11 ms·

Why gravity is not like the other forces

134 points by kjhughes 3y ago

iaw 3y ago
I've always naively assumed gravity to be a facet of momentum/movement through time. The math between acceleration, gravity, and time dilation are all too cute to be coincidental in my book. I suspect that gravity manifests as a force but does not stem from the same underlying drivers as other forces.
- m3kw9 3y ago
  Yeah gravity seem to have a time dimension to it
  - ars 3y ago
    Gravity itself does not have a time dimension - if you had a highly changed particle acting on another one, the time dilation would be identical to that caused by gravity acting with the same force. i.e. it's force and acceleration that cause it, not gravity.
- magicalhippo 3y ago
  Lee Smolin is working on an idea where the building blocks are events where momentum is exchanged.Space, and hence position, is an emergent phenomena. Here's a colloquium talk[1] and here's a recent paper[2]. Not exactly what you said but perhaps interesting. As mentioned it's work in progress so remains to be seen if the idea pans out. [1]: https://pirsa.org/21030005 https://pirsa.org/21030005 [2]: https://arxiv.org/abs/2303.15546 https://arxiv.org/abs/2303.15546
  - nemo44x 3y ago
    I love how this word “emergent” has become popular by people that want to pretend they have an answer but it’s really just the “…” thing. There is no such thing as “emergent”. The motion of an automobile isn’t an “emergent” thing you can claim because you don’t know how engines and transmissions work. Everything we can’t explain recently has people, who I assume don’t know how to say I don’t know, proclaiming “it’s emergent”. As if that’s an answer to anything. Much less a claim to knowledge. Might as well claim “god made it happen”.
    - lanstin 3y ago
      System availability is an emergent property - I can build five nines of availability out of components with just two nines. Retries, algorithmic handling of failure, and the like.
    - Dylan16807 3y ago
      > The motion of an automobile isn’t an “emergent” thing you can claim because you don’t know how engines and transmissions work. That's the opposite of what emergent means, isn't it? Because of force on the transmission, you can say that the motion of the vehicle is emergent rather than needing its own framework or explanation.
    - CaptainNegative 3y ago
      Seems like pretending to know what the word emergent means in physics is an emergent phenomenon on HN. It's not wishy washy, it has a precise meaning that can be both derived and, typically, refuted.
    - nextaccountic 3y ago
      Emergent just means that it's not the basic building block; it's made up of other, more fundamental things. So for example thermodynamics is emergent (even though it is fully studied and relatively well understood) because it arises from interactions of particles. If you say that spacetime is emergent that is a big deal: it means that, while it can be studied on its own, it's actually composed of other, more fundamental things. But you still need to show that a) this other fundamental thing actually exists, and b) we can show mathematically how the spacetime works, in terms of those other fundamental thing (just like you can show how thermodynamics works in terms of statistical mechanics) And that's where your "because you don't know how it works" thing breaks apart: one can't claim to have solved this problem without tacking b).
      - nemo44x 3y ago
        > it's made up of other, more fundamental things Then explain those things and prove they exist. Otherwise this just sounds like bullshit. There’s a difference between observing thermodynamics (and not trying to answer why but rather how we observe it - big difference!) and writing equations we can measure and just saying “it’s emergent” to “…” things we are unable to answer “I don’t know to”.
- wiml 3y ago
  That is the basic idea that gives rise to the theory of general relativity — that gravity isn't really a force, it's just the observable effect of being in curved spacetime. (Take a look at the "equivalence principle" and its role in the development of GR as a theory.)
  - denton-scratch 3y ago
    > gravity isn't really a force Yeah, this fact made me cringe at explanation #2: the guy keeps saying things like "gravity is the only force that is felt by all kinds of matter". Stuff doesn't "feel" gravity; it's not a force, it's just the shape of spacetime. Also, gravity doesn't just affect matter; photons are not matter, but are affected by the curvature of spacetime (i.e. gravity). /me not a physicist; but I'm not a baby either, and I don't appreciate babytalk oversimplifications.
- alanfalcon 3y ago
  Gravity is an effect of the curve of space and relativity coming together in what we now call spacetime. Right? Einstein told us so. But... Maybe not? There's promising math behind the idea that there are multiple additional dimensions, but where are they? How can we perceive them? What do they have to do with gravity? The unifying theory of dimensional geometry and interaction: https://youtu.be/fvqXshyuvOg https://youtu.be/fvqXshyuvOg
jenadine 3y ago
But gravity isn't really a force. In the same way that inertia is not really a force. It's just an artifact of the coordinate system being curved.
- deanCommie 3y ago
  But WHY is the coordinate system curved?
  - jenadine 3y ago
    Because of mass. But WHAT is mass?
    - drsnow 3y ago
      A measure of a particle's energy as pertains to the spacetime curvature it induces
      - ars 3y ago
        This is incorrect all the energy of a particle curves space. Even velocity and potential energy.
        drsnow 3y ago
        Right, so those kinds of energy also induce spacetime curvature and are included in what I was saying
    - irrational 3y ago
      Wikipedia says “the property of "mass" is a manifestation of potential energy transferred to fundamental particles when they interact ("couple") with the Higgs field, which had contained that mass in the form of energy.”
    - ars 3y ago
      Energy, not mass. Photons also curve space in this way. Mass is just one kind of energy.
jacknews 3y ago
"We can picture many entities called fields superimposed on top of one another and extending throughout space. " And all affected in exactly the same way by the mysterious gravity 'force' or field. Or, we can picture just a single field, that has many dimensions or traits, which we call the other forces.
guender 3y ago
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BlackFly 3y ago
> But near the center of a black hole or in the first moments of the universe, Einstein’s equations break. Not really. The formation and continuous description of a singularity is somewhat unique in physics, but general relativity continues to provide a description, allowing even accurate descriptions of black hole mergers. The problem with the cosmic singularity is instead a lack of observation of anything before it: maybe our universe really did begin there. Of course, inflation, dark matter and dark energy are all outstanding questions, but they don't break general relativity, even if they represent some missing extension like general relativity represents to the precession of the perihelion of Mercury.
