4 ms·

Does gravity CAUSE the bending of spacetime, or IS it the bending of spacetime?

49 points by rrauenza 8y ago

iamjdg 8y ago
I think a mass bends/distorts spacetime the effect of this bent/distorted spacetime on other masses is what we describe as gravity.
- vardump 8y ago
  IANAPhysicist. In other words, most likely the following is totally wrong armchair physicist speculation. :) If there are any experts (=physicists) around, I'd love to know why it's wrong. What if mass doesn't even distort spacetime, but only "slows" down time locally. Maybe there's a some sort of "quantum progression" limit per volume of space. You'd get gravity out of local time slowdown, because of gradient effects. The object under influence of gravity would experience time slightly slower on the side that is closer to a gravity well. This should cause acceleration towards the larger mass. Gravity's inverse square law could be a result of this local effect depending somehow on wavelength. Longer wavelengths would affect objects further away. Perhaps you could derive theory of relativity out of a local quantum progression limit.
  - raattgift 8y ago
    The problem here is that you can't represent tidal deformations purely through time dilation, since they bring (parts of) objects closer together. The tl;dr is: "how does your idea explain that Earth is oblate, with objects at the two poles slightly closer together than objects on the equator but on opposite sides of the planet?" General Relativity explains this as tidal forces acting on the planet, which is in approximate hydrostatic equilibrium, but rotating. The same features explain why a measurement of the local gravity (the field strength "g") at the poles gives a higher number than at the equator. It also explains the shape of the moon in its synchronous orbit. In General Relativity we can decompose the Riemann curvature tensor into the Weyl tensor, the Ricci tensor, and the metric tensor. The Weyl tensor encodes the squash-stretch deformation of moving objects experiencing shear, for example, a rotating spherical body deforming into an oblate spheroid, or a spherical body moving close to (including synchronously orbiting) a large mass deforming into a scalene ellipsoid. The Ricci tensor encodes the volume deformation of a body experiencing tidal forces; if we drop a ball of dust (think very loosely packed coffee grounds) onto a planet, the Ricci tensor describes the tendency of the individual dust particles to converge into a smaller volume as they each fall "straight down" on individual lines converging at the centre of the planet. Naively, we could say that there is a clear relationship between the Ricci tensor and gravitational time dilation, since time is running slower in the direction where the individual fall paths ("geodesics") converge. However, we can make the Ricci tensor go to zero by dropping the dustball onto a planet with enormous radius, such that it's effectively flat. Similarly, ultra massive black holes have essentially no Ricci curvature just outside the horizon, while low-mass black holes have significant Ricci curvature. However, gravitational time dilation is still very strong just outside the horizon of such a massive black hole. In this way we can distinguish between spatial curvature (or if you like, length contraction) and curved time (or if you like, time dilation) in the region around massive bodies. Gravitational time dilation is not a tidal effect, but rather depends on the gravitational potential, which in General Relativity is encoded in the metric tensor. If we orbit our dust ball very rapidly around a heavy planet, it will have substantial Weyl curvature (squashed along its axis of motion, stretched on the other two spatial axes). If the planet has a big enough radius, the Ricci tensor vanishes, leaving us with just the Weyl curvature. In that case, if we measure periodic microscopic processes ("clocks") closest to the planet and furthest from the planet, we will see that the latter are running faster than the former, and the difference is precisely that of gravitational time dilation. Again, this lets us distinguish between changes in length and changes in duration in the region around massive bodies. Gravitational effects are the result of curved spacetime, although in special circumstances practically all of the curvature can be in the timelike axis, and in other special circumstances practically all of the curvature can be in the three spacelike axes. I don't understand what you mean by "quantum progression".
    - vardump 8y ago
