques by observing the spectral signal of free oxygen in the planet’s atmosphere. But Earth, for example, would have provided a positive answer to such a search criterion for the past 500 million years, but only has only possessed a species able to receive and detect such a signal for the past 50 years. Based on these odds, ET would have to set up one million transmitters to living planets have a 10 percent chance that one of its signal units would be sending at the right time. This would require a transmitter system power of 1000 terawatts, about two orders of magnitudes higher than the total power produced by human civilization today. If they wanted to keep their odds as good, but reduce the number of transmitters, they could do so, but only at the cost of continuing to power the transmission program for millions of years.
|In short, I am suggesting… that rather than transmit information across interstellar distances using radio waves, that solid objects containing records be used instead.|
Furthermore, to receive the signal, the inhabitants of the target planet would have to be focusing their 70-meter dish at precisely the right star at the right time, be listening at the right frequency, and be technologically and intellectually prepared to recognize and decipher the signal. Using lasers instead of radio to transmit would reduce the power requirement significantly, but it would still be huge and, furthermore, it would impose a requirement that the receiving civilization have a giant telescope focused on the transmitter’s narrow beam at the right time, with its optics limited to the transmitting frequency to avoid having the transmitter outshone by its home system’s star. This seems like a rather farfetched hope upon which to expect ET to spend such a large investment in infrastructure and energy.
Surely there must be a more efficient approach. What could it be?
One problem of data transmission by radio is that it occurs in real time, leaving no record behind. Once the transmission is over, it’s gone. If the intended recipients miss it the first time, they’ve missed it for all time. As a result, the transmitting party is forced to transmit repeatedly, perhaps endlessly, in the hope that, on at least one occasion, someone might be listening.
Imagine that you want to tell a story to children. So you walk onto the front porch of a house and tell the story, whether any children are there are not. In fact, they are usually out running around, so you probably have missed your audience. But the odds against you are worse, because while the house is suitable for children, they may not have been born yet, or those that are there may be too young to understand your story, or they could have grown up and moved out. Now, you could increase your odds by going from porch to porch, reading the story again and again in the hope that someone might be there to listen. But this could get exhausting.
A better strategy would be to go about slipping storybooks into houses through their mail slots. Even if most of them ended up on the shelf or sold to used book stores, sooner or later many of them would likely get read. The only problem with this strategy is its cost; giving out a lot of storybooks could be expensive. But if you could get them for free, and have them delivered for you cheaply, it could be a very good approach, allowing you to reach many children not only today, but also for decades and generations to come.
In short, I am suggesting, in agreement with Davies (Davies, 2010), that rather than transmit information across interstellar distances using radio waves, that solid objects containing records be used instead. This may seem like it would be very inefficient, but in fact every planetary civilization orbiting a star has available to it an engine that it can use to send information across space at little or no energy cost. This is the star itself.
Let us consider our own Sun as a case in point. At one astronomical unit, the Sun attracts objects to it with a gravitational acceleration of 0.006 m/s2. It also repels objects from it via its light pressure with a force of 0.000009 N/m2. If these two forces are equal, an object will feel no attraction from the Sun, and fly out of the solar system in a straight line with the Earth’s velocity of 30 kilometers per second. (Both of these forces change with the inverse square of their distance from the Sun, so if they are equal at one astronomical unit, they remain equal at any distance.) Assuming the object has a radius r and a density d, and setting these two forces as being equal we find:
Gravity = (4πr3d/3)(0.006) = Light pressure = (πr2)(0.000009)
Cancelling terms, this simplifies to:
r = 0.001125/d
If d = 1,000 kg/m3 (water density), we find that r = 1 micron. This means that such an object would have a diameter of 2 microns. However, it is not necessary to cancel all the force of gravity to escape the solar system. If half of the gravitational attraction is cancelled, an object with the orbital velocity for the full gravity object will escape. In the case above, this would imply a maximum diameter of 4 microns.