- elashri 3y ago
  But actually what happens when they merge is that the event horizon are the ones which gets merged and the then the two singularities comea into contact and they merge together. The resulting singularity will be be more massive of the individual singularities. You will get a singularity where GR breaks down. The process of black holes merge is somehow can be understood without referring at all to singularity.
  - BlackFly 3y ago
    Yes, due to the causal structure, but you can also refer to the singularities. The simulation of black hole mergers can be carried out both by excision method and moving puncture method. The existence of black swans does not disprove white swans, it simply proves that black swans exist. General relativity has absolutely no problem describing singularities. The existence of a singularity is not in and of itself evidence of a breakdown. Annihilation of matter via singularity formation may be the correct description of the laws of physics. We have no evidence to the contrary and it is a prediction of our best theory of gravity.
    - elashri 3y ago
      I agree with you here. Just I was referring that Black hole mergers is not something that can be taken as an example here talking about singularities. But yes, existence of singularities does not mean a breakdown of GR. It suggests that GR is not a complete theory of gravity and means that ww cannot make a meaningful prediction about behavior of the system at the singularity points. We can even consider that GR predicts singularities when the curvature of the spacetime becomes so strong that it becomes infinite.
divbzero 3y ago
Isn’t gravity also an outlier in that it is far weaker than the other fundamental forces? The article only mentions that in passing.
- taejavu 3y ago
  Yes, and it's not too much of a stretch to imagine the author might reasonably assume readers of Quanta Magazine don't need more than a passing mention of that fact.
eviks 3y ago
Because it's not a force?
AITree 3y ago
your article showcases the perspectives of different quantum gravity researchers on this topic, highlighting the complexity and uncertainty surrounding the search for a truer theory of gravity.
wanda 3y ago
I think the first lesson here is that the term "force" is pretty unhelpful. I think the term "interaction" is generally preferred to avoid confusion. What I would say is that to bundle gravity in with electromagnetic, strong and weak interaction as "four fundamental interactions" – without explaining the rationale for doing so – is also pretty unhelpful. The reason the other three fundamental interactions seem distinct from gravity is that, theoretically, we can "unify" them at a high enough energy scale. That is, at an energy scale that reflects conditions within a fraction of a second after the big bang, these three interactions can be understood as one interaction, and only as energy distributes and temperature decreases do the forces become distinct. If I am not mistaken, we have experimentally demonstrated that the electromagnetic and weak interactions unify at high enough energy, and we have theories that make predictions towards unifying the strong interaction at higher energy scales. Gravity doesn't slot into this picture quite so neatly at all because gravity resists definition at extremely high energies, and by extension resists any effort to understand it as a component of a unified interaction. I think it is a mistake to dismiss unified interaction as impossible because 'gravity is not a force'. Whenever anyone says this, they make an analogy to the centrifugal force and local reference frames, and that's a fair point to highlight, but I think this indicates more of a linguistic problem in the context of gravity as a fundamental interaction. Similarly, people are quick to dismiss the graviton because it is impossible to detect and philosophically counter-intuitive – how can the effects on matter by the curvature of spacetime be mediated by a particle? But remember: there is no particle. The graviton is a mathematical abstraction, it's not some magic ball of gravity that you can fire from a gravity gun or surround your spaceship with. A graviton is just a... I don't want to say "position" so I'll say "probability density" of spacetime at a QM scale where uncertainty is part of the business. (Someone will correct me if that is wrong). It makes sense that we should want to use that as way to describe gravity because it would use the same framework and methods that we use to successfully describe the rest of the universe. Of course, there are deep seated problems with gravitons, and that's another thing that sets gravity apart, but I wouldn't say that these problems arise because the notion of the graviton is in any way absurd. Gravity is different, and it perhaps doesn't help that at human-relevant scales, Newtonian or "classical" gravity is usually sufficient for our needs in application. It lets us believe gravity is intuitive, simple, and then when we look at galaxies and subatomic particles, and we encounter consequences of GR and QM – which can be rather surprising. But all we have to bear in mind is that at relativistic scales, the uncertainty aspect of QM is still there, it's just incredibly improbable that any quantum weirdness will occur at the macro scale. Similarly, relativistic effects are accounted for at low energy scales, but their influence is so minute that we can remove them from our equations and leave ourselves with classical solutions that are faster to reason about. As for why gravity is different to other interactions, I think any insightful answer to that question would probably require more ground than has yet been covered in efforts to develop quantum gravity. At the moment, the difference is technical, it's different because our tools and methods don't account for it very well as long as we're trying to fit it into our current, well-tested models of physics, and to try and fit all of our physics into a model that makes gravity more sensible is even more daunting a task. --— [IANAP but I am keen to learn so please please please correct me if I am wrong]
ISL 3y ago
As an experimentalist who worked in the field for a long time, my answer to the title is: The Equivalence Principle. Because gravitational charge and inertial mass are seemingly inseparable from one another, gravity is special. It is the concept that makes General Relativity possible and the empirical fact that makes gravitational experiments difficult and unlike any other.
Xcelerate 3y ago
