> It’s become dogma. All the other fields in nature are quantized. There’s a sense that there’s nothing special about gravity — it’s just a field like any other — and therefore we should quantize it.
I keep on citing Stephen Hawking here on HN, but it again seems very appropriate:
> It would be rather boring if this were the case. Gravity would be just like any other field. But I believe it is distinctively different, because it shapes the arena in which it acts, unlike other fields which act in a fixed spacetime background.[0]
I'm definitely into the "it's an emergent phenomena" camp. I think it's inherently relational, as that's a more efficient way to encode geometry rather than space itself being a quantized "thing".
Afaik this theory is the "leading"/imho most promising theory of "quantum gravity", that being the "gravity = entanglement" conjecture and the related ideas of "Complexity= action/volume/whatever" that susskind and many others have been developing for the past 20 so years.
All that said I'm nowhere near a physicist and am probably just spewing a total misunderstanding of the situation from my armchair.
That said, I've been incessantly watching lectures in this space to try to beat an understanding into my dumb dumb brain because it's super, super fucking cool.
I'm also fascinated by the idea of phase transitions, which seems to be how the laws of physics have "evolved" so far. It's crazy how much quantum computation is coming into play with this stuff, ex last year's Nobel prize with the bell inequality. That said I'm sure being a programmer I'm biased to think the universe is inherently computation/math.
It's not relational. There's several rigorous proofs against Spinoza's relational theory. Namely that of Kant and Einstein:
"even if space is composed of nothing but relations between observers and events, it would be conceptually possible for all observers to agree on their measurements, whereas relativity implies they will disagree" [0]
Doesn't relativity mean there's no difference between that observer accelerating in one direction and everything else accelerating in the opposite direction? So wouldn't everyone's observation be equally privileged? I'm asking honestly as am (obviously) not a physicist.
There are some pretty strict limitations on emergent phenomena, like Weinberg–Witten theorem. It doesn't rule out emergence of gravity but it makes it less likely
To my understanding that's only if you're trying to quantize the graviton as an emergent particle. I'm personally of the belief that spacetime itself is an emergent phenomena
If gravity is not quantized, would that not mean that it has infinite information (the accuracy to represent its values to infinite digits of precision), and thus cause a black hole due to such high information density?
IIUC, quantized doesn't mean finite, it just means discrete. Energy being quantized in bound states in quantum mechanics means that the eigenvalues are discrete, but a state can still be any linear combination whatsoever of the eigenstates.
And since spacetime is continuous, it's determines by its values on a dense subset, in particular a countable sense subset, which would make both sets of possibilities, gravity and quantum, have cardinality something like R^N.
What determines the amount of information? Is it the total values the gravity could have had with unlimited mass? The number of values that are less than the one it is observed to be? All the values in the probability distribution for some uncertainty thing?
If you have a carbon atom, does it have unlimited information because it theoretically could instead have been some other number of atoms?
Noise. Your ability to distinguish between states.
> If you have a carbon atom, does it have unlimited information because it theoretically could instead have been some other number of atom
You don't even need that. A single hydrogen atom has an infinite number of bounded energy states. In principle, you could store infinite information by putting an electron in the nth energy state and keeping it there.
Keep in mind in GR singularities don't really make sense either: i.e. when you pass the event horizon of a black hole, under GR the rules say that you must always been traveling towards the singularity. But the corollary of this is that it's not actually possible - mathematically - to arrive at the singularity (because then you'd be moving parallel with it rather then towards it).
So while we can define what happens very well under GR for the "most" of the inside of a black hole, we can't actually explain what happens to objects which reach the center. There has to be some type of discontinuity whatever happens - i.e. if the object disappears, that's still a discontinuity even if it would resolve the paradox.
EDIT: Which is of course where GR and QM need to reconcile as well - as you get infinitely close to the singularity, the size scale is getting small enough that QM should be well in play.
Your first question is the problem: they must always be moving towards the center - which implies they must be getting closer to it. Because if they can never actually reach the center, then in 4D spacetime they're no longer moving towards it - they'd be traveling parallel with it.
