Solution They Are Signaling But We Do Not Know How to Listen

The world should listen then — as I am listening now!

Percy Bysshe Shelley, To a Skylark

Perhaps large-scale interstellar travel is unattainable, either for crewed star-ships or for probes. This would explain why we have not been visited, but not why we have not heard from them. Fermi simply asked: "Where is everybody?" The question refers to more than the mere absence of visitors; it refers to the absence of any evidence that they exist.

If interstellar travel is indeed unattainable — something ETCs would presumably quickly discover — then why should they hide? An ETC need not fear invasion by an aggressive neighbor, since any neighbors would be too distant to pose a threat. They have nothing to lose by signaling, and the potential reward is huge: mutually satisfying dialogs with equally advanced civilizations. Furthermore, telecommunication is cheaper than travel. (You are more likely to use the telephone or e-mail to keep in touch with antipodean relatives than you are to travel there by jet.) But if advanced civilizations are out there, educating each other, gossiping, holding conversations that are the Galactic equivalent of the Algonquin Round Table — then why do we not overhear them occasionally?

One extremely plausible answer is that we do not know how an ETC would choose to send a signal. We therefore do not know how to listen.

It is certainly true that we cannot know what communication technology ETCs might possess. As my editor pointed out, if a radio engineer from 1939 were somehow transported into the New York of 2002, he could build a radio receiver and conclude there were almost no useful radio broadcasts being made: he would not know about FM. Similarly, he would be blissfully unaware of communication devices employing lasers, fiber optics or geosynchronous satellites. So it is conceited of us to suppose we can know what communication channels are available to a technical culture that may be a million years in advance of our own. If they wanted to talk to each other secretly (maybe they do not want to influence the development of young species like our own?) then presumably they could maintain secrecy without difficulty. But things are very different if they want to be heard, and heard widely. We can assume that every civilization must obey the laws of physics; moreover, any ETC will know that other ETCs must obey those same laws. Since we all have to pay our energy bills, the number and types of signal that can reasonably be sent is quite restricted. Let us examine the advantages and disadvantages of four methods of communication: signals using electromagnetic waves, gravitational waves, particle beams, and hypothetical tachyon beams.

Electromagnetic Signals

The obvious way to send information is via electromagnetic (EM) radiation. Not only does it propagate at c, the fastest possible speed, it propagates over interstellar and intergalactic distances. (We know that EM signals can operate over interstellar distances because natural objects indicate their presence in this way over vast reaches of space. Astronomy is essentially the science of recording and interpreting these signals. We use visible light when we look at stars with our eyes or photograph them with optical telescopes. We use radio waves when we study the sky with radio telescopes. Increasingly we use the infrared, ultraviolet, X-ray and gamma-ray wave lengths, particularly in satellite experiments. If we can study natural objects over interstellar distances using the EM radiation they emit, then presumably we can do the same with artificial objects.)

For many years the working assumption made by researchers looking for ETCs is that technological civilizations will build powerful EM transmitters, broadcast a signal, and modulate it in order to convey useful information — perhaps, if we are lucky, they will broadcast their "Encyclopedia Galactica." In the next section I will discuss in detail how we might detect purposeful EM signals. Here, I want to argue that it may even be possible to detect EM radiation that leads to the discovery of inadvertent markers or beacons of K2 civilizations. (Detecting inadvertent markers of a K3 civilization might be even easier.) Even an inadvertent beacon would convey a tremendous amount of information: that life exists on another world, that it is technologically advanced, the location of that world, and so on.

We have already discussed why K2 civilizations might construct Dyson spheres. A Dyson sphere would radiate just as much energy as the central star — the energy has to go somewhere — but it would presumably do so in the infrared. In essence, the sphere would radiate because it is warm — about 200-300 K. So one way to search for an ETC would be to look for bright infrared sources at a wavelength of around 10 microns: such sources might be the waste heat from astroengineering projects.

