Alternate View Column AV-19
Keywords: cosmic strings, gravitational lensing, angle deficit, superconductor, space drive
Published in the April-1987 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 9/19/86 and is copyrighted © 1986, John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of the author.
This Alternate View column is about cosmic strings, strange massive objects that may have formed shortly after the Big Bang when the energy saturated space of the early universe was being replaced by the more normal space in which we now live. (Warning: cosmic strings should not be confused with the sub-microscopic supersymmetric strings now fashionable in particle theory [see Margaret Silbar's recent article in Analog]; both share the name "string" but are quite different.) If they exist at all, cosmic strings will be infintesimally small in cross section but very long, perhaps forming loops that encircle an entire galaxy. And they will be quite massive, producing strong and very odd gravity fields. Cosmic strings can be loosely described as "seams" or "cracks" in space, long closed-loop tangles in the fabric of space itself. They are (or at least may be) geometrical imperfections in topology produced as the universe was unfolded out of the Big Bang. Cosmic strings are now of great interest because they may be the "seeds" that caused the formation of galaxies. Moreover, they may also resolve at least part of the "dark matter problem", which was discussed in my Alternate View column "The Dark Side of the Force of Gravity" in the February-1985 issue of Analog.
Since the dark matter problem was featured in that column, I'll just remind you that there are good reasons to think that most of the mass in the universe, between 30 and 300 times more than the normal "visible" matter in stars, is in some mysterious "dark matter" form. The 2/85 AV column discussed the idea that hypothetical particles called "axions" might be responsible for the extra mass of the universe. This now seems unlikely. Since 1985 an amazing new picture of the large-scale structure of the universe has emerged from careful measurement of the recession velocities of a large number of very faint galaxies. These studies show that when viewed on a sufficiently large distance scale the universe resembles a sponge. There are large holes or voids hundreds of mega-light years across that are almost empty of stars and galaxies. These enormous holes are bounded and connected by planes and filaments dense with galactic clusters.
This emerging picture of a hole-riddled universe has caused axions to lose favor with cosmologists as a dark-matter candidate. Axions are slow-moving "cold" particles that cannot be responsible for the large-scale hole structures. What is needed is a model of dark matter that provides a mechanism for forming both the large holes and the finer-grained clumping of matter into galaxies and galactic clusters. A favored scenario explaining both structures suggests that fast-moving "hot" dark matter particles, perhaps massive neutrinos, are responsible for the large hole structure while cosmic strings are responsible for the granularity of galaxies and galactic clusters within this overall sponge-like structure. This latter idea is reinforced by a recent analysis of the space distribution of stars within nearby galaxies using the new mathematical technique of fractal analysis. This shows that the "fractal dimensionality" of the galaxies analysed is about 1.2, strongly suggesting a one-dimensional string as the underlying object producing galactic structure.
What are these cosmic strings? One good analogy is that strings are like the lines formed in a magnetic material like iron when several magnetic domain-boundaries meet. We can also make the geometrical analogy that lines are to points as cosmic strings are to magnetic monopoles. Both strings and monopoles are imperfections in space itself that may have occurred in the initial phases of the Big Bang, when normal space was precipitated from the energy saturated Higgs space of the primeval universe. Strings are line-defects in space and monopoles are point-defects. Both monopoles and strings should be extremely massive. Suppose you had a section of cosmic string that was 1 meter in length. The mass of this meter length of string may be as large as 1 × 1025 kilograms. This is about 1.6 times the mass of the entire earth. When we consider that a hypothetical cosmic string that may have caused the formation of our galaxy would have a length of thousands of light years, it becomes clear that a single such cosmic string could indeed be the dark matter responsible for a large part of the mass of our galaxy.
While a string should show normal Newtonian inverse square law gravity at distances much larger than its loop-size, general relativity tells us that the close-up gravity field of a string will be quite weird. Despite the string's enormous mass, it will exert no gravitational attraction on objects close to it. It has a peculiar "conical" gravity field that exerts no force on a nearby mass. Instead space is distorted into a sort of conical shape, rather as if a wedge of space starting at the string had been snipped out and the cut edges joined together. If you travel in a closed circular path around a length of cosmic string and measure the total angle through which you traveled, you will find that you come back to your starting point before you have travelled a full 360o. For example, the 1.6 earth-mass per meter string just mentioned would have a total angle for a closed loop surrounding of only about 350o. In other words, the string distorts the very geometry of space so that there are less than 360o in a circle when the circle includes a string within its loop.
