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The Mouse that Boomed

by John G. Cramer

Alternate View Column AV-34
Keywords: sonic boom, plasma, alien, spaceship, shock wave
Published in the August-1989 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 1/14/89 and is copyrighted © 1989, John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of the author.


    Perhaps, as many science fiction writers have suggested in their writings, our galaxy is populated by advanced civilizations that go about their business in the "downtown" area near the center of the galaxy and can't be bothered to communicate with technologically and culturally unsophisticated rubes that, like the human race, reside far out in the galactic boondocks. If this is the way things are, it may be hopeless to search for such civilizations, as SETI researchers have been attempting to do, by trying to receive their radio messages. A better way to do a SETI search might be to look for detectable radio-wave by-products of the activities of alien civilizations.

    SETI scientists have done this by looking for the equivalent of television signals that might emanate from the planet of a civilization that uses radio-wave broadcasts as we do. They have found no evidence of the equivalent of our radio/TV signals in our galactic neighborhood, but that result is inconclusive. Galactic civilizations may be so different or so far ahead of us that they don't use radio waves to communicate. Or perhaps it's just that TV-watching is incompatible with advanced intelligence.

    In any case, we need a less culture-dependent way of searching for advanced civilizations. So let's turn our attention from communication, which may be quite culture-dependent, to transportation, which should be more universal. It seems very likely that any advanced space-faring civilization would on occasion need to transport material objects from one star system to another for colonization, exploration, or trade. This would require a starship moving through space at a high velocity.

    Even given a worm-hole transport system like that described in my recent AV column [Analog, June '89], it would still be necessary to carry one worm-hole portal by starship to a distant location before the wormhole could be used for faster-than-light transport. Advanced space-faring civilizations will inevitably need to move mass from one place to another, and it seems reasonable that this will be done at the highest speed permitted by available resources (energy, reaction mass, ...). As it happens, massive objects moving rapidly through most regions of our galaxy will leave radio waves in their wake that may be detectable.

    Consider a supersonic jet airplane flying overhead. Even if the plane's engines were completely silent, the plane's passage would be apparent because of the sonic boom it produces, the shock wave left in its wake because it is traveling faster than about 0.3 kilometers per second, the speed of sound in air. The shock wave phenomenon is not limited to high performance jet planes. A motorboat traveling faster than the wave-speed on the water surface leaves a vee-shaped wake that is also a shock wave. A fast charged particle traveling through transparent plastic at a speed that exceeds the speed of light in the plastic medium (perhaps 60% of the speed of light in vacuum) makes Cêrenkov radiation, a kind of optical shock wave. And a massive body traveling rapidly through our galaxy should make a plasma shock wave that may produce detectable radio waves.

    In most regions of our galaxy the "empty" space between the stars is not empty at all; it is occupied by a plasma, a very thin gas of ionized hydrogen atoms and free electrons. Any traveling pressure variation (or sound) moves through this plasma in much the same way sound waves move through air. The molecules of air are electrically neutral, however, and to transmit a sound wave the molecules must physically bump together, requiring close contact. The analogous components of a plasma, ions and electrons, have electrical charges that can act over large distances, so they can jostle one another with their electric fields while they are well separated. Because of this difference, the speed of sound is very much greater in a plasma than in air.

    The actual speed of sound in a plasma depends on its temperature, which is related to its degree of ionization. A "hot" plasma (temperature ~ 106 K) such as might be found near the center of the galaxy has a sound velocity (S) of about 100 km/sec or 3.3 × 10-4 of the velocity of light (c). A "cold" plasma (temperature ~ 104 K) like that to be found in our galactic neighborhood has a lower sound velocity of S ~ 10 km/sec or 3.3 × 10-5 c. In either case any object moving through the plasma at a speed faster than S will produce a shock wave in the plasma medium, and this in turn will generate very characteristic and possibly detectable radio waves.

    The shock wave or "bow shock" has a characteristic vee-shape like the wake left in the water behind a speedboat. The shape of the bow shock provides a kind of speedometer for objects moving faster than the speed of sound S. The opening angle a of the "vee" (measured center-to-edge) is related to the speed V of the object by the relation: V = S/Sin a. For example, a bow shock making an angle of a = 30o (remember that Sin 30o = 1/2) means that V = 2.0 S, and so an object making a 30o bow shock is traveling at twice the speed of sound. This relation is independent of which medium is involved.

    In the case of a plasma shock wave, the acceleration of electrons in the plasma by the shock generates radio waves by the process called synchrotron radiation. Such radio waves are easily recognized because they have a distinctly non-thermal spectrum of frequencies and are strongly polarized. Therefore, to the extent that a massive object moving through the interstellar plasma energizes a bow shock, we have a method of looking for any interstellar space vehicle that moves faster than the speed of sound in the local plasma. We simply look for the radio waves from its "sonic boom".

