We are always travelling in time to a future at a near constant rate. This is the most trivial form of time travel. It is currently unknown if the laws of physics allow time travel to the past. Backward time travel also introduces a whole set of problems and paradoxes.But we will come to that later, now we will look at various possibilities of time travel which are theoretically allowed by the current day physics.
- Wormholes - A wormhole is a hypothetical feature of the space time curvature. According to the general relativity gravity curves the space time. There are theoretical solutions of general relativity which contain wormholes. A structure in which space time curves onto itself looking something like the following picture. In such a structure we can travel through the hole as a shortcut in space time. No such structure has been observationally detected yet. The possibility of engineering such a structure for time travel is quite far fetched.
- Primordial Black Holes - If we cannot create a wormhole the other possibility is that there may already exist some really old black holes. These black holes formed due to collapse of extremely dense matter in the early stages of the universe. Some of these black holes could be traversable due to their geometry and form candidates for time travel. Even though they are considered as a candidate for dark matter none has been detected observationally yet.
- Cosmic Strings - These are large 1-D topological defects associated with early universe.The formation of cosmic strings is somewhat analogous to the imperfections that form between crystal grains in solidifying liquids, or the cracks that form when water freezes into ice. They are also predicted by some string theories. Cosmic strings can also exist with negative tension and hence negative mass. If this is wrapped around a wormhole it can be used for time travel.
- The Special Theory of Relativity - Time dilation is a form of time travel permitted by Einstein's special (and general) theory of relativity. Time elapsed in two frames of reference can be different and this can be perceived as a forward time travel by one of the observer. The following gif shows the difference between time experienced by green and red clock. Time experienced by a moving body is slower than experienced by an inertial frame at rest. This effect is observed and accounted for in satellites used for GPS.
- Negative Energy, Mass and Flux - Exotic forms of matter like negative pressure, energy etc can be in theory manipulated to create possibilities for time travel. Study of dark matter and energy is one of the hot topics in current research in physics.
- Tachyons - These are hypothetical faster than light particles. If they exist they may be used to send signals which can travel in time. There have been some faster than light neutrino anomalies detected in experiments but they turned out to be due to some errors in the setup and calculation.
- Time Reversal - This phenomenon is not really related to time travel but an interesting possibility. Most laws of physics are invariant under time reversal transformation except maybe the second law of thermodynamics. We are used to a universe where the time increases in the direction in which entropy increases. But the question is if would you experience time backwards if entropy was always decreasing in the universe ?