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"Every other force besides gravity is polarised, so why not also gravity?" Gravity is mediated by a spin-2 gauge boson, the graviton, whereas the other known f

"Every other force besides gravity is polarised, so why not also gravity?" Gravity is mediated by a spin-2 gauge boson, the graviton, whereas the other known fundamental forces are mediated by vector (spin-1) bosons. There is an exercise in Peskin & Schroeder (I believe) working out the lack of a gravitation "repulsion" resulting from this difference. That being said, I think this kind of phenomenology is excellent and raises lots of interesting questions. There may be reason to rule out this author's new idea, or maybe rethink an established tenet. That's why science is such an exciting field.

zamalek 8y ago
> raises lots of interesting questions Exactly. What I liked about MOND was that it basically amounted to "we don't know." The populist notion that dark matter is conclusively matter, and dark energy is energy seems a bold claim for such a young species. Crazy theories like non-universal constants are probably crazy, but they do explore unconventional lines of thought. One of those, most likely an unknown unknown, may be answer. Focusing all our energy on one explanation (that has been falsified more than once) seems like a waste.
- acqq 8y ago
  “What’s most impressive is that the predictions of dark matter were first made in the 1970s and 1980s, and were observationally confirmed later. This is not a case of tweaking the model to fit the data; this is a case of the best kind of science you hope for: where you make predictions, make the observations, and what you see validates and confirms the predictions you had made. And yet, even 35 years later, there are no modifications of gravity that achieve the galaxy-scale successes of MOND that also explain these other observations. The best tests of dark matter vs. MOND, which are on large, cosmic scales, have a clear winner and a clear loser.” https://medium.com/starts-with-a-bang/theres-a-debate-raging-over-whether-dark-matter-is-real-but-one-side-is-cheating-d25f28319d70 https://medium.com/starts-with-a-bang/theres-a-debate-raging... In short: looking at everything we can observe and can model, we do know that the “dark matter” is matter. Because we first made predictions (“it should behave as matter”), then confirmed them, many times. And MOND can’t even explain that what’s already observed.
  - guscost 8y ago
    "Every genuine test of a theory is an attempt to falsify it, or to refute it. Testability is falsifiability; but there are degrees of testability: some theories are more testable, more exposed to refutation, than others; they take, as it were, greater risks." - Karl Popper
  - zamalek 8y ago
    That's putting the effect before the cause. The dark matter map can also be a map of "how wrong we are," not only "how much invisible mass is there." An incomplete periodic table of physics, if you will. We need to produce a WIMP here, on Earth, and watch it decay. That's a "thruthifiable" experiment, but it'd be enough to convince me that we've found the correct line of thinking (which the popular opinion could very well be, don't get me wrong - I simply believe that we're overconfident).
    - acqq 8y ago
      > We need to produce a WIMP here, on Earth, and watch it decay. No. You can't "need" something if that is simply unreachable. There’s reasonable chance that to produce the particles (which still don’t have to be the WIMPs as we imagined them, and still would be „matter“) the energies needed are simply beyond those that will ever be available to humanity. It wold not mean more than that: that we, humans, just can't repeat with our technology the extreme conditions that the Universe had/has. The WIMPs in the form discoverable by the energies that are available to us at the moment would have been nice luck, but the existence of dark matter is not anything less certain when the dark matter is simply different from that. It would still be "matter" and not something else.
- antidesitter 8y ago
  > that has been falsified more than once What has been falsified more than once?
  - zamalek 8y ago
    We haven't observed any of the predicted WIMPs at the LHC, as one example. They were subsequently relegated to a higher energy level.
    - acqq 8y ago
      That doesn’t disprove the “matter-like” observed behavior, it just shows that the particular assumptions under which exactly these non-found WIMPs were imagined were false.
      - dnautics 8y ago