      Thanks for your reply, I'll try to digest all that. :) I don't yet understand why "local" slowing of time wouldn't generate all of those tensor components. > I don't understand what you mean by "quantum progression". Just hypothetical local limitation on quantum state changes (=progression).
      - mikhailfranco 8y ago
        I think any computer scientist naturally and naively thinks about your 'quantum progression', since it relates to a transactional rate on interactions experienced in the local proper time of a particle, as it interacts with other real/virtual particles/fields. For example, a particle in deep space experiences faster relative time, compared to one on Earth, because it has 'fewer transactions slowing it down'. The discrete 'tick' of proper time is then one iteration of the spinning polling loop the particle executes while waiting for something to interact with. So if existence is (or requires) computation, then ....
        mikhailfranco 8y ago
        P.S. There is also a slightly related Transactional Interpretation of Quantum Mechanics, which is more about '2-phase commit' of past and future :) https://en.wikipedia.org/wiki/Transactional_interpretation https://en.wikipedia.org/wiki/Transactional_interpretation
        raattgift 8y ago
        > a particle in deep space experiences faster relative time, compared to one on Earth, because it has 'fewer transactions slowing it down'. Transactions with what? Other matter? A precise weather and waterproof clock on the ground (so in air) on the surface at the north pole will tick more slowly than an identical clock immersed in seawater several metres below the sea level at the equator, thanks to the oblation of the Earth (and the rotation that causes the oblation). The equatorial water is much denser and warmer than the arctic air, so surely there are more interactions between the water and the clock? See the Early observations subsection of the Gravity measurement section of, starting with 1672 : https://en.wikipedia.org/wiki/Pendulum?oldformat=true#Early_observations https://en.wikipedia.org/wiki/Pendulum?oldformat=true#Early_...
dsnuh 8y ago
I try to be well read on physics, but a lot is admittedly beyond my grasp at the moment, so forgive me if this is a silly question. Is this saying that gravity would not be possible without the curvature of space because if space-time paths, then no matter as we know it would be possible because all particles would be on their own timeline orthogonal to all others? And since no collisions could happen if this were the case, no energy in the system would be converted to mass to bend the space-time and create the geodesic curvature gravity emerges from? If I have this right, does this mean all energy in this system would be massless (photons?)
- raattgift 8y ago
  I'm not sure what you're asking, exactly. Please ask again, if this doesn't help: Photons have "gravitational charge", in that they both feel other bodies' gravitational influence, and induce a gravitational influence on other bodies. One sees the "feeling" part in observations of gravitational lensing even around our own sun; the generating part is harder to demonstrate, but consider https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics) https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics) In General Relativity, we explain the gravitational charge in terms of spacetime curvature; light both generates curvature (because it has momentum) and responds to curvature (because of the universality of free fall). An isolated photon (or even a non-extreme classical beamlike pulse of light) generates so little curvature that we can safely ignore it in an otherwise Minkowski (flat) spacetime; however, in both cases if it traverses a region of spacetime that is not flat, the isolated photon and classical pulse will respond accordingly. More technically, light contributes to the matter tensor, whose components encode the flux of momentum from one direction to another. Although light has no rest mass, it does have energy-momentum (in special relativity, E = pc; additionally, the Planck-Einstein relation for photon energy E = h\nu, where h is Planck's constant and \nu is the frequency of the photon). So a higher frequency photon contributes more to the stress-energy tensor than a lower frequency photon, and even the lowest-frequency imaginable contributes more than vacuum. (Aside: General Relativity runs into localization problems with extremely low frequency photons if one piles up a lot of them in a region of space comparable in diameter to their wavelengths; they should mutually gravitate, and possibly collapse, but exactly how is so hard to show that probably nobody's done so successfully. How do gravitational interactions among the photons change their wavelengths? (This is an aspect of quantum gravity; there are related practical problems with Hawking radiation, especially near final evaporation, for instance)). > particles would be on their own timeline orthogonal to all others You can contrive a "dimpled" mostly-flat spacetime in which small masses are so widely separated that the curvature they induce is not felt by their neighbours or vice-versa. The contrivance is not especially awful, because we do something similar with "swiss cheese" models of cosmology, where the holes in the cheese are galaxy clusters generating a non-expanding or even collapsing metric (like Schwarzschild or Lemaître-Tolman-Bondi) and the body of the cheese is an expanding Robertson-Walker metric. Already separated holes "float" in the cheese, and the expansion just separates them with ever more cheese, without interfering in the evolution of the galaxies etc. in the holes. Holes which are too close together coalesce into a single larger hole. The spacetime curvature of an expanding "swiss cheese" is extremely strong, but we can turn off the expansion and it is similar to the "dimpled" mostly-flat one; in fact you could model the "dimples" as tiny patches of Schwarzschild space stitched into Minkowski space [1]. The "holes" that are sufficiently separated by the non-growing cheese would stay separated forever. In all these cases, the worldlines of the centres of masses of the holes are essentially entirely timelike. - -- [1] This kind of "stitching" using junction conditions is straightforward if extremely complicated and subject to a number of conditions/assumptions. Misner, Thorne & Wheeler's _Gravitation_ has a chapter on Israel junction conditions, and various authors notably Darmois, O'Brien, Synge, Bonnor and Raju have catalogued stitchings of different types of metrics into one another. Most of these would likely require a trip to sci-hub, as they were written largely in the 1970s to 1990s. Unfortunately, wikipedia seems very sparse on this topic (see for example the Inhomogenous cosmology entry, which is desperate for some love [<- possible note to self]).
6nf 8y ago
So correct me if I'm wrong, but if gravity is merely a curvature in space-time, does that mean there's no such thing as a graviton?
- raattgift 8y ago
  General Relativity is a classical field theory, so there's no graviton in it. However, there are various quantum gravity research programmes which have tried to quantize the (classical) tensor fields of General Relativity in one way or another, for instance by turning (classical) gravitational waves or position and momentum variables into quantum particles. In these theories the graviton is a spin-2 gauge boson and induce an attractive force on fermions with like gravitational charge. This compares with the photon, a spin-1 gauge boson that induces a repulsive force on fermions with like electric charge (and an attractive force on fermions with opposite electric charge). The spin also corresponds with the rank of the (classical) fields, where in General Relativity we have rank-2 tensors and in electrodynamics we have vectors (which are rank-1 tensors). Unfortunately this approach has a problem: particles other than fermions also have a gravitational charge, including gravitons themselves. (Photons do not have an electric charge, but they do have a gravitational charge). Quantum field theories use a variety of tricks to remain linear theories, but General Relativity's Einstein Field Equations are non-linear. That non-linearity manifests in graviton-graviton interactions in this sort of quantized General Relativity, and we do not know how to stabilize the resulting quantum field theories. In more concrete terms, let's "drop" an electron from higher potential to lower potential. The electron is a fermion with electric and gravitational charge. When we drop it from higher to lower electric potential, out pops a photon. When we drop it from higher to lower gravitational potential, out pops a graviton. But we can't move the photon around the electric field and get it to produce another photon, because the photon is not self-interacting. A graviton, on the other hand, can be dropped down a gravitational potential causing it to emit another graviton. That in turn influences the potential, making it easier for those two gravitons