A heartfelt letter physicist Albert Einstein wrote to his son is up for sale through a Boston auction house.
The one-page letter was written around 1929 to Eduard Einstein. It could fetch more than $100,000 when it's sold this week through RR Auction.
Einstein wrote in the letter he longed to see his son, who was finishing high school. He described a deep kinship with him and invited him to visit for Easter.
Einstein added 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, he continued to work on the problem and never fully mastered it.
The hand-written letter is signed "Papa."