What is time? By Martin Bojowald

This post is also available in: Spanish

In the initial mathematical formulation of loop quantum gravity space and time play different roles, which has sometimes led to the charge that the theory breaks the covariance of general relativity. There are atoms of space but no atoms of time. Time or change arises when atoms of space and matter interact with each other, forming new atoms by subdivision or exciting existing atoms. An excited spatial atom occupies a larger amount of volume, and so by creating new spatial atoms and exciting existing ones a growing space, just like the expanding universe, arises. Even though space and time seem to play different roles in this active interacting picture, there are consistency conditions which ensure that the dynamical processes described in this way respect covariance properties as they are known from general relativity. Showing that these consistency conditions are realized in all situations remains one of the major questions in the development of the theory.

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