The Equivalence Principle

Ah, equivalence. A property that our lives revolve around. Yet, some people don't appreciate the relativity of equivalence, possibly because they don't understand how it works.

First, think of an elevator in space. Even though this is not physically possible, pretend that the elevator is free of all gravitational forces. There are two people in this elevator. Without any gravity, these two unlucky people are floating freely. Now, let's bring the elevator up to the free fall rate, or the speed required to emulate the effect of Earth's gravity (9.8 meters per square second, I believe). Next, pretend that the observers still believe they're on Earth (not remotely possible, hopefully). Will they know the difference between gravity and acceleration? Most likely not, as in this case acceleration has simulated gravity.

For the next example, let's take that elevator and the two brave astronauts and place them in the Willis Tower (Sears Tower, whatever). Now, we'll drop the elevator into a free fall. Not counting air resistance, the elevator should fall at a constant rate of approximately 9.8 meters per square second, as mentioned before. What will happen to the people inside the elevator? In this case, acceleration has cancelled gravity out as it is going toward gravity's pull.

Finally, picture the same elevator one last time, but instead of two travelers there is a beam of light stretching across the elevator in a perfectly horizontal line. Now, we'll raise the elevator one last time. The beam of light will appear to arc downward to an observer due to the motion of the elevator, even though its path actually remains undisturbed.

This theory usually is updated to use a rocket ship, but you know me. I'm old fashioned.

Credit: Leonard Susskind's Black Hole Wars. An excellent, highly recommended book!