The problem of time is one of the greatest puzzles of modern physics. The first bit of the conundrum is cosmological. To understand time, scientists talk about finding a “First Cause” or “initial condition”—a description of the Universe at the very beginning (or at “time equals zero”). But to determine a system’s initial condition, we need to know the total system. We need to make measurements of the positions and velocities of its constituent parts, such as particles, atoms, fields and so forth. This problem hits a hard wall when we deal with the origin of the Universe itself, because we have no view from the outside. We can’t step outside the box in order to look within, because the box is all there is. A First Cause is not only unknowable, but also scientifically unintelligible.
The second part of the challenge is philosophical. Scientists have taken physical time to be the only real time—whereas experiential time, the subjective sense of time’s passing, is considered a cognitive fabrication of secondary importance. The young Albert Einstein made this position clear in his debate with philosopher Henri Bergson in the 1920s, when he claimed that the physicist’s time is the only time. With age, Einstein became more circumspect. Up to the time of his death, he remained deeply troubled about how to find a place for the human experience of time in the scientific worldview.