Dear “Rufus”:

I made two serious blunders. One is signing the letter to President Franklin Delano Roosevelt suggesting that the Nazis might be ahead of the U.S. in weaponizing nuclear fission. If Hitler developed the atomic bomb before us, he might have ruled the world. As it turned out the Nazis were behind us. I regretted the loss of life at Hiroshima and Nagasaki, but the bomb shortened World War II. Had I known that the Germans would not succeed in producing an atomic bomb, I never would have lifted a finger.
The second is the cosmological constant. I added this constant to my gravitational field equations to account for the observation at the time that the Universe appeared to be neither expanding nor contracting.

I added a term in my equation to produce some kind of repulsive gravitational force to keep everything in a steady state. Ironically, once Hubble discovered that the Universe is expanding, we found that the constant added as a “blunder” at the time turned out to be prescient, in that it became needed to produce what is now called “dark energy” of repulsion which makes up about 85 percent of the observable mass-energy of the Universe! What I thought was a blunder turns out to be profound. The joke is “I was wrong once; on that occasion I said I was wrong and I was right”.

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