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A quantum theory of gravity would by definition explain the curvature of spacetime, since said curvature (and its dynamics) is how gravity is expressed. Asking

A quantum theory of gravity would by definition explain the curvature of spacetime, since said curvature (and its dynamics) is how gravity is expressed. Asking how a quantum theory of gravity would explain curvature is then asking someone to give you a completed quantum theory of gravity, and it is not at all clear that the first successful theory will describe the world terms of particles. However, it can be instructive to consider electromagnetism as as classical theory and then as a quantum theory: waves in fields become enormous showers of particles, and these photons then carry the force that attracts or repels and can sometimes interact with each other.

choeger 6y ago
Ah, you caught me there ;). I was hoping someone would spill their secret quantum gravity theory on Hackernews.
8note 6y ago
A quantum theory of gravity doesn't have to explain curvature of spacetime. It needs to produce better experimental results than modeling gravity as the curvature of spacetime. Whether that still describes it in a way that can be mapped to a curvature based explanation is irrelevant. It should for some cases give the same results?
- raattgift 6y ago
  A quantum gravity theory might not use the vocabulary of curvature or the tensor fields and scalar objects found in the Einstein Field Equations of General Relativity, but that's not quite the same as "doesn't have to explain curvature". For the theory to be viable it would have to explain why General Relativity is such a good effective theory at solar system scales. One can do this by treating General Relativity as a theory which emerges in some limit of the more fundamental theory of gravitation (or everything). One can compare that with how Newtonian gravitation emerges from General Relativity as one takes c to infinity, or as one takes v^2/c^2 (where c is usually set to one; one can likewise expand in other small parameters) to zero. The first is the basis for the first section of https://en.wikipedia.org/wiki/Post-Newtonian_expansion https://en.wikipedia.org/wiki/Post-Newtonian_expansion The second section has more explanatory power: Newtonian gravitation emerges from General Relativistic systems when multipole moments are small and the distance to the sources is large. The key to the success of General Relativity compared to many of its competitors is that it very straightforwardly produces the Newtonian solution for the well-studied orbits of the Jovian system, while explaining the anomaly in the precession of Mercury's orbit. One would have to be able to show conditions in the new theory in which General Relativity emerges (and for which General Relativity has observational support), or the theory is not going to get much practical use in astrophysics or GPS-style engineering, even if it appears to get the microscopic details right. Practically[1] nobody -- there are several serious contenders like Loop Quantum Gravity -- who has developed an alternative theory of gravitation (or everything) has been able to show at all that General Relativity emerges from it. Lastly, because we don't have the technology to probe the quantum gravitation sector directly, but we do have the technology to probe General Relativity very finely (such is the topic of the article), I can't see how a failure to explain General Relativity's results can do anything but undermine quantum gravity candidates. - -- [1] People have proposed turning various constants into spacetime-dependent functions, and find that such adjustments can solve some things but generally have to vanish practically everywhere, so such theories are not viable. Among these are Palatini and others https://en.wikipedia.org/wiki/F(R)_gravity https://en.wikipedia.org/wiki/F(R)_gravity and Brans-Dicke https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory. These families of theories usually show how General Relativity could emerge, but further investigation almost always shows that the parameters or features that distinguish these theories from General Relativity are incompatible with observation and so must vanish.
- qubex 6y ago
  The ‘funniest’ part of all the talk of curvature et al as if they were fundamental aspects of nature is that they’re nothing but a gradient across space, and are (currently, at least) best described as curvature in spacetime, but that’s just a physical interpretaton.

choeger 6y ago

Ah, you caught me there ;). I was hoping someone would spill their secret quantum gravity theory on Hackernews.

8note 6y ago

A quantum theory of gravity doesn't have to explain curvature of spacetime. It needs to produce better experimental results than modeling gravity as the curvature of spacetime. Whether that still describes it in a way that can be mapped to a curvature based explanation is irrelevant. It should for some cases give the same results?

raattgift 6y ago
A quantum gravity theory might not use the vocabulary of curvature or the tensor fields and scalar objects found in the Einstein Field Equations of General Relativity, but that's not quite the same as "doesn't have to explain curvature". For the theory to be viable it would have to explain why General Relativity is such a good effective theory at solar system scales. One can do this by treating General Relativity as a theory which emerges in some limit of the more fundamental theory of gravitation (or everything). One can compare that with how Newtonian gravitation emerges from General Relativity as one takes c to infinity, or as one takes v^2/c^2 (where c is usually set to one; one can likewise expand in other small parameters) to zero. The first is the basis for the first section of https://en.wikipedia.org/wiki/Post-Newtonian_expansion https://en.wikipedia.org/wiki/Post-Newtonian_expansion The second section has more explanatory power: Newtonian gravitation emerges from General Relativistic systems when multipole moments are small and the distance to the sources is large. The key to the success of General Relativity compared to many of its competitors is that it very straightforwardly produces the Newtonian solution for the well-studied orbits of the Jovian system, while explaining the anomaly in the precession of Mercury's orbit. One would have to be able to show conditions in the new theory in which General Relativity emerges (and for which General Relativity has observational support), or the theory is not going to get much practical use in astrophysics or GPS-style engineering, even if it appears to get the microscopic details right. Practically[1] nobody -- there are several serious contenders like Loop Quantum Gravity -- who has developed an alternative theory of gravitation (or everything) has been able to show at all that General Relativity emerges from it. Lastly, because we don't have the technology to probe the quantum gravitation sector directly, but we do have the technology to probe General Relativity very finely (such is the topic of the article), I can't see how a failure to explain General Relativity's results can do anything but undermine quantum gravity candidates. - -- [1] People have proposed turning various constants into spacetime-dependent functions, and find that such adjustments can solve some things but generally have to vanish practically everywhere, so such theories are not viable. Among these are Palatini and others https://en.wikipedia.org/wiki/F(R)_gravity https://en.wikipedia.org/wiki/F(R)_gravity and Brans-Dicke https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory. These families of theories usually show how General Relativity could emerge, but further investigation almost always shows that the parameters or features that distinguish these theories from General Relativity are incompatible with observation and so must vanish.
qubex 6y ago
The ‘funniest’ part of all the talk of curvature et al as if they were fundamental aspects of nature is that they’re nothing but a gradient across space, and are (currently, at least) best described as curvature in spacetime, but that’s just a physical interpretaton.