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This is not a duplicate. I do not ask about the gravitational field of the black hole, or why it extends farther out then the event horizon. I do understand that the gravitational field is described by a mathematical model called virtual gravitons. And that it has nothing to do with actual real gravitons. I do understand that nothing (no info, no real particles) can escape the event horizon. My question is about real gravitons (hypothetical).

I have read this question:

How does gravity escape a black hole?

Where John Rennie says:

The answer to your question is that nothing can travel faster than light, and light can't escape through the event horizon. Therefore gravitational waves can't escape either.

Now GWs are said to be made of real gravitons. They are said to be the quanta of gravity, just like photons are the quanta of electromagnetism, and the classical EM wave is built up by a herd of photons.

What is the difference between gravitational waves and gravitational distortions in spacetime?

where Bob Bee says:

On gravitons, probably a quantum theory of gravity will have to include gravitons, but till we figure out that theory we don't know for sure. All points to there probably being gravitons as the carriers of the gravitational waves, but we don't really know yet what that means.

What is the difference between gravitons and gravitational waves?

where annav says:

Gravitons are to gravitational waves the theoretical analogue of photons for electromagnetic waves. They are the proposed carriers of the gravitational interactions at the quantum level, and are expected to appear naturally in a future theory of quantized gravity. At the moment quantization of gravity can be accommodated in string theories, which are at the frontier of research for particle physics. The standard model involves only the three other forces , not the gravitational. A future standard model should have both the present standard model and quantization of gravity, a Theory Of Everything (TOE). So photons are the building blocks of light, and gravitons are (hopefully) the building blocks of gravitational waves.

Gravitons do interact with matter, but the interaction has a low crossection.

Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector.[17] The reason is the extremely low cross section for the interaction of gravitons with matter. For example, a detector with the mass of Jupiter and 100% efficiency, placed in close orbit around a neutron star, would only be expected to observe one graviton every 10 years, even under the most favorable conditions.

https://en.wikipedia.org/wiki/Graviton

Real gravitons do have an extremely low cross section when interacting with matter.

Thus, they would easily just fly through a BH. The BH would seem transparent to a real graviton.

Question:

  1. if the real graviton has a extremely low cross section when interacting with matter, does that mean that the real graviton will just fly through the BH, like it (the BH) was transparent?
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  • $\begingroup$ Think about a similar question for photons and a transparent black hole. The photons fly in, but not back out. $\endgroup$ – mmesser314 Jul 4 at 14:47
  • $\begingroup$ You don't need to say "real gravitons" to contrast with "virtual" ones. Just remember that most stories about virtual particles are just kind of made up to get some intuition about equations. In the actual formalism, only "real" particles are actually present at any time, so there's no need to specifically call them real. $\endgroup$ – knzhou Jul 4 at 16:59
  • $\begingroup$ "the gravitational field is described by a mathematical model called virtual gravitons" - No, gravitons are hypothetical, because there is no valid theory that predicts them. "nothing (no info, no real particles) can escape the event horizon" - Only for true horizons, but they can't form in a finite time and don't exist. Stuff can escape real apparent horizons. A "BH would seem transparent to a real graviton" - A hypothetical graviton will never reach the horizon and for all practical purposes would appear absorbed 100%, unless it misses the BH (as @mmeent has correctly answered below). $\endgroup$ – safesphere Jul 6 at 6:44
  • $\begingroup$ Based on your reference to virtual gravitons, you may be under the impression that the gravitational field of a black hole is sourced from inside the horizon (e.g. from the singularity), so that virtual gravitons can escape. This is a widespread misconception. A singularity doesn't cause gravity even inside and definitely not outside the horizon. The gravity of a black hole is sourced from outside the horizon. $\endgroup$ – safesphere Jul 6 at 6:57
  • $\begingroup$ @safesphere "The gravity of a black hole is sourced from outside the horizon." I thought that the source of gravitational field is stress-energy. Most of the stress-energy of the BH comes from the matter it has inside the EH right? Is that a contradiction? $\endgroup$ – Árpád Szendrei Jul 6 at 6:59
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The scattering of gravitons of black holes is really no different than that photons (or massless scalars for that matter). The scattering/absorbtion cross-sections are readily calculated using modern black hole perturbation theory. The only difference is the spin of the underlying Teukolsky wave equation.

The absorbtion crossection depends on the wavelength of the graviton. If the wavelength of the graviton is much larger than the size of the black hole horizon, the absorbtion cross-section will be very small. However, rather than saying the black hole is transparent to the graviton, it is more accurate to think of this as the wave going around the black hole.

If the wavelength is much smaller than the size of the black hole horizon (but still larger than the planck length), the absorbtion crossection will be much bigger, and gravitons will have a high probability of being absorbed by the black hole, just as high as for photons. I.e. the effective crosssection of the black hole will be approximately the area of the black hole shadow (to first approximation the projection of the photon sphere).

(If the wavelength becomes comparable to the Planck length, the tacit approximation used above that non-linear effect can be ignored ceases to be valid and we would a full theory of quantum gravity to obtain the answer.)

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  • $\begingroup$ What would be the energy of a graviton (in kilograms) with the wavelength comparable to the Planck length? (+1) $\endgroup$ – safesphere Jul 6 at 6:48
  • $\begingroup$ @safesphere That would be a Planck mass i.e. circa $2\cdot 10^{-8}$ kg. (Note that this assumes that the scale for quantum gravity is set by the Planck scale, in principle quantum gravity effects could set in earlier, e.g. if you have "large" extra dimensions.) $\endgroup$ – mmeent Jul 8 at 7:19

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