A search by Japanese astronomers for artificial infrared sources out to a distance of 80 light years found no plausible signatures from Dyson spheres.107 Although several stars show a large excess emission in the infrared, this happens to be because they are shrouded in dust. However, we cannot conclude from this that there are no ETCs within 80 light years; ETCs may choose not to construct Dyson spheres there for a variety of reasons. Even if Dyson spheres are common, really advanced civilizations — as Marvin Minsky pointed out108 — would consider radiation at any temperature above the cosmic background temperature of 3 K to be wasteful. Perhaps an ETC advanced enough to construct a Dyson sphere is advanced enough to squeeze every last drop of useful work out of a star's radiation, leaving waste heat at 4 K. Perhaps we should be looking for points in space that possess a small temperature excess over the microwave background.

In 1980, Whitmire and Wright gave another example of how inadvertent beacons can be transmitted by electromagnetic radiation.109 They asked what would happen if a civilization used fission reactors as an energy source over long periods of time. One of the problems with fission reactors is the need to dispose safely of radioactive waste material. And one proposed disposal method is to launch it into the Sun (though I, for one, would not be too thrilled at the prospect of having tons of radioactive waste perched on top of a chemical rocket). If an ETC used its star as a dumping ground for radioactive waste, then the spectrum of the star could exhibit characteristics that would not easily be interpreted as natural. For example, if we saw a stellar spectrum containing large amounts of the elements praseodymium and neodymium, then our interest would be caught. Furthermore, the alteration in the spectrum would not be a brief flicker; the spectral evidence of their nuclear waste disposal policy would be visible for billions of years. (A civilization might deliberately alter its star's spectrum in this way to create a beacon. This possibility was first suggested by Drake. Another method of using one's home star as a beacon was suggested by Philip Morrison: put a large cloud of small particles in orbit around the star in such a way that the cloud cuts off the starlight for a viewer who is in the plane of the cloud's orbit. Move the plane of the cloud and the distant viewer sees the star flash on and off. Variable stars naturally alter in brightness, but if the star flashed in a pattern that represented prime numbers, for example, then the distant viewer could quickly rule out a natural phenomenon.110 )

So far, no EM beacons — inadvertent or not — have been identified.

Gravitational Signals

Besides electromagnetism, the only other force we know that acts over astronomical distances is gravity. It too propagates at the speed of light, so perhaps ETCs might use gravitational waves to signal each other? Gravity, however, is a much weaker force than electromagnetism. To build a gravity-wave transmitter you have to be able to take large masses (of the order of a stellar mass) and shake them violently. It is debatable whether a K2 civilization would possess such technology. A K3 civilization might be able to build such a gravity wave transmitter, but why would it bother when EM waves do the job just as well and EM transmitters are so easy to construct?

The complementary problem of detecting gravitational waves is also much more difficult than the equivalent problem of detecting EM waves. It is so difficult, in fact, that terrestrial science has yet to build a functioning gravitational-wave detector. (Detectors such as LIGO will soon come online, but even if they are successful they figure 30 ligo, in Washington State, consists of two 4-km arms at right angles, each with laser beams in high vacuum. There is an identical observatory in Louisiana, and the two installations will work in tandem. The objective will be to detect gravity waves by searching for changes in length a thousand times smaller than an atomic nucleus.

figure 30 ligo, in Washington State, consists of two 4-km arms at right angles, each with laser beams in high vacuum. There is an identical observatory in Louisiana, and the two installations will work in tandem. The objective will be to detect gravity waves by searching for changes in length a thousand times smaller than an atomic nucleus.

will have the sensitivity to detect gravitational waves from only the most violent astronomical phenomena.111 The detectors will collect exceptionally interesting scientific data, but they will not find modulated signals.) So, given the difficulties of transmitting and receiving gravitational waves, it seems unlikely that an ETC would use them for communication.

