Alternate View Column AV-24
Keywords: cosmic string, oscillation, gravity wave, spiral galaxy
Published in the January-1988 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 6/26/87 and is copyrighted © 1987, John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of the author.
This column is about gravitational beams, beams of antigravity that may exist in our universe and may have important consequences for the formation of spiral galaxies like our own Milky Way. The object that generates these antigravity beams is a loop of cosmic string. Cosmic string should not be a new notion for regular readers of this column because I discussed them in a fairly recent Alternate View column published in the April, 1987 issue of Analog. However, for those of you who may have missed or forgotten that column, let me give you a brief recap of the idea.
Cosmic strings are hypothetical remnants of the Big Bang which have been described by popularizers as "cracks in space" or as "strings of pure energy". Their predicted properties are now being examined with growing interest by theoretical astrophysicists because they show promise of solving a number of vexing astrophysical problems. For example, it may be that the accumulation of matter that eventually formed our galaxy and others was initially clumped together by the gravitational pull of a circular loop of cosmic string. Cosmic strings are predicted to be infinitesimally small in cross section but very long, perhaps forming closed loops a few hundred thousand light years long that encircle entire galaxies. They should be extremely massive, perhaps as much as an Earth-mass per meter of length. They are expected to have very strong and very odd gravity fields and should also be electrical superconductors. They can be loosely described as "seams" or "cracks" in the fabric of space. They are a consequence of geometrical imperfections in space itself, long closed-loop tangles in topology that may have occurred when the universe was unfolding out of the Big Bang. Although there has been speculation about extensive networks of cosmic strings (with a magnetic monopole at each "knot" of the net), the work we will discuss in this column considers only a simple roughly circular loop of cosmic string.
While a string should show normal Newtonian inverse square law gravity at distances much larger than its loop-size, at short distances it should exert no gravitational attraction at all on objects close to it. But it distorts space in another way. Travel in a closed circular path around a length of cosmic string brings you back to your starting point before you have travelled a full 360o. For example, a string with 1.6 earth-masses per meter string would have a total angle for a closed loop encircling it of only about 350o. This distortion of space has bizarre consequences that were discussed in the 4/87 AV column.
Now we want to go beyond our previous discussion and consider what happens when a cosmic string vibrates, makes gravity waves, and loses mass-energy. The theoretical work described here was performed by Dr. Tanmay Vashaspati of the University of Delaware. It was published recently in the Physical Review. It may provide a clue to a long standing mystery in astrophysics, the question of why most galaxies have a double spiral shape. Vashaspati combined the equations of general relativity with computer simulation to show some dramatic effects of the vibrating cosmic string on the nearby matter.
Why should a cosmic string vibrate? We know from every day experience that tightly stretched strings can and do vibrate. Stretched rubber bands make the familiar twanging vibration. All stringed musical instruments, the violin, the harp, the electric guitar, use vibrating strings to make their sounds. And, as it turns out, it would be very difficult to make a cosmic string that was not in some state of vibration when it was produced. A cosmic string is created by a sort of turbulence in the early radiation dominated era of the Big Bang. This turbulence would have to be unusually symmetric and uniform to produce a string that did not have some amount of vibrational motion. Therefore if cosmic strings exist at all, they should vibrate, and so it becomes important to investigate the implications of such vibrations.
Vashaspati has done this, and he finds that during a part of the vibration cycle of a cosmic string there is a sort of "crack-the-whip" motion that produces on opposite sides of the loop a pair of very special regions which he calls "cusps". These cusps are special because at a single instant during the vibration cycle a single point on each cusp reaches the velocity of light. The result of this motion is to produce a very intense and tightly directed beams of gravity waves. At the instant of maximum velocity a huge quantity of energy is transferred from the mass-energy of the cosmic string to the twin gravity wave beams that are radiated out on tangents from the two cusps on opposite sides of the cosmic string.
But wait a minute! What's a gravity wave? Gravity waves are to the force of gravity as photons, light, radio waves, gamma rays, etc. are to electromagnetism. They are travelling disturbances in the geometry of space itself, moving with the speed of light and carrying energy and momentum. Gravity resembles electromagnetism in many ways, but the gravitational force is a factor of 4.3 x 10-40 weaker making gravity waves far more difficult to detect than electromagnetic waves (light). Despite occasional reports to the contrary, gravity waves have not yet been detected in any laboratory on earth.
