Relativity, Cosmology, and Time

A Scenario for Time Travel

Recently in 2006, a scenario for time travel has been proposed without relying on rotating black holes or exotic wormhole tunnels. The idea is based heavily on the Superstring theory, according to some versions of which, our universe is a four-dimensional membrane or "brane" embedded in a higher-dimensional hyperspace called the bulk. Almost all matter and force-carrying particles are trapped on the 3-D brane, where they are contrained to travel at the speed of light or lower. However, sterile neutrinos and gravitons are particles that can access the hidden dimensions and travel faster than light (Figure 09s). From some view points or frames of reference, this is equivalent to time travel (see Figure 04c, event 4).

Figure 09s Time Travel [view large image]

The main ingredients of this theory are summarized in the followings:

For many years, physicists believed that extra dimensions were acceptable, but only if they were finite in size, either curled up or bounded between branes. An infinite extra dimension was supposed to destablilize everything around us. It is now known that infinite extra dimension from a single brane (like our 3D brane) is possible if that extra dimension is heavily warped so that the gravitons are localized near the brane in a small region no more than the Planck length scale of 10-33 cm (see Figure 09t). Thus, instead of the requirement for a heavily curved space-time in our 3D universe, or a folding one such as shown in Figure 09k, a model can be conceived such that the extra dimension in the bulk is seriously warped. It is found that the extra dimensions can be distorted by exotic matter in such a way that anything moving through the bulk can travel faster than the speed of light. Such off-brane short cuts can appear as "closed time-like curves" (CTC) as shown in Figure 09q.

Figure 09t Extra Dimension [view large image]

• This has dramatic consequences for inhabitants stuck on the 3D universe. To them, any entity that takes a short cut through the bulk appears to vanish and then pops up again at some point far sooner than it could have had it kept to the 3D universe, and in some other frames of reference, it has travelled backward in time (see Figure 04c, event 4).
• However, according to the Superstring theory, only closed strings such as the sterile neutrinos and gravitons can wander into the bulk. We may not be able to do time travel ourselves, but we can study time travel experimentally by manipulating these particles. The problem is that no one has ever spotted a graviton or a sterile neutrino.
• Nevertheless, experiment to test time travel has been proposed via the mechanism of neutrino oscillation, which could change ordinary neutrinos into sterile neutrinos and vice versa. The odds of this happening increase whenever the density of the material the neutrinos are travelling through changes abruptly. Thus, it is suggested that a beam of ordinary neutrinos would be sent through the Earth, from South Pole to a detector located at the equator. Some of them flipped into sterile neutrinos going through the bulk and emerge as ordinary neturino (via another flipping), which may appear as moving at greater than light speed or arrived before setting off.
• Critics point out that even if future technology can manipulate sterile neutrino in the next 50 years or so, the problem with the hypothetical exotic matter (to produce the warped space-time) is only shifted from our universe to unknown dimensions. Concealing the problem in the bulk is little better than a scenario in which it is out in the open. However, if the experiment is successful, it will verify many conjectures such as higher dimensions, some versions of the Superstring theory, sterile neutrinos, exotic matter in higher dimensions, and time travel albeit only for sterile neutrinos and gravitons.

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