    Have such radio waves ever been observed? As a matter of fact, radio waves from the bow shock of a rapidly moving galactic object have recently been discovered and reported in the journal Nature. The object making the waves has the catchy name of G359.3-0.82. Not satisfied with this designation, its discoverers, Farhad Yusef-Zadeh of NASA's Goddard Laboratory and John Bally of AT&T's Bell Labs, have nicknamed their discovery the Mouse because of its mouse- or tadpole-like shape. The Mouse's radio image shows a symmetric conical body trailing a tail which, viewed with 6 cm radio waves, shows a steady decay of surface brightness along its 12 arc-minute length.

    Yusef-Zadeh and Bally discovered the Mouse while using the Very Large Array (VLA), an interferometer array of large radio-telescope dishes located on the desert near Socorro, New Mexico, to study the radio waves coming from the center of our galaxy. They used the VLA because of the excellent angular resolution that it can provide. The Mouse, located in the line of sight toward the center of our galaxy, is in the constellation Sagittarius at right ascension 17h 44'04" and declination -29o57'05" in the southern sky. Its radio image is not far from that of a prominent ring-like supernova remnant radio source, G539.2-05.

    Determination of the distance of objects observed is often a difficult problem in astronomy, and is particularly so for unusual objects like the Mouse that cannot be referenced to standard stars and cannot be observed visually because of the dust that obscures our galactic center. Since the radio image of the Mouse is located near the supernova remnant G539.2-05, which is established to be near the center of the galaxy, Yusef-Zadeh and Bally have suggested that the Mouse is associated with G539.2-05 and is therefore the same distance from us, some 27,700 light years (or 8,500 parsecs) away. While this distance assumption is reasonable, it has not yet been verified by independent consistency checks. The Mouse could be much closer, lying anywhere along the line of sight between us and the center of the galaxy.

    The Mouse exhibits the polarized non-thermal radio spectrum expected from bow shock synchrotron radiation. As measured from head to extreme tail, it shows an opening angle of about a =10o, which corresponds to V = 5.8 S. That means that if the Mouse is located in the hot plasma of the galactic center, it is moving through that plasma with 5.8 times the speed of sound, 580 km/sec or 0.2% of the velocity of light, and making a sonic boom as it goes. This makes the Mouse unique. No other astrophysical object in our galaxy has ever shown evidence of such a high velocity relative to its surroundings.

    The quantity or energy in radio waves in the 1.4 GHz band received from the Mouse is about 2 Janskys (2 × 10-26 watts/m2-Hz), a very large energy flux considering the distance of the object. If the Mouse lies at the galactic center, it is continuously emitting about 1025 watts of energy as radio waves. The length of the wake, given that distance, implies that it has been radiating at about that energy level for perhaps the past 26 thousand years. Yusef-Zadeh and Bally speculate that the very strong power plant of the Mouse may be a rapidly rotating neutron star. They suggest that it might be a former member of a binary star system that has been propelled to a high velocity by the supernova explosion of its binary companion. However, the actual physical processes that could accomplish this are unclear, and there is some skepticism that this scenario can work. The Mouse is a mystery.

    Readers of this magazine should by now be wondering if the Mouse could be an alien starship. If the distance scale suggested by Yusef-Zadeh and Bally is correct, the answer is no. The Mouse would have to be a star-size object with star-class energy output and a huge kinetic energy that could only have been produced in some ancient cataclysm. But there remains the possibility that its discovers are wrong about how far away it is.

    Perhaps the Mouse is much closer, maybe only a few dozen light years distant, and it just happens to lie along the line of sight leading to the galactic center. In that case, to account for the energy reaching us, the energy output of the Mouse would be only about 1019 watts, equivalent to direct conversion of matter to energy at the rate of about 100 kilograms per second. Such energy generation, though inconceivably large by contemporary standards (1010 watts is the output of a very large nuclear power plant), might be within the capabilities of an advanced civilization.

    Because the local plasma is cooler and its sound velocity slower than that near the galactic center, the velocity of the object would also drop if it was close by in our galactic neighborhood. The speed of the Mouse would be only about 58 km/sec or 0.02% of light speed. The required duration of its energy output, based on the length of the wake, would drop to about 200 years, long enough to have traveled about 0.04 light years during that period. Starship Mouse would be expending a huge amount of energy for a long time while not getting very far.

    Is the Mouse likely to be an alien starship? Of course not. In astrophysics the hypothesis that "little green men" are producing a new astrophysical phenomenon is always tempting; remember the discovery of pulsars. But a reasonable and plausible explanation involving only natural processes has, up to now, always been found. Probably this will be so for the Mouse also. A reasonable mechanism for producing its large velocity will be worked out.

    But someday, if there is intelligent life in the universe, the "little green men" explanation will have to be used. Because someday we will observe an astrophysical phenomenon that has been produced by an alien civilization.


The Mouse:
Farhad Yusef-Zadeh and John Bally, Nature 330, 455 (1987).

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