The best evidence that gravity waves actually exist comes from the stars. Radio astronomers have obtained this evidence from studies of a binary pulsar system. This very special astronomical object, a pair of massive neutron stars in a tight 8 hour binary orbit, is rotating so fast that the movement and acceleration of these two compact masses provides a very prolific source of gravity waves. The rapidly varying gravitational field generated by the moving stars becomes a travelling gravitational disturbance that moves away from the system at the speed of light, removing large quantities of energy. One of the neutron stars in this binary system is a pulsar, and its periodic bleeps of radio waves have permitted radio astronomers to track the spin-down and orbit shrinkage of the system caused by the energy drain from gravity waves. They find that the system loses energy and rotational angular momentum at just the rates predicted by Einstein's theory of gravity. Because of these remarkable observations we are pretty sure that gravity waves exist and that we can calculate their properties, even though none have yet been detected in the laboratory.
Because gravity waves interact so weakly with matter, the usual rule to thumb is that their effects, except for the energy loss they represent, can be ignored. Vashaspati has demonstrated that this rule does not apply to the beamed gravity waves from the vibration of a cosmic string. These gravity waves have incredible intensity, and they carry vast quantities of energy and momentum away from the cusps of the cosmic string. Some of this energy and momentum is transferred to the matter that lies in the twin beams. Vashaspati describes radiated gravity waves as "antigravity beams" because of this effect. He finds that particles near the beam are strongly repelled from the cosmic string and are dispersed along twin tangents from the loop. He has tested the effects of these beams of antigravity on the star distribution in a model "galaxy" by starting with a uniform distribution of particles (i.e., stars) forming a planar disk about twice the diameter of a vibrating loop of cosmic string. The particle positions are calculated as the string oscillates a few dozen times. The patterns generated by this computer program show the growth of two "spiral arms" gradually smearing out from the central disk as the oscillations proceed. The pattern looks very much like a typical type SA double arm spiral galaxy. This does not, of course, constitute any sort of proof that cosmic strings exist, that they are responsible for the formation of galaxies, or that their vibrations cause the double arm shape that is so often observed. Nevertheless, the calculations are interesting and suggestive.
They also tell us other interesting about the probable history of a cosmic string. If it vibrates at all it should eventually "die", ultimately giving up all of its mass energy to gravity waves. A string of a size appropriate to the formation of our galaxy should have one complete vibration cycle every 40,000 years. At each cycle the string will consume some of its mass energy in producing a large burst of gravity waves. Unless there are some processes not considered by Vashaspati that will damp this process, in about 109 years the string will have consumed all of its mass and will disappear. The age of the universe is estimated at about 15 x 109 years, so a cosmic string formed by the Big Bang and vibrating in this way should no longer be around. It would last just long enough to form a galaxy and determine its characteristic shape before disappearing, leaving behind only the Cheshire Cat smile of the spiral arms as a signature.
OK, this is a science fiction magazine after all, so what SF use can we make of the antigravity beams of Vashaspati's calculations? First of all, don't count on producing such beams with a laboratory device. The intentional generation of antigravity beams requires true cosmic engineering and consumes unthinkably large amounts of mass-energy. It is perhaps the most inefficient method of producing "antigravity" and acceleration ever conceived. Only an extremely tiny fraction of the beam's energy can be recovered as kinetic energy by an object accelerated in the beam.
But suppose you could get in your spaceship and manage to find your way into an antigravity beam. It might take you for a very interesting ride. Since all of the particles comprising you and your ship would receive acceleration from the gravity wave beam exactly in proportion to their masses, there would be no sense of acceleration at all. You and your ship would remain in free fall, but would begin to move faster and faster along the beam with respect to the rest of the universe with no perception of gee forces. Moreover, Vashaspati points out that it is possible to gain a very large net acceleration by "riding the beam", rather like a surfer on a wave. The repulsion force builds to a peak as the vibration cycle builds to the instant of cusp formation. This buildup of repulsion moves out along the beam at the velocity of light as a sort of travelling lump. An object in the beam moving at nearly the velocity of light could stay at or near the repulsion maximum for a very long time, receiving acceleration all the while and acquiring a very high velocity and kinetic energy. This arrangement might be considered a sort of galactic slingshot for launching very high velocity probes and vehicles from one galaxy to another.
Slowing down at the other end of the ride would, of course, be a problem. Another beam from a vibrating string in another galaxy might be used for deceleration, but then the wave riding technique would work backwards, prevent use of the beam for more than a very short time. Slowing down by interacting with matter locally at rest would probably be necessary, and this would be difficult, dangerous, and slow.
Of course all of this may be moot if all the cosmic strings did evaporate and
the antigravity beams switch off 109 years after the Big Bang. In
any case, these are very new ideas. In the near future there will certainly be development and refinement of these concepts. There may also be more
discoveries, both theoretical and experimental, about the existence and nature
of cosmic strings. I'll try to keep you informed as these unfold.
A. Vilenkin, Physics Reports 121, 263, (1985).
Antigravity Beams from Cosmic Strings:
T. Vachaspati, Physical Review D35, 1767, (1987).
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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