Is Time Travel Possible?
Time travel is the idea of moving backwards or forwards in time. It is a concept which has featured heavily in fiction, in films such as Back to the Future and Terminator, and books such as The Time Machine (H.G. Wells) and A Christmas Carol (Charles Dickens). Apart from it being used in such non-serious contexts, the concept of time travel is also much discussed in science and philosophy. Even though most would accept that we do not have the facilities to be able to travel through time right now, the question of whether we will ever be able to, or whether time travel is even an intelligible concept, is a much debated one.
When we talk about time travel, we often use the analogy of space. We are all familiar with travelling from A to B on a map and maybe we can think of time in the same way. Using this analogy we can perhaps map different points in time (in the past, present and future) and potentially travel to them like how we travel in space. However, time seems to be a very different and mysterious thing, not observable in the same way we observe space. Without knowing what kind of thing time is and whether it exists, we cannot say whether time travel is possible.
What is the nature of time?
In order to fully understand if time travel is possible, it is necessary to have an intelligible conception of what time is. In fact, if it so happens that there is no such thing as past and future, only the present (a view which has been known as presentism), then it is clear that time travel is impossible. Indeed, many philosophers such as J.M.E. McTaggart have tried to put forward proofs that time is unreal and, while many believe his proofs are sound, debate rages on.
In his book, The Unreality of Time(1908), McTaggart argues that there are two ways of looking at time, the A series and the B series. The A series is where we have something resembling a timeline with past, present and future, and the present 'moves' along it, so the present is always changing and therefore has no fixed position in time. If we look at it by using the B series conception, on the other hand, we are no longer considering past, present, and future, but rather, we are considering relative positions in time. The B series is concerned with 'before', 'simultaneous with', and 'after', and there is no such thing as 'the present'. So for example, the Battle of Hastings was in 1066, 1066 years after the birth of Christ and 934 years before the millennium. None of these dates are considered to be the present, they just have fixed relations to each other.
None of the two views seem satisfactory as there are problems with both. For example, the B series seems not to do justice to our psychological view of the present, and the A series seems contradictory as all events are past, present and future. Different philosophers have held different positions on the matter over the years. It seems certain that time travel is only possible on the B series of time given that on the B series every event has the same kind of 'existence' as any other. If we adopt the A series view, how can we go back in time to the past and yet be in the present? The present will have 'moved on' so to speak. But, using the B series case, all past and future events are already set out like points on a map, so we can conceive of visiting different points in time without having a confused notion of 'present'.
Scientific views on time travel
Philosophical theorising aside, it is important to note what science has to say on the matter, as it has made the most progress where actual cosmology is concerned. According to various scientists, there are at least three ways in which time travel may be possible.
Travelling faster than the speed of light
These days, our understanding of things that move close to the speed of light is shaped by our understanding of Albert Einstein's Theory of Relativity. This theory, which is one of the most successful and widely accepted of all scientific theories, suggests that movement faster than light (if that were possible) would entail movement backwards in time too. It is highly unlikely, however, that any object could go at a speed faster than light. According to the theory, it would take an infinite amount of energy to accelerate any object to that kind of speed.
The use of black holes
In theory, it has been said that we can use black holes to travel to the future. Black holes are surrounded by very strong gravitational fields, and the closer that you draw towards the centre of a black hole, the stronger the field. In these extreme conditions, travellers experience an effect known as time dilation. This is a phenomenon which has been shown to be true by the Scout rocket experiment of 1976 and it causes travellers to appear to move more slowly through time for an outside observer, whilst they are in a black hole. Everything outside the black hole at this point appears to move faster for the traveller. Therefore if a traveller managed to both enter and escape from a black hole, they would end up in the future. However, this is obviously very difficult. For more information on theories about black holes, check the work of Cambridge University Professor Stephen Hawking, who has written much about the nature of space-time.
The use of wormholes
A wormhole is a hypothetical fold in space-time which gives travellers the potential to shortcut different points in the universe. It is often referred to as an Einstein-Rosen bridge named after those whose theories have been used to formulate the concept. The idea of a wormhole was first introduced by the American theoretical physicist John Wheeler in 1957. According to his theory, time travel may be possible using wormholes if the following occurs. If one end of the tunnel is accelerated to a speed approaching that of light, it will undergo a time dilation effect, meaning that it will age less than the other end. As such, if a traveller went through the wormhole from the younger end to the older he'd be travelling forwards in time, and vice versa. If wormholes do indeed exist, they are considered to have the most potential for time travel, but currently we have no evidence for them besides pure speculation. Perhaps the most prominent physicist postulating this view is Amos Ori, who argues that wormholes are a "realistic" model for timetravel.
The paradoxes of a time travel
Many philosophers and scientists say that time travel is impossible even on a conceptual level. There are certain paradoxes involved in thinking about time travel which could mean that those who believe in the possibility of time travel are not even making any sense, let alone formulating cogent and constructive arguments.
The grandfather paradox
By far the most popular and well-known paradox involving time travel is the grandfather paradox. It is as follows. Suppose you go back in time and come across your grandfather when he was a child and kill him. That would mean that you were never born, since there is no way someone who didn't exist at the time could have had grandchildren. But then, if you were never born, you couldn't have gone back to kill him, which would mean he would have stayed alive and then you would have been born.
This paradox involves what is known as a closed time loop and can be best illustrated by reference to the film Terminator 2. In this film, a superadvanced robot is sent into the past, where it is destroyed. From the debris, scientists learn of the advanced technology and are able to build such robots. They would not have been able to build them had the robot not come back in the first place. But he would not have come back if they had not built it.
Suppose you go to the future, and you discover something you do not like. You will inevitably then try and change it when you go back to the present to ensure it does not happen in the future. But if you are successful in altering something in the present that actually changes the future, then you'd have visited that future. But if you'd visited the future that you liked, you wouldn't have gone back and changed the present after all.
Although there are potential solutions to such paradoxes, it is inevitably a great difficulty where theorising about time travel is concerned. Any theory of time travel must be able to successfully account for what would happen in these situations.
There are many conflicting theories about time travel, both philosophical and scientific, and there is inevitably much confusion given that it is a very difficult topic and we are talking about something we cannot make good sense of. Perhaps scientific advances in the future will be able to determine whether things like wormholes really do exist but, even then, to be able to use them effectively still seems like a thing of fiction.