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To me, intuitively, it makes sense for gravity to be classical, since gravity is not a force but a curvature of space-time. I hope we see results of their tests

To me, intuitively, it makes sense for gravity to be classical, since gravity is not a force but a curvature of space-time. I hope we see results of their tests soon.

jerf 3y ago
Gravity is a curvature of space-time in the theory of General Relativity. It is not known what it will be in the final theory. It could end up being a "force" after all, or it could be that all forces are "curvatures" of a sufficiently complicated understanding of "space time", in which case force would be likely be redefined to be "curvatures" of space time anyhow, or other possibilities. It may be easier to understand this point by looking at the current theory of Quantum Mechanics, in which gravity doesn't exist at all. We are not entitled to make conclusions about gravity from that fact, since the conclusion would be "gravity doesn't exist", which is clearly false.
DiogenesKynikos 3y ago
In General Relativity, the curvature of spacetime is affected by matter. Matter is governed by quantum mechanical physical laws, so General Relativity has to interface with quantum mechanics. To illustrate the problem: If an electron is not in a fixed position, but is rather spread over a range of possible locations (described by a wavefunction), then what is the gravitational effect of the electron? General Relativity, being a classical theory, contains the assumption that the electron's location and velocity can be precisely determined, so that its effect on the curvature of spacetime can be calculated. There's no way in General Relativity (as presently formulated) to take into account the electron wavefunction. If you start thinking about ways to interface General Relativity (GR) with quantum mechanics, you quickly realize that GR itself has to become quantum mechanical. Quantum mechanics is like a virus: once it gets into one part of the theory, it spreads everywhere. There's no way to build a logically coherent theory of the Universe that is only partly quantum mechanical. It's 100% or 0%.
- eigenket 3y ago
  This is not completely accurate. The theory proposed by Oppenheim and his collaborators appears to be a consistent theory with classical gravity interacting with quantum stuff. It is subjectively rather ugly, but it does appear to be a counterexample to this 100% or 0% dichotomy.
- kolinko 3y ago
  Why can’t gravity take into account the electron wavefunction? Electromagnetism and other forces work well with electrons in a superposition
  - eigenket 3y ago
    Gravity can and must "take this into account" somehow. The problem is we don't really have a working theory of gravity which does this. There are a bunch of proposals, including this one but none of them are really complete or fully satisfying right now.
  - DiogenesKynikos 3y ago
    The theory of electromagnetism had to be completely reworked in order to take quantum mechanics into account. The modern theory, developed from the 1920s-40s, is called "Quantum Electrodynamics." The older theory of electromagnetism (Maxwell's Equations) is the classical limit of the modern quantum theory. General Relativity is in the same situation now as Maxwell's Equations were before Quantum Electrodynamics was developed. Everyone knows that a quantum version of the theory is necessary, and that that theory will have a classical limit that is equivalent to the old theory. It's just that finding a quantum version of General Relativity is way more difficult than finding a quantum version of Maxwell's Equations. The tricks that worked for electromagnetism don't work for gravity.
    - kolinko 3y ago
      Thanks for the reply!
    - AnimalMuppet 3y ago
      > Everyone knows that a quantum version of the theory is necessary "Everyone knows" is not any kind of evidence. It's not even really an argument. It's, essentially, a statement that an assumption is widely shared. And, if I understand the article correctly, it may not in fact be necessary.
      - DiogenesKynikos 3y ago
kzrdude 3y ago
The paper uses terms like "classical general relativity" but for me, that's a contradiction. Do you know what exactly they mean by classical, then?
- atombender 3y ago
  It refers to Einstein's original formulation of GR, without the later quantum theories.

To me, intuitively, it makes sense for gravity to be classical, since gravity is not a force but a curvature of space-time. I hope we see results of their tests soon.

jerf 3y ago

Gravity is a curvature of space-time in the theory of General Relativity. It is not known what it will be in the final theory. It could end up being a "force" after all, or it could be that all forces are "curvatures" of a sufficiently complicated understanding of "space time", in which case force would be likely be redefined to be "curvatures" of space time anyhow, or other possibilities. It may be easier to understand this point by looking at the current theory of Quantum Mechanics, in which gravity doesn't exist at all. We are not entitled to make conclusions about gravity from that fact, since the conclusion would be "gravity doesn't exist", which is clearly false.

DiogenesKynikos 3y ago

In General Relativity, the curvature of spacetime is affected by matter. Matter is governed by quantum mechanical physical laws, so General Relativity has to interface with quantum mechanics. To illustrate the problem: If an electron is not in a fixed position, but is rather spread over a range of possible locations (described by a wavefunction), then what is the gravitational effect of the electron? General Relativity, being a classical theory, contains the assumption that the electron's location and velocity can be precisely determined, so that its effect on the curvature of spacetime can be calculated. There's no way in General Relativity (as presently formulated) to take into account the electron wavefunction. If you start thinking about ways to interface General Relativity (GR) with quantum mechanics, you quickly realize that GR itself has to become quantum mechanical. Quantum mechanics is like a virus: once it gets into one part of the theory, it spreads everywhere. There's no way to build a logically coherent theory of the Universe that is only partly quantum mechanical. It's 100% or 0%.

eigenket 3y ago
This is not completely accurate. The theory proposed by Oppenheim and his collaborators appears to be a consistent theory with classical gravity interacting with quantum stuff. It is subjectively rather ugly, but it does appear to be a counterexample to this 100% or 0% dichotomy.
kolinko 3y ago
Why can’t gravity take into account the electron wavefunction? Electromagnetism and other forces work well with electrons in a superposition
- eigenket 3y ago
  Gravity can and must "take this into account" somehow. The problem is we don't really have a working theory of gravity which does this. There are a bunch of proposals, including this one but none of them are really complete or fully satisfying right now.
- DiogenesKynikos 3y ago
  The theory of electromagnetism had to be completely reworked in order to take quantum mechanics into account. The modern theory, developed from the 1920s-40s, is called "Quantum Electrodynamics." The older theory of electromagnetism (Maxwell's Equations) is the classical limit of the modern quantum theory. General Relativity is in the same situation now as Maxwell's Equations were before Quantum Electrodynamics was developed. Everyone knows that a quantum version of the theory is necessary, and that that theory will have a classical limit that is equivalent to the old theory. It's just that finding a quantum version of General Relativity is way more difficult than finding a quantum version of Maxwell's Equations. The tricks that worked for electromagnetism don't work for gravity.
  - kolinko 3y ago
    Thanks for the reply!
  - AnimalMuppet 3y ago
    > Everyone knows that a quantum version of the theory is necessary "Everyone knows" is not any kind of evidence. It's not even really an argument. It's, essentially, a statement that an assumption is widely shared. And, if I understand the article correctly, it may not in fact be necessary.
    - DiogenesKynikos 3y ago

kzrdude 3y ago

The paper uses terms like "classical general relativity" but for me, that's a contradiction. Do you know what exactly they mean by classical, then?

atombender 3y ago
It refers to Einstein's original formulation of GR, without the later quantum theories.