7 ms·

It's too bad the top-rated answer on Reddit says "GR does not say Gravity is not a force (or if you do say it’s not a force, then none of the other forces are f

It's too bad the top-rated answer on Reddit says "GR does not say Gravity is not a force (or if you do say it’s not a force, then none of the other forces are forces either)" rather than explaining what people mean when they say gravity is not a force (basically, it affects the geometry of spacetime in such a way that an unaccelerated particle can still move along a path that's not a straight line in the traditional sense) and why nonetheless we can treat gravity approximately (that is, perturbatively) as if it were a force, and why this perturbative description when quantized predicts a spin-2 boson, the graviton. Oh well.

JumpCrisscross 2y ago
Help me with the last bit, where the perturbations when quantised predict spin-2 bosons.
- brylie 2y ago
  [flagged]
  - krsrhe 2y ago
    [dead]
- CamperBob2 2y ago
  Honestly, the flagged/dead answer from ChatGPT makes a lot of sense to me. Can someone explain where it goes wrong, without resorting to the usual snide remarks about parrots and other whistling-past-the-graveyard rhetoric? Is it just making up BS regarding the attributes imparted by various spin values, or is that a reasonable explanation of why gravitons are presumed to be spin-2 particles?
  - brylie 2y ago
    Indeed. I shared what I thought was a detailed and helpful generative response to further the discussion and hopefully receive feedback from those knowledgeable on the topic. It's disappointing to see it met with downvotes and flagging, rather than constructive conversation.
    - CamperBob2 2y ago
      Same old story throughout history. People feel threatened by technology, and they lash out. It's disappointing here, for sure.
krsrhe 2y ago
I recognize your name and believe you know what you are talking about, so please, please tell us the why! You or someone you know probable has a blog post or article you can link, maybe?
- morcus 2y ago
  Wow I never look at usernames but maybe I should start. I believe I read his `Gauge Fields, Knots, and Gravity` back in college (or tried to, anyways. it was a touch above my level at the time)
sandworm101 2y ago
I had a prof explain it using basic analogy. Forces are things that can move other things. An electrical field can move a metal ball. So too can a metal bat. put a g-meter on the ball and it will register acceleration as force is applied to the ball. Gravity is different. Put a g-meter on a falling metal ball and it will detect zero acceleration. No force is acting upon a ball. Put that metal ball in an eccentric orbit around the earth and it will speed up and slow down during each orbit, but the g-meter will register zero acceleration. The ball falls but is no not accelerated. So gravity is not a force because it doesn't move things. Gravity is something different.
- gwd 2y ago
  But the acceleration meter won't measure any force because gravity is acting on every part of it uniformly. If you had an acceleration meter entirely made out of the same magnetic substance, and you brought a magnet near it, would the acceleration meter register anything, or would it read zero acceleration, since all parts of it were being acted on uniformly (and thus didn't "notice" any acceleration)?
  - gwd 2y ago
    (Replying to myself) I guess one answer to this is that particles which are (supposedly?) massless, like the photon, are affected by the space-time warping effects of gravity. A parallel construction wouldn't be true of magnetism or an electric field. Furthermore, when we detect gravity waves, they come at the same time as corresponding gamma ray bursts; since the gamma rays are affected by the space-time warping effects of gravity, this means that the gravity waves themselves are affected by the space-time warping effects of gravity. So gravity probably is something else. But who knows!
    - lupire 2y ago
      That could also be explained by light having momentum which is affected by (but does not generate) the force of gravity acts like mass.
  - 2y ago
gnramires 2y ago
I've recently learned about Kaluza–Klein theories (just curious wiki browsing), do they unify electromagnetism and gravity in the sense that electromagnetism also "works by changing the geometry of spacetime" ? How does this relate to QFT?
- elashri 2y ago
  > do they unify electromagnetism and gravity in the sense that electromagnetism also "works by changing the geometry of spacetime" ? Yes, somehow actually. KK theories introduce an extra spatial dimension beyond our usual (3+1) which is postulated to be "compactified". This just means that it's curled up on itself at such a tiny scale that we don't directly perceive it. The way this extra dimension is curled and shaped affects the geometry of the overall 5-D spacetime. How it is connected to EM is that now these geometrical variations in the 5-D spacetime, when viewed from our 4-D perspective, manifest as the EM force and its associated field. So you can say like in GR which have gravity arises as consequence of geometry, it is in KK that EM is consequence of geometry. However, the geometry and details are different. > How does this relate to QFT? Not much in the sense that they can provide useful information to each other. QFT describes forces in terms of interactions mediated by particles (e.g., photons for EM). KK, while primarily geometrical, give hints that perhaps these force-carrying particles can be associated with specific vibrational modes of the extra dimension. Of course KK theory only include EM and gravity. So we know for sure that we need to go beyond KK. This was the actual motivation for people to think about string theory to expand the original KK work.
  - Karellen 2y ago
    > KK theories introduce an extra spatial dimension beyond our usual (3+1) which is postulated to be "compactified". This just means that it's curled up on itself You've just explained "compactified" in terms of being "curled up", but that doesn't really help (for me, at least). What does it mean for a dimension to be "curled up"?
    - elashri 2y ago
      I'm sorry if I'm confusing. That's the term I use, usually assuming that it is obvious (like referring to C++ templates to a python developer without explaining what is that). To be honest, it is hard to visualize, as it is counterintuitive of what we think of space. While I know many people would disagree, I really like the garden hose analogy [1]. The idea is to simply imagine a very long garden hose. From a great distance, it looks like a one-dimensional line. Now you get closer, and realize it has a second dimension, which is its circumference curled around that seemingly 1-D line. An ant walking on the hose can move along its length, but also in a circle around it. The extra dimension in KK is like this circumference. It is tiny, curled up so we don't directly notice it, but still potentially there. [1] https://www.preposterousuniverse.com/blog/2004/06/30/extra-dimensions/ https://www.preposterousuniverse.com/blog/2004/06/30/extra-d...
