Time travel has been a popular trope for decades, in books, TV shows and movies. It is not uncommon to see people going back and forth in time and being heroes (or dicks) in general. Recently I have been reading up on and thinking about the scientific aspects of it.

After doing some research and reading up various journals, here are my thoughts on time travel:

Time travel to the past

I am a firm believer of the notion that time flows in one direction and it is impossible to reverse the flow of time and “travel” into the past. As such, the various temporal paradoxes are null-and-void. It is impossible to go into the past and change what has already happened.

The biggest proof? If this was possible, someone from the future would have come to our present or past and left evidence.

What I believe you can do is “look” into the past. You can do that today when you look at distant stars. Whether you can see past events at places not far away (like earth)? I am not sure but I don’t think so.

At some level, I feel comfort in the fact that the events in the past are set in stone and cannot be changed.

Time travel to the future

I believe time travel to the future is possible, but not in the way you think. It is a well known fact that gravity and velocity influence how you perceive the passage of time. Such effects have already been observed, albeit at a small scale.

So technically, you can travel (say) 10 years into the future at the cost of only 1 year of your personal time. But, as per the previous topic, this is a one way street. When you do this, you leave behind your time forever and cannot go back.

How?

You can do this in 2 ways

1. Travelling every fast at relativistic speeds (Special relativity) :
   
   Eg you can travel to earth as it will be in 200 years, if you travel 2 years at 99.995% the speed of light. However, since the closer you get to the speed of light, the higher your mass becomes, it may be impossible to accelerate a human to such speeds.
   
   However, lower returns, like travelling to earth 4.5 years from now at the expense of 2 years of personal time is possible at just 90% of the speed of light. At 90% of the speed of light, a 100kg mass will only increase its mass to 229.4kg.
   
2. Being in close proximity to a huge gravitational well (General relativity):
   
   This was beautifully demonstrated in the movie Interstellar, in which, due to their close proximity to a black hole, the protagonist travels 14 years into the future from the reference frame of earth, at the expense of only 45 minutes of his personal time.

Again, none of this is science fiction; all this has been repeatedly confirmed by experiments. At this point, it is not a physics problem, but an engineering problem. Will take a few massive leaps in spacecraft propulsion technology and all this will become everyday affair.

Mathematical Predictive Time Window

You know how you can calculate (on paper) that when one billiard ball hits another, what path they both travel? You can do that with 3,4 balls, too. As you scale up, the problem becomes too difficult to calculate on paper, but computers can do this at a much higher scale already.

Now imagine a computer powerful enough to calculate/predict the path and behaviour of individual atoms. Now, if you were to build a sufficiently powerful computer that can predict the path and behaviour of all the atoms on earth, you could easily predict the future and see what is going to happen days, months and years into the future.

Similarly, if you reverse the billiard ball concept, and you measure what speed and angle each ball is travelling at, you can predict the starting position of each ball (and the cue stick). Extending this concept to extremely powerful computers, you could theoretically look back in time into the past.

The Roadblocks

There are however the following problems with this concept:

• Measurement : Measuring the position and movement of each atom instantaneously is almost impossible, because by the time you measure them, these parameters have already changed.
• Storage : If you were to store the data of each atom (1 bit), even with the most efficient storage mechanism, it would require a like-for-like atom in the computer. So to store the data of each atom on earth, you need at least a computer with the same number of atoms. To simulate the whole universe, you need at least another universe.
• Free Will : All of this assumes that a living creature’s behaviour is purely a product of the neurons firing in the brains, in turn their behaviour at the atomic level. Which also assumes that everyone’s behaviour is pre-determined and there’s no such thing as free will.