Is Time Travel Logically Possible?

(This is an extended version of the essay for an online introductory course on philosophy I took in Coursera.)

Introduction

In H. G. Wells' The Time Machine, the protagonist travels both backward and forward in time. The Doraemon animation series which was created in Japan in the 60s features a robotic cat who travels 2 centuries backward. In Hollywood, we have Terminator series which is predominantly based on the theme of travel. A web-search on 'mythology' and 'time travel' would bring up many resources. But human's fascination with time-travel is not new. Einstein's special theory of relativity (1905) and then General theory (1915) gave a hint that time is not a physical absolute that keeps all universe chained to one single point which moves at its own personally chosen pace. Different objects experience different personal times depending on how close their relative speeds are to the speed of light. It's over a 100 years since then. Design of time machine hasn't been achieved yet. That prompts us look at the phenomenon of time-travel with some more scepticism; to investigate what would logically entail when the external and personal times of an object start diverging.

Here I present my thoughts on time travel. In this essay, I present three arguments: Unique existence, impossibility of passive existence and causal loops. My analysis makes me believe that time travel may not be a logically well-founded notion, at least not in the form we popularly perceive it in.

The Issue of Unique Existence

This argument rests on an assumption that, given any point and time in all space -- the (x, y, z, t) co-ordinates -- there can be only one particle occupying it. Suppose I (means, the collection of all particles in me) am occupying a particular space at a given point in time. What happens if an object or person travelling through time pops out this very moment -- apparently from nowhere -- to occupy the same space that I occupy. What would happen to the particles in me? Would they be displayed here and there? Would they just disappear giving place to the newly arrived object? At any rate, two particles occupying the same coordinates in the space is incomprehensible.

Impossibility of Passive Existence

The phenomenon of objects popping out of thin air has yet another issue. Each such event is bound to create counterfactual changes. Even if an object pops out at a point previously occupied by nothing else (i.e. vacuum), it's sure to change the course of history of the universe. If it's a charged particle, it will set an electric field around itself. A radiating object will change the illumination of the place. If not anything else, the very fact of an object having a mass immediately causes it to interact with all other particles of the universe through the force of gravity. The fact of something's existence is fundamentally determined by its ability to interact with the universe surrounding it in someway. Even a passive spectator of events around it must be stimulated by physical stimuli (e.g. light, sound waves etc.) to sense them. Absorbtion of physical signals results in changes in the surrounding that wouldn't have happened if the signals hadn't been absorbed. In other words, an absolutely passive existence sounds like a vacuous idea. May be, invoking Descartes' idea of substance dualism may succeed in modelling such a 'ghost' existence. But, in my mind, I am unable to do so.

Causal Loops

The most serious difficulty in the notion of time-travel, according to me, comes from the notion of causality. If we write out all the events in the universe, and for every pair of events A and B, we draw an arrow from A and B if A directly causes B This exercise (of drawing arrows between events) would give us a directed acyclic graph (DAG) of causality. As per this idea, all events in the history of the universe can be tracked back to a set of events, which in turn, aren't caused by any other event. There can exist no cycles of causality in this graph. Put another way, no event can directly or indirectly cause itself.

However, time travel can create cycles in this causality graph. Several examples are present in the Terminator series of movies. For example, a soldier named Reese is sent back in time by John himself from the Terminator. John is born out of the love between Reese and his mother Sarah. Does this mean that John caused his own existence? In fact, it would have been possible for John to father himself, had the storyteller decided to tighten the loop of causality a bit further! Even more interesting is the advent of the Skynet -- the AI network which wages a war on humans. Skynet develops out of the remains of the first Terminator, the one which had travelled to 1984 from 2029 to assassinate Sarah. And its Skynet which creates Terminator. The question is: wherefrom comes the idea Skynet or Terminator. They seem to have begotten each other! The trouble here is more serious than that a machine invents a machine. We may imagine, at least in theory, that AI machines sophisticated enough may even invent other machines. But the acyclicity of causality is an even more important requirement in the case of ideas and thoughts than in the case of physical events like birth of people.

Conclusion

We have presented three arguments which speak against the logical possibility of time travel. The first objects to time travel based on the incomprehensibility of two particles occupying the same co-ordinates in space at any given instance of time. The second rests on the notion that existence of a physical object can't be passive in true sense; it must interact with the outside world. And therefore, it must have a role in the course of history. The third objection rests on the assumption that causation is acyclic: Time travel makes it possible for events to cause themselves.

There are other difficulties which are physical. For instance, how would time travel as we understand it, fit in with laws of conservation of mass and energy?

It also appears to me that the objections raised above, more than proving a fundamental impossibility of time-travel, brings forth a certain incomprehensibility of the reality that would result from it. To prevent time-travel from creating an anomalous universe, one must imagine a concept of time travel very different from the one popularly held.

An Ode to Mediocrity

Consider any quality: Physical, intellectual, emotional, economic, social, spiritual. Create any grading scale of that quality that you feel suitable. Start placing the members of the human population at various grades. You are well likely to end up with a bell-curve. And where will you find yourself? Well, if you are somewhat like me, I'm sure it will be somewhere close that crowded median. Repeat the exercise with another virtue. Again, most likely, you will be loafing around close to the median.

