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Maybe I am missing something, but how could a quantum theory of gravity explain curvature of spacetime? If a photon follows a curve around a large mass, it does

Maybe I am missing something, but how could a quantum theory of gravity explain curvature of spacetime? If a photon follows a curve around a large mass, it does so because that curve is the shortest path through spacetime, if I am not mistaken. But how would a theoretical graviton explain this effect? Would it interact with the photon to carry over a force? And if so, how would it "know" the direction of said force (wouldn't it be itself subject to gravity?). Or would it interact with space itself?

irjustin 6y ago
All good questions. Fast answer: we have no clue. Until we have a good quantum theory of gravity, we can't explain how it affects the curvature of spacetime at the quantum level. All the current quantum gravity theories, i.e. loop quantum gravity, string theory, etc... work if you squint just right, otherwise they fall apart really fast. Spacetime has a bunch of episodes on going theories. A very large elephant in the room is, a lot of these ideas don't have a method of testing. And sure, a lot of theories start out without a practical method until decades later where technology can catch up, but we don't even have a theoretical test to start with! Spacetime recently did an episode touching on some of your thoughts at the classical level and even then it's... relative[0]. [0] https://www.youtube.com/watch?v=OHdV9aO6jaE https://www.youtube.com/watch?v=OHdV9aO6jaE
- ArtWomb 6y ago
  >>> we don't even have a theoretical test to start with Every quantum computing calculation, is a desktop quantum experiment ;) Non-Gaussianity as a signature of a quantum theory of gravity https://arxiv.org/abs/2004.01189 https://arxiv.org/abs/2004.01189 Another cool aspect of the optomechanical tabletop device research was that it wasn't entirely fruitless at all. Although the original mission of finding gravity in quantum space may have failed. The behavior exhibited was so extremely non-classical that an explosion of waveguide manipulating metamaterials was born!
contravariant 6y ago
The idea is that it would be similar to the way that photons mediate the effect of the electromagnetic field. Though I've personally always thought that this description wasn't too good at explaining large gradual changes in the electromagnetic field, similar to how gravitons are a bit of weird model for the large gradual curvature of spacetime (it's not that it can be done but it is a bit awkward). Anyway the most straightforward way to implement gravitons in this sense doesn't pan out mathematically so we're not even sure if this model is at all accurate.
peignoir 6y ago
My very light understanding of quantum physics is that it does not really translate in words. It s like trying to explain what’s apple times i in English. That being said the standard model is very good at predicting outcomes at a small scale. The issue with gravity is things where General relativity applies need to be huge to be observe. If we want to find a quantum model of gravity we need to observe what gravity does at a small scale and try to fit a model within these observations. That’s why this experiment is a step in the right direction. Maybe gravity does “weird” things at a smaller scale… Back to your question we don’t know we just observe :)
- martijn_himself 6y ago
  > My very light understanding of quantum physics is that it does not really translate in words. It s like trying to explain what’s apple times i in English. This is really one of the best comments I have read on an article like this ever. Having had a cursory introduction to quantum physics in math terms and not understanding much of it, I find it impossible to read any sort of popular science literature on the subject. It's always going to be a very vague description with analogies that raise more questions than give answers. That's not to say I don't applaud the effort of quantum physicists to explain it to a broader public- I just think that most of the time they come away thinking it's some sort of black magic rather than solid science.
  - Koshkin 6y ago
    A book about QM by Zagoskin is one of the more recent attempts to be open and honest in this regard. It is packed with math.
  - FriendlyNormie 6y ago
    You could have just said you’re retarded instead of typing all those words.
- Koshkin 6y ago
  > apple times i To be fair, Apple has been doing this (multiplying things by i) for a while now. Not that they have given any explanation why.
deleted 6y ago
[deleted]
codethief 6y ago
> If a photon follows a curve around a large mass, it does so because that curve is the shortest path through spacetime, if I am not mistaken. Correct! > Maybe I am missing something, but how could a quantum theory of gravity explain curvature of spacetime? […] how would a theoretical graviton explain this effect? Note that a quantum theory of gravity need not involve gravitons. The latter only appear in approaches that assume that gravity is a more or less ordinary [string] quantum field theory (QFT). Personally, I've never found this very convincing. Neither did Hawking by the way, who once wrote > It would be rather boring if this were the case. Gravity would be just like any other field. But I believe it is distinctively different, because it shapes the arena in which it acts, unlike other fields which act in a fixed spacetime background.