What do we want?! Time Travel! When do we want it?! It’s irrelevant. 

Time, by far my favourite abstract notion to think about whilst also the one that can easily cause the most damage to one’s sense of reality if over-dwelled on. This post will cover thoughts on our perception of time and then move on to consider sci-fi’s ever favourite tool, time travel; the common misconceptions, problems and theoretical consistent possibilities.

As human beings our perception of the passage of time is fixed, it is an ever forward-flowing river from which we can never escape. Einstein’s general relativity teaches us that time is a dimension, as are the three of space, however although we are able to freely traverse the spatial dimensions (we can go up/down, backwards/forwards, left/right) we cannot exercise such control in time. We are confined to travel ever ‘forward’ through this dimension. From our limited human perception we perceive the universe in these four-dimensions but this is not necessarily to say the universe exists only in four-dimensions. Such thought is the foundation of string-theory which suggests extra-dimensions exist ‘curled up’ unable for us to probe. This is a broad theoretical leap, into which we shall not digress, but the point is that the existence of higher dimensional beings, those which can perceive 5, 6 dimensions or more, is not an impossibility, and such beings could be able to traverse the dimension of time as we are able to space. However we are not fortunate to have such extra-dimensionality and as such this option for time travel is most definitely ruled out.

Second consideration: time dilation. Ok so what is time dilation? Time dilation is a by-product of Einstein’s theory of Special Relativity. Let me try and explain the consequences of Special Relativity as simply as possible. (Funnily enough the mathematics behind the theory allows one to comprehend it much clearer and if you’re ready for that any half decent textbook will explicate the key principles well, but i’ll try to stay qualitative). Everything, every human, every star, every bug, every single particle is moving through spacetime at the speed of light, 3×10^8 m/s. This a very fast speed indeed. Now, think  of our expenditure through spacetime, at the speed of light c, as divided up into the space and time components. Us humans, we move around in space very slowly compared to this colossal speed and because we travel so slowly through space, a lot of the fraction of c is left over to travel through the time dimension. A photon however, (a particle of light) obviously travels at the speed of light, so it’s fraction leftover for the time component is 0, i.e. photons do not experience the passage of time!

Anyway the point of this is that if an observer travels at a high speed, a significant fraction of the speed of light their fraction of c leftover for the time component will be less and, as seen by another observer, their time will run slow, their time will dilate. Now here is the catch, the observer travelling so fast will not notice this themselves, for them and everything in their system  travelling with them at c, life will continue as normal. Let’s turn to the famous twin paradox. One twin stays on planet earth whilst a second zooms of at the speed of 80% of c, to a distant planet and back again. As measured by Earth Twin 50 years may have passed before Space Twin returns, though on Space Twin’s clock only 5 years have passed. (The maths is not accurate here). Through this method of travelling at close to light speeds the traveller does in some sense time travel but only relative to another stationary observer. The traveler manages to dilate their passage of time by travelling so fast in comparison to the one who stays still and as such manages to ‘travel’ into their future at a less year-cost compared to them. Again however, with this method we are still travelling forward in time, just slower relative to another reference frame. This method does not allow us to manipulate time and we experience no extended life, although 50 years may have passed for the Earth Twin the Space Twin only enjoys 5 years of life by his watch, he manages to do no more than the average human being on earth in 5 years. The last point to consider is that we are so incredibly far from being able to actually travel at such significant fractions of the speed of light that this may well never become an actuality for human beings. It remains as nothing but a theoretical possibility.

The second idea to turn to is within the framework of General Relativity. General Relativity theoretically permits certain solutions which contain what are known as closed timelike curves (CTCs). These occur in regions of spacetime where the governing metric is the Godel metric (we approximately live in the Minkowski metric… more on this if you study GR). Ok what on earth does this mean? General Relativity is the most accurate theory in describing the macroscopic universe and motion through the universe is traced out by worldlines – a worldline is a trace of a particle’s motion in spacetime (quite simply its motion through space plus time). Now the workings get a bit mathsy but a closed timelike curve is a worldline where the particle manages to return to it’s starting point – it goes on a journey and that journey leads it back to the same point in space (easy enough, take a walk in a circle you can achieve this) but also in time! Such a curve and its consistency with the theory of general relativity, as discovered by Kurt Godel, makes their existence plausible given our current understanding of the universe. So, traveling along a CTC implies the ability to travel backwards in time.  Now that the ability to travel back in time is a theoretical possibility many a paradox arise. The most famous being the Grandfather Paradox. The idea that if a traveler were able to go back in time, they could kill their grandfather, thus never being born, in which case never being able to back in time to kill his grandfather in the first place. Messy right? Now this all might seem like a load of nonsense but philosophy and its paradoxes have their place as they seek to deduce with logic what kind of time travel may be plausible. As such the most well-accepted response to the grandfather paradox is the Novikov self-consistency principle. This principle states that all actions taken by the time traveller must be consistent with events that follow  and any events that give rise to a paradox would have a zero probability of occurring. A principle of self-consistency which states the only solution to the laws of physics that can occur locally in the universe are those which are globally self-consistent.  So the grandfather paradox could never occur because it is self inconsistent! (i.e. it makes no logical sense!) This does not mean the traveler cannot be the cause of events in their own past, this is plausible as long as it is consistent, this is known as circular causation – a boy may come across a time travel machine planted by the grown boy 10 years later on his trip back in time outside his younger self’s front door, allowing him to figure out how to use it and then travel back in the first place (mind-boggling but still logical). However a grown boy cannot go back in time and kill his younger self – the Novikov self-consistency principle would assign the probability of such an event occurring as null. Through this principle we retain the possibility of time travel whilst making it impossible to create time paradoxes.

A final point to touch on for this post (if you’ve made it this far i’m impressed but you’re in luck because it’s the best bit) – wormholes! A wormhole, otherwise (rarely) known as a Einstein-Rosen bridge, is a topological feature of spacetime that, theoretically, connects two separate points in spacetime itself. Wormholes warp spacetime and are again permitted by the equations of general relativity. My favourite way to conceptualise a wormhole is taken from Interstellar. Romily, whilst aboard the Endurance, takes a piece of paper and tells Cooper to imagine it as representing a region of space (just like a map). First the spacetime is flat, (as normal) then something (mass) causes a warping so giant in the spacetime that the piece of paper folds over. Romily then punches a hole through the piece of paper with a pencil (which as it’s folded goes through both sides) and this my friends, is the wormhole. The hole connects two points of spacetime which when the paper is then unfolded are seemingly very far away (perhaps billions of light years) from each other but due to the warping of spacetime manage to be connected.  Now, if the wormhole is traversable (how this could be possible is a different topic entirely, involving fun terms such as exotic matter) seeing as it connects two points in spacetime, if an observer were to be able to travel through it they would in principle be able to time travel (as well as taking a great spatial leap). Whether such topological features do exist in our universe is another question, but remember the universe is incomprehensibly large and we know not what wonders lie out there, or what advanced civilisations, who may be able to create such phenomena…

So there we have it, the three (most well-supported) theoretically consistent possibilities of time travel. Special relativity’s time dilation (arguably not time travel but time dilation relative to another reference frame), General relativity’s Closed Time-like Curves and the wonders of the Wormhole. I’ll leave you with final thought to muse on though, is time truly real and fundamental in nature?  Or is just a human construct to better articulate our experience of reality that we have so tightly woven into experiences and our mathematics? A thought I shall return to in another post… for now I think we’ve all spent enough time thinking about time.