In this section, addressed to all public, we intend to give a simple answer to such an elusive question. The answer will not be based on any theory or technical knowledge but on the daily experience. The aim is that anybody could understand it, expressing in plain words the intuitive idea we all have about time.
Before answering to this question we should answer another one, simpler and, at some extent, related with the previous one: what is space? Likewise, the answer has to be simple and based on our own experience
As we know, we can move from the place where we are to the beach, the mountains, the countryside, another town or any other place we wanted, on foot or using any transport means. Perhaps, because of the action of gravity it were more difficult moving upwards, but we can also get up in an aeroplane or climb on top of a mountain without too much difficulty.
These facts suggest the space are three dimensions: high, wide, and long where we can move freely. Easy, right? It is something we all know and we have been able to express with simple words.
Once we have answered this question we are ready to answer the main question, what is time?
So, as many of you had intuited, time is another dimension in which we cannot move freely. As simple as that. As we all know, this is that way because time goes always forward and, in principle, in an immutable and unalterable fashion.
In order to deeply understand this dimension, it is very clarifying this video from Carl Sagan about fourth dimension, we can watch on youtube.
The fourth dimension is time. Back in this fourth dimension is the past and forward is the future, we are moving relentlessly with no freedom. We neither see the past nor the future because we are trapped in three dimensions, as such as the flat beings in the video do not see the things up and down.
But, why cannot we move freely in time and we can in space? What really happens is not so intuitive. Movement though time depends on movement through space. As an example, if you and me meet each other in a park at a specific distance, then time for you and me elapses in an identical way. However, if I move away or get closer to you, then, I’m not only moving through space, but I’m also moving through time respect to you.
In the following video two children discuss friendly about this topic.
In the aforementioned situation, my temporal movement is very small. However, if I were able to move at high speed, close to the speed of light, then this difference would be really important. It is known by the physicists a long time ago.
Despite we don’t have the freedom to move through time, it doesn’t mean I couldn’t do it. In order to move through time, I have only to move also through space, because both movements are connected each other. Nevertheless, despite I moved through space, the temporal movement I’m experiencing cannot be free, in principle. This is because both movements are bound each other and therefore, I still don’t have freedom to move through the time.
On the other hand, it happens that speed moving forward in time depends on the speed moving through the space as is shown in the equation (1.25) in the following section.
From this relationship we derive that the faster I move through space, the faster I move through time. Ultimately, and more interesting, if I were able to get the speed of light, then I could go back in time.
The very sceptics can argue that no material body can achieve the speed of light. Of course, they can be right, but only in part of it; the Theory of Special Relativity avoids any material particle reached the speed of light but it does not avoid it in other cases, as it is demonstration below.