Wednesday, October 16, 2013

On falling apples and whether tomatoes really are fruit

Imagine a film of an apple accelerating toward the ground under the influence of gravity. If we played this film backwards, what would we see? It may seem odd, but a physicist will tell us that when the film is played backwards, the apple will continue to accelerate downwards! Regardless of whether the film is played forwards or backwards, the apple will accelerate downwards.

It is because physicists use the word accelerate with a slightly different meaning to the one in everyday use that an otherwise trivial fact about the universe can be cast as such a curious statement.