I think this article is missing the biggest thing that makes gravity different: it arises from and affects everything with energy, which includes everything that we know exists. See this graph of the known standard model interactions: https://upload.wikimedia.org/wikipedia/commons/4/4c/Elementary_particle_interactions.svg https://upload.wikimedia.org/wikipedia/commons/4/4c/Elementa... Gravity isn’t shown because every type of particle in that graph interacts with every other type of particle via gravity (i.e., the stress-energy tensor). A lot of people may not realize that a perfectly mirrored box containing photons would have “weight”. Similarly, a spinning top weighs more than the same top that isn’t spinning, a compressed spring weighs more than the same spring in an uncompressed state, and the earth+moon system weighs less as a unit than if you pulled them apart and added together their individual weights. In fact, most of the “weight” of an atom isn’t due to rest mass at all but rather the kinetic energy of the motion of its constituent particles. For electron/positron collisions, it isn’t so much “annihilation” as it is a change of form. After the collision, total momentum, angular momentum, and energy all remain the same. Even though there are no longer two particles that have the property called “rest mass”, the effect on the gravitational field remains exactly the same the moment before and after the collision.
- gpderetta 3y ago
  I think that gravity isn't shown simply because it isn't part of the standard model. IANAP, but standard model fields can only interact with other quantum fields and we do not yet have a quantum field version of GR.
  - zachf 3y ago
    This is one of those things that laymen often misunderstand, actually. (I don’t blame you, it’s often not well explained.) General relativity is easy to make into a QFT. There’s an action and you can easily derive amplitudes from it. And it works well, as long as you don’t ask questions of what happens at arbitrarily high energies. That’s what “nonrenormalizable” actually means: it only answers questions up to some energy scale, after which it stops being predictive. But believe it or not, physics has been here before, so we’re not totally clueless what to do! Fermi’s theory of beta decay [0] had the exact same problem. And the moral there was that there was something new that had been missed in Fermi’s theory that was only important at high enough energy. The theory was correctly predicting its own limitations. In fact we figured out what was missing. Fermi didn’t know about the W boson, and at the energy where W boson exchange becomes important, that’s where the old theory breaks down! But Fermi’s theory works well up to that scale. It’s like how Einstein’s relativity didn’t mean that everything in Newtonian physics was totally wrong, but just that you need it if you want to think about really fast moving objects. Newton’s theory is still an outstandingly good approximation. So if history is any guide, we don’t need to change the low energy theory of GR (at least not much), that works just fine. We just have to find out what’s the high energy phenomenon that we’re missing. That’s stuff that comes into play with black holes and very high energy collisions. Anyway we have some guesses here, too. In string theory, that’s the stringyness of the strings, their 2D nature, that fixes the problems automatically. (This was not by design, it was completely unexpected! You start playing with dimensions and it just falls out.) In LQG it’s about the fuzziness or discretization of space (which sounds appealing, although it’s really tough in practice to make this work). See also [1]. [0] https://en.m.wikipedia.org/wiki/Fermi%27s_interaction https://en.m.wikipedia.org/wiki/Fermi%27s_interaction [1] https://en.m.wikipedia.org/wiki/Effective_field_theory https://en.m.wikipedia.org/wiki/Effective_field_theory
sghiassy 3y ago
Gravity isn’t a force, it’s a curvature of space time
- moffkalast 3y ago
  It's weird how mass just somehow perturbs spacetime creating a 3D whirlpool down which everything falls without using any energy in the process. That theory about mass actually only accelerating time and then the time dilation between atoms causing the force of gravity as a consequence is pretty nuts. Makes you think there really ought to be a way to "stick" onto spacetime and stop yourself from being accelerated, much like pulling the parking brake on a hill. Maybe one just needs a time machine.
  - hollerith 3y ago
    You can put the surface of a planet between you and the bottom of the whirlpool like us humans do.
    - moffkalast 3y ago
      Yeah but that's the lazy solution that's not really all that useful. I mean it's definitely useful to not keep falling towards the planet core, but for other less mundane uses.
  - ISL 3y ago
    If you want to "stick" to spacetime, there must be a reference frame that is somehow different from all the others. There is presently no experimental evidence for any such frame, despite a century or two of searches by very clever people.
    - moffkalast 3y ago
      Well maybe not literally, just in practical terms. If it's a time gradient that causes an object to accelerate, then if you can apply a reverse gradient that would just cancel out the forces. But yes, one would need to first figure out how to do that without an equivalent amount of mass on the other side, if it's even physically possible. Here's a stupid idea that's based on my definitely flawed understanding of relativity: If having high relative velocity causes time dilation, then wouldn't spinning a disc at relativistic speeds cause that same time gradient? Maybe we just need 99.9% c rad/s fidget spinners to float in the air just like bricks don't. Found the video on this theory: https://www.youtube.com/watch?v=UKxQTvqcpSg https://www.youtube.com/watch?v=UKxQTvqcpSg
stainablesteel 3y ago
just wanna put this out there before someone else gets the nobel prize it's a push, not a pull thank me later
pontus 3y ago