Which would imply that singularity isn't a singularity - i.e. the "hole" it makes would in fact be highly curved spacetime in one dimension, but completely flat in another (i.e. it would be a cylinder).
Which creates a hole lot of weird infinities in the system: i.e. a black hole becomes a finite bounded volume on the outside, but contains an infinite amount of space on the inside (since no matter how much you distort spacetime, the molecules of whatever falls in can just pull themselves together more tightly provided the distortion doesn't happen too quickly).
Right, time doesn't slow down for them, but it would appear so for an outside observer. So would the "experience" of a particle sent in to be that it circles closer and closer for some time on the order of years and at some point when it is arbitrarily close to the center it just pops out some billion or trillion years later once the black hole has evaporated? Assuming it isn't destroyed and somehow can experience it's own time.
It would be more than a few trillion years I think, but that is the thinking unless you are a certain contentious Berkeley astrophysicist.
The particle does see itself “reach” the black hole though. And so would an outside observer if they had infinite time.
The black hole dissipating sure would make some of the experience odd. I would need to revisit my notes to make a claim about that, but discontinuities like that arent uncommon.
Singularities aren't believed to be real by physicists to my understanding. Which if true, would mean the existence of them in GR is an error of the formulation or an error of comprehension.
If you look at the tangent lines facing toward the middle of the torus they would point to a single middle point, a singularity. But if you follow those lines around the surface they would never get to that extrapolated point of singularity. Probably nonsense, but the 2d creatures can't see in 3d and all that philosophy stuff.
They aren't real in the physical sense. Physicists do not believe that the singularity is a real physical phenomena at the centre of a black hole. They are only an artefact of the incomplete maths. When a singularity appears in the math of a theory, it is seen as an error or incompleteness in the theory. GR breaks down and fails to describe what happens at the centre of a black hole. It's like how a "divide by 0" doesn't actually equal "infinity", it's answer is undefined.
In the spirit of Oppenheim's CQ (classical gravity, quantum matter) work which is discussed in the fine article at the top, I'll say that your first sentence is a bit too strong. Curvature singularities (in the Kretschmann (or other curvature scalar) divergence sense) aren't ruled out by astronomical observation. Indeed, practically no observational data sheds any light on the question. What is easily ruled out is Schwarzschild black holes (known black hole candidates all have significant angular momentum) and Kerr black holes (Kerr & Schwarzschild are eternal without beginning while our universe appears to have a finite age; they exist in an energy-free and non-fluctuating vacuum rather than in a local region full of at least CMB photons but also gas and dust and nearby stars; and they exist in a larger volume filled with stars, black holes in their host galaxy, galaxies with other black holes, and so on).
At the following link is Roy Kerr giving a 2016 invited talk where he points out that despite the success of the metric that carries his name, especially for round spinning bodies (planets, stars) and even the then-current evidence favouring Kerr as a good description of large black holes (evidence since then has also been supportive), nobody should feel comfortable about the Kerr interior solution because there will be matter inside an astrophysical BH that is for practical purposes Kerr for outside observers. <https://youtu.be/nypav68tq8Q?t=2884> (around the 48 minute mark).
This is not that General Relativity is wrong, merely that GR itself depends on a solution to the Einstein Field Equations (EFEs) for a given spacetime, and black hole spacetimes commonly known by non-specialists (including physicists who aren't relativists) have features which are unphysical and which actually matter. There are lots of somewhat recondite solutions to the Einstein Field Equations (EFEs) which look like Kerr or Schwarzschild black holes to families of observers, but which have very different geometrical structure (these model black holes might have formed a finite time in the past by collapse of matter, for instance, or they might couple to a more realistic expanding spacetime with gravitational radiation from distant souces sloshing around).
Known black holes (rather, things that in telescopes etc are for all practical purposes black holes) are in such a complicated environment by comparison that we simply do not have an exact solution for the Einstein Field Equations for any of them. We are stuck with approximations, and those approximations often don't even solve the EFEs (even numerically) but rather some cut-down version.
The Raychaudhuri focusing theorem and similar results make it pretty clear that singularities form rather generically in fairly generic curved spacetimes equipped with matter. