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Gravity is not a force

53 points by Luc 2y ago

taylodl 2y ago
First off, the author doesn't appear to be a crank. Here's the Wikipedia entry: https://en.wikipedia.org/wiki/Jonathan_Oppenheim https://en.wikipedia.org/wiki/Jonathan_Oppenheim Second, we should note that even Einstein himself cautioned against believing spacetime was actually curved. His writings inform us he didn't believe it. I don't want to appeal to authority, that's just to say smart people, including the main developer of general relativity, didn't believe it. But he didn't believe in the non-local nature of quantum theory either, which we have now, since Einstein's death, proven to be true. Third, the claim that only gravity can be described using geometry is false, which the author himself notes later in this article. The stress-energy-momentum tensor simply makes gravity universal, unlike the other forces. I don't see any reason why that universality confers something special to gravity with regards to interpreting it as geometry. Just because we can model gravity as geometry, doesn't make gravity a result of geometry, and the author notes that modeling gravity that way makes it so we can't unify the forces. Finally, as the old saying goes, if you think gravity isn't a force, drop a brick on your toe! :) I'll also point out that singularities are generally considered to be a sign of issues with a model. GR has singularities. Maybe that should tell us something.
- BeetleB 2y ago
  > First off, the author doesn't appear to be a crank. Is this the first most people are hearing about this? In the 90's, I had a physics textbook for the layperson (i.e. non-STEM fields). It had a fantastic chapter on general relativity, and it also went with "not a force but a spacetime curvature".
  - taylodl 2y ago
    Popular scientists have been saying it for decades and always use the flexible rubber sheet as the learning aid. Serious physicists tended to stay out of the fray, preferring to just "shut up and compute", as they do for quantum mechanics. It's a relatively recent development that physicists have entered the fray and say maybe curved spacetime isn't a model, maybe it's reality. I urge caution to all as we have no evidence that the model of curved spacetime is indeed reality. I think the caution is warranted considering that GR contains singularities, which is a sign the theory has issues.
    - Filligree 2y ago
      GR is a theory of the bulk. It’s like fluid mechanics, which treats fluids as infinitely divisible and doesn’t acknowledge the existence of molecules: There’s no way it’s accurate at small enough scales that molecules matter, which is what would be happening inside a black hole. At larger scales, however, fluid mechanics is extremely reliable.
    - api 2y ago
      What if singularities do in fact exist? What issues does that create? They might not be observable if superextremal black holes don't exist and thus naked singularities do not exist.
      - Filligree 2y ago
        We’d have no way to predict what they do, since the math breaks down. This is fine if you treat the math as only a model, and you’re willing to accept that it can’t predict the behaviour of a thing that can’t affect you anyway, but given your wording — “do in fact exist” — I assume that’s not what you mean. A theory that purports to describe how the universe actually functions, can’t have places where the theory says “and now the universe bluescreens”. At a minimum it would need additional postulates about computational order that allows the inside of the black hole to be NaN without that causing external reality to seize up.
        taylodl 2y ago
        > “and now the universe bluescreens”. I love it!
        toast0 2y ago
        > A theory that purports to describe how the universe actually functions, can’t have places where the theory says “and now the universe bluescreens”. A theory that provides useful predictions in some conditions and doesn't provide useful predictions in other conditions is a useful theory if you can determine which conditions are which. My lay understanding is general relativity provides useful predictions as long as mass isn't too dense (black hole) and as long as nothing breaks the speed limit; which is pretty general for everyday use, even if it doesn't cover the whole universe.
- Filligree 2y ago
  > But he didn't believe in the non-local nature of quantum theory either, which we have now, since Einstein's death, proven to be true. We have not proven quantum theory to be non-local. We’ve only proven that it can’t be both local and contain particles, as opposed to particles being emergent from the wavefunction’s interactions. MWI chooses the latter, and is therefore a local theory. (Alternately, collapse. But collapse theories are largely nonsensical.)
- wongarsu 2y ago
  > we should note that even Einstein himself cautioned against believing spacetime was actually curved. [...] I don't want to appeal to authority Another example that comes to mind is Max Planck believing that light being absorbed in discrete packets of energy was only a neat mathematical hack he came up with. It took Albert Einstein to say "but what if light is discrete packets". And then as you say Einstein having major reservations against the field of quantum physics that he himself spawned. > if you think gravity isn't a force, drop a brick on your toe By that logic the centrifugal force has to be a force. If you don't believe it, just drive a vehicle around a curve, or whirl a rock on a string.
  - marcosdumay 2y ago
    > By that logic the centrifugal force has to be a force. Are you trying to claim it's not?
    - ivansavz 2y ago