to each emit a further graviton, which influenes the potential, making it easier for those ... and so forth to infinity. Our original electron is in that potential too, so it in turn can emit further gravitons as well. Fortunately this explosion of gravitons only happens in strong gravity, and we only find that deep inside black holes (or in the very early universe), rather than somewhere we can outright observe (assuming the cosmic censorship principle is true), so we can still use this type of theory to describe most of the universe. Unfortunately, gravitational waves are hard enough to detect; when they are quantized, they turn into lots and lots and lots of gravitons, each of which has a tiny fraction of the detectability of a gravitational wave. Detecting an individual graviton experimentally is beyond the ability of present human technology, or anything reasonably likely to appear in the next few decades. So if we ever figure out how to stabilize this type of approach to quantum gravity, we will only be able to assess it against observations of many-graviton systems, likely at extreme events like black hole/neutron star mergers or asymmetrical hypernovae. This is just one broad example of quantum gravity; there are several other approaches, some with a "graviton" dynamics (e.g. string theory's gravitons are a string state) and some without. All these approaches share a common problem: General Relativity describes our universe exceptionally well in practice, and its fundamental components are among the best-tested of any scientific theory to date. Where there is weak gravity, barring significant masses in superpositions of position, any type of quantum gravity has to exactly reproduce the results of General Relativity. It is likely that we will be able to bring microgram masses into such superpositions in the next few years, and we will be able to directly probe one aspect of quantum field theory on (classical) curved spacetime (aka semiclassical gravity). However, if General Relativity fails in that situation, it would certainly allow some useful comparisons between competing theories (with and without gravitons) hoping to extend General Relativity as it in turn extended Newtonian gravity. If only graviton-containing theories match the experimental results, that would be evidence in favour of the existence of a graviton, even without direct detection; conversely, perhaps only non-graviton theories match, in which case the existence of a graviton would seem pretty dubious. Right now, though, we just don't know: in the absence of evidence for or against, gravitons are purely theoretical.
quandarium 8y ago
If you reduce all energy exchange to particles in a vacuum, most of the particles transfering energy are photons. Energy transfer, in general is mostly reducible to photons at a fundamental level. In other words, across the vacuum of space, in an idealized model, where there is nothing but void, any exchange or transfer is via particles traversing the emptiness, and most often, those particles are photons. If two bodies at absolute zero are suspended in a void, and the limits of that void are walls of matter frozen at absolute zero, light years away, and nothing in the scene is emitting photons, what reason would drive the to objects to be attracted toward one another? This is to say, if the arrow of time is a deterministic effect of entropy, and the irreversible aspect of time is a result of the chaotic interactions of many discrete subatomic entities, why would spacetime still be curved by two dark, massive, cold bodies, such that, even in the absence of photon exchange between the two, there are forces acting upon them at a distance, leading them to cross a void, travel down some unseen curvature or gradient and collide? If empty space truly is nothingness and void, then entropy, and by corollary, the arrow of time, only exists in the quantum state of each material body. This would seem to herald a fact that the bodies each move themselves toward eachother, as an incidental phenomenon of being condensed from raw unbound energy to systems of materialized standing waves, (subatomic particles trapped in fixed discrete systems of balanced energy, which does not readily decay into other particles without action from external entities). In total darkness, without the exchange of photons, why would two massive, cold, inert objects move inexorably toward one another? How would each massive object find the other's location and accelerate toward the other, if there is nothing but empty space between?

