Why can’t we solve the Theory of Everything (Grand Unified Theory)? What are some missing links? Do we think it can be solved?

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I’m not sure if this is the place to ask this or if anyone has any idea, but I just watched [this video](https://www.youtube.com/watch?v=eNhJY-R3Gwg) by minutephysics and it got me curious about the missing links. I’ve procrastinated enough on my homework but I am curious, so if anyone has some input it would be greatly appreciated 🙂

In: Physics

We have two freakishly accurate theories: general relativity and quantum mechanics. They both make incredibly accurate predictions that have stood up to almost every experimental test we’ve thrown at them so far, which is an enormous achievement for any scientific theory. And they mostly don’t overlap….quantum mechanics only really shows up at very small scales (the level of individual atoms & molecules) while general relativity describes gravity, which is generally so weak over quantum distances that it doesn’t matter but is the major force dominating very large scale things like solar systems, galaxies, etc.

The major problem is that the two theories are incompatible…they can’t both be true because one contradicts the other in those spheres where they overlap. We don’t have a quantum mechanical explanation for gravity and we don’t have a “general relativistic” explanation for quantum phenomenon. In almost all practical situations this doesn’t matter, because the theories only overlap in places we generally can’t go, like black holes, but it drives physicists nuts because there’s only one universe so there ought to be one set of physical laws, not two (or more).

We have no reason to think it *can’t* be solved, but we can’t solve it yet because nobody has come up with a theory that explains *both* general relativity and quantum mechanics (which is a requirement for any grand unified theory) that we can also experimentally test. String theory might be it, but we’ve got no way (yet) to test the predictions. It might be something else entirely. So the challenge is that we need to come up with one theory that covers the phenomenal accuracy of both quantum and general relativity, but also makes predictions we can test that would allow us to tell them apart.