# Question #0098e

Jul 29, 2017

The kilogram (symbol $\text{kg}$) is standardized via the mass of an iridium-platinum alloy located at the International Bureau of Weights and Measures in Sèvres, France:

What is unique about the kilogram is that not only is it the only base SI unit with a prefix, but is also the only base SI unit standardized by an object and not by fundamental physical properties.

However, this platinum alloy (the International Prototype Kilogram, or IPK) does vary mass ever slightly over the years, so there have been recent debates about whether it should be standardized with a constant of nature.

The second is currently based on an atomic clock, which uses the energy difference between the two lowest energy states of a cesium atom. If the atom is bombarded with microwaves of just the right frequency, the atom undergoes a transition from these two hyperfine energy states.

The second is defined as the time it takes for $9 , 192 , 631 , 770$ transitions between these two hyperfine energy levels of a $\text{Cs}$ atom.

The meter is currently defined as the distance light travels in $\frac{1}{299792458}$ of a second. This value comes from the speed of light, which is measured and standardized as $299 , 792 , 458$ $\text{m/s}$.