[0] Anyway, assuming we're indeed looking at a quantum theory of gravity that involves gravitons, let's tackle your other questions: > Would it interact with the photon to carry over a force? Yes, the graviton would interact (very weakly) with all particles we know, including itself. > And if so, how would it "know" the direction of said force (wouldn't it be itself subject to gravity?). My first, rather theoretical answer would be: This is not a well-defined question. After all, gravity as a force is a Newtonian concept which does not exist in General Relativity. As you already said, in Relativity matter is "guided" by the curvature of spacetime, not by a force. Moreover, in quantum mechanics Newtonian forces don't (really) exist, either, and while people often speak of "forces" as if they were Newtonian, these forces are really mediated by bosons. My second, more direct answer would be: In two-particle interactions in quantum field theory, the outgoing particles usually leave the scenery at a different angle (and momentum) than their initial one. So the photon might get deflected by interacting with the graviton. > Or would [the graviton] interact with space itself? As already mentioned, the graviton would interact with itself in the same way as gravitational waves can interact with one another. As for space(-time) itself, however, note that the aforementioned "theories involving gravitons" usually assume that spacetime itself is flat (i.e., it is your run-of-the-mill Minkowski spacetime) and that gravitons are just another quantum field on that (so-called) background spacetime. As I personally don't like this idea a lot (see above), I can only guess as to whether its proponents really think spacetime is flat or whether they only assume this to simplify things. After all, we have no idea how to do QFT on curved spacetime in a mathematically precise way. (In fact, we don't know how to do QFT on a flat Minkowski background in a mathematically precise way, either.) [0]: Hawking: The Nature of Space and Time, https://arxiv.org/abs/hep-th/9409195v1 https://arxiv.org/abs/hep-th/9409195v1
s1dev 6y ago
It's generally expected that the spacetime geometry is emergent as some coherent state of gravitons Unpacking that statement a bit: - In quantum mechanics there is a procedure called "quantization" that takes in a classical theory and gives you a quantum theory. - The things (excitations) that follow your classical intuition are waves [*] and when you quantize them, you get excitations that carry discrete quanta of energy (ex. photons / gravitons / <particle name of choice>) - It's convenient to talk about states with a definite number of particles because of the way the math works out. - To get back to something that follows your classical intuition (waves) you have to consider superpositions of the states with definite particle number. These particular states are known as "coherent states" and very much look like your normal notion of a classical wave which is something with a definite phase and amplitude As an example: you can recover something that looks like a classical radio wave in quantum electrodynamics by taking a particular superposition of states with a definite number of photons. Since it's in superposition, the number of photons in the state isn't well defined. By well defined here, I mean if you measure the number of particles, you can get different answers due to Heisenberg uncertainty. You can even get standing waves in a cavity (like a microwave oven) which is an oscillator and the state has a definite phase and amplitude that you can measure. Everything looks nice and classical but if you look hard enough you find that actually you can't put in an infinitesimally small amount of energy, you can only add it in multiples of some constant (h-bar). Now replace the electromagnetic field with a field that represents the curvature of spacetime and away you go, your theory works with small excitation quanta (gravitons) but the actual things in everyday life that you see are complicated superpositions of these things [**]. So the geometry (spacetime) is some superposition of states with definite graviton numbers that have no classical description. Much like how the electric field around an electron or an oscillating electromagnetic field inside of a microwave oven is a superposition of states with definite photon number. Sidenote: it’s also easy to talk about a single graviton in isolation sitting on top of that emergent geometry [*] This is "backwards" from pop-sci / intro physics because we are talking about electromagnetism which shares some superficial similarities with gravity [**] For some technical reasons this viewpoint doesn't apply to particles which aren't "force-carriers" ex. electrons, protons, etc
- Koshkin 6y ago
  The idea of (the first) quantization has always looked suspicious to me. It feels like a theoretical hack, a leap of faith, as it were. No wonder it's not working as well in gravity's case.
SiempreViernes 6y ago
A quantum theory of gravity would by definition explain the curvature of spacetime, since said curvature (and its dynamics) is how gravity is expressed. Asking how a quantum theory of gravity would explain curvature is then asking someone to give you a completed quantum theory of gravity, and it is not at all clear that the first successful theory will describe the world terms of particles. However, it can be instructive to consider electromagnetism as as classical theory and then as a quantum theory: waves in fields become enormous showers of particles, and these photons then carry the force that attracts or repels and can sometimes interact with each other.
- choeger 6y ago
  Ah, you caught me there ;). I was hoping someone would spill their secret quantum gravity theory on Hackernews.
- 8note 6y ago
  A quantum theory of gravity doesn't have to explain curvature of spacetime. It needs to produce better experimental results than modeling gravity as the curvature of spacetime. Whether that still describes it in a way that can be mapped to a curvature based explanation is irrelevant. It should for some cases give the same results?
  - raattgift 6y ago
