© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

24.2.1  Alternative Channels:  HEPs, Neutrinos, Gravitons and Tachyons

In 1977, D. M. Jones proposed using high energy particles (HEPs) in the attention-getting or acquisition mode of interstellar communication.²²⁰¹ To avoid the disturbing effects of planetary magnetic fields, receivers and transmitters should be located in space or on nonmagnetic bodies such as Luna. Taking into account the problem of beam spreading, Jones calculates that to signal across a distance of 10 light-years ETs will need a 10 ampere transmitter beam consisting of protons, electrons, or ether HEPs. The beam energy should average about 1000 TeV (10¹⁵ eV).* The transmitter will require about 10¹¹ watts of power, about two orders of magnitude higher than the largest accelerators constructed by humanity to date. The detector area should span an area of about 10 km².

HEP beams, according to Jones, are so unusual that they must necessarily call attention to themselves as artificial in origin. Information may be impressed on these beams in at least three different ways. Most primitive is content modulation -- 10 minutes of electrons, then 5 minutes of protons, then 7 minutes of electrons, and so on. This would be technically difficult because of the large mass difference between protons and electrons (1836:1). The bit rate also would be low. Another possibility is energy modulation of the particle stream, which can be accomplished at the transmitter end quite accurately. But changing energy alters velocity. Packets will be delayed to varying degrees, so large pauses must be left between pulses. Receiver technology is complicated, and again the bit rate is low. Perhaps the third alternative -- pulse modulation -- is best. In this scheme, an homogeneous beam of particles with equal energy is pulsed in "dots" and "dashes" something like Morse code. If the beam is powerful enough, maximum bit rates may approach 1000 bits/second.

HEP beam communication has already been reduced to practice on Earth. In April 1972, Dr. Richard C. Arnold at the Argonne National Laboratory conducted an experiment in remote signaling using 0.012 TeV beams of muon particles.³¹¹² Using Morse code, Arnold successfully transmitted a series of "V"s (dot-dot-dot-dash) via muon beam. The stream of information-bearing particles passed through a two-meter-thick wall and traveled 150 meters before reaching a "receiver" consisting of two coincidence counters. The message was encoded using pulse modulation by mechanically interposing a heavy brass barrier alternately to block or to pass muon packets emerging from the accelerator. This experiment is the first known use of a particle accelerator to transmit a message.³¹⁶¹

Dr. Arnold is of the opinion that muon-HEP beam communication systems are potentially competitive with radio and microwave over planetary distances. This is in part due to the fact that charged particles curve in a magnetic field and thus can be made to follow trajectories parallel to the surface of a world. Unfortunately, this very property proves to be the principle disadvantage of the HEP scheme in interstellar communications. As the author has pointed out, the Galactic magnetic field gives rise to unacceptable drifts in beam alignment even over fairly modest distances.²⁷¹¹ Solutions include increasing the beam energy to 10,000,000 TeV (suggested by Jones) or switching to uncharged HEPs such as neutrons.²⁷¹² Recent studies indicate that it may be possible to latch onto neutral particles and control their motion by coupling to the nonuniform positive and negative regions within their internal structure.²⁸²²

Another exciting possibility for interstellar communication is neutrino beams.²⁸²⁵ Neutrinos are extremely stable, neutral, massless particles which travel at the speed of light just like photons. They are ubiquitous throughout the universe, produced first at the time of the Big Bang of universal creation (power flux about equivalent to that of the photonic 3 K background radiation)²¹⁹⁸ and later by the hot fusion reactions occurring in the interior of Sol and other stars.³¹¹⁴ The principal advantage of neutrinos is their tremendous penetrating power. A very high-energy photon can burrow a few centimeters into a chunk of solid lead before it is absorbed, but neutrinos can pass through about 50 light-years of lead before there is a 50% chance they will be captured. It is estimated that only one particle of every trillion passing through the Earth is absorbed by our planet.¹⁵⁵⁷

Here, possibly, is the equivalent of the international cable traffic passing around Sagan’s New Guinea islanders. Each second many tens of thou sands of these phantom particles zip though our bodies without interaction. Could this be the "seashell communications channel" we are looking for?

First-generation neutrino detectors were understandably primitive in design. The earliest model was constructed 1500 meters below ground level in the Homestake gold mine in Lead, South Dakota in 1970 by Raymond Davis. It has since been used to detect neutrinos emitted by Sol. The subterranean location serves to provide a natural shield from cosmic ray "noise."