If the star were brighter than the Sun compared to its gravity, which would be the case with F stars, the objects could be somewhat larger. If it were dimmer than the Sun compared to its gravity, as would be the case with K stars, the objects would be need to be smaller. If their initial orbits were elliptical, rather than circular, the objects could be bigger. So, depending on assumed conditions, the diameter of the objects could range from 1 to 10 microns, and be readily projectible across interstellar distances using no other mechanism than the pressure of the star’s light. This is precisely the size range of many typical bacteria. (Arrhenius, 1908)
|So, bacteria can be projected across interstellar space at essentially no power cost to the transmitting party, beyond that required to launch them to planetary escape velocity. But can they carry useful information?|
It is also possible that the solar wind, which moves at a velocity of about 500 kilometers per second, could be used to propel particles out of the solar system at very high velocity. However, in order for a particle to interact effectively with the solar wind, it would need be strongly magnetized, allowing it to function as a miniature magnetic sail. (Zubrin and Andrews, 1989). Under normal circumstances, even such a strongly magnetized particle would be propelled away from the Sun by the solar wind with less force than that provided by light pressure. During solar flare events, which greatly increase the force of the plasma wind emanating from the Sun, however, this could change radically.
In Table 1, we show the relationship between star type, population fraction, star mass, luminosity, orbital distance and velocity and maximum particle diameter for stellar system escape for stars of various types. In each case the launch planet is assumed to be in a circular orbit in the given star type’s habitable zone. It can be seen that for type F, G, and K stars, collectively amounting to about 22.5 percent of the stellar population, that starlight can readily propel bacteria-sized objects to system escape velocity. At half the diameters shown, the objects will be projected outward from the stellar system at the orbital velocity given, at less than half, still greater velocity. Objects larger than that shown will not escape the system by light pressure alone, but could still do so if the light pressure is enough to drive them into an elliptical orbit that intersects a large planet capable of delivering a gravity assist.
Upon reaching a destination solar system, the bacteria-sized particles could be decelerated using the same mechanism.
|Star Type||Fraction||Star Mass||Luminosity||Distance||Velocity||Diameter|
|F||0.03||1.2 Suns||2 Suns||1.44 AU||27 km/s||7 microns|
In Table 1, the particles are assumed to be simple spheres. If more complex shapes are used—for example, shapes with a spherical core surrounded by thinner wings—the core spheres can be thicker and still achieve the same velocities. For example, a sphere with a diameter of 4 microns surrounded by a disc with a diameter of 8 microns and a thickness of 1 micron would have the same surface/mass ratio as a simple sphere with a diameter of 1.2 microns, and would therefore be able to escape a K star. Such “microsailcraft” designs could be readily mass-produced artificially or potentially created through natural crystallization processes, as exemplified by snowflakes. They could be stabilized in a manner to effectively serve as sails either by spinning (as some snowflakes do), or by having inherently stable shapes, as exemplified by a badminton birdie.
So, bacteria can be projected across interstellar space at essentially no power cost to the transmitting party, beyond that required to launch them to planetary escape velocity. The latter could be accomplished either by artificial technological means that are well within our means today—and therefore clearly feasible for advanced extraterrestrials—or potentially through natural processes such as asteroidal impacts. They also may be cheaply mass produced (McDowell, 2003). But can they carry useful information?
The answer is most certainly yes. The genetic material of individual common bacteria is estimated to contain between 130 kilobytes to 14 megabytes of information. Current estimates that bacteria can be used to store data with a density of 900 terabytes per gram, about 500 times greater than current state-of-the-art electromagnetic hardware. This means that a bacterium 5 microns on a size could store about 120 kilobytes of information (Wilkins 2010, Herkewitz 2016). Taking 60 kilobytes as typical, this means that a single bacterium can carry a record of information about equal in size to a 10,000-word (~30 page) booklet. In experiments done to date, scientists have demonstrated such capabilities by encoding entire books in DNA, and showing that bacteria can be made to replicate encoded information when they reproduce (Ayre 2012). Most recently, researchers at Columbia University and the New York Genome Project have shown that they can encode information with a density of 215,000 terabytes of information per gram in DNA, with some of the items successfully encoded including a movie, a computer operating system, and an Amazon gift card (Service, 2017).
