Heisenberg's uncertainty principle
The uncertainty principle came from Werner Heisenberg's matrix mechanics. Max Planck  already knew that the energy of a unit of light is proportional to the frequency of that unit of light (), and that its amount of energy can be expressed in familiar terms such as the joule by using a proportionality constant. The constant he gave the world is now called the Planck constant and is represented by the letter h.  When matrices are used to express quantum mechanics, frequently two matrices have to be multiplied to get a third matrix that gives the answer the physicist is trying to find. But multiplying a matrix such as P (for momentum) by a matrix such as X (for position) gives a different answer matrix from the one you get when you multiply X by P. The result of multiplying P by X and X by P and then comparing them always involves the Planck constant as a factor. The number used to write the Planck constant will always depend on the system of measurement in use. (With a certain system of measurement, its numerical value is one.) The slope of the line in the diagram to the right that shows the ratio of frequency to energy will also depend on the system of measurement chosen.