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Loop quantum gravity Explained

ARYAN BANERJEE
Sept. 1, 2021

The closest thing we have to an all-encompassing framework that explains all particles and forces and is represented by the quality Model of high-energy physics. But this model is flawed because it doesn't explain gravity. A theory of everything has got to be ready to reconcile quantum physics with the overall theory of relativity. This unification appears to be possible with a theory called “Loop Quantum Gravity” -- or LQG.

What is it?

The theory of LQG attempts to point out that this geometric fabric of space-time isn't continuous as Einstein presumed, but is itself quantum, made from discrete quanta. This is like the garments or sweaters you would possibly be wearing immediately. Even though it's smooth from distance, if you checked out it closely, you'd see threads, nodes, and loops woven together. These are the quanta or bits, that your clothing is formed of.

This is different from every other theory including string theory. Because even in string theory, space is that the background, on which strings vibrate. In LQG this space-time background is itself quantized. So this means that distance has a minimum quantity about equivalent to the plank length or 10^-33 cm.

Similarly, the area has a minimum value of 10^-66 cm squared, and the volume has a minimum value of 10^-99 cm cubed - below which it cannot go. This is super tiny. LQG says that there are about 10^99 quanta of volume in every cubic centimeter of space. This quantum of volume is so tiny that there are more such quanta during a milliliter than there are cubic centimeters within the entire visible universe -- (10^85) And Time itself has a minimum quantity as well which is 10^-43 seconds or close to plank time.

And what do these spacetime quanta look like? They are like loops. The nodes that intersect are where the quanta volumes of space reside. It has a volume that's a multiple of the plank volume 10^-99 cubic centimeters. The loops in between the nodes represent 2D areas. And large quantities of these loops and nodes are called “spin networks.” -- because their properties are related to particle qualities called spins. And Time is defined by the moves that rearrange this spin network.

The spin network, when combined with these quantum movements of the time, is called a spin foam. Time does not flow like a river in the spin foam and it ticks like a digital clock. And each such quantum tick or movement of the spin network is 1*10^-43 second. Every location in the spin network where a quantum move takes place, the time has ticked once. When mass and energy are added to the present spin foam, the form of the volumes of the spin network is distorted. This distorts space and time because any movement of these quanta also affects the time quanta.

Time is essentially a movement of these volume quanta. And this distortion of space and time is what we perceive as gravity. The exciting thing about LQG is that it appears to explain some of cosmology’s greatest questions, like, what came before the big bang. According to LQG, the big bang could not have started with a singularity that would have been infinitesimally small, and of infinite density…because there is a limit to how small space can become 10^-99 cubic centimeters.

This is the littlest volume of space consistent with LQG. This also implies a maximum energy density that this minimum volume of space could have had. And once that maximum energy density was reached, it would have repelled any additional energy added to it -- like a sponge that gets saturated with water, will repel any additional water. So LQG predicts that the universe didn't start with an enormous bang, but with an enormous bounce.

The universe was big at one time. It then contracted. It bounced and then exploded from this bounce. So before what we call the large bang, the universe was during a contracting phase of a previous universe. And when it reached a minimum plank volume, all additional energy bounced off and created what looks to us sort of an explosion.

Conclusion

So LQG has a lot going for it. Is it the theory of everything? No, not quite yet, because it needs to be confirmed by observation. And, its mathematics must be further developed to form dark energy and substance emerge. But, in my view, Loop Quantum Gravity has brought us closer to a theory of everything than anything over the past 100 years.