In the main experiment, a huge tank filled with 400,000 liters of perchloroethylene or C₂Cl₄ (commercial dry cleaning fluid) was placed at the bottom of the mine shaft. When a neutrino struck one of the chlorine atoms (of which there were about 10³⁰ in the tank) it is captured. The chlorine transmutes into an atom of chemically inert but radioactive argon gas. Even though Sol gives off about 10³⁸ neutrinos/second, the interaction of these particles with matter is so weak that only about one argon atom is produced in the tank each week. After a period of months, the experimenters flush the vat with helium to concentrate the argon and then carefully measure its concentration with delicate radiation counters.

Second-generation neutrino detectors have now been constructed which permit the sending of messages by neutrinos. Neutrino telecommunication has been reduced to practice on Earth today.³²²⁹ In 1977, A. W. Sáenz and his coworkers of the Naval Research Laboratory in Washington, D.C., established the world’s first neutrino communications link over a distance of 1 kilometer.³¹¹³

The 0.4 TeV proton accelerator at Fermilab, using 40 megawatts of power, was used to generate a pulsed beam of 10¹³ protons per pulse at the rate of one pulse every eight seconds. Each pulse lasted only 0.02 millisecond. The protons were sharply focused by a magnetic horn and directed onto an aluminum target. A variety of short-lived mesons were produced, flying off down a 400-meter tunnel wherein they decay into a 0.015 TeV neutrino beam with a spread of only 206 arcsec (about one-twentieth of a degree). There were about 10¹⁰ neutrinos in every pulse.

The beam was aimed at a liquid neon bubble chamber measuring 4 meters in diameter. It was located about one kilometer away from the source and served as the receiver for the neutrino transmissions. The particle beam generated by the Fermilab accelerator was sufficiently intense to produce one observable neutrino interaction per pulse in the 25 tons of detector material. The liquid neon receiver picked up a series of Morse "dots" transmitted to it from Fermilab. Communication was established.

When neutrinos interact with water, they emit a forward cone of decay products giving rise to what is called Cerenkov radiation. Cerenkov rays are photons emitted by any material object which is traveling faster than the speed of light in that medium. Sáenz and his colleagues suggest using Cerenkov counters to observe neutrino-induced interaction events in large bodies of water located 10,000 kilometers from the source (here assumed to be Fermi lab). Here’s how such a communications link might work.

Neutrinos generated by Fermilab would be directed thousands of kilometers straight through the mantle of the Earth to arrive in a large body of water at the surface containing 100 million tons of liquid. This might be a lake 1 kilometer wide and 100 meters deep. As the particles interact with the fluid, muons are produced. These travel about 50 meters in water, emitting along their path a 41°-wide Cerenkov cone consisting of about 200 photons per centimeter of path length in the visible wavelengths of light. These flashes of light would be picked up by photomulipliers and registered as information-carrying, communicative neutrino signals. Since a single counter can monitor water volumes of about 10⁶ tons, only 100 Cerenkov counters are needed in the "receiving lake." About 2500 events/hour should be detectable, a bit less than one per second.

In order feasibly to use such a method for interstellar communications, substantial technological improvements must be made. If we optimistically assume a beam spread of only 10^-6 arcsec using huge spaceborne transmitters, a beam energy of 1000 TeV, and require a bit rate of 1 bit/second across a range of 100 light-years, the receiver must consist of a giant sphere of water weighing 10¹⁵ tons and measuring about 124 kilometers in diameter. (This is about one-tenth of all the water stored in Earth’s polar icecaps.) About 10⁹ Cerenkov counters would have to be deployed, a mission which a Type II society could probably handle with ease if it chose to do so. Going still further afield, imagine that a Type III civilization sets up a Library World near the Galactic Core and receives broadcasts from planets like Earth at a rate of 10¹³ bits/year using a water receiver as described above. The average distance to the Library will be 30,000 light-years, so detectors must monitor the equivalent of 50 Earth-oceans’ worth of water to ensure adequate reception. While this certainly is not impossible, and xenologists recognize that big problems require big solutions, the neutrino-transmitter/water-detector scheme does seem rather tendentious and inelegant.

