Our ability to solve integrals is much more limited when the dx represents a slight change in a function, rather than a small change in a real number. As a result, a lot of things that are easy to say in English such as "quantized curvature in spacetime," or "strongly coupled gauge theory," turn into a big mess when they're written down more precisely. One of the consequences of this limitation is that we have a model for quantized vibrations in spacetime that only works when they do not interact with each other. General relativity says that no, gravitational fields do interact with each other - so the picture we have at present is incomplete. The model of non-self-interacting gravity is a particle we call a "graviton," and it probably describes reality very well when the gravitation involved is so weak that its self-interaction is undetectable.
String theory and loop quantum gravity fit into this picture by trying to replace the integral over something we can't handle with an integral that matches it at large scales, but turns into something more tractable at small scales. Maybe the fact that we still can't make sense of the integral is Nature's way of telling us that she does not do the integral either...
> The model of non-self-interacting gravity is a particle we call a "graviton," and it probably describes reality very well when the gravitation involved is so weak that its self-interaction is undetectable.
Can you please elaborate, the first part of the sentence says graviton is for non-self-interacting gravity, the second part of the sentence says graviton is for self-interacting (if 'its' in 'its self-interaction' refers to the graviton).
I don't intend to nitpick the sentence, just trying to understand the theory and I don't even know if particle means self interaction or the opposite and can't parse it here either...
If the answer is graviton is for non-self-interacting: what is the model for the other case (where gravity does self interact) and what would cause that self interaction if not the graviton?
This needs to be emphasized more, by the TFA too — most (theoretical) physicists think that detecting gravitons is an engineering exercise that has no implications* for quantum gravity (as understood by the public)
>The model of non-self-interacting gravity is a particle we call a "graviton,"
This needs to be emphasized even more, because it has
>when the dx represents a slight change in a function
*see the discussion around sharikous’ comment below
> The model of non-self-interacting gravity is a particle we call a "graviton," and it probably describes reality very well when the gravitation involved is so weak that its self-interaction is undetectable.
I disagree with that part. For the strong force we have the "gluons" and they are considered particles and they have a strong self-interaction. The strong self interaction makes it a huge mess and a lot of things that involve gluons are impossible to calculate.
It's more like:
fake quote> Let's pretend for 30 minutes that the strong field don't self-interact, so we have this nice particles call gluons. Now we add this interaction to the Lagrangian to make gluons interact with other gluons, and now we have a problem.
I agree that that when gravity is small enough, then gravitons give an easy to calculate aproximation. IIRC at high enough energies calculations with gluons get not impossible to calculate too.
I don't know if it was just sheer luck that your comment fit my particular flavor of ignorance perfectly, but it struck me as great writing! I think I learned a little thing today. When I read the article I thought, too bad I can't ever understand anything of this, but now my personal model of the universe is just a little bit richer.
In theory, if gravitons exist, they should reproduce the same effects as the curvature of spacetime at larger scales. So, while they seem contradictory, they're actually complementary. Gravitons would be the "quantized" particles that, in large numbers, create the effect we observe as curved spacetime.
The problem is that nobody has successfully combined these two views into a single unified theory, known as "quantum gravity". General Relativity and quantum mechanics don't naturally fit together, and that's why we don't yet fully understand gravity in a way that reconciles both the spacetime curvature and graviton perspectives.
> I thought gravity was basically the curvature of spacetime.
Classically, it is. But most physicists believe that there is a quantum theory of gravity that underlies the classical theory, and that that quantum theory will include, at some level of description, a spin-2 gauge boson that mediates the quantum gravitational interaction, called the "graviton". Our classical theory of gravity, General Relativity, would then be the classical limit of that quantum theory, just as classical Maxwell electrodynamics is the classical limit of quantum electrodynamics.
Electromagnetism is both a continuous wave and a discrete particle, so it makes sense to me that a continuous spacetime curvature could also be a discrete particle at the same time. (Keeping in mind we're not talking about tangible shapes but mathematical models that describe aspects of reality that are hard for humans to intuitively conceptualize.)