49 points by rrauenza 8y ago

I think a mass bends/distorts spacetime the effect of this bent/distorted spacetime on other masses is what we describe as gravity.

vardump 8y ago
IANAPhysicist. In other words, most likely the following is totally wrong armchair physicist speculation. :) If there are any experts (=physicists) around, I'd love to know why it's wrong. What if mass doesn't even distort spacetime, but only "slows" down time locally. Maybe there's a some sort of "quantum progression" limit per volume of space. You'd get gravity out of local time slowdown, because of gradient effects. The object under influence of gravity would experience time slightly slower on the side that is closer to a gravity well. This should cause acceleration towards the larger mass. Gravity's inverse square law could be a result of this local effect depending somehow on wavelength. Longer wavelengths would affect objects further away. Perhaps you could derive theory of relativity out of a local quantum progression limit.
- raattgift 8y ago
  The problem here is that you can't represent tidal deformations purely through time dilation, since they bring (parts of) objects closer together. The tl;dr is: "how does your idea explain that Earth is oblate, with objects at the two poles slightly closer together than objects on the equator but on opposite sides of the planet?" General Relativity explains this as tidal forces acting on the planet, which is in approximate hydrostatic equilibrium, but rotating. The same features explain why a measurement of the local gravity (the field strength "g") at the poles gives a higher number than at the equator. It also explains the shape of the moon in its synchronous orbit. In General Relativity we can decompose the Riemann curvature tensor into the Weyl tensor, the Ricci tensor, and the metric tensor. The Weyl tensor encodes the squash-stretch deformation of moving objects experiencing shear, for example, a rotating spherical body deforming into an oblate spheroid, or a spherical body moving close to (including synchronously orbiting) a large mass deforming into a scalene ellipsoid. The Ricci tensor encodes the volume deformation of a body experiencing tidal forces; if we drop a ball of dust (think very loosely packed coffee grounds) onto a planet, the Ricci tensor describes the tendency of the individual dust particles to converge into a smaller volume as they each fall "straight down" on individual lines converging at the centre of the planet. Naively, we could say that there is a clear relationship between the Ricci tensor and gravitational time dilation, since time is running slower in the direction where the individual fall paths ("geodesics") converge. However, we can make the Ricci tensor go to zero by dropping the dustball onto a planet with enormous radius, such that it's effectively flat. Similarly, ultra massive black holes have essentially no Ricci curvature just outside the horizon, while low-mass black holes have significant Ricci curvature. However, gravitational time dilation is still very strong just outside the horizon of such a massive black hole. In this way we can distinguish between spatial curvature (or if you like, length contraction) and curved time (or if you like, time dilation) in the region around massive bodies. Gravitational time dilation is not a tidal effect, but rather depends on the gravitational potential, which in General Relativity is encoded in the metric tensor. If we orbit our dust ball very rapidly around a heavy planet, it will have substantial Weyl curvature (squashed along its axis of motion, stretched on the other two spatial axes). If the planet has a big enough radius, the Ricci tensor vanishes, leaving us with just the Weyl curvature. In that case, if we measure periodic microscopic processes ("clocks") closest to the planet and furthest from the planet, we will see that the latter are running faster than the former, and the difference is precisely that of gravitational time dilation. Again, this lets us distinguish between changes in length and changes in duration in the region around massive bodies. Gravitational effects are the result of curved spacetime, although in special circumstances practically all of the curvature can be in the timelike axis, and in other special circumstances practically all of the curvature can be in the three spacelike axes. I don't understand what you mean by "quantum progression".
  - vardump 8y ago
    Thanks for your reply, I'll try to digest all that. :) I don't yet understand why "local" slowing of time wouldn't generate all of those tensor components. > I don't understand what you mean by "quantum progression". Just hypothetical local limitation on quantum state changes (=progression).
    - mikhailfranco 8y ago
      I think any computer scientist naturally and naively thinks about your 'quantum progression', since it relates to a transactional rate on interactions experienced in the local proper time of a particle, as it interacts with other real/virtual particles/fields. For example, a particle in deep space experiences faster relative time, compared to one on Earth, because it has 'fewer transactions slowing it down'. The discrete 'tick' of proper time is then one iteration of the spinning polling loop the particle executes while waiting for something to interact with. So if existence is (or requires) computation, then ....
      - mikhailfranco 8y ago
        P.S. There is also a slightly related Transactional Interpretation of Quantum Mechanics, which is more about '2-phase commit' of past and future :) https://en.wikipedia.org/wiki/Transactional_interpretation https://en.wikipedia.org/wiki/Transactional_interpretation
      - raattgift 8y ago
        > a particle in deep space experiences faster relative time, compared to one on Earth, because it has 'fewer transactions slowing it down'. Transactions with what? Other matter? A precise weather and waterproof clock on the ground (so in air) on the surface at the north pole will tick more slowly than an identical clock immersed in seawater several metres below the sea level at the equator, thanks to the oblation of the Earth (and the rotation that causes the oblation). The equatorial water is much denser and warmer than the arctic air, so surely there are more interactions between the water and the clock? See the Early observations subsection of the Gravity measurement section of, starting with 1672 : https://en.wikipedia.org/wiki/Pendulum?oldformat=true#Early_observations https://en.wikipedia.org/wiki/Pendulum?oldformat=true#Early_...

dsnuh 8y ago

I try to be well read on physics, but a lot is admittedly beyond my grasp at the moment, so forgive me if this is a silly question. Is this saying that gravity would not be possible without the curvature of space because if space-time paths, then no matter as we know it would be possible because all particles would be on their own timeline orthogonal to all others? And since no collisions could happen if this were the case, no energy in the system would be converted to mass to bend the space-time and create the geodesic curvature gravity emerges from? If I have this right, does this mean all energy in this system would be massless (photons?)

