One of the hottest topics in science fiction is time travel. The classic H.G. Wells story about time machine fascinated many generations of science fiction enthusiasts.
Philosophically, time travel in backward direction violates the principles of causality. This problem is sometimes referred to as the grandfather/grandmother paradox.
Does time travel have any place in theoretical physics? Do the laws of physics allow traveling back in time? Or perhaps as the Chronology Protection Conjecture states “the laws of physics somehow conspire and prevent Macroscopic objects (like human beings) to travel backward in time.”
Einstein’s Theory of Relativity allows time travel in forward direction, which is a consequence of time dilation. Also, the laws of nature including those of Maxwell’s electromagnetism and Einstein’s Special and General Theories of Relativity are all T-invariance. In other words, if you replace t (time) with -t (time in reverse direction) in equations governing the laws of the universe, nothing will be changed.
Classical concept of time travel is directly related to the laws of gravitational physics (solutions of the Einstein’s equations) and the nature of space-time itself.
It has been known that a wormhole works as a shortcut or a tunnel to connect two distant regions of the universe together. However, recently wormholes are used in some theories for time travel purposes as well. Kip Thorne and his collaborators at California Institute of Technology have theorized that time travel in backward direction might indeed be possible with the aide of wormholes and exotic matter — hypothetical material that exists inside of a wormhole, which is different from the kind of matter we know currently. The trick is in keeping the two ends of a wormhole (the mouths) stay open sufficiently long enough for a macroscopic object such as human beings to enter and exit.