Of course, our idea of how to reconcile quantum gravity with general relativity is much less developed than our understanding of electromagnetism and the nuclear forces.
My understanding is the particle model of electromagnetism, the photon, really only shows up where the em field interacts with matter(electrons really), the em field itself is not quantized, or at least not quantized at the level of the photon. Not that this really matters(intentional), we can only interact with the em field as matter so that is what matters.
The most elegant description of electromagnetism is also in terms of curvature, but the curvature of a certain mathematical structure called "connection on fiber bundles" and the math field is called differential geometry.
When you mention nuclear forces, are you referencing weak force and strong force? Do we understand these forces at the same level that we understand electromagnetism?
That gravity is curvature of spacetime is one view of two equivalent views, but it is the standard view. The other view is that you have flat spacetime with different distortions (of things other than spacetime) than the distortions you get in curved spacetime. The Schwarzschild metric essentially lets you do exactly that projection of curved spacetime to flat, and vice-versa. When you watch an animation like https://www.youtube.com/watch?v=hF7zltx7Ecc or https://www.youtube.com/watch?v=E1mD4C7dBKc you're watching a flat spacetime representation of GR's effects, and the reason for using flat spacetime in these representations is <drum-roll/> that that is what us humans understand.
So if you take the gravity curves spacetime view, then gravity is not a force and all that. But if you take the alternative view then gravity is a force. Now, I'll leave what the distortions are that gravity produces in flat spacetime for another time, or for the reader. But I'll say this: this view is both controversial (perhaps replies will show this) and not (see above -and many other- animations).
I last took a physics course when Pluto was a planet, so excuse my possibly outdated question, but isn't the detection of gravitational waves proof of gravity being a force?
I follow a few educators/communicators in this field and I have a feeling they're using this "gravity isn't really a force" to bridge the gap between their deep understanding and us mortals that don't poses the language / understanding to get the entire meaning behind it. Is that feeling correct or am I missing something?
Not an expert, but: the curvature of spacetime is modeled as a tensor field (the metric tensor). That field can have (classical) waves in it, which is what LIGO detects (I believe). Then you can certain hypothetically quantize that field, in which case it definitely has to be a spin-2 particle and it seems likely that there will be a way to do it since all the rest were.
The "geometry" comes from the fact that the way we measure distances (or, well, experience time) uses the metric tensor field to do it. But it is still ultimately just a value attached to every point like any other field.
The YT algorithm is so damn bizarre sometimes. I'm subscribed to probably a dozen different astronomy and physics channels, some for 8 years, and never once saw this channel you recommended. It recommends plenty of AI-TTS bunk too. But somehow didn't decide to show me that channel. Thanks for recommending it.
There are some ideas that spacetime is an emergent phenomenon. One such proposal is that it is produced by the large-scale presence of entanglement between particles: that entanglement creates spacetime. Where entanglement between regions of spacetime is stronger, the space is closer together, and where that entanglement is cut things get farther apart. This idea is known as "ER=EPR" [0].
That's the link bridging gravity as a particle (small-scale) and gravity as a feature of a manifold (large-scale). Physicists are trying to find a way to make spacetime emerge from quantum field theory, or make both emerge from some common framework.
Gravity is similar to an electric field here. A wave function for a field consists of an amplitude for each field configuration, where a “field configuration” refers to a value for the electric field for each point in space. In GR each field configuration would correspond to a space time geometry for the universe. We have quantized excitations as distinct “valid” solutions to the wave function, which we call a particle, though it is nothing like an electron. The notion of space time geometry holds throughout. (Edit: in practice, people never calculate wave functions for fields like electric fields. That would be too hard. Different methods are used in calculations. Second edit: the wave function wouldn’t be composed of complete space-time configurations, histories of the universe, but time slices from it, like space geometries. Maybe this can be expanded in responses/comments.)
To my understanding (not the best) there's a huge disconnect between the physics of the very small (quantum mechanics and the standard model) and that of the very large (general relativity).
