The Search for Extraterrestrial Intelligence

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Pulse Code Neutrino Communication

For SETI, neutrino communications are a beguiling region for investigation because neutrinos penetrate massive interstellar clouds and solid bodies with ease. And even if these near-zero mass particles cannot travel faster than the speed of light, they can travel at very close to that speed. So neutrinos are proposed as a means of interstellar communication or navigation that could be used by advanced extraterrestrial civilizations. Detecting neutrino transmissions carrying information would provide evidence of such civilizations. The challenge is the ease with which neutrinos travel through materials. This makes them extremely difficult to detect and means huge detectors are required. Large as they are, current neutrino detectors operate at very low efficiencies and would have difficulty in separating intelligent signals out of the neutrino background.A recent demonstration of information transmission using neutrinos illustrates the scale of the challenge.

The possibility of using neutrino beams to transfer information was suggested by A. W. Saenz and his colleagues in 1977. They pointed out that the Fermilab 400 Gev accelerator was sending neutrinos across 1 km to a bubble chamber detector, demonstrating the basics of neutrino communication. The transmitter was the Fermilab particle accelerator, itself of considerable size. It accelerated protons around its 2.5 mile track to 120 GeV and sent them through the main injector into a carbon target. This generated a dense stream of neutrinos carrying a message by means of a pattern of pulses achieved by switching the proton stream on and off. The neutrinos then travelled 1,035 meters, including 240 meters of earth, to the MINERVA neutrino detector, where the message was decoded. A massive surplus of neutrinos was required because only about one in ten billion could be detected.

The 170-ton detector is located in a cavern about 100 meters underground. Analyses in the communication experiment focused on neutrino interactions in the central tracker, comprising a three-ton detector that is fully sensitive to the neutrinos produced by the beam collisions. Detection of the signal made use of interactions that provided muons in the final state. At the reduced accelerator intensity used in this test, an average of 0.81 events was registered during each pulse of 2.25 x1013 protons, produced at 2.2 second intervals. With a time resolution of a few nanoseconds for the muons, it was possible to disentangle multiple events in the 8.1 microsecond beam pulse. The communications link achieved a decoded data rate of 0.1 bits/sec with a bit error rate of 1 per cent.

For communication a “1” or a “0” was represented by the presence or absence of a beam pulse, implemented by controlling proton beam pulses from the accelerator’s main injector. A “1” bit corresponded to a beam pulse with an observed event or events, and a “0” bit was a pulse with no events. The message was the word “neutrino” expressed in an abbreviated 5-bit ASCII code. This 40-bit message was subsequently encoded using a NASA/ESA Planetary Standard that expands the message to 92 bits. It was then concatenated with a 64-bit pseudo-noise synchronization sequence. The data received consisted of 3,454 records spanning an interval of just over 142 minutes, along with time stamps for each record.

The beam was a few meters wide at the face of the detector. Over 90 per cent of the neutrinos had energies below 10 GeV, although some reached 80 GeV. The energy spectrum peaked at 3.2 GeV. The authors report that ‘the accelerator throughout this study was operated at 25 pulses spaced by 2.2 microseconds, followed by a 6.267 s interval to form a 61.267 s supercycle.

 

Neutrino Communication System

The neutrino’s extremely small interaction with matter means that intense beams and massive detectors are needed for any feasible information link. More recently, John Learned and his colleagues have suggested that an advanced extraterrestrial civilization could readily overcome these problems of scale. They might therefore be making use of neutrinos for information transmission, which we might detect with our present resources. The ability of neutrinos to pass through matter with little or no attenuation is particularly advantageous within the galaxy because of the presence of large interstellar clouds of gas and dust.

In examining a communication system in which an advanced civilization aims neutrino beams directly at planets, Learned and his colleagues calculated that a 10 PeV neutrino linear accelerator 1,000 km long would be required, probably operating in space. A gigawatt of electrical power would be needed, perhaps derived from the star’s own radiation. This is very much an activity for a civilization advanced well beyond Earth in engineering capabilities, but no new principles are involved.

The neutrinos would be obtained by accelerating charged pions to about 30 PeV so that their decay would yield neutrinos in the 6.3 PeV range. The muons generated would be stopped by a thin shield. Depending on whether positively or negatively charged pions were used, a beam of neutrinos or anti-neutrinos would be produced. No natural mechanism to produce neutrinos in this energy range is known to exist, so their detection in our present neutrino detectors would indicate the activity of an extraterrestrial civilization. The direction of arrival would indicate its location. About 1,000 pages of information might be transmitted to Earth in a year by this means.

 

Modulating Cepheid Variables

Learned and his colleagues suggest that, as an alternative, the broadcasting of information in all directions could be achieved by an advanced civilization using a similar neutrino beam to modify the output of a class of stars called Cepheid variables. A Cepheid star dims and brightens in a repeated, regular cycle over days or months. Most importantly, the period of the cycle indicates the absolute brightness of the star. When the actual brightness as it appears on Earth is compared with this, the distance of the star can be calculated. These stars act as mileposts throughout the universe, indicating the distances separating galaxies and how the speed of separation changes with distance. Advanced civilizations will know that other civilizations will be observing these stars carefully, so that if they wish to reveal their presence to every one in their galaxy, or in other galaxies, they can do this by systematically altering the Cepheid brightness cycle by small amounts.

To make this change requires depositing pulses of energy into the core of the Cepheid star, which is theoretically possible with an intense neutrino beam. Again, the advantage of neutrinos is that they can pass through the outer layers of the star with very little attenuation. In contrast, other types of radiation, such as radio waves or light, are rapidly absorbed by the outer layers of the star. A coded sequence of neutrino energy pulses will rely on amplification in the Cepheid to modify its pulsations slightly, which will function as a form of modulation. This is comparable to the way acoustic or visual information is modulated on to radio waves for terrestrial transmission. If this is actually happening, the modulation will be detectable in records gathered on Earth during the past century. It can be analyzed, possibly enabling the information being broadcast to be decoded.

 

Neutrino Navigation

A possible system of neutrino interstellar navigation that might be implemented by an extraterrestrial civilization was suggested in 1994 by J.G. Learned and his colleagues at the University of Hawaii. The possibility of intercepting signals was based on the idea that a civilization traveling among the stars would make use of a navigation system that would require accurate synchronization of clocks over interstellar distances. The reason for synchronization is that chaos intrudes into the motion of astronomical objects and degrades their value for position fixing. Their proposed system made use of isotropic pulses of monoenergetic (45.6 GeV) neutrinos and anti-neutrinos from the decay of a Z0 boson at rest. The neutrino system was calculated to have a large signal-to-noise ratio advantage over an electromagnetic system using gamma rays.

These neutrino signals would be distinctive and unlikely to be mistaken for naturally occurring processes. The authors calculated that neutrino telescopes being developed on Earth might be able to detect such signals if the transmitter were within three thousand light years of Earth.