Tell them that I came, and no one answered. Walter de la Mare, The Listeners
The evidence that ETCs are currently visiting Earth is not compelling. But maybe they visited Earth, or at least our Solar System, some time in the past — perhaps long ago, at a stage in human development when no one could recognize them for what they were. Is there any evidence for this? Let us work through the Solar System, beginning at home.
The famous Tunguska explosion of 1908 — an event that felled acres of trees across the Siberian taiga — was long thought to be the result of an asteroid strike. Researchers, however, found none of the debris that one would expect from such an impact. It was a mystery. Once the immense power of nuclear explosions became apparent, soon after World War II, the notion circulated that the Tunguska event had been a nuclear blast — the impact of an alien nuclear-powered spacecraft that had crashed. The idea was taken semi-seriously, and there was a simple means of testing it: go to Tunguska and search for traces of radioactivity. This was done, and scientists found no traces of radioactivity that could have come from a nuclear engine. (They also ruled out an antimatter engine.) We now know the Tun-guska event was probably the result of a stony meteoroid that exploded in the atmosphere (although the evidence is still not conclusive, and several scientists believe Tunguska was hit by a comet). There have been similar events in the past, and they have a similar explanation: meteorite impact. There is no need to invoke the hypothesis of a downed spaceship. If a spaceship ever did crash-land in the past, we have not found the evidence (Roswell notwithstanding).
In the 1970s, Erich von Daniken became famous for a series of books in which he claimed that extraterrestrial visitors built many of the enigmatic structures dotted around the world — Stonehenge, the lines on the Nazca Plain in Peru, the Easter Island statues, and so on.39 None of the books contained proof to back up his claims. Nevertheless, his large reading public supported him during his lengthy spell in prison for fraud; they supported him after his claims were painstakingly and thoroughly debunked; only when they became bored and taste and style moved on did they drop him. Now, like several pop groups from that era, von Daniken and his ideas are back in fashion even though, in the thirty-odd years since the books were first published, no proof has been produced to support his speculations — something that von Daniken himself cheerfully admits and seems to find irrelevant. Since the supporters of von Daniken are unlikely to be swayed by rational argument, we may as well move on — and accept that there is no evidence that members of an ETC have ever been on Earth. (This, of course, is not to say that they definitely have not been here. If they visited Earth 1 billion years ago, say, who knows what signs — if any — would remain of their visit? But in the absence of any evidence to the contrary, we may as well assume that Earth has been untouched.)
Until fairly recently, some people claimed to see evidence for ETCs on the Moon. In 1953, for example, the astronomer Percy Wilkins discovered what appeared to be an artificial structure — a bridge.40 However, other astronomers could not see the structure through more powerful telescopes and decided, quite reasonably, that the bridge was a trick of the light. This did not dampen the enthusiasm of those who believed in the Moon as an abode of alien life. Enthusiasts pointed out that the Moon shows only one side to Earth (to be precise, due to the phenomenon of libration we see only 59% of the Moon's surface). If we never see 41% of the lunar surface, who knows what might be hiding on the far side of the Moon? It was not until the late 1970s, well after the many landers and orbiters had mapped the entire surface of the Moon, that "life" enthusiasts finally stopped promoting the idea of bridges and other artifacts. (At least, I think they have stopped promoting the idea.)
As we shall see later (page 79), one can argue that an ETC wishing to explore our Solar System would send small unmanned (unaliened?) probes rather than a fleet of crewed spacecraft. Where might we find such probes? There are three cases to consider. First, the probes could be programmed to attract our attention. Since we see no evidence for beacons, it is safe to assume that such probes are not here. Second, the probes could be programmed to hide from us. The Solar System is a large place, and there are plenty of places where they could hide. Since we are unlikely ever to find such probes, we need not spend time discussing the best strategy to observe them. Third, an ETC might send probes and not care whether humans observe them. If that is the case, where might we find them?41
We can reasonably argue that of all the planets in the Solar System, ours is most worthy of study. Earth is an interesting planet for a variety of reasons — most importantly it is, as far as we know, the only planet to harbor life. So probes would most likely be programmed to investigate Earth. (This argument of course reeks of anthropocentricism. Who knows what an alien mind might want to investigate? Who knows what technology it might employ? But such logic is all we have, so we lose nothing if we continue the argument and see where it leads us.) The surface of the Earth would be a poor site for long-term studies of our planet. It would make more sense to view the entire planet from space, where solar energy is more readily available, and where there is no need for the probe to protect itself against the effects of the Earth's geological activity.42
Several types of orbit are suitable for long-term parking of observational probes, but perhaps the best known are the Lagrangian points L4 and L5.43 If a small mass is near two large orbiting masses, then there are five points at which the small mass can orbit at a fixed distance from the larger masses. These five Lagrangian points mark the positions where the gravitational pull of the two larger masses exactly balances the centripetal force required to rotate with them. At first glance, then, there are five points where ETCs might place a small probe in the hope that it maintains a fixed distance from Earth and Moon. However, three of the Lagrangian points — L1, L2 and L3 — are unsuitable because they are unstable: nudge the small mass and it will move away from the L point. But L4 and L5 are stable: nudge the small mass and it will return to the L point. (To be precise, L4 and L5 are stable only if the most massive of the three bodies is at least 24.96 times as massive as the intermediate body. This condition is satisfied in the Sun-Earth system, since the Sun is much more massive than Earth. It is also satisfied in the Earth-Moon system, since Earth is 81 times as massive as the Moon. The Sun's gravitational influence tends to destabilize the L4 and L5 points of the Earth-Moon system; however, it smears the stable points into volumes of space in which stable orbits exist.)
