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Alternate View Column AV-69
Keywords: NASA Workshop
relativity quantum mechanics wormholes FTL
Published in the
Mid-December-1994 issue of Analog Science Fiction & Fact
Magazine;
This column was written and submitted 5/22/94 and is
copyrighted ©1994 by John G. Cramer.
All rights reserved. No part may be
reproduced in any form without
the explicit permission of the author.
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This column is about the Advanced Quantum/Relativity Theory Propulsion Workshop, a gathering held at the Caltech Jet Propulsion Laboratory which I attended last week (May 16-17, 1994). It was sponsored by NASA's Office of Advanced Concepts and Technology, and its purpose was to bring together a group of experts and interested parties to review quantum mechanics and relativity theory as applied to concepts such as space-time wormholes, trans-relativistic physics, spacetime structure, and quantum nonlocality. In other words, we were to review possible physics routes to faster-than-light (FTL) travel and/or communication.
The 13 participants at the workshop were from varied backgrounds: NASA personnel, NASA consultants and contractors, academic physicists, and science fiction writers. Several physicists who also write science fiction were workshop participants, including Gregory Benford, Robert Forward, Geoffery Landis, and myself. Dr. Gary L. Bennett, a SF novelist and expert on nuclear power systems for spacecraft, was the Workshop's official NASA sponsor. The workshop was organized by Dr. Robert H. Frisbee of JPL, with some help from Bob Forward.
The areas of physics covered during the workshop fall into several broad areas: wormholes, nonlocality and other quantum effects, tachyons, and extra dimensions. As often happens when physicists get together, we probably generated more questions than answers, but we made progress. In this column I'll summarize the discussion about wormhole physics that took place on the first day of the Workshop. In my next AV Column [Analog, February '95] I'll cover other FTL topics discussed on the second day of the Workshop.
Constructing Wormholes: At the Workshop we discussed two types of wormholes, the "standard" Morris-Thorne (MT) spherical wormhole and the somewhat more housebroken Visser wormhole. Both wormhole types are shortcuts through space-time, completely consistent with standard gravity theory, that appear to offer the possibility of faster-than-light travel and even time travel.
Michael Morris from Butler University in Indianapolis gave the first AQRTP Workshop presentation. Mike did his Caltech PhD thesis with Kip Thorne on the original formulation of spherical wormholes. Morris-Thorne wormholes, first described in a 1988 paper in the American Journal of Physics, are spatial shortcuts that an advanced civilization, one capable of manipulating planet-masses of energy, might be able to snatch from the quantum foam, inflate to a useful size, and stabilize by placing a Casimir-effect spherical capacitor in the wormhole mouth. [See "Wormholes and Time Machines," my AV Column in the June-'89 Analog for details.] These could, in principle, be used for faster-than-light travel and even time travel.
There are some problems, however, with the MT spherical wormhole scenario: (1) the Casimir capacitor needs to provide a very large quantity of negative energy, perhaps a Jupiter-mass in size, and this must be in delicate balance with the equivalent positive energy of the wormhole's spatial curvature, (2) large radial tension (stretching force) and tangential pressure (squeezing force) develop in the wormhole mouth, which would probably destroy an observer attempting to traverse the wormhole, and (3) recent work by Steven Hawking and others indicates that if a wormhole becomes a time machine, it will probably destroy itself. Morris discussed these problems.
Matt Visser from Washington University in St. Louis was also a participant in the AQRTP Workshop. Visser's wormholes provide the FTL space traveler with a more benign environment. Visser conceives the creation of his wormhole geometry as "cutting similar holes in two regions of space-time and then sewing the edges together". Instead of distributing the curvature of space at the wormhole mouth over a broad region, including the space through which the traveler must pass, Visser would frame a flat-space wormhole connection with "struts" that contain a region of very sharp curvature. The material needed to make the struts is not ordinary matter, but in a sense it is perhaps available.
Over the past decade, cosmologists have speculated about cosmic strings, linear fractures in the fabric of space that are solutions of Einstein's equations. [See my AV Column "Strings and Things", Analog, April-'87.] The usual cosmic strings are very massive, perhaps an Earth-mass per meter. To frame his flat wormholes, Visser needs a slightly different string solution that has the property of negative mass. [See my AV Column "Antigravity I: Negative Mass", Analog, July-'86.] The negative mass of the strut edges (as in the MT wormhole) is balanced by the positive mass of the mouths themselves, leaving the wormhole structure with a relatively small (perhaps zero) mass.
Visser suggests making a wormhole mouth in the form of a cube, with flat-space wormhole connections on the square sides and strings as the edges. Each cube-face may connect to the face of another wormhole-mouth cube, or the 6 cube faces may connect to 6 different cube faces in 6 separated locations. The Visser Cube of wormhole mouths is reminiscent of the familiar SF notion of a star gate.
Natural Wormholes: Our technical capabilities at present are not up to the task of creating either of MT or Visser wormholes. Therefore, the question arises of whether they might occur naturally. In this context it is very interesting that cosmic strings are a component of Visser wormholes. Cosmologists have speculated that loops of cosmic string might be produced in the early phases of the Big Bang, particularly the so-called inflationary phase when the emerging universe undergoes very rapid expansion. The usual scenario is that if such string-loops were created in the early universe they would probably oscillate, disperse their large mass-energy as gravity waves, and disappear. However, if a cosmic string had negative mass this decay process would not be possible because negative energy gravity waves presumably cannot be generated.
