Since the advent of films and TV shows, all of humanity has long fantasised of travelling in time. The wish arises from the one dimensional picture of time we have in our minds. We imagine time flowing at a constant rate – that the interval between the two ticks of a clock remains same irrespective of the location or the motion of the clock. When one imagines time travel, we often picture ourselves getting into a time machine and setting a destination – this time not a city or a famous landmark- but a desired ‘point’ in time. The time machine energizes, lifts up into the air and vanishes. When you come to your senses up you find yourself in a different time. Depending on your setting, you have either arrived in the past or the future.
This concept stems from the fact that we think of time as we think of two locations in space. Just like in space, we can go from point A to point B, similarly, we can travel in from the present into the past or the future – employing any means that suit you. But that is not the case.
There is a basic concept of thermodynamics called ‘Irreversibility’. To do away with scientific jargon, it simply means that every natural process in the universe is irreversible. Logically stated, everything that happens in the universe cannot be reversed.
To demonstrate the proof of this principle, assume you dropped one of your mom’s favourite vase. She is going to be home in a matter of minutes. You think of every possible alternative to fix the problem, to make the vase as new as it was before you broke it. In simple words, you are only looking to reverse the unfortunate event of dropping the vase. One idea is to try to fix the pieces with glue. But then you think ‘ That won’t do it.’ Now that some pieces of the vase have turned into powder on impact, while some are simply thrown away in unreachable corners of the house, the vase can never look whole again. The other idea is to collect what remained of the vase, melt the ceramics and mould an identical vase. It would be identical in all respects but its outer finish. Come what may, you can never replicate the exact finish of the particular artisan that moulded the vase. Nor can you perfectly replicate the sceneries painted on it by an experienced artist. You can think of innumerable ideas even if you had infinite time, but the vase won’t be like it was before.
Simply put, you can never reverse the process. In scientific terms, if a natural process goes like this :
A => B => C
Then it is impossible to reverse the process It will never happen that something achieves its initial state starting from it’s final state following the same route it took to attain the final state from the initial state.
ie. the above natural process cannot be reversed in the following way :
C => B => A
Now, lets come back to our beloved time machine. Back to the beginning, we make ourselves comfortable in the time machine. We set the date to some day in 1992. Then you press the big red button that has GO boldly written on it. Again, the time machine energizes, rises above the ground and vanishes in a bright flash of light. Now your journey from 2015 to 1990 begins. Let’s assume that if and when you land in 1990, everything is exactly as it was when you were 25 years younger. If you consider yourself as a reference point ie. if you observe what you see through your own eyes – you will see the whole course of events from the present ( 2015) to the point in the past (1990) being reversed. You will see every event that has happened, happening backward – taking the same path but the opposite direction in time.
ie. for every event A to B to C, you will see the event taking place as C to B to A. But this itself is not possible. Because we have earlier established that no natural process can be completely reversed. The only logical conclusion is that we have erred in assuming that we do land in 1990. Fact is that, we cannot arrive in 1990. To generalise the conclusion, it is logically impossible to travel into the past.
I’m extremely sorry to have spoiled the hopes of time travel enthusiasts of shaking hands with history’s’ greats. But there is one more aspect of time travel left untouched : Travelling into the future. Which is an altogether different story.