Letter to Michele Besso (1916)
"You are right that time is not as I previously thought. Let me explain it to you again."
Dear Michele,
As I finalize the general theory of relativity, I find the correspondence between the principles of inertia and gravity becoming increasingly elegant. You remember our many discussions in the past when these concepts appeared irreconcilable. Now, they stand together in a new kind of harmony.
Perhaps you have followed the recent development where the equations of the field are now fully formed in their final symmetry. Though the mathematics is complex, the underlying idea has become deeply simple to me. Just as you were one of the first to grasp the ideas of the special theory with me, I hope you will continue to follow my reasoning here.
I think of the future of this theory—how it may help to explain the nature of light and mass more deeply than ever before.
With warm regards to your wife and to yourself.
— Albert Einstein