What we need is more sensitive receivers. One oft-mentioned possibility for third-generation devices involves using gallium to capture a neutrino (which then transmutes into germanium). A large tank filled with liquid gallium metal could be about 100 times more sensitive than Davis’ chlorine/argon system. Unfortunately, 20 tons of material would be required.¹⁹⁸⁷ This represents about 5 years of the current world production of the element,³¹¹⁴ and at the present price of $550/kilogram the receiver will not be cheap. A related proposal, in which indium metal is used to absorb a neutrino and change into tin, would require only 3 tons of the pure element because of its high natural isotopic purity.¹⁵⁶² An indium receiver of this size would be about three times more sensitive than the gallium detector.

Many other kinds of advanced receivers have been discussed in the literature. One such was elaborated by Dr. M. Subotowicz of Poland at the 3rd International CETI Review Meeting, held in Amsterdam on 4 October 1974 as part of the 25th International Astronautical Congress.¹⁰⁹³ In his paper "The Use of Neutrinos in Interstellar Communication," Subotowicz discussed the technical problems involved in generating, modulating, collimating, transmitting and receiving neutrino beams, and he outlined some possible communication system designs. One of these designs involved a detector consisting of a single crystal of absolutely pure cobalt-60, weighing about 9 tons (a cubic meter in volume) and maintained at a constant cryogenic temperature of 0.01 K. Such a system would provide a favorable signal-to-noise ratio against the normal solar/stellar neutrino background and would permit extraterrestrial neutrino communications to be detected across interstellar distances. The receiver would be small enough, to fit in a modest-sized starship, and at least one writer has speculated that an advanced civilization might use such means to talk only with its peer races, to the deliberate exclusion of the humbler "peasant societies" using primitive radio wave systems.**¹¹⁴²

Xenologists can imagine still more sophisticated devices. In theory, to see a neutrino you need a receiver-target which is very rich in neutrons. The optimal detector configuration might possibly involve a free neutron gas (see Chapter 19). A specially shaped ultradense neutronium sheet might serve as a kind of lens to concentrate and to detect neutrino impulses. If magnetic monopole particle accelerators are available and neutron gas receivers are already set up, an advanced galactic neutrino telecommunications system may be as cheap to operate as an electromagnetic network -- and with competitive performance characteristics as well.

While both HEPs and neutrinos are in some sense "proven" or "practical" systems, the concept of gravity wave signaling remains at the theory stage of development.¹³⁴⁵ Much as accelerated charged particles give rise to electromagnetic fields, General Relativity theory predicts the existence of a radiation of gravity generated by accelerated masses. Theorists are firmly convinced that gravitational radiation does exist, and there is now growing experimental confirmation of this supposition.^3227,3228

Harnessing gravitational radiation will open up a whole new communication band from DC up to GHz frequencies and beyond -- but made up of gravity fields rather than electromagnetic fields. Gravity waves should have penetrating power even better than neutrinos. They experience very little attenuation when they pass through material objects, and they travel at the speed of light.

To set up a working communication system, we first must have a gravity wave transmitter. In principle, any mass which undergoes translational or circular acceleration emits gravitational radiation. But gravity energy is far weaker than electromagnetic energy. For example, a locomotive engine spinning so fast that it is just about to fly apart from centrifugal force (about 6 revolutions per second) would generate only 10³² watts of gravity wave energy.³¹¹⁶ The waves radiated by the whole Earth have an energy of only 10^-3 watts, and the total emission from the entire solar system amounts to just a few hundred watts. This is insufficient for the purpose of inter stellar communication.

To make a good transmitter we need lots of mass concentrated in very dense clumps, and those clumps must be moved very rapidly (close to the speed of light) in order to generate significant quantities of gravitational radiation.²⁰¹⁴ Hawking black holes (HBHs) probably would be ideal for this purpose. Once one has been captured or manufactured and an electrical charge imposed upon it, it may be manipulated by electric fields powered by the spontaneous evaporative energy thrown off by the HBH. For instance, a 10¹² kilogram HBH spinning with a tangential velocity of 130 meter/second should emit 2 x 10¹² watts of gravitational energy. This is equal to the spontaneous evaporative power output. (Such a system should be stable for about 1 eon.) Larry Niven has discussed a similar idea in his science fiction story "The Hole Man."⁶⁸⁶

At the end of every communications system there is a receiver. In a radio receiver, an electromagnetic wave accelerates free electrons in the antenna; in a gravity receiver, the passage of gravitational radiation will cause the physical deformation of the mass comprising the receiver. Pioneering work on such detectors was initiated by Joseph Weber at the University of Maryland, starting in the early 1960’s.⁶⁵⁸ Weber built a gravity wave detector consisting of a 1-ton aluminum cylinder which could be driven into oscillation by passing waves. These oscillations would then be picked up by piezoelectric strain sensors mounted on the surface of the cylindrical bar. The apparatus was supposed to be able to measure physical deformations on the order of 10^-17 meter.

