Alternate View Column AV-70
Keywords: Casimir effect negative energy quantum nonlocality tachyons extra dimensions
Published in the February-1995 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 7/13/94 and is copyrighted ©1994 by John G. Cramer.
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This column is the second of two about the Advanced Quantum/Relativity Theory Propulsion Workshop (AQRTP), 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 like space-time wormholes, trans-relativistic physics, space-time structure, and quantum nonlocality. In other words, we were to brainstorm possible physics routes to faster-than-light (FTL) travel and/or communication. The AQRTP Workshop was organized by Dr. Robert H. Frisbee of JPL, with some help from Bob Forward.
In my last AV Column I wrote about our new ideas on wormholes, which has now resulted in a physics paper already submitted for publication, with six of the Workshop attendees as co-authors. In this column I want to discuss the other items on our FTL agenda. Like wormholes, some of the other FTL loopholes that we discussed have been featured in my previous AV columns.
The Casimir Effect [see also my column in Analog, Mid-December, 1990]: Quantum theory tells us that "empty space" is not empty at all. At the quantum level it is full of fireworks made with virtual particles - quarks and antiquarks, electrons and positrons, photons and neutrinos, that flash into a brief existence and disappear again, their short life spans made possible with energy borrowed on Heisenberg's credit card, i.e. from the time-energy part of Heisenberg's Uncertainty Principle. The existence of this quantum fireworks gives a certain energy to the vacuum, energy that is in principle there but that cannot normally be measured or changed.
There is, however, a tricky way of changing the vacuum energy. It is called the Casimir Effect. If one places a pair of grounded conducting parallel plates close together, with only a narrow gap separating them, some of the quantum fireworks is suppressed. The virtual particles cannot exist if they have a half-wavelength greater than the gap between the plates and interact electrically with the plates. The energy reduction result is that the vacuum energy between the plates is reduced as the fourth power of the plate gap distance. This energy reduction results in a net force pushing the grounded plates together, a force that has been measured in the laboratory.
The Casimir Effect is relevant to FTL considerations because it has been predicted from quantum electrodynamics that the speed of light is greater in the energy-depleted vacuum between the Casimir plates than in free space. This increase in velocity is a tiny effect because the amount of vacuum energy depletion possible in practical Casimir experiments is small and because the speedup of the light velocity c (normally a fundamental constant defined to be exactly 2.99792458 × 108 m/s) depends on the amount of energy removed.
The idea suggested in our AQRTP Workshop was that one might conceivably surround a space vehicle with a "bubble" of highly energy depleted vacuum, in which it could travel at FTL velocities, carrying the bubble along with it. This speculation produced the following thought experiment. Suppose that one produces an energy-depleted region of space using the Casimir Effect and then somehow makes the parallel conducting plates disappear. What happens to the "bare" volume of energy-depleted space left behind? Does it return to normal space? If so, where does the energy come from to make up for the energy that is missing? Does the volume of energy-depleted space persist as a "bubble"? If so what shape does it assume? Does it expand or contract?
We were not able to answer these questions at the Workshop. Physicists often generate more questions than answers.
Quantum Nonlocality [see also my columns in Analog, January, 1990 and October, 1991]: Quantum mechanics, the theory describing the behavior of matter and energy at the smallest distance scale, implies the persistence of correlations between the separated parts of an overall quantum system, even when the parts are separated by light-years of distance. These correlations manifest themselves instantaneously in all parts of the system whenever a measurement is performed on any part of it. This property is called nonlocality, and it carries with it the implication that Nature arranges such correlations using some faster-than-light linking process. Einstein called nonlocality "spooky actions at a distance" and regarded it as demonstrating a fundamental flaw in the formulation of quantum mechanics. However, from the experimental tests of Bell's Inequality, we now understand that quantum nonlocality is "not a bug but a feature", as the programmers say.
At the AQRTP Workshop we considered the question of whether quantum nonlocality was a possible medium for FTL communication. In the context of standard quantum mechanics there is good reason for believing that it is not. Eberhard has proved a theorem demonstrating that the outcomes of separated measurements of the same quantum system, correlated by nonlocality though they are, cannot be used for FTL observer-to-observer communication. A possible loophole in Eberhard's theorem could arise if, following the work of Nobel Laureate Steven Weinberg, one modifies conventional quantum mechanics by introducing a small non-linear element into the standard QM formalism. It has been shown that in slightly non-linear quantum mechanics, the observable nonlinear effects that would arise would make possible FTL communication through nonlocality.
