The concept of time is the most important one to find a unification of quantum mechanics and gravity.
On the first glance these two seem to be incompatible because they take care of time in two complete different ways.
Quantum mechanics needs absolute time to be consistent and Einstein's general relativity tells us on the other hand, that time is a
relativistic phenomena depending on velocities and the matter surrounding us, i.e. depend on the fact, where we are.
The first approach, by Wheeler and DeWitt, were able to combine these two and created the Wheeler-DeWitt-equation to honor them for their work.
But this equation opened a complete new problem, because it predicts on the first glance, that there is absolute NO time in our universe, that is that our universe
should appear static, what is in complete contradiction to our everyday experience.
In the year 1983 Don Page and William Wooters had an interesting solution to this problem:
They showed that an observer inside of the universe can use the entangled particles to build a clock, and on the other hand, that an external observer of the
universe is not able to measure time with these two particles.
This might seem confusing and extremely philosophic but in the year 2013 E. Moreva, G. Brida, M. Garmegna, V. Giovannetti, L. Maccone and M. Genovese
made an experiment to show that these theoretic ideal really describes the properties of time in our universe.
(For more details I refer to their publication: )
This new feature has to be extend further and to be integrated in the most recent theories of quantum gravity.
I'm curious to see what comes out of it.