5 ms·

We already know for a fact that quantum mechanics is an incomplete theory. It fails to accurately describe gravity. A fundamental force of nature. Therefore

We already know for a fact that quantum mechanics is an incomplete theory. It fails to accurately describe gravity. A fundamental force of nature. Therefore we should expect that it fails to explain other phenomena as well.

greglindahl 10y ago
Yes, and it doesn't appear that any of the weird things under discussion in this case have anything to do with a GUT or TOE. The important thing to remember is that theories like Newton's Law do a great job in a large number of regimes -- you have to look at things like the precession of Mercury or an atomic clock in orbit to see why Einstein is needed.
- lisivka 10y ago
  I believe, if we take in account wave-particle duality, then we will be able to explain large parts of General Relativity using Newtonian physics, because waves are propagating, so second postulate is obvious, for example.
  - duaneb 10y ago
    Newtonian laws are excellent for intuition about human-scale problems. What do we gain from modeling GR that way?
    - lisivka 10y ago
      Intuition at GR-scale problems. Why I need to believe in Einstein postulates if I can just look at walking droplet 2D model of QM and see that these postulates are obvious?
      - raattgift 10y ago
        Why do you need to consider Einstein's postulates if you can just look at a ball traversing a rubber sheet with a depression in it, for that matter? Same answer: because these rough analogues are not background-independent -- you would see a very different picture in each case if you moved the walking droplet or rubber sheet setups to the moon or to the ISS. Einstein GR postulate number one: microscopic physics is background-independent. These experiments' microscopic details behave the same on Earth, on the moon, and in the ISS, but their macroscopic behaviours sure don't. You have to input details of the local structure of spacetime into each of them if you want to predict their results (e.g., what trajectory the walk takes, or how long it takes for a ball to reach the bottom of a depression in the rubber sheet) successfully.
        duaneb 10y ago
        This was really well explained; thank you.
SomeStupidPoint 10y ago
Also, we know that QM (as it stands) doesn't have a good way to deal with topological effects and their (low-energy) contributions to various things. TQFTs in 3+1D are an active area of research. Heck, TQFTs in 2+1D are still under active research (and are behind Microsoft's quantum computer efforts).
Florin_Andrei 10y ago
Not sure why you're singling out QM like this. General relativity would also be incomplete in this sense. It fails to describe quantum phenomena. A better approach is to say that, to describe fundamental things in nature, what we have now is a patchwork of independent theories that are not quite meshing with each other.
- jcoffland 10y ago
  I'm singling out QM only because that was what the poster above mentioned. I completely agree that all of our theories are equally flawed. Our patchwork of theories has holes in it so we cannot immediately discount ideas just because they don't fit with any of our existing theories.
  - poelzi 10y ago
    In fact, we have two fundamental bad/wrong assumptions in both theories and that's why they can never be united without going into mathematical/physical mumbojumbo like strings theory. In fact, humans build unified models that are not abstract and seem to work wherever you look - they are just not discussed... See http://www.pnas.org/content/112/24/7426.abstract http://www.pnas.org/content/112/24/7426.abstract for the phenomena.
- raattgift 10y ago
  Perturbatively quantized gravity exists and completely reproduces General Relativity as a quantum phenomenon. Study of perturbatively quantized gravity is precisely why we know that General Relativity is not renormalizable by power-set counting in strong gravity, and precisely why we have a good definition for strong gravity (more than one loop of gravitons in a Feynman diagram) and also why we have a good idea about where to expect strong gravity (very close to the centre of mass-momentum of a black hole, and very close to the big bang). Although the behaviour of arbitrarily high energy and high density matter may be found in a very different theory from General Relativity, there is no presently known reason why approaches involving renormalization group flow (e.g. asymptotically safe gravity) cannot "save" perturbatively quantized gravity by providing a renormalization scheme that works into the UV limit. One could likewise point to GUT programmes and say that at those energy scales a theory very different from the Standard Model will be more useful. In the meanwhile, perturbatively quantized gravity is a perfectly good effective field theory, in the sense of the work by Kenneth G Wilson. The point you're replying to is better addressed this way: the Standard Model doesn't even properly consider gravity, and it simply does not need to if in the UV completion of General Relativity you can recover the Minkowski metric in sufficiently tiny regions of spacetime. That's because the Poincaré group, the symmetry group of region of spacetime with the Minkowski metric induced on it, is baked right into the group theory of the Standard Model, and into General Relativity. One can read this as the Standard Model subsuming Einsteinean gravitation into it at that level, and thanks to perturbatively quantized gravity one could equally pencil in a gluon on the particle zoo tables and do gauge theory on that, and expect consistent results outside strong gravity as described above. Finally, even if other symmetries are added (or the three matter ones are unified) in a Beyond the Standard Model Yang-Mills group theory, the couplings with the graviton outside strong gravity will be straightforward. So, I don't agree that the fundamental theories of gravity and matter are a patchwork, nor that they mesh poorly, even though we do not have a meshing that works to arbitrarily high energy levels. But crucially, those arbitrarily high energy levels might not even be physically observable; we aren't certain about that yet. Finally, whatever dark energy and dark matter are, how they generate the metric and how they interact with the rest of the Standard Model are subjects of various research programmes competing with radically different ideas for a theory of quantum gravity.
novalis78 10y ago
isn't that what the Lipinski brothers suggest they have figured out (http://adsabs.harvard.edu/abs/2008AcPPB..39.2823L http://adsabs.harvard.edu/abs/2008AcPPB..39.2823L) that was mentioned here just recently on HN (unifiedgravity.com)? - would be interesting to hear from someone with physics background to comment on this one

greglindahl 10y ago

Yes, and it doesn't appear that any of the weird things under discussion in this case have anything to do with a GUT or TOE. The important thing to remember is that theories like Newton's Law do a great job in a large number of regimes -- you have to look at things like the precession of Mercury or an atomic clock in orbit to see why Einstein is needed.

