In one of Einstein's
famous thought experiments
he realized that
gravity and acceleration
appear to be
the same phenomenon.
Think about
what would happen
if an elevator suddenly
went into a free-fall.
The person inside would
effectively weigh nothing.
Next, imagine that
same person
in a motionless rocketship,
so far from earth
that the force of gravity
is practically zero.
Just like in the
free-falling elevator
he would also weigh nothing.
Now, put the rocket in motion.

As the rocket speeds up
the passenger's
weight increases.
To him, it feels
just as if gravity
had planted his feet
firmly on the floor. Einstein realized
that the force of gravity
is actually the
acceleration you feel
as you move
through space-time.