Again, if you are like me and every other mediocre person, every single day or your life, you aspire – no, agonise – to break free of your mediocrity. You too, at least sometimes, dream of being a star.

Stars are those who once, at least once, become the best in something. Their glitter may fade after a period – brief or long. Rajesh Khanna was a star. He shone bright for a while, and faded away into oblivion, to come back briefly once more to the limelight when he died. But we have others like Gandhi, Einstein, Vinci, Tolstoy etc. whom people will probably never tire of talking about. Their stardom may go. With time, they may even get surpassed by someone else in their achievements. But the stories of their achievements endure well beyond the relatively brief period of their stardom. These are legends.

Legends are legends because they somehow end up doing something which changes the world. But you don't need to change the world to be a star. You don't need to invent a medicine for AIDS. You don't need to sacrifice your life for the freedom of your nation. You don't need to reveal to the world the metaphysical secret behind their living and dying. You just need to take something – however trivial, however insignificant, however irrelevant to the current concerns of people – and become good at it. You have to be so good that you become a benchmark in that. The importance of your achievement isn't in the choice of what you have done, but to what extent you went in perfecting your ability to do it. A person who has trained himself all his life to swim as fast as anyone can – which will hardly ever mean anything for the alleviation of one man's misery – becomes a Michael Phelps. Someone specialising in painting horses, naked dieties, and erotic scenes involving them becomes a reverred as well as controversial artist. There seem to be seemingly an infinite number of things to choose from to achieve your stardom.

And yet, stars would probably make a meagre 0.00000001% (I typed a string of zeros that just looked long enough; don't read too much into it) of the population. Legends make up a vanishingly tinier fraction of that. Rest of populations constitutes of wannabes and losers.

Then why on Earth, like me, does every mediocre person perhaps wants to become a star, knowing full well that, by the very definition of the term, stars will always constitute a minuscule part of the population? Logically speaking, the probability of success in becoming one is that much small for all of us. But still we keep trying and trying and trying.

Trying endlessly is one thing. But we go beyond that. We hate ourselves, our lives, everything until we get stardom in at least something. We call ourselves mediocres, losers. The wish to be a star has probably brought out the best in a bunch of people. But for the remaining teeming millions, it makes life a hellish business.

The world is fully justified in celebrating its stars, in worshipping its heroes. Because heroes line the limits to which humans are capable of going in the infinite space of activities. But it's a sheer tragedy that the remaining population, which does the very important task of filling up the space in between these boundaries, frets and whines about getting there at the boundary at least once, to get a photo taken of themselves in that moment of stardom, and to think of that brief moment as the summary of their life. How much more stupid could humanity get?!

How can 70-80 years of living be about vying for a moment of glory that mostly doesn't come? And even if it does, what value does it really have if earned through mere suffering, not just of the self, but of innumerable others trying uselessly to clinche that moment. That moment becomes precious, often not by its intrinsic value, but by the very fact that most won't get it. Just as gold is considered more valuable than iron because very few have it. I don't fully understand the logic behind considering something valuable merely because it's rare. And yet, I know, that's what we humans keep doing.

Tell me, isn't it a huge, colossal mistake? I know, humanity has been committing it since ever, and is probably cursed genetically to continue committing it forever. Yet, I don't think Phelps could have made it if swimming hadn't given him a day to day sense of well-being. I can't imagine M. F. Hussain churning out so many paintings till the last day of his long life without each painting giving him the pleasure of having said something of his own. Phelps' best laps can't be his medal-winning ones. Hussain's best paintings may never have been sold.

Feeling of worth is a very private feeling. Experience of beauty is a very solitary experience. It doesn't get displayed to a clapping audience; it can't be auctioned to rich bidders. And those moments of beauty are the blessings we all humans have got, whether we are stars or mediocres. The privilege to create is afforded to every one of us. That privilege is available each and every moment of our life. The principle output of that creation is the experience of creating. Not how it changes the world, or how people clap, or how much it is sold for. The reason for an act of creation is fulfilled much before it ever comes before any audience, at the very moment of its happening. And that fulfilment doesn't depend on whether you are a star or a mediocre. It just asks you: did you feel the joy? Did you feel it enough?

The thought of being mediocre is a liberating one in a sense. It frees you from the pressure of trying to be the best, the first, the quickest. We know that the burden of changing the world is not on our shoulders. Being mediocre, you can just focus on what you like doing, and enjoy yourself. What the world wants to make out of your work is purely their worry. From your perspective, it's at best a side effect of your actions. Beyond the condition where the world perceives it practically worth their while to let you exist, I don't think you owe anything to it. Your life is yours to live. Your strengths are yours to use; your frailties are yours to fight.

Also, every one of us isn't born to paint a Mona Lisa, to discover the laws of motion. OK, if fixing that broken tap in your house is the best thing you can do at the moment, why not do it and feel as happy? That's your contribution to making the world a better place (and for sure, for your wife this contribution will count more than Newton's laws of motion). So, why be judgemental?

In short: you think you are a mediocre? Good! Happy Independence Day!

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