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 co-exist, 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.
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