        If you have an infinite space of particle parameters and a series of tests keeps eliminating large swaths of this space, at some point it feels like you're playing a god of inbetweens game a la creationists arguing using gaps in the fossil record. There is a categorical difference, of course, but as a practical matter the parallels are useful, at what point do we table (for now) the search for the populist position and break for alternatives?
        acqq 8y ago
        It's not "populist" anything. It's the best science that can be done: https://medium.com/starts-with-a-bang/five-reasons-we-think-dark-matter-exists-a122bd606ba8 https://medium.com/starts-with-a-bang/five-reasons-we-think-... "Five Reasons We Think Dark Matter Exists: No other idea explains even two of these." "1.) Galaxy Clusters" "2.) Galactic Rotation Curves" "3.) The Cosmic Microwave Background" "4.) The Bullet Cluster" "5.) Large-Scale Structure Formation" From these 5, only dark matter matches all. Whereas MOND theories match only one, the "item number 2" and only as an attempt to after-the-fact-explanation of what's anyway observed, not as any prediction of anything new. As I've quoted in my another message here: "the predictions of dark matter were first made in the 1970s and 1980s, and were observationally confirmed later. This is not a case of tweaking the model to fit the data; this is a case of the best kind of science you hope for: where you make predictions, make the observations, and what you see validates and confirms the predictions you had made." Moreover, there's nothing "large" of the possible space that is now excluded. The possible space includes everything that "behaves like matter". What's (still just "probably") eliminated is just what were for us, humans, in this particular moment of our history very convenient to happen: that the energies needed are exactly such that WIMPs that were conveniently postulated to be such can be generated by the device which design and maintenance costs less than 2% of the total cost of the F-35 fighter planes (the money invested and planned to be invested in F-35 is 75 times bigger than the international investment in LHC). It would have been luck if it had happened, but the possibilities are much bigger and include such which are going to remain physically unreachable to be produced by our technologies. That we, humans, can't produce some particles doesn't disprove that dark matter exists when we have precise observations that can only be explained by dark matter and effectively by nothing else.
- bmh100 8y ago
  The Bullet Cluster seems pretty conclusive though.
raverbashing 8y ago
Given there have been no detections of individual gravitons, saying it's a spin-2 gauge boson is speculation at best (though it might be what the model predicts)
- duality 8y ago
  If it weren't, general relativity would be wrong as well and we know that (in its domain of applicability) it's a highly accurate model. GR models gravity as fluctuations in the metric tensor of spacetime, which is a symmetric rank-2 tensor. This is all you need to know to conclude that a graviton (a quantize fluctation of that tensor) must be spin-2. https://en.wikipedia.org/wiki/Graviton https://en.wikipedia.org/wiki/Graviton mentions "[I]t can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field would couple to the stress–energy tensor in the same way that gravitational interactions do. This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton." It links to other resources from there.
  - raverbashing 8y ago
    Yes but while we don't have the unification between QM and GR we don't have the full picture. We know what fits GM, but we don't know if it's the only solution.
  - codeflo 8y ago
    Well, in classical GR, which is still our best and most accurate theory of gravity, gravity is only an apparent force and thus isn't (or can't be) mediated by anything. Stating as fact that a graviton must exist or otherwise GR "would be wrong" is a bit of stretch IMHO. The graviton is plausible speculation, backed by some theoretical justifications, but nothing more yet.
    - gpderetta 8y ago