Second generation detectors, currently under construction or already in operation, aim for two orders of magnitude improvement in sensitivity -- down to about 10^-19 meter.³¹¹⁵ At least six different schemes have been proposed.

The first approach is to take a Weber-type bar, make it more massive (say, 10 tons instead of 1 ton), and cool it down to liquid helium temperatures to reduce thermal noise. The system is further isolated from extraneous external vibrations by using superconductive magnetic levitation.^3321,3319 A second approach involves using a dielectric monocrystal -- for example, a large, very pure, single crystal sapphire -- instead of a massive metal bar. The gain in the receiver is far higher using monocrystals, which compensates for the reduced mass. A third approach is to set up a laser interferometer with two arms laid perpendicular. The passage of gravity waves shortens one branch and causes a measurable visual fringe shift. The fourth detector, called the Braginsky notch capacitor, records the change in electrical capacitance of a metal bar with a notch cut in it, as a gravity wave passes and changes very slightly the length of the notch. Fifth, there is the Doppler shift tracking technique. The Doppler shift in signals emitted from a spacecraft are carefully monitored. When a gravity wave passed by, it jiggles the ship relative to Earth and thus produces a blip in the Doppler shift measurement,³³¹⁶ Finally, there is the superconducting ring magnet detector. In this scheme, an electric current induced in the superconducting metal ring flows indefinitely. This persistent current generates a magnetic field perpendicular to the plane of the ring. When the shape of the ring is disturbed by the passage of a gravity wave, the magnetic flux through the ring changes and can be measured by a device called SQUID (Superconducting Quantum Interference Device).³¹¹⁷

Dr. Kip Thorne, theoretical physicist at CalTech, estimates that within a decade or so third generation devices will become available and extend sensitivity down to 10^-21 meter.³¹¹⁵ This should be sensitive enough to map the entire "gravity wave sky."⁶⁵⁷ Assuming an HBH gravitational radiation transmitter as discussed above with a power output of 10¹² watts and a directional gain of 10⁸ (about 1 arcmin focus), Thorne’s proposed devices could probably receive communications from a sender located 100 light-years away. High gravitational antenna gains could be achieved by using an entire star as a lens.³¹¹⁸ Using a body the mass of Sol, a nearly parallel beam of gravity waves could be created, a beam with a diameter of 1000 kilometers that would not diverge appreciably out to distances on the order of 10,000 light-years. The focal length of such lenses would be of the same order as the dimensions of the solar system. Xenologists suspect that if the transmitter problem can be licked, gravity wave communication may be quite feasible across interstellar or even galactic distances.

The technology for tachyon signaling does not yet exist on Earth. Indeed we do not even know if tachyons exist at all. But the concept,, which has only just moved from the idea to the theory stage of development quite recently, offers the fascinating possibility of hyperoptic (faster-than-light) communications throughout the entire universe. (See especially Alväger and Kreisler,¹⁴⁷⁹ Antippa,¹⁴⁹⁵ Baltay et al,¹⁴⁹⁸ Bers, Fox, Kuper and Lipson,¹⁴⁷⁶ Bilaniuk, Deshpande and Sudarshan,¹⁵¹⁵ Bilaniuk and Sudarshan,¹⁵¹⁶ Bilaniuk et al,¹⁵¹⁷ Everett and Antippa,¹⁴⁷⁷ Feinberg,^648,1492 Fox,¹⁵⁰⁴ Kreisler,¹⁵¹⁸ Mignani and Recami,^1507,1519 Newton,⁶⁴⁵ Parmentola and Yee,¹⁴⁹³ Raychaudhuri,¹⁵²¹ Recami,³²⁵² Recami and Mignani,¹⁵¹¹ Taylor,¹¹⁹⁰ Trefil,²⁰²⁶ and the short bibliography by Feldman complete through 1973.¹⁵¹⁴) As Stephen L. Brown at Stanford Research Institute points out:

