In everyday life 1D space and 1D time are typically used. We are concerned with how far away something is (travel length), how long will it take to get there (travel time), what the speed is (apart from direction). Unless we are doing something in which size or direction are important, 2D is all we need.

Only two dimensions are necessary: 1D space and 1D time. Anything beyond this is possible but not necessary – up to a limit of 3D space and 3D time. Thus it is possible to define, measure, and adopt conventions in which there is exactly 3D space and 1D time. But it is also possible to define, measure, and adopt conventions in which there is exactly 1D space and 3D time. And it may be possible to define, measure, and adopt conventions in which there is exactly 3D space and 3D time.

So it is no objection to 3D time that it can be defined to be 1D. It is also no objection to 1D space that it can be defined to be 3D. Other conventions are possible. And a 3D space and 1D time convention is symmetric with a 1D space and 3D time convention.

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