All quantum fields have their vacuum energy fluctuations.
When we change the parameters of the space (e.g. stretch or compress it), the current state is no longer the ground state but becomes a so-called squeezed vacuum state.
This effect is used in laser physics to "split photons" in spontaneous parametric downconversion. That is, an intense laser changes the refractive index of a medium periodically. These oscillations generate a squeezed vacuum state.
You seem educated on some matters, so please reply here. My physics education stopped at modern physics (optics, general relativity) so I'm missing the core mathematics of quantum mechanics and deeper dives into particle physics.
In the actual summation, does the calculation of vacuum energy take into account a cross section of neutrinos and extended "particles". I've always found the concept of vacuum energy lacking in that most(even physicists) do not fundamentally conceptualize a permanent neutrino and elementary particle flux in all but the most astoundingly shielded vacuum.
If there is a medium in which to effect the refractive index, then isnt this an interaction of the molecular structures therein?
FWIW, so inspired, I continued to write a few better prompts for all of this.
"""Explain how specific (for example Fedi and Turok's) theories of gravitons and superfluidity differ from General Relativity, the Standard Model, prevailing theories of Quantum Gravity and dark matter and/or dark energy non-uniform correction coefficients, classical fluid dynamics, and quantum chaos theory in regards to specific phenomena in the quantum foam.
Also, are is there one wave function or are there many; and are they related by operators expressible with qubit quantum computers or are qudits and qutrits necessary to sufficiently model this domain of n body gravity gravitons in a fluid field?
If graviton fields result in photons, what are the conservation symmetry relations in regards to the exchange of gravitons for photons?"""
And then (though apparently currently one must remove "must use `dask_ml.model_selection.GridSearchCV`" presumably due to current response length limits of Google Bard):
"""Design an experiment as a series of steps and Python code to test (1) whether Bernoulli's equations describe gravitons in a superfluidic field; and also (2) there is conservational symmetry in exchange of gravitons and photons. The Python code must use `dask_ml.model_selection.GridSearchCV`, must use SymPy, define constants in the `__dict__` attribute of a class, return experimental output as a `dict`, have pytest tests and Hypothesis `@given` decorator tests, and a `main(argv=sys.argv)` function with a pytest `test_main`, and use `asyncio`."""
Is there now, ironically, a cycle in the comment graph like there are cycles of fluidic nonlinearities in graphs of relations in real complex - possibly adaptive - systems?
FWIR there are Degrees of curl: convergence, divergence
But are paths of photon particles always non-intersecting?
So the trick to creat this light in todays environment would be to make a gravity amplifying device similiar to a https://en.wikipedia.org/wiki/Gravity_laser and overlay the amplitudes? That can be easy. Just have a microscopic black hole particle go back and forth in a field..
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 that gravitational waves - the spacetime curvature changes with time - carry energy, so can be transformed into light. Do we still know if spacetime alone can create light?.. I'm not sure we know it today. So... we have a great experiment here, which shows something known in a different way - is it correct?
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.
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
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.
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 ?
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.
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).
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.
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.
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>. An accessible HTML5 version is available at <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.
> 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
I am not a physicist, but I understand we are talking about Universe so early after Big Bang that it wasn't yet transparent to light. There simply wasn't vacuum yet if by vacuum you mean electromagnetic waves being able to travel long distances.
I am not sure what you're getting at exactly. Although the paper does touch on early universe cosmology, the authors do their principal analysis using the refractive value for water, and there were no interstellar water clouds before early supernovae started generating oxygen. The authors also explicitly contemplate observables generated by LIGO-accessible compact binary mergers ("compact" here means black holes and neutron stars), all of which postdate the first stars.
In physical cosmology (and especially considering alternatives to General Relativity) it is very common to consider the possibility that some effect is strong in the very early universe and so weak as to be undetectable at present times (or even as early as the first galaxies or the surface of last scattering). Examples include auxiliary gravitational fields ("bimetric" theories, for example) that decay in the early universe, variable-speed-of-light/variable-Newton's-constant theories, and so forth.
