Every great and deep difficulty bears in itself its own solution. It forces us to change our thinking in order to find it.

He also appears on a banknote in 1997 when the Danish National Bank started circulating the 500-krone banknote with the portrait of Bohr smoking a pipe. In Kalckar Jørgen. He helped develop quantum mechanics in which electrons move from one energy level to another in discrete steps instead of continuously.

Later the element Bohrium was named after him. After Denmark was occupied by the Germans he had a dramatic meeting in Copenhagen with Heisenberg who had become the head of the German nuclear energy project. He conceived the principle of complementarity: that items could be separately analysed as having contradictory properties like behaving as a wave or a stream of particles.