Since the development of relativity theory, space and time have been combined in a four-dimensional continuum. Because the speed of light is an absolute value in relativity theory, it acts as a conversion factor between space and time. Accordingly, the four dimensions may be understood as any combination of space and time:

4 + 0: Four dimensions of space and none of time. The invariant spacetime interval is commonly expressed in spatial terms only as *s*² = *x*² + *y*² + *z*² – *ct*² with signature (+++–). The opposite signature is also used (+–––). The factor *c* converts the time coordinate into a distance coordinate. Note that there is an implicit 1-3 split in dimensions.

3 + 1: Three dimensions of space and one of time. This has been the common conception of space and time for centuries.

2 + 2: Two dimensions of space and two of time. This was discussed in the previous post *here*. It is a pictorial representation of four dimensions, two at a time.

1 + 3: One dimension of space and three of time. This has been discussed in many posts such as *here*. It may have been the ancient conception.

0 + 4: No dimension of space and four of time. This is the invariant spacetime interval expressed in temporal terms only as *τ*² = *t*² – *x*²/*c*² – *y*²/*c*² – *z*²/*c*². There is an implicit 1-3 split in dimensions.

The above should be understood in the context of an implicit six-dimensionality, as was discussed *here*. As soon as rates are considered, three dimensions of either space or time must be compressed to one so that only four dimensions remain.