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I don't get why people are so attracted to idea of "modified gravity". So far it is a body of knowledge without predictive power struggling to adjust itself to

by Svoka 5y ago

I don't get why people are so attracted to idea of "modified gravity". So far it is a body of knowledge without predictive power struggling to adjust itself to things predicted by GR.

enkid 5y ago
Is that a swipe against the article? I think she would argue Dark Matter is so over parametrized it no longer makes meaningful predictions. And the argument that any new particle requires a new quantum field makes total sense. So Dark Matter is Modified Gravity, there's not clear distinction because of the need for new fields anyways. Of course, IANAP, so I could be totally misreading it.
- akvadrako 5y ago
  The difference between dark matter and modified gravity is how it varies throughout space. Modified gravity is a formula that applies exactly at every point based on other mass, like Newtonian gravity F=G(m₁m₂)/r^2. The density of dark matter varies throughout space and it isn't based on a simple formula, but all the complicated dynamics of the past.
- Svoka 5y ago
  How exactly Dark Matter is overparameterized? So far there was just constraints on what it could be from actual physical representation, not from what effects it has in terms of spacetime curvature.
  - enkid 5y ago
    I'm not a physicist, I'm just trying to represent what's in the article.
joeberon 5y ago
One huge motivation is the disturbingly fierce incompatibility between the Standard Model and GR
- bunje 5y ago
  How are the theories incompatible? My basic understanding is that GR describes how the space looks like and Standard Model describes fields defined in this space.
  - joeberon 5y ago
    That's a very complicated question really and kind of beyond my specialisation, but it mostly comes down to something called "renormalization", which is a technique used to construct a Quantum Field Theory. Unfortunately it doesn't seem to be possible to construct a renormalizable field theory for gravity, which makes it seem incompatible with the approach for the Standard Model. In their basic forms, GR and the Standard Model are basically entirely different languages, as you said, one using a model of motion in a curved spacetime, and the other using a formalism of quantum fields. However this brings up a big question: why are the electromagnetic, weak, and strong interactions described by a totally different formalism to gravity? You can do quantum field theory in a curved spacetime, but that is more of a band-aid approach to the problem than anything and doesn't really help to unify the two approaches. It could be that the answer is that two different Gods designed gravity and the other forces, they certainly seem that disparate. But as a Physicist is it quite hard to accept that gravity and the other interactions are fundamentally separate, we would like a full theory of all four of the interactions. EDIT: I should point out that QFT is hardly very nice anyway. We don't have evidence for a specific ontology/interpretation for Quantum Mechanics yet, we don't know what Quantum Mechanics is or means. We may come closer in the years coming though.
    - raattgift 5y ago