      disclaimer: IANAP. as far as I understand, the space-time curvature is just an interpretation of the math. Another perfectly acceptable interpretation is as a relativistic classical field that couples with the stress-energy tensor field (thus, indirectly, with any other field). In fact I think it is possible to come up with a space-time curvature interpretation of the EM field, for a toy universe where everything has a EM charge. Now, given that the gravitational field couples with other fundamental fields and those are quantized, the gravitational field must necessarily also be quantized. The math straight forward quantization works fine for low energies, thus the spin-2 boson described elsethread. The issue is that at low energy, the gravitational effects are so small that it is impossible to come up with an experiment that would detect the difference between a classical field and a quantized field. At high energy the straightforward derivation breaks down because of infinities (other quantum fields had similar issues, but the math tricks used to resolve them do not work with the gravitational field). There are multiple theories (string theory, loop gravity, etc) that try to resolve this problems, but the experimental apparatus required to distinguish between them are colossal (as in particle accelerators with radii measured in AU).
AnimalMuppet 8y ago
Um, correct my amateur understanding if it's wrong, but... we don't actually have a working quantum theory of gravity, do we? So how certain are we that gravity is mediated by a spin-2 gauge boson?
- deleted 8y ago
  [deleted]
- duality 8y ago
  Yes, because they are fluctuations of the metric tensor field, a symmetric rank-2 tensor. Take a look at https://en.wikipedia.org/wiki/Graviton#Gravitons_and_renormalization https://en.wikipedia.org/wiki/Graviton#Gravitons_and_renorma... for details.
  - AnimalMuppet 8y ago
    That does not appear to have any details that relate to your comment whatsoever. All it says is that renormalization doesn't work on gravitons. However, if I understand your comment correctly, you're saying that the underlying non-quantum theory (GR) uses a symmetric rank-2 tensor, therefore you have to have a spin 2 particle - as opposed to photons, whose underlying non-quantum theory has an antisymmetric rank-2 tensor, and therefore give rise to a spin 1 particle. Do I have that right? If so, could you point me to something that would explain why?
    - krastanov 8y ago
      I recollect that Feynman's Lectures on Gravitation (Frontiers in Physics) has an amazing and detailed explanation.
      - drjesusphd 8y ago
        "Quantum field theory in a nutshell" also covers this well.
    - AnimalMuppet 8y ago
blincoln 8y ago
My understanding is that the entire existence of the graviton is still theoretical. Has that changed recently?
- aurailious 8y ago
  I don't think its even a part of the standard model, still just a hypothesis.
- duality 8y ago
  Please see my comment above. You can use the language of gravitons and Feynman diagrams even for classical gravitational physics, but the calculations will be intractable for strong fields. See https://en.wikipedia.org/wiki/Graviton#Gravitons_and_renormalization https://en.wikipedia.org/wiki/Graviton#Gravitons_and_renorma... for example
- auntienomen 8y ago
  It's hypothetical, but less so than a lot of the things theoretical physicists consider. Any quantum theory which reduces to general relativity in the classical limit will look like a theory of gravitons when quantum effects are small. This is just how the math works out in effective quantum field theories involving massless spin-2 particles.
lmilcin 8y ago
The definitions you brought are tailored to what is our best understanding of the universe. Nobody ever said that these are immutable. Newtonian physics also must have felt rock solid, time and time again explaining new phenomena until it did not and there came about a better explanation.
- dwaltrip 8y ago
  Sidenote, but I think it is very important to emphasize that Newtonian physics is actually still quite rock-solid under certain conditions. These conditions are actually quite common and useful for humans. Many engineers utilize Newtonian physics everyday.
FabHK 8y ago
That's what the article (https://arxiv.org/pdf/1712.07962.pdf https://arxiv.org/pdf/1712.07962.pdf) says: > In theories of quantum gravity, gravitation is mediated by the graviton – a massless, spin-2, boson. This means that any pair of negative masses would attract, and not repel as suggested in this theory. However, there are also theoretical arguments that gravitons cannot, and will not ever, be detected experimentally (Rothman & Boughn 2006). There appear to be two options: either it is possible that the graviton could be modelled as a bound state of a positive and a negative mass, in a theory of composite gravity or some other mechanism which provides a modification of graviton properties. Alternatively, this could also indicate that the proposed theory cannot be modelled by real, physical, particles, but rather by the presence of effective negative masses within a superseding theory.

