Unifying General Relativity and Quantum Mechanics has been the dream and goal of many of the greatest minds of the past century. But we are not able to unify it even now. So, why is it so difficult to unify these two theories?
We have unified many forces and theories. First, we have unified electricity and magnetism as electromagnetism. Also, we have unified electromagnetism and weak nuclear force as electroweak force. Now, what’s left is to unify the two other forces i.e. nuclear strong force and gravity with electroweak force.
For strong nuclear force, it’s not so hard to unify because we have a quantum theory for the strong nuclear force. The real difficulty comes when unifying gravity with the other forces or having a quantum theory of gravity.
In General Relativity, gravity is the curvature of spacetime. There are some misconceptions about this. So I wrote a blog explain ‘How Time Causes Gravity?’. I strongly suggest you read the above blog because you will understand better what’s the source of gravity. Now, back to topic. For the other three forces except gravity, we have a quantum theory for each of them. That theory is called Quantum Field Theory. In Quantum Field Theory, we imagine that the fields exist on top of a flat, smooth coordinate system called spacetime. If you wanna know more about Quantum Field Theory, read this blog. But things are not so easy in quantizing gravity.
Gravity cannot be quantized by QFT. Remember, gravity is the curvature of spacetime. It is not a force lying on top of spacetime, it is spacetime. To quantize gravity, you have to quantize spacetime itself. When you try to quantize spacetime, there is no clean coordinate to make predictions and make calculations.
To understand better, let us take an analogy. In order to quantize gravity, we imagine like this. Suppose you are writing on a paper. This is what other forces except gravity do in QFT. In quantizing gravity, we imagine like the letter which is written on the paper can exist independently of the paper. Also, the letter can be bent, curved, etc.
This is what we must imagine in order to quantize gravity. That’s why it’s so hard to unify General Relativity and Quantum Mechanics.
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Knowledge Glutton 
Newton’s Gravity – is interaction between 2 bodies
Einstein’s Gravity – is effect of one single, local body
In order to quantize gravity, we need to know the quantum forces and the quantum matter that cause curving of spacetime
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