Killing time: is time travel possible?

From Doctor Who to Back to the Future, humans have shown a fascination for time travel. But is it really possible?

Philosophy of time

Since the publication of H.G. Wells’ The Time Machine, time travel as a concept has become littered throughout nerd and pop culture, from the classic Back to the Future franchise to more recent movies like Interstellar and Arrival, creating a litany of paradoxes in its wake. Perhaps these nostalgia-driven fantasies are just our mind’s fixation on slowing down the pace at which our lives seem to pass.

In order to define any other time, we need to define now. It is the impossibly thin razor on which we live our lives. It acts as a boundary between the past and the future. How do you distinguish what is on one side of now from what is on the other? The only thing that defines the direction in which we experience time is the second law of thermodynamics, which states that any system will gain disorder or entropy as it moves “forward” through time. That is why a sandcastle disintegrates into its surroundings and the idea of a pile of sand spontaneously jumping into the shape of a sandcastle feels so improbable.

As living organisms, we like to think that we slow entropy because we consume apparently less complex organisms and produce ourselves. But if you were to view humans as a system, only a small number of what we consume becomes a part of us. Most of it is broken down into more disordered forms to provide us with energy. Whether we like it or not, we are engines of entropy. That is possibly why the idea of time travel is so appealing, if you can cheat entropy, you can stall death and live in a million timelines.

So theoretically, if one wanted to travel through time how would they do it? Well, we are always travelling through time, tripping over the past and stumbling into the future. But if we wanted to change the pace or direction with which we do so, Einstein’s theories of relativity need consultation.

Time and relativity

Einstein’s field equations describe our universe as a spacetime: a fusion of three spatial dimensions with one time dimension, forming a four-dimensional continuum. This spacetime is all made of the same stuff and is bent or curved by mass. Imagine a bowling ball placed on a trampoline. Now picture placing marbles at the edges of the trampoline. They would be pulled towards the bowling ball in a manner similar to the gravitational attraction of a moon to a planet or a planet to a star. This curving of spacetime also slows down the passage of time. If you were to fall backwards towards something with a large mass such as a black hole, provided that you do not get turned into a spaghetti string of atoms by its crushing gravity, you would watch the passage of time for the rest of the universe speed up more and more as you approached the event horizon.

So you could go forward in time through approaching large masses – well, your atoms could, you are unlikely to survive. But you could also do it through acceleration. If there were two twins and one twin got in a spaceship and taken away from the earth approaching the speed of light, and returned in a similar fashion, less time will have passed to her than her terrestrially-bound sibling. This is known as the Twin Paradox.

That said, we should not get too excited about how we might change our own past yet.

Now, how would you move back in time? It would similarly be bound by Einstein’s theories.  There are these hypothetical things called wormholes that are permitted by the field equations; they are basically two distant positions in space that are warped in so that they are connected. You may have seen them in the movie Interstellar. If you were able to create a wormhole and move one end to a point within a strong gravitational field or accelerate it to a percentage of the speed of light, then this end would experience the passage of time at a slower rate that the other end. There would be a chronological lag between this end and the static end of the wormhole. This means that if you were to enter this side of the wormhole, you would arrive at an earlier point in time on the other side.

That said, we should not get too excited about how we might change our own past yet. In a 1992 paper, Stephen Hawking proposed the “chronological protection conjecture” which hypothesised that this backwards time travel could not work on a human scale as this would be a violation of causality. Essentially, if these wormholes were to exist, it would have to be at a distance so great that no information could travel across space from the older to the younger end without it already existing at the young end.  

Quantum time

Observations of one of two quantumly entangled particles seem to reach through time to influence the patterns and behaviour of its partner. This is conveyed excellently by the “delayed choice quantum eraser” experiment performed in 1999. There is a lot of lingo involved in this paper, but the important thing is that there are two sets of outcomes: decoherence or interference for one quantumly entangled partner and preserved measurement or scrambled measurement.  

These two sets of outcomes are entangled. A preserved measurement retroactively leads to decoherence: if we measure a property of one entangled particle and preserve the information, then we alter the behaviour of its partner throughout time as if it had been destined to act in a decoherent manner. This is like the particle measured first is whispering to its partner in the past: “I got measured! You better act decoherently.” However, if we measure it but use beam splitters to scramble the signal, ridding it of any information, it’s partner would have the property of interference. It is as if the first particle is saying: “No one is looking! Have interference.” This implies that the collapsing of a wave function produced by its observation will cause ripples extending into the past of its quantumly entangled partner.  

“Your atoms may travel through time in a non-linear fashion but you and your DeLorean almost certainly won’t.”

Quantum mechanics can stretch and exhaust the mind because we live on an entirely different and literally massive scale that is ruled by causality. The heuristics employed by our brains to understand the world are not very useful on the quantum scale. As things get smaller, values like causality and locality start to disperse into mists of probability. So again, your atoms may travel through time in a non-linear fashion but you and your DeLorean almost certainly will not.

On the bright side, you are unlikely to ever kill your grandfather and prevent yourself from being born. Einstein said: “Time and space are modes by which we think and not conditions in which we live”, so try not to get too stressed if you are running late for a lecture. It is important to learn from the past and prepare for the future, but one is just a story we tell ourselves and the other does not even exist (yet). We spend our lives in the now and if we spend that time dwelling on the past or worrying about the future, we will have less of both. Time is just a measurement of change, and for there to be a second there has be a first.