General Relativity & Quantum Theory: Where's the Limit?

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Discussion Overview

The discussion revolves around the relationship between General Relativity (GR) and Quantum Theory (QM), specifically exploring the limits of their applicability and potential coexistence. Participants examine whether there is a scale at which both theories can be reconciled or if they fundamentally operate in separate domains.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions the limit where General Relativity ceases to exist and Quantum Theory provides a better representation of reality, suggesting a potential coexistence scale.
  • Another participant argues that GR and QM are not incompatible but rather operate differently across scales, with GR being relevant at large scales and QM at small scales.
  • A further contribution emphasizes that while GR and QM can coexist in practical applications, the perceived incompatibility is a significant motivation for developing a theory of quantum gravity.
  • One participant mentions that the Newtonian approximation serves as a scale where GR and QM can coexist effectively.

Areas of Agreement / Disagreement

Participants express differing views on the compatibility of GR and QM, with some asserting that they can coexist under certain conditions, while others highlight the ongoing debate regarding their fundamental incompatibility. No consensus is reached on the specifics of their relationship.

Contextual Notes

Participants acknowledge that the discussion involves complex assumptions about the applicability of GR and QM, particularly regarding the conditions under which they can be reconciled or approximated. The nuances of quantum field theory in curved spacetime and the implications of the Newtonian approximation are also noted but remain unresolved.

Giuliano97
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I've always read that these two theories are incompatible, and how General Relativity works for large scales while Quantum Theory represent reality in extra-small cases.
So my question is where is the limit where General relativity ceases to exist and Quantum Theory gives a better portrait of reality?
There is a scale in which those can coexist?
 
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It isn't that they are incompatible, just very different.
They coexists at all scales, but the effects of relativity are commonly seen at very large scales and the effects of QM are commonly seen at very small scales.

In some cases, such as linear accelerators, the effects can be seen working together.
 
.Scott said:
It isn't that they are incompatible, just very different.

I think many physicists would disagree with this as you state it, since perceived incompatibility between GR and QM in their current states is a major motivation for seeking a theory of quantum gravity.

I would say that practically speaking, GR and QM as we actually apply them are compatible, because in any system of practical interest, we can mesh the two without incompatibility using suitable approximations. For example, we can study quantum field theory in curved spacetime without having to quantize spacetime itself; and if we need to model the "back reaction" of the energy in the quantum field on the spacetime geometry, we do it using a suitable expectation value of stress-energy. But that's a weaker statement than just saying that GR and QM are not incompatible, period.
 
Giuliano97 said:
There is a scale in which those can coexist?

Anywhere that the Newtonian approximation can be applied with success.
 
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