Inside the black hole resides a mystery. What happens at its centre? What happens if we squeeze the observable universe in size smaller than an atom? The physics that we know seem to be inadequate. We need a new theory, the quantum theory which unites the general theory of relativity and quantum mechanics (Quantum Gravity). After publishing the general theory of relativity Einstein published one more paper which indicates it’s not just the end of the story. We have to incorporate quantum mechanics into it. In physics, we end up with unwanted infinities in many places. Renormalization is a technique that was invented to get rid of these infinities and it gives much better results. But the theory of gravity is not renormalizable. The problem of quantum gravity remained a nightmare for Physicists.
There comes the leading candidate of quantum gravity, which is called the loop quantum theory. What makes it’s a success in dealing with infinities? It tried to use the concept called discrete mathematics; An example is something simple as counting. Where quantum mechanics also use the same type of math. By approaching it with the idea of discrete mathematics it was possible to eliminate the infinities and conceptual difficulties.
Loop quantum gravity then imposed two core principles of the general theory of relativity
- Background independence
- Diffeomorphism invariance
Background independence
What is background independence? Let’s look at an analogy. If you are writing on a piece of paper you can assume the paper is there, then you write on it. You can also assume that paper has a shape, it’s flat. It’s the most common assumption we make in physics. In loop quantum gravity we are not making any assumptions on the nature of space it is like the world can exist independently of the paper and also they can bend or change just like space-time in the general theory of relativity.
Diffeomorphism invariance
It says that what is going on at a point in space depends only on that point and not where it is located in space nor what’s happening at other spots in space. If we are combining these two ideas and don’t require that the mathematics be continuous we can found out that the space-time is quantized. Let’s don’t bother over the crazy mathematics.
Another consequence of loop quantum gravity is that it implies there is a small length area volume time which has a value of 10-35 m, 10-70 m2, 1010-103 m3,10-43 seconds. It strongly suggests that you cant have a volume which smaller than 10^-103m^3 and time 10^-43 seconds That means these are the smallest bit of volume and time.
How does the space-time quanta look like?
Space-time is made up of finite loops, the nodes where the loop intersects is the quantum volume of space resides. It has a volume that is multiple of 10^-103 m^3. Loops in between nodes represent the two-dimensional area in multiple of 1×10^-66 m^2. Quantities of these loops and nodes are called a spin network.
What is space in loop quantum gravity? Space is the geometry of the spin network. What is time? It is the movement that rearranges spin networks. The spin network combined with this quanta movement of time is called a spin foam. Every locality in space-time where quanta move occurs, the time has ticked once (1×10^-43) seconds. When mass and energy are added it distorts the shape of the spin foam which we perceive as gravity.
Loop Quantum Gravity & the Big Bang
Another success of loop quantum gravity is explaining the big bang theory. According to loop quantum gravity, the big bang couldn’t have started with a singularity which would have been infinitesimally small and infinite density, because there is a limit to how small space can become. This is the smallest volume of space according to loop quantum gravity. It also has the maximum energy this volume can have. Once, more energy has reached it would have repelled additional energy added to it. The best analogy would be a sponge that gets saturated with water will repel any additional water.
The prediction of loop quantum gravity is that it was not a big bang as we assume but something called a big bounce. The universe was big at one time it then contracted bounced to be the big bang that we know. So before the big bang, the universe was in a contraction phase of a prior universe and when it reached a minimum Planck volume then all the additional energy bounced off and created the big bang as we know.
In the latest formulation of quantum loop gravity, the loops are replaced by graphs which yielded much better results.
Is loop quantum gravity a complete success? The answer is no. It still has to make predictions that could be proven experimentally and also the mathematics needs to be further developed.
Click here to read another interesting article: QUANTUM GRAVITY: A NEW WAY TO UNDERSTAND GRAVITY
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