Q: How many general-relativity theorists does it take to change a light bulb?
A: Two. One to hold the bulb and one to rotate space.
This one is based on Einstein’s General Relativity.
Before Einstein, physicists tried to explain motion and everything in relation to a certain point of view, called the Frame of Reference. Their failed attempts to find a universal Frame of Reference inspired Einstein to suggest that such a Frame must not exist.
General Relativity deals with different actions as observed from different Frames of Reference.
An analogy given by Einstein himself was this:
When a man sits with a pretty girl for an hour, it seems like a minute. but let him sit on a hot stove for a minute and it’s longer than any hour. That’s relativity.
When we talk about replacing a light bulb, from our point of view, it may look like we are turning the light. But from another view, perhaps of the light bulb itself, it looks like it is not the light bulb which is turning but everything else.
Einstein said that to be able to understand the universe, we need to be able to talk about things from every Frame of Reference, because there is no single Frame of Reference which can properly compare different phenomena.