What are the SI units for length, mass, time, and temperature?

The SI units for length, mass, time, and temperature are metre, kilogram, second, and Kelvin respectively.

In the SI system, length is measured in metres (m). The metre is defined as the distance travelled by light in a vacuum in 1/299,792,458 seconds. This definition ensures that the metre is a constant value and not dependent on any physical object.

Mass is measured in kilograms (kg). The kilogram is defined as the mass of the International Prototype of the Kilogram, a platinum-iridium cylinder kept at the International Bureau of Weights and Measures in France. However, a new definition based on fundamental constants is being developed to replace the physical prototype.

Time is measured in seconds (s). The second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

Temperature is measured in Kelvin (K). The Kelvin scale is based on the thermodynamic temperature of an ideal gas at constant pressure. The Kelvin scale has its zero point at absolute zero, the theoretical temperature at which all matter has zero thermal energy.

In summary, the SI units for length, mass, time, and temperature are metre, kilogram, second, and Kelvin respectively. These units are defined based on fundamental constants and ensure consistency and accuracy in scientific measurements.