What if a glass of water was, all of a sudden, literally half empty? —Vittorio Iacovella A. The pessimist is probably more right about how it would turn out than the optimist. When people say “glass half empty,” they usually mean a glass containing equal parts water and air. Traditionally, the optimist sees the glass as half full while the pessimist sees it as half empty. This has spawned a zillion joke variants—for example, the engineer sees a glass that’s twice as big as it needs to be, the surrealist sees a giraffe eating a necktie, etc. But what if the empty half of the glass were actually empty—a vacuum?1 The vacuum would definitely not last long. But exactly what happens depends on a key question that nobody usually bothers to ask: Which half is empty? For our scenario, we’ll imagine three different half-empty glasses, and follow what happens to them microsecond by microsecond.
