On this day in 1916, Albert Einstein (books by this author) published his theory of general relativity. The theory proposed that massive objects, like planets, create a distortion in space-time that is felt as gravity.
Relativity is important for many technologies today. For example, GPS satellites need to take into account imperceptible changes in time described by relativity — just a few microseconds — due to their high speed. If they did not do this, they would be off in their distance calculations by several miles after only one day.
When Einstein was asked what he would have done if his theory of general relativity had not held up, he replied, “Then I would have been sorry for the dear Lord. The theory is correct.”