Time Travel - Is it possible?
Possibility of time travel
The Theory of Relativity of Albert Einstein offers several options for time travel:
Traveling into the future
If you travel with nearly the speed of light in a spaceship (under circumstances speeds by 10% the speed of light should be enough) from earth and return after a certain period of time, on earth, a longer period of time will have elapsed as on board the spacecraft. The reason for this is the time dilation that occurs after the theory of relativity by Albert Einstein at such high speeds. More on that under twin paradox.
At sufficiently high cruising speed and acceleration any distant future on earth would thereby be reachable in any (short) time. However considering a reasonable acceleration acceptable for the human body, a time shift of years requires a travel time from the perspective of the spaceship crew of over a year (about 347 days each for acceleration and deceleration of 9.81 m / s²).
According to the general theory of relativity, the passage of time is also dependent on the gravitational acceleration and conditions to which a system is subjected. Meaning the time on a high mountain passes slightly faster than at sea level. This phenomenon could be described as time travel into the future, but not only faster, a travel at slower speed is also possible.
On a neutron star, the gravitational time dilation can be substantial. A hypothetical resident of a neutron star could do a time-consuming task in an orbit around the star in order to keep an appointment on the surface.
Travel into the past
In the current state of science time travel into the past is principally not possible. Existing theories, according to which such travel is possible, are speculative and controversial. Undeniably though, in any event, is that the practical implementation of such theories is impossible in the foreseeable future.
1949 Kurt Gödel discovered that a solution of general relativity in which the universe is rotating, that its possible for an object to return to its own past. Such a universe is known as a Gödel universe (R-universe).
Even though it can be proved that our universe is not rotating, the R-universe shows that Einstein's field equations admit a universe with closed time like curves. Consequently, this means that it’s not necessary to have a uniform passage of time.
According to the general theory of relativity, it is conceivable that two different regions of space-time could be interconnected via so-called wormholes. If the two outputs of such wormhole would connect two regions of different time, time travel into the past would be possible. However, calculations show that wormholes are not normally stable and collapse so quickly that a passage is not possible. However, with negative energy density, the so-called exotic matter, one could stabilize a wormhole. Nevertheless the required amount of exotic matter is, according to current knowledge, not available in the entire known universe.
Possibly a journey into his past would be possible on a specific trajectory in the environment of a sufficiently rapidly rotating black hole. However, it is believed that there are no such a rapidly rotating black holes.
A time travel into the past would be possible with the vicinity of two cosmic strings that pass sufficiently fast to each other. However the existence of such strings is controversial.
After a very controversial interpretation of a time reversal was achieved by Superluminare tunnels for particles or photons, that is, from the experimental arrangement – in the "tunnel" the particles seemed to come out before they were blasted into it. However, in these experiments, a very controversial definition of signal timing was used.
Should it be possible to travel into the past, questions would arise as how the paradoxes are to be avoided which may arise in this context from the violation of causality, such as the Grandfather paradox. A possible answer would be Everettsche Much worlds theory. In that theory, the past, in which one travels, is located in a parallel world. The original course of events and by intervening in the past modified sequence would both happen. In particular, it would be impossible for the traveler to return to his original version of the present, but into a parallel world that would be almost identical to his.
Occasionally, if traveling back in time is mentioned, hypothetical “faster than light” particles called tachyons, are brought into play. When a particle with superluminal velocity moves from A to B, you will always find an observer who sees the movements from B to A. Since the observers evaluate the temporal order of the events of A and B different, the tachyon moves from the past to the future for all participating.