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> even though we've supposedly observed [gravitons] now Source? Or are you saying that the existence of gravitational waves constitute a proof of the existence

by wanda 10y ago

> even though we've supposedly observed [gravitons] now Source? Or are you saying that the existence of gravitational waves constitute a proof of the existence of gravitons, due to wave/particle duality? I'm not a physicist, but all I thought we had detected were gravitational waves.

deleted 10y ago
[deleted]
TeMPOraL 10y ago
> the existence of gravitational waves constitute a proof of the existence of gravitons, due to wave/particle duality I always thought this is the case. But IANAP either. Could anyone with actual knowledge chime in?
- ars 10y ago
  No, a gravitational wave has mass (carries mass is more accurate), but a graviton does not. They are not two aspects of the same thing.
  - empath75 10y ago
    In a quantum field theory, a wave implies a force carrying particle, does it not?
    - ars 10y ago
      Yes, but the particle can be virtual, not actual.
      - codethief 10y ago
        Since QFT usually involves perturbation theory of a free field, whether or not particles are virtual (= intermediate products of interactions) is beside the point.
    - codethief 10y ago
      To be precise, particles (with precise momentum) are certain modes or excitations of a quantum field which is often governed by something like a wave equation. However, not all quantum fields (and their associated particles) are force carriers. These are usually just the gauge bosons. In any case, though, noone has managed to quantize gravity yet (i.e. describe gravity as a quantum field theory with gravitons as force carriers), so we've detected just gravitational waves but no gravitons.
      - raattgift 10y ago
        Well, we've quantized gravitation extremely well - it's just perturbative quantum gravity, of which there are a couple flavours. In weak gravity it makes predictions identical to General Relativity, but is taken to be an effective field theory (in the Charles Wilson sense of "effective) in that we do not know how to deal with Feynman diagrams with more than a loop or two of gravitons, which only happens in strong gravity. In these theories, gravitons are quantizations of the weak-field perturbations. The non-renormalizability by power-set counting of gravity because it is a long-range force is a good result of perturbatively quantized gravity. Who knows if there is a workable way of renormalization by other methods? Not me. Since we don't get strong gravity except close to a black hole singularity or in the extremely early universe, it's unfair and premature to say that the EFT approach has been unsuccessful.
  - joadha 10y ago
    Can you elaborate on what you mean by "a gravitational wave has mass (carries mass is more accurate)"? That doesn't fit my understanding of gravitational waves.
    - ars 10y ago
      They can carry energy see: https://en.wikipedia.org/wiki/Sticky_bead_argument https://en.wikipedia.org/wiki/Sticky_bead_argument
  - maverick_iceman 10y ago
    Gravitational wave doesn't carry mass. It travels at the speed of light, so it has to be massless. In fact, in 4D gravitational waves can arise even without any mass/energy source.
    - ars 10y ago
      See here: https://en.wikipedia.org/wiki/Sticky_bead_argument https://en.wikipedia.org/wiki/Sticky_bead_argument I suppose I should have said energy, not mass, but unlike a photon which can disappear into nothing, leaving just the energy, a gravitational wave can't ever vanish entirely, so it's more like mass in that regard. And don't forget that a wave can travel at the speed of light even if the wave is carried by massy particles that can not.
      - snissn 10y ago
        > And don't forget that a wave can travel at the speed of light even if the wave is carried by massy particles that can not. I would love an example of that!
        ars 10y ago
        Electricity. The electrons barely move, but the wave (the voltage in this case) travels at almost the speed of light.
      - gus_massa 10y ago
        > I suppose I should have said energy, not mass, but unlike a photon which can disappear into nothing, leaving just the energy, a gravitational wave can't ever vanish entirely, so it's more like mass in that regard. This sentence is wrong. I'm not sure what you mind with "photon which can disappear into nothing, leaving just the energy", but gravitons [1] can do the same thing. A gravitational wave is made of a huge amount of gravitons, it's like the electromagnetic wave produced by radio transmitter. I´m not sure if with your classification the electromagnetic wave produced by radio transmitter "can't ever vanish entirely". [1] If they exist. I'm almost almost sure gravitons exist. There is still not a good quantum gravity theory, but I strongly believe that we will discover it in the future an it will include gravitons.