      Not the OP, but I think I know what wongarsu is referring to. In order to make an object turn, it needs to experience an centripetal acceleration (towards the centre of rotation). This is the force causing objects to change trajectory. If there is another object inside the turning object (like clothes inside a washer, or a person inside a car) they will "feel" like they are being flug out as if a centrifugal force existed, but actually that is just the effect of Newton's first law: the natural tendency of every moving body is to continue to move in a straight line, so when the containing object is changing direction (due to the centripetal force), Newton's first law tends to push you outwards. All of the above is from the static (world) frame of reference. It is also possible to put a coordinate system on the rotating object, in which case something like a centrifugal force will exit, but we kind of created it by choosing an accelerating reference frame, so it's not real. Sometimes called a pseudoforce.
  - cryptonector 2y ago
    "There is no centrifugal force" is often found together with "centrifugal forces appear as a term when using the frame of reference of the object going around", and this implicitly says that some reference frames are more privileged than others which -in a world that accepts the principle of Relativity- is just not acceptable. So of course gravity is a force, and of course centrifugal forces are real. These dogmas serve to do nothing more than to scare away students, and to make the dogmatic seem like geniuses because only they can understand these things.
- fxj 2y ago
  It is possible to unify the electromagnetic field into a geometric framework via the Kaluza-Klein theory which is just rewriting GR + ED. Then electromagnetic forces are also of geometric origin. How do you interpret this? Is electromagnetism then also not a force? I am not a physicist, so I am asking the experts. https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory
- fxj 2y ago
  I found this paper which details how a charged particle falling into a black hole radiates. How is this possible when there is no acceleration on the particle? https://adsabs.harvard.edu/full/1971PASP...83..633R https://adsabs.harvard.edu/full/1971PASP...83..633R Title: Radiation from Particles Falling into Black-Holes Authors: Ross, D. K. Journal: Publications of the Astronomical Society of the Pacific, Vol. 83, No. 495, p.633 Bibliographic Code: 1971PASP...83..633R
  - cryptonector 2y ago
    I'm going to have to read that one. Thanks for the link!
- cryptonector 2y ago
  > First off, the author doesn't appear to be a crank. Perhaps, but the author is being dogmatic. See commentary elsewhere in this post.
ars 2y ago
"is to say that when no force acts on a test particle in curved space, it should move along a geodesic" Every single author who calls gravity not a force, just hand waves right past this: why should the particle move at all? Sure if the particle is moving it will follow a curved path thinking the path is straight. But if the particle just sits there, okay the path is curved, but it just sits there. I've heard explanations having to do with the fact that particles move through time, that doesn't really answer the question because it can continue moving through time while just sitting there. "in the absence of being pushed or pulled, test particles in a curved spacetime will free fall" Really? Why exactly will they free fall? Why can't they just stay exactly where they are? I've also never heard anyone explain what the issue is with calling gravity a force. One person said it can't be a force because it makes light move, and light is massless. However gravity does not act on mass it acts on energy, and mass is just a form of energy. Since photons have energy obviously they gravitate.
- zmgsabst 2y ago
  You’re moving in the time direction — and when spacetime is curved, you end up moving in the space direction as well, to follow the geodesic.
  - AlexandrB 2y ago
    Not arguing for or against this explanation, but it's hard enough (impossible for me) to visualize a curved 3d space, once you try to think of what curved 4d space might be like any intuitive sense of what should happen goes out the window. Classic diagrams of curved spacetime portray space as a 2d sheet with divots in it. I wonder what a similar visualization of curved 1d time + 1d space would look like. Long valleys for gravitational wells and everything moving along the sheet in the direction of positive time?
  - ars 2y ago
    > and when spacetime is curved, you end up moving You did the same as everyone else: You hard waved right past "end up moving". Why do I end up moving instead of just sitting there? And if I "end up moving" that means I exchange momentum, if it's just bent space that magically makes me move, what did I exchange momentum with, if not via a force toward another particle? Gravity is a force that acts on energy. If you disagree please give me a counter example.
TheAceOfHearts 2y ago
Sabine Hossenfelder chimed in [0] on this discussion a couple days ago. I generally find her to be trustworthy on topics related to physics: > The easiest way to see that gravity is not a force is to note that a force causes acceleration, but gravity does not. > Acceleration is measurable with a device called an accelerometer. Acceleration is not relative (like velocity), it's absolute. > If you are standing on the surface of Earth, an accelerometer will show that you are accelerated in the upward direction. That's because a force is acting on you from below, it's the solidity of Earth's crust (or whatever you are standing on), going back to a combination of electromagnetic forces and the Pauli principle. > If you take away that support from Earth, eg by jumping off a plane, you are not accelerated. You are freely falling. Since you are not accelerated, there is no force acting on you. You experience gravity but no force, hence gravity is not a force. > We can assign a pseudo-force to gravity by defining it as acceleration relative to the surface of Earth. This is how Newtonian gravity works. One can derive it from general relativity as an approximation. > Physicists frequently do refer to gravity as a force anyway -- even I do -- because that's linguistically simpler. But it's like we say "internet" rather than "world wide web" even though we know that the two aren't the same, just because "internet" is simpler. > So I usually don't pick on this. But strictly speaking, gravity is indeed not a force. If you have doubts about it, buy an accelerometer and do your own research... [0] https://x.com/skdh/status/1850120005070799153 https://x.com/skdh/status/1850120005070799153