Particle Signals

Cosmic rays, in the form of electrons, protons and atomic nuclei, can reach Earth over interstellar distances — and cosmic-ray astronomy is a thriving research field. However, charged particles like these would constitute a poor choice of communication channel because a transmitting civilization could not guarantee where the particles would end up: twisting magnetic fields throughout the Galaxy make the paths of these particles quite tortuous. Neutrinos are electrically neutral, so at first glance they seem a better choice for a communication channel. Unfortunately, neutrinos are difficult to study because they react so infrequently with matter; typically, a neutrino will pass through 1000 light years of lead before stopping! Nevertheless, despite the tremendous difficulties involved, astronomers have developed neutrino telescopes. 112

Neutrino Telescopes

The first such telescope was developed by Ray Davis, who wanted to study the neutrinos that are generated in nuclear fusion reactions in the heart of the Sun. His telescope was in essence a 100,000-gallon vat of perchloroethy-lene (dry-cleaning fluid) buried almost a mile beneath the ground in the Homestake gold mine in South Dakota. It was the strangest telescope anyone had ever constructed (there are stranger telescopes nowadays), but the setup was necessary because neutrinos are so elusive. The deep mine shielded the vat from other subatomic particles that bombard Earth; the dry-cleaning fluid provided enough chlorine atoms to guarantee detectable numbers of neutrinos.

Theory predicted that when a chlorine nucleus captured a neutrino it would turn into a nucleus of radioactive argon. So by detecting argon atoms, Davis was able to detect solar neutrinos. Of 1021 neutrinos passing through the vat each day, theory suggested that 6 events should take place; but the experiment found only 2 events per day. Davis' experiment continues to detect solar neutrinos, but only one third of the expected number — a finding that is of great significance for particle physics.

Mediterranean. Similar detectors are situated in mine shafts and figure 31 A virtual reality deep view of the

0.1-km2 Antares neutrino telescope, which will be located beneath the underneath mountains.

Mediterranean. Similar detectors are situated in mine shafts and figure 31 A virtual reality deep view of the

0.1-km2 Antares neutrino telescope, which will be located beneath the underneath mountains.

In February 1987, the Kamiokande detector in Japan and the IMB detector in America between them stopped 20 neutrinos in a period of a few seconds. Those neutrinos were produced in the famous supernova of that month: SN1987A. Now, SN1987A occurred in the Large Magellanic Cloud, about 170,000 light years away. Demonstrably, then, it is possible for neutrinos to travel interstellar, even intergalactic, distances and for a primitive technological civilization like ours to detect them. Perhaps ETCs use modulated neutrino beams to communicate with each other? Well, perhaps. But again we have to ask why they would do this when electromagnetic waves do the job far better and much more cheaply.

Tachyon Signals

We can speculate that extremely advanced ETCs will use tachyons to signal each other. If tachyons exist, and if it is possible to modulate a beam of them to carry signals, then no doubt they will be an attractive option for interstellar communication. Tachyon-based communication would obviate that irritating delay between asking a question and receiving an answer — a delay that can be hundreds or thousands of years. Unfortunately, as we saw earlier (see page 67), there is absolutely no evidence that tachyons exist, let alone that it is possible to use them to send signals.

Perhaps there are lots of civilizations out there, communicating with each other using gravitational waves, neutrinos and tachyons. Or perhaps they send signals using techniques we have not yet dreamed of — techniques that break no laws of physics but that are as exotic to us as fiber-optic com-

munication channels would be to a 1939 radio engineer. Since we cannot detect such signals it would explain why we have not heard from them; it would explain the "great silence" — if not the full Fermi paradox itself.

On the other hand, even for advanced civilizations, communication by EM waves seems to be a logical choice: EM signals are cheap to produce, the message moves as fast as is possible in a relativistic Universe, and the signals are easy to receive. If an ETC wanted to make its presence known to other perhaps less developed civilizations (civilizations like us, who can only listen for electromagnetic signals), then the EM spectrum might be their only option.

For these reasons, although it may seem conceited and it may mean we are missing out on Galactic conversations, many physicists would argue that we know how to listen for signs of extraterrestrial civilization: we should listen for their EM radiation. (In fact, given the level of our present technology, we have little option but to try and detect such radiation.) But at what frequency should we listen?

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