figure 12 The five Lagrangian points are places in the vicinity of two orbiting masses where a third smaller body can maintain a fixed distance from the larger masses. The points L1, L2 and L3, which lie on a line connecting the two large masses, are unstable: after a perturbation, the small body will move away from the Lagrangian point. Under certain circumstances, the points L4 and L5 are stable: after a perturbation, the small body will return to the Lagrangian point. The L4 and L5 points are stable for the Earth-Moon system, so they are a good place to park probes for long-term study of the Earth.
NASA is already using the Lagrangian points of the Sun-Earth system as parking places for its satellites. The L1 point is home to SOHO (Solar and Heliospheric Observatory); from the L1 vantage point, SOHO has an uninterrupted view of the Sun. The L2 point is home to MAP (Microwave Anisotropy Probe); from there, MAP will study wrinkles in the cosmic microwave background and uncover information about the Big Bang. If NASA finds it convenient to park satellites at L points, then perhaps ETCs
would do so too. Perhaps we might find probes at Lagrangian points in the Earth-Moon system? Well, at least one dedicated search has been made. Furthermore, astronomers have already studied the L4 and L5 points of the Earth-Moon system, since the points are interesting from a general astronomical viewpoint — material will tend to accumulate there. (The Trojan asteroids Agamemnon, Achilles and Hector, for example, orbit in the L4 and L5 points of the Sun-Jupiter system.) However, in neither the dedicated search nor the general scans was any evidence of probes found.
Increasingly, other near-Earth orbits are being scanned — this time by astronomers searching for potentially lethal asteroids. As a by-product of this research we might hope to find artifacts; so far, though, none have been found. Probes would give off heat, but no anomalous infrared signals have been observed; probes might be expected to transmit messages back to their creators, but no such transmissions have been detected.
Some people have claimed that long-delayed radio echoes (LDEs) are transmissions from ETC probes. The LDE phenomenon — radio echoes that appear between 3 and 15 seconds after transmission of the signal — has been observed since the dawn of radio, and it remains somewhat mysterious. Radio echoes from the Moon are common, but the echo appears 2.7 seconds after transmission of the main signal — this being the time it takes light to travel to the Moon and back. Echoes from Venus, the nearest planet, can only appear 4 minutes after the main signal. So neither the Moon nor Venus can be the cause of LDEs. One explanation is that they are radio returns from ETC probes that are beyond the distance of the Moon. A more prosaic explanation is that they are a natural phenomenon caused by plasma and dust in the Earth's upper atmosphere.44
Although the search for probes is not complete — indeed, the search has hardly begun, as Earth could be bathed in signals at certain frequencies and we would not necessarily know about them — all observations to date have given a negative result. (Interestingly, our telescopes have occasionally detected transmissions from a probe in the depths of our Solar System; but they are from the Pioneer spacecraft, not from an ETC craft.)
Mars has long been thought to be home to life,45 but much of the fuss stemmed from a mistranslation.46 Giovanni Schiaparelli, in a series of observations beginning in 1877, saw features on Mars that he called canali — an Italian word meaning "channels" or "canals." It is clear from his writings that Schiaparelli, when he named these features, thought natural processes had formed them. English-speaking astronomers, however, translated the word as "canals" — artificial structures connecting two bodies of water.
figure 13 Mars as photographed by the Hubble Space Telescope, when the red planet was at its closest to Earth.
