A theory that developed from Max
Planck's quantum principle and Werner
Heisenberg's uncertainty principle.
The quantum principle states that waves (light, infrared, X rays, etc.) are emitted from any body in "packets" known as quanta, rather than arbitrarily. Each quantum would have more energy depending on the frequency of the waves.
The uncertainty principle states that the more accurately you know the position of a particle, the less accurately you can know its velocity, and vice versa.
Heisenberg, Erwin Schrodinger, and Paul
Dirac created a theory called quantum mechanics, which was based on the uncertainty principle. Particles were now thought to have a quantum state, a combination of position and velocity.
In a nutshell, it brought randomness into physics, which many physicists (Einstein, for example) objected. Still, many others accepted it.