- tomp 2y ago
  I don't understand her comment. This is really easy to verify. You drop an iPhone, the gravity "acceleration" really goes to 0 - so far so good. But at rest (i.e. holding the iPhone in your hand), the acceleration is pointing downwards, not upward as she claims.
  - Filligree 2y ago
    Your iPhone lies to you. The acceleration is indeed upwards, as you’ll see if you look at raw accelerometer data… or the feeling from your feet, which counts.
  - recursive 2y ago
atemerev 2y ago
“If you accept that we live in spacetime, and it can be curved, then I think you should accept that gravity cannot be a force.” I am on the opposite time of thinking — where spacetime itself emerges from particle interaction events. Pairwise distance (i.e. metric) is just yet another interaction parameter in the world graph.
- cryptonector 2y ago
  The metrics in GR relate the effects on spacetime of massive bodies, but... the effects relative to... what? Well, relative to flat spacetime. Those metrics are really projections. We need them to understand the effects on waves and matter (which is standing waves anyways, so it's all waves, all the time). But we don't have to take the view that curved spacetime is more fundamental than flat spacetime . There is an equivalence between them, therefore we can say that neither is more fundamental. That means that we can use the interpretation that is easiest to understand.
  - atemerev 2y ago
    There is no such thing as “flat spacetime”, or, above all, “absolute” background spacetime. This is what relativity explicitly denies. Spacetime appears as pairwise metrics between objects (i.e. quanta/particles), and doesn’t exist beside objects.
    - cryptonector 2y ago
      There are no absolute coordinates. But things like the Schwarzschild metric very much map between flat and curved spacetime. You should search for "conformal mappings to flat spacetime". This is not a controversial thing.
theamk 2y ago
He lost me there: > Is this purely a semantic difference? You could argue that it doesn't really matter whether we describe gravity as a force or through geometry, and we should conflate these two concepts. But I think this distinction is important to make because it has predictive power. If you believe that gravity is manifest through spacetime bending, then you will never find two different test particles that follow different geodesics. Don't we get the same conclusion if we believe gravity is a force _and_ equivalence principle is true?
- cryptonector 2y ago
  We do. The argument here is that there are two equivalent interpretations but that one is more fundamental (or closer to actual reality anyways) than the other. But this is mostly a result of the starting point taken by Einstein and then everyone understandably accepting that as the more fundamental thing. Even if no one ever finds a way to mathematically and equivalently derive GR from flat spacetime + gravity is a force first principles, it is certainly a lot easier for humans to understand gravity as being a force, and the curvature of spacetime as being other distortions on flat spacetime, not unlike the various 3D->2D projections we use for maps of Earth. Pedagogy matters. [Notice that I'm not describing the distortions that one gets when mapping GR to flat spacetime. I want to leave that to the reader, though I might pop up and reply with a list later if someone asks.]
- whatshisface 2y ago
  This is all pretty far up the semantic creek anyways because forces are only defined in the Newtonian framework.
photochemsyn 2y ago
I like the model of an insulated tunnel bored through the Earth from the north to the south pole and filled with a vacuum (technologically implausible, yes). If we drop a steel ball into the tunnel at the north pole, what forces does it experience? From Newton's perspective, F = ma and the ball accelerates towards the center of the Earth. The value of g diminishes to zero at the center, the ball is at its maximum velocity, and then enters the negative acceleration regime until it just reaches the surface of the Earth at the south pole. This will continue indefinitely in harmonic motion. It's not a perpetual motion machine because machines do work and we're not doing any work on the ball; it's similar to an orbiting sphere. (ChatGPT-o1 claims the period is 84.4 minutes, assuming uniform density) The general relativity perspective seems to be the ball is just rolling up and down a bowl of spacetime, without any friction or drag, which isn't all that satisfying a picture, since it implies a restorative force being involved to keep the ball from escaping the bowl. It helps to consider the state of the ball right before it is kicked into the tunnel - it is being prevented from following its natural geodesic trajectory by the electromagnetic forces of the rocks of the Earth's crust upon which it is being held up - that's the only relevant force in this picture.
whatshisface 2y ago