zamalek 8y ago

> raises lots of interesting questions Exactly. What I liked about MOND was that it basically amounted to "we don't know." The populist notion that dark matter is conclusively matter, and dark energy is energy seems a bold claim for such a young species. Crazy theories like non-universal constants are probably crazy, but they do explore unconventional lines of thought. One of those, most likely an unknown unknown, may be answer. Focusing all our energy on one explanation (that has been falsified more than once) seems like a waste.

acqq 8y ago
“What’s most impressive is that the predictions of dark matter were first made in the 1970s and 1980s, and were observationally confirmed later. This is not a case of tweaking the model to fit the data; this is a case of the best kind of science you hope for: where you make predictions, make the observations, and what you see validates and confirms the predictions you had made. And yet, even 35 years later, there are no modifications of gravity that achieve the galaxy-scale successes of MOND that also explain these other observations. The best tests of dark matter vs. MOND, which are on large, cosmic scales, have a clear winner and a clear loser.” https://medium.com/starts-with-a-bang/theres-a-debate-raging-over-whether-dark-matter-is-real-but-one-side-is-cheating-d25f28319d70 https://medium.com/starts-with-a-bang/theres-a-debate-raging... In short: looking at everything we can observe and can model, we do know that the “dark matter” is matter. Because we first made predictions (“it should behave as matter”), then confirmed them, many times. And MOND can’t even explain that what’s already observed.
- guscost 8y ago
  "Every genuine test of a theory is an attempt to falsify it, or to refute it. Testability is falsifiability; but there are degrees of testability: some theories are more testable, more exposed to refutation, than others; they take, as it were, greater risks." - Karl Popper
- zamalek 8y ago
  That's putting the effect before the cause. The dark matter map can also be a map of "how wrong we are," not only "how much invisible mass is there." An incomplete periodic table of physics, if you will. We need to produce a WIMP here, on Earth, and watch it decay. That's a "thruthifiable" experiment, but it'd be enough to convince me that we've found the correct line of thinking (which the popular opinion could very well be, don't get me wrong - I simply believe that we're overconfident).
  - acqq 8y ago
    > We need to produce a WIMP here, on Earth, and watch it decay. No. You can't "need" something if that is simply unreachable. There’s reasonable chance that to produce the particles (which still don’t have to be the WIMPs as we imagined them, and still would be „matter“) the energies needed are simply beyond those that will ever be available to humanity. It wold not mean more than that: that we, humans, just can't repeat with our technology the extreme conditions that the Universe had/has. The WIMPs in the form discoverable by the energies that are available to us at the moment would have been nice luck, but the existence of dark matter is not anything less certain when the dark matter is simply different from that. It would still be "matter" and not something else.
antidesitter 8y ago
> that has been falsified more than once What has been falsified more than once?
- zamalek 8y ago
  We haven't observed any of the predicted WIMPs at the LHC, as one example. They were subsequently relegated to a higher energy level.
  - acqq 8y ago
    That doesn’t disprove the “matter-like” observed behavior, it just shows that the particular assumptions under which exactly these non-found WIMPs were imagined were false.
    - dnautics 8y ago
      If you have an infinite space of particle parameters and a series of tests keeps eliminating large swaths of this space, at some point it feels like you're playing a god of inbetweens game a la creationists arguing using gaps in the fossil record. There is a categorical difference, of course, but as a practical matter the parallels are useful, at what point do we table (for now) the search for the populist position and break for alternatives?
      - acqq 8y ago
        It's not "populist" anything. It's the best science that can be done: https://medium.com/starts-with-a-bang/five-reasons-we-think-dark-matter-exists-a122bd606ba8 https://medium.com/starts-with-a-bang/five-reasons-we-think-... "Five Reasons We Think Dark Matter Exists: No other idea explains even two of these." "1.) Galaxy Clusters" "2.) Galactic Rotation Curves" "3.) The Cosmic Microwave Background" "4.) The Bullet Cluster" "5.) Large-Scale Structure Formation" From these 5, only dark matter matches all. Whereas MOND theories match only one, the "item number 2" and only as an attempt to after-the-fact-explanation of what's anyway observed, not as any prediction of anything new. As I've quoted in my another message here: "the predictions of dark matter were first made in the 1970s and 1980s, and were observationally confirmed later. This is not a case of tweaking the model to fit the data; this is a case of the best kind of science you hope for: where you make predictions, make the observations, and what you see validates and confirms the predictions you had made." Moreover, there's nothing "large" of the possible space that is now excluded. The possible space includes everything that "behaves like matter". What's (still just "probably") eliminated is just what were for us, humans, in this particular moment of our history very convenient to happen: that the energies needed are exactly such that WIMPs that were conveniently postulated to be such can be generated by the device which design and maintenance costs less than 2% of the total cost of the F-35 fighter planes (the money invested and planned to be invested in F-35 is 75 times bigger than the international investment in LHC). It would have been luck if it had happened, but the possibilities are much bigger and include such which are going to remain physically unreachable to be produced by our technologies. That we, humans, can't produce some particles doesn't disprove that dark matter exists when we have precise observations that can only be explained by dark matter and effectively by nothing else.
bmh100 8y ago
The Bullet Cluster seems pretty conclusive though.

raverbashing 8y ago

Given there have been no detections of individual gravitons, saying it's a spin-2 gauge boson is speculation at best (though it might be what the model predicts)

