A QUESTION OF QUANTUM TIME

The closer we look at time, the stranger it gets

- PROF JIM AL-KHALILI

The nature of time is one of the most profound and longstanding problems in physics – one that no one can agree on. From our perspective, time seems to steadily progress forward with each tick of the clock. But the closer we look, the more bizarre time becomes – from equations that state time should flow as freely backwards as it does forwards, to the strange quantum realm where cause and effect can flip on their heads. Could it even be that time itself is an illusion?

What makes time so confounding is that we have three very different ways of defining it, which don't easily fit together.

The first definition comes from the equations that describe how things change over time. We have many such equations describing everything from the motion of tennis balls to the decay of atomic nuclei. In all these equations, time is a quantity, referred to as 'coordinate time'. Time is no more than a mathematical label to which we can assign a particular value.

The second definition of time comes from Einstein's theories of relativity, where it's a dimension in addition to the three we're familiar with. It's a direction in four-dimensional spacetime. Our picture of reality then becomes one in which all times - past, present and future - are equally real and coexist, just as all points in space are equally real. More than that; time has a deep connection with gravity according to General Relativity, where the shape of spacetime is influenced by gravity.

Much of the effort at the forefront of theoretical physics over the past half-century has been devoted to unifying General Relativity with the strange world of quantum mechanics. Mathematical frameworks that attempt to do this are known as theories of quantum gravity. But how do we reconcile these two notions of time - the quantum mechanical idea, in which time is a mere parameter, versus the relativistic idea that time is a dimension in spacetime? I call this 'the first problem of physical time'.