The first three dimensions are height, width and depth. Picture a vertical line being one-dimensional, a square being two-dimensional and a box being three-dimensional. It's the added sense of depth that transforms a regular movie into a 3-D one. But what is the fourth dimension? Time. All events taking place in the universe need both space and time in which to occur, so space and time have an interconnected relationship known as the space-time continuum.
So far we're unable to see the fourth dimension -- except in the fleeting glimpses we know as the present. It's like being able to see a just single point instead of the entire line. And we venture only in one direction we know as going toward the future. Even physicists and mathematicians, who try to solve problems based on the conditions of other dimensions, can have trouble visualizing worlds beyond 3-D. Most likely, we remain unable to see the fourth dimension because our brains haven't been trained or taught to look for it. It's possible that with practice or advanced modeling we may one day see the fourth dimension, but it's just as likely that we'll remain incapable of such sight.
A further barrier to our understanding of other dimensions is that scientists generally use them to explain objects and events infinitely larger and almost unimaginably smaller than we are. Einstein held that the gravitational force of planets warped space-time. We know that for each day an astronaut spends in orbit, he or she returns 0.000023 seconds younger than someone who remained on Earth NASA. But few of us will ever experience that -- and the accelerated aging effect of spending time at the DMV is only an illusion.
Theorists also think in more than three dimensions at the other end of the size scale. Quantum physicists are up to 11 dimensions in attempting to explain the behavior of subatomic particles -- 10 dimensions of space, plus time Groleau.
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