lubesGordi 2y ago
What does it mean to 'treat gravity approximately (that is, perturbatively)'? That sounds like something we do to model, which only approximates reality. That model sounds like it shouldn't be used to predict anything else? Or at least whatever is predicted shouldn't be expected to exist in 'reality'.
- canjobear 2y ago
  It’s models all the way down.
- nyrikki 2y ago
  John von Neumann: "... the sciences do not try to explain, they hardly even try to interpret, they mainly make models. By a model is meant a mathematical construct which, with the addition of certain verbal interpretations, describes observed phenomena. The justification of such a mathematical construct is solely and precisely that it is expected to work—that is, correctly to describe phenomena from a reasonably wide area." Both GR and QFT are insanely accurate models, but they are just models. The N-body problem is undecidable, and Gödel, Turing, Church and other s proved that is the best we can do. https://philsci-archive.pitt.edu/13175/1/parker2003.pdf https://philsci-archive.pitt.edu/13175/1/parker2003.pdf Western reductionism or Laplacian determinism is a good framework for practical, computable models. QFT actually is actually one of the counterexamples to Western reductionism. But models are reductive and scientific models are just models. Don't confuse the map for the territory. All models are wrong, some are useful; is another way of saying the same thing.
  - arter 2y ago
    Could you elaborate or share some articles on why QFT is a counterexample to western reductionism ? In laymans terms that is.
    - nyrikki 2y ago
      The oversimplified version is Laplac's deamon can't split quantum superposition. Superposition being inseparable is a large part on why the many words concept is popular with some people. It is about regaining a form of determinism.
pdonis 2y ago
I think that more generally the answer is not responsive to the actual question, which is: why do GR and QM need to be unified? To be fair, the question itself wrongly conflates "gravity needs a gauge boson" with the question about GR and QM being unified. Not all theories of quantum gravity involve a "gauge boson" for gravity along the lines of the other Standard Model interactions. Nor does the basic rationale for why we think gravity needs to be quantized involve a "gauge boson". It involves simple reasoning about how QM works. Say we have an experiment which puts an object with non-negligible stress-energy into a superposition of being in two different positions (for example, we make its position depend on the outcome of a spin measurement on a qubit). QM would say that spacetime would then need to also be in a superposition of two different geometries. But GR, as a classical theory, has no way to handle that. We would need a quantum theory of gravity, i.e., a quantum theory that can handle superpositions of different spacetime geometries.
- trhway 2y ago
  >Say we have an experiment which puts an object with non-negligible stress-energy into a superposition of being in two different positions (for example, we make its position depend on the outcome of a spin measurement on a qubit). QM would say that spacetime would then need to also be in a superposition of two different geometries. The energy to move the object into a given position is an additional element here unaccounted for in your model. 2 different positions to move object into - 2 different energies (more specifically 2 different changes to the starting, before the experiment, stress energy distribution of the Universe). When corresponding moving energies (ie. their GR effects) are accounted for in those 2 cases it may as well be that those 2 cases are indistinguishable from the GR point of view, ie. those 2 supposedly different spacetime geometries happen to be the same. The superposition of 2 indistinguishable cases - it doesn't really matter is it superposition or not.
  - qnleigh 2y ago
    There is an entire sub-field of experimental science that involves putting ever larger objects in superpositions of different locations. These experiments are no closer to testing quantum gravity, but they falsify whatever it is you're trying to say here. See https://www.nature.com/articles/srep13884 https://www.nature.com/articles/srep13884 for a random example.
me_me_me 2y ago
it would be so much easier to visualize it if we could imagine 4th dimension.
layer8 2y ago
You can say similar things about the field of the other forces too, though. The path of a charged particle in the EM field could be described as that particle experiencing a different space-time geometry, arising from the EM and gravitational field combined, and thus EM could also be seen as a geometry and not a force. In fact, the impulse of photons, which are vibrations in the EM field, does affect the curvature of space-time, similar to how particles with mass do.
- pdonis 2y ago
  > The path of a charged particle in the EM field could be described as that particle experiencing a different space-time geometry No, it can't, because the EM interaction does not obey the equivalence principle, as gravity does. The geometric interpretation of gravity relies on the equivalence principle. To state this another way: if I put two objects with different masses at a given point in spacetime and give them both the same initial velocity in the same direction, their paths through spacetime under gravity will be the same. But if I put two objects with different charges at a given point in the same electromagnetic field and give them both the same initial velocity in the same direction, their paths through spacetime will not be the same. And this remains true even if I add "extra dimensions" to "spacetime" along the lines of Kaluza-Klein theory, to represent the EM field.
codexb 2y ago
I feel like it's easier to explain that gravity is a force between matter and space(time) and not necessarily between matter and matter, like we presume the other forces are.
Biganon 2y ago
Woaw, a top comment by John Carlos Baez himself!
machina_ex_deus 2y ago
Non gravity forces are quite linear and simple, the coupling with fermions is only through covariant derivative. Gravity is extremely non linear, when you look at the expression for Ricci tensor, it is much more complicated than other forces.