Accelerometers don't measure acceleration, they measure the difference between the force on a mass on springs and the body of your phone. If you drilled a tiny hole and pulled the mass up, the sensor would think the phone was being pulled down. Acceleration due to gravity has an important property of acceleration in general: radiation. Spiraling black holes emit gravitational waves, in the same way that an accelerating electric charge emits light. There are free falling reference frames where uniform gravitational fields can go away, but the gravitation of a massive body isn't uniform and can't be eliminated by changing the coordinates. The relationship between gravity and fictitious forces is an important stepping stone, but it does not have all the properties of a fictious force, only some of them.
cvoss 2y ago
I am sympathetic to the author's thesis. I favor the idea that gravity is a different thing from the other fundamental forces, and possibly an emergent phenomenon rather than a fundamental thing in its own right. But, I don't buy the argument made here: > To call gravity a force, is to privilege flat space as somehow being special. Flat space is special, and we didn't make it special. This is taking an important aspect of known physics---that there exist various symmetries and all elements of the corresponding group are equal players (there is no privileged reference frame, positive charge and negative charge are indistinguishable save for their oppositeness, etc.)---and attempting to apply this principle to spacetime curvature. But the zero curvature state is a unique one that is differentiatable from the others. It's the only one where a circle is perfect, having circumference 2 * pi * r. And pi is a fundamental invariant of geometry, curved or otherwise. The mathematics privileges flat space. Further, experiments can be constructed to detected whether we are in flat space or not [1]. That wouldn't be possible if the whole concept of flat were only relative to an arbitrary frame. [1] https://en.m.wikipedia.org/wiki/BOOMERanG_experiment https://en.m.wikipedia.org/wiki/BOOMERanG_experiment
- TeMPOraL 2y ago
  I tend to think about this in via a simple 0-1-many heuristic - there's infinitely many ways to have a curved space, but there is exactly one way to have a flat space. That by itself makes it special. Is this heuristic wrong?
  - cryptonector 2y ago
    > Is this heuristic wrong? I would say it is, but in a subtle way. There is only one way in which gravity curves spacetime, and also one set of effects it has when seen from the lens of flat spacetime.
    - TeMPOraL 2y ago
      > There is only one way in which gravity curves spacetime, and also one set of effects it has when seen from the lens of flat spacetime. Is there though? Isn't that the holy grail of science - expressing all of physics, particularly all of "fundamental constants", in a formula where there is one clearly preferred answer? One function with a single global minimum in the parameter space, that defines our universe? Perhaps let me put my heuristic differently: when there exists multiple (especially infinitely many) solutions, you still need to explain why a particular solution would be the solution, and not any of the other ones. A single solution is naturally privileged, because there is only one and there can be no other.
fxj 2y ago
Question to the physicists out there: When an electron gets accelerated it emits "Bremsstrahlung" because it radiates away photons when it changes its velocity vector. So for an electron on a circular path in a magnetic field, we know that it emits this radiation because this is the synchrotron radiation. Now what happens to an electron on a circular path around a black hole? Does it emit synchrotron radiation or not?
- cryptonector 2y ago
  Bremsstrahlung radiation involves accelerating electric charges through non-zero electromagnetic fields. When all charged particles are accelerated by gravity in the same way by a [close to] uniform gravitational field then they are not being accelerated through non-zero electromagnetic fields because the causes of those fields are also accelerating in the same way. However, if an electron were falling into a charged, rotating black hole then I'd expect some Bremsstrahlung radiation indeed. That said, IANAP. And when I say "accelerated by gravity" I am taking the interpretation that gravity is a force, which is a valid interpretation (see other commentary above).
  - deleted 2y ago
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stonethrowaway 2y ago
Whatever comes out of this kerfuffle, the based dial will be set to 11. It’s unfortunate that we can’t monetize this stuff in the same vein as crypto and ChatGPT because I’d love to hear YouTube grifters telling you how you can make money overnight from quarks and neutrinos.
tzot 2y ago
This is not article-content-related but article-presentation-related: the trouble with 3-pixel-wide scrollbars is that you have to be very exact when you try to use them. Yes, there are scroll wheels and keyboard buttons, but forcing the mobile experience (on a mobile I probably wouldn't notice) on non-mobile setups is at least annoying.
amai 2y ago
Actually even Newtonian gravity is no force, because Newtonian spacetime is curved: https://youtu.be/IBlCu1zgD4Y?feature=shared https://youtu.be/IBlCu1zgD4Y?feature=shared It must be so, because it is just a approximation to general relativity.
amai 2y ago
The author is 50% correct. As John Wheeler stated "Spacetime tells matter how to move; matter tells spacetime how to curve." The author is unfortunately forgetting the second part. If matter is quantized spacetime curvature must also be quantized, because matter defines our spacetime. In principle one can even shorten the sentence and say: matter tells matter how to move. Spacetime appears only to be a convenient calculation tool. And in that sense spacetime=gravity isn't a force. It actually doesn't even exist.