    A quantum gravity theory might not use the vocabulary of curvature or the tensor fields and scalar objects found in the Einstein Field Equations of General Relativity, but that's not quite the same as "doesn't have to explain curvature". For the theory to be viable it would have to explain why General Relativity is such a good effective theory at solar system scales. One can do this by treating General Relativity as a theory which emerges in some limit of the more fundamental theory of gravitation (or everything). One can compare that with how Newtonian gravitation emerges from General Relativity as one takes c to infinity, or as one takes v^2/c^2 (where c is usually set to one; one can likewise expand in other small parameters) to zero. The first is the basis for the first section of https://en.wikipedia.org/wiki/Post-Newtonian_expansion https://en.wikipedia.org/wiki/Post-Newtonian_expansion The second section has more explanatory power: Newtonian gravitation emerges from General Relativistic systems when multipole moments are small and the distance to the sources is large. The key to the success of General Relativity compared to many of its competitors is that it very straightforwardly produces the Newtonian solution for the well-studied orbits of the Jovian system, while explaining the anomaly in the precession of Mercury's orbit. One would have to be able to show conditions in the new theory in which General Relativity emerges (and for which General Relativity has observational support), or the theory is not going to get much practical use in astrophysics or GPS-style engineering, even if it appears to get the microscopic details right. Practically[1] nobody -- there are several serious contenders like Loop Quantum Gravity -- who has developed an alternative theory of gravitation (or everything) has been able to show at all that General Relativity emerges from it. Lastly, because we don't have the technology to probe the quantum gravitation sector directly, but we do have the technology to probe General Relativity very finely (such is the topic of the article), I can't see how a failure to explain General Relativity's results can do anything but undermine quantum gravity candidates. - -- [1] People have proposed turning various constants into spacetime-dependent functions, and find that such adjustments can solve some things but generally have to vanish practically everywhere, so such theories are not viable. Among these are Palatini and others https://en.wikipedia.org/wiki/F(R)_gravity https://en.wikipedia.org/wiki/F(R)_gravity and Brans-Dicke https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory. These families of theories usually show how General Relativity could emerge, but further investigation almost always shows that the parameters or features that distinguish these theories from General Relativity are incompatible with observation and so must vanish.
lmilcin 6y ago
Indeed, we have no idea what is the connection between time, space, mass and other forces. We have some findings (hey! mass is intimately connected to curvature of spacetime!) and approximations in forms formulae we know, but that's just it. I think our physics is akin to epicycles before we found actual place of Earth in solar system and shape of planet orbits.
ganzuul 6y ago
PBS Space Time on Youtube just did an episode on light interacting with gravity. You should check it out.
Koshkin 6y ago
Well, if you take into account also the fact that the "monochromatic" photon (one that has a definite wavelength, or momentum) does not even have a certain position in space, the idea of its interaction with an actual (non-virtual) graviton near a mass located at a certain point in space will start looking problematic.
- tinus_hn 6y ago
  How is it more problematic than a photon interacting with a normal matter particle?
kjrose 6y ago
Excellent question. And in this lies one of the biggest reasons successfully quantizing gravity would be huge. The two theories fundamentally don't mesh. You quickly end up with infinities all over the place. There's a variety of ideas on how to do this but none have really played out.
drran 6y ago
Spacetime is mathematical abstraction, not a real thing. Photon changes trajectory because of differences in conductivity: light propagates a bit slower in stronger gravitational force.
Iwan-Zotow 6y ago
> But how would a theoretical graviton explain this effect? there would be more gravitons if the field is stronger. No gravitons in the empty space. Gravitons are quantas of the always attractive forces. So traveling in the gravitation field, photon constantly absorbs and emits gravitons, more gravitons - more interactions, thus "bending" photon trajectory in the presence of the gravitation. If you a bit math inclined, you could look at Feynman's lectures on gravitation (Caltech, fall semester of 1962). They are quite outdated, a lot of mistakes - he was building it as he is preparing lectures, but approach itself is very interesting He starts with MICROSCOPIC description - lets introduce gravitons, we want bosons which always attract, so no spin 1, only spin 0 or 2. Dropped spin 0 (don't remember why) and start building interaction diagrams form spin 2 bosons like in any standard quantum field theory. Discussed exactly the topic of "trajectory bending", got to quasi-classic limit and showed that this is Einstein field eqs, got to weak field limit and Newton, ... And all this (even his mistakes) in beautiful style and logic.
cma 6y ago
I think static forces in quantum mechanics have a little bit different part of the framework: https://en.wikipedia.org/wiki/Static_forces_and_virtual-particle_exchange https://en.wikipedia.org/wiki/Static_forces_and_virtual-part... Even though nothing is fully static, it is a way of approximating. So the theories would presumably have something similar for the static gravity case you are describing.

