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The title is about gravity, but the first line is about wave of gravity. Hmm. We know that gravity alone can create radiation - the Hawking one. We also know th

• adastra22 4y ago
  > We know that gravity alone can create radiation - the Hawking one To be pedantic, we don't know this. Hawking radiation has never been observed.
  • PartiallyTyped 4y ago
    Relevant: https://phys.org/news/2021-02-stationary-hawking-analog-black-hole.html https://phys.org/news/2021-02-stationary-hawking-analog-blac...
    • dogma1138 4y ago
      Not particularly. This isn’t an actual observation, this is just an experiment that attempts to create some analogous phenomena.
  • deciplex 4y ago
    to be even more pedantic, hawking radiation isn't exactly created by gravity
    • neatze 4y ago
      furthermore, so far in universe (after big bang), nothing is really created, only transformed.
      • adastra22 4y ago
        Dark energy is adding energy to the universe (mechanism unknown).
        • neatze 4y ago
          How does expansion of universe increases total energy ? If anything seems like due to dark energy total energy dissipation per area is increasing until death of universe.
          • adastra22 4y ago
            It takes work to separate two masses from each other (increasing gravitational potential energy). The universe is not just expanding, which might possibly be understood in an energy conserving way if the rate of expansion is slowing, but it is expanding at an ever increasing rate. Ergo enter guy is being dumped into the universe from some repository/field/whatever that we don’t understand. That’s what Dark Energy is.
      • eep_social 4y ago
        and (depending how time actually works) transformed also may be wrong, rather, everything simply is and time is our hallucination of reality
        • neatze 4y ago
          This does not compute in my mind, energy transfer (in some form) it is observed phenomena (my understanding (primitive) of time is energy transfer), even on quantum level we can measure it, what am I missing ?
          • adastra22 4y ago
            You aren’t missing anything. He’s playing definitional word games, a common pastime of philosophers.
    • kordlessagain 4y ago
      In theory it’s created by the singularity boundary. Related, I posted a paper on William Sidis earlier today where he explored the idea of a black hole before they were theorized or discovered: https://www.sidis.net/animate.pdf https://www.sidis.net/animate.pdf
  • chasd00 4y ago
    didn't the LHC create a black hole that immediately evaporated due to Hawking radiation? ..maybe it was only a theory that the LHC could do that but i thought it actually did create one.
    • twawaaay 4y ago
      No. That's far beyond our capabilities. Somebody calculated that LHC would have to be over 1000 light years in diameter to do this (and then still we would have to wait for thousands of years for particles to get accelerated).
  • mfer 4y ago
    Not only has Hawking radiation not been observed but the article starts with a "may have" and concludes by pointing out that there aren't conditions to observe the phenomenon described today. Maybe I'm just not a fan of strong language, making it appear we know something, where we don't.
• marginalia_nu 4y ago
  If gravity would quantize into photons, that would have very weird implications for the standard model. It would threaten to equate gravity with the electromagnetic force, and mass with charge.
  • pa7x1 4y ago
    Gravity cannot be exchanged by photons or any other spin 1 particle, for that matter. Spin 1 particles lead to repulsive forces for like charged particles. This leaves you with spin 0 and spin 2 as the simplest alternatives. Spin 0 doesn't result in light bending and gives a wrong result for Mercury's perihelion precession. Spin 2 gives you General Relativity.
  • kordlessagain 4y ago
    That would imply there could be devices that can create thrust with electricity. Faraday ran experiments to try to test this…in the 1800s. https://skullsinthestars.com/2009/03/06/michael-faraday-grand-unified-theorist-1851/ https://skullsinthestars.com/2009/03/06/michael-faraday-gran...
• antonvs 4y ago
  > We know that gravity alone can create radiation - the Hawking one. What does "gravity alone" mean here? Hawking radiation depends on a black hole that has energy to radiate. The radiation is not due to "gravity alone".
