Section 3.1

Time Travel:

By the 26th century time-travel pods constructed of plasticized tritanium mesh came into use for historians to chart the past. We know that previous to this in the 23rd century Starfleet had employed standard starships to travel into the past to observe key events, but this seems neither to have been standard procedure or particularly safe. By the 29th century dedicated timeships came into use by Starfleet. The 29th century timeships were capable of navigating through time using artificially generated temporal rifts. see also History of the Federation.

Since the introduction of Special Relativity in 1905 the concept of time as a constant has become unworkable. It has been proven that time is in fact relative to the observer. The most simple examination of this is the Fitzgerald Lorentz theory of time transformation. The Special Theory (of Relativity), by Einstein confirms the Galilean equivalence of inertial frames of reference (that to stationary observations and those in a frame at constant uniform velocity will be the same) and adds a second postulate which states:

"...light is always propagated in empty space with a definite velocity [c] which is independent of the state of motion of the emitting body."

There is at least an implied ambiguity in this postulate since he later states -

"Any ray of light moves in the "stationary" system of coordinates with the determined velocity [c], whether the ray be emitted by a stationary or by a moving body"

(W. Babin)

The symbol c is used for the speed of light (in vacuum) and has been for a some time. Einstein used other symbols such as V in his early relativity papers but most early books on relativity used c. The symbol probably stands for the Latin word celeritas meaning swift. Celerity is also an infrequently used word in English for speed and the same root is used in the words accelerate and decelerate.

The possibility of time travel is not possible under Special relativity (though manipulations of the passage of time are) for true time travel we must instead turn to the more complex (General Relativity) in General Relativity has been discussed since at least 1949 (by Kurt Godel). The General Relativity spacetime found by Godel has what are now called "closed timelike curves". A closed timelike curve is a worldline that a particle or a person can follow which ends at the same spacetime point as it started. A solution to General Relativity which contains CTCs cannot have a spacelike embedding - space must have "holes" (as in donut holes, not holes punched in a sheet of paper). These open the possibility for exotic solutions including the use Einstein-Rosen-Poldolsky-Bridges.

Two recent proposals, one by Morris, et al. propose the use of Einstein-Rosen-Poldolsky-Bridges to travel through time. If one end remains fixed in an inertial reference frame but the opposing end is moved at relativistic velocities then the two ends would no longer exist at the same point in time. Got uses the conical geometry generated by an infinitely long string of mass. If two strings pass by each other, a trajectory is created whereby an infalling object might be moved through time if its course describes a figure of eight around the strings. In this scenario, if the string has non-zero diameter and finite mass density, there is a CTC without any unusual topology.

Arguments against time travel sometimes include the laws of thermodynamics. There is a concern that time travel violates conservation laws. The first law of thermodynamics states that the amount of matter/energy is fixed and that the existing amounts can be changed from one form to another but never destroyed. The second law states that the distribution of useable energy must become more chaotic with time. Unfortunately, sending mass back in time increases the amount of energy that exists at that previous time. Doesn't this violate conservation of energy? If timetravel were possible through "natural" but exotic phenomena such as interacting strings then the amount of energy can no longer be considered fixed. There would be considerable influx into the system from every point in the future and thus the Universal background Radiation would rise. Unfortunately the work by Smoot in the early 1990's especially by the use of the COBE satellite showed that the detected radiation was to exceptionally high accuracy in agreeement with the predicted by Dicke and Peebles in the 1960's radiation from the Big Bang. The radiation is at 2.735 K and is almost perfectly isotropic, the only anisotropy coming from Earth's movement around the Sun, the Sun's motion in the Galaxy and the Wrinkles in Space-Time from the seeds of the Galaxies.

The Appearance of Time and the Apparent Differences between the Beginning and Ending of the Universe.

The theory that a high density Universe may lead to a "big crunch" is widely accepted in the scientific community. A theory related to this has been debated for some time though. If the Universe stopped expanding and began to collapse the question was raised as to how this would affect the arrow of time. Would time flow backwards and if so what about thermodynamics? Many people began to accept that the end and the beginning would be identical. Raymond Laflamme, of the Los Alamos National Laboratory in New Mexico, has carried out a new calculation which suggests that the Universe cannot start out uniform, go through a cycle of expansion and collapse, and end up in a uniform state.

Many years ago, Thomas Gold suggested that these two arrows might be linked. That would mean that if and when the expansion of the Universe were to reverse, then the everyday arrow of time would also reverse,

For a long time this has formed a debate between the two great men of Quantum Cosmology Stephen Hawking and Roger Penrose. Stephen believes in a theory that he and James Hartle put forward in 1983 known as the No-Boundary Porposal. He believes that the ends of the Universe will be different but embraces a more symmetrical approach to Roger Penrose.

More recently, these ideas have been extended into quantum physics. There, the arrow of time is linked to the so-called "collapse of the wave function"

Unfortunately, Laflamme has now shown that this will not work. He has proved that if there are only small inhomogeneities present in the Big Bang, then they must get larger throughout the lifetime of the Universe, in both the expanding and the contracting phases.

"A low entropy Universe at the Big Bang cannot come back to low entropy at the Big Crunch"

John Gribbin HomepageRoger 

Penrose and Hawking offer very different views on the topic of the Arrow of Time. To Stephen's belief there is a very clear distinction between the forward and the backward directions of time in our region of the universe. The local laws that physical fields obey are time symmetric, or more precisely CPT (charge-parity-time) invariant. Thus, the observed difference between the past and the future must come from the boundary conditions of the universe. 

