What is quantum gravity in simple terms

Relativity and Quantum / Beginners Tour Part 4: Gravity in Loops

From the point of view of Einstein's theory, it is actually not surprising that attempts to describe gravity as a quantum force in the same way as electromagnetism and nuclear forces fail. For Einstein, gravitation is not a force like other forces, but a property of space-time itself. An attempt to develop a quantum theory of gravitation that is closely based on geometry is the so-called Loop quantum gravity.

The details of this approach are quite difficult to visualize - this is partly due to the mathematical formalism, but on the other hand it also reveals a question that loop quantum gravity cannot yet fully answer: How the universe that we around us ultimately emerges on the quantum basis see and that obeys the classical general theory of relativity is not yet clarified and is the subject of current research.

One aspect of the loop models, on the other hand, is quite clear: the spatial geometry of general relativity is continuous. Sub-areas of any small volume can be defined in each room area, and each room area can be further subdivided as often as desired. In the loop models, the basic structure of spacetime proves to be discrete: the values ​​of surface areas and volumes can only be changed in (admittedly tiny) jumps, and there are such things as minimal areas and spatial contents, below which the space can no longer extend can be subdivided - similar to how a Lego construction kit cannot be used to build an object that is smaller than the smallest available Lego brick. The basic structure of the room is a so-called spin network of connected nodes, as shown in the following figure:

Spin network sketch

The smallest conceivable volume just contains a node; if further nodes are added, the volume for each node increases by a characteristic volume value.

Discrete time structures also result from loop quantum gravity. In a simplified model in the loop formalism for an expanding universe, for example, the big bang singularity has disappeared - instead, the history of the universe can be followed step by step into the past.