A group of tadpoles swim around in their very shallow puddle of a home. Their world is two dimensional. They can move widthwise, they can move lengthwise. They have no concept of depth, or height, but they are quite aware that it exists. They can see the sky above them, they can feel the warmth of the sun in the water. However, they cannot full understand the third dimension, the dimension of depth.
We humans are accustomed to the three dimensional world. We understand the depths of water, the altitudes of air, even the deepness of space. But what if we, like the tadpoles, are missing something that we cannot comprehend?
It is frequently believed that the fourth dimension is Time. Some would believe that time is simple to comprehend, that we can measure it, count it, and witness it. However, we are really only feeling the effects of time. Unlike the other three dimensions, we cannot simply point at part of time. We cannot traverse time, as we can length, width, and depth. Time is always in the present, we cannot move forward through time, we cannot move backwards through time.
We already have a sort of influence of time. Time is not absolute. Gravity has an effect on time, making it slower the closer you are to the surface of a large mass. The difference is very minute, but if you were to travel away from the Earth at near lightspeeds for some time, and then return, you will find that you have aged normally, but time on Earth has passed many years. Therefore, time is not just a static constant, but is dependant on where you are, and the speed you are moving.
It isn't possible to imagine a fourth dimension physically. Perhaps it is possible that we may one day be able to travel through time. Then, the 4th dimension may be open for us to explore. Or it could be quite possible that there is no other dimension, and that what we see is all there is. Or, after traversing time, there may be some higher plane, another level of existence, a fifth dimension.
Works cited and consulted:
Stephen Hawking, A Brief History of Time
Stephen Hawking, The Cambridge Lectures
Ken Grimes and Alison Boyle, "The Universe Takes Shape" Astronomy Magazine