Many differences are proposed between space and time. This post briefly indicates how all of them are a matter of convention, and so not real. For details, consult posts on this blog.

(1) There are three space dimensions but only one time dimension.

Directionality can be associated with either length (distance) or time (duration). 3D time is as legitimate as 3D space.

(2) The time is always changing (“flowing”) but space position need not change.

Time seems to “flow” because of its association with continual change, as of a river. But change can just as well be associated with length. Something floating down a river continually changes it length of travel. So a continual increase of length can be associated with change.

(3) There is an “arrow” of time, but not of space.

As in (2) above, if length is associated with change, then there is an “arrow” of length. Compare a vehicle’s odometer, which never decreases. An odologue shows length always increasing, like a clock.

(4) Time has a past, present, and future, but space does not.

Space includes where one was in the past, where one is now, and where one will be in the future.

(5) Travel takes place in space in all directions, but only in one direction in time.

Travel takes place in time in all directions since there is a travel time for each direction of travel. Space may be associated with the travel distance, which has one direction.

(6) Entropy grows with time, but not with space.

One can equally well define an entropy of space and show that it tends to increase with the distance of motion.

(7) Causality is ordered in time, not in space.

Causality is ordered in space as in time. What happens in one place is causally related to what happens in adjoining places, and on to other places.

(8) Causally connected events are timelike or lightlike, but cannot be spacelike.

In a 3D time and 1D space context, the properties of timelike and spacelike are interchanged.