Paradoxes of the theory of relativity
Time Travel
In 1905, Einstein published his theory of Special Relativity. This theory predicted a limited form of time travel because it showed that time is not absolute and universal, but that time is dependent on an observer's state of motion. Thus, two observer's moving at different speeds will measure different time intervals between the same two events. The classical twin paradox problem illustrates this idea of time dilation. Throughout the 20thcentury, experimental evidence such as the extended lifetime of unstable subatomic particles and the background radiation in the Earth's atmosphere has supported Einstein's theory.
Although the theory of relativity does not explicitly show or prove that backwards time travel is possible; it does not deny the idea either. In fact, if an object were to travel faster than the speed of light then it would be able to travel back in time. Travelling faster than the speed of light, however, cannot be attained by simple acceleration. One way to break the "light barrier" is to shortcut the distance. By shortcutting the distance, the journey could be made in less time, which could cause the speed of an object to be greater than that of light. An Einstein-Rosen Bridge or wormhole could provide this short cut
. A wormhole is a geometry of four-dimensional space-time in which two regions of the universe are connected by a short narrow throat. According to general relativity, a wormhole is possible if the correct gravitational arrangement is available. This idea, along with the science fiction craze of time travel of the 1980s, prompted Michael Morris, Kip Thorne, and Ulvi Yurtsever to further investigate the possibility of time travel through wormholes.
On September 26, 1988, Michael Morris, Kip Thorne, and Ulvi Yurtsever published an article in Physical Review Letters titled, "Wormholes, Time Machines, and the Weak Energy Condition." Inspired by Carl Sagan's Contact, the goal of their research was to figure out the physical constraints on the possibility of time travel through a wormhole.
What they discovered, to their surprise, was that in the solution for Einstein's equations for a black hole, a passage could exist between two universes or two parts of the same universe. In the standard mathematical solution, the apparent wormhole in the black hole solution represents only a single moment in time. Therefore, the solution requires a very specific environment. Throughout the paper, they are able to show mathematically that time travel is possible. Conceptually and physically, however, their argument suffered.
First, they had to explain how a wormhole could even exist. Because both the entrance and exit of a wormhole are black holes, such a tunnel cannot be used as a short cut. The gravitational field of a black hole is so strongly attractive that it would cause the wormhole to collapse almost as soon as it forms.
If, however, the attractive force could be reduced within the wormhole, Morris, Thorne, and Yurtsever claim that the wormhole could be kept open long enough for something material to pass through. According to general relativity, both mass and pressure are sources of gravitation. Although mass cannot be negative, pressure can be negative. Thus, if matter could exert a negative pressure, it would create "anti-gravity."
Although the positive contribution of mass will most likely dominate, it is conceivable that there is a form of "exotic" matter existing in the universe. This would have the effect of negative pressure exceeding the positive mass, and therefore resulting in a net gravitational repulsion. With these special conditions in a wormhole, the anti-gravity could perhaps be enough to prevent the tunnel from collapsing.