Traveling at a velocity of 30 kilometers per second (0.0001c), bacteria would take 100,000 years to fly 10 light-years. This would expose them to cosmic ray doses between 1 and 10 megarads, which is close to the limit for survivability of hardy microcrobial species such as radiodurans. This need not be a showstopper. A message sent using bacteria storage would no doubt use billions of individuals, and if even a few survived the trip the message could still get through. While ultraviolet light would kill unshielded bacteria in days, effective shielding against this hazard can be provided by a half micron of soot (Hoyle, 1981). Such protection would be provided by design in any artificial microsailcraft, but could conceivably also occur naturally.
Such long trips might not be necessary, however. Once they reach a planet, bacteria will multiply to vast numbers. They can then be ejected again into space via cometary impact. Such impacts are most likely to occur during periods when a foreign star is passing through the Oort Cloud of the bacteria’s home star, as such a passage would destabilize Oort Cloud object orbits orbiting both stars, and thereby causing impacts to occur. Like frigates in the age of fighting sail, which could span the globe with their movements but only reach a few hundred yards with their guns, roving solar systems discharge their broadsides at each other only during rare close approaches. As a result, ejected bacteria might typically only need to travel distances on the order of 0.1 light-years to reach a new planetary home, with radiation doses accordingly reduced by two orders of magnitude compared to those postulated above.
Given the density of stars in our own region of the galaxy, and assuming a random velocity of stars relative to each other of five kilometers per second, it can be shown that a star is likely to experience such a close encounter about once every 20 million years, a frequency strikingly close to the observed mean time of 26 million years between of mass extinctions on Earth (Zubrin, 2001). It may be noted that microbes traveling embedded in impact debris would be well shielded from ultraviolet and soft x-rays, thereby increasing their survival odds (Melosh, 1988, Hornek, Klaus, and Mancinelli, 2010).
The Milky Way galaxy is 13 billion years old. Allowing three billion years for several generations of early stars to seed the place with heavy elements, that leaves 10 billion years, or 500 stellar close-encounter doubling times of 20 million years each, for life to spread from its first planet of origin to everywhere else. So the answer to Fermi’s paradoxical question is almost certainly this: They’re here.
At this point, we need to reexamine the question of what might be the purpose of interstellar communication. Certainly, if a species were spacefaring and had sent out expeditions that established settlements in nearby solar systems, it would want to maintain communication to exchange or trade information among its various worlds. For such purposes, high-gain directed electromagnetic transmissions would be the most practical, as they are the fastest possible, the most secure, and all the required technological and linguistic conventions would be known and mutually understood between the parties involved.
|So the question is, what kind of information is really worth broadcasting that is distributed as widely as possible to people who we don’t know and are not likely to hear back from? If human experience is any guide, the answer is propaganda.|
But if we are talking about communication between different species originating in different and distant worlds, what is the point? In speculative SETI literature, it is frequently supposed that there are intelligent aliens out there, who for some reason want others to know that they exist, and therefore transmit signals such as the value of pi into the void, so that other smart folks won’t confuse them with astrophysical phenomenon. Then, assuming that someone picks up the signal, they will transmit back the value of e, or the golden mean, or some other special number, thereby completing the freemasonic handshake. This done, the two parties could then proceed to methodically expand their mutual vocabulary, eventually allowing them to exchange QST cards, recipes, celebrity gossip, novels, scientific theories, starship designs, and treaties of alliance against the barbarians from the galactic rim.
However, as noted above, both experimental searches and theoretical considerations suggest that such a picture does not correspond to reality. Microbial data transmission is possible, but it does not lend itself to conversations of the types described above. Rather, it is a superior method of interstellar broadcasting.