Here's a fun thought experiment / apparent paradox. In high school physics we learn that a 1kg mass accelerates as quickly as a 2kg mass when only subjected to the force of gravity. When I used to teach physics, the intuitive explanation I gave for this hinged on a thought experiment. Suppose that you have three 1kg masses falling side by side after being dropped from the same height. Clearly they are all going to fall at the same rate since they're equivalent. Now imagine redoing the experiment but this time taking two of the masses and placing them closer together. Does anything change? Clearly not, they're still all equivalent and ought to fall at the same rate. Now imagine doing this until those two masses are right next to each other, touching. Does anything change? Well no, all three should still fall at the same rate. But now, why not glue those two masses together and call it a 2kg mass? Once you do that you've shown that a 1kg mass and a 2kg mass fall at the same rate. This usually convinces people, but there's actually a flaw in the argument that gets to the heart of why gravity is so different from the other forces. To see the flaw, replace the above masses by three electrons falling next to each other in an electric field. Everything goes through in exactly the same way. You end up gluing together two of the electrons and these two electrons will accelerate at the same rate as the single electron. But if you're not careful you'd conclude that all electric charges fall at the same rate in an electric field, something we know is false. Where's the flaw? Well, all of matter is built from some particles, and as long as you restrict yourself to particles that have the same "charge/mass ratio", the argument above works. It is true that one electron accelerates the same as 100 electrons tied together but that's just because e/m is the same for all those constituents. So, the thing that's glossed over in my high school explanation for why 1kg and 2kg accelerate at the same rate is that the constituent particles all have the same "gravitational charge / inertial mass" ratio. Because this ratio is the same for all particles, we may as well absorbed that ratio into the gravitational constant and just use "m" in place of both of them. It's this "universal coupling" that's really responsible for the equivalence principle and what sets gravity apart from the other forces.
- splitwheel 3y ago
  1kg mass and a 2kg mass do not fall at the same rate. The Gravitational force is (G*m1*m2)/r^2. You are observing that m1 (the earth) is much much greater than m2 (the 1 or 2 kg masses), and you are simplifying to (G*m1)/d^2 because of the precision of the measuring device. Also, d is the same for both masses.
dist-epoch 3y ago
> If there were any type of particle that did not feel gravity, we could use that particle to send out a message from the inside of the black hole, so it wouldn’t actually be black. What about gravitational waves? Could you propagate a gravitational wave from inside the black hole to the outside? Or is it stopped at the event horizon?
- MaxikCZ 3y ago
  Gravitational waves travel trough spacetime at the speed of light. So no, not even gravitational waves can escape black hole.
ChicagoDave 3y ago
I just had four physicists in my Uber last weekend and we got into this a bit. The one guy said the biggest problem with gravity is we’ve never figured outs its opposite or if there even is an opposite. He mentioned something about negative mass, which is maybe the handwavium equivalent of anti-matter in science fiction? Gravity only attracts. It never pushes away. Yeah I’m way in over my head but it’s fun to poke around in things we don’t understand.
- codethief 3y ago
  > He mentioned something about negative mass, which is maybe the handwavium equivalent of anti-matter in science fiction? Well, anti-matter actually exists but its mass is positive. Also, the anti-particle of the (hypothetical) graviton is the graviton itself[0], so all in all nothing fancy happening there. > Gravity only attracts. It never pushes away. Cosmic expansion would like to have a word. (Though, admittedly, expansion doesn't happen in gravitationally bound systems, so from this point of view your statement is of course correct.) > The one guy said the biggest problem with gravity is we’ve never figured outs its opposite or if there even is an opposite. Interesting. Do you know whether he was an active researcher in the field of relativity? I can think of a lot of "big problems" in relativity but this wouldn't be one of them. I mean, don't get me wrong, it would be nice to have something to cancel out gravity since this would inspire lots of applications that we so far deem science fiction. But we already have a pretty good idea of the limits of what's possible[1]. [0]: https://physics.stackexchange.com/questions/273918/is-there-a-possibility-for-discovery-of-anti-graviton-i-e-the-graviton-antipar https://physics.stackexchange.com/questions/273918/is-there-... [1]: https://arxiv.org/abs/1611.01808 https://arxiv.org/abs/1611.01808 + the rigidity case of the https://en.wikipedia.org/wiki/Positive_energy_theorem https://en.wikipedia.org/wiki/Positive_energy_theorem suggest that one could theoretically "shield" / cancel out gravity to some degree but not everywhere (in all directions).
codethief 3y ago
My favorite "fun fact" about gravity is still this one[0]: > [Gravity] shapes the arena in which it acts, unlike other fields which act in a fixed spacetime background. From what I've seen, this fact is rather underappreciated by many of the folks doing (what they call) "quantum gravity" these days. They often try to treat gravity as a rather ordinary quantum (or string) field theory, living on a fixed background. As Hawking says in [0], "It would be rather boring if this were the case. Gravity would be just like any other field." [0]: Hawking: The Nature of Space and Time (1994), p. 2, https://arxiv.org/abs/hep-th/9409195 https://arxiv.org/abs/hep-th/9409195
tlogan 3y ago
I have no clue about this but my layman’s idea is: Energy is entanglement. Energy creates gravity. So gravity is created by entanglement (gravity can be considered an emergent property stemming from entanglement).
OnlyMortal 3y ago
It’s not a “force”. I’d argue that it’s more to do with lowering entropy.
darken 3y ago
Respin of a comment I posted a while back: This is what helped me wrap ahead around the fact that gravity "is not really a force": when you are in free fall you feel 0 acceleration. You appear to be accelerating relative to the ground-- but you're actually motionless in an "inertial reference frame". (Similar to how the astronauts on the ISS don't "feel" acceleration despite accelerating rapidly relative to the earth.) The "force" of gravity is often modeled as "gravity pulling you down" and the ground "pushing you back up". This works mathematically, but isn't quite logically consistent. In reality, on the ground you're in a region of warped spacetime, so you feel constant upward acceleration despite not actually accelerating. (Thinking of this another way, standing on earth feels identical to being in a far away spaceship accelerating at 9.8 m/s².) This is also why time "speeds up" near more massive objects. (Separate from "acceleration".) We're so used to gravity this it doesn't seem weird. But when you consider the fact that free-fall is when you truly do not experience acceleration... well pondering that from many angles is what ultimately led Einstein to his model of relativity. (This is me trying to condense what could be a 10 minute explanation into a few sentences, so apologies if it's not particularly clear.) Veritasium had a good video on this topic: https://www.youtube.com/watch?v=XRr1kaXKBsU https://www.youtube.com/watch?v=XRr1kaXKBsU (Thanks to u/badocr for commenting with this link last time.)

