Pfhorrest wrote:I think what you really want is Planck time, which has already been mentioned here before. The Planck time is the amount of time it takes something moving as fast as theoretically possible to to cross the shortest theoretically measurable distance, both of which are experimentally quantifiable values not dependent on measurements of any particular physical objects.
The Planck units are theoretically interesting because they simplify many fundamental equations, but they are not so useful as the basis for practical systems of units because it isn't easy to determine their values to a high precision.
Note that we can't measure the Planck length and time units directly. Like all the Planck units, their values are derived by simple algebraic manipulation of fundamental constants (in this case, G, the gravitational constant, h-bar, the reduced Planck constant, and c, the speed of light), so the precision of the Planck units is dependent on the precision in our measurements of those constants. In particular, our current estimate of G is not very precise - we know what the first 4 digits are, but after that it gets a bit fuzzy.
The gravitational constant is a physical constant that is difficult to measure with high accuracy. In SI units, the 2010 CODATA-recommended value of the gravitational constant (with standard uncertainty in parentheses) is:
G = 6.67384(80) × 10-11 m3 kg-1 s-2
with relative standard uncertainty 1.2×10−4.
The Planck length isn't exactly the shortest theoretically measurable distance. If spacetime is quantised, then it's quite possible that it happens somewhere around the Planck scale, and if so, spatial distances smaller than the Planck length and temporal durations shorter than the Planck time may not even make theoretical sense, since the structure of spacetime could be a weird non-linear fractal at that scale. Of course, we need a working theory of quantum gravity to address such issues. Still, it's fairly safe to say that we may never be able to measure the Planck length directly.
However, there might be be some physical significance to the Planck length, since the Bekenstein-Hawking entropy
of a black hole is kA / 4, where k is Boltzmann's constant and A is the surface area of the black hole's event horizon measured in square Planck lengths. OTOH, as mentioned above, many equations take on simple form when using Planck units, so maybe this is of no special significance.
As for leap seconds, I've never been very happy with them because they are messy and insufficiently predictable. The notion of saving them up for a leap hour seems ludicrous to me, but a leap minute would be OK, IMHO.