Although one might think "hm, it's very convenient that an important effect only happens so early that we cannot use telescopes to see it", there is very good evidence for electroweak unification and cosmic inflation, both of which terminated (in different ways) in the very early universe, and are (or arguably were) too difficult to directly observe this late in the universe's history. Additionally there is ample indirect evidence that (if it exists) is within our reach.
That the hypothesized graviton-photon mechanism cannot work in vacuum makes it at least very difficult to test (or observe with telescopes) today, however the final section of the paper does suggest that if it happens in nature, where it happens is likely to become accessible to us in due course. This is not a theory that has a hard cut-off in the early universe; it is just a hypothesis that to be realized requires a configuration of e.g. binaries and molecular clouds that is not very close to what we commonly observe. (Double-binary compact objects in dusty environments might end up being commonplace though, and in those settings one could expect changes in "multimessenger" signals if the authors' ideas are correct. It's amazing how many star systems are turning out to be triples, and we know of triple-compact-star systems; there are a number of known quadruples like DI Chamaelontis; and Gamma Cassiopeiae is a system of at least seven ~stellar mass bodies.)
AFAIK there exists no popular belief that physics was different in early universe. The physics was the same, the only thing that was different was physical conditions. Meaning everything was densely packed together.
If you, even for a moment, assume that laws were different in early universe then you essentially lost any possibility to predict anything.
It's not that things become unpredictable, it's that it can capture mispredictions (actual and possible) of things colloquially called "laws" of physics.
Spontaneous symmetry breaking has been at the root of at least three Nobel prizes, and is crucial to understanding the differences in physical systems at very high energies both in laboratories and in extreme astrophysical settings, at both early and approximately present times in the universe.
The early universe was in a high energy state, being very much hotter and denser than the later universe, as you say. There are several epochs -- notably the https://en.wikipedia.org/wiki/Electroweak_epoch -- where symmetry breaking is important, and using the lower energy theory (electromagnetism, in this example) simply does not work: results are (if even calculable) manifestly wrong, leading to a universe with a very different cosmic microwave background, and very different chemistry and nuclear physics.
I think at best one might say that theories with broken symmetries could still have those symmetries (i.e., the breaking may be reversible under "different ... physical conditions", like if our universe surprisingly evolved to a Big Crunch), however treating that as a denial of the possibility of different physics in the early universe is probably something you'd have to take up with philosophers or lexicographers for now.
Additionally, there is no reason to just assume (and refuse to trace out implications if wrong, or to validate) that physical constants are constants everywhere and everywhen. Putting some spacetime-location-dependent function on constants like G, k_B, \alpha, \Lambda, c has at the very least proven instructive in further understanding the concordance (standard) models of particle physics and cosmology, where those constants are taken as constant everywhere and at all times in the universe. Indeed paramaterizing apparent constants is outright productive science. See e.g. <https://en.wikipedia.org/wiki/Test_theories_of_special_relat...> for a scratch-the-surface set of details, and additionally <https://en.wikipedia.org/wiki/Variable_speed_of_light#Relati...> are at least [a] interesting [b] testable and [c] improves testability of the families of theories in which these constants are assumed truly constant (i.e, everywhere and everywhen).
> popular belief
Well, I guess your popular is could outweigh a literature search. But for scientists:
It's been about half a century since Kenneth Wilson and Nikolay Bogolyubov explored rescaling and renormalization, and nowadays practically every physical theory is written down as, considered as, or is being adapted towards <https://en.wikipedia.org/wiki/Effective_field_theory> (EFT). It is common that different EFTs apply to the same physical configuration as some characteric scale is crossed, and it is possible that physical theories will be EFTs all the way down (and all the way up), with the concept of fundamental becoming a relation between families of theories. (For example, Newtonian gravitation is less fundamental than General Relativity, because the former can be derived from the latter (and not the reverse), not because General Relativity is known to be correct at all scales).
> 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".
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.
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.