This geometry distortion can produce dramatic effects when the string is moving. If a string cuts through a star, for example, all matter in the wake of the string's passage will be highly compresses by the angle-squeeze effect. The material of the star in this situation will be compressed to enormous density. Then it will rebound and literally explode, releasing a large amount of energy supplied by a slight reduction in the kinetic energy of the string. It is possible that some of the novas reported by astronomers are the result of such string-star collisions. If all galaxies contain such strings and if this mechanism for producing novas accounts for a significant fraction of all novas, then there should be a "nova-prone" zone in galaxies in the vicinity of the string. This may provide one observational means of testing the string hypothesis.
A less violent effect of a moving string is that it causes light travelling in its wake to receive a sort of geometrical push. This "blue-shifts" the light, increasing the frequency and energy of its photons. Such a blue shift of the primordial 2.7o K microwaves from the Big Bang should be quite apparent in microwave measurements presently planned and in progress. This can used to set limits on the mass and motion combinations of strings within our galaxy.
A related gravitational effect of a string crossing the line of sight of more distant objects is that it should produce double images of those objects. The conical space-warp of the string acts as a sort of prism, deflecting light so that objects behind the string are seen twice, with images both above and below the line of the string. There was a recent report that such a double image had been found. The May 6, 1986 issue of the New York Times trumpeted the headline "Powerful Source of Gravity Detected Deep in the Universe". The article concerned a report in the journal Nature published by a group of astronomers at Princeton University of observation of a double quasar image. Such double quasars have been seen before and are explained as the gravity-lensing effects of galaxies between the object and the observer. But the Princeton double quasar showed images separated by 156 arc seconds, a separation more than 20 times larger than that of any previously observed double quasar. Such a huge separation could only be the result of an extremely massive galaxy or a cosmic string. For a week or so there was great excitement in the astrophysics community.
It is unfortunate that perhaps half the time when there is a new and exciting result in science it is new and exciting because it is incorrect. That's what happened in this case. Other astronomers found in examining the Princeton "double quasar" image over a wider range of light wavelengths that the double quasar really seemed to be two unrelated quasars that happened to have similar brightness and wavelength spectrum characteristics and to be fairly close in the sky. Before this was realized, however, a number of science-oriented magazines (fortunately not including our own Analog) had run major articles on the amazing double quasar and the super-massive object it revealed. But let's press forward and refrain from drawing moral lessons from the misfortunes of others.
Despite this non-discovery, it is possible that strings have already been observed. Another property that cosmic strings should have, if they exist, is that they should be excellent superconductors. This means that if a string spans two regions of the galaxy that are at different electrical potentials, perhaps because of effects of the galactic magnetic field, a large current will begin to flow in the string. The growth of such a current should produce intense and detectable microwave radiation. It may be significant that recent microwave images of the center of our galaxy have shown threadlike structures that seem to be sources of microwaves. This had led to speculation that these "threads" are cosmic strings.
This column appears in a science fiction magazine. So let's assume for the moment that the cosmic strings are real and consider their science fiction implications. What we could have here is a sort of galactic railroad. The Japanese are presently devoting a large development effort to propelling railroad cars and ships with linear induction motors, devices which propel vehicles along by "paddling" with a changing magnetic field against magnetic materials embedded in a rail line or against the conductivity of salt water. We can work the same trick with a space vehicle using a string in place of the rail line or the water. Suppose we travel in space to the nearest point where a cosmic string passes and surround it with an array of magnet coils attached to our space ship. We can induce travelling magnetic pulses in the coils and the superconducting string will respond with opposing magnetic fields which will move us along.
This process requires energy, of course, but it does not require that we shoot reaction mass out the back, rocket-style, to gain momentum. This is a "reactionless" drive, for the huge mass of the string provides a convenient base to push against in order to gain momentum and kinetic energy. And the energy should be recoverable. It should be possible to push against the string until we are half-way to our desired destination along the string, accumulating velocity by "investing" stored energy in the kinetic energy we develop. Then we can decelerate for the remainder of the distance withdrawing the energy and returning it to storage as we reach our destination.
Of course, the problem here may be that we don't happen to be located near any
of the "string-roads" that perhaps span our galaxy. Maybe we live out in the
boondocks, far from the nearest string rail-line. Perhaps that's why
nobody ever comes to visit us.
Theory of Cosmic Strings:
A. Vilenkin, Astrophys. Journal 282, L51 (1984);
J. R. Gott, Astrophys. Journal 288, 422 (1985).
Double Quasar Report:
E. L. Turner, D. P. Schneider, B. F. Burke, J. N. Hewitt, G. I. Langstrom, J. E. Gunn, C. R. Lawrence, M. Schmidt, Nature(London) 321, 142 (1986).
J. A. Tyson and C. A. Gullixson, Science 233, 1183 (1986).
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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