The disconnect seems to be unresolvable (I don't understand this part at all) and so efforts are being made to quantise gravity and incorporate it into the standard model.
it's important to realise that particles are an artefact of living in a monkey sized body. at the basic level, the equations are useful if they match observations, not if they make sense intuitively.
> So I thought gravity was basically the curvature of spacetime. But if there's a "gravity" particle, those two things seem mutually exclusive?
It does not have to be either or. It can be both. Both models can be useful to understand the nature of gravity and make predictions about natural phenomenon.
Yes. I've seen lots of twitter/X posts lately about how Gravity is not actually a force. But how can that be true if there is a force carrying "gravity" particle? Or is the word 'force' being used loosely here?
> Yes. I've seen lots of twitter/X posts lately about how Gravity is not actually a force.
That is true. Classically, gravity is a fictitious force, merely a result of inertia from moving in a curved space-time.
> But how can that be true if there is a force carrying "gravity" particle? Or is the word 'force' being used loosely here?
Because we _suspect_ that the classical view is not correct. And there's a quantum description that may or may not involve curved space-time.
It's not impossible that the spacetime curvature is a mathematical artifact of a deeper theory. Merely a kinematic explanation, just like epicycles.
It's also possible that the space-time _is_ really curved, and gravitons simply cause the curvature by somehow coupling with it. And then other matter experiences this, in the manner described above.
You can make a lot of pseudo particles in semiconductors which definitely exist, but also aren't "real" - i.e. semiconductor electron holes are capably modelled as positive particles which can move freely with momentum/position within a semiconductor.
> if there is a force carrying "gravity" particle?
There is not. Or maybe there is not. At least so far there is not. We have never observed one. "Graviton" particle is just a hypothesis. Outside of some people theorizing that graviton could exist, there is no observations that it exists.
"In theories of quantum gravity, the graviton is the hypothetical quantum of gravity"
In a sim, it would fall into the "configurable parameter" category and dynamically altered parameter whos laws depended on locations are function lookups. And to execute performant, it would be a constant factor field only updated onAlteration with fun(x)
So you have a thing, that gets interpolated updated with various functions, that overlap, and those functions only get updated at lightspeed, cause caching.
Cachesize limit should show as farway gravity sources getting bundled into lower density information functions.
whatshisface|1 year ago
String theory and loop quantum gravity fit into this picture by trying to replace the integral over something we can't handle with an integral that matches it at large scales, but turns into something more tractable at small scales. Maybe the fact that we still can't make sense of the integral is Nature's way of telling us that she does not do the integral either...
Aardwolf|1 year ago
Can you please elaborate, the first part of the sentence says graviton is for non-self-interacting gravity, the second part of the sentence says graviton is for self-interacting (if 'its' in 'its self-interaction' refers to the graviton).
I don't intend to nitpick the sentence, just trying to understand the theory and I don't even know if particle means self interaction or the opposite and can't parse it here either...
If the answer is graviton is for non-self-interacting: what is the model for the other case (where gravity does self interact) and what would cause that self interaction if not the graviton?
gradschoolfail|1 year ago
>The model of non-self-interacting gravity is a particle we call a "graviton,"
This needs to be emphasized even more, because it has
>when the dx represents a slight change in a function
*see the discussion around sharikous’ comment below
https://news.ycombinator.com/item?id=42003116
gus_massa|1 year ago
I disagree with that part. For the strong force we have the "gluons" and they are considered particles and they have a strong self-interaction. The strong self interaction makes it a huge mess and a lot of things that involve gluons are impossible to calculate.
It's more like:
fake quote> Let's pretend for 30 minutes that the strong field don't self-interact, so we have this nice particles call gluons. Now we add this interaction to the Lagrangian to make gluons interact with other gluons, and now we have a problem.