It's a very large horizon, so the Hawking radiation is very weak (it's always a weak effect). We can't see it because the cosmic microwave drowns it out.
Broadly Hawking radiation intensity goes with curvature - i.e. a more curved surface has a better chance of separating a particle pair then a flatter one. This is because a sharper curve means more vectors which carry you away from the event horizon.
This is also why small black holes evaporate faster then big ones - as the black hole shrinks, the Hawking radiation intensity increases because it's curving more and more (hence why microscopic black holes don't devour everything).
Correct, that's a key fact about the underlying effects in question, of which Hawking radiation is just a consequence: different observers see different particles in the vacuum.
In 1976, Bill Unruh published "Notes on black-hole evaporation"[1], in which he showed that "an accelerated detector even in flat spacetime will detect particles in the vacuum" - now known as the Unruh effect This means that an observer in an accelerated reference frame will observe particles in the vacuum where an inertial observer will observe none. The presence of certain particles - the ones we call Hawking radiation in the context of a black hole - is a relative phenomenon. This is known as the Unruh effect. The equation for the Hawking temperature is essentially the same as the equation for the Unruh temperature, where the acceleration value is the acceleration due to gravity of the black hole.
Then in 1977, Gibbons and Hawking published "Cosmological event horizons, thermodynamics, and particle creation"[2], which showed that "the close connection between event horizons and thermodynamics which has been found in the case of black holes can be extended to cosmological models with a repulsive cosmological constant" and that "An observer with a particle detector will indeed observe a background of thermal radiation coming apparently from the cosmological event horizon." This is known as the Gibbons-Hawking effect.
There's a fairly complex relationship between the two effects which I won't try to describe, but if you're interested then [3] discusses it. The abstract itself gives some sense of the connection.
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?]
The paper summarized at the link at the top hypothesizes that in a setup like this:
Bb --- Earth --- molecular cloud --- B'b'
where Bb and B'b' are very similar (from the molecular cloud's perspective) inspiralling binary black hole pairs, and Earth is where LIGO (and Virgo and other detectors) are, then we will see characteristic bright flashes from within the molecular cloud and a change in the B'b' detected waveform because it will have lost some energy to the production of the flashes of light.
(The light flash will trail behind the dampened B'b' waveform detection because this hypothesized mechanism only works when a refractive medium slows light from c (its speed in vacuum) but does not slow gravitational waves from c. Although the authors do not touch on the matter, I suspect that we would also be interested in <https://en.wikipedia.org/wiki/Light_echo>s.)
The paper focuses on an analysis of water as the dominant molecule. There are known astrophysical water megamasers (see second paragraph at <https://en.wikipedia.org/wiki/Megamaser>), so this is far from ridiculous.
E=mc^2. If you have enough energy, in any form, it is equivalent to mass and should disturb spacetime equivalently.
Light can also be used to push objects. Interestingly, sunlight exerts a pressure of 6.56e-10 [psi = lbs/in^2] or 4.53e-6 [N/m^2] on the Earth. This is roughly 5.75e8 [N] or 46 Space Shuttle SRBs. The gravitational force between the Earth and Sun is 3.52e22 [N], or + ~14 orders-of-magnitude.
Objects on Earth at the rotational equator at MSL weigh 0.2% less than at the rotational poles due to the centrifugal force.
Maybe better as "momentum-energy" than as "energy and momentum". "Stress-energy" is probably better still.
I don't think we have to consider the photons as such because large-photon-number beams sent from, to, and between spacecraft have been shown to deflect around masses in our solar system, sunlight generates measurable radiation pressure (and greybody radiation contributes to the Yarkovsky effect), and because your parent comment just asked about "light".
However, your t and my t can differ if we are in a quasilocally-different gravitational field, or accelerated or boosted with respect to each other. In that case one of us may prefer coordinates where some of the quantity in T^{tt} is instead in T^{tj} (the latter being momentum), or even in the pressure diagonal T^{ij}, i == j, i != t.(e.g. when considering a Shapiro test setting, or a "mirrored box of light").
