Warped Passages - Lisa Randall [49]
Figure 36. An observer in a falling elevator who releases a ball will not see it drop. However, when a freely falling elevator meets the stationary Earth, the observer will not be very happy.
The reason that Einstein’s conclusion seems so surprising and strange is that our upbringing here on Earth, with a stationary planet beneath our feet, biases our intuition. When the force of the Earth keeps you stationary on the ground, you notice the effects of gravity because you are not following the path towards the center of the Earth that gravity would have you follow. On Earth, we’re accustomed to gravity making things fall. But “falling” really means “falling relative to us.” If we were falling along with a dropped ball, as we would be in a free-falling elevator, the ball would not go down any faster than we would. We therefore would not see it drop.
In your freely falling reference frame, all the laws of physics would coincide with the laws of physics that would be obeyed if you and everything near you were at rest. A freely falling observer would observe that motion is described by the same equations, consistent with special relativity, that apply for observer in an inertial, non-accelerating reference frame. In the review paper he wrote in 1907 about relativity, Einstein explains how the gravitational field has only a relative existence, “because for an observer falling freely from the roof of a house there exists—at least in his immediate surroundings—no gravitational field.”*
This was Einstein’s major insight. The equations of motion for a freely falling observer are the equations of motion for an observer in an inertial reference frame. A freely falling observer does not feel the force of gravity—only objects that are not in free fall experience a gravitational force.
In our lives we don’t generally encounter things or people in free fall. When free fall happens, it looks scary and dangerous. But, as an Irishman said to the physicist Raphael Bousso when he was visiting Ireland’s Cliffs of Moher, “It’s not the fall that kills you, but the %&!# crash when you stop.” And when I broke several bones in a rock-climbing accident and had to miss a conference I had organized, there were quite a few jokes about my testing the theory of gravity. I can state with complete confidence that gravitational acceleration agrees with predictions.
Tests of General Relativity
There’s more to general relativity; soon we’ll get to the rest, which took considerably longer to develop. But the equivalence principle alone explains many results from general relativity. Once Einstein had recognized that gravity could be canceled in an accelerating reference frame, he could calculate gravitational influence by imagining an accelerating system equivalent to the one with gravity. This allowed him to calculate the gravitational effects for some interesting systems which others could use to check his conclusions. We’ll now consider a few of the most significant experimental tests.
First is the gravitational redshift of light. A redshift causes us to detect light waves at a lower frequency than the frequency at which they were emitted. (You’ve probably encountered the analogous effect in sounds waves when a motorcycle roared past you and the sound waves rose and fell in pitch.)
There are several ways to understand the origin of the gravitational redshift, but probably the simplest is through an analogy. Imagine that you throw a ball up into the air. The rising ball slows down as it moves against the force of gravity. But the ball’s energy is not lost, even though the ball is slowing down. It is converted into potential energy, which is then released as kinetic energy, or energy of motion, when the ball falls back down.
The same reasoning applies to the particle of