In the eighth instalment of his exclusive monthly series for the South China Morning Post, American theoretical physicist and Nobel laureate Frank Wilczek explains how physicists consider the contradictions of time. Read his previous articles here.
“The test of a first-rate intelligence is the ability to hold two opposed ideas in mind at the same time, and still retain the ability to function,” wrote F. Scott Fitzgerald, famed American author of The Great Gatsby. Time is one of those concepts that forces us to hold opposed ideas in mind. To grasp the scientific concept of time, let alone the cluster of psychological ideas that go by the same name, requires a supple mind indeed.
Modern physics asks us, in different situations, to describe time in ways that seem opposed or even contradictory. When, as a physicist, I look at time, I see three faces. I will call them ideal time, stochastic time and manufactured time.
The essence of ideal time was defined and used by Isaac Newton in his Principia, the great work upon which classical physics was erected. “Absolute, true, and mathematical time, of itself and from its own nature, flows equably without regard to anything external, and by another name is called duration,” he wrote.
Time, according to Newton, is a primary ingredient of reality. It cannot be analysed into anything simpler or more basic. Though quantum mechanics and relativity brought revolutionary charges to physics, and its scope has expanded vastly, Newton’s concept of time as a primary ingredient of reality remains foundational.
Indeed, within the basic equations that summarise today’s fundamental physics – the so-called Standard Model – there is a variable quantity that we denote by “t” and call time. A definite value of t attaches to every event that happens in the universe. The equations of the theory – its “equations of motion” – tell us, in principle, how the state of the world at a given value of t evolves into its state at a slightly larger value of t.