      Firstly, I note your > as a Physicist So, while I could let your "basic forms" do some heavy lifting in your second sentence, I would like to draw your attention to: > entirely different languages First, let's start with the Hamiltonian formulation : https://en.wikipedia.org/wiki/Initial_value_formulation_(general_relativity) https://en.wikipedia.org/wiki/Initial_value_formulation_(gen... which leads us to https://en.wikipedia.org/wiki/Canonical_quantum_gravity https://en.wikipedia.org/wiki/Canonical_quantum_gravity cf. https://en.wikipedia.org/wiki/Second_quantization https://en.wikipedia.org/wiki/Second_quantization or if you're not looking deep into compact objects (astrophysically) or concerned with theoretical UV divergence problems, Perturbative quantum gravity (quick lecture) https://webspace.science.uu.nl/~hooft101/lectures/erice02.pdf https://webspace.science.uu.nl/~hooft101/lectures/erice02.pd... vs https://en.wikipedia.org/wiki/Perturbation_theory_(quantum_mechanics)#Strong_perturbation_theory https://en.wikipedia.org/wiki/Perturbation_theory_(quantum_m... Numerical methods can be even more similar: there's several approaches to gravitation on the lattice, for instance, that would be familiar to lattice QCD people. Programmes to make use of objects used by HEP (Lie algebras, configuration/phase/state spaces) for strong gravity are not accidental. > You can do quantum field theory in a curved spacetime Birrell & Davies likes to stress "Curved Space" (as in the textbook's title). It's not a band-aid at all, it's a first approximation, up to the so-called one-loop level. There are higher-order approximations. It's not the 1960s any more, and especially after the 1982 Nobel (the same year as Birrell & Davies was published), I think it's not super-controversial to argue that every good physical theory is an effective theory, even if the characteristic scale has not been determined. > why are the electromagnetic, weak, and strong interactions described by a totally different formalism to gravity? > certainly seem disparate Teaching tradition! Also fossilized successes of https://en.wikipedia.org/wiki/Abductive_reasoning https://en.wikipedia.org/wiki/Abductive_reasoning But to me a variational approach on the Ricci curvature tensor (following Pirani https://journals.aps.org/pr/abstract/10.1103/PhysRev.105.1089 https://journals.aps.org/pr/abstract/10.1103/PhysRev.105.108... ) and on the Faraday electromagnetic tensor (Bondi & Pirani started this in http://www.theory.physics.ubc.ca/530-19/planewave-bondi.pdf http://www.theory.physics.ubc.ca/530-19/planewave-bondi.pdf ) are very similar, not very disparate. Indeed, you can see how one arrives at the spin-2 gauge boson for the former (symmetric rank-2 tensor) in the same way one arrived at the spin-1 gauge boson for the (totally antisymmetric rank-2) Faraday tensor. But this is certainly not a successful approach to a "full theory of all four of the interactions", however it led to "just" an effective field theory that is good to shorter lengths than we likely will be able to probe any time soon.
idlewords 5y ago
The appeal of modified gravity lies in all the observations Sabine outlines here, that don't fit our understanding. You need something to explain them, and the choice is either positing lots of invisible stuff, or modifying the laws of gravity. It seems reasonable to try both approaches, particularly since positing lots of invisible stuff has a pretty checkered history in science.

