Encouraged by his successes with Special Relativity and Quantum Mechanics,
Albert tackled the thorny problem of General Relativity
(the behaviour of Physics in accelerated reference frames )
with his characteristic élan. The first difficulty
was in distinguishing between truly accelerated frames
(like a compartment in a rocket) and frames that only seem
to be accelerated (like where you are sitting). Consider: if you
can't look out the window, how do you know you are
being pressed into the seat of your chair by the Earth's gravity,
as opposed to being in a rocket somewhere in interstellar space
accelerating ``up'' at 9.81 m/s
? Well,
yes, you walked into
the room from outside and sat down just a short while ago; but
suppose you had lost your memory? How can you tell
(by experiment) which is the case?