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So ELI5. In physics there are 4 fundamental forces. Strong, Weak, Electromagnetic and Gravitational.
Strong and Weak act on quarks, protons, electrons, etc.
Gravitational act on large masses. Electromagnetic act on charged particles.
The issue is we haven’t been able to link all 4 of these forces together in a workable model yet. We can do the general relativity stuff of large masses and gravity but when you take it down to the quantum level where the weak and strong forces are, the math doesn’t work out any more and if you take the quantum level stuff up to large scale, the same thing. This has been something scientists have been working towards for decades and where string theory and n-theory and loop quantum gravity all arose from is trying to get all 4 of these forces into 1 universal model. We still have yet to achieve this.
In physics, there is a conflict between relativity, which applies on large-scales, and quantum mechanics, which applies to small-scales.
The problem is that when you measure things on a quantum mechanics level experiments will give results that say, “X will happen”. When you measure on a larger scale, relativity experiments will show that “Y will happen”. But nobody can properly explain how, for example, a bunch of particles, all demonstrably working under the “X will happen” theory (quantum mechanics) when taken one at a time will somehow do Y when they are measured as a group as predicted by relativity, or how they decide which rules to follow.
The Unified Field Theory, sometimes known as “the Theory of Everything”, is a much sought-after theory that will somehow explain how the two connect. It will explain how, for example, a group of particles that individually do X will somehow end up doing Y once enough of them are together, how to tell when they will do X or Y and why this will change, and otherwise connect quantum dynamics and relativity in a way that explains the disconnect.
Old physics joke:
Albert Einstein died and went to heaven. When he arrived he was taken before the Lord God Almighty.
“Albert, I was most impressed with your attempts to understand my creation, and very pleased that you, in your words, tried to ‘understand the mind of God’. I am minded to give you a boon. Of course, even you could not understand everything, so ask me a single question, no matter how complex, and I will tell you the anwer.”
Einstein says, “Oh Almighty, I spent the last years of my life seeking the answer to one question. If it pleases you, may I have the Unified Field Theory?”
God smiles. “I thought that might be what you would request. Here you are”, and hands Einstein a stack of paper.
Einstein took the stack in his trembling hands. He read the first page, then the second, growing ever more excited with each page. The equations were so beautiful! The knowledge so profound!
When he reached the fifty-seventh page, he stopped. He looked puzzled. He flipped back to the beginning and began to re-read page ten. Then he sighed and put the stack down.
“Wrong again”.
There are 4 fundamental forces. The electromagnetic force, the strong and weak nuclear forces, and gravity.
Unified field theories aim to “unify” these 4 forces into different aspects of the same force.
We have already succeeded in the unification of the electromagnetic force and the weak nuclear force.
The main obstacle in creating a unified field theory is gravity: our current description of gravity is based on classical mechanics, whereas everything else is based on quantum mechanics.
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