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FTL Travel:
The Realities of an SF Cliche

                                                  by Brian A. Hopkins

In 1676, Danish astronomer Ole Christensen Roemer noticed something odd about the moons of Jupiter. Because Jupiter's moons orbit at a constant rate, one would expect the times at which they appear to pass behind Jupiter to be consistent. However, Roemer noticed that this was not the case. Roemer knew that the distance between the Earth and Jupiter varies as the two planets orbit the sun. The eclipses of Jupiter's moons appeared later the farther the Earth was from Jupiter. Roemer reasoned that this was because the light from the moons took longer to reach us when we were farther away. Because his measurements of the variations in the distance of the Earth to Jupiter were inaccurate, Roemer's value for the speed of light was also off, but his insight was accurate: the speed of light is a constant. It would be 1865 before British physicist James Clark Maxwell proposed a proper theory for the propagation of light, and later yet before Einstein's Theory of Relativity and the infamous E=mcc implied that because bodies have infinite mass at the speed of light, no amount of energy can make them travel faster.

Faster Than Light (FTL) travel has long been a staple of Science Fiction (SF). SF authors, myself included, have constructed various pseudo-scientific explanations to explain how FTL travel might be possible, but are any of them achievable?

There have been a number of proposed FTL techniques which, over the years, have failed to deliver. Tachyons were once thought to hold the key. Tachyons are theoretical FTL particles compatible with the Theory of Relativity because they are created already traveling at speeds faster than light. There is no general consensus among physicists as to whether tachyons would permit the transmission of either information or matter, but it hardly matters because no tachyon has ever been detected. Physicists seem to have a habit of speculating on the existence of particles or exotic materials simply to explain natural phenomena which they cannot understand. Having said that, even if tachyons exist, it seems obvious that they would lend themselves to the transmission of information and not matter. Personally, I don't relish the thought of having my composite particles converted to tachyons and squirted across the universe.

Superluminal quantum effects were a hot topic for a while. These include the Einstein- Podolsky-Rosen (EPR) paradox, quantum teleportation, and that sort of conjecture. These approaches rely on transmitting information via the wave function and often rely on an accompanying classical light signal as well, ultimately limiting the transmission to the speed of light. Physicists argue passionately about the reality of the wave function and whether it collapses. There is no consensus. No quantum superluminal effect has ever been demonstrated.

Quantum theory tells us that "empty space" is not empty at all. At the quantum level, there exists a soup of virtual particles that wink in and out of existence, their short life spans made possible with energy borrowed courtesy of the Heisenberg Uncertainty Principle. The existence of these particles gives vacuum a certain energy, but it's an energy that cannot normally be measured or changed. There is, however, a tricky way of changing the vacuum energy via the Casimir Effect. If one places a pair of grounded conducting parallel plates close together, some of the quantum energy is suppressed within the gap that separates the plates. The energy reduction results in a net force pushing the plates together, a force that has been measured in a lab. The Casimir effect is relevant to FTL travel because it has been predicted that the speed of light is greater in the energy-depleted vacuum between the Casimir plates. One might conceivably surround a spaceship with a bubble of energy-depleted vacuum in which the spaceship could travel at FTL velocities, carrying the bubble with it. Of course, no one has any idea how this might be accomplished.

Quantum mechanics also implies a correlation between the individual quanta of an overall system, even when those elements are separated by light years of distance. This property is called nonlocality. The assumption is that Mother Nature works this magic using some yet-to-be-discovered FTL communication. Einstein called nonlocality "spooky actions at a distance" and regarded it as demonstrating a fundamental flaw in quantum mechanics. Since Einstein's days, physicists have decided to refer to nonlocality as a feature of quantum mechanics, rather than an anomaly in their theories. If nonlocality could somehow be harnessed, it might provide a means of FTL communication, but more and more it appears that it does not provide even a theoretical avenue for FTL travel.

The existence of alternate dimensions has often been proposed as a means of FTL travel. The most inspiring work done in this arena is the Klein-Kaluza theory, which uses compacted (rolled-up) extra dimensions to explain fundamental forces. The basic problem with hypothesizing extra dimensions is to explain why we are not aware 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. With this approach, however, there is no direct opportunity for FTL travel. Recently, Matt Visser of Washington University in St. Louis has proposed a variant approach to extra dimensions. Visser proposes that the extra dimensions form a bowl-shaped energy well with our everyday reality at the bottom of the well. Since the well's bottom lies at a single point in these dimensions, we have no evidence of their existence. Rising up through the well (i.e., through the other dimensions) would require massive energy and the acquisition of momentum. Visser himself has no suggestions as to how either might be accomplished.

SF writers have made much use of black holes and wormholes. In a black hole, however, infalling radiation blueshifts to infinity, frying our intrepid explorer, if the tidal forces don't shred him or her first. Wormholes were first hypothesized in the form of Einstein-Rosen bridges. A bridge connects two otherwise widely separated regions of space. Unfortunately, they are short-lived and pinch off so quickly that only tachyons -- if they exist -- could travel through them.

The hot topic these days is the Morris-Thorne (MT) spherical wormhole. Shortcuts through space, consistent with gravity theory, MT wormholes appear to offer the possibility of FTL travel. However, MT wormholes would require planet-sized masses of energy to create, inflate, and stabilize. Stabilization could be accomplished with a Casimir Effect spherical capacitor placed in the mouth of the wormhole. There are some problems with this concept. The Casimir capacitor must provide a large quantity of negative energy, perhaps a Jupiter-sized mass, and this must be in delicate balance with the equivalent positive energy of the wormhole's spatial curvature. Without this, large radial tension (stretching) and tangential pressure (squeezing) develop in the wormhole mouth, destroying anyone attempting to traverse the wormhole.