53 points by Luc 2y ago

First off, the author doesn't appear to be a crank. Here's the Wikipedia entry: https://en.wikipedia.org/wiki/Jonathan_Oppenheim https://en.wikipedia.org/wiki/Jonathan_Oppenheim Second, we should note that even Einstein himself cautioned against believing spacetime was actually curved. His writings inform us he didn't believe it. I don't want to appeal to authority, that's just to say smart people, including the main developer of general relativity, didn't believe it. But he didn't believe in the non-local nature of quantum theory either, which we have now, since Einstein's death, proven to be true. Third, the claim that only gravity can be described using geometry is false, which the author himself notes later in this article. The stress-energy-momentum tensor simply makes gravity universal, unlike the other forces. I don't see any reason why that universality confers something special to gravity with regards to interpreting it as geometry. Just because we can model gravity as geometry, doesn't make gravity a result of geometry, and the author notes that modeling gravity that way makes it so we can't unify the forces. Finally, as the old saying goes, if you think gravity isn't a force, drop a brick on your toe! :) I'll also point out that singularities are generally considered to be a sign of issues with a model. GR has singularities. Maybe that should tell us something.

BeetleB 2y ago
> First off, the author doesn't appear to be a crank. Is this the first most people are hearing about this? In the 90's, I had a physics textbook for the layperson (i.e. non-STEM fields). It had a fantastic chapter on general relativity, and it also went with "not a force but a spacetime curvature".
- taylodl 2y ago
  Popular scientists have been saying it for decades and always use the flexible rubber sheet as the learning aid. Serious physicists tended to stay out of the fray, preferring to just "shut up and compute", as they do for quantum mechanics. It's a relatively recent development that physicists have entered the fray and say maybe curved spacetime isn't a model, maybe it's reality. I urge caution to all as we have no evidence that the model of curved spacetime is indeed reality. I think the caution is warranted considering that GR contains singularities, which is a sign the theory has issues.
  - Filligree 2y ago
    GR is a theory of the bulk. It’s like fluid mechanics, which treats fluids as infinitely divisible and doesn’t acknowledge the existence of molecules: There’s no way it’s accurate at small enough scales that molecules matter, which is what would be happening inside a black hole. At larger scales, however, fluid mechanics is extremely reliable.
  - api 2y ago
    What if singularities do in fact exist? What issues does that create? They might not be observable if superextremal black holes don't exist and thus naked singularities do not exist.
    - Filligree 2y ago
      We’d have no way to predict what they do, since the math breaks down. This is fine if you treat the math as only a model, and you’re willing to accept that it can’t predict the behaviour of a thing that can’t affect you anyway, but given your wording — “do in fact exist” — I assume that’s not what you mean. A theory that purports to describe how the universe actually functions, can’t have places where the theory says “and now the universe bluescreens”. At a minimum it would need additional postulates about computational order that allows the inside of the black hole to be NaN without that causing external reality to seize up.
      - taylodl 2y ago
        > “and now the universe bluescreens”. I love it!
      - toast0 2y ago
        > A theory that purports to describe how the universe actually functions, can’t have places where the theory says “and now the universe bluescreens”. A theory that provides useful predictions in some conditions and doesn't provide useful predictions in other conditions is a useful theory if you can determine which conditions are which. My lay understanding is general relativity provides useful predictions as long as mass isn't too dense (black hole) and as long as nothing breaks the speed limit; which is pretty general for everyday use, even if it doesn't cover the whole universe.
Filligree 2y ago
> But he didn't believe in the non-local nature of quantum theory either, which we have now, since Einstein's death, proven to be true. We have not proven quantum theory to be non-local. We’ve only proven that it can’t be both local and contain particles, as opposed to particles being emergent from the wavefunction’s interactions. MWI chooses the latter, and is therefore a local theory. (Alternately, collapse. But collapse theories are largely nonsensical.)
wongarsu 2y ago
> we should note that even Einstein himself cautioned against believing spacetime was actually curved. [...] I don't want to appeal to authority Another example that comes to mind is Max Planck believing that light being absorbed in discrete packets of energy was only a neat mathematical hack he came up with. It took Albert Einstein to say "but what if light is discrete packets". And then as you say Einstein having major reservations against the field of quantum physics that he himself spawned. > if you think gravity isn't a force, drop a brick on your toe By that logic the centrifugal force has to be a force. If you don't believe it, just drive a vehicle around a curve, or whirl a rock on a string.
- marcosdumay 2y ago
  > By that logic the centrifugal force has to be a force. Are you trying to claim it's not?
  - ivansavz 2y ago
    Not the OP, but I think I know what wongarsu is referring to. In order to make an object turn, it needs to experience an centripetal acceleration (towards the centre of rotation). This is the force causing objects to change trajectory. If there is another object inside the turning object (like clothes inside a washer, or a person inside a car) they will "feel" like they are being flug out as if a centrifugal force existed, but actually that is just the effect of Newton's first law: the natural tendency of every moving body is to continue to move in a straight line, so when the containing object is changing direction (due to the centripetal force), Newton's first law tends to push you outwards. All of the above is from the static (world) frame of reference. It is also possible to put a coordinate system on the rotating object, in which case something like a centrifugal force will exit, but we kind of created it by choosing an accelerating reference frame, so it's not real. Sometimes called a pseudoforce.
- cryptonector 2y ago
  "There is no centrifugal force" is often found together with "centrifugal forces appear as a term when using the frame of reference of the object going around", and this implicitly says that some reference frames are more privileged than others which -in a world that accepts the principle of Relativity- is just not acceptable. So of course gravity is a force, and of course centrifugal forces are real. These dogmas serve to do nothing more than to scare away students, and to make the dogmatic seem like geniuses because only they can understand these things.
fxj 2y ago
It is possible to unify the electromagnetic field into a geometric framework via the Kaluza-Klein theory which is just rewriting GR + ED. Then electromagnetic forces are also of geometric origin. How do you interpret this? Is electromagnetism then also not a force? I am not a physicist, so I am asking the experts. https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory
fxj 2y ago
I found this paper which details how a charged particle falling into a black hole radiates. How is this possible when there is no acceleration on the particle? https://adsabs.harvard.edu/full/1971PASP...83..633R https://adsabs.harvard.edu/full/1971PASP...83..633R Title: Radiation from Particles Falling into Black-Holes Authors: Ross, D. K. Journal: Publications of the Astronomical Society of the Pacific, Vol. 83, No. 495, p.633 Bibliographic Code: 1971PASP...83..633R
- cryptonector 2y ago
  I'm going to have to read that one. Thanks for the link!
cryptonector 2y ago
> First off, the author doesn't appear to be a crank. Perhaps, but the author is being dogmatic. See commentary elsewhere in this post.