134 points by kjhughes 3y ago

iaw 3y ago

I've always naively assumed gravity to be a facet of momentum/movement through time. The math between acceleration, gravity, and time dilation are all too cute to be coincidental in my book. I suspect that gravity manifests as a force but does not stem from the same underlying drivers as other forces.

m3kw9 3y ago
Yeah gravity seem to have a time dimension to it
- ars 3y ago
  Gravity itself does not have a time dimension - if you had a highly changed particle acting on another one, the time dilation would be identical to that caused by gravity acting with the same force. i.e. it's force and acceleration that cause it, not gravity.
magicalhippo 3y ago
Lee Smolin is working on an idea where the building blocks are events where momentum is exchanged.Space, and hence position, is an emergent phenomena. Here's a colloquium talk[1] and here's a recent paper[2]. Not exactly what you said but perhaps interesting. As mentioned it's work in progress so remains to be seen if the idea pans out. [1]: https://pirsa.org/21030005 https://pirsa.org/21030005 [2]: https://arxiv.org/abs/2303.15546 https://arxiv.org/abs/2303.15546
- nemo44x 3y ago
  I love how this word “emergent” has become popular by people that want to pretend they have an answer but it’s really just the “…” thing. There is no such thing as “emergent”. The motion of an automobile isn’t an “emergent” thing you can claim because you don’t know how engines and transmissions work. Everything we can’t explain recently has people, who I assume don’t know how to say I don’t know, proclaiming “it’s emergent”. As if that’s an answer to anything. Much less a claim to knowledge. Might as well claim “god made it happen”.
  - lanstin 3y ago
    System availability is an emergent property - I can build five nines of availability out of components with just two nines. Retries, algorithmic handling of failure, and the like.
  - Dylan16807 3y ago
    > The motion of an automobile isn’t an “emergent” thing you can claim because you don’t know how engines and transmissions work. That's the opposite of what emergent means, isn't it? Because of force on the transmission, you can say that the motion of the vehicle is emergent rather than needing its own framework or explanation.
  - CaptainNegative 3y ago
    Seems like pretending to know what the word emergent means in physics is an emergent phenomenon on HN. It's not wishy washy, it has a precise meaning that can be both derived and, typically, refuted.
  - nextaccountic 3y ago
    Emergent just means that it's not the basic building block; it's made up of other, more fundamental things. So for example thermodynamics is emergent (even though it is fully studied and relatively well understood) because it arises from interactions of particles. If you say that spacetime is emergent that is a big deal: it means that, while it can be studied on its own, it's actually composed of other, more fundamental things. But you still need to show that a) this other fundamental thing actually exists, and b) we can show mathematically how the spacetime works, in terms of those other fundamental thing (just like you can show how thermodynamics works in terms of statistical mechanics) And that's where your "because you don't know how it works" thing breaks apart: one can't claim to have solved this problem without tacking b).
    - nemo44x 3y ago
      > it's made up of other, more fundamental things Then explain those things and prove they exist. Otherwise this just sounds like bullshit. There’s a difference between observing thermodynamics (and not trying to answer why but rather how we observe it - big difference!) and writing equations we can measure and just saying “it’s emergent” to “…” things we are unable to answer “I don’t know to”.
wiml 3y ago
That is the basic idea that gives rise to the theory of general relativity — that gravity isn't really a force, it's just the observable effect of being in curved spacetime. (Take a look at the "equivalence principle" and its role in the development of GR as a theory.)
- denton-scratch 3y ago
  > gravity isn't really a force Yeah, this fact made me cringe at explanation #2: the guy keeps saying things like "gravity is the only force that is felt by all kinds of matter". Stuff doesn't "feel" gravity; it's not a force, it's just the shape of spacetime. Also, gravity doesn't just affect matter; photons are not matter, but are affected by the curvature of spacetime (i.e. gravity). /me not a physicist; but I'm not a baby either, and I don't appreciate babytalk oversimplifications.
alanfalcon 3y ago
Gravity is an effect of the curve of space and relativity coming together in what we now call spacetime. Right? Einstein told us so. But... Maybe not? There's promising math behind the idea that there are multiple additional dimensions, but where are they? How can we perceive them? What do they have to do with gravity? The unifying theory of dimensional geometry and interaction: https://youtu.be/fvqXshyuvOg https://youtu.be/fvqXshyuvOg

jenadine 3y ago

But gravity isn't really a force. In the same way that inertia is not really a force. It's just an artifact of the coordinate system being curved.

deanCommie 3y ago
But WHY is the coordinate system curved?
- jenadine 3y ago
  Because of mass. But WHAT is mass?
  - drsnow 3y ago
    A measure of a particle's energy as pertains to the spacetime curvature it induces
    - ars 3y ago
      This is incorrect all the energy of a particle curves space. Even velocity and potential energy.
      - drsnow 3y ago
        Right, so those kinds of energy also induce spacetime curvature and are included in what I was saying
  - irrational 3y ago
    Wikipedia says “the property of "mass" is a manifestation of potential energy transferred to fundamental particles when they interact ("couple") with the Higgs field, which had contained that mass in the form of energy.”
  - ars 3y ago
    Energy, not mass. Photons also curve space in this way. Mass is just one kind of energy.

jacknews 3y ago

"We can picture many entities called fields superimposed on top of one another and extending throughout space. " And all affected in exactly the same way by the mysterious gravity 'force' or field. Or, we can picture just a single field, that has many dimensions or traits, which we call the other forces.

