The clock at the frontal plane of the picture has a relatively high accuracy, depicted by its sharply defined hands. The uncertainty of time reading for this clock is inversely proportional to the energy gap

ΔE of the internal degree of freedom that constitutes the clock (Clock Model). By the mass–energy equivalence, the energy of the clock will produce gravitational time dilation effects on nearby clocks. Because the energy is not well defined but has an uncertainty

ΔE, nearby clocks will have an uncertainty in their time dilation with respect to the main clock, as depicted by the “fuzzy” hands in a superposition. There exists, therefore, a limitation to the possibility of defining time accurately at nearby points, given by the joint effects of quantum mechanics (superposition principle) and general relativity (gravitational time dilation). This effect is fundamental and independent of the energy gap

ΔE of the clock, as stated in Eq. 3.

http://www.pnas.org/content/early/2017/03/06/1616427114.full