ars 2y ago

"is to say that when no force acts on a test particle in curved space, it should move along a geodesic" Every single author who calls gravity not a force, just hand waves right past this: why should the particle move at all? Sure if the particle is moving it will follow a curved path thinking the path is straight. But if the particle just sits there, okay the path is curved, but it just sits there. I've heard explanations having to do with the fact that particles move through time, that doesn't really answer the question because it can continue moving through time while just sitting there. "in the absence of being pushed or pulled, test particles in a curved spacetime will free fall" Really? Why exactly will they free fall? Why can't they just stay exactly where they are? I've also never heard anyone explain what the issue is with calling gravity a force. One person said it can't be a force because it makes light move, and light is massless. However gravity does not act on mass it acts on energy, and mass is just a form of energy. Since photons have energy obviously they gravitate.

zmgsabst 2y ago
You’re moving in the time direction — and when spacetime is curved, you end up moving in the space direction as well, to follow the geodesic.
- AlexandrB 2y ago
  Not arguing for or against this explanation, but it's hard enough (impossible for me) to visualize a curved 3d space, once you try to think of what curved 4d space might be like any intuitive sense of what should happen goes out the window. Classic diagrams of curved spacetime portray space as a 2d sheet with divots in it. I wonder what a similar visualization of curved 1d time + 1d space would look like. Long valleys for gravitational wells and everything moving along the sheet in the direction of positive time?
- ars 2y ago
  > and when spacetime is curved, you end up moving You did the same as everyone else: You hard waved right past "end up moving". Why do I end up moving instead of just sitting there? And if I "end up moving" that means I exchange momentum, if it's just bent space that magically makes me move, what did I exchange momentum with, if not via a force toward another particle? Gravity is a force that acts on energy. If you disagree please give me a counter example.

TheAceOfHearts 2y ago

Sabine Hossenfelder chimed in [0] on this discussion a couple days ago. I generally find her to be trustworthy on topics related to physics: > The easiest way to see that gravity is not a force is to note that a force causes acceleration, but gravity does not. > Acceleration is measurable with a device called an accelerometer. Acceleration is not relative (like velocity), it's absolute. > If you are standing on the surface of Earth, an accelerometer will show that you are accelerated in the upward direction. That's because a force is acting on you from below, it's the solidity of Earth's crust (or whatever you are standing on), going back to a combination of electromagnetic forces and the Pauli principle. > If you take away that support from Earth, eg by jumping off a plane, you are not accelerated. You are freely falling. Since you are not accelerated, there is no force acting on you. You experience gravity but no force, hence gravity is not a force. > We can assign a pseudo-force to gravity by defining it as acceleration relative to the surface of Earth. This is how Newtonian gravity works. One can derive it from general relativity as an approximation. > Physicists frequently do refer to gravity as a force anyway -- even I do -- because that's linguistically simpler. But it's like we say "internet" rather than "world wide web" even though we know that the two aren't the same, just because "internet" is simpler. > So I usually don't pick on this. But strictly speaking, gravity is indeed not a force. If you have doubts about it, buy an accelerometer and do your own research... [0] https://x.com/skdh/status/1850120005070799153 https://x.com/skdh/status/1850120005070799153

