# Why is time considered a (fourth) dimension?

Since dimensions usually refer to space and we naturally acknowledge three of them, we also perceive time and we separate space from time ("space AND time"), why is it, that time has ended up in the same shelf as spatial dimensions. Is time really a dimension "by nature"? Or is it a concesus, shifting our natural understanding what dimensions are, for practical reasons, similarly to defining particles?

EDIT: by Is time really a dimension "by nature"? I asked if there are good intrinsic reasons, given by to label time as a dimension, as opposed to us labeling it as such, to make it more tangible, explainable.

• "Is time really a dimension "by nature"? Or is it a concesus, shifting our natural understanding what dimensions are, for practical reasons, similarly to defining particles?" It's the latter. In order to accommodate the special theory of relativity, we need to expand our definition of what a dimension is. However, time is nothing like space, and that distinction is still made when talk about a temporal dimension and a spatial dimension (even in relativity). This answer summarizes the point very succinctly I think. – Maximal Ideal May 6 '19 at 18:46
• @User123 What do you mean by "true dimension?" If you make an attempt to actually define what you mean, you will end up presupposing your own conclusion. This is sort of the issue with the question to begin with. What is meant by "by nature?" If you say a dimension has to be something you can measure with a ruler, you will end up presupposing that all dimensions have to be spatial dimensions. If you say a dimension is any point of data (as mathematicians do), then you will get the opposite conclusion. Physicists use the latter definition of dimension. – Maximal Ideal May 6 '19 at 18:57
• We all always travel at the speed of light (even when we stand because at that time we travel through time with a speed of light). – User123 May 7 '19 at 16:56
• Gotta name it something. and it fits into a lot of equations exactly as a dimension would. – Wayfaring Stranger May 9 '19 at 14:48
• This question depends on "our natural understanding what dimensions are" and I'm far from sure if there really is such an understanding shared at all widely. – Steve Linton May 9 '19 at 15:45

$$\mathbf {G} ={\frac {8\pi G}{c^{4}}}\mathbf {T}$$