BIPM |

Many of the units in Physics are defined in terms of others. Energy is often measured in Joules, but these are equivalent to kilogram meters squared per second squared. You still need a base set of units for a few fundamental quantities to define the others. The outer ring of wheel above shows the quantities in the International System of Units (SI) that are used as a basis for all others:

- mass (kilogram)
- distance (meter)
- time (second)
- current (Ampere)
- temperature (Kelvin)
- number of particles (mole)
- luminous intensity (candela)

Using these 7 quantities, all other units can be defined, but the problem remains of how to define these 7 precisely. To ensure consistency across the planet (and indeed, the universe) these 7 units are each derived from a measurable universal constant, shown in the inner circle.

For example, the meter is defined in terms of the speed of light,

That brings us to the kilogram: Until last week, mass was defined in terms of an actual bar of platinum and iridium referred to as Le Grand K:

The problem with this method is that any change to that lump of metal effectively changes what a kilogram is. Comparing the base model to duplicates in 1989 showed that the duplicates were heavier. It's impossible to know though whether the duplicates had gained weight since their creation, or Le Grand K had lost weight, since no matter what, it weighed 1 kg. The redefinition resolves that problem.

The new definition uses Planck's constant,

One of the things I love about science is how we have a good understanding of how things work now, but we still strive for greater precision, which in turn reveals new areas to explore. The universe is a beautiful place, and I'd like to appreciate every second, meter, kilogram, mole, candela, Kelvin, and Ampere of it!

*c*: 1 meter is the distance light travels in 1/299,792,458 second. The second, in turn, is defined as 9,192,631,770 oscillations of a caesium atom. These unwieldy numbers come from the fact that the units existed before they were formally defined in terms of constants: Most people would say a second is 1/(60 * 60 * 24) of a day, but a day is not a precise enough quantity for certain measurements.That brings us to the kilogram: Until last week, mass was defined in terms of an actual bar of platinum and iridium referred to as Le Grand K:

Reuters/Benoit Tessier |

The new definition uses Planck's constant,

*h*, to set the kilogram. This is a quantity used in quantum mechanics, which relates the frequency of light to the energy it carries. It was first used to describe blackbody radiation, the light emitted by heated objects like the sun, or even an incandescent bulb. Since*h*has units of energy * time, we can use the other definitions to find the kilogram. Here's a nice diagram similar to the one at the top, but showing how the units interconnect:Wikipedia |