tomp 2y ago
I don't understand her comment. This is really easy to verify. You drop an iPhone, the gravity "acceleration" really goes to 0 - so far so good. But at rest (i.e. holding the iPhone in your hand), the acceleration is pointing downwards, not upward as she claims.
- Filligree 2y ago
  Your iPhone lies to you. The acceleration is indeed upwards, as you’ll see if you look at raw accelerometer data… or the feeling from your feet, which counts.
- recursive 2y ago

atemerev 2y ago

“If you accept that we live in spacetime, and it can be curved, then I think you should accept that gravity cannot be a force.” I am on the opposite time of thinking — where spacetime itself emerges from particle interaction events. Pairwise distance (i.e. metric) is just yet another interaction parameter in the world graph.

cryptonector 2y ago
The metrics in GR relate the effects on spacetime of massive bodies, but... the effects relative to... what? Well, relative to flat spacetime. Those metrics are really projections. We need them to understand the effects on waves and matter (which is standing waves anyways, so it's all waves, all the time). But we don't have to take the view that curved spacetime is more fundamental than flat spacetime . There is an equivalence between them, therefore we can say that neither is more fundamental. That means that we can use the interpretation that is easiest to understand.
- atemerev 2y ago
  There is no such thing as “flat spacetime”, or, above all, “absolute” background spacetime. This is what relativity explicitly denies. Spacetime appears as pairwise metrics between objects (i.e. quanta/particles), and doesn’t exist beside objects.
  - cryptonector 2y ago
    There are no absolute coordinates. But things like the Schwarzschild metric very much map between flat and curved spacetime. You should search for "conformal mappings to flat spacetime". This is not a controversial thing.

theamk 2y ago

He lost me there: > Is this purely a semantic difference? You could argue that it doesn't really matter whether we describe gravity as a force or through geometry, and we should conflate these two concepts. But I think this distinction is important to make because it has predictive power. If you believe that gravity is manifest through spacetime bending, then you will never find two different test particles that follow different geodesics. Don't we get the same conclusion if we believe gravity is a force _and_ equivalence principle is true?

cryptonector 2y ago
We do. The argument here is that there are two equivalent interpretations but that one is more fundamental (or closer to actual reality anyways) than the other. But this is mostly a result of the starting point taken by Einstein and then everyone understandably accepting that as the more fundamental thing. Even if no one ever finds a way to mathematically and equivalently derive GR from flat spacetime + gravity is a force first principles, it is certainly a lot easier for humans to understand gravity as being a force, and the curvature of spacetime as being other distortions on flat spacetime, not unlike the various 3D->2D projections we use for maps of Earth. Pedagogy matters. [Notice that I'm not describing the distortions that one gets when mapping GR to flat spacetime. I want to leave that to the reader, though I might pop up and reply with a list later if someone asks.]
whatshisface 2y ago
This is all pretty far up the semantic creek anyways because forces are only defined in the Newtonian framework.

photochemsyn 2y ago

I like the model of an insulated tunnel bored through the Earth from the north to the south pole and filled with a vacuum (technologically implausible, yes). If we drop a steel ball into the tunnel at the north pole, what forces does it experience? From Newton's perspective, F = ma and the ball accelerates towards the center of the Earth. The value of g diminishes to zero at the center, the ball is at its maximum velocity, and then enters the negative acceleration regime until it just reaches the surface of the Earth at the south pole. This will continue indefinitely in harmonic motion. It's not a perpetual motion machine because machines do work and we're not doing any work on the ball; it's similar to an orbiting sphere. (ChatGPT-o1 claims the period is 84.4 minutes, assuming uniform density) The general relativity perspective seems to be the ball is just rolling up and down a bowl of spacetime, without any friction or drag, which isn't all that satisfying a picture, since it implies a restorative force being involved to keep the ball from escaping the bowl. It helps to consider the state of the ball right before it is kicked into the tunnel - it is being prevented from following its natural geodesic trajectory by the electromagnetic forces of the rocks of the Earth's crust upon which it is being held up - that's the only relevant force in this picture.

