BOSTON >> A heartfelt letter that physicist Albert Einstein wrote to his son is up for auction.

The one-page letter was written around 1929 to Einstein's son, Eduard. It could fetch more than $100,000 when it's sold this week through Boston-based RR Auction.

Einstein wrote in the letter that he longed to see his son, who was finishing high school, and invites him to visit for Easter. He describes a deep kinship with his son.

Einstein added that he had solved a problem that concluded his life's work. The problem became known as the Unified Field Theory, an attempt to explain gravity in terms of the laws of electromagnetism. Despite his excitement, Einstein continued to work on the problem and never fully mastered it.

The hand-written letter is signed only as "Papa."