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So I want to try to answer what I can, despite being a layman on this. Gravity exists, it manifests as the warping/geometry of space. This is in contrast to t

So I want to try to answer what I can, despite being a layman on this. Gravity exists, it manifests as the warping/geometry of space. This is in contrast to the other fundamental forces which get explained via Quantum Field Theory. That's the very high level difference of the two, our current understanding of gravity does not work the same way as the way everything else does, and so far we can't find a provable theory (yet) that makes the two work together at all scales. String theory purported as a way to create a quantum theory of gravity and explain everything else, but my understanding is that it's fallen out of favor because it mostly turned into a tunable mathematical framework that could just change to fit any observations that were made, so it doesn't have the same kind of predictive power that people want (i.e. too much freedom so it can be used to explain anything, not just everything). I believe this is where predictions about a possible gravitational force carrier generally come from, aka the graviton. Then there's theories like Loop Quantum Gravity, where the way it works is that space-time itself is quantized and that's how you get things to mesh because you can now use the same wave-function style of things that all other quantum theories use. Though I think this doesn't predict much about a quantum field for gravity on it's own. I believe one of the other things that runs into everything being difficult is that with relativity you end up with a lot of infinities in the equations and results and so there's a "new" kind of math for it that gets called "renormalization" that prevents them from coming out but it also has issues when translating between quantum theories and relativity.

nyssos 2y ago
> it doesn't have the same kind of predictive power that people want (i.e. too much freedom so it can be used to explain anything, not just everything). That's the popsci version that's been disseminated, yes. It's not exactly wrong, but it's misleading. First a bit of background. Quantum field theories like the standard model are effective theories, not fundamental ones. We know we don't know the real high-energy physics, so we treat it as a black box and loosely speaking "average it out" as a new free parameter. This is analogous to how an engineer designing a bridge can ignore the fact that iron has a crystal structure and treat it as a continuum with bulk properties like tensile strength. In reality this having a particular tensile strength is a state, not an intrinsic property, and you could end up with a different tensile strength if you melted the iron and let it resolidify (I'm not a metallurgist, substitute some other material if that's not true for iron), but we can build bridges without knowing that. In the same way, Standard Model is a particular form of "solidified" string theory. It's true that there are many, many, many others, but they're not free parameters in the same way. You can write down perfectly reasonable looking quantum field theories that string theory can't produce, and if our best effective theory was one of them then we would have good reason to reject string theory. But it's not. So the situation we're in is that we have some solid material, and we want to know what a single molecule of it looks like, but we can't see anything other than the bulk properties. What the "string theory is unfalsifiable" crowd is demanding is that whatever molecule we predict have only a few possible crystal structures. And maybe it does. That would be convenient. But sometimes nature inconveniences us: it might be some crazy carbon allotrope. It might be glass.

nyssos 2y ago

> it doesn't have the same kind of predictive power that people want (i.e. too much freedom so it can be used to explain anything, not just everything). That's the popsci version that's been disseminated, yes. It's not exactly wrong, but it's misleading. First a bit of background. Quantum field theories like the standard model are effective theories, not fundamental ones. We know we don't know the real high-energy physics, so we treat it as a black box and loosely speaking "average it out" as a new free parameter. This is analogous to how an engineer designing a bridge can ignore the fact that iron has a crystal structure and treat it as a continuum with bulk properties like tensile strength. In reality this having a particular tensile strength is a state, not an intrinsic property, and you could end up with a different tensile strength if you melted the iron and let it resolidify (I'm not a metallurgist, substitute some other material if that's not true for iron), but we can build bridges without knowing that. In the same way, Standard Model is a particular form of "solidified" string theory. It's true that there are many, many, many others, but they're not free parameters in the same way. You can write down perfectly reasonable looking quantum field theories that string theory can't produce, and if our best effective theory was one of them then we would have good reason to reject string theory. But it's not. So the situation we're in is that we have some solid material, and we want to know what a single molecule of it looks like, but we can't see anything other than the bulk properties. What the "string theory is unfalsifiable" crowd is demanding is that whatever molecule we predict have only a few possible crystal structures. And maybe it does. That would be convenient. But sometimes nature inconveniences us: it might be some crazy carbon allotrope. It might be glass.