It’s the year 2100. You wake up alone in a small, windowless room. The only other thing in the room is a small ball. Maybe the room is located in your city, but maybe it’s inside that new spaceship everyone’s talking about. How can you tell?
You pick up the ball and drop it. It falls vertically to your feet. You time the fall and calculate that the ball accelerates at 9.8 metres per second per second, exactly the acceleration of gravity at the surface of the Earth.
But a spaceship in the middle of deep space can also accelerate by that much, producing the exact same results. So where are you?
In 1911, Einstein formally proposed that gravitational mass (that which produces a gravitational field) and inertial mass (that which resists acceleration) were one in the same, and this became known as the “equivalence principle”. According to this principle, you can’t tell whether you’re in a gravitational field (such as on the surface of the Earth) or experiencing constant acceleration (a spaceship speeding up, pushing you to the floor, like the g-force of a roller-coaster).