Loop quantum gravity

From the point of view of Einstein´s theory, it comes as no surprise that all attempts have failed to treat gravity simply like one more quantum force, on par with electromagnetism and the nuclear forces. According to Einstein, gravity is not a force - it is a property of space-time itself. Loop quantum gravity is an attempt to develop a quantum theory of gravity based directly on Einstein´s geometrical formulation.

This approach is hard to explain in detail without resorting to the language of mathematics. Part of the problem is an as yet unanswered question: This approach leads to a quantum formulation for gravity, but researchers are still busy trying to work out how, from such a quantum foundation, a universe like ours can arise, which obeys the classical (i.e. non-quantum) general theory of relativity.

One aspect of the loop models, though, is easy to grasp: Space, in general relativity, is a continuum. In every part of it, one can define regions of arbitrarily small volume, and every little region can be divided further into smaller regions still, ad infinitum. In the loop models, the basic structure of space-time turns out to be discrete. Just as, with a children´s lego set where you cannot build a structure smaller than the smallest block, in such discrete space times, there are smallest values for volumes and areas that are not divisible any farther. The fabric of space is what is called a spin network with lines and nodes, as pictured here:

The smallest possible volume is that of a region containing only a single node. Enlarging the region, the region´s volume grows in a specified way for each additional node enclosed.

But not only space acquires a grainy, discrete structure - so does time. In simplified models used for cosmological explorations, it turns out that within loop quantum gravity, there is no big bang singularity - instead, the universe´s history can be traced infinitely far into the past, step by step.

Next page: Superstrings and universal harmony