One of the most important principles in Einstein's theory of gravity is the so called Equivalence principle. Roughly, this states that wherever we are, there exists a freely falling coordinate system in which there are no effects of gravity. An example of such a coordinate system would be coordinates that are fixed to a freely falling elevator (suppose the cables have snapped). If we just drop a coin in such an elevator, it will not fall to the elevator floor. Rather it will just float in the air before us. If we instead throw the coin, it will just float away - following a straight line and moving with constant speed - as if there was no gravity at all. Seen from the outside of the elevator, the coin and the thrower of the coin are both falling. But relative the elevator, there is no sign of gravity except for an outside observer that for some reason accelerates upwards.

With this principle in mind, consider the following two questions:

1. Suppose that we are going to shoot an arrow at an apple. All is arranged such that exactly when we release the arrow, the apple itself is released and starts to fall. How should we aime. Will we hit the apple if we aim straight for the apple?

2. Suppose that we in stead shoot a photon, that is a light-particle, at the apple. All is still arranged such that exactly when we emit the foton from our flash light, the apple itself is released and starts to fall. How should we aim now? Will we hit the apple if we aim straight for it? Can the photon fall?

Below is a link to an animation that answers these questions, and explains generally what the equivalence principle is all about. Play around with the various settings: Arrow, Photon, Net, and Relative/Vanlig, and all should become clear.

The animation may not work for older web browsers. For newer versions (with whatever extensions necessary to run Java applets) it should work fine, just don't resize the window and only use the link once!