guender 3y ago

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BlackFly 3y ago

> But near the center of a black hole or in the first moments of the universe, Einstein’s equations break. Not really. The formation and continuous description of a singularity is somewhat unique in physics, but general relativity continues to provide a description, allowing even accurate descriptions of black hole mergers. The problem with the cosmic singularity is instead a lack of observation of anything before it: maybe our universe really did begin there. Of course, inflation, dark matter and dark energy are all outstanding questions, but they don't break general relativity, even if they represent some missing extension like general relativity represents to the precession of the perihelion of Mercury.

elashri 3y ago
But actually what happens when they merge is that the event horizon are the ones which gets merged and the then the two singularities comea into contact and they merge together. The resulting singularity will be be more massive of the individual singularities. You will get a singularity where GR breaks down. The process of black holes merge is somehow can be understood without referring at all to singularity.
- BlackFly 3y ago
  Yes, due to the causal structure, but you can also refer to the singularities. The simulation of black hole mergers can be carried out both by excision method and moving puncture method. The existence of black swans does not disprove white swans, it simply proves that black swans exist. General relativity has absolutely no problem describing singularities. The existence of a singularity is not in and of itself evidence of a breakdown. Annihilation of matter via singularity formation may be the correct description of the laws of physics. We have no evidence to the contrary and it is a prediction of our best theory of gravity.
  - elashri 3y ago
    I agree with you here. Just I was referring that Black hole mergers is not something that can be taken as an example here talking about singularities. But yes, existence of singularities does not mean a breakdown of GR. It suggests that GR is not a complete theory of gravity and means that ww cannot make a meaningful prediction about behavior of the system at the singularity points. We can even consider that GR predicts singularities when the curvature of the spacetime becomes so strong that it becomes infinite.

divbzero 3y ago

Isn’t gravity also an outlier in that it is far weaker than the other fundamental forces? The article only mentions that in passing.

taejavu 3y ago
Yes, and it's not too much of a stretch to imagine the author might reasonably assume readers of Quanta Magazine don't need more than a passing mention of that fact.

eviks 3y ago

Because it's not a force?

AITree 3y ago

your article showcases the perspectives of different quantum gravity researchers on this topic, highlighting the complexity and uncertainty surrounding the search for a truer theory of gravity.

wanda 3y ago

I think the first lesson here is that the term "force" is pretty unhelpful. I think the term "interaction" is generally preferred to avoid confusion. What I would say is that to bundle gravity in with electromagnetic, strong and weak interaction as "four fundamental interactions" – without explaining the rationale for doing so – is also pretty unhelpful. The reason the other three fundamental interactions seem distinct from gravity is that, theoretically, we can "unify" them at a high enough energy scale. That is, at an energy scale that reflects conditions within a fraction of a second after the big bang, these three interactions can be understood as one interaction, and only as energy distributes and temperature decreases do the forces become distinct. If I am not mistaken, we have experimentally demonstrated that the electromagnetic and weak interactions unify at high enough energy, and we have theories that make predictions towards unifying the strong interaction at higher energy scales. Gravity doesn't slot into this picture quite so neatly at all because gravity resists definition at extremely high energies, and by extension resists any effort to understand it as a component of a unified interaction. I think it is a mistake to dismiss unified interaction as impossible because 'gravity is not a force'. Whenever anyone says this, they make an analogy to the centrifugal force and local reference frames, and that's a fair point to highlight, but I think this indicates more of a linguistic problem in the context of gravity as a fundamental interaction. Similarly, people are quick to dismiss the graviton because it is impossible to detect and philosophically counter-intuitive – how can the effects on matter by the curvature of spacetime be mediated by a particle? But remember: there is no particle. The graviton is a mathematical abstraction, it's not some magic ball of gravity that you can fire from a gravity gun or surround your spaceship with. A graviton is just a... I don't want to say "position" so I'll say "probability density" of spacetime at a QM scale where uncertainty is part of the business. (Someone will correct me if that is wrong). It makes sense that we should want to use that as way to describe gravity because it would use the same framework and methods that we use to successfully describe the rest of the universe. Of course, there are deep seated problems with gravitons, and that's another thing that sets gravity apart, but I wouldn't say that these problems arise because the notion of the graviton is in any way absurd. Gravity is different, and it perhaps doesn't help that at human-relevant scales, Newtonian or "classical" gravity is usually sufficient for our needs in application. It lets us believe gravity is intuitive, simple, and then when we look at galaxies and subatomic particles, and we encounter consequences of GR and QM – which can be rather surprising. But all we have to bear in mind is that at relativistic scales, the uncertainty aspect of QM is still there, it's just incredibly improbable that any quantum weirdness will occur at the macro scale. Similarly, relativistic effects are accounted for at low energy scales, but their influence is so minute that we can remove them from our equations and leave ourselves with classical solutions that are faster to reason about. As for why gravity is different to other interactions, I think any insightful answer to that question would probably require more ground than has yet been covered in efforts to develop quantum gravity. At the moment, the difference is technical, it's different because our tools and methods don't account for it very well as long as we're trying to fit it into our current, well-tested models of physics, and to try and fit all of our physics into a model that makes gravity more sensible is even more daunting a task. --— [IANAP but I am keen to learn so please please please correct me if I am wrong]

ISL 3y ago

As an experimentalist who worked in the field for a long time, my answer to the title is: The Equivalence Principle. Because gravitational charge and inertial mass are seemingly inseparable from one another, gravity is special. It is the concept that makes General Relativity possible and the empirical fact that makes gravitational experiments difficult and unlike any other.