So the question is, what kind of information is really worth broadcasting that is distributed as widely as possible to people who we don’t know and are not likely to hear back from? If human experience is any guide, the answer is propaganda. Think of Radio Free Europe, or its Cold War opposite number, Radio Moscow, for example, and their persistent messaging: “We are good. You should admire us. You should be like us. You should join us.” Another example would be the Gideons, placing Bibles in hotel rooms in the hope that their unknown readers would see the light and become Christians. We also try to broadcast ourselves to worlds we will never see, by using art to send our message across deep time. Thus Pericles at the Parthenon: “Future ages will wonder at us, even as the present age wonders at us now.”
The key to propaganda is in the root of the word itself: propagate. Through propaganda we seek to propagate ourselves across both space and time. This can be in spirit, as in the cases described above, or in the flesh, through physical reproduction. Indeed, while only a relative handful of people have been able to message the future through monuments, literary works, or art, the great majority of those of the past world who have sent us something of themselves have done so by propagating themselves biologically. Using this method, they have transmitted to our world not only their genotypes and phenotypes, but even their languages, beliefs, and traditions as well. Propagation is propaganda. Propaganda is propagation. It is the most desired form of communication. It is how the past has communicated with us, and how we seek to communicate with the future. This is a key point, because interstellar communication through any means must perforce be communication across time.
So, it should be clear. If we are going to transmit across the ages, we need to send instructions on how to make ourselves. Such messages are not sent via radio. They are sent using genes.
The code of life is the code of the cosmos.
It is a striking fact that, despite several centuries of microbial research by thousands of competent investigators, no free-living organisms have been found on Earth that are simpler than bacteria. This is truly remarkable because, as simple as bacteria may be compared to more complex organisms, they are certainly not simple in any absolute sense, incorporating as they do, among other things, the entire elegant double-helix scripted language of DNA. Believing that bacteria were the first life forms to emerge from chemistry is like believing that the iPhone was the first human-invented machine. This is incredible. Just as the development of the iPhone had to be preceded by the development of computers, radio, telephones, electricity, glassware, metallurgy, and written and spoken language, to name just a few necessary technological predecessors, so the creation of the first bacterium had to be preceded by the evolution of a raft of preceding biological technologies. But we see no evidence of any such history.
We still see devices all around us that use one or more of the iPhone’s ancestor technologies—telephones, light bulbs, batteries, glass windows, and steel knives, for example—but we see no pre-bacteria organisms. This observation has led many investigators, dating back to Arrhenius (Arrhenius 1908) over a century ago, to postulate that life on Earth is an immigrant phenomenon. According to this “panspermia” hypothesis, bacteria did not originate on Earth, but came here from space, after which they gave rise via generally understood evolutionary processes to all other life forms.
The panspermia hypothesis is generally disliked by origin of life researchers, because it completely ducks their central question of how life originated from chemistry in the first place. This is particularly the case for the original form of the panspermia hypothesis offered by Arrhenius, who believed that the universe and life had existed eternally, thus making the question of the origin of either meaningless. However, if the panspermia hypothesis is taken to simply open the question as to the location of life’s planet of origin, then it is by no means useless. Consider that an investigator seeking to explain the origin of Americans would be crippled in his or her research if he or she had to accept as axiomatic the conceit that humans evolved independently in North America (and even more so if Golden, Colorado, were specified.)
No, the fact is that humans originated in Africa, and only came to the Americas much later. This is why we can find evidence of humanity’s closest relatives, primate ancestors, and earliest cultures and technologies in Africa but not in North America. Knowing this, an investigator would not be bound to try to explain the independent origin of humans from native North American (or Goldenian) fauna, but instead could focus on conditions and biological foundations that were present in Africa in the relevant period. Similarly, there have been origin of life experiments, such as the famed Miller-Urey experiment, that have been discounted because they postulated conditions that did not exist on the early Earth. If the possibility of an extraterrestrial origin of life is accepted, such objections lose their force.
|The key question for microbial SETI is whether there is artificial intelligent input being inserted into the vast flood of genetic information traveling around the Earth. Are there any real letters of importance to be found in the deluge of junk mail?|
Indeed, insistence on geospermia by assumption puts origin of life researchers in the same absurd position as the above described unfortunate paleontologist, whose assumption of a local origin for humanity forces him or her to reject the theory that humans evolved from higher primates because there were no such species in evidence in Golden, Colorado, at the time of humanity’s appearance. There are innumerable planets where the spontaneous formation of amino acids from chemistry, as demonstrated by the Miller-Urey experiment, could readily have occurred, as opposed to the early Earth, where it could not. Science needs to follow the data, not defy it. Therefore it is the Miller-Urey experimental results that discredit the assumption of geospermia, rather than the reverse.
