Ask someone what time is and they may point to a clock.
Ask physics, and the answer becomes stranger.
A clock can measure the duration accumulated along its path.
Relativity can tell us which events could have influenced which others.
A coordinate can label an event.
An equation can describe how a physical system changes.
One quantum system can sometimes be used as a clock for another.
And a spacetime can possess a temporal orientation that allows physicists to distinguish two possible directions through it.
All of these concern time.
But they are not the same physical thing.
That distinction is the starting point of Physical Time.
Rather than beginning with another grand definition of what time ultimately “is,” the paper asks a more disciplined question.
When physicists say that time has been reconstructed, derived, or made to emerge from something more fundamental, what exactly has been reconstructed?
The answer matters because several of the deepest problems associated with time may have been bundled together simply because they share the same word.
Consider a few apparently simple statements.
One event happened before another.
One event could have caused another.
Five seconds elapsed along a particular path.