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Imagine a 4-dimensional object… Unfortunately, you soon realize that you can’t. But why? Our brains are not hardwired to picture anything beyond 3 dimensions. Life on Earth only goes until the 3rd dimension. The human brain cannot imagine something that it has never been exposed to (such as the 4th dimension). It would be like envisioning a new color out there in the universe that has not yet been discovered by humans. How would you describe it? The inexplicable nature of this mathematical and physical concept makes it a true wonder of physics. However, many theoretical physicists have proposed several theories as to what the 4th dimension is and what it would look like. Scientifically, we can describe this dimension but we may never experience it in the physical realm.

Before we delve into the details of the 4 dimensions, we need to understand what the first few dimensions are. To begin, take a point that has no spatial extent – we’ll say this is a 0-D space. Stretching this point out creates the first dimension which is a straight line with 0 width and only length. You can only travel in 2 ways – forwards or backward. 2-D space is a stack of infinite 1-D space spread out lengthwise or breadthwise. An example of a 2-D shape would be a square. There are more ways by which one can travel in 2 dimensions – forwards, backward, left, and right. 3-D space is in fact an infinite heap of 2-D space stacked upon each other. In 3-D space, there are three coordinate axes—usually labeled x, y, and z—with each axis orthogonal (i.e. perpendicular) to the other two. The six directions in this space are called: up, down, left, right, forwards, and backward. Lengths measured along these axes can be called length, breadth, and height.

Many physicists, including Einstein as part of his ‘Special Theory of Relativity, proposed that the fourth dimension is time. He said time should be a dimension like the other spatial dimensions because space and time are inseparable. If you wish to move through space, you cannot do it instantaneously; you have to move from where you are right now to another spatial location, where you’ll only arrive at a certain point in the future. If you’re here now, you cannot be in a different place at this same moment, you can only get there later. Moving through space necessitates you to move through time as well. Hence, they argue that time is the 4th dimension since, without it, we cannot construct any meaningful position vector with an unchanging length. Time’s dimension is a line going from the past to the present to the future. Thus, time as the fourth dimension locates an object’s position at a particular moment. If we had the ability to see an object’s fourth-dimensional space-time (or world-line) it would resemble a spaghetti-like line stretching from the past to the future showing the spatial location of the object at every moment in time. Unlike the other spatial dimensions, we can only move forwards in time. The other dimensions allow you to move both ways. Hence, they separate time from spatial dimensions and call it a temporal dimension. On the other hand, some researchers, using the logic of other dimensions, still hold out hope for finding wormholes in the universe that connect to different sections of space-time (i.e. the past).

Credit: Ridddle, The 4th Dimension