What you always need to know about Quantum Gravity?

Before I answer this question, let’s go back to the time of Sir. Issac Newton. Newton sees space, time and gravity are the three different things and are not connected to each other. He thought of our universe to be a space consisting of matters and matters does not play any role to define the structure of space. Because at that time, there was no concept that space has some structure. Newton says, our universe and space are the interchangeable terms. He said space does not change due to anything so, he called it as static space.

Now comes with the conception of time, if different observers are observing the same events happening somewhere in the space then, Newton says the time noted by different observers are same and he believed that there is one universal clock and it gives the information that events have happened in this specific time. He then called time to be static. This means you cannot control that clock. It is said to be static time.

With that concept of space and time, he developed an approach that explains the behavior of heavenly bodies in our universe which we now called Classical Mechanics.

Let’s comes with gravity, the conception of gravity was realized thousands of years ago, even to monkeys, they know how to jump from one tree to another tree in projectile motion😃. But there was no formalism for gravity which we can be thought of as a force. Newton gave a description of gravity in the form of Newton laws of gravitation by observing an apple falling towards the earth and questioning, how much force does the earth needed to pull the apple towards it!

After 200 years, Prof. Einstein was there to oppose the Newton concept of static time by giving an example. Suppose I have two light bulbs in my left and right hand then I set up these light bulbs in such a way that they light up simultaneously, this means I will see these two events happened at the same time. But if the observer who is driving and observing these events from the left side from me. He says the left light bulb lighten up first then the right one. And Einstein answered the “why” by saying: light also takes some time to take the information from the events happened somewhere in the universe to the observer’s retina. This causes him to give a value for the velocity of light and is the highest velocity you can ever imagine. So, time is not the same for different observers and suggested that time should be relative and different from different observers. Since space consists of matters and Einstein said, matters are the source of creating events like dead/birth of stars. And events depends upon the observer’s observations! i.e. the variable for observations is “relative time’. And he said these two things are not different but the same i.e. from space and time, he gave us the term spacetime. And he gave the suggestion that spacetime and our universe are interchangeable terms, not space! With this concept, he developed the approach called Special Relativity which can explain the behavior of particle which has high velocity compared to the speed of light. But, he hasn’t touched on gravity. At the same time, there was also a revolution in the quantum theory that can explain the behavior of sub-atomic particles.

In 10 years, Einstein again put a question like this: Okay! An apple is falling towards the earth because I’m on the earth observing the motion of that apple. But what if I somehow live on the surface of that apple and observing the motion of earth then, will I see the earth is falling towards the apple? His answered was YES! Because of his Special Theory of Relativity, he said everything in the universe is in the motion relative to each other. To have a conclusion like this is seems doubt to see gravity as a force. This means Einstein understands that spacetime and gravity are the same things. Einstein realized that spacetime is a container that stores events and matters, and this matters defined the structure of spacetime/our universe. This means our universe has also some structure and from the observations, our universe is rapidly expanding. With this concept, Einstein formulated the gravity in-terms of geometry and which is presented in the General Theory of Relativity. On the other side, there was the development of the different approaches for quantum theory i.e. Matrix mechanics, Schrodinger picture of wave functions, Feynman path integral and now we have Quantum field theory. One must also need to be aware that General relativity is also a field theory.

Now, what is remaining?

If we are probing our universe on a very small scale i.e. nearly to the Planck scale or even below, then we cannot neglect the quantum effects. And even if we assume our universe is empty but due to vacuum fluctuation, the universe can get enormous fluctuating energies and this is the source of strong gravity. This means we have our universe to be “superposition of curved spacetimes”. Superposition means you cannot predict the geometry of our universe and curved spacetimes mean, our universe will have a strong gravitational field due to vacuum fluctuations. And such a universe is our experimental lab for quantum gravity.

So, we need an approach to give a better understanding of the behavior of such quantum spacetimes.