THE QUEST FOR LOOP QUANTUM GRAVITY

Nipumoni Kalita

BSc 5^th Sem(Physics Major)

Loop quantum gravity is one of the several concepts under development by theoretical physicists to provide a descriptive of gravity in terms of Quantum mechanics.

For decades, physicists have sought to reconcile the two major pillars of modern physics theory: general relativity and quantum mechanics. General relativity postulated by Albert Einstein, advanced beyond Issac Newton’s gravitational dynamics by stating that gravitational masses distort the space-time around them and that the resulting curvature in space-time is synonymous with gravity. Because gravity is the weakest of the four fundament forces of nature - the others being electromagnetism and the weak and strong nuclear interactions that predominate inside the atomic nucleus - general relativity is extremely useful for describing the gravitational effects associated with massive objects, including planets, stars and galaxies. Quantum mechanics in constant describes objects and their properties in terms of probabilistic wave functions. It excels at describing the sub-microscopic universe where unusual quantum effects, uncertainty, diffraction and tunnelling become highly significant.

The problem is that the theoretical frame-works for relativity and quantum mechanics are different and irreconcilable. In mathematical terms, the obstacle is that when physicists apply the quantum field theories for electromagnetism and the two nuclear interactions they can use a technique called renormalization to avoid problematic infinites arising in the calculations. The equations for relativistic gravity however are not renormalizable. Physicists would prefer a single comprehensive framework that could encompass all scales of mass and size accurately: a theory of quantum gravity.

Loop quantum gravity addresses this problem by posting that space itself has a discrete granularity on the order of the Plank scale (about 10-35 meters). The granularity is defined in terms of minutely looping gravitational fields interwoven into a so-called spin network or spin form. A conceptual advantage of loop quantum gravity is that unlike some competing ideas. Such as string theories (also called superstring theories, which are specialized cases of border M-theory), it doesn’t invoke the existence of additional, as-yet-unseen spatial dimensions. On the other hand, direct observation of Planck’s-scale granularity is also essentially impossible. So testing of the hypothesis is difficult and may eventually depend on indirect evidence from astronomical observations and gravity wave detector. Moreover out of multiple possible variation of loop quantum gravity, no signal formulation is yet favoured by a consensus of theorists, and all of the variations have difficulties with predicting how the phenomena of general relativity should emerge at large scales. Even, if loop quantum gravity does reconcile relativity and quantum mechanics. It wouldn’t help with the unification of gravity, electromagnetism and the weak and strong nuclear interactions as manifestations of a single underlying force.