Note: This web version is derived from an earlier draft of the paper and may possibly differ in some substantial aspects from the final published paper.
Sir, D. M. Jones has suggested that high energy particles in the 1010-1015 eV range might be used in CETI work . Although he makes passing mention of the "distorting effects of interstellar magnetic fields," I find no reference to the drift (caused by the Galactic magnetic field) in alignment of the particle beam itself.
Qualitatively, if such a beam is aimed exactly parallel to the local Galactic field, there will be no VxB force whatsoever. However, if the beam is normal to the field, deflection is maximal. Instead of travelling in a straight path from Earth to target star, the particles (assuming they're charged) trace out a gigantic loop in space, circling the magnetic field lines at some radius R. A simple calculation reveals that even across relatively short transmission distances, curvature of the beam is unacceptable for any reasonable particle energy E.
The relevant relativistic equations are as follows:
Lines of magnetic force are known roughly to follow the spiral arm structure of the Milky Way and to possess a strength of about 10-5 gauss [2, 3]. Assuming the bean is properly aimed, it will curve around and just barely reach the target if R is one-half the distance thereto. If we send message via high energy protons across ten light-years as Jones suggests, E must be at least 1.4 x 1016 eV if q = 90o (This assumes, of course, a nice, uniform B field all the way a dubious proposition at best).
If 1010 eV particles are used, the target must lie within q ~ 0.00002o of the Galactic field vector - probably not the direction in which we wish to transmit! Even if 1015 eV protons are used, the situation is not much. improved. To each a target 10 light-years distant, it must lie within q ~ 2o, which is still a miniscule CETI 'window,' and transmissions farther than 10 light-years are even more restricted.
In view of the Galactic deflection problem, it is difficult to see how
charged particle beams could represent the preferred communication mode for
advanced extraterrestrial civilisations.
Mr. Jones replies:
I would like to thank Mr. Freitas for pointing out the distorting effects of a uniform magnetic field along the path of the beam. In my initial analyses I had assumed that such a field would be very weak, or that it would be confined to small volumes of space with random orientations. This would cause small tweaks in the direction of travel but over small distances, 10 light years say, the distortion should not be excessive.
The effect of a uniform field throughout the length of the beam would be much more disturbing as the beam would travel in a large circle. This bending could be taken into account by the sender when aiming the beam, the receiver would receive the signal (provided that the receiver was not farther away from the sender than the diameter of the circle). However, one vital piece of information would be denied the receiver: from what part of the Galaxy did the beam originate? Knowing that the beam was bent by magnetic fields, and that the sender would be aware that the beam would be bent, the receiver would be forced to the conclusion that the transmitter was nearby, close enough that the arc of a circle traced out approximated a straight line. An electromagnetic search could then be made of the area of the sky from which the beam came, electromagnetic radiation being the preferred mode of communication due to the relative ease of transmission and the much higher data rate. I suggest the use of high energy particles as the attention getting mode, the main message being "LOOK OVER HERE", since they would be very easy to observe, although the correct interpretation of their origin may not be so easy.
Because of the curvature and dispersion of the beam its use is a strictly local affair. The condition that the path of the beam approximates a straight line sets a lower limit on the energy of the particles used, dependent on the mass of the particles used, distance between sender and receiver, the angle the beam makes with the galactic field and the strength of that field. What is an acceptable level of curvature for the beam? I think that 20' is a reasonable deviation, remembering that the receiving intelligence will be aware of the curvature of the beam and will accordingly widen the search to nearby stars centred around the direction of the beam.
The strength of the galactic field is still in dispute, but it is believed to be between 10-1-10-5 gauss, its effect on the beam being proportional to sinq where q is the angle between the direction of the beam and the magnetic field.
Taking these facts into consideration, I would like to increase my
original estimate of the particle energy to the 1015-1019
eV range, and to suggest that protons be used in preference to electrons.