JumpCrisscross 2y ago

Help me with the last bit, where the perturbations when quantised predict spin-2 bosons.

brylie 2y ago
[flagged]
- krsrhe 2y ago
  [dead]
CamperBob2 2y ago
Honestly, the flagged/dead answer from ChatGPT makes a lot of sense to me. Can someone explain where it goes wrong, without resorting to the usual snide remarks about parrots and other whistling-past-the-graveyard rhetoric? Is it just making up BS regarding the attributes imparted by various spin values, or is that a reasonable explanation of why gravitons are presumed to be spin-2 particles?
- brylie 2y ago
  Indeed. I shared what I thought was a detailed and helpful generative response to further the discussion and hopefully receive feedback from those knowledgeable on the topic. It's disappointing to see it met with downvotes and flagging, rather than constructive conversation.
  - CamperBob2 2y ago
    Same old story throughout history. People feel threatened by technology, and they lash out. It's disappointing here, for sure.

krsrhe 2y ago

I recognize your name and believe you know what you are talking about, so please, please tell us the why! You or someone you know probable has a blog post or article you can link, maybe?

morcus 2y ago
Wow I never look at usernames but maybe I should start. I believe I read his `Gauge Fields, Knots, and Gravity` back in college (or tried to, anyways. it was a touch above my level at the time)

sandworm101 2y ago

I had a prof explain it using basic analogy. Forces are things that can move other things. An electrical field can move a metal ball. So too can a metal bat. put a g-meter on the ball and it will register acceleration as force is applied to the ball. Gravity is different. Put a g-meter on a falling metal ball and it will detect zero acceleration. No force is acting upon a ball. Put that metal ball in an eccentric orbit around the earth and it will speed up and slow down during each orbit, but the g-meter will register zero acceleration. The ball falls but is no not accelerated. So gravity is not a force because it doesn't move things. Gravity is something different.

