Alternate View Column AV-12
Keywords: Cygnus X-3, particles,cosmic rays, proton decay, underground detectors
Published in the March-1986 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 8/16/85 and is copyrighted © 1985, John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of the author.
Deep under Mt. Blanc, the largest mountain in the Alps, is a highway tunnel connecting France with Italy. In a big side room constructed near the tunnel midpoint is a large 150 ton cube of metal plates and photomultiplier light sensors. This apparatus, which physicists have named NUSEX, was designed to observe the extremely rare proton decay, the final self-destructive act of a basic building block of the universe (discussed in my first AV column "When Proton Meets Monopole", ANALOG, July, 1984). Up to now NUSEX has seen no proton decays. But it has detected something else, something coming out of the summer night sky of the northern hemisphere from the direction of the constellation Cygnus, the Swan.
In the remote deserts of Utah, at a location selected for clear air and remoteness from the lights of civilization is the Fly's Eye, a group of "compound eye" light detectors with photomultipliers for facets. It watches the night sky for the light made by the most energetic cosmic rays as they enter the upper atmosphere. Physicists using the Fly's Eye have found an exciting new result, the upper energy limit of cosmic rays. And they have also observed something else, something entering the atmosphere from the direction of the Swan.
Beneath the rolling hills of Ohio miners have burrowed deep into subterranean salt domes seeking salt for the round blue boxes on the supermarket shelves. In one of these, the Morton Thiokol salt mine near Cleveland, physicists have placed a large tank of water watched by photomultiplier light sensors to observe proton decays and energetic neutrinos from supernovas. Proton decays have so far been absent and neutrino counts sparce, but something else has appeared in their detectors, something coming from the Swan.
A third of a mile below the surface of Minnesota in the Soudan Iron Mine another proton decay detector named Soudan-I, a stack of 3456 ionization sensor tubes, is detecting something coming from the Swan. And similar reports are arriving from other underground detectors around the world ...
A new branch of experimental physics has recently emerged which some call "mineshaft physics" or even "troglodyte physics". Its key element is the mounting of large (and expensive) physics experiments deep underground so that sensitive detectors can be isolated from the strong cosmic ray bombardment at the earth's surface. Typically a few thousand feet of rock serves as a radiation shield providing a low radiation environment for these super-sensitive measurements. Experiments now operating in this "troglodyte mode" are probing the stability of the proton, or counting neutrinos from our sun, or searching for super-energetic neutrinos from supernovae.
The detector responses of such experiments are called "events". These must be carefully sorted to separate the interesting "signal" events from the unwanted "background" events. But now some of the background events are beginning to look more interesting than the signal. A maxim of physics is that when you look where none have looked before you may see what none have seen before. These underground experiments indeed seem to be seeing something new and unexpected. Some of their "background" seems to be an unknown kind of extremely energetic neutral particle which has been christened the "cygnon".
The source of the cygnons has been traced to an unusual binary star system in the constellation Cygnus. In recent years space-borne instruments have been able to examine the universe through a new window, the x-ray part of the electromagnetic spectrum. Bright sources of x-rays have been located and catalogged, and it has been found that the constellation Cygnus contains three bright x-rays objects. One of these called Cygnus X-3 is probably the most powerful source of high energy photons in the galaxy and has become the hottest topic in astrophysics today. Cygnus X-3 is on the other side of our galaxy, about 30,000 light-years from Earth. It is a binary star system, probably consisting of a neutron-star supernova remnant orbiting a normal star which feeds it hydrogen. The system has an orbital period of 4.79 hours. That's a remarkably short period: if a neutron star of 1 solar mass were orbiting our sun with that period, its orbit would be less than one solar radius above the sun's surface!! The 4.79 hour period can be used as a sort of "fingerprint" to tag radiation from Cygnus X-3, which should change in strength with this characteristic period. This period has been seen in Cygnus X-3 infrared, visible, x-ray, and gamma-ray emissions. The cygnons in the underground experiments have also been found to fluctuate with the same 4.79 hour period. This is confirming evidence that they come from Cygnus X-3. It also means that they travel at essentially the velocity of light; otherwise a spread of lower velocities straggling out across 30,000 light years would wash out the time variations.
Cygnons events observed with the Fly's Eye have truly enormous kinetic energies: up to 20 million times the mass-energy of a proton at rest, or 20,000 times more energy than particles from even the largest earthbound accelerators. They must have no electric charge because they travel in a straight line path from Cygnus X-3. Their path is not curved by the magnetic field of the galaxy, as the path of a proton or any other charged particle would be. Further, the cygnons are found to make many µ-mesons in their collisions with the atmosphere, suggesting that they are strongly interacting particles (like protons) rather than electromagnetic particles (gamma rays) or weak particles (neutrinos).