Xcelerate 3y ago

I think this article is missing the biggest thing that makes gravity different: it arises from and affects everything with energy, which includes everything that we know exists. See this graph of the known standard model interactions: https://upload.wikimedia.org/wikipedia/commons/4/4c/Elementary_particle_interactions.svg https://upload.wikimedia.org/wikipedia/commons/4/4c/Elementa... Gravity isn’t shown because every type of particle in that graph interacts with every other type of particle via gravity (i.e., the stress-energy tensor). A lot of people may not realize that a perfectly mirrored box containing photons would have “weight”. Similarly, a spinning top weighs more than the same top that isn’t spinning, a compressed spring weighs more than the same spring in an uncompressed state, and the earth+moon system weighs less as a unit than if you pulled them apart and added together their individual weights. In fact, most of the “weight” of an atom isn’t due to rest mass at all but rather the kinetic energy of the motion of its constituent particles. For electron/positron collisions, it isn’t so much “annihilation” as it is a change of form. After the collision, total momentum, angular momentum, and energy all remain the same. Even though there are no longer two particles that have the property called “rest mass”, the effect on the gravitational field remains exactly the same the moment before and after the collision.

gpderetta 3y ago
I think that gravity isn't shown simply because it isn't part of the standard model. IANAP, but standard model fields can only interact with other quantum fields and we do not yet have a quantum field version of GR.
- zachf 3y ago
  This is one of those things that laymen often misunderstand, actually. (I don’t blame you, it’s often not well explained.) General relativity is easy to make into a QFT. There’s an action and you can easily derive amplitudes from it. And it works well, as long as you don’t ask questions of what happens at arbitrarily high energies. That’s what “nonrenormalizable” actually means: it only answers questions up to some energy scale, after which it stops being predictive. But believe it or not, physics has been here before, so we’re not totally clueless what to do! Fermi’s theory of beta decay [0] had the exact same problem. And the moral there was that there was something new that had been missed in Fermi’s theory that was only important at high enough energy. The theory was correctly predicting its own limitations. In fact we figured out what was missing. Fermi didn’t know about the W boson, and at the energy where W boson exchange becomes important, that’s where the old theory breaks down! But Fermi’s theory works well up to that scale. It’s like how Einstein’s relativity didn’t mean that everything in Newtonian physics was totally wrong, but just that you need it if you want to think about really fast moving objects. Newton’s theory is still an outstandingly good approximation. So if history is any guide, we don’t need to change the low energy theory of GR (at least not much), that works just fine. We just have to find out what’s the high energy phenomenon that we’re missing. That’s stuff that comes into play with black holes and very high energy collisions. Anyway we have some guesses here, too. In string theory, that’s the stringyness of the strings, their 2D nature, that fixes the problems automatically. (This was not by design, it was completely unexpected! You start playing with dimensions and it just falls out.) In LQG it’s about the fuzziness or discretization of space (which sounds appealing, although it’s really tough in practice to make this work). See also [1]. [0] https://en.m.wikipedia.org/wiki/Fermi%27s_interaction https://en.m.wikipedia.org/wiki/Fermi%27s_interaction [1] https://en.m.wikipedia.org/wiki/Effective_field_theory https://en.m.wikipedia.org/wiki/Effective_field_theory

sghiassy 3y ago

Gravity isn’t a force, it’s a curvature of space time

moffkalast 3y ago
It's weird how mass just somehow perturbs spacetime creating a 3D whirlpool down which everything falls without using any energy in the process. That theory about mass actually only accelerating time and then the time dilation between atoms causing the force of gravity as a consequence is pretty nuts. Makes you think there really ought to be a way to "stick" onto spacetime and stop yourself from being accelerated, much like pulling the parking brake on a hill. Maybe one just needs a time machine.
- hollerith 3y ago
  You can put the surface of a planet between you and the bottom of the whirlpool like us humans do.
  - moffkalast 3y ago
    Yeah but that's the lazy solution that's not really all that useful. I mean it's definitely useful to not keep falling towards the planet core, but for other less mundane uses.
- ISL 3y ago
  If you want to "stick" to spacetime, there must be a reference frame that is somehow different from all the others. There is presently no experimental evidence for any such frame, despite a century or two of searches by very clever people.
  - moffkalast 3y ago
    Well maybe not literally, just in practical terms. If it's a time gradient that causes an object to accelerate, then if you can apply a reverse gradient that would just cancel out the forces. But yes, one would need to first figure out how to do that without an equivalent amount of mass on the other side, if it's even physically possible. Here's a stupid idea that's based on my definitely flawed understanding of relativity: If having high relative velocity causes time dilation, then wouldn't spinning a disc at relativistic speeds cause that same time gradient? Maybe we just need 99.9% c rad/s fidget spinners to float in the air just like bricks don't. Found the video on this theory: https://www.youtube.com/watch?v=UKxQTvqcpSg https://www.youtube.com/watch?v=UKxQTvqcpSg

stainablesteel 3y ago

just wanna put this out there before someone else gets the nobel prize it's a push, not a pull thank me later

pontus 3y ago

Here's a fun thought experiment / apparent paradox. In high school physics we learn that a 1kg mass accelerates as quickly as a 2kg mass when only subjected to the force of gravity. When I used to teach physics, the intuitive explanation I gave for this hinged on a thought experiment. Suppose that you have three 1kg masses falling side by side after being dropped from the same height. Clearly they are all going to fall at the same rate since they're equivalent. Now imagine redoing the experiment but this time taking two of the masses and placing them closer together. Does anything change? Clearly not, they're still all equivalent and ought to fall at the same rate. Now imagine doing this until those two masses are right next to each other, touching. Does anything change? Well no, all three should still fall at the same rate. But now, why not glue those two masses together and call it a 2kg mass? Once you do that you've shown that a 1kg mass and a 2kg mass fall at the same rate. This usually convinces people, but there's actually a flaw in the argument that gets to the heart of why gravity is so different from the other forces. To see the flaw, replace the above masses by three electrons falling next to each other in an electric field. Everything goes through in exactly the same way. You end up gluing together two of the electrons and these two electrons will accelerate at the same rate as the single electron. But if you're not careful you'd conclude that all electric charges fall at the same rate in an electric field, something we know is false. Where's the flaw? Well, all of matter is built from some particles, and as long as you restrict yourself to particles that have the same "charge/mass ratio", the argument above works. It is true that one electron accelerates the same as 100 electrons tied together but that's just because e/m is the same for all those constituents. So, the thing that's glossed over in my high school explanation for why 1kg and 2kg accelerate at the same rate is that the constituent particles all have the same "gravitational charge / inertial mass" ratio. Because this ratio is the same for all particles, we may as well absorbed that ratio into the gravitational constant and just use "m" in place of both of them. It's this "universal coupling" that's really responsible for the equivalence principle and what sets gravity apart from the other forces.

