Photons

The kilogram has been officially redefined.  No longer is its mass equal to that of the special platinum-iridium alloy cylinder locked in a vault beneath a building in Paris.  Now it officially weighs the same as 1.4755214×10^40 photons, a type of elementary particle, with frequencies matching a cesium atomic clock.

-- Oliver Roeder, Significant Digits, 21 May 2019