4 DOES TIME EXIST?

Time is what stops everything happening at once,” said American physicist John Wheeler. But time is a slippery concept. Most of what we think we know is false. For instance, we imagine time flowing. However, for something to flow, it must flow with respect to something else, just as a river flows with respect to a river bank. Does time flow with respect to something else – a second type of time? The idea seems nonsensical. Most likely, the flow of time is an illusion created by our brains to organise the information constantly flooding in through our senses.

We also have a strong sense of a shared past, present and future. However, the idea of a common present appears nowhere in our fundamental description of reality: relativity. Precisely how someone else’s time is sliced up depends on how fast they are moving relative to you or the strength of the gravity they are experiencing. These effects are noticeable only at relative speeds close to that of light or in ultra-strong gravity, which is why they are not obvious in the everyday world. Nevertheless, they lead to the idea that one person’s interval of time is not the same as another person’s, and that one person’s interval of space is not the same as another’s.

Actually, it is worse. Space and time are inextricably intertwined. In our Universe, all events – from the Big Bang to the death of the Universe – are laid out in a preexisting four-dimensional space-time map. Nothing actually ‘moves’ through time. As Einstein wrote after the death of his friend Michele Besso: “Now he has departed from this strange world a little ahead of me. That means nothing. People like us, who believe in physics, know that the distinction between past, present and future is only a stubbornly persistent illusion.”

If the expansion of the Universe is imagined running backwards like a movie in reverse, in its earliest moments space and time are both ripped apart. Physicists therefore suspect that in the Big Bang time emerged from something more fundamental. As yet, no one knows what that might be.