Time travel has been an integral part of the yesteryear’s fictional work, and it provokes even the dumbest of the reader to question- Is it really possible? If yes, how would it work? What impact could it have upon life as we see it? And, we as engineers with a penchant for some complex physics decided to take up the issue and present to you in a nutshell, the DYNAMICS OF TIME TRAVEL!

To the uninitiated, Time Travel, as the name suggests, is defined as moving between different points in time analogous to moving between different points in space. Although novelists have been enchanted with the phenomenon since a long time back, it wasn’t until Albert Einstein came out with his “Theory of Relativity” that the possibility of Time Travel was even theoretically possible. For our topic of discussion, relativity established that “if a particle moved at a speed close to the speed of light, an observer’s clock would tick faster than a clock attached to the particle”. What this effectively means is that the particle’s time will appear “dilated” or “stretched” to an observer. To visualize it, just imagine yourself in a place where everything around you is moving in slow-motion, while you operate at the normal speed (Read: Stretching of time). We shall discuss about the mass-energy inconsistencies later on, for now let us assume that a particle can indeed be projected at speeds comparable to the speed of light.

One hypothetical situation, which highlights the contradiction in this theory of time dilation is the “Twin Paradox”. Let us consider two twins A and B. At t=0, twin A goes on a spaceship which moves at very high speed, and returns home to find he has aged less than his identical twin B who stayed on Earth. This result has been verified experimentally, using precise clocks in airplanes and satellites. However, it appears perplexing because each twin sees the other twin as traveling, and so, according to a naive application of time dilation, each should paradoxically find the other to have aged more slowly! Over the years, scientists have tried to explain the paradox. The most plausible explanation is that – Twin A uses two inertial frames, one on the way up, one on the way down. So, the difference arises from this change in inertial frames and not acceleration per se. In the twin experiment, we observed that if the time taken by Twin A to complete his journey was ∆t_{1 }and time elapsed for Twin B was ∆t_{2}, then ∆t_{1 }is less than ∆t_{2}. It effectively implies that A has travelled into the future of B, since he is still younger compared to B. However, based on the theory of relativity, we cannot go into the absolute past i.e. before t=0, but only the future.

The most basic flaw for Time-Travel is the famous E=mc^{2 }equation. It is ironical that Einstein’s one famous equation should serve as hindrance for the second equation to completely excel. In layman’s terms, the equation implies that “for speeds close to the speed of light, all forms of matter must convert into energy”. The solution to this obstacle is actually simple. Instead of actually transporting a human body into the past or future, one can merely use signals/light waves as carriers of information, accelerated at a very high speed. What this would mean is that, we could view the future via light signals. And, as far as “Nothing can travel faster than light” statement is concerned, even the Special theory of relativity allows for the existence of particles travelling faster than light (Read: tachyons).

Other theories for time travel include “Gravitational time dilation”, “Time travel using portals/wormholes”, “Time-machines”. Stephen Hawking, one of the most brilliant minds in field of theoretical physics in the recent years has given his backing to existence of portals and wormholes, and has proposed the idea of a time-machine that allows one to go into absolute past (up to the time the machine was first built).

Now that the basic principles for Time Travel have been established, let us ponder over the impact it could have on our lives. If, hypothetically, it was possible for man to venture back and forth in time, fiddling with the space-time continuum, what would the result be? Say, if someone were to go into the past, and kill your younger self, would you continue to exist in the present or the future? What if your future self was given huge sums of money, would it have any impact on your present financial situation? It is obvious that these questions cannot be answered with absolute logic, since it is beyond the scope of our physics as of now. And yet, we shall try to answer with all the various possibilities. Scientists in the past have favored the theory of existence of infinite universes, oscillating parallel to each other, where a change in one timeline of a universe has an impact on other timelines of that universe only. Amongst other arguments is – changing the past shall have no impact on the present or future, it shall unfold just as it was meant to in the first place. Another possibility is that by changing the past, we shall have altered the course of the events thereafter. Think about it!

Time Travel is still in infancy. There is still a long way to go. Whether one of these days, man will actually be able to come up with a practical way to travel through time, only time will tell!