by wanda 10y ago

deleted 10y ago

[deleted]

TeMPOraL 10y ago

> the existence of gravitational waves constitute a proof of the existence of gravitons, due to wave/particle duality I always thought this is the case. But IANAP either. Could anyone with actual knowledge chime in?

ars 10y ago
No, a gravitational wave has mass (carries mass is more accurate), but a graviton does not. They are not two aspects of the same thing.
- empath75 10y ago
  In a quantum field theory, a wave implies a force carrying particle, does it not?
  - ars 10y ago
    Yes, but the particle can be virtual, not actual.
    - codethief 10y ago
      Since QFT usually involves perturbation theory of a free field, whether or not particles are virtual (= intermediate products of interactions) is beside the point.
  - codethief 10y ago
    To be precise, particles (with precise momentum) are certain modes or excitations of a quantum field which is often governed by something like a wave equation. However, not all quantum fields (and their associated particles) are force carriers. These are usually just the gauge bosons. In any case, though, noone has managed to quantize gravity yet (i.e. describe gravity as a quantum field theory with gravitons as force carriers), so we've detected just gravitational waves but no gravitons.
    - raattgift 10y ago
      Well, we've quantized gravitation extremely well - it's just perturbative quantum gravity, of which there are a couple flavours. In weak gravity it makes predictions identical to General Relativity, but is taken to be an effective field theory (in the Charles Wilson sense of "effective) in that we do not know how to deal with Feynman diagrams with more than a loop or two of gravitons, which only happens in strong gravity. In these theories, gravitons are quantizations of the weak-field perturbations. The non-renormalizability by power-set counting of gravity because it is a long-range force is a good result of perturbatively quantized gravity. Who knows if there is a workable way of renormalization by other methods? Not me. Since we don't get strong gravity except close to a black hole singularity or in the extremely early universe, it's unfair and premature to say that the EFT approach has been unsuccessful.
- joadha 10y ago
  Can you elaborate on what you mean by "a gravitational wave has mass (carries mass is more accurate)"? That doesn't fit my understanding of gravitational waves.
  - ars 10y ago
    They can carry energy see: https://en.wikipedia.org/wiki/Sticky_bead_argument https://en.wikipedia.org/wiki/Sticky_bead_argument
- maverick_iceman 10y ago
  Gravitational wave doesn't carry mass. It travels at the speed of light, so it has to be massless. In fact, in 4D gravitational waves can arise even without any mass/energy source.
  - ars 10y ago
    See here: https://en.wikipedia.org/wiki/Sticky_bead_argument https://en.wikipedia.org/wiki/Sticky_bead_argument I suppose I should have said energy, not mass, but unlike a photon which can disappear into nothing, leaving just the energy, a gravitational wave can't ever vanish entirely, so it's more like mass in that regard. And don't forget that a wave can travel at the speed of light even if the wave is carried by massy particles that can not.
    - snissn 10y ago
      > And don't forget that a wave can travel at the speed of light even if the wave is carried by massy particles that can not. I would love an example of that!
      - ars 10y ago
        Electricity. The electrons barely move, but the wave (the voltage in this case) travels at almost the speed of light.
    - gus_massa 10y ago
      > I suppose I should have said energy, not mass, but unlike a photon which can disappear into nothing, leaving just the energy, a gravitational wave can't ever vanish entirely, so it's more like mass in that regard. This sentence is wrong. I'm not sure what you mind with "photon which can disappear into nothing, leaving just the energy", but gravitons [1] can do the same thing. A gravitational wave is made of a huge amount of gravitons, it's like the electromagnetic wave produced by radio transmitter. I´m not sure if with your classification the electromagnetic wave produced by radio transmitter "can't ever vanish entirely". [1] If they exist. I'm almost almost sure gravitons exist. There is still not a good quantum gravity theory, but I strongly believe that we will discover it in the future an it will include gravitons.