Feynman writes:
One very important feature of pseudo forces is that they are always proportional to the masses; the same is true of gravity. The possibility exists, therefore that gravity itself is a pseudo force. ... It might seem all right to consider gravity to be a pseudo force, to say that we are all held down because we are accelerating upward... Einstein found that gravity could be considered a pseudo force only at one point at a time...
Now, how can gravity be a pseudo force? What did Feynman meant by saying this statement? Is gravity really a pseudo force? If so, why?