The formation of such negative-energy string loops framing a wormhole during the Big Bang could result in a stable object that had a net positive mass. This, then, is a plausible scenario for the formation of stable natural wormholes that might still exist in our universe. As this scenario emerged from our discussion, the focus of the Workshop turned to the question of how, if such natural wormholes exist, we might search for and find them. And we invented a way.
If a positive electric charge Q passes through a wormhole mouth, the electric lines of force radiating away from the charge must thread through the aperture of the wormhole. The net result is that the entrance wormhole mouth has lines of force radiating away from it, and the exit wormhole mouth has lines of force radiating toward it. In effect, the entrance mouth has now been given a positive electric charge Q, and the exit mouth acquires a corresponding negative charge -Q. Similarly, if a mass M passes through a wormhole mouth, the entrance mouth has its mass increased by M, and the exit mouth has its mass reduced by an amount -M.
In the early universe these mass changes might create a dynamically unstable situation. If one natural wormhole mouth begins to increase in mass, its twin will correspondingly be reduced in mass until it acquires a net negative mass. The mouth with positive mass will attract more mass to it, while its negative-mass twin will gravitationally repel any nearby mass. Thus, this mass imbalance should grow until it eventually it is damped by the growing distance scales from the expansion of the universe.
The net conclusion of this line of reasoning is that there might be natural wormhole mouths of planet-size or star-size negative mass within our galaxy. These would repel nearby masses but, like normal stars or planets, would be attracted to the mass of our galaxy and orbit around it. The large negative mass of such objects might be exploited in searching for them.
There are presently three groups conducting astronomical searches for non-luminous or dim Jupiter-size masses or MACHOs (MAssive Cosmic Halo Objects) that if they occur in sufficient numbers may provide a partial solution to the Dark Matter Problem. [See my AV Column "Searching for MACHOs", Analog, May-'94.] The groups have all recently reported successful detections. Their technique is to use the gravity field of the objects as a lens that will briefly, over a period of days or weeks, increase the brightness of a more distant background star as the MACHO passes through the line of sight between observer and background star.
At the AQRTP Workshop we realized that a negative mass object would also produce a gravitational lensing effect, but in the form of a diverging rather than converging lens. Thus, the background star should be briefly dimmed or extinguished if a negative mass object passed through the line of sight. During my week stay at JPL I did quick calculation of the shape of this profile. The star intensity first increases,then vanishes, then returns, producing a time profile that is quite distinctive and, if it exists, should be discernable in the MACHO search data. Forward, Benford, and I are now writing a physics paper to publish this new prediction.
Time Holes and Catastrophe: The original paper describing MT wormholes also described a way in which they could be made into time machines by using relativistic time dilation to create a time difference between one mouth and the other. Hawking has suggested that while Nature does not abhor a vacuum She may very well abhor a time machine. His calculations indicate that vacuum fluctuations of drastically increasing energy, rather like the audio feedback we experience with a PA system when we bring a microphone too close to the speaker, will arise just as the wormhole connection becomes "timelike".
This arises because the magnitude of quantum vacuum fluctuation depends on the space-time interval (square root of distance squared minus c*time squared) around a circuit threading through the wormhole and then back in normal space. As a wormhole becomes a time machine this interval first becomes very small, then becomes zero, and then becomes negative. During this transition in the region near zero interval it crosses what is called "the Cauchy Horizon". There the quantum fluctuations should, according to Hawking's calculations, grow without limit and destroy the wormhole.
It is interesting to note that it is the transition through the Cauchy Horizon that produces this catastrophe, not the existence of a time-spanning wormhole. If somehow the time-spanning wormhole (negative interval) could be established, it would perhaps be as stable as a space-spanning wormhole (positive interval).
An interesting corollary of this is that even when two wormholes each span a positive interval, a path threading both of them can have a zero interval leading to the Cauchy instability. Visser in a recent paper showed that under certain circumstances the instability might be evaded, but only for very small aperture wormholes separated by a distance approaching the diameter of the universe. If there are naturally occurring wormholes, it was suggested that because of back-reaction they might arrange themselves to fill all of the available wormhole "space" and to vigorously resist, though quantum fluctuations and back reaction, any attempt to create new wormholes or to significantly alter the space-time connections of existing ones.
Summary: The first day of the Workshop provided stimulating discussion about wormholes, both natural and artificial, and several interesting new ideas emerged. We invented a way of searching for matural wormholes, using data that is already available.
From the point of view of SF, new "ground rules" for the construction and operation of wormholes seem to be emerging from the active theoretical work on the subject. Attempting to convert FTL wormholes to time machines may be spectacularly dangerous. Time travelers beware!
References:
Spherical Wormholes:
W. G. Morris and K. S. Thorne, American Journal
of Physics 56, 395-412 (1988);
W. G. Morris, K. S. Thorne, and U.
Yurtsever, Phys. Rev. Letters 61, 1446-9 (1988).
Visser Wormholes:
Matt Visser, Physical Review D 39, 3182-4
(1989).
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issue index.This page was created by John G. Cramer on 7/12/96.