gwd 2y ago
But the acceleration meter won't measure any force because gravity is acting on every part of it uniformly. If you had an acceleration meter entirely made out of the same magnetic substance, and you brought a magnet near it, would the acceleration meter register anything, or would it read zero acceleration, since all parts of it were being acted on uniformly (and thus didn't "notice" any acceleration)?
- gwd 2y ago
  (Replying to myself) I guess one answer to this is that particles which are (supposedly?) massless, like the photon, are affected by the space-time warping effects of gravity. A parallel construction wouldn't be true of magnetism or an electric field. Furthermore, when we detect gravity waves, they come at the same time as corresponding gamma ray bursts; since the gamma rays are affected by the space-time warping effects of gravity, this means that the gravity waves themselves are affected by the space-time warping effects of gravity. So gravity probably is something else. But who knows!
  - lupire 2y ago
    That could also be explained by light having momentum which is affected by (but does not generate) the force of gravity acts like mass.
- 2y ago

gnramires 2y ago

I've recently learned about Kaluza–Klein theories (just curious wiki browsing), do they unify electromagnetism and gravity in the sense that electromagnetism also "works by changing the geometry of spacetime" ? How does this relate to QFT?

elashri 2y ago
> do they unify electromagnetism and gravity in the sense that electromagnetism also "works by changing the geometry of spacetime" ? Yes, somehow actually. KK theories introduce an extra spatial dimension beyond our usual (3+1) which is postulated to be "compactified". This just means that it's curled up on itself at such a tiny scale that we don't directly perceive it. The way this extra dimension is curled and shaped affects the geometry of the overall 5-D spacetime. How it is connected to EM is that now these geometrical variations in the 5-D spacetime, when viewed from our 4-D perspective, manifest as the EM force and its associated field. So you can say like in GR which have gravity arises as consequence of geometry, it is in KK that EM is consequence of geometry. However, the geometry and details are different. > How does this relate to QFT? Not much in the sense that they can provide useful information to each other. QFT describes forces in terms of interactions mediated by particles (e.g., photons for EM). KK, while primarily geometrical, give hints that perhaps these force-carrying particles can be associated with specific vibrational modes of the extra dimension. Of course KK theory only include EM and gravity. So we know for sure that we need to go beyond KK. This was the actual motivation for people to think about string theory to expand the original KK work.
- Karellen 2y ago
  > KK theories introduce an extra spatial dimension beyond our usual (3+1) which is postulated to be "compactified". This just means that it's curled up on itself You've just explained "compactified" in terms of being "curled up", but that doesn't really help (for me, at least). What does it mean for a dimension to be "curled up"?
  - elashri 2y ago
    I'm sorry if I'm confusing. That's the term I use, usually assuming that it is obvious (like referring to C++ templates to a python developer without explaining what is that). To be honest, it is hard to visualize, as it is counterintuitive of what we think of space. While I know many people would disagree, I really like the garden hose analogy [1]. The idea is to simply imagine a very long garden hose. From a great distance, it looks like a one-dimensional line. Now you get closer, and realize it has a second dimension, which is its circumference curled around that seemingly 1-D line. An ant walking on the hose can move along its length, but also in a circle around it. The extra dimension in KK is like this circumference. It is tiny, curled up so we don't directly notice it, but still potentially there. [1] https://www.preposterousuniverse.com/blog/2004/06/30/extra-dimensions/ https://www.preposterousuniverse.com/blog/2004/06/30/extra-d...

lubesGordi 2y ago

What does it mean to 'treat gravity approximately (that is, perturbatively)'? That sounds like something we do to model, which only approximates reality. That model sounds like it shouldn't be used to predict anything else? Or at least whatever is predicted shouldn't be expected to exist in 'reality'.

