I have written about the characteristic (modal) rate for a mode of travel. This rate provides a factor for converting spatial into temporal measures and vice versa. It is possible that the characteristic rate is independent of any particular rate but often it is a function of many rates, such as the minimum, maximum, or mean. These functions may have some similarities with independent rates but they should be distinguished as relative characteristic rates.

An absolute characteristic rate is absolutely independent of any and all particular rates in a mode. This rate may be a limit of the rates but it must not be dependent on them in any way. So an absolute characteristic rate may be the infimum (greatest lower bound) or supremum (least upper bound) of the rates but strictly speaking may not be their minimum or maximum.

The speed of light in a vacuum is widely considered the maximum speed for signals. What is important is not so much that it is the speed of light as that it is the speed that is the supremum for the speeds of all bodies with non-zero rest mass. It so happens that the photon travels at this speed, called *c*.

I wrote recently about a lower limit on speed equal to the inverse of the speed of light. This is better described as the infimum of the speeds of all bodies with non-zero rest mass. Is there a particle that travels at this speed, 1/*c*? It might be a “lygon” for twilight particle (from *lygo*, Greek root for twilight).