lisivka 10y ago
I believe, if we take in account wave-particle duality, then we will be able to explain large parts of General Relativity using Newtonian physics, because waves are propagating, so second postulate is obvious, for example.
- duaneb 10y ago
  Newtonian laws are excellent for intuition about human-scale problems. What do we gain from modeling GR that way?
  - lisivka 10y ago
    Intuition at GR-scale problems. Why I need to believe in Einstein postulates if I can just look at walking droplet 2D model of QM and see that these postulates are obvious?
    - raattgift 10y ago
      Why do you need to consider Einstein's postulates if you can just look at a ball traversing a rubber sheet with a depression in it, for that matter? Same answer: because these rough analogues are not background-independent -- you would see a very different picture in each case if you moved the walking droplet or rubber sheet setups to the moon or to the ISS. Einstein GR postulate number one: microscopic physics is background-independent. These experiments' microscopic details behave the same on Earth, on the moon, and in the ISS, but their macroscopic behaviours sure don't. You have to input details of the local structure of spacetime into each of them if you want to predict their results (e.g., what trajectory the walk takes, or how long it takes for a ball to reach the bottom of a depression in the rubber sheet) successfully.
      - duaneb 10y ago
        This was really well explained; thank you.

SomeStupidPoint 10y ago

Also, we know that QM (as it stands) doesn't have a good way to deal with topological effects and their (low-energy) contributions to various things. TQFTs in 3+1D are an active area of research. Heck, TQFTs in 2+1D are still under active research (and are behind Microsoft's quantum computer efforts).

Florin_Andrei 10y ago

Not sure why you're singling out QM like this. General relativity would also be incomplete in this sense. It fails to describe quantum phenomena. A better approach is to say that, to describe fundamental things in nature, what we have now is a patchwork of independent theories that are not quite meshing with each other.

jcoffland 10y ago
I'm singling out QM only because that was what the poster above mentioned. I completely agree that all of our theories are equally flawed. Our patchwork of theories has holes in it so we cannot immediately discount ideas just because they don't fit with any of our existing theories.
- poelzi 10y ago
  In fact, we have two fundamental bad/wrong assumptions in both theories and that's why they can never be united without going into mathematical/physical mumbojumbo like strings theory. In fact, humans build unified models that are not abstract and seem to work wherever you look - they are just not discussed... See http://www.pnas.org/content/112/24/7426.abstract http://www.pnas.org/content/112/24/7426.abstract for the phenomena.
raattgift 10y ago
Perturbatively quantized gravity exists and completely reproduces General Relativity as a quantum phenomenon. Study of perturbatively quantized gravity is precisely why we know that General Relativity is not renormalizable by power-set counting in strong gravity, and precisely why we have a good definition for strong gravity (more than one loop of gravitons in a Feynman diagram) and also why we have a good idea about where to expect strong gravity (very close to the centre of mass-momentum of a black hole, and very close to the big bang). Although the behaviour of arbitrarily high energy and high density matter may be found in a very different theory from General Relativity, there is no presently known reason why approaches involving renormalization group flow (e.g. asymptotically safe gravity) cannot "save" perturbatively quantized gravity by providing a renormalization scheme that works into the UV limit. One could likewise point to GUT programmes and say that at those energy scales a theory very different from the Standard Model will be more useful. In the meanwhile, perturbatively quantized gravity is a perfectly good effective field theory, in the sense of the work by Kenneth G Wilson. The point you're replying to is better addressed this way: the Standard Model doesn't even properly consider gravity, and it simply does not need to if in the UV completion of General Relativity you can recover the Minkowski metric in sufficiently tiny regions of spacetime. That's because the Poincaré group, the symmetry group of region of spacetime with the Minkowski metric induced on it, is baked right into the group theory of the Standard Model, and into General Relativity. One can read this as the Standard Model subsuming Einsteinean gravitation into it at that level, and thanks to perturbatively quantized gravity one could equally pencil in a gluon on the particle zoo tables and do gauge theory on that, and expect consistent results outside strong gravity as described above. Finally, even if other symmetries are added (or the three matter ones are unified) in a Beyond the Standard Model Yang-Mills group theory, the couplings with the graviton outside strong gravity will be straightforward. So, I don't agree that the fundamental theories of gravity and matter are a patchwork, nor that they mesh poorly, even though we do not have a meshing that works to arbitrarily high energy levels. But crucially, those arbitrarily high energy levels might not even be physically observable; we aren't certain about that yet. Finally, whatever dark energy and dark matter are, how they generate the metric and how they interact with the rest of the Standard Model are subjects of various research programmes competing with radically different ideas for a theory of quantum gravity.

novalis78 10y ago

isn't that what the Lipinski brothers suggest they have figured out (http://adsabs.harvard.edu/abs/2008AcPPB..39.2823L http://adsabs.harvard.edu/abs/2008AcPPB..39.2823L) that was mentioned here just recently on HN (unifiedgravity.com)? - would be interesting to hear from someone with physics background to comment on this one