In 1907, or so it has sometimes been written, Albert Einstein saw a workman fall off a roof and began to think about gravity.
Alas, like many good stories this one appears to be apocryphal.
According to Einstein himself, he was simply sitting in a chair when the problem of gravity occurred to him.
Actually, what occurred to Einstein was something more like the beginning of a solution to the problem of gravity,
since it had been evident to him from the outset that one thing missing from the special theory was gravity.
What was "special" about the special theory was that it dealt with things moving in an essentially unimpeded state.
But what happened when a thing in motion—light, above all—encountered an obstacle such as gravity?
It was a question that would occupy his thoughts for most of the next decade
and lead to the publication in early 1917 of a paper entitled "Cosmological Considerations on the General Theory of Relativity."
The special theory of relativity of 1905 was a profound and important piece of work, of course,
but as C. P. Snow once observed, if Einstein hadn't thought of it when he did someone else would have, probably within five years;
it was an idea waiting to happen.
But the general theory was something else altogether.
"Without it," wrote Snow in 1979, "it is likely that we should still be waiting for the theory today."