irjustin 6y ago

All good questions. Fast answer: we have no clue. Until we have a good quantum theory of gravity, we can't explain how it affects the curvature of spacetime at the quantum level. All the current quantum gravity theories, i.e. loop quantum gravity, string theory, etc... work if you squint just right, otherwise they fall apart really fast. Spacetime has a bunch of episodes on going theories. A very large elephant in the room is, a lot of these ideas don't have a method of testing. And sure, a lot of theories start out without a practical method until decades later where technology can catch up, but we don't even have a theoretical test to start with! Spacetime recently did an episode touching on some of your thoughts at the classical level and even then it's... relative[0]. [0] https://www.youtube.com/watch?v=OHdV9aO6jaE https://www.youtube.com/watch?v=OHdV9aO6jaE

ArtWomb 6y ago
>>> we don't even have a theoretical test to start with Every quantum computing calculation, is a desktop quantum experiment ;) Non-Gaussianity as a signature of a quantum theory of gravity https://arxiv.org/abs/2004.01189 https://arxiv.org/abs/2004.01189 Another cool aspect of the optomechanical tabletop device research was that it wasn't entirely fruitless at all. Although the original mission of finding gravity in quantum space may have failed. The behavior exhibited was so extremely non-classical that an explosion of waveguide manipulating metamaterials was born!

contravariant 6y ago

The idea is that it would be similar to the way that photons mediate the effect of the electromagnetic field. Though I've personally always thought that this description wasn't too good at explaining large gradual changes in the electromagnetic field, similar to how gravitons are a bit of weird model for the large gradual curvature of spacetime (it's not that it can be done but it is a bit awkward). Anyway the most straightforward way to implement gravitons in this sense doesn't pan out mathematically so we're not even sure if this model is at all accurate.

