the problem of time

To unify quantum mechanics and general relativity, the "problem of time" crops up when encompassing the theory of quantum gravity. 

Time's steady ticks dictate the evolving entanglements between particles.
But in general relativity (Albert Einstein’s theory of gravity), time is relative and dynamical, a dimension that’s inextricably interwoven with directions x, y and z into a four-dimensional “space-time” fabric. The fabric warps under the weight of matter, causing nearby stuff to fall toward it (this is gravity), and slowing the passage of time relative to clocks far away.

Unifying quantum mechanics and general relativity requires reconciling their absolute and relative notions of time.

To learn more, click on the source article: Quantum Gravity Research