Loop Quantum Gravity

Meet Loop Quantum Gravity: the direct competitor and fated rival of String Theory (that other weird attempt at a Grand Unified Theory I made a post on). While String Theory starts with what is essentially Quantum Field Theory and builds upon that to accommodate for gravity, LQG does it the other way round: starts with General Relativity and tries to add quantum effects to it. The reason for this is that, according to what Einstein postulated, gravity is not simply just another force, like they are trying to treat it in String Theory. It is actually an intrinsic property of spacetime. So according to the people doing Loop Quantum Gravity and Einstein, String Theory is getting nowhere. (On a side note, LQG is much more testable than String Theory, which is also another reason to its popularity because LQG requires the simple  4-dimensional spacetime postulated in General Relativity while String Theory requires us to assume that at least 10-dimensions exist).

In this theory, spacetime is quantized and is divided into discrete units. This means that you can't move an infinitesimally small distance and call it movement. There is a limit to the minimum distance that we move. So the discrete units they've divided up spacetime into are (clue's in the title) 'loops'. Spacetime is just a finely woven fabric of loops all connected together into a spin network. The size of these loops is estimated to be the Planck length (10^-35). If we go any smaller, there is no meaning to the distances smaller than the Planck scale, just like there is no meaning to mass smaller than a quark because there is nothing smaller than it. So there is no point in talking about it. Or how talking of exceeding light in classical physics is deemed impossible and not even meaningful. So space, just like matter, has an 'atomic structure'.

The prerequisites of this theory were first laid by (of course) Albert Einstein, but his work was reformulated by Abhay Ashtekar. Lee Smolin used this reformulation to figure out that space could be made  up of loops and worked further with Carlo Rovelli to expand this theory. Today, there are two branches this theory has taken: the more traditional canonical version and the new covariant version called spin foam theory.

Loop Quantum Gravity differs from String Theory, mainly, in the fact that LQG is a background independent theory while String Theory is background dependent. What background independence means is that the equations are not based on some predetermined spacetime coordinate system. Background dependence (as you would guess)  means the opposite so it is dependent on other coordinate systems. LQG researchers would say that background independence is necessary if we need to get general relativity to work for small scales.