The zero charge of the cygnons is intriguing, for all of the known stable neutral particles can be counted on the fingers of one hand with a few fingers left over. The only truly stable neutral particles are photons, neutrinos, and neutral atoms. For good measure we could include the neutron, which is unstable to beta decay with a half life of 10.6 minutes. There are good reasons for eliminating each of these as cygnon candidates. As all good Analog readers know, relativity makes clocks run slower. Neutrons could possibly make it from Cygnus X-3 to Earth before decaying if they travelled so fast that relativistic time dilation slowed their internal clock until 10 minutes of internal neutron time became equivalent to 30,000 years of earth time. But this time dilation factor needs neutrons with 100 times more energy than the most energetic cygnon events which the Fly's Eye has seen.
Neutral atoms can be eliminated because the "empty space" between Earth and Cygnus X-3 is not completely empty. A pipe with a cross section one centimeter square stretching across this distance would contain about 5 grams of interstellar hydrogen. This is several thousand times more matter than required to strip some electrons from any energetic neutral atom and give it a net electrical charge. Neutrinos can be eliminated because they interact with matter too weakly, and also because the detected cygnons show a "horizon effect", diminished counts when Cygnus X-3 drops below the horizon. The gamma rays from Cygnus X-3 have about the right energy, but should, because they are electromagnetic particles, produce only 1/300 of the µ-mesons observed in cygnon events. No known neutral particle has all the characteristics of the cygnons. The inevitable conclusion is that the cygnon must be a new and previously unknown kind of particle.
So let's summarize the properties of this new particle. (1) It is has no electric charge (and no magnetic charge); (2) it has a rest mass estimated to be somewhere between zero and about 1/20 of a proton mass; (3) it is a strongly interacting particle; and (4) It must be stable or at least have a half-life greater than a day or so. As we have discussed in previous AV columns, the several variants of modern particle theory provide us with a whole beastiary of predicted but so-far unobserved particles: Higgs bosons, axions, gravitinos, monopoles, squarks, photinos, winos, gluinos, etc. Many of these could-be particles fit most of the criteria listed above, but none seem able to accommodate all. The predicted strongly interacting particles are either too unstable or too massive (or both). There is also the problem of how a stable strongly interacting particle with a mass less than that of a pi meson could possibly have been overlooked up to now. Cygnons are a profound puzzle, with no solution in sight at this writing (8/85).
An equal mystery is how Cygnus X-3 could possibly be producing such an enormous number of hyper-energetic particles, photons and protons as well as cygnons. The raw energy dumped by Cygnus X-3 is ten time greater than that seen by any other identified source of high energy particles. Half a dozen similar objects sprinkled around the galaxy would account for all of the cosmic rays which we observe. Cygnus X-3 is a double mystery.
The sociology of modern physics is perhaps not as widely appreciated as it might be. In the pre-twentieth-century Good Old Days physicists like Galileo and Newton could develop theories and perform experiments with equal facility. But in the modern era, this Age of Specialization, there are few renaissance men who can excel in both theoretical and experimental physics. The world of physics is now divided between two specialties, theorists and experimentalists, working opposite sides of this street which leads to the understanding of the universe. And their goals are somewhat opposite also. The dream of the theorist is to discover some regularity of nature which will allow him to predict what has not yet been observed, to reduce nature to a few beautifully simple equations which will explain everything. The dream of the experimentalist, on the other hand, is to discover a "new phenomenon" which is completely unexpected, is not understood, and which no theory had predicted. Recent examples of this are the Moessbauer effect, the CP violation of the KLo meson, and superluminal objects from quasars. Experimentalists keep busy when new phenomena are not at hand by checking theoretical predictions. Nevertheless, it is the possibility of finding a new phenomenon which provides a sizable part of the excitement and intellectual stimulation of experimental science and which compensates for the long hours and the less-than-you-could-get-as-a-plumber pay scale.
In the past decade the theorists have definitely been winning in this game. Theory breakthroughs like electro-weak unification, quantum chromodynamics, and GUTs have surpassed anything that experimentalists have been able to discover. So perhaps it is time for the experimentalists to have their inning. Cygnons have all the trappings of an important new phenomenon. The testing now begins to see if they are real, and if they are indeed a new kind of particle. And the mysterious energy engine driving Cygnus X-3 demonstrates our need for deeper understanding in astrophysics.
We all are very fortunate to be alive in this age of discovery and scientific
adventure. New phenomena are being discovered, mysteries of the universe
unravelled as we watch. Even now workers laboring in the deepest mines are
unearthing nuggets of the purest gold, discoveries of new and unknown particles
coming from the stars, particles which promise changes in our fundamental
understanding of the microcosm. I tell you, Readers, it's an experimental
physicist's dream come true!!
Followup note: Shortly after this column was written, Cygnus X-3 "switched off". Although newer and better detectors have subsequently come on line, they have not provided any more evidence about the mysterious particles that the system seemed to be producing during its active period. Observational astrophysics can be frustrating.
M. M. Waldrop, Science 228, 1298 (1985);
A. Dar, J. J. Lord, and R. J. Wilkes, Physical Review D (to be published).
M. L. Marshak, et al., Physical Review Letters 54, 2079 (1985).
G. Battistoni, et al., Physics Letters 155B, 465 (1985).
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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