peignoir 6y ago

My very light understanding of quantum physics is that it does not really translate in words. It s like trying to explain what’s apple times i in English. That being said the standard model is very good at predicting outcomes at a small scale. The issue with gravity is things where General relativity applies need to be huge to be observe. If we want to find a quantum model of gravity we need to observe what gravity does at a small scale and try to fit a model within these observations. That’s why this experiment is a step in the right direction. Maybe gravity does “weird” things at a smaller scale… Back to your question we don’t know we just observe :)

martijn_himself 6y ago
> My very light understanding of quantum physics is that it does not really translate in words. It s like trying to explain what’s apple times i in English. This is really one of the best comments I have read on an article like this ever. Having had a cursory introduction to quantum physics in math terms and not understanding much of it, I find it impossible to read any sort of popular science literature on the subject. It's always going to be a very vague description with analogies that raise more questions than give answers. That's not to say I don't applaud the effort of quantum physicists to explain it to a broader public- I just think that most of the time they come away thinking it's some sort of black magic rather than solid science.
- Koshkin 6y ago
  A book about QM by Zagoskin is one of the more recent attempts to be open and honest in this regard. It is packed with math.
- FriendlyNormie 6y ago
  You could have just said you’re retarded instead of typing all those words.
Koshkin 6y ago
> apple times i To be fair, Apple has been doing this (multiplying things by i) for a while now. Not that they have given any explanation why.

deleted 6y ago

[deleted]

codethief 6y ago

> If a photon follows a curve around a large mass, it does so because that curve is the shortest path through spacetime, if I am not mistaken. Correct! > Maybe I am missing something, but how could a quantum theory of gravity explain curvature of spacetime? […] how would a theoretical graviton explain this effect? Note that a quantum theory of gravity need not involve gravitons. The latter only appear in approaches that assume that gravity is a more or less ordinary [string] quantum field theory (QFT). Personally, I've never found this very convincing. Neither did Hawking by the way, who once wrote > It would be rather boring if this were the case. Gravity would be just like any other field. But I believe it is distinctively different, because it shapes the arena in which it acts, unlike other fields which act in a fixed spacetime background.[0] Anyway, assuming we're indeed looking at a quantum theory of gravity that involves gravitons, let's tackle your other questions: > Would it interact with the photon to carry over a force? Yes, the graviton would interact (very weakly) with all particles we know, including itself. > And if so, how would it "know" the direction of said force (wouldn't it be itself subject to gravity?). My first, rather theoretical answer would be: This is not a well-defined question. After all, gravity as a force is a Newtonian concept which does not exist in General Relativity. As you already said, in Relativity matter is "guided" by the curvature of spacetime, not by a force. Moreover, in quantum mechanics Newtonian forces don't (really) exist, either, and while people often speak of "forces" as if they were Newtonian, these forces are really mediated by bosons. My second, more direct answer would be: In two-particle interactions in quantum field theory, the outgoing particles usually leave the scenery at a different angle (and momentum) than their initial one. So the photon might get deflected by interacting with the graviton. > Or would [the graviton] interact with space itself? As already mentioned, the graviton would interact with itself in the same way as gravitational waves can interact with one another. As for space(-time) itself, however, note that the aforementioned "theories involving gravitons" usually assume that spacetime itself is flat (i.e., it is your run-of-the-mill Minkowski spacetime) and that gravitons are just another quantum field on that (so-called) background spacetime. As I personally don't like this idea a lot (see above), I can only guess as to whether its proponents really think spacetime is flat or whether they only assume this to simplify things. After all, we have no idea how to do QFT on curved spacetime in a mathematically precise way. (In fact, we don't know how to do QFT on a flat Minkowski background in a mathematically precise way, either.) [0]: Hawking: The Nature of Space and Time, https://arxiv.org/abs/hep-th/9409195v1 https://arxiv.org/abs/hep-th/9409195v1