by Svoka 5y ago

I don't get why people are so attracted to idea of "modified gravity". So far it is a body of knowledge without predictive power struggling to adjust itself to things predicted by GR.

enkid 5y ago

Is that a swipe against the article? I think she would argue Dark Matter is so over parametrized it no longer makes meaningful predictions. And the argument that any new particle requires a new quantum field makes total sense. So Dark Matter is Modified Gravity, there's not clear distinction because of the need for new fields anyways. Of course, IANAP, so I could be totally misreading it.

akvadrako 5y ago
The difference between dark matter and modified gravity is how it varies throughout space. Modified gravity is a formula that applies exactly at every point based on other mass, like Newtonian gravity F=G(m₁m₂)/r^2. The density of dark matter varies throughout space and it isn't based on a simple formula, but all the complicated dynamics of the past.
Svoka 5y ago
How exactly Dark Matter is overparameterized? So far there was just constraints on what it could be from actual physical representation, not from what effects it has in terms of spacetime curvature.
- enkid 5y ago
  I'm not a physicist, I'm just trying to represent what's in the article.

joeberon 5y ago

One huge motivation is the disturbingly fierce incompatibility between the Standard Model and GR

bunje 5y ago
How are the theories incompatible? My basic understanding is that GR describes how the space looks like and Standard Model describes fields defined in this space.
- joeberon 5y ago
  That's a very complicated question really and kind of beyond my specialisation, but it mostly comes down to something called "renormalization", which is a technique used to construct a Quantum Field Theory. Unfortunately it doesn't seem to be possible to construct a renormalizable field theory for gravity, which makes it seem incompatible with the approach for the Standard Model. In their basic forms, GR and the Standard Model are basically entirely different languages, as you said, one using a model of motion in a curved spacetime, and the other using a formalism of quantum fields. However this brings up a big question: why are the electromagnetic, weak, and strong interactions described by a totally different formalism to gravity? You can do quantum field theory in a curved spacetime, but that is more of a band-aid approach to the problem than anything and doesn't really help to unify the two approaches. It could be that the answer is that two different Gods designed gravity and the other forces, they certainly seem that disparate. But as a Physicist is it quite hard to accept that gravity and the other interactions are fundamentally separate, we would like a full theory of all four of the interactions. EDIT: I should point out that QFT is hardly very nice anyway. We don't have evidence for a specific ontology/interpretation for Quantum Mechanics yet, we don't know what Quantum Mechanics is or means. We may come closer in the years coming though.
  - raattgift 5y ago
    Firstly, I note your > as a Physicist So, while I could let your "basic forms" do some heavy lifting in your second sentence, I would like to draw your attention to: > entirely different languages First, let's start with the Hamiltonian formulation : https://en.wikipedia.org/wiki/Initial_value_formulation_(general_relativity) https://en.wikipedia.org/wiki/Initial_value_formulation_(gen... which leads us to https://en.wikipedia.org/wiki/Canonical_quantum_gravity https://en.wikipedia.org/wiki/Canonical_quantum_gravity cf. https://en.wikipedia.org/wiki/Second_quantization https://en.wikipedia.org/wiki/Second_quantization or if you're not looking deep into compact objects (astrophysically) or concerned with theoretical UV divergence problems, Perturbative quantum gravity (quick lecture) https://webspace.science.uu.nl/~hooft101/lectures/erice02.pdf https://webspace.science.uu.nl/~hooft101/lectures/erice02.pd... vs https://en.wikipedia.org/wiki/Perturbation_theory_(quantum_mechanics)#Strong_perturbation_theory https://en.wikipedia.org/wiki/Perturbation_theory_(quantum_m... Numerical methods can be even more similar: there's several approaches to gravitation on the lattice, for instance, that would be familiar to lattice QCD people. Programmes to make use of objects used by HEP (Lie algebras, configuration/phase/state spaces) for strong gravity are not accidental. > You can do quantum field theory in a curved spacetime Birrell & Davies likes to stress "Curved Space" (as in the textbook's title). It's not a band-aid at all, it's a first approximation, up to the so-called one-loop level. There are higher-order approximations. It's not the 1960s any more, and especially after the 1982 Nobel (the same year as Birrell & Davies was published), I think it's not super-controversial to argue that every good physical theory is an effective theory, even if the characteristic scale has not been determined. > why are the electromagnetic, weak, and strong interactions described by a totally different formalism to gravity? > certainly seem disparate Teaching tradition! Also fossilized successes of https://en.wikipedia.org/wiki/Abductive_reasoning https://en.wikipedia.org/wiki/Abductive_reasoning But to me a variational approach on the Ricci curvature tensor (following Pirani https://journals.aps.org/pr/abstract/10.1103/PhysRev.105.1089 https://journals.aps.org/pr/abstract/10.1103/PhysRev.105.108... ) and on the Faraday electromagnetic tensor (Bondi & Pirani started this in http://www.theory.physics.ubc.ca/530-19/planewave-bondi.pdf http://www.theory.physics.ubc.ca/530-19/planewave-bondi.pdf ) are very similar, not very disparate. Indeed, you can see how one arrives at the spin-2 gauge boson for the former (symmetric rank-2 tensor) in the same way one arrived at the spin-1 gauge boson for the (totally antisymmetric rank-2) Faraday tensor. But this is certainly not a successful approach to a "full theory of all four of the interactions", however it led to "just" an effective field theory that is good to shorter lengths than we likely will be able to probe any time soon.

idlewords 5y ago

The appeal of modified gravity lies in all the observations Sabine outlines here, that don't fit our understanding. You need something to explain them, and the choice is either positing lots of invisible stuff, or modifying the laws of gravity. It seems reasonable to try both approaches, particularly since positing lots of invisible stuff has a pretty checkered history in science.