Niels Bohr is famous for the Bohr model of the atom based on the electron shell structure that chemists use as a short hand for the rather more complex picture of electron orbitals.

Bohr famously said

"Those who are not shocked when they first come across quantum theory cannot possibly have understood it."

Bohr knew that quantum mechanics asked us to make seemingly paradoxical statements about reality that are deeply troubling.

Bohr thought this meant our view of reality was wrong.

He clashed with Einstein over quantum mechanics. Bohr won in the sense that his view predominated amongst physicists but the contradictions at the heart of quantum mechanics have never been resolsved.

It has proven to be the most successful scientific theory of the 20th century. But the most controversial at its heart.

According to Max Born the wave associated with a paritcle can be thought of as a probability wave that describes the probability of finding the particle at a given time and position.

This is the modern interpretation of the electron cloud descrition of electron orbitals.

But it means we do not know where the electron is until we observe it.

This causes lots of problems interpreting what this means.

This led Erwin Schrodinger to come up with the paradox of the live-dead cat trapped in a box until the observer opens the box and kills the cat.

Schrodinger and Einstein were repulsed by this description of quantum mechanics.

Albert Einstein produced his famous work in 1905 when he published three papers that changed our view on reality.

He produced his version of the photoelectric effect in 1905 which confirmed Max Planck's view of the quantum theory of light.

This started the quantum revolution which is now firmly established as the basis for all sub-atomic physics.

But Einstein was never happy with the consequneces of quantum theory and preferred to it as incomplete.

"quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the “old one”. I, at any rate, am convinced that He is not playing at dice."

This is usually simplified to

"God does not play dice"

Einstein rejected the Born interpretation of the probability wave associated with matter.

Einstein did not win this argument and as it stands the probability wave is part of the modern interpretation of quantum mechanics used today with great success.

Einstein believed a better more complete theory would remove this paradox and bring causality back into physics.

We are still waiting for a resolution of the contradications at the heart of quantum mechanics.