s1dev 6y ago

It's generally expected that the spacetime geometry is emergent as some coherent state of gravitons Unpacking that statement a bit: - In quantum mechanics there is a procedure called "quantization" that takes in a classical theory and gives you a quantum theory. - The things (excitations) that follow your classical intuition are waves [*] and when you quantize them, you get excitations that carry discrete quanta of energy (ex. photons / gravitons / <particle name of choice>) - It's convenient to talk about states with a definite number of particles because of the way the math works out. - To get back to something that follows your classical intuition (waves) you have to consider superpositions of the states with definite particle number. These particular states are known as "coherent states" and very much look like your normal notion of a classical wave which is something with a definite phase and amplitude As an example: you can recover something that looks like a classical radio wave in quantum electrodynamics by taking a particular superposition of states with a definite number of photons. Since it's in superposition, the number of photons in the state isn't well defined. By well defined here, I mean if you measure the number of particles, you can get different answers due to Heisenberg uncertainty. You can even get standing waves in a cavity (like a microwave oven) which is an oscillator and the state has a definite phase and amplitude that you can measure. Everything looks nice and classical but if you look hard enough you find that actually you can't put in an infinitesimally small amount of energy, you can only add it in multiples of some constant (h-bar). Now replace the electromagnetic field with a field that represents the curvature of spacetime and away you go, your theory works with small excitation quanta (gravitons) but the actual things in everyday life that you see are complicated superpositions of these things [**]. So the geometry (spacetime) is some superposition of states with definite graviton numbers that have no classical description. Much like how the electric field around an electron or an oscillating electromagnetic field inside of a microwave oven is a superposition of states with definite photon number. Sidenote: it’s also easy to talk about a single graviton in isolation sitting on top of that emergent geometry [*] This is "backwards" from pop-sci / intro physics because we are talking about electromagnetism which shares some superficial similarities with gravity [**] For some technical reasons this viewpoint doesn't apply to particles which aren't "force-carriers" ex. electrons, protons, etc

Koshkin 6y ago
The idea of (the first) quantization has always looked suspicious to me. It feels like a theoretical hack, a leap of faith, as it were. No wonder it's not working as well in gravity's case.

SiempreViernes 6y ago

A quantum theory of gravity would by definition explain the curvature of spacetime, since said curvature (and its dynamics) is how gravity is expressed. Asking how a quantum theory of gravity would explain curvature is then asking someone to give you a completed quantum theory of gravity, and it is not at all clear that the first successful theory will describe the world terms of particles. However, it can be instructive to consider electromagnetism as as classical theory and then as a quantum theory: waves in fields become enormous showers of particles, and these photons then carry the force that attracts or repels and can sometimes interact with each other.

