Spacetime theories

Silence is olden  ::  Michael Brooks  ::  New Scientist, 14 June 2014  ::

string-theory_50290aef7a637Summary and review of the above article

An understanding of the beginning of the universe requires an integration of general relativity and quantum theory or in other words a theory of quantum gravity. String theory has been the best known attempt at this with quanta described as the vibrations of one dimensional strings. However, alternative theories have emerged in recent years.

Loop quantum gravity

A theory known as loop quantum gravity views spacetime as a woven fabric of braids and knots. The different ways in which the braids intertwine yields the different particles and forces of quantum theory. An extension of this is loop quantum cosmology (LQC) which extends the theory to try to describe the conditions of the early universe. Close to the singularity, light can’t move at all, which means that there is no communication or action of forces at all between different parts of the very early universe. At least two other theories of the early universe, dilation gravity and causal dynamical triangulation (CDT) point towards the same conclusion.

Directionality of time

With LQC the period of inflation thought to have occurred near the beginning of the universe would be a consequence of the braids and knots of spacetime becoming overloaded with energy and setting up a repulsive force. quantum-gravity-nature-onlineLQC also appears to predict an early state where time is turned into a spatial dimension, giving a total of four spatial dimensions. At present, the only feasible test for theories about the very early universe lies in studying features of the cosmic microwave background radiation. The Planck satellite has been creating a more detailed map of the radiation, but this still lacks sufficient accuracy to check the loop quantum predictions, and greater accuracy appears to await future satellites.

In the meantime, there appear to be some pointers from laboratory work. So-called metamaterials involving columns of magnetic particles are suggested to simulate the conditions of the early universe with randomly aligned quantum spins and no net magnetic field, thus supposedly removing the directionality of time.

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