duality 8y ago
If it weren't, general relativity would be wrong as well and we know that (in its domain of applicability) it's a highly accurate model. GR models gravity as fluctuations in the metric tensor of spacetime, which is a symmetric rank-2 tensor. This is all you need to know to conclude that a graviton (a quantize fluctation of that tensor) must be spin-2. https://en.wikipedia.org/wiki/Graviton https://en.wikipedia.org/wiki/Graviton mentions "[I]t can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field would couple to the stress–energy tensor in the same way that gravitational interactions do. This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton." It links to other resources from there.
- raverbashing 8y ago
  Yes but while we don't have the unification between QM and GR we don't have the full picture. We know what fits GM, but we don't know if it's the only solution.
- codeflo 8y ago
  Well, in classical GR, which is still our best and most accurate theory of gravity, gravity is only an apparent force and thus isn't (or can't be) mediated by anything. Stating as fact that a graviton must exist or otherwise GR "would be wrong" is a bit of stretch IMHO. The graviton is plausible speculation, backed by some theoretical justifications, but nothing more yet.
  - gpderetta 8y ago
    disclaimer: IANAP. as far as I understand, the space-time curvature is just an interpretation of the math. Another perfectly acceptable interpretation is as a relativistic classical field that couples with the stress-energy tensor field (thus, indirectly, with any other field). In fact I think it is possible to come up with a space-time curvature interpretation of the EM field, for a toy universe where everything has a EM charge. Now, given that the gravitational field couples with other fundamental fields and those are quantized, the gravitational field must necessarily also be quantized. The math straight forward quantization works fine for low energies, thus the spin-2 boson described elsethread. The issue is that at low energy, the gravitational effects are so small that it is impossible to come up with an experiment that would detect the difference between a classical field and a quantized field. At high energy the straightforward derivation breaks down because of infinities (other quantum fields had similar issues, but the math tricks used to resolve them do not work with the gravitational field). There are multiple theories (string theory, loop gravity, etc) that try to resolve this problems, but the experimental apparatus required to distinguish between them are colossal (as in particle accelerators with radii measured in AU).

AnimalMuppet 8y ago

Um, correct my amateur understanding if it's wrong, but... we don't actually have a working quantum theory of gravity, do we? So how certain are we that gravity is mediated by a spin-2 gauge boson?

deleted 8y ago
[deleted]
duality 8y ago
Yes, because they are fluctuations of the metric tensor field, a symmetric rank-2 tensor. Take a look at https://en.wikipedia.org/wiki/Graviton#Gravitons_and_renormalization https://en.wikipedia.org/wiki/Graviton#Gravitons_and_renorma... for details.
- AnimalMuppet 8y ago
  That does not appear to have any details that relate to your comment whatsoever. All it says is that renormalization doesn't work on gravitons. However, if I understand your comment correctly, you're saying that the underlying non-quantum theory (GR) uses a symmetric rank-2 tensor, therefore you have to have a spin 2 particle - as opposed to photons, whose underlying non-quantum theory has an antisymmetric rank-2 tensor, and therefore give rise to a spin 1 particle. Do I have that right? If so, could you point me to something that would explain why?
  - krastanov 8y ago
    I recollect that Feynman's Lectures on Gravitation (Frontiers in Physics) has an amazing and detailed explanation.
    - drjesusphd 8y ago
      "Quantum field theory in a nutshell" also covers this well.
  - AnimalMuppet 8y ago

blincoln 8y ago

My understanding is that the entire existence of the graviton is still theoretical. Has that changed recently?

aurailious 8y ago
I don't think its even a part of the standard model, still just a hypothesis.
duality 8y ago
Please see my comment above. You can use the language of gravitons and Feynman diagrams even for classical gravitational physics, but the calculations will be intractable for strong fields. See https://en.wikipedia.org/wiki/Graviton#Gravitons_and_renormalization https://en.wikipedia.org/wiki/Graviton#Gravitons_and_renorma... for example
auntienomen 8y ago
It's hypothetical, but less so than a lot of the things theoretical physicists consider. Any quantum theory which reduces to general relativity in the classical limit will look like a theory of gravitons when quantum effects are small. This is just how the math works out in effective quantum field theories involving massless spin-2 particles.

lmilcin 8y ago

The definitions you brought are tailored to what is our best understanding of the universe. Nobody ever said that these are immutable. Newtonian physics also must have felt rock solid, time and time again explaining new phenomena until it did not and there came about a better explanation.

dwaltrip 8y ago
Sidenote, but I think it is very important to emphasize that Newtonian physics is actually still quite rock-solid under certain conditions. These conditions are actually quite common and useful for humans. Many engineers utilize Newtonian physics everyday.

FabHK 8y ago

That's what the article (https://arxiv.org/pdf/1712.07962.pdf https://arxiv.org/pdf/1712.07962.pdf) says: > In theories of quantum gravity, gravitation is mediated by the graviton – a massless, spin-2, boson. This means that any pair of negative masses would attract, and not repel as suggested in this theory. However, there are also theoretical arguments that gravitons cannot, and will not ever, be detected experimentally (Rothman & Boughn 2006). There appear to be two options: either it is possible that the graviton could be modelled as a bound state of a positive and a negative mass, in a theory of composite gravity or some other mechanism which provides a modification of graviton properties. Alternatively, this could also indicate that the proposed theory cannot be modelled by real, physical, particles, but rather by the presence of effective negative masses within a superseding theory.