This post is another in a series on the duality of space and time. I have emphasized that the basis for space is length and the basis for time is duration. What, then, about direction? Does direction apply to both space and time? Yes, and in the same manner.
If someone says, “The hotel is 10 minutes away by car” how is that different from saying “The hotel is 5 miles away by car”? One provides a duration and the other a distance. Neither provides a direction. Both require movement to measure. They are exactly parallel.
If someone says, “The hotel is 10 minutes north by car” how is that different from saying “The hotel is 5 miles north by car”? One provides a duration and the other a length, each with a direction. Both require movement to measure. They are again exactly parallel.
Is the direction “north” part of space in one case but not in the other case? Then what does “north” mean in the sentence “The hotel is 10 minutes north by car”? It means that the direction “north” and the duration “10 minutes” are combined, just as we combined the direction “north” and the length “5 miles”. It would be arbitrary to say that direction applies to space (length) and not to time (duration).
So what is direction? It is something independent of length and duration, that is, it is independent of space and time but can be applied to either space or time. Direction is what makes the scalar “lengths” into a vector of directed lengths, often called a displacement. In the same way, direction is what makes the scalar “duration” into a vector of directed duration, which could be called a temporal displacement.
Is the concept angle only related to space? Look at the hand of a clock. Is it measuring an angle of space or of time? Both. We read a clock directly as time, a duration measured by revolving hands. But we recognize the spatial angles, too, and can use clock numbers to indicate space, as in “10 o’clock high” for a direction in space.
But if there are three dimensions for direction in space, does that mean there are three dimensions for direction in time? Yes, and they are the same three dimensions. For example, an isochrone map shows contour lines (isolines) for durations in two dimensions. It is like an isodistance map which shows travel lengths in two dimensions. The only difference between these maps is whether durations or lengths are shown; the two dimensions are the same.
So when we say that looking into outer space is looking back in time, that includes the three dimensions we see.