Is Quantum Gravity the Key to Understanding the Universe?

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Discussion Overview

The discussion revolves around the nature of spacetime and the quantization of gravity, exploring whether spacetime is a tangible entity or merely an abstract concept. Participants examine the implications of treating gravity as a quantum field and the challenges associated with this approach, particularly in relation to existing theories like General Relativity and quantum field theory.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question whether spacetime is a literal entity or an abstract idea, suggesting that it serves as a useful abstraction in General Relativity.
  • There is a belief among some that gravity can be treated as a quantum field due to the success of quantum physics in other areas, though the quantization of gravity presents significant challenges.
  • One participant notes that while quantum gravity could potentially predict phenomena such as primordial B modes, the actual detection and confirmation of such predictions remain uncertain.
  • Another participant emphasizes that the effectiveness of quantum field theory in other domains provides no reason to exclude gravity from this framework, though the complexities of non-linear field equations in General Relativity complicate the matter.
  • There is acknowledgment that beyond current theoretical cutoffs, the nature of quantum gravity remains speculative, with possibilities including string theory or other unknown theories.

Areas of Agreement / Disagreement

Participants express differing views on the nature of spacetime and the feasibility of quantizing gravity, indicating that multiple competing perspectives exist without a clear consensus.

Contextual Notes

The discussion highlights limitations related to the definitions of spacetime and gravity, as well as unresolved mathematical complexities in the context of General Relativity and quantum field theory.

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Is Timespace a literal real thing? or is it an abstract idea?

and what makes physisists so sure that gravity is a quantum field? and can be quantised?
 
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brianhurren said:
Is Timespace a literal real thing? or is it an abstract idea?

Spacetime is a useful abstraction, it is a dynamical "thing" in Einstein's General Relativity (GR), the modern theory of gravity.

brianhurren said:
and what makes physisists so sure that gravity is a quantum field? and can be quantised?

Because quantum physics has shown itself to be the most accurate model for all other branches of physics ... thus the expectation is that it also applies to gravity.

The difficulty is in finding an appropriate way to do it - Special Relativity (SR) is already taken into account within quantum field theory, but the spacetime of SR is not a dynamic entity.

Also the field equations for GR are non-linear; this was not the case for Maxwell's electrodynamics - those equations are linear. That makes the mathematics much more difficult to work with.
 
Gravity is can be treated theoretically as a quantum field, and can be quantized to the point where it already makes predictions. For example, quantum gravity is a possible source of primordial B modes that Planck or BICEP2 might detect.

http://motls.blogspot.com/2014/03/bicep2-primordial-gravitational-waves.html It's still not known if BICEP2 really observed primordial B modes, but Liam McAllister discusses how quantum gravity could be a source of such modes.
 
brianhurren said:
and what makes physisists so sure that gravity is a quantum field? and can be quantised?

The issue with quantum gravity is not what some popularisations make it out to be:
http://arxiv.org/pdf/1209.3511v1.pdf

The answer is its works for everything else, produces a theory in accord with observation up to a cutoff so there is zero reason to think QFT is an odd man out.

Now peeking behind that cutoff is another issue - and all our theories are up for grabs beyond that - it may be string theory - it may be something else - we don't know.

Thanks
Bill
 

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