Time Travel Research Center © 2005 Cetin BAL -
GSM:+90 05366063183 - Turkey / Denizli
The Realities of an SF Cliche
by Brian A. Hopkins
1. The Constant in the Chaos
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
2. Tachyons, Extra Dimensions, and Other Pretensions
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
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
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
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.
4. Distortions of Spacetime
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
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.
6. For Further Reading
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
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.
Travel: The Realities of an SF Cliche" is © copyrighted 1998 by Brian A.
Hopkins. All rights reserved.
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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