Percival Lowell also saw the surface features recorded by Schiaparelli, and he finally counted 437 of them.47 However, Lowell failed to acknowledge he was working at the limits of observation; he did not realize evolution has primed the human visual system to look for familiar features in random patterns. He became convinced he saw artificially constructed linear canals, and he speculated that the canals supplied water from the polar caps to a desert world. The notion of canals was in the public consciousness anyway — the Suez Canal, a modern wonder of the world, had opened to navigation in 1869 — and the general figure 14 Percival Lowell. public was gripped by the possibility that intelligent beings had constructed the Martian canals. Science fiction writers were quick to use it as a source of stories. It was a popular and romantic notion, and even as late as 1960 some maps of the planet showed oases and canals; and several astronomers continued to believe that seasonal changes in the Martian surface markings might be due to changing vegetation patterns.
Meanwhile, in the early 1960s,48 Shklovsky discussed a peculiarity in the orbit of Phobos, the larger of Mars' two moons, and offered an ingenious explanation.
The orbit of Pho-bos is decaying. The peculiarity was that, according to observations made by Bevan Sharpless in the 1940s, the rate of decay was difficult to explain. A number of mechanisms was suggested — the effect of a hypothetical large Martian magnetic field, tidal interaction with Mar^ figure 15 Phobos, the larger of Mars' two moons, is a a possible solar influ- potato-shaped rock about 16 miles by 10 miles in size. It is almost ence _ but none of certainly a captured asteroid.
them were feasible.
Neither was the obvious explanation that Phobos was passing through the thin outer regions of the Martian atmosphere, as drag would not affect a rock the size of Phobos to the extent observed by Sharpless. The audacious Shklovsky wondered whether Phobos were hollow. A hollow Phobos would be less massive than its size would suggest, so its orbit would be affected more by the Martian atmosphere. If Phobos really were hollow, then it could not be natural: Shklovsky thus suggested that the satellite was artificial — the product of a Martian civilization. (It was a suggestion more imaginative than anything in the books of von Daniken, yet it was based on the best available observational data.) Shklovsky thought the satellite would have been launched millions of years ago, but other scientists thought the launch could have been more recent. Frank Salisbury pointed out that the Martian moons were discovered in 1877 by Asaph Hall, who used a 26-inch telescope.49 Fifteen years earlier, when Heinrich d'Arrest trained a larger telescope on the red planet, the conditions for viewing Mars had been better. How could d'Arrest have missed the moons in 1862? Was it possible, Salisbury asked, that the moons were artificial satellites launched between 1862 and 1877?
The romantic notion of an advanced Martian civilization capable of building canals and launching satellites did not survive the 1960s. It was laid to rest when the early Mariner spacecraft flew by at close range, returning photographs that showed none of the canals seen by Lowell. The Viking
landers of 1976 and the Pathfinder and Mars Global Surveyor missions of 1997 also failed to find canals. Similarly, the flyby missions showed that there is nothing artificial about Phobos. It is a small pockmarked piece of rock — almost certainly a captured asteroid. Furthermore, although its orbit is indeed decaying, recent measurements indicate that the rate of decay is only half that calculated by Sharpless. With this improved measurement, theorists can now explain the origin of the drag on Phobos: it is the result of tidal interaction with Mars. (Phobos draws closer to Mars by about 1 inch every year. The satellite will hit Mars some time within the next 40 million years, leaving a basin the size of Belgium. Although 40 million years is a short time on the astronomical scale, it is a long time on the human scale. A pity — it would be a spectacular event.)
The evidence from the various flyby, orbiting and lander missions almost killed the belief in an ancient Martian civilization. Almost, but not quite. In 1976, Viking photographed the Cydonia region on Mars, and NASA released the photographs soon afterward. Almost immediately, enthusiasts pointed out that one of the low-resolution photographs appeared to show a human face. You could make out an eye, a mouth, and a nostril (though the enthusiasts often failed to point out that the "nostril" was actually an artifact of the way the image had been processed, and did not
correspond to any physical structure on Mars). The face was large, roughly a square of 1 km, and seemingly carved out of stone. NASA scientists emphasized that this was a natural formation; the image was simply the result of sunlight falling on a hill one Martian afternoon. Others argued that the formation was an artificial structure; the stone "face" was proof that Mars was once home to an ancient civilization.