choeger 6y ago
Ah, you caught me there ;). I was hoping someone would spill their secret quantum gravity theory on Hackernews.
8note 6y ago
A quantum theory of gravity doesn't have to explain curvature of spacetime. It needs to produce better experimental results than modeling gravity as the curvature of spacetime. Whether that still describes it in a way that can be mapped to a curvature based explanation is irrelevant. It should for some cases give the same results?
- raattgift 6y ago
  A quantum gravity theory might not use the vocabulary of curvature or the tensor fields and scalar objects found in the Einstein Field Equations of General Relativity, but that's not quite the same as "doesn't have to explain curvature". For the theory to be viable it would have to explain why General Relativity is such a good effective theory at solar system scales. One can do this by treating General Relativity as a theory which emerges in some limit of the more fundamental theory of gravitation (or everything). One can compare that with how Newtonian gravitation emerges from General Relativity as one takes c to infinity, or as one takes v^2/c^2 (where c is usually set to one; one can likewise expand in other small parameters) to zero. The first is the basis for the first section of https://en.wikipedia.org/wiki/Post-Newtonian_expansion https://en.wikipedia.org/wiki/Post-Newtonian_expansion The second section has more explanatory power: Newtonian gravitation emerges from General Relativistic systems when multipole moments are small and the distance to the sources is large. The key to the success of General Relativity compared to many of its competitors is that it very straightforwardly produces the Newtonian solution for the well-studied orbits of the Jovian system, while explaining the anomaly in the precession of Mercury's orbit. One would have to be able to show conditions in the new theory in which General Relativity emerges (and for which General Relativity has observational support), or the theory is not going to get much practical use in astrophysics or GPS-style engineering, even if it appears to get the microscopic details right. Practically[1] nobody -- there are several serious contenders like Loop Quantum Gravity -- who has developed an alternative theory of gravitation (or everything) has been able to show at all that General Relativity emerges from it. Lastly, because we don't have the technology to probe the quantum gravitation sector directly, but we do have the technology to probe General Relativity very finely (such is the topic of the article), I can't see how a failure to explain General Relativity's results can do anything but undermine quantum gravity candidates. - -- [1] People have proposed turning various constants into spacetime-dependent functions, and find that such adjustments can solve some things but generally have to vanish practically everywhere, so such theories are not viable. Among these are Palatini and others https://en.wikipedia.org/wiki/F(R)_gravity https://en.wikipedia.org/wiki/F(R)_gravity and Brans-Dicke https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory. These families of theories usually show how General Relativity could emerge, but further investigation almost always shows that the parameters or features that distinguish these theories from General Relativity are incompatible with observation and so must vanish.

lmilcin 6y ago

Indeed, we have no idea what is the connection between time, space, mass and other forces. We have some findings (hey! mass is intimately connected to curvature of spacetime!) and approximations in forms formulae we know, but that's just it. I think our physics is akin to epicycles before we found actual place of Earth in solar system and shape of planet orbits.

ganzuul 6y ago

PBS Space Time on Youtube just did an episode on light interacting with gravity. You should check it out.

Koshkin 6y ago

Well, if you take into account also the fact that the "monochromatic" photon (one that has a definite wavelength, or momentum) does not even have a certain position in space, the idea of its interaction with an actual (non-virtual) graviton near a mass located at a certain point in space will start looking problematic.

tinus_hn 6y ago
How is it more problematic than a photon interacting with a normal matter particle?

kjrose 6y ago

Excellent question. And in this lies one of the biggest reasons successfully quantizing gravity would be huge. The two theories fundamentally don't mesh. You quickly end up with infinities all over the place. There's a variety of ideas on how to do this but none have really played out.

drran 6y ago

Spacetime is mathematical abstraction, not a real thing. Photon changes trajectory because of differences in conductivity: light propagates a bit slower in stronger gravitational force.

Iwan-Zotow 6y ago

> But how would a theoretical graviton explain this effect? there would be more gravitons if the field is stronger. No gravitons in the empty space. Gravitons are quantas of the always attractive forces. So traveling in the gravitation field, photon constantly absorbs and emits gravitons, more gravitons - more interactions, thus "bending" photon trajectory in the presence of the gravitation. If you a bit math inclined, you could look at Feynman's lectures on gravitation (Caltech, fall semester of 1962). They are quite outdated, a lot of mistakes - he was building it as he is preparing lectures, but approach itself is very interesting He starts with MICROSCOPIC description - lets introduce gravitons, we want bosons which always attract, so no spin 1, only spin 0 or 2. Dropped spin 0 (don't remember why) and start building interaction diagrams form spin 2 bosons like in any standard quantum field theory. Discussed exactly the topic of "trajectory bending", got to quasi-classic limit and showed that this is Einstein field eqs, got to weak field limit and Newton, ... And all this (even his mistakes) in beautiful style and logic.

cma 6y ago

I think static forces in quantum mechanics have a little bit different part of the framework: https://en.wikipedia.org/wiki/Static_forces_and_virtual-particle_exchange https://en.wikipedia.org/wiki/Static_forces_and_virtual-part... Even though nothing is fully static, it is a way of approximating. So the theories would presumably have something similar for the static gravity case you are describing.