Although I don't think that much of that particular stack exchange discussion, the accepted answer is right to aim readers at <https://en.wikipedia.org/wiki/Electromagnetic_stress%E2%80%9...>, which is almost wholly classical in its outlook. If one were really keen on thinking about gravitation and sufficiently small numbers of photons, it can get a bit messy or drive one towards the canonical quantization and canonical quantum gravity. As far as I can see nothing at your link goes anywhere close to that, or even really discusses the active or passive gravitational behaviour of an individual photon.
There's better in the sense of correctness, and there's better in the sense of "words that will be useful to someone who has never heard that light can create gravity". My inference is the GGP does not already have a deep technical background in general relativity. This guides the words I choose.
In General Relativity, any momentum is encoded in the stress-energy tensor in the Einstein Field Equations, which relates curvature (mainly described by the Einstein tensor) and matter (mainly described by the stress-energy tensor). The vacuum of General Relativity has the stress-energy tensor filled with zeroes in all its components; a wave or beam of light introduces one or more nonzeroes. In suitable coordinates and the flat spacetime of special relativity, this is encoded in the "p" (p for momentum) in e.g. E^2 = (pc)^2 + (mc^2)^2, a fuller version of the famous E = mc^2. See <https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relati...> for details.
Alternatively, we can consider the active and passive gravitational charges of a given mass, also commonly called the active gravitational mass and the passive gravitational mass. The passive charge describes a mass's response to a known source of gravitation; the active charge describes the strength of the gravitational effects generated by an object. In General Relativity the version of the equivalence principle that says that all objects fall identically no matter what their internal composition is ("universality of free fall") ensures that the passive and active charges are identical for all matter. Moreover, in the approximately three hundred years before General Relativity was first written down, there were many successful tests of the equality of the active and passive gravitational charges for many masses; many of these tests were motivated by the work of Newton.
In both Newtonian gravity and General Relativity, light is deflected around large masses (e.g. light from distant stars, or radio beams from <https://en.wikipedia.org/wiki/MESSENGER> grazing the sun, and also lunar laser ranging experiments), so in both theories light has a passive gravitational charge (or passive gravitational mass). If passive and active gravitational charges are identical or at least totally equivalent, light must also source gravitation.
Isn't gravity just a fundamental force? Could it be matter reacting to gravity? Light is energy but gravity is not energy and has no mass so how can it create something out of nothing? How is hawking radiation even possible unless something else was involved?
Gravity is most accurately described by the general theory of relativity... which describes gravity not as a force, but as the curvature of spacetime, caused by the uneven distribution of mass, and causing masses to move along geodesic lines.[1]
You have to remember that General Relativity is wrong. We don't know exactly how it's wrong, but it's wrong. In light of that fact, it's not really a proper answer to say that "General Relativity says it isn't so it isn't." It is true that GR says gravity isn't a force, but the implication doesn't follow. We don't know whether a Grand Unified Theory would have gravity as a force or not. Given that a great deal of the answer to "Is gravity a force?" depends heavily on the definition of force being used, we don't even know what the final definition will be in light of a GUT. There may or may not be a meaningful distinction between gravity and other forces in the real universe.
In QM, gravity isn't a force because gravity doesn't exist. Clearly this is wrong as well. Saying "GR says gravity isn't a force" is effectively the same thing as saying "Gravity doesn't exist because QM says it doesn't." This is exactly where both theories break, so we can't use them to answer this question.
I'm not seeing how these distinctions are helpful. Maybe there's a miscommunication?
When we say "gravity isn't a force", it's shorthand for "our best understanding of gravity, with the widest explanatory power that most accurately matches experimental results, says that gravity isn't a force." But that's an exhausting, verbose way to talk. Knowing that GR is incomplete doesn't change that.
No falsifiable theory is 100% guaranteed Truth. Not GR, QM, evolution, or any widely accepted future Grand Unified Theory. Science is one big exercise in affirming the consequent, verifying contrapositives, and finding surprises. Maybe all mental models are wrong. Maybe we're brains in vats. Aaaah!...so what?