canjobear 2y ago
It’s models all the way down.
nyrikki 2y ago
John von Neumann: "... the sciences do not try to explain, they hardly even try to interpret, they mainly make models. By a model is meant a mathematical construct which, with the addition of certain verbal interpretations, describes observed phenomena. The justification of such a mathematical construct is solely and precisely that it is expected to work—that is, correctly to describe phenomena from a reasonably wide area." Both GR and QFT are insanely accurate models, but they are just models. The N-body problem is undecidable, and Gödel, Turing, Church and other s proved that is the best we can do. https://philsci-archive.pitt.edu/13175/1/parker2003.pdf https://philsci-archive.pitt.edu/13175/1/parker2003.pdf Western reductionism or Laplacian determinism is a good framework for practical, computable models. QFT actually is actually one of the counterexamples to Western reductionism. But models are reductive and scientific models are just models. Don't confuse the map for the territory. All models are wrong, some are useful; is another way of saying the same thing.
- arter 2y ago
  Could you elaborate or share some articles on why QFT is a counterexample to western reductionism ? In laymans terms that is.
  - nyrikki 2y ago
    The oversimplified version is Laplac's deamon can't split quantum superposition. Superposition being inseparable is a large part on why the many words concept is popular with some people. It is about regaining a form of determinism.

pdonis 2y ago

I think that more generally the answer is not responsive to the actual question, which is: why do GR and QM need to be unified? To be fair, the question itself wrongly conflates "gravity needs a gauge boson" with the question about GR and QM being unified. Not all theories of quantum gravity involve a "gauge boson" for gravity along the lines of the other Standard Model interactions. Nor does the basic rationale for why we think gravity needs to be quantized involve a "gauge boson". It involves simple reasoning about how QM works. Say we have an experiment which puts an object with non-negligible stress-energy into a superposition of being in two different positions (for example, we make its position depend on the outcome of a spin measurement on a qubit). QM would say that spacetime would then need to also be in a superposition of two different geometries. But GR, as a classical theory, has no way to handle that. We would need a quantum theory of gravity, i.e., a quantum theory that can handle superpositions of different spacetime geometries.

trhway 2y ago
>Say we have an experiment which puts an object with non-negligible stress-energy into a superposition of being in two different positions (for example, we make its position depend on the outcome of a spin measurement on a qubit). QM would say that spacetime would then need to also be in a superposition of two different geometries. The energy to move the object into a given position is an additional element here unaccounted for in your model. 2 different positions to move object into - 2 different energies (more specifically 2 different changes to the starting, before the experiment, stress energy distribution of the Universe). When corresponding moving energies (ie. their GR effects) are accounted for in those 2 cases it may as well be that those 2 cases are indistinguishable from the GR point of view, ie. those 2 supposedly different spacetime geometries happen to be the same. The superposition of 2 indistinguishable cases - it doesn't really matter is it superposition or not.
- qnleigh 2y ago
  There is an entire sub-field of experimental science that involves putting ever larger objects in superpositions of different locations. These experiments are no closer to testing quantum gravity, but they falsify whatever it is you're trying to say here. See https://www.nature.com/articles/srep13884 https://www.nature.com/articles/srep13884 for a random example.

me_me_me 2y ago

it would be so much easier to visualize it if we could imagine 4th dimension.

layer8 2y ago

You can say similar things about the field of the other forces too, though. The path of a charged particle in the EM field could be described as that particle experiencing a different space-time geometry, arising from the EM and gravitational field combined, and thus EM could also be seen as a geometry and not a force. In fact, the impulse of photons, which are vibrations in the EM field, does affect the curvature of space-time, similar to how particles with mass do.

pdonis 2y ago
> The path of a charged particle in the EM field could be described as that particle experiencing a different space-time geometry No, it can't, because the EM interaction does not obey the equivalence principle, as gravity does. The geometric interpretation of gravity relies on the equivalence principle. To state this another way: if I put two objects with different masses at a given point in spacetime and give them both the same initial velocity in the same direction, their paths through spacetime under gravity will be the same. But if I put two objects with different charges at a given point in the same electromagnetic field and give them both the same initial velocity in the same direction, their paths through spacetime will not be the same. And this remains true even if I add "extra dimensions" to "spacetime" along the lines of Kaluza-Klein theory, to represent the EM field.

codexb 2y ago

I feel like it's easier to explain that gravity is a force between matter and space(time) and not necessarily between matter and matter, like we presume the other forces are.

Biganon 2y ago

Woaw, a top comment by John Carlos Baez himself!

machina_ex_deus 2y ago

Non gravity forces are quite linear and simple, the coupling with fermions is only through covariant derivative. Gravity is extremely non linear, when you look at the expression for Ricci tensor, it is much more complicated than other forces.