"I am discouraged, if not repelled by matrix mechanics".

Erwin Schrodinger(1926)

Until 1926, every one believed in determinism. Determinism simply means that an observer can actually predict everything that would happen in the universe if:
    1.He knew the complete state of the universe at one time,
    2.He knew the set of scientific laws that would allow him to calculate the function state.

Heisenberg showed that the first assumption is impossible. He proved that nature does not allow us to measure the position and velocity of a single particle (let alone the whole universe) with perfection no matter how precise our measuring instruments.

This picture illustrates Heisenberg's uncertainty principle

In order to measure the present position of a particle, the obvious way is to shine light on the particle. Some of the waves of light will be scattered by the particle and this will indicate its position. However, one will not be able to determine the position of the particle more accurately than the distance between the wave crests of light, so one needs to use light of a short wavelength in order to measure the position of the particle precisel. Now, by Planck’s quantum hypothesis, one cannot use an arbitrarily small
amount of light; one has to use at least one quantum. This quantum will disturb the particle and change its velocity in a way that cannot be predicted. Moreover, the more accurately one measures the position, the shorter the wavelength of the light that one needs and hence the higher the energy of a single quantum. So the velocity of the particle will be disturbed by a greater amount. In other words the more accurately you try to measure the position of the particle, the less accurately you can measure its speed, and vice versa. Heisenberg showed that the uncertainty in the position of the particle times the uncertainty in its velocity times the mass of the particle can never be smaller than a certain quantity, which is nothing but Planck’s constant. Moreover, this limit doesn’t depend on the way in which one tries to measure the position or velocity of the particle, or on the type of particle. Heisenberg’s uncertainty principle is a fundamental inescapable property of the world.

The uncertainty principle has profound implications for the way in which we view the world. The uncertainty principle signaled an end to the old dream of a theory of science, a model of the universe that would be completely deterministic: one certainly cannot predict future events exactly if he cannot even measure the present state of the universe precisely!