Why is it so difficult to quantize gravity?
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Basically, because our best theory of gravity (general relativity) is that gravity is really distortions in space-time. So if gravity is quantized, space-time is quantized.
But that causes quantum mechanics to fall apart; quantum mechanics is quantum phenomenon happening with particles & fields on top of *non* quantized space-time.
So we *can* quantize gravity, but then we kill our other best theory (quantum mechanics), which isn’t much of a victory.
We need to figure out solutions for both gravity and quantum mechanics that don’t break each other. This has, so far, proved…elusive.
Not a trained physicist but will try for an ELI5.
One of the biggest problems with gravity is that it is very very small relative to the other fundamental forces. Electromagnetism, gravity, strong and weak nuclear forces are the 4 known fundamental forces that act in our universe. The usual illustration is to say that it takes a small magnet the size of a pea to hold a nail against the entire gravity force of our planet. So gravity is very very very weak (to the order of 10^(-34) weaker than electromagnetism)
This makes it difficult to study and experiment with. The explanation is that the weaker the force, the more “concentrated” we need to make it in order to study it. At the level of quantum effects, this “concentration” is done by pumping a lot of energy into a very small space to conduct the experiment – this is why particle physicists use huge accelerators to obtain high concentrations of energy in a very small volume to conduct the experiments – and this is barely sufficient to study the other 3 forces.
We’d need equipment that are many orders of magnitude (possibly trillions of times) more powerful to experiment and verify the existence of gravity particles at the quantum level. This is beyond today’s technology for human construction.
Our other option is observation of gravity’s effects at the cosmological level. That, by its nature, depends on coincidence and detection of effects from very far away. So now we need equipment that is highly sensitive etc. Again not so easy – even things like LIGO (miles large) can (in my own words) really only start probing some of these very long distance gravity effects and we are only now starting to understand how to build these things.
This is the problem then – theory runs FAR FAR AHEAD of experimental capability. So there are multiple theories of quantum gravity but no way to test any of them today. And without testing, it is difficult to know how to proceed.
Again this is ELI5 as I understand it.
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