Evolution naturally developed eyeballs in mammals. My niece just turned 5. The atoms composing me won't rearrange into a facsimile of Abraham Lincoln tomorrow. I'm not going to add "to our best understanding, given the evidence, granting my incomplete knowledge, etc" about these facts in service of reality obviously being unknowable. It's pointless.
Gravity isn't a force. That's a perfectly "proper answer" -- no need to pontificate about philosophical relativism.
This isn't philosophical relativism. I don't deal in that.
This is precisely where GR doesn't work, so using it to declare with confidence what gravity "is" is as silly as using QM to do it. They don't work on this matter. General Relativity is definitely not a complete description of how the universe works. It's a good approximation, even a very good one.. but no more than that.
We know this. It is well understood. This is not some sort of whacky Time Cube position. The whacky Time Cube position is really the one that says GR is correct so we can declare with confidence that gravity isn't a force.
In the end I don't expect gravity to be different from electromagnetism and the other forces. There's been plenty of efforts to build GUTs from nothing but geometry in which case nothing is a "force". Efforts from the QM side tend to incorporate "gravitons" as the force mediation particle just like the other forces have mediation particles. It seems very likely to me that GR's "gravity is specially not a force" will end up being an error, in that either all the other forces turn out not to be forces either, in which case we'll change the meaning, or it will be a force like every other. I don't think there's a lot of reason to expect this to be anything other than an artifact of GR being an approximation.
We don't have a single test that shows that GR is wrong in any way and we have many tests that show that it is exactly right to any arbitrary precission we can dream of achieving.
GUT does not unify gravitation; that would be a Theory of Everything (TOE). GUTs only seek to unify the strong and electroweak forces. TOEs are even more speculative than GUTs. AFAIK there are no purported GUTs which do not use gauge groups, so I don't understand your point about "force".
> In QM ... gravity doesn't exist
The Standard Model of Particle Physics, a particular relativistic quantum field theory, is Lorentz-covariant by design (and with good reason; Lorentz symmetry is a highly-tested feature of our universe). General Relativity as a physical theory of our universe is defined on a Lorentzian spacetime. Where the radius of curvature is larger than the Compton wavelengths involved, adapting the flat-spacetime Standard Model to curved spacetime is straightforward textbook stuff. (Wald 1995 ISBN 0-226-87025-1; Birrell & Davies 1982 ISBN 0-521-23385-2 and several others).
For smaller radiuses of curvature, of if the semiclassical approach is unsuitable because of nontrivial superpositions, one can take a second-quantization approach ("CQG", very definitely a QM theory in which gravity "exists" as a quantum field with potentials and dynamics, and which admits the correspondence principle) or <https://webspace.science.uu.nl/~hooft101/lectures/erice02.pd...> both of which match General Relativity as effective theories (the characteristic energy cutoffs are usefully large and/or the characteristic cut-off lengths are transplanckian) are verified to high precision in manifestly relativistic systems (e.g. triple PSR J0337+1715). Are these "wrong"? Prove it. You'll likely need to look into a region of strong gravity, and for that you have to deal with a cosmic censorship conjecture.
> General Relativity is wrong.
Prove it?
I'm with Clifford Will <https://en.wikipedia.org/wiki/Clifford_Martin_Will> on this; penultimate paragraph of his 2020 book with Nicolas Yunes ISBN 9870198842125, <https://global.oup.com/academic/product/is-einstein-still-ri...>: "It would not surprise me if the solution to the universal acceleration turned out to be simply Einstein's original cosmological constant, a constant of nature like Planck's constant or Newton's constant of gravitation ... And it would not surprise me if general relativity turned out to be absolutely correct according to any future experiment accessible to humankind." The authors' following paragraph is a challenge to you which is not met by your bold assertion at the top of your comment.
General Relativity may be wrong, but that is not proven. Will would be happy to be surprised by it being proven wrong experimentally. (A huge amount of his research has been aimed at what exactly it means for General Relativity to be wrong).
