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My impression was that QFT includes gravity and "only" breaks down when things get extreme, i.e. black holes/big bang/etc.

by pka 10y ago

pja 10y ago
No, there is no quantum theory of gravity & QFT doesn’t include gravity at all. The best you can do is add gravitational effects as an after-thought, but the things you might analyse with the full QFT are generally far too small & short lived for gravitational effects to matter anyway.
- raattgift 10y ago
  > No, there is no quantum theory of gravity Here's a couple of examples of quantum theories of gravity. http://www.phys.lsu.edu/faculty/pullin/talks/pire1.pdf http://www.phys.lsu.edu/faculty/pullin/talks/pire1.pdf http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pdf http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pd... http://einrichtungen.ph.tum.de/T31/seminars-past/seminar-talks/FrankSaueressig13.pdf http://einrichtungen.ph.tum.de/T31/seminars-past/seminar-tal... http://www.damtp.cam.ac.uk/research/gr/public/qg_ss.html http://www.damtp.cam.ac.uk/research/gr/public/qg_ss.html The extent to which these are complete, consistent, natural (in the fine-tuning sense), and so forth is debatable but these are certainly existing examples of quantum theories of gravity, and indeed the first is a perfectly reasonable Effective Field Theory that people work in regularly. > QFT doesn’t include gravity at all I think you mean "The Standard Model of Particle Physics", which is a quantum field theory (as is e.g. perturbative quantum gravity). > the things you might analyse with the full QFT Atoms and molecules have gravitational fields; when you send one through a double slit, which way does their gravitational field go? http://www.nature.com/nnano/journal/v7/n5/abs/nnano.2012.34.html http://www.nature.com/nnano/journal/v7/n5/abs/nnano.2012.34.... A quantum theory of gravity is needed to answer that. Assemble a huge number of particles in superposition with 1/(sqrt 2) (|M @ a> + |M @ b>), with a & b separated. A quantum theory of gravity is needed to describe the gravitational influence of M on a small test object (General Relativity's answer is just wrong :( ).
  - pja 10y ago
    I’m not sure I believe that a mathematical formalism that is unable to make useful real world predictions deserves the “theory” moniker. Hence your list of quantum theories of gravity aren’t. But perhaps that’s me being picky :) > I think you mean "The Standard Model of Particle Physics", which is a quantum field theory (as is e.g. perturbative quantum gravity). Sure: I was just quoting the parent comment & using the term informally to stand in for the mouthful that is TSMoPP. (I’d love to see an experimental setup that was capable of detecting the gravitational field of a single molecule: that would be impressive!)
    - raattgift 10y ago
      > I’m not sure I believe that a mathematical formalism that is unable to make useful real world predictions deserves the “theory” moniker. Hence your list of quantum theories of gravity aren’t. On the contrary, the ones I listed are all completely in accord with General Relativity up to strong gravity and absent superposed sources, which is found from studying the renormalization group flow of perturbative quantum gravity and is four loops of gravitons in a 3+1 dimensional spacetime. Strong gravity can only be found very close to the singularity of black holes (and well inside event horizons, except at the final evaporation), or in the very early universe. So we're good for neutron stars, and have no problems studying things around the event horizons of astrophysical black holes. The only new mathematical formalism in perturbative quantum gravity is renormalization, and that goes back to the 1980s. Perturbative quantum gravity itself comes from the 1990s. Sean Carroll has a good explanation of renormalization and effective field theory here: http://www.preposterousuniverse.com/blog/2013/06/20/how-quantum-field-theory-becomes-effective/ http://www.preposterousuniverse.com/blog/2013/06/20/how-quan... Asymptotically safe gravity posits an ultraviolet fixed point at which one can take a finite number of measurements, producing a strong gravity completion that perturbative renormalzation cannot; this is prompted by asymptotic safety in QCD. Below that limit, ASG completely matches perturbative quantum gravity, and so in the EFT limit it's the same as General Relativity. There are five or six of other viable families of quantum theories of gravity, where viability means they accord exactly with perturbatively quantized General Relativity in its effective field limit, and thus agree completely with GR in the classical limit and weak gravity, and additionally are candidates as fundamental theories because they do not rely on perturbative renormalization by power counting and thus are expected to be useful to arbitrarily high energies. Additionally it is not wildly irresponsible to think that mathematical research (perhaps not driven by physics!) will produce a tractable renormalization that does not require nature to select a convenient effect to suppress the explosion of parameters at high energies. > I’d love to see an experimental setup that was capable of detecting the gravitational field of a single molecule: that would be impressive! Everyone would. We're down below milligrams and yoctoNewtons: https://arxiv.org/abs/1602.07539 https://arxiv.org/abs/1602.07539 http://newscenter.lbl.gov/2014/06/26/smallest-force-ever-measured/ http://newscenter.lbl.gov/2014/06/26/smallest-force-ever-mea... I'm not as au fait about how the other side of the tunnel is approaching the ultimate meeting point, but it's not unreasonable to think of nanogram masses in superposition. Experiments were only at thousands of atomic mass units a few years ago, though: https://arxiv.org/abs/1310.8343 https://arxiv.org/abs/1310.8343 Unfortunately General Relativity can only have the whole gravitational influence of these molecules go through one or the other slits. However all of the quantum theories in my previous message have the distribution of the gravitational influence follow distribution of the matter, as one would expect.

