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>Thank you for your response. What's the source of incompatibility between standard model and general relativity? Gravity is about acceleration and flow of time

>Thank you for your response. What's the source of incompatibility between standard model and general relativity? Gravity is about acceleration and flow of time. What's the problem with plugging general relativity in place of classical dynamics whenever you care about flow of time and acceleration in quantum physics? As if those gravity and other forces and particle had nothing to do with each other. The difficulty is that the standard model uses "quantum field theory" (QFT) as the physical model in the background to describe it. However QFT is actually quite hacky in a lot of ways, the main one being that actually it predicts that every point of space should have an infinite free energy (see here for example https://en.wikipedia.org/wiki/Vacuum_energy https://en.wikipedia.org/wiki/Vacuum_energy). The way we get around this is by subtracting off this infinite energy, which is one of many similar techniques called "Renormalization" (https://en.wikipedia.org/wiki/Renormalization https://en.wikipedia.org/wiki/Renormalization). Well it turns out that if you produce a quantum field theory for gravity, it is not actually possible (or at least in any way easy) to "renormalize" it (i.e. it's really hard to subtract off these infinities) The issue is more that QFT is really weird, rather than that general relativity is >Why does general relativity is more incompatible with quantum mechanics than with fluid dynamics for example? Well general relativity (GR) and QFT both describe fundamental interactions, whereas fluid dynamics describes an emergent property of electromagnetic (EM) interactions between many particles. Gravity is also usually very irrelevant in fluid dynamics unless we are looking at very large objects like the flow of matter in stars for example, so in most cases it is just EM interaction which is fully described by QFT. In other cases we have relatively weak gravity + QFT, for example in those stars, in which cases we can actually already describe the dynamics fully using approximate versions of gravity and not have any incompatibility. In cases where gravity is very strong though we do have these issues as we just don't know what happens due to lacking a proven theory of quantum gravity. One example is how the matter behaves in the super-dense core of a neutron star, known as the neutron star "equation of state", which is currently unknown, as in these cases both quantum and gravitational effects are very strong. >My understanding is that general relativity says that gravity is not even a real force. It's just a function of acceleration caused by misshapen space-time (deformed by mass/energy). Yes that is completely correct to our understanding, but we could maybe model it as a "real force" by writing a quantum field theory for it >Some other things I don't understand... How gravity is considered to be a fundamental force, while Pauli exclusion principle is not, despite the fact that it can keep neutron stars from collapsing. Is it because there's no hope for finding mediating particle in the math that describes the principle because the math is too simple? Honestly the fact that the Pauli exclusion principle isn't a force has always been confusing to me too as it behaves like one in so many situations, but it is similar to gravity really which as you said isn't really a force, but more like an influence. In fact GR says that we follow the geodesic (roughly translated to the "shortest path") along the curved space-time, i.e. it is by minimising the so-called "action". This isn't really a force, more like a fundamental law of motion. I think the Pauli exclusion principle is similar in this way, but I don't think it is a satisfactory explanation

scotty79 6y ago
> Well it turns out that if you produce a quantum field theory for gravity, it is not actually possible ... I heard about that. But why would you want to have quantum field theory of gravity? Why can't you just consider gravity to be only about the shape of space-time in which everything including quantum field theories of everything else happen? > Well general relativity (GR) and QFT both describe fundamental interactions .. Does GR really describe fundamental interactions? For me it seem that it only describes shape of space-time where all the fundamental interactions take place. > In cases where gravity is very strong though we do have these issues as we just don't know what happens due to lacking a proven theory of quantum gravity. Isn't it just because we lack data from systems where gravity is extremely strong and systems are small/isolated enough to show any quantum effects? How can we be sure that we need quantum gravity? Why it's not enough to calculate QFT in curved spacetime instead of flat spacetime? > One example is how the matter behaves in the super-dense core of a neutron star, known as the neutron star "equation of state", which is currently unknown, as in these cases both quantum and gravitational effects are very strong. I see. > Yes that is completely correct to our understanding, but we could maybe model it as a "real force" by writing a quantum field theory for it Apparently we can't due to problems with renormalization. And I argue we shouldn't be able to because I believe it's completely different thing from a real force and if same math worked for two dissimilar things I'd be worried. :-) I'm happy to hear that Pauli principle is unsettling not only to grumpy weirdo like me. :-)
- joeberon 6y ago
  >I heard about that. But why would you want to have quantum field theory of gravity? Why can't you just consider gravity to be only about the shape of space-time in which everything including quantum field theories of everything else happen? No, because it turns out that curved space-time is actually incompatible with quantum field theory in certain extreme parameter regimes, in a way that it actually makes it impossible to make predictions on scales where both gravitational and quantum effects are very strong (e.g. neutron stars). We want a quantum field theory of gravity as then it will "play nice" with other quantum field theories. >Does GR really describe fundamental interactions? For me it seem that it only describes shape of space-time where all the fundamental interactions take place. I think it is quite fundamental that energy curves space-time and that it affects the paths of objects (even light), but yes maybe I wouldn't call it a fundamental interaction in the same sense as quantum field theories, particularly because it isn't quantised so it isn't really a single "thing" being exchanged (like a gauge boson). Most physicists do call it an interaction or a force though. I mean that it describes nature on a fundamental and indivisible level (at least so far as our current/most popular understanding) >Isn't it just because we lack data from systems where gravity is extremely strong and systems are small/isolated enough to show any quantum effects? How can we be sure that we need quantum gravity? Why it's not enough to calculate QFT in curved spacetime instead of flat spacetime? Of course we lack this data, which is why measurements of, for example, the aforementioned neutron star equation of state will give us more understanding. We can't be sure that we need a quantum theory of gravity, it could be that it is actually just impossible to describe both gravity and QFT at the same time. QFT in a curved spacetime is only a first approximation of general relativity and doesn't capture all of the effects of GR (see here for example https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved_spacetime https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved...). I don't really know much about quantum gravity, but I don't believe that QFT in curved space-time is sufficient, at the very least it doesn't solve the cosmological constant problem (https://en.wikipedia.org/wiki/Cosmological_constant_problem https://en.wikipedia.org/wiki/Cosmological_constant_problem) > And I argue we shouldn't be able to because I believe it's completely different thing from a real force It's worth considering that what we call a force in quantum field theory is very different from what Newton would have considered a force! Probably in fact the gravitational "force" due to curved space-time would seem more like a force to him, certainly QFT is much weirder than GR after all...