raattgift 8y ago
I'm not sure what you're asking, exactly. Please ask again, if this doesn't help: Photons have "gravitational charge", in that they both feel other bodies' gravitational influence, and induce a gravitational influence on other bodies. One sees the "feeling" part in observations of gravitational lensing even around our own sun; the generating part is harder to demonstrate, but consider https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics) https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics) In General Relativity, we explain the gravitational charge in terms of spacetime curvature; light both generates curvature (because it has momentum) and responds to curvature (because of the universality of free fall). An isolated photon (or even a non-extreme classical beamlike pulse of light) generates so little curvature that we can safely ignore it in an otherwise Minkowski (flat) spacetime; however, in both cases if it traverses a region of spacetime that is not flat, the isolated photon and classical pulse will respond accordingly. More technically, light contributes to the matter tensor, whose components encode the flux of momentum from one direction to another. Although light has no rest mass, it does have energy-momentum (in special relativity, E = pc; additionally, the Planck-Einstein relation for photon energy E = h\nu, where h is Planck's constant and \nu is the frequency of the photon). So a higher frequency photon contributes more to the stress-energy tensor than a lower frequency photon, and even the lowest-frequency imaginable contributes more than vacuum. (Aside: General Relativity runs into localization problems with extremely low frequency photons if one piles up a lot of them in a region of space comparable in diameter to their wavelengths; they should mutually gravitate, and possibly collapse, but exactly how is so hard to show that probably nobody's done so successfully. How do gravitational interactions among the photons change their wavelengths? (This is an aspect of quantum gravity; there are related practical problems with Hawking radiation, especially near final evaporation, for instance)). > particles would be on their own timeline orthogonal to all others You can contrive a "dimpled" mostly-flat spacetime in which small masses are so widely separated that the curvature they induce is not felt by their neighbours or vice-versa. The contrivance is not especially awful, because we do something similar with "swiss cheese" models of cosmology, where the holes in the cheese are galaxy clusters generating a non-expanding or even collapsing metric (like Schwarzschild or Lemaître-Tolman-Bondi) and the body of the cheese is an expanding Robertson-Walker metric. Already separated holes "float" in the cheese, and the expansion just separates them with ever more cheese, without interfering in the evolution of the galaxies etc. in the holes. Holes which are too close together coalesce into a single larger hole. The spacetime curvature of an expanding "swiss cheese" is extremely strong, but we can turn off the expansion and it is similar to the "dimpled" mostly-flat one; in fact you could model the "dimples" as tiny patches of Schwarzschild space stitched into Minkowski space [1]. The "holes" that are sufficiently separated by the non-growing cheese would stay separated forever. In all these cases, the worldlines of the centres of masses of the holes are essentially entirely timelike. - -- [1] This kind of "stitching" using junction conditions is straightforward if extremely complicated and subject to a number of conditions/assumptions. Misner, Thorne & Wheeler's _Gravitation_ has a chapter on Israel junction conditions, and various authors notably Darmois, O'Brien, Synge, Bonnor and Raju have catalogued stitchings of different types of metrics into one another. Most of these would likely require a trip to sci-hub, as they were written largely in the 1970s to 1990s. Unfortunately, wikipedia seems very sparse on this topic (see for example the Inhomogenous cosmology entry, which is desperate for some love [<- possible note to self]).

6nf 8y ago

So correct me if I'm wrong, but if gravity is merely a curvature in space-time, does that mean there's no such thing as a graviton?