What I would agree with is that we do not know at present how to avoid divergences of the curvature scalars in the limit of strong gravity, and that this means we can't always fully solve a whole spacetime from initial values. But we also don't have any idea how to test whether that actually happens in our universe (that such divergences occur in stellar remnants or at the centres of galaxies is untested hypothesis) so GR is a perfectly sound, mathematically complete effective field theory at the very least.
And moreover the answer may come from new physics in the matter sector, rather than new gravitational physics. What does the Standard Model say about particle energies above YeV? Discovering more about the mechanisms that generate the stress-energy tensor is not the same task as changing the theory of General Relativity.
Meanwhile, there are hundreds of pages in Will & Yunes's excellent book which explain to lay readers the match between GR and observation and experiment, and a wide body of specialist literature on a century's worth of experimental and observational tests of General Relativity, every single one of which has supported the theory or was inconclusive.
Gravity couples with anything that carries energy (in more precise terms with the energy-momentum tensor). Photons carry energy. Therefore there is interaction between photons and gravity. In particular, you can write Feynman diagrams for the interaction between a quanta of gravity (a graviton) and a photon. Some of the simplest ones look like this:
Regarding Hawking radiation, it's not gravity itself that causes it. It's a consequence of the existence of an event horizon. In fact, you can have a similar effect in flat spacetime (i.e. no gravity), an accelerating observer will see an event horizon and it will feel itself immersed in a thermal bath of particles at certain temperature that has to do with its acceleration. This is called the Unruh effect. Further reading: https://en.wikipedia.org/wiki/Unruh_effect
It's just unfortunate nomenclature. Space(time) is a set of dimensions. It's not a substance, like a ruler, anymore than temperature is a substance in a thermometer. The word "bend" misleads us by equivocating "bending" a ruler (forcefully rearranging its atoms into a new shape) with "bending" spacetime (e.g. changing how fast someone's clock ticks).
General relativity doesn't really say forces don't exist, it just says that they're a secondary, and relative, effect caused by a more fundamental phenomenon, i.e. the curvature of spacetime due to the presence of mass/energy.
Just as centrifugal force is a relative effect that you only observe in a rotating (i.e. accelerated) reference frame, the force of gravity is also a relative effect that you only observe in an accelerated reference frame.
Standing on Earth, we're accelerating towards the center at 9.8 m/s^2, and we feel that as weight due to a force of gravity that we experience in our reference frame.
If you jump out of a plane, though, you feel weightless (especially if you can arrange to do so in a vacuum), because you're in "free fall" - you're following the curvature of spacetime. In your reference frame, you're not experiencing the force of gravity.
This demonstrates that gravity, like centrifugal force, is a "fictitious" force - which doesn't really mean that it's not real, but rather that it's a secondary effect that depends on your reference frame.
the simplest way to describe it is that an object in freefall moves in geodesic in spacetime, which is a is not straight line in 3dimensional classical physics but produces gravitational motion.
The basic idea is that depending on your frame of reference, you might experience what is called a fictitious or psuedo force that disappears in a more simple frame of reference. Examples of these include centrifugal force, the Coriolis effect.
Einstein's great revelation in the theory of general relativity was that there was no difference between gravity and free fall - which is called the equivalence principle.
In special relativity, einstein's theory had a preference for inertial frame observers where the observer essentially became the origin of coordinate system to map space by, but in general relativity einstein does away with inertial frames as privileged, and an observer is literally decided by what light it can see, and their coordinate system cannot be universal because of curved space.
They attribute it to the fact that space around the object is bent. Which is causing the other object to drop, accelerate or bent. Check General Relativity's explanation of Gravity.
That might not answer all of your or my question. But maybe we didn't give enough time to ponder on that line.
This effect is used in laser physics to "split photons" in spontaneous parametric downconversion. That is, an intense laser changes the refractive index of a medium periodically. These oscillations generate a squeezed vacuum state.
https://en.wikipedia.org/wiki/Squeezed_coherent_state