My impression was that QFT includes gravity and "only" breaks down when things get extreme, i.e. black holes/big bang/etc.

by pka 10y ago

My impression was that QFT includes gravity and "only" breaks down when things get extreme, i.e. black holes/big bang/etc.

pja 10y ago

No, there is no quantum theory of gravity & QFT doesn’t include gravity at all. The best you can do is add gravitational effects as an after-thought, but the things you might analyse with the full QFT are generally far too small & short lived for gravitational effects to matter anyway.

raattgift 10y ago
> No, there is no quantum theory of gravity Here's a couple of examples of quantum theories of gravity. http://www.phys.lsu.edu/faculty/pullin/talks/pire1.pdf http://www.phys.lsu.edu/faculty/pullin/talks/pire1.pdf http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pdf http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pd... http://einrichtungen.ph.tum.de/T31/seminars-past/seminar-talks/FrankSaueressig13.pdf http://einrichtungen.ph.tum.de/T31/seminars-past/seminar-tal... http://www.damtp.cam.ac.uk/research/gr/public/qg_ss.html http://www.damtp.cam.ac.uk/research/gr/public/qg_ss.html The extent to which these are complete, consistent, natural (in the fine-tuning sense), and so forth is debatable but these are certainly existing examples of quantum theories of gravity, and indeed the first is a perfectly reasonable Effective Field Theory that people work in regularly. > QFT doesn’t include gravity at all I think you mean "The Standard Model of Particle Physics", which is a quantum field theory (as is e.g. perturbative quantum gravity). > the things you might analyse with the full QFT Atoms and molecules have gravitational fields; when you send one through a double slit, which way does their gravitational field go? http://www.nature.com/nnano/journal/v7/n5/abs/nnano.2012.34.html http://www.nature.com/nnano/journal/v7/n5/abs/nnano.2012.34.... A quantum theory of gravity is needed to answer that. Assemble a huge number of particles in superposition with 1/(sqrt 2) (|M @ a> + |M @ b>), with a & b separated. A quantum theory of gravity is needed to describe the gravitational influence of M on a small test object (General Relativity's answer is just wrong :( ).
- pja 10y ago
  I’m not sure I believe that a mathematical formalism that is unable to make useful real world predictions deserves the “theory” moniker. Hence your list of quantum theories of gravity aren’t. But perhaps that’s me being picky :) > I think you mean "The Standard Model of Particle Physics", which is a quantum field theory (as is e.g. perturbative quantum gravity). Sure: I was just quoting the parent comment & using the term informally to stand in for the mouthful that is TSMoPP. (I’d love to see an experimental setup that was capable of detecting the gravitational field of a single molecule: that would be impressive!)
  - raattgift 10y ago
    > I’m not sure I believe that a mathematical formalism that is unable to make useful real world predictions deserves the “theory” moniker. Hence your list of quantum theories of gravity aren’t. On the contrary, the ones I listed are all completely in accord with General Relativity up to strong gravity and absent superposed sources, which is found from studying the renormalization group flow of perturbative quantum gravity and is four loops of gravitons in a 3+1 dimensional spacetime. Strong gravity can only be found very close to the singularity of black holes (and well inside event horizons, except at the final evaporation), or in the very early universe. So we're good for neutron stars, and have no problems studying things around the event horizons of astrophysical black holes. The only new mathematical formalism in perturbative quantum gravity is renormalization, and that goes back to the 1980s. Perturbative quantum gravity itself comes from the 1990s. Sean Carroll has a good explanation of renormalization and effective field theory here: http://www.preposterousuniverse.com/blog/2013/06/20/how-quantum-field-theory-becomes-effective/ http://www.preposterousuniverse.com/blog/2013/06/20/how-quan... Asymptotically safe gravity posits an ultraviolet fixed point at which one can take a finite number of measurements, producing a strong gravity completion that perturbative renormalzation cannot; this is prompted by asymptotic safety in QCD. Below that limit, ASG completely matches perturbative quantum gravity, and so in the EFT limit it's the same as General Relativity. There are five or six of other viable families of quantum theories of gravity, where viability means they accord exactly with perturbatively quantized General Relativity in its effective field limit, and thus agree completely with GR in the classical limit and weak gravity, and additionally are candidates as fundamental theories because they do not rely on perturbative renormalization by power counting and thus are expected to be useful to arbitrarily high energies. Additionally it is not wildly irresponsible to think that mathematical research (perhaps not driven by physics!) will produce a tractable renormalization that does not require nature to select a convenient effect to suppress the explosion of parameters at high energies. > I’d love to see an experimental setup that was capable of detecting the gravitational field of a single molecule: that would be impressive! Everyone would. We're down below milligrams and yoctoNewtons: https://arxiv.org/abs/1602.07539 https://arxiv.org/abs/1602.07539 http://newscenter.lbl.gov/2014/06/26/smallest-force-ever-measured/ http://newscenter.lbl.gov/2014/06/26/smallest-force-ever-mea... I'm not as au fait about how the other side of the tunnel is approaching the ultimate meeting point, but it's not unreasonable to think of nanogram masses in superposition. Experiments were only at thousands of atomic mass units a few years ago, though: https://arxiv.org/abs/1310.8343 https://arxiv.org/abs/1310.8343 Unfortunately General Relativity can only have the whole gravitational influence of these molecules go through one or the other slits. However all of the quantum theories in my previous message have the distribution of the gravitational influence follow distribution of the matter, as one would expect.