Just before Schrodinger proposed his theory, a German physicist Werner Heisenberg, in 1925, had a theory of his own called matrix mechanics which also explained the behavior of atoms. The two theories seemed to be based on entirely different sets of assumptions yet they both worked. Heisenberg's theory was based on mathematical quantities called matrices and fit with the conception of an electron as a particle whereas Schrodinger's theory was based on waves. Both theories were really mathematically the same.
In 1927 Heisenberg formulated an idea which said that the position and momentum of a quantum particle can never be simultaneously determined. This idea is called the ‘Heisenberg uncertainty principle’.
This implies that as the certainty of the position of a particle is known, the uncertainty in the momentum becomes correspondingly larger. Or, if momentum is determined with great accuracy, the knowledge about the particle's position is correspondingly less.