  • XorNot 4y ago
    Hawking Radiation didn't depend on gravity either, it depends on an event horizon. Any phenomenon which would separate virtual particle pairs would produce it - i.e. the edge of the observable universe would do it too.
    • fallingknife 4y ago
      That doesn't make sense. Everywhere is the edge of the observable universe to someone.
      • XorNot 4y ago
        Yes, but because space is expanding - i.e. there's a value of meters per meter per second for the expansion of space, then every point in space has a sphere around it of locations which are sufficiently far away that they are growing more distant at the speed of light. Any virtual particle pair which appears along that horizon can potentially be caught on the wrong side of it - i.e. one virtual particle is in a part of space which is now far enough way to be expanding faster then light, whereas the other particle is at a location close enough that it is not. At the moment that happens, there's a particle or photon which is now inside the light cone of a distant object, paired with a particle that it will never meet again because it's outside of it. As a result, you get Hawking radiation: because one particle can go off and interact with your universe, but it's partner will never be able to causally effect anything inside that horizon again. So the virtual particle has to become real.
        • dmbche 4y ago
          Thanks for writing this, I hadn't realised that. How cool!
• gus_massa 4y ago
  I never thought about Hawking radiation in this way! How is the Feynman diagram??? [Ok, there is no Quantum Gravity theory yet, but is there a good guess?]
• raattgift 4y ago
  The preprint of the paper that is the subject of the (not so great) phys.org article at the top is <https://arxiv.org/abs/2205.08767 https://arxiv.org/abs/2205.08767>. An accessible HTML5 version is available at <https://ar5iv.org/abs/2205.08767 https://ar5iv.org/abs/2205.08767> (arxiv->ar5iv, the latter expands to a link within ar5iv.labs.arxiv.org). Hawking radiation is a semiclassical result: the curved spacetime is classical General Relativity and the scalar field (in which Hawking quanta arise near the central black hole) is quantum. The dynamical spacetime creates -- through the equivalence principle -- an acceleration between past observers and future observers, and this acceleration corresponds with the Unruh effect. The Unruh effect rests on the definition of a vacuum as a state in which an observer sees no particles, and that when an observer accelerates a no particle state may be transformed into a state with particles. Equivalently, differently-accelerated observers will count different numbers of particles in a spacetime-filling quantum field. (A family of observers may count no particles, i.e., it's vacuum.) The important part here is that a dynamical spacetime ("gravity") and a relativistic quantum field is needed for Hawking radiation. So, "[can] spacetime alone ... create light?" No. There must be a matter field filling the spacetime. That matter field, if quantum, can look like it has no particles in it to some observers, but not all observers. The dynamical evolution of the spacetime can cause observers' counts of particles to evolve. > gravitational waves ... carry energy, so can be transformed into light The paper is about how, given: * a massless quantum field theory proxying for light * a quantum field theory in which gravitation is mediated by a massless spin-2 boson * a dense medium with a (light-) refractive index greater than 1 * standing gravitational waves of significant amplitude occur in cases where gravitational radiation from widely separated sources converge within the dense medium and somehow [a] cancel out polariation and [b] are within a wide (compared to the wavelength) patch of flat spacetime * the non-light massive and massless particles within the medium couple very weakly to the incoming gravitational radiation * the particles of the refracting medium couple weakly to the "light" field, and generate practically no spacetime curvature even in bulk then the light-proxying particles may be produced via a process which the authors compare with electron-positron pair production and Cherenkov radiation. (Although they do the latter comparison very very breezily, not delving into the cross section of light-by-light scattering). There are weaknesses in this list of requirements, some of which the authors admit requires further study. The key point though is that their mechanism cannot work in vacuum. It absolutely requires that the light travels significantly slower than the gravitational radiation (which in turn is assumed to travel at c, even in the non-vacuum in which light travels slower than that) and that a far-from-negligible momentum is lost by the incoming gravitational radiation as it passes through the refracting medium. > great experiment here The last paragraph in the Conclusions and Discussion section suggests there may be avenues for experimenting with the ideas in the paper.