A century-old letter in which Albert Einstein defended a friend who assassinated an Austrian politician has gone up for auction.

Einstein had a friend named Friedrich Adler from the school days. The two were relatively close but grew popular for very different reasons.

While Einstein made headlines for his theory of special relativity that explained the relationship between space and time and redefined modern physics, Adler assassinated Austrian minister-president Karl von Stürgkh in 1916.

On 29 April, 1917, days before the trial of his friend, Einstein wrote a letter to Michele Besso, another friend of his and the only person credited on his paper of theory of special relativity, Live Science reported.

In that note, written in German, the Nobel Prize winner touched on topics like general relativity and quantum physics, but also brought up Adler – looking for a possible way to help him.

The man had "proved himself a selfless, calm, hard-working, good-hearted, conscientious man who was highly esteemed by everybody, and that it is my heartfelt desire, therefore, to intercede for him", Einstein wrote defending Adler's personality.

He called Adler a "sterile rabbinical mind, obstinate, without a sense of the real. Ultra-selfless with a strong tinge of self-torture, even suicide. A real martyr-type."

The effort did not do much of a good as Adler was found guilty during the trial. However, the man was sentenced to 18 years of hard labour instead of being executed, according to a 1978 report published in German Studies Review.

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The letter, a crucial piece from Einstein's life, also highlights how the scientist sought Besso's help to reply to Adler's letters in which he had been reiterating theories proposed by Ernst Mach, an Austrian physicist, "to the point of exhaustion".

"I am sure you will know what chords to strike since you are much wiser about human affairs than I am," Einstein said to Besso. The note is up for auction at the Nate D Sanders Auctions in Los Angeles and the bidding ends on Thursday, 25 January, 2018.