This result is not particularly satisfying because the experimental work, mainly in atomic systems, that followed Weinberg's description of non-linear observables placed very low upper limits on the possible size of such effects. Our AQRTP Workshop participants turned to the question of where one might look for quantum nonlinearities with a better chance of finding them than in atoms. We concluded that such a breakdown in quantum linearity would most likely be observed at the smallest scales of time and distance, i.e. in experiments conducted at the highest available energies. To our knowledge, however, no consideration has been given to Weinberg-type quantum linearity tests by the high-energy physics community.
Tachyons [see also my column in Analog, October, 1993]: Measurements of the mass of the electron neutrino in tritium beta-decays experiments at six different laboratories suggests that the electron neutrino may have a mass-squared that has a negative value. The statistical uncertainties in these mass-squared determinations are presently too large to be compelling, but the measured negative mass-squared values, if taken seriously, would require that the electron neutrino is a tachyon.
Tachyons are hypothetical could-be particles that always travel faster than the velocity of light and that slow down to closer to the velocity of light when they are given additional kinetic energy. The rest mass of a tachyon is imaginary (like the square root of -1), but the net mass energies of tachyons are always real because they are never at rest.
At the AQRTP Workshop we considered the possibility of tachyonic neutrinos and their implications for FTL communication. Indeed, it would seem that a FTL signal could be transmitted by creating an information-modulated beam low-energy neutrinos, perhaps using an accelerator, and then detecting the modulation of the neutrino intensity at some distant location. The creation and modulation of such neutrinos are more-or-less feasible with current technology. However, the detection of the particles is not currently feasible. The only working neutrino detectors operate on neutrinos that have energies of around 1,000,000 electron volts, preferably much more. If electron neutrinos are tachyons, the experiments suggest an imaginary rest mass of around 10 electron volts (10 eV). Therefore, in order to exploit their tachyonic properties, one would have to be able to detect electron neutrinos with kinetic energies around 10 eV, or preferably much less. We have no way of doing this.
The Workshop kicked around the idea of accelerating target nuclei in a storage ring so that the nucleus to be transmuted by the incoming 10 eV neutrino beam would be accelerated into the neutrinos, providing enough energy for a nuclear reaction. This method is semi-plausible but would undoubtedly have severe problems with detection rates and with background processes. Our conclusion was that even if the electron neutrino is confirmed to be a tachyon in the next few years, the prospects for using it soon as a medium for FTL communication are very dim.
Extra Dimensions [see also my column in Analog, April, 1985]: The NASA organizers of the AQRTP Workshop asked that we consider whether some of the recent work on extra dimensions, for example the Klein-Kaluza theory that uses compactified (rolled-up) extra dimensions to explain the fundamental forces, has implications for FTL communication or travel. Our first-order answer was "no". Even if infinitesimal superstrings and/or compactified dimensions do provide the inner structure of the universe, they do not offer any direct opportunity for breaking the lightspeed barrier.
However, Matt Visser offered a variant approach to extra dimensions that could provide such an opportunity. The basic problem with hypothesizing the existence of extra dimensions in which our universe is embedded is to explain why we are not aware of them, and in fact see no evidence of them. The Klein-Kaluza theory solves this problem by "compactification", by looping the extra dimension on itself and reducing the radius of the loop to a distance scale at which it would have little or no experimental consequences. Visser has another solution: the extra dimension (or dimensions) form a bowl-shaped energy well, and our everyday world lies at the bottom of that well. Since the well-bottom lies at a single point in the extra dimension, we have no evidence of its existence.
To rise above the bottom of the well would require (a) adding energy to the system, i.e., doing work against the restoring force that holds the system at the well-bottom, and (b) acquiring momentum in the extra dimension that moves the system away from the well-bottom. We agreed that (a) might be easier than (b), but we had no real suggestions about how to do either.
The implications of this approach to extra dimensions is that the velocity of light might be different from c away from the well-bottom where we live and perform experiments, just as it is in the energy-depleted space produced by the Casimir Effect and discussed above. This, then, is a rationalization for the "hyperspace" described in countless SF stories. There could be a hyperspatial domain in which the velocity of light is higher than c.
Summary: On the second day of the AQRTP Workshop we covered many FTL possibilities. None of them have sent us running for a patent application form, but I think we came away from our discussions convinced that Nature's door is not absolutely closed to the possibility of FTL communication and travel.
This page was created by John G. Cramer on 7/12/96.