scotty79 6y ago

> Well it turns out that if you produce a quantum field theory for gravity, it is not actually possible ... I heard about that. But why would you want to have quantum field theory of gravity? Why can't you just consider gravity to be only about the shape of space-time in which everything including quantum field theories of everything else happen? > Well general relativity (GR) and QFT both describe fundamental interactions .. Does GR really describe fundamental interactions? For me it seem that it only describes shape of space-time where all the fundamental interactions take place. > In cases where gravity is very strong though we do have these issues as we just don't know what happens due to lacking a proven theory of quantum gravity. Isn't it just because we lack data from systems where gravity is extremely strong and systems are small/isolated enough to show any quantum effects? How can we be sure that we need quantum gravity? Why it's not enough to calculate QFT in curved spacetime instead of flat spacetime? > One example is how the matter behaves in the super-dense core of a neutron star, known as the neutron star "equation of state", which is currently unknown, as in these cases both quantum and gravitational effects are very strong. I see. > Yes that is completely correct to our understanding, but we could maybe model it as a "real force" by writing a quantum field theory for it Apparently we can't due to problems with renormalization. And I argue we shouldn't be able to because I believe it's completely different thing from a real force and if same math worked for two dissimilar things I'd be worried. :-) I'm happy to hear that Pauli principle is unsettling not only to grumpy weirdo like me. :-)

joeberon 6y ago
>I heard about that. But why would you want to have quantum field theory of gravity? Why can't you just consider gravity to be only about the shape of space-time in which everything including quantum field theories of everything else happen? No, because it turns out that curved space-time is actually incompatible with quantum field theory in certain extreme parameter regimes, in a way that it actually makes it impossible to make predictions on scales where both gravitational and quantum effects are very strong (e.g. neutron stars). We want a quantum field theory of gravity as then it will "play nice" with other quantum field theories. >Does GR really describe fundamental interactions? For me it seem that it only describes shape of space-time where all the fundamental interactions take place. I think it is quite fundamental that energy curves space-time and that it affects the paths of objects (even light), but yes maybe I wouldn't call it a fundamental interaction in the same sense as quantum field theories, particularly because it isn't quantised so it isn't really a single "thing" being exchanged (like a gauge boson). Most physicists do call it an interaction or a force though. I mean that it describes nature on a fundamental and indivisible level (at least so far as our current/most popular understanding) >Isn't it just because we lack data from systems where gravity is extremely strong and systems are small/isolated enough to show any quantum effects? How can we be sure that we need quantum gravity? Why it's not enough to calculate QFT in curved spacetime instead of flat spacetime? Of course we lack this data, which is why measurements of, for example, the aforementioned neutron star equation of state will give us more understanding. We can't be sure that we need a quantum theory of gravity, it could be that it is actually just impossible to describe both gravity and QFT at the same time. QFT in a curved spacetime is only a first approximation of general relativity and doesn't capture all of the effects of GR (see here for example https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved_spacetime https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved...). I don't really know much about quantum gravity, but I don't believe that QFT in curved space-time is sufficient, at the very least it doesn't solve the cosmological constant problem (https://en.wikipedia.org/wiki/Cosmological_constant_problem https://en.wikipedia.org/wiki/Cosmological_constant_problem) > And I argue we shouldn't be able to because I believe it's completely different thing from a real force It's worth considering that what we call a force in quantum field theory is very different from what Newton would have considered a force! Probably in fact the gravitational "force" due to curved space-time would seem more like a force to him, certainly QFT is much weirder than GR after all...