splitwheel 3y ago
1kg mass and a 2kg mass do not fall at the same rate. The Gravitational force is (G*m1*m2)/r^2. You are observing that m1 (the earth) is much much greater than m2 (the 1 or 2 kg masses), and you are simplifying to (G*m1)/d^2 because of the precision of the measuring device. Also, d is the same for both masses.

dist-epoch 3y ago

> If there were any type of particle that did not feel gravity, we could use that particle to send out a message from the inside of the black hole, so it wouldn’t actually be black. What about gravitational waves? Could you propagate a gravitational wave from inside the black hole to the outside? Or is it stopped at the event horizon?

MaxikCZ 3y ago
Gravitational waves travel trough spacetime at the speed of light. So no, not even gravitational waves can escape black hole.

ChicagoDave 3y ago

I just had four physicists in my Uber last weekend and we got into this a bit. The one guy said the biggest problem with gravity is we’ve never figured outs its opposite or if there even is an opposite. He mentioned something about negative mass, which is maybe the handwavium equivalent of anti-matter in science fiction? Gravity only attracts. It never pushes away. Yeah I’m way in over my head but it’s fun to poke around in things we don’t understand.

codethief 3y ago
> He mentioned something about negative mass, which is maybe the handwavium equivalent of anti-matter in science fiction? Well, anti-matter actually exists but its mass is positive. Also, the anti-particle of the (hypothetical) graviton is the graviton itself[0], so all in all nothing fancy happening there. > Gravity only attracts. It never pushes away. Cosmic expansion would like to have a word. (Though, admittedly, expansion doesn't happen in gravitationally bound systems, so from this point of view your statement is of course correct.) > The one guy said the biggest problem with gravity is we’ve never figured outs its opposite or if there even is an opposite. Interesting. Do you know whether he was an active researcher in the field of relativity? I can think of a lot of "big problems" in relativity but this wouldn't be one of them. I mean, don't get me wrong, it would be nice to have something to cancel out gravity since this would inspire lots of applications that we so far deem science fiction. But we already have a pretty good idea of the limits of what's possible[1]. [0]: https://physics.stackexchange.com/questions/273918/is-there-a-possibility-for-discovery-of-anti-graviton-i-e-the-graviton-antipar https://physics.stackexchange.com/questions/273918/is-there-... [1]: https://arxiv.org/abs/1611.01808 https://arxiv.org/abs/1611.01808 + the rigidity case of the https://en.wikipedia.org/wiki/Positive_energy_theorem https://en.wikipedia.org/wiki/Positive_energy_theorem suggest that one could theoretically "shield" / cancel out gravity to some degree but not everywhere (in all directions).

codethief 3y ago

My favorite "fun fact" about gravity is still this one[0]: > [Gravity] shapes the arena in which it acts, unlike other fields which act in a fixed spacetime background. From what I've seen, this fact is rather underappreciated by many of the folks doing (what they call) "quantum gravity" these days. They often try to treat gravity as a rather ordinary quantum (or string) field theory, living on a fixed background. As Hawking says in [0], "It would be rather boring if this were the case. Gravity would be just like any other field." [0]: Hawking: The Nature of Space and Time (1994), p. 2, https://arxiv.org/abs/hep-th/9409195 https://arxiv.org/abs/hep-th/9409195

tlogan 3y ago

I have no clue about this but my layman’s idea is: Energy is entanglement. Energy creates gravity. So gravity is created by entanglement (gravity can be considered an emergent property stemming from entanglement).

OnlyMortal 3y ago

It’s not a “force”. I’d argue that it’s more to do with lowering entropy.

darken 3y ago

Respin of a comment I posted a while back: This is what helped me wrap ahead around the fact that gravity "is not really a force": when you are in free fall you feel 0 acceleration. You appear to be accelerating relative to the ground-- but you're actually motionless in an "inertial reference frame". (Similar to how the astronauts on the ISS don't "feel" acceleration despite accelerating rapidly relative to the earth.) The "force" of gravity is often modeled as "gravity pulling you down" and the ground "pushing you back up". This works mathematically, but isn't quite logically consistent. In reality, on the ground you're in a region of warped spacetime, so you feel constant upward acceleration despite not actually accelerating. (Thinking of this another way, standing on earth feels identical to being in a far away spaceship accelerating at 9.8 m/s².) This is also why time "speeds up" near more massive objects. (Separate from "acceleration".) We're so used to gravity this it doesn't seem weird. But when you consider the fact that free-fall is when you truly do not experience acceleration... well pondering that from many angles is what ultimately led Einstein to his model of relativity. (This is me trying to condense what could be a 10 minute explanation into a few sentences, so apologies if it's not particularly clear.) Veritasium had a good video on this topic: https://www.youtube.com/watch?v=XRr1kaXKBsU https://www.youtube.com/watch?v=XRr1kaXKBsU (Thanks to u/badocr for commenting with this link last time.)