#### So whose theory is more on track: Dr. Sheldon Cooper or Dr. Leslie Winkle (from The Big Bang Theory)? Loop quantum gravity or string theory?

#### Asked by Jamie Guinn via Facebook

Loop quantum gravity and string theory are two prominently competing answers to a problem that has been at the forefront of physics since the early 20^{th} century: the reconciliation of general relativity and quantum mechanics. Unification of the two major advances in modern physics would provide scientists with a theory, labelled quantum gravity, which has the potential to describe any and all observable interactions in the universe, regardless of whether they occur on the cosmic or quantum scales. Deciphering the universe with such a theory would be one of, if not the greatest scientific discoveries of all time. It easy to see why The Big Bang Theory’s Sheldon Cooper and Leslie Winkle are so at odds; in many ways this is a true reflection of their real life counterparts, with prejudices and subject bias often deciding which side of the debate a scientist favours.

On the surface, loop quantum gravity has modest ambitions – at least when compared to some of its rivals. Ignoring the electromagnetic force, the theory makes no attempt to unify gravity with the other forces of nature. Instead loop quantum gravity chooses to focus on constraining the gravitational field that permeates the universe into discrete values.

In much the same way that matter is comprised of atoms, loop quantum gravity suggests that the fabric of space consists of closed loops knitted together into networks. Loops are not considered objects, they are simply abstract relationships and connections in the quantum force fields. Networks link together, forming what scientists have dubbed ‘spin foam’ and on a large enough scale the smooth and continuous framework of space emerges.

This is one of loop quantum gravity’s key concepts: the idea that gravity should *not* be thought of as residing within space, being contained by it. Instead, gravity is the defining physical property of the space itself.

String theory on the other hand has higher aspirations. A contender for the long sought after ‘theory of everything’, it sees itself marrying together all the fundamental forces, including gravity, with the theory of general relativity. In principle this would allow string theory to achieve what no other theory has come close to achieving: an elegant explanation of *all known physical phenomena* and the capability to predict the outcome of any conceivable experiment.

Current quantum mechanics paints a picture of the building blocks of matter as **dimensionless ‘point-like’ particles**. String theory proposes that nature’s basic constituents are comprised of a tangle of one-dimensional vibrating strings, themselves braided together on an infinitesimally small scale to form a particle.

Requiring more than three dimensions (with current calculations putting it at 10!) to construct the universe, string theory has been able to predict the existence of particles that carry gravity through space and time.

String theory as has been around on the quantum gravity scene much longer and as a result it is perhaps considered the more established of the two. However, loop quantum gravity has a firm base and is gathering momentum. The fact is, while both theories are moving ahead fast, we do not yet know if either one or some combination of both of them works.

Neither loop quantum gravity or string theory have ever been experimentally tested. In fact, they haven’t produced a single number that can be investigated in a lab, even one as high tech as the Large Hadron Collider at CERN. Critics say that this means they are simply theoretical speculation, not worthy of the time, effort and most importantly money that is spent on them. However, historically theory has often preceded experimental evidence by years if not decades. There is plenty still to do before we can think about reaching a conclusion on which one is more on track, the only sure thing is that they are both fascinating concepts.

Therefore – for the moment Leslie Winkle and Sheldon Cooper can carry on arguing. The winner is unlikely to emerge in the near future!

Answered by Toby Brown