Carl Sagan asked some theoretical physicists for plausible methods of FTL travel when he was writing Contact. Among the team that worked on this problem was Thorne and his grad students at Caltech. They approached the problem by asking what forms of matter would hold a wormhole open permanently. The answer: "exotic matter," a highly stressed material with enormous tensile strength. The tension or pressure of such a material would exceed the energy density. We have no familiarity with such matter today, but theoretically it existed under conditions of extraordinary pressure in the early universe.

Theoretically, MT wormholes could use 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 well abhor a time machine. His calculations indicate that vacuum fluctuations of drastically increasing energy will arise when the wormhole connection becomes "timelike," collapsing the wormhole. Despite much debate, no general consensus has emerged on this paradox. Elaborate and very convincing papers by Thorne's group and others reconcile time travel with quantum theory, while others, like Hawking, propose a Chronological Protection Conjecture (CPC) which says the universe shall not allow time travel. One of the time travel skeptics is Matt Visser. Early in 1993, he showed that wormholes do not enable time travel by proposing physical mechanisms that enforce CPC. At a point where a paradox could develop, quantum field and gravitational effects build up as the two ends of a wormhole approach the critical point and either collapse the wormhole or induce a mutual repulsion. Regardless, it's more physics mumbo-jumbo thrown at a structure we haven't the ability to create or sustain.

An alternative is a wormhole proposed by Visser. Visser proposes the creation of a wormhole geometry by "cutting similar holes in two regions of space-time and then sewing the edges together." Instead of disturbing the curvature of space at the wormhole mouth over a broad region, including the space through which a traveler must pass, Visser would frame a flat-space wormhole connection with "struts" that constrain it. For struts, Visser proposes the use of cosmic strings with negative mass. Thus, the negative mass of the strut edges balance the positive mass of the mouth. Cosmologists speculate that loops of cosmic string might have been produced in the early phases of the Big Bang, but we've yet to discover how to create or use them.

Recently, Miguel Alcubierre has mathematically shown that, within the framework of general relativity and without the introduction of wormholes, it's possible to modify spacetime in a way that allows a spaceship to travel at FTL velocities. By a purely local expansion of spacetime behind the spaceship and an opposite contraction in front of it, motion faster than the speed of light is possible. However, just as it happens with travel through wormholes, exotic matter will be needed in order to generate a distortion of spacetime.

If this seems confusing, consider the inflationary phase of the early universe. It's easy to convince oneself that, if we define the relative speed of inflation as the rate of change of spatial distance over time, we obtain a value that is much greater than the speed of light. This doesn't mean that two observers comoving in the expanding universe are traveling faster than light. The enormous speed of separation comes from the expansion of spacetime itself. (This superluminal speed is very often a source of confusion. It is also a very good example of how an intuition based on special relativity can be deceiving when one deals with dynamic spacetimes, says Alcubierre.) This example shows how one can use an expansion of space time to move away from some object at an arbitrarily large speed. In the same way, one can use a contraction of spacetime to approach an object at any speed. This is the basis for Alcubierre's FTL model: create a local distortion of spacetime to produce an expansion behind the ship and an opposite contraction ahead of it. In this way, a spaceship would be pushed away from Earth and pulled toward a distant star by spacetime itself. This model, as Alcubierre is quick to admit, violates all three energy conditions, weak, dominant, and strong, requiring, once again, exotic matter.

By proposing exotic materials and speculating on the existence of certain cosmic phenomena (which, at best, scientists cannot substantiate the existence of, let alone say how such could be created, harnessed, or sustained) it's possible to make a case for, quite literally, anything. However, the question remains: How feasible, how achievable, how probable is it that we will ever travel at velocities exceeding the speed of light? The theories discussed in this article have seen physicists and cosmologists published and paid, but they've taken no one even so far as across their living room. Perhaps wormholes or expansions of spacetime or even the sheer human power of wishing on a star hold the key, but until such time as one of these methods is founded on demonstrated science and materials, FTL travel remains unlikely.

1. Alcubierre, Miguel, "The Warp Drive: Hyper-Fast Travel Within General Relativity," Classical and Quantum Gravity, N11, 1994.

2. Cramer, John G., "NASA goes FTL, Parts 1 and 2," Analog Science Fiction and Science Fact, Dec 94 and Feb 95.

3. Hawking, Stephen W., A Brief History of Time: From the Big Bang to Black Holes, Bantam, 1988.

4. Hawking, S., "Chronology Protection Conjecture," Physical Review D, V46, N2, July 92.

5. Hawking, S.W. and Ellis, G.F.R., The Large Scale Structure of Spacetime, Cambridge University Press, 1973.

6. Morris and Thorne, "Wormholes in Spacetime and Their Use for Interstellar Travel," America Journal of Physics, V56, 1988.

7. Price, Michael Clive, "Transversable Wormholes: Some Implications," Extropy, N11, date unknown.

8. Thorne, et al, "Cauchy Problems in Spacetimes with Closed Timelike Curves," Physical Review D, V12, 1990.

9. Visser, Matt, "From Wormholes to Time Machines: Remarks on Hawking's Chronology Protection Conjecture," Physical Review D, V47, N2, Jan 93.

10. Visser, Matt, "Transversable Wormholes: Some Simple Examples," Physical Review D, V39, N10, May 89.

11. Visser, Matt, "Wormholes, Baby Universes and Casuality," Physical Review D, V41, N4, 1990.

Author's notes: This article was commisioned by and first appeared in Just Because, a publication for which I am one of two Science Editors, the other being Richard Dunbar. Richard and I were to write pro and con sides of the FTL argument. I drew the con side of the argument; hence the article above. Hey, I want to believe in FTL travel as much as the next SF writer! I was paid to say all those negative things...

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