The Cosmic Now
Anthony Aguirre: Birth — an event that happens just once in a person’s lifetime — happens every second somewhere in the world! Similarly, the explosive death of a star as a supernova, which happens just once in the 100-million-year lifetime of just a small fraction of stars, occurs many times per second somewhere in the observable universe. (I follow the convention that ‘Universe’ means roughly all that exists; ‘observable universe’ means that which we can currently probe using observations with telescopes and other experiments’, and ‘universe’ means the space-time region encompassing, and with similar properties to, the observable universe. Is the Universe bigger than the universe? Bet on it!) It is a big place! (And getting bigger, to the tune of about 1062 cubic metres per second.) When we contemplate all of these things happening now, it feels very intuitively clear what we mean by ‘now’: some event either happens now, or it does not, right? Wrong.