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As I understand it (not being a physicist): the graviton, if it exists at all, would have to be a massless chargeless spin-2 boson for it to fit our understandi

As I understand it (not being a physicist): the graviton, if it exists at all, would have to be a massless chargeless spin-2 boson for it to fit our understanding of what particles are. But the graviton has never been observed, nor has any quantized behavior of gravity or its General Relativistic effects, so it doesn't count as an observation by String Theory; there are other models that also give rise to gravity-as-we've-observed-it, up to and including just gluing the Standard Model to General Relativity and calling it a day.

evanb 9y ago
While it's true that the graviton has never been observed, the point is that string theory was not, originally, a "theory of everything" candidate! It was originally a way of trying to understand QCD and strong dynamics. In 1974 people discovered that the massless chargeless spin-2 boson "popped out" of the theory accidentally! Since people knew that such a particle behaves like a graviton, and is manifestly quantum-mechanical, suddenly it occurred to them that string theory might be a theory of quantum gravity. In fact, General Relativity can be viewed simply as the low-energy effective theory of massless chargeless spin-2 bosons, as emphasized in Feynman's Lectures of Gravitation, where he derives GR from a particle-physicist point of view, rather than a differential-geometry point of view. This led to the tongue-in-cheek saying that string theory post-dicted the existence of gravity at all---since it isn't built into the foundational assumptions of the theory at all (which is more than a lot of the other models, such as the "gluing" you propose, can say). And since string theory is guaranteed to be quantum-mechanical, a lot of interest developed towards understanding it as a theory of quantum gravity (most other models fail to account for how GR can be made compatible with quantization and how the renormalization works sensibly).
- daxfohl 9y ago
  So given Higgs bosons exist, doesn't that account for mass? Why are gravitons needed as well?
  - techdragon 9y ago
    Gravity != Mass Gravity isn't the same as mass. It's related, but not equivalent.
    - daxfohl 9y ago
      So are gravitons related to higgs bosons in the same way? Or are gravitons, assuming higgs boson === inertial mass, more like the difference between (higgs bosons aka inertial mass) and gravitational mass (thus unrelated to / independent of higgs bosons)?
      - wayn3 9y ago
        gravity and inertia are 2 different properties of mass. inertia is conveyed by the higgs field. gravity is conveyed by the gravitational field. we know that those fields exist, because we can observe them. if the fields exist, the exchange particles (higgs boson and graviton) have to exist, too - or our theory is wrong. thats kind of circular reasoning, but we cant derive physics from first principles anyway.
        AstralStorm 9y ago
        Can you explain then why no gravitons have been found yet? Otherwise it is a baseless untestable assumption that the particle even exists. We probably could derive physics from first principles if we knew them.
        baq 9y ago
        We've only measured gravitational waves directly last year. The theoretical graviton is still ways off - the energies required are incomprehensible to mere mortals.
        wayn3 9y ago
        the graviton is too small to be observed directly by our machinery. if you can provide a detector the size of the solar system, proving/disproving its existence will be simple. we could derive anything from first principles if we knew them. theres nothing probable about that. thats the whole point of first principles. do you want me to call you captain obvious?
guygurari 9y ago
As far as I know, string theory is the only complete theory that incorporates these two observations: 1. Nature includes the force of gravity, described at low energies by Einstein's theory with a gravitational field. 2. Matter in Nature is quantum mechanical. We believe that spin 2 particles will exist in any such theory (as they do in string theory), but even without this assumption it is still difficult to come up with such a theory. The example you give of gluing the Standard Model to General Relativity does not achieve this goal and does not correctly describe nature. This is because the quantum matter of the Standard Model does not correctly back-react on the classical gravitational field. When we couple the classical theory of General Relativity to classical matter, the gravitational field pushes matter around, and the matter in turn back-reacts on the gravitational field and changes it. When coupling quantum matter to classical General Relativity, the classical gravitational field can act on matter just fine, but it does not work in the other direction. The problem is that when matter is in a quantum superposition, it is not clear how to update the classical gravitational field (which cannot itself be in superposition). This missing part of the interaction means that SM+GR is not a complete theory.
- evanb 9y ago
  Hey Guy! We should finish the horizon paper :D -- Evan
kobeya 9y ago
> But the graviton has never been observed, nor has any quantized behavior of gravity or its General Relativistic effects The MOND folks would disagree here. They'd say that the phenomenon we call "dark matter" and "dark energy" are the result of quantized gravity.
skookumchuck 9y ago
> would have to be a massless chargeless spin-2 boson Sure it's not a pointless mass?
zodiac 9y ago
I don't know much about this, but I've read that the standard model and GR are mutually inconsistent? How do you "glue them together"?
- PhantomGremlin 9y ago
  I also don't know much about this. But I don't think there's anything serious yet that glues them together. Years ago someone on slashdot made an attempt. It's silly humor. Your moma so fat even if I'd entangle with her no information would be able to leave her event horizon. Nobody has managed to put gravitation and QM together yet, and you want to do it in a your-momma-so-fat-joke? Wow. https://science.slashdot.org/story/09/02/19/2338245/human-eye-could-detect-spooky-action-at-a-distance https://science.slashdot.org/story/09/02/19/2338245/human-ey... Sigh. I couldn't resist. It's one of my favorites. Let the downvotes begin.
- freeflight 9y ago
  >standard model and GR are mutually inconsistent? I'm not an expert, but from what I understand GR and standard model don't exactly contradict each other, they are both considered "valid" and self-consistent on their own as they merely describe different processes on different scales? On their own, they work just fine but if we try to stick the two of them together, to get a "unifying theory", we get different outcomes than we expected, that's why we are now trying to find the "glue" that fits the two of them together while still making sense in the end. At least that's my layman's take on this whole situation.

