The spatial dimensions are related to each other via the Euclidean metric (AKA the Pythagorean formula): d^2^ = x^2^+y^2^+z^2^.

The time dimension is related to the spatial dimensions via the Minkowski metric, which differs only by one sign: d^2^ = x^2^+y^2^+z^2^-t^2^. So it’s kind of the same thing as a spatial dimension, but with a difference.

Among other things, the change in sign means that, where spatial rotations result in circular transformations, spacetime rotations with a time component (AKA acceleration) result in hyperbolic transformations. And the asymptotes of the underlying hyperbola are the light cone of the center of the transformation—which is why no amount of acceleration can cause an object’s future path to leave its current light cone, and why faster-than-light travel is impossible.

Get 3 sticks, try and arrange them all perpendicular to each other. You can do it. Now try with 4, you can't. We live in 3D space.

Time is a line, at least as we experience it. Just one, it's hard to even figure out what 2D time would mean, although physicists have thought about it.

So, 3+1=4. It turns out they're kind of interchangeable, though, in that swapping time for a kind of angle made up of some space and some time, if done in a certain way, leaves the predictions of physics the same. That's special relativity, and it is itself connected to the way everything is a wave.

That'd be spacetime, from Einstein's general relativity. I suppose different names for the dimensions are warranted, since we can move freely forwards and backwards in the space dimensions, but the same does not apply for time. I can't go backwards with that, or stop. It's all part of the universe. Or at least solving gravity made Einstein realize space and time are connected.