Hawking's belief is that if we take a spatially closed universe model that is allowed to expands to a maximum size and collapse again then the universe will be very different at the two ends of its history. At the beginning of the universe, it seems to have been very smooth and regular. However, when it collapses again, we expect it to be very disordered and irregular due to the fact that there are so many more disordered configurations than ordered ones, this means that the initial conditions would have had to be chosen incredibly precisely. Hawking's interpretation is that therefore there must be different boundary conditions at the two ends of time. Penrose's proposal is that the Weyl tensor should vanish at one end of time but not the other. The Weyl tensor is that part of the curvature of space-time that is not locally determined by the matter through the Einstein equations. It would have been small in the smooth, ordered early stages but large in the collapsing universe. Thus, this proposal would distinguish the two ends of time and so might explain the arrow of time.

Penrose's theory is not CPT invariant and means that if the Weyl tensor had been exactly zero in the early universe, it would have been exactly homogeneous and isotropic and would have remained so for all time. Hawking argues that this homogeny could not have given rise to the clumping of matter and can not explain the fluctuations in the background. Hawking states that the Weyl tensor can and will be small but can never reach zero because this would be a direct violation of the uncertainty principle and instead there would have been small fluctuations that later grew into galaxies and bodies. By contrast, the universe would have been very irregular and chaotic at the other end of time with a Weyl tensor that was typically large. T

Penrose, however believes that both he and Hawking are arguing much closer theories than Hawking implies. For an initial singularity the Weyl curvature is approximately zero Stephen argued that there must be small quantum fluctuations in the initial state and thus pointed out that the hypothesis that the initial Weyl curvature is zero at the initial singularity is classical, and there is certainly some flexibility as to the precise statement of the hypothesis. Small perturbations are acceptable from Penrose' point of view, certainly in the quantum regime. Penrose simply argues that it just need something to constrain it very near to zero. (Scientific American July 1996). The arrow of time has also been a topic of debate in elementary particle physics. It is still unclear whether the arrow of time seen in particle decays is related to any sort of "ageing" process for particles. Therefore, understanding the nature of time-reversal symmetry, and its violation, appears essential in the search for a full understanding of the concept of time.

Most physicists believe that the violation of time-reversal symmetry is linked to the symmetry between matter and antimatter. Mathematically, this latter symmetry is expressed through a fundamental theorem stating that under the assumptions of locality (i.e. that interactions are local in space-time), Lorentz invariance (the laws of physics are the same in all reference frames) and unitarity (all probabilities always add up to one), all known quantum field theories possess a symmetry under the combined operation of CPT: C denotes the charge conjugation, which changes the quantum numbers of every particle into those of its antiparticle; P is a reflection operation known as parity, which turns an object into its mirror image and rotates it by 180 about an axis perpendicular to the mirror; and T denotes time reversal. Therefore, assuming that CPT symmetry is always preserved, any violation of time-reversal symmetry should always be accompanied by a violation of CP symmetry and vice versa.

The laws of electrodynamics and the strong interaction preserve CP symmetry to great accuracy. However, it is known that weak interactions do not conserve CP symmetry. This was confirmed experimentally by Christenson, Cronin, Fitch and Turlay in 1964 when they measured rare (1-in-500) decays of long-lived kaons into pairs of pions. The neutral-kaon system is still the only system in which CP violation has been verified experimentally. Although this result implies that time-reversal symmetry should also be violated, it is nevertheless very important to demonstrate T violation directly, as such an experiment could allow us to test the CPT theorem itself, and thus shed light on the issue of entropy production in a particle system.

Such an independent measurement of T violation has now been provided by the CPLEAR collaboration at CERN, the European particle physics laboratory in Geneva. The test essentially amounts to a comparison of probabilities for the neutral kaon to transform into its antiparticle, the antikaon, and vice versa. The neutral kaon is a bound state of a down quark and an anti-strange quark, and has a strangeness quantum number of -1; its antiparticle has a strangeness quantum number of +1. Weak interactions are known not to conserve strangeness, so kaon can transform into antikaon in the course of time and vice versa.

In the CPLEAR experiment, headed by Panagiotis Pavlopoulos of the University of Basle in Switzerland, the neutral kaons are produced through the strong interactions in collisions of antiprotons with the protons in hydrogen atoms:

antiproton + proton --> kaon- pion+ kaon0
or antiproton + proton --> kaon+ pion- antikaon0

The strangeness of the neutral kaon - that is, whether it is kaon or antikaon- at the time of production can be "tagged" by the charge of the second kaon produced in the collision. The strangeness of the kaon at the moment of decay can be tagged through its decay products: kaon particles decay into a positron, negative pion and a neutrino, while antikaon particles decay into an electron, positive pion and anti-neutrino. (These decays are called semileptonic because some of the products are leptons and some are hadrons.) If there is an asymmetry, AT, between the number of initial antikaon decaying into positrons and the number of initial kaon decaying into electrons, then, under certain assumptions that have been supported by the current data, this is a measure of the violation of time-reversal symmetry in the sense described above.

The CPLEAR collaboration, following a series of preliminary announcements at conferences since 1995, has now reported a final result for the asymmetry of AT = (6.6 1.3stat 1.0syst) X 10-3 (A Angelopoulos et al. 1998 Physics Letters B at press). The result demonstrates clearly a departure from time invariance in the semileptonic decays of neutral kaons.

Within the experimental accuracy of the CPLEAR experiment, the amount of T violation is found to be equal to the amount of CP violation observed in the neutral-kaon system, thereby providing a non-trivial consistency check of the validity of the CPT theorem to this level of accuracy.

(Physics Review 1998)

See Einstein Rosen Bridge

Return to home page