evanb 9y ago

While it's true that the graviton has never been observed, the point is that string theory was not, originally, a "theory of everything" candidate! It was originally a way of trying to understand QCD and strong dynamics. In 1974 people discovered that the massless chargeless spin-2 boson "popped out" of the theory accidentally! Since people knew that such a particle behaves like a graviton, and is manifestly quantum-mechanical, suddenly it occurred to them that string theory might be a theory of quantum gravity. In fact, General Relativity can be viewed simply as the low-energy effective theory of massless chargeless spin-2 bosons, as emphasized in Feynman's Lectures of Gravitation, where he derives GR from a particle-physicist point of view, rather than a differential-geometry point of view. This led to the tongue-in-cheek saying that string theory post-dicted the existence of gravity at all---since it isn't built into the foundational assumptions of the theory at all (which is more than a lot of the other models, such as the "gluing" you propose, can say). And since string theory is guaranteed to be quantum-mechanical, a lot of interest developed towards understanding it as a theory of quantum gravity (most other models fail to account for how GR can be made compatible with quantization and how the renormalization works sensibly).

daxfohl 9y ago
So given Higgs bosons exist, doesn't that account for mass? Why are gravitons needed as well?
- techdragon 9y ago
  Gravity != Mass Gravity isn't the same as mass. It's related, but not equivalent.
  - daxfohl 9y ago
    So are gravitons related to higgs bosons in the same way? Or are gravitons, assuming higgs boson === inertial mass, more like the difference between (higgs bosons aka inertial mass) and gravitational mass (thus unrelated to / independent of higgs bosons)?
    - wayn3 9y ago
      gravity and inertia are 2 different properties of mass. inertia is conveyed by the higgs field. gravity is conveyed by the gravitational field. we know that those fields exist, because we can observe them. if the fields exist, the exchange particles (higgs boson and graviton) have to exist, too - or our theory is wrong. thats kind of circular reasoning, but we cant derive physics from first principles anyway.
      - AstralStorm 9y ago
        Can you explain then why no gravitons have been found yet? Otherwise it is a baseless untestable assumption that the particle even exists. We probably could derive physics from first principles if we knew them.
        baq 9y ago
        We've only measured gravitational waves directly last year. The theoretical graviton is still ways off - the energies required are incomprehensible to mere mortals.
        wayn3 9y ago
        the graviton is too small to be observed directly by our machinery. if you can provide a detector the size of the solar system, proving/disproving its existence will be simple. we could derive anything from first principles if we knew them. theres nothing probable about that. thats the whole point of first principles. do you want me to call you captain obvious?

guygurari 9y ago

As far as I know, string theory is the only complete theory that incorporates these two observations: 1. Nature includes the force of gravity, described at low energies by Einstein's theory with a gravitational field. 2. Matter in Nature is quantum mechanical. We believe that spin 2 particles will exist in any such theory (as they do in string theory), but even without this assumption it is still difficult to come up with such a theory. The example you give of gluing the Standard Model to General Relativity does not achieve this goal and does not correctly describe nature. This is because the quantum matter of the Standard Model does not correctly back-react on the classical gravitational field. When we couple the classical theory of General Relativity to classical matter, the gravitational field pushes matter around, and the matter in turn back-reacts on the gravitational field and changes it. When coupling quantum matter to classical General Relativity, the classical gravitational field can act on matter just fine, but it does not work in the other direction. The problem is that when matter is in a quantum superposition, it is not clear how to update the classical gravitational field (which cannot itself be in superposition). This missing part of the interaction means that SM+GR is not a complete theory.

evanb 9y ago
Hey Guy! We should finish the horizon paper :D -- Evan

kobeya 9y ago

> But the graviton has never been observed, nor has any quantized behavior of gravity or its General Relativistic effects The MOND folks would disagree here. They'd say that the phenomenon we call "dark matter" and "dark energy" are the result of quantized gravity.

skookumchuck 9y ago

> would have to be a massless chargeless spin-2 boson Sure it's not a pointless mass?

zodiac 9y ago

I don't know much about this, but I've read that the standard model and GR are mutually inconsistent? How do you "glue them together"?

PhantomGremlin 9y ago
I also don't know much about this. But I don't think there's anything serious yet that glues them together. Years ago someone on slashdot made an attempt. It's silly humor. Your moma so fat even if I'd entangle with her no information would be able to leave her event horizon. Nobody has managed to put gravitation and QM together yet, and you want to do it in a your-momma-so-fat-joke? Wow. https://science.slashdot.org/story/09/02/19/2338245/human-eye-could-detect-spooky-action-at-a-distance https://science.slashdot.org/story/09/02/19/2338245/human-ey... Sigh. I couldn't resist. It's one of my favorites. Let the downvotes begin.
freeflight 9y ago
>standard model and GR are mutually inconsistent? I'm not an expert, but from what I understand GR and standard model don't exactly contradict each other, they are both considered "valid" and self-consistent on their own as they merely describe different processes on different scales? On their own, they work just fine but if we try to stick the two of them together, to get a "unifying theory", we get different outcomes than we expected, that's why we are now trying to find the "glue" that fits the two of them together while still making sense in the end. At least that's my layman's take on this whole situation.