Although the study of time travel is a fairly new topic of scientific and philosophical investigation, it is so well-known because of different literature, films and stories in our daily life. It is believe that most locals have got access to at least one such production related to time travel. Yet, the possibility of time travel is still a question mark to them. Is it possible to have a time travel?
The answer is definitely yes. Although not every scientist agrees with the answer, supporters of relativity would also be supporters of time travel. Moreover, some scientists or philosophers give explanations to the paradoxes and make the answer almost certain.
In this article, we give a clear explanation to some kinds of time travel with high possibility such as time travel to the future. For time travel to the past, we discuss the possibility and point out the impossibility of some counter arguments as well as the contradiction of some ‘paradoxes’.
Among lots of definitions of time travel, David Lewis’s version is easy for everyone to understand. An object time travels if and only if the difference between its departure and arrival times in the surrounding world does not equal the duration of the journey undergone by the object (1). Although the definition of the phrase ‘surrounding world’ is indefinite, we give such definition: the environment at sea level on the Earth.
Yet, things are not clear. What is time? Philosophers offer a few explanations such as eternalism, possibilism, and presentism(2). Eternalism and possibilism give same explanation for the past and present The eternalist thinks that time, correctly understood, is a fourth dimension essentially constitutive of reality together with space. All times, past, present and future, are actual times just like all points distributed in space are actual points in space, while possibilism suggests that future is the only possible. They are in fact the determinism and indeterminism version of the similar theories. The presentist thinks that only temporally present objects are real. ‘Whatever is, exists now. The past was, but exists no longer; the future will be, but does not exist yet.’(3) Therefore, the past does not exist and we cannot travel to the past. We would put presentist aside and consider eternalism (or possibly possibilism).
The above gives the concept of time-flow. Yet, the time-flow of a person might not be the same with the others. Time could distinguish from physical time or external time (which we define as the time at the surrounding world) and personal time or inner time. Put this into the definition of time travel, we could get the combination: An object time travels if and only if the inner time during the journey is different from the external time(4). To be more detail, if the inner time is longer than the external time, we call it ‘time travel to the past’, while we called it ‘time travel to the future’ for the opposite.
Relativity and time travel
It is important to understand general relativity before studying this issue. In Newtonian gravity, which we usually learn in high school, the source of gravity is mass(5). In special relativity, we replace the term mass with more accurate terms, including energy, momentum densities, pressure and shear, called energy-momentum tensor(6). Using the equivalence principle, this tensor could curve space-time since time and distance are not the invariance but the speed of light is. This breaks many people’s common sense. The distance and time are not fixed but variable when under different gravitational force or experiencing different ‘speed’. The theory shows that if the gravitational force and the speed change, the inner time could also change and the way of time travel is to change the force and the speed(7). This is explained in later chapter.
Real case of relativity in daily life
Do you believe that we could have time...