If you search through a large collection of random data long enough and hard enough, conveniently ignoring arrangements of the data that are of no interest and not defining beforehand what you are looking for, then eventually you will find something that seems remarkable. The surface of Mars covers 150 million km2; it would be strange if one of those square kilometers did not vaguely resemble something familiar. Planetary scientists argued that the Martian "face" has as much significance as the patterns you see in the coals on a fire. It was another instance of an observer imposing meaning on a meaningless pattern.
Mars Global Surveyor revisited the Cydonia region and took a more detailed photograph. The evidence for the face, of course, evaporated. (It is only fair to point out that the illumination is different in the two photographs. Nevertheless, modern computer imaging techniques can retain the detail of the Global Surveyor photograph while simulating the feature in the same afternoon light that Viking saw. If I scrunch my eyes, then I can just about make out Chewbacca from Star Wars — but no human face.)50
Michael Papagiannis argued that we must rule out the possibility that ETCs are in the Asteroid Belt before we can conclude that they are not here.51 The Asteroid Belt would be an ideal place for ETCs to set up space colonies. They could mine the asteroids for natural resources, and they would have plentiful supplies of solar energy. Who knows — perhaps the fragmentation of the Asteroid Belt components is the result of large-scale mining projects by ETCs? If space colonies were in the Asteroid Belt, we would not necessarily know about them; craft that were, say, 1 km or less in size would be difficult to distinguish from natural asteroids.
On the other hand, if they really are in the Asteroid Belt, there are questions to ask. Why have we detected no leakage of electromagnetic radiation? Why have we not observed a single object that possesses an effective temperature higher than is justified by its distance from the Sun? And why, if they are there, have they remained silent for so long?
Beyond the asteroids we can see numerous "anomalies" — like the axial tilt of Uranus or the retrograde orbit of Triton — that could be taken as evidence for tampering by ETCs. David Stephenson, for example, suggested that Pluto's unusual orbit is the result of an astroengineering project.52 These anomalies, however, can be explained more prosaically as the result of collisions and interactions that took place in the early history of the Solar
System. There is simply no need to invoke other explanations.
When we begin to discuss the outer planets we also begin to realize just how big the Solar System is. There are 50 billion billion billion cubic miles of space within a sphere that encloses the orbit of Pluto; and the Solar System extends to the Oort Cloud of comets, far beyond Pluto. The chances of finding a small alien artifact by accident are essentially zero. Only if an artifact draws attention to itself — by signaling us, perhaps, or by being in a visible location — will we detect it. We therefore cannot rule out the possibility that observational probes were once in the Solar System nor, indeed, that they are still here. Some would argue that until we can rule out that possibility, there is no Fermi paradox.
We can say with confidence, however, that no evidence for alien artifacts has yet been uncovered.53 If we do not observe them, why assume they might be there? (Besides, if probes are in the Solar System, we are still left with the problem of why they have left Earth alone.)
Perhaps we are looking in entirely the wrong place. The discussion has revolved around alien artifacts — evidence of engineered objects. Perhaps an ETC has been here and left information rather than things?
An entertaining SF story from the 1950s suggested that the reason so many people dislike spiders is that the class Arachnida consists of alien creatures. They were carried here on some spacecraft, and then escaped; humans, instinctively recognizing the spiders' alien heritage, recoil from them. (As we shall see later [page 189], all life on this planet is related; however much you may dislike spiders, you share a large part of your DNA with them.) In the 1970s, some scientists finally caught up with SF writers and made the suggestion that biological material might carry a coded message from an ETC. In theory, this would be possible: after all, the whole point of DNA is that it encodes information.
A message encoded in DNA seems an unlikely communication channel. For one thing, the sender could convey a message only to a planet that possessed the same biochemistry. (In our case, the sender's biochemistry would have to be based on L-amino acids, have protein synthesis based on the same genetic code as ours, and so on.) Even if it were possible for the recipient to distinguish between a natural sequence and an artificial message, over time the content of the message might become garbled through random mutations. And the vagaries of evolution might erase the message altogether. Nevertheless, a few investigations have been performed to test the idea,54 and analysis of certain types of viral DNA has found nothing resembling an artificial pattern. Now that biologists have sequenced the entire genome of several creatures, including man, more detailed searches could be performed for coded messages. Such searches must be low on the list of priorities for geneticists, but eventually someone will sift through the genome data looking for patterns. My guess is that patterns will be found, but they will have the same source as the Martian canals and the Cydonian face. Such patterns are evidence of intelligence — but at the observer's end of the telescope or microscope.
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