Further support is offered to the panspermia hypothesis by discoveries of bacterial fossils known as stromatolites, dating back approximately 3.5 billion years, and residues of biological activities dating back 3.8 billion years, that is practically right back the end of the heavy bombardment that previously made the early Earth uninhabitable. In fact, as this is being written, a team of researchers have just reported microfossils that date back 4.28 billion years, that is, to the middle of the heavy bombardment (Drake 2017). In short, life appeared on our planet virtually as soon as it possibly could (and possibly several times, before it could last), suggesting that it was already around, waiting to land and spread as soon as conditions on the ground were acceptable.
The primary counterargument offered against the panspermia hypothesis is that there may once have been prebacteria on Earth, but that they have since been wiped out by their more developed descendants. While this may be possible, it is not consistent with the history of life on Earth, in which simpler forms generally continue to exist in abundance even after they give rise to higher or more complex varieties. In any case, this argument is only an excuse for the lack of any evidence for any prebacterial history of life on Earth. Accordingly, it has no power or potential to falsify the panspermia hypothesis.
Furthermore, it needs to be understood that the conceit that life originated on Earth is quite extraordinary. There are over 400 billion of stars in our galaxy, with multiple planets orbiting many of them. There are 51 billion hectares on Earth. The probability that life first originated on Earth, rather than another world, is thus comparable to the probability that the first human on our planet was born on any particular 0.1-hectare lot chosen at random; for example, my backyard. It really requires evidence, not merely an excuse for lack of evidence, to be supported.
The panspermia hypothesis could be falsified however, if we were to send explorers to Mars and find either a) no evidence of any past or present life, b) evidence for the presence of prebacteria, or c) evidence of life of sufficiently different type as to imply a second genesis. Condition (a) would falsify panspermia because Mars had liquid water on its surface during the period when life appeared on Earth, so that if Earth were seeded via panspermia, Mars should have been seeded too. Condition (b) would refute panspermia directly by revealing the prior evolutionary history of Earth life on Mars, from whence it could readily have been transmitted here via meteoric impact. Condition (c) would refute panspermia by showing two independent origins. However, if none of these conditions are met, and we find evidence of past or present bacteria on Mars similar in structure to Earth bacteria dating back to the planet’s early history, with no evidence of prebacteria, the panspermia hypothesis would be strongly supported.
In the absence of falsification, we are presented with three possibilities for interstellar microbial transmission.
With respect to the above listed possibilities, the one that seems most difficult to defend is (2), because if bacteria can survive interstellar trips, there will be natural transmission, regardless of whether artificial transmission is also going on. Indeed, it is hard to escape the conclusion that natural transmission has been going on for at least 3.6 billion years, if from no other original source than the Earth. If the average time between close stellar encounters is 20 million years, with number of microbe inhabited system doubling each time, we could expect 2180 solar systems in our galaxy to have been Earth-progeny-microbe-invaded by now, which is to say all of them, many times over. (This being the case, the probability that the Earth was actually the first of these billions of microbe-inhabited worlds would be vanishingly small.)
So, the question is not whether interstellar microbial transmission is going on; it almost certainly is. Further, even if is not occurring naturally, it is still clearly possible through artificial means. So indeed, far from being a necessary condition for microbial SETI, even the possibility of natural panspermia creates difficulties, as it introduces the potential for noise that could drown out the signal.
The key question for microbial SETI is whether there is artificial intelligent input being inserted into the vast flood of genetic information traveling around the Earth. Are there any real letters of importance to be found in the deluge of junk mail? If so, how could we pick them out?
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