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I am a little confused about dimensions.

People say we have three spatial dimensions and one temporal dimension but I don't know how this is possible. How can there be different "types" of dimensions? Maybe we just can't perceive the fourth one properly.

Let's take two-dimensional space for example. Two-dimensional space is composed of many one-dimensional slices.

Three-dimensional space is composed of many two-dimensional slices.

Four-dimensional space, therefore, should be composed of many three-dimensional slices.

Assume that the universe has four spatial dimensions, time being the speed at which objects pass from "one slice" of three-dimensional space to the next. And us being humans, we just can't perceive movement in that fourth dimension. We can only detect when we change discrete positions, i.e in moments.

And time can't go backwards because the universe as a whole has a velocity in one direction in this fourth dimension.

I just want to know how time is a separate thing.

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  • $\begingroup$ I'm n00b. Why is this in astronomy SE instead of physics SE or maths SE? $\endgroup$
    – BCLC
    Commented Dec 8, 2022 at 15:31
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    $\begingroup$ I’m voting to close this question because it belongs on physics.stackexchange.com. $\endgroup$
    – Paul T.
    Commented Dec 10, 2022 at 15:37

1 Answer 1

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Take a look at this equation from special relativity. You don't need to know what it represents; it's good enough to know that $t$ is time, and $x, y, z$ are the three spatial dimensions.

$$ds^2 = -dt^2 + dx^2 + dy^2 + dz^2$$

Note that although time & space are on equal footing (you can add them), they are NOT the same - there is a relative sign difference between time and space. In other words, there's a negative sign (-) in front of $dt$ while the three spatial dimensions have a positive sign (+). Hence time is not a spatial dimension.

(Another difference between time and space is that there is only one time dimension, but there are three spatial dimensions.)

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  • $\begingroup$ Hi Allure, why is this in astronomy SE instead of physics SE or maths SE? $\endgroup$
    – BCLC
    Commented Dec 8, 2022 at 15:32
  • $\begingroup$ Don't know, ask the OP =) $\endgroup$
    – Allure
    Commented Dec 8, 2022 at 20:32

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