Tachyons could be used for communication systems. Such systems would be useful only where ordinary electromagnetic radiation is too slow, as in interstellar communication. It would seem likely that any extraterrestrials with high technology would be aware of tachyons (if they exist) and would use them for communications instead of waiting centuries for replies at the speed of light. Perhaps the Project OZMA concept of monitoring electromagnetic radiation for intelligible patterns will turn out to have much less potential for interstellar contact than a tachyon monitoring system.¹⁵¹⁷

Tachyons may represent close to the most perfect signaling system we can imagine. The message carriers will always travel faster than light and in fact may go arbitrarily close to infinite velocity. The faster a tachyon goes the less energy it requires -- a transcendental tachyon moving at infinite speed has zero energy. So far as we know there is very little tachyonic background noise, and these particles (if they exist) should not interact appreciably with ordinary matter or galactic magnetic fields. The only requirement for optimal message carrier not yet satisfied is that message particles should be easy to generate, launch, and detect. But ETs may well possess the requisite technical knowledge to make the dream of hyperoptic signaling a reality.

The theoretical superiority of tachyons over photons rarely is fully appreciated. Dr. Martin Harwit of the Center for Radiophysics and Space Research at Cornell University has provided several mathematical formulae from which a simple calculation of the comparative bit rates of the two message channels may be made.³¹¹⁹ If bandwidth and detector size are held constant, the ratio of tachyon bit rate to photon bit rate is equal to (mlc/h)³/N, where m is tachyon mass, l is photon wavelength, N is tachyon velocity in units of c (speed of light), and h is Planck’s constant. This ratio may be called the Tachyon Advantage, and is tabulated in Table 24.1 above for various wavelengths of electromagnetic radiation, assuming tachyonic mass equal to the mass of the electron. Note the tremendous theoretical advantages of transmitting information with tachyons rather than with radio waves or photons of visible light. The tachyons are also traveling incredibly swiftly, yet another advantage over photonic signals.

What about energetic efficiency? Although the calculations are highly speculative, it would appear that the ratio of the theoretical tachyonic efficiency in bits/joule-sec to the theoretical photonic efficiency in bits/joule-sec is equal to (mlc/h)². For tachyons having the mass of an electron, only high-energy x-rays (10²⁰ Hz and higher) are more efficient. If proton-mass tachyons are transmitted, only powerful gamma-ray photons (10²³ Hz and higher) should be more energy-efficient per bit.

If the existence of tachyons is finally verified,*** and if the generation, transmission and detection of these fleeting particles can be achieved with reasonable equipment., sentient ETs races may have at their disposal one of the cheapest and fastest communications systems imaginable. Perhaps someday human scientists may learn to tap this channel. The first tachyonic interstellar signals we receive may say: "Greetings! Welcome to the Galactic Club!"

* The largest machines on Earth today can achieve about 0.5 TeV, and 10 TeV machines are in the planning stage.

** According to the Conference Report, strange neutrino "signals" have already been detected on first- or second-generation equipment:

Professor G. Marx {Department of Physics, Budapest University, Hungary) was interested in Dr. Subotowicz’ theories of neutrino communication and reported that on 4 January 1974, a Philadelphia team had received bursts of neutrino pulses, where groups of pulses (approximately 1 microsecond pulse duration and 1000 pulses per burst) were received. Twelve such groups were received and then the counters saturated. Although obviously open to the wildest speculation, these signals were almost certainly due to the initial stages of supernovae explosion and stellar gravitational collapse.¹⁰⁹³

*** Nuclear physicists have searched for evidence of tachyons for more than a decade. For the records of these experimental investigations, see Alväger and Kreisler,¹⁴⁷⁹ Ashton et al,³¹²⁰ Baltay et al,¹⁴⁹⁸ Bartlett and Lahana,¹⁵⁰⁰ Clay and Crouch,⁶⁵⁴ Danburg and Kalbfleisch,¹⁵⁰² Danburg et al,¹⁴⁹⁶ Davis, Kreisler and Alväger,³¹²¹ Feinberg,⁶⁴⁸ Kreisler,¹⁵¹⁸ Murthy,¹⁵¹⁰ and Thomsen.⁶⁴⁶

Last updated on 6 December 2008