whatshisface 2y ago

Accelerometers don't measure acceleration, they measure the difference between the force on a mass on springs and the body of your phone. If you drilled a tiny hole and pulled the mass up, the sensor would think the phone was being pulled down. Acceleration due to gravity has an important property of acceleration in general: radiation. Spiraling black holes emit gravitational waves, in the same way that an accelerating electric charge emits light. There are free falling reference frames where uniform gravitational fields can go away, but the gravitation of a massive body isn't uniform and can't be eliminated by changing the coordinates. The relationship between gravity and fictitious forces is an important stepping stone, but it does not have all the properties of a fictious force, only some of them.

cvoss 2y ago

I am sympathetic to the author's thesis. I favor the idea that gravity is a different thing from the other fundamental forces, and possibly an emergent phenomenon rather than a fundamental thing in its own right. But, I don't buy the argument made here: > To call gravity a force, is to privilege flat space as somehow being special. Flat space is special, and we didn't make it special. This is taking an important aspect of known physics---that there exist various symmetries and all elements of the corresponding group are equal players (there is no privileged reference frame, positive charge and negative charge are indistinguishable save for their oppositeness, etc.)---and attempting to apply this principle to spacetime curvature. But the zero curvature state is a unique one that is differentiatable from the others. It's the only one where a circle is perfect, having circumference 2 * pi * r. And pi is a fundamental invariant of geometry, curved or otherwise. The mathematics privileges flat space. Further, experiments can be constructed to detected whether we are in flat space or not [1]. That wouldn't be possible if the whole concept of flat were only relative to an arbitrary frame. [1] https://en.m.wikipedia.org/wiki/BOOMERanG_experiment https://en.m.wikipedia.org/wiki/BOOMERanG_experiment

TeMPOraL 2y ago
I tend to think about this in via a simple 0-1-many heuristic - there's infinitely many ways to have a curved space, but there is exactly one way to have a flat space. That by itself makes it special. Is this heuristic wrong?
- cryptonector 2y ago
  > Is this heuristic wrong? I would say it is, but in a subtle way. There is only one way in which gravity curves spacetime, and also one set of effects it has when seen from the lens of flat spacetime.
  - TeMPOraL 2y ago
    > There is only one way in which gravity curves spacetime, and also one set of effects it has when seen from the lens of flat spacetime. Is there though? Isn't that the holy grail of science - expressing all of physics, particularly all of "fundamental constants", in a formula where there is one clearly preferred answer? One function with a single global minimum in the parameter space, that defines our universe? Perhaps let me put my heuristic differently: when there exists multiple (especially infinitely many) solutions, you still need to explain why a particular solution would be the solution, and not any of the other ones. A single solution is naturally privileged, because there is only one and there can be no other.

fxj 2y ago

Question to the physicists out there: When an electron gets accelerated it emits "Bremsstrahlung" because it radiates away photons when it changes its velocity vector. So for an electron on a circular path in a magnetic field, we know that it emits this radiation because this is the synchrotron radiation. Now what happens to an electron on a circular path around a black hole? Does it emit synchrotron radiation or not?

cryptonector 2y ago
Bremsstrahlung radiation involves accelerating electric charges through non-zero electromagnetic fields. When all charged particles are accelerated by gravity in the same way by a [close to] uniform gravitational field then they are not being accelerated through non-zero electromagnetic fields because the causes of those fields are also accelerating in the same way. However, if an electron were falling into a charged, rotating black hole then I'd expect some Bremsstrahlung radiation indeed. That said, IANAP. And when I say "accelerated by gravity" I am taking the interpretation that gravity is a force, which is a valid interpretation (see other commentary above).
- deleted 2y ago
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stonethrowaway 2y ago

Whatever comes out of this kerfuffle, the based dial will be set to 11. It’s unfortunate that we can’t monetize this stuff in the same vein as crypto and ChatGPT because I’d love to hear YouTube grifters telling you how you can make money overnight from quarks and neutrinos.

tzot 2y ago

This is not article-content-related but article-presentation-related: the trouble with 3-pixel-wide scrollbars is that you have to be very exact when you try to use them. Yes, there are scroll wheels and keyboard buttons, but forcing the mobile experience (on a mobile I probably wouldn't notice) on non-mobile setups is at least annoying.

amai 2y ago

Actually even Newtonian gravity is no force, because Newtonian spacetime is curved: https://youtu.be/IBlCu1zgD4Y?feature=shared https://youtu.be/IBlCu1zgD4Y?feature=shared It must be so, because it is just a approximation to general relativity.

amai 2y ago

The author is 50% correct. As John Wheeler stated "Spacetime tells matter how to move; matter tells spacetime how to curve." The author is unfortunately forgetting the second part. If matter is quantized spacetime curvature must also be quantized, because matter defines our spacetime. In principle one can even shorten the sentence and say: matter tells matter how to move. Spacetime appears only to be a convenient calculation tool. And in that sense spacetime=gravity isn't a force. It actually doesn't even exist.