raattgift 8y ago
General Relativity is a classical field theory, so there's no graviton in it. However, there are various quantum gravity research programmes which have tried to quantize the (classical) tensor fields of General Relativity in one way or another, for instance by turning (classical) gravitational waves or position and momentum variables into quantum particles. In these theories the graviton is a spin-2 gauge boson and induce an attractive force on fermions with like gravitational charge. This compares with the photon, a spin-1 gauge boson that induces a repulsive force on fermions with like electric charge (and an attractive force on fermions with opposite electric charge). The spin also corresponds with the rank of the (classical) fields, where in General Relativity we have rank-2 tensors and in electrodynamics we have vectors (which are rank-1 tensors). Unfortunately this approach has a problem: particles other than fermions also have a gravitational charge, including gravitons themselves. (Photons do not have an electric charge, but they do have a gravitational charge). Quantum field theories use a variety of tricks to remain linear theories, but General Relativity's Einstein Field Equations are non-linear. That non-linearity manifests in graviton-graviton interactions in this sort of quantized General Relativity, and we do not know how to stabilize the resulting quantum field theories. In more concrete terms, let's "drop" an electron from higher potential to lower potential. The electron is a fermion with electric and gravitational charge. When we drop it from higher to lower electric potential, out pops a photon. When we drop it from higher to lower gravitational potential, out pops a graviton. But we can't move the photon around the electric field and get it to produce another photon, because the photon is not self-interacting. A graviton, on the other hand, can be dropped down a gravitational potential causing it to emit another graviton. That in turn influences the potential, making it easier for those two gravitons to each emit a further graviton, which influenes the potential, making it easier for those ... and so forth to infinity. Our original electron is in that potential too, so it in turn can emit further gravitons as well. Fortunately this explosion of gravitons only happens in strong gravity, and we only find that deep inside black holes (or in the very early universe), rather than somewhere we can outright observe (assuming the cosmic censorship principle is true), so we can still use this type of theory to describe most of the universe. Unfortunately, gravitational waves are hard enough to detect; when they are quantized, they turn into lots and lots and lots of gravitons, each of which has a tiny fraction of the detectability of a gravitational wave. Detecting an individual graviton experimentally is beyond the ability of present human technology, or anything reasonably likely to appear in the next few decades. So if we ever figure out how to stabilize this type of approach to quantum gravity, we will only be able to assess it against observations of many-graviton systems, likely at extreme events like black hole/neutron star mergers or asymmetrical hypernovae. This is just one broad example of quantum gravity; there are several other approaches, some with a "graviton" dynamics (e.g. string theory's gravitons are a string state) and some without. All these approaches share a common problem: General Relativity describes our universe exceptionally well in practice, and its fundamental components are among the best-tested of any scientific theory to date. Where there is weak gravity, barring significant masses in superpositions of position, any type of quantum gravity has to exactly reproduce the results of General Relativity. It is likely that we will be able to bring microgram masses into such superpositions in the next few years, and we will be able to directly probe one aspect of quantum field theory on (classical) curved spacetime (aka semiclassical gravity). However, if General Relativity fails in that situation, it would certainly allow some useful comparisons between competing theories (with and without gravitons) hoping to extend General Relativity as it in turn extended Newtonian gravity. If only graviton-containing theories match the experimental results, that would be evidence in favour of the existence of a graviton, even without direct detection; conversely, perhaps only non-graviton theories match, in which case the existence of a graviton would seem pretty dubious. Right now, though, we just don't know: in the absence of evidence for or against, gravitons are purely theoretical.

quandarium 8y ago

If you reduce all energy exchange to particles in a vacuum, most of the particles transfering energy are photons. Energy transfer, in general is mostly reducible to photons at a fundamental level. In other words, across the vacuum of space, in an idealized model, where there is nothing but void, any exchange or transfer is via particles traversing the emptiness, and most often, those particles are photons. If two bodies at absolute zero are suspended in a void, and the limits of that void are walls of matter frozen at absolute zero, light years away, and nothing in the scene is emitting photons, what reason would drive the to objects to be attracted toward one another? This is to say, if the arrow of time is a deterministic effect of entropy, and the irreversible aspect of time is a result of the chaotic interactions of many discrete subatomic entities, why would spacetime still be curved by two dark, massive, cold bodies, such that, even in the absence of photon exchange between the two, there are forces acting upon them at a distance, leading them to cross a void, travel down some unseen curvature or gradient and collide? If empty space truly is nothingness and void, then entropy, and by corollary, the arrow of time, only exists in the quantum state of each material body. This would seem to herald a fact that the bodies each move themselves toward eachother, as an incidental phenomenon of being condensed from raw unbound energy to systems of materialized standing waves, (subatomic particles trapped in fixed discrete systems of balanced energy, which does not readily decay into other particles without action from external entities). In total darkness, without the exchange of photons, why would two massive, cold, inert objects move inexorably toward one another? How would each massive object find the other's location and accelerate toward the other, if there is nothing but empty space between?