Letters penned by Albert Einstein in which he discusses everything from the breakdown of his marriage to general relativity are going under the hammer.

The celebrated scientist wrote the letters, which offer a “rare insight” into his life and thoughts, to his closest friend, Michele Besso.

He discusses in detail some of the “key scientific concepts of his career”, such as special and general relativity, the ‘cosmological constant’, the redshift of spectral lines, unified field theory and quantum mechanics.

Lot 90 at Christie’s auction (Christie’s)

But the theoretical physicist also writes about the breakdown of his first marriage, the League of Nations, fame,  getting old, walking in the mountains with his young son –  and makes humorous remarks about his colleagues.

Auction house Christie’s say the letters reveal  “delight in his work, his relish for a new theory” and sense of “getting closer to God” when grasping at fundamental truths.

The last letter in the correspondence is written to Besso’s family a few days after his friend’s death in 1955, shortly before Einstein’s own death at 76.

The letter ends: “Now he has again preceded me a little in parting from this strange world. This has no importance. For people like us who believe in physics, the separation between past, present and future has only the importance of an admittedly tenacious illusion.”

A letter going under the hammer (Christie’s)

Einstein, famous for developing the theory of relativity, first met Besso when they were students in Zurich in the  1890s.

They cemented their friendship in the early 1900s, working together in the Swiss federal patent office in Bern.

In 1905, when Einstein published four ground-breaking papers, Besso was his only acknowledged collaborator.

Estimates range £800 in an online auction from July 6-13, to £150,000 in a live auction on July 12.