I agree that that when gravity is small enough, then gravitons give an easy to calculate aproximation. IIRC at high enough energies calculations with gluons get not impossible to calculate too.
actionfromafar|1 year ago
Iolaum|1 year ago
https://en.wikipedia.org/wiki/Soliton
LeoPanthera|1 year ago
The problem is that nobody has successfully combined these two views into a single unified theory, known as "quantum gravity". General Relativity and quantum mechanics don't naturally fit together, and that's why we don't yet fully understand gravity in a way that reconciles both the spacetime curvature and graviton perspectives.
pdonis|1 year ago
Classically, it is. But most physicists believe that there is a quantum theory of gravity that underlies the classical theory, and that that quantum theory will include, at some level of description, a spin-2 gauge boson that mediates the quantum gravitational interaction, called the "graviton". Our classical theory of gravity, General Relativity, would then be the classical limit of that quantum theory, just as classical Maxwell electrodynamics is the classical limit of quantum electrodynamics.
gary_0|1 year ago
Of course, our idea of how to reconcile quantum gravity with general relativity is much less developed than our understanding of electromagnetism and the nuclear forces.
somat|1 year ago
dist-epoch|1 year ago
Willingham|1 year ago
cryptonector|1 year ago
So if you take the gravity curves spacetime view, then gravity is not a force and all that. But if you take the alternative view then gravity is a force. Now, I'll leave what the distortions are that gravity produces in flat spacetime for another time, or for the reader. But I'll say this: this view is both controversial (perhaps replies will show this) and not (see above -and many other- animations).
NitpickLawyer|1 year ago
I follow a few educators/communicators in this field and I have a feeling they're using this "gravity isn't really a force" to bridge the gap between their deep understanding and us mortals that don't poses the language / understanding to get the entire meaning behind it. Is that feeling correct or am I missing something?
ajkjk|1 year ago
The "geometry" comes from the fact that the way we measure distances (or, well, experience time) uses the metric tensor field to do it. But it is still ultimately just a value attached to every point like any other field.
renegade-otter|1 year ago
Not just some dumbed down Discovery show - it pushes the limits of what a layperson can understand.
gosub100|1 year ago
philipov|1 year ago
That's the link bridging gravity as a particle (small-scale) and gravity as a feature of a manifold (large-scale). Physicists are trying to find a way to make spacetime emerge from quantum field theory, or make both emerge from some common framework.
0: https://en.wikipedia.org/wiki/ER_=_EPR
gpsx|1 year ago
unknown|1 year ago
[deleted]
yarg|1 year ago
The disconnect seems to be unresolvable (I don't understand this part at all) and so efforts are being made to quantise gravity and incorporate it into the standard model.
exe34|1 year ago
https://arxiv.org/abs/1204.4616
blenderob|1 year ago
It does not have to be either or. It can be both. Both models can be useful to understand the nature of gravity and make predictions about natural phenomenon.
dcl|1 year ago
cyberax|1 year ago
That is true. Classically, gravity is a fictitious force, merely a result of inertia from moving in a curved space-time.
> But how can that be true if there is a force carrying "gravity" particle? Or is the word 'force' being used loosely here?
Because we _suspect_ that the classical view is not correct. And there's a quantum description that may or may not involve curved space-time.
It's not impossible that the spacetime curvature is a mathematical artifact of a deeper theory. Merely a kinematic explanation, just like epicycles.
It's also possible that the space-time _is_ really curved, and gravitons simply cause the curvature by somehow coupling with it. And then other matter experiences this, in the manner described above.
unknown|1 year ago
[deleted]
XorNot|1 year ago
sampo|1 year ago
There is not. Or maybe there is not. At least so far there is not. We have never observed one. "Graviton" particle is just a hypothesis. Outside of some people theorizing that graviton could exist, there is no observations that it exists.
"In theories of quantum gravity, the graviton is the hypothetical quantum of gravity"
https://en.wikipedia.org/wiki/Graviton
InDubioProRubio|1 year ago
So you have a thing, that gets interpolated updated with various functions, that overlap, and those functions only get updated at lightspeed, cause caching.
Cachesize limit should show as farway gravity sources getting bundled into lower density information functions.
1024core|1 year ago
swayvil|1 year ago
ruthmarx|1 year ago
That's just part of the picture.
I always thought that Veritasium video did more harm than good.