Time is just a convention we use to describe events. "Time" gives us order and proximity. We use time to say that this happened "first" and that this other event happened "almost immediately after" this event. However, unlike length, area, and volume, time does not measure a dimension. It can be compared to a set of encyclopedias. Volume 1 comes before Volume 2, and page 65 comes before page 78. King Louis V and King Louis VI are on the same page, while aardvarks and zoos are at opposite ends. However, this "encyclopedia" is not ordered alphabetically, but in the order the events "happened". Nor is it finished. More and more volumes are constantly added. But the first volumes have already been written and cannot be rewritten.
If the fourth dimension was time, then it would allow time travel. Just like you can go forward and backward in the three spatial dimensions, you could travel forwards and backwards in the "time" dimension as well. While this may sound fascinating, it allows for some unavoidable paradoxes.
One example is what is often called the "Grandfather Paradox". It states basically that if you go back in time and kill your grandfather before he married your grandmother, then your parents wouldn't be born, and neither would you. But if you were never born, then you couldn't have traveled back in time to kill your grandfather!
There are countless other paradoxes and impossible situations that would arise if time travel was possible, and that is yet another reason to believe the fourth dimension is not time.
Stretch out a "string" of points perpendicular to it, and you get a line.
This is one dimension.
If we take this line and stretch it into a dimension perpendicular to the line,
we create a square. This is two dimensions.
If we take a square and stretch it in a direction that is perpendicular to all of its sides,
then we create a cube. This is three dimensions.
So what if we stretch
this cube in some fourth direction (dimension) that is perpendicular to all of
the cube's sides? Then you have a "Hypercube".
We cannot visualize this "Fourth Dimension" very well, because we see and live in the third. Try to imagine what a "living square" could see in his or her two-dimensional world. They would have no way to experience the third dimension, unless a three-dimensional object (say, a sphere) "passed through" their own two-dimensional world. And even then, all you would see would be a circle growing, then becoming smaller and smaller and then disappearing entirely. Literally, all you would see would be a crude two-dimensional interpretation of this object.
This would be the same with a four-dimensional object in our space. You could only see the "slice" of this object that happened to be in our three-dimensional space.
However, Einstein himself believed time was the fourth dimension. He said that gravity slows down light and thus any object or wave that passes by it. But if space is curved like he said, then there is another explanation. If the light must travel around this "curve" in space, then it will take longer to reach its destination. Therefore, it appears to "slow down".
Imagine traveling on a train from Town A to Town B. And say you know the exact (straight-line) distance between Town A and Town B, and that the train travels at a constant speed. Then you should be able to calculate how long it takes the train to get from A to B. But if it has to curve around a mountain, then it will take longer to reach Town B. So observers in Town B will think the train went slower than it really did. However, the mountain did not slow the train down, it simply made the path longer.
The same could be said for curved three-dimensional space. Gravity is the force that curves space, but it does not slow down time. It simply makes the path longer, which in turn makes the trip take more time.
However, we are neglecting one key question: How does our three-dimensional space "curve", and what does it "curve" into? To answer this question, we will use the analogy of our two-dimensional world. Imagine this world as a flat sheet of rubber. If we place a softball on this sheet of rubber, it will "curve" around the ball. In fact, it will be "curved" into the third dimension, while it still appears to be a two-dimensional world to our square friend living there. This is the same way our space is "curved" by gravity. Our three-dimensional world still appears to be three-dimensional to us, while it is being "curved" in a fourth direction -- a fourth dimension. So scientific studies seem to support the idea of a fourth spatial dimension.
However, these two-dimensional worlds do not have to be identical or even similar. For example, imagine two dimensional worlds as layers of a cake. There's the frosting, then the cake, then possibly another layer of frosting, etc. until you get to the plate. But the frosting layers are drastically different from the cake layers, and the plate layer is different from both of the other layers. And the "layer" of atmosphere around the cake is virtually empty, quite different from anything in the cake. If you lived in the frosting and tried to hypothesize about the cake layer, you would most likely be wrong.
Now let's take this analogy into the third and fourth dimensions. The fourth dimension is just infinitely many three-dimensional worlds put together "on top of each other". So a three-dimensional creature traveling through the fourth dimension would see only one three-dimensional "slice" at a time. And these slices may be radically different from his or her "home space", or they may be quite similar. There is no way to speculate about these matters.
However, the train track could curve left or right, but the people on the train could
only see the train track below, and for all they know the train is either traveling
forward or backward throughout the entire trip. So while they believe they are traveling
in one dimension, they are actually curving through the second.
And the shortest distance between two points is not a "straight line" (down the track)
through their one-dimensional world, but a line through the second dimension.
However, the road could curve up and down a mountain or even hills (through the third
dimension), but to our "two-dimensional" observer, the car constantly travels in only
two dimensions.
Again, the shortest distance between any two points may be through the third dimension
(assuming you could travel through mountains and such).
Or maybe we should use a spaceship for our analogy. We have seen that our three-dimensional space curves through the fourth dimension, even though we cannot detect it.
So the shortest distance between two points may not be through our three dimensions
after all. Instead it would be through the fourth dimension, bypassing all of the
"curves" in our way. This is the essence of Hyperdrive -- to cut through all of the
curves of our limited three-dimensional world by passing out of it and into others.
The debate on what the Fourth Dimension really is, or whether it exists at all, will continue for years. However, I believe this theory is, if not the best theory, at least a theory that is consistent with other scientific knowledge. Everyone is free to decide what he or she thinks about the fourth dimension, but until we are able to either study it directly or prove that it never existed at all, we will never know.
For more on the Fourth Dimension and things related to it, please check out my friend's
home page: The Fourth Dimension.
Here you will find theories about what the Fourth Dimension is as well as a forum for discussion
about topics related to the Fourth Dimension.
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