The first three dimensions covered the three variables that determine space: length, width and depth. Then what could the fourth dimension possibly add? The answer is duration. The third dimension effectively becomes a point and the 4th dimension connects different 3D points to form a line that we refer to as time. For example, the “you” at this exact time is different to the “you” in five years time. These two “you”s are different (3D) points that lie on a 4D line (more specifically, your life). This is exactly the same principle as the 0th dimension being points in a 1D line, except three dimensions higher.
As a “moment” in 3D space is just a point on a 4D line, travelling from one point in time to another (i.e. time-travelling) would be as easy as walking along a straight line for a four-dimensional being. This concept is mind-blowing for us as we cannot fathom the concept of manipulating time. We are unable to see time as a dimension as we exist in a lower dimension. Every single moment in our lives is a brick that paves the road called time, meaning that we can only see each individual brick and not the overall picture. This is exactly the same as how a Flatlander could not understand the concept of depth and how we can be above them.
Although we see time as linear and straight, we are under the same illusion as the ant walking down the Mobius strip. Time is actually twisting and turning in the fifth dimension, creating multiple timelines that branch out like a tree of possibilities. These branches are influenced by our own choice, chance and the actions of others.
In other words, if a man proposes to girl A then he will go down the branch where he marries girl A. However, if he chose not to propose, he would end up marrying a different girl (or not at all). Therefore, he has entered a different branch than the girl A branch.
Now, if the man wanted to go from a timeline where he married girl A to another timeline where he married girl B, what could he do?
One method would be if he bent the 4th dimension (time) on itself through the 5th dimension to travel back in time to when he met the girl and not ask her number. This is exactly like folding a 1D line into a 2D circle to make the end point meet the starting point. However, to marry girl B he would have to make the right sequence of choices that lead to him marrying girl B (much like a role-playing game). This is the long-way round that would be too time-consuming and complex.
A simpler approach would be if he folded the 5th dimension through the sixth dimension and jump from the ending of timeline A to timeline B. This would be like the finger-lifting analogy we have been using time after time. If we pretend that the 5th dimension was a piece of paper, then we could fold it into a 6D cylinder so that the two edges meet. Now we are able to jump from one ending to another effortlessly, just as we did in the 2nd dimension.
It is easy to confuse the 5th and 6th dimension as they both deal with “alternate realitites”. Here is one way to differentiate the two: 5D space is like a 2D space for time - a flat plane where different timelines cross each other. Therefore, it can contain all of the possible outcomes from an initial condition - that depends on an action, choice or chance - such as your conception (visualise a dot on a piece of paper with many lines radiating out from it).
If we were to put a dot on a piece of paper above the first piece of paper (in the 3D space for time), we have entered a universe where there is a completely different starting point (before your conception), such as dinosaurs not existing. This means that the line called “you” may not even exist, and the two pieces of paper would never meet. The only way to jump from one piece of paper (5D) to the other would be through the 6th dimension.
Therefore, by jumping up a dimension, we gain a degree of freedom where we can move in yet another direction. This is seen between every dimension, such as 2D versus 3D. The 6th dimension merely lets us travel between different sheets of 5D paper.
So somehow we have reached the 6th dimension where one can not only time-travel, but jump from one alternate reality to another. Shall we venture further into the seventh dimension - infinity - and beyond?
(Stick figures from xkcd)