The international system of physical units, introduced in 1960. It is based on seven fundamental units; in the context of Einstein Online, the interesting ones are the metre as a measure of length and distance, the second as the unit of time, the kilogram as the unit of mass and the Kelvin as the unit of temperature.

By multiplication and division, the seven fundamental units can be used to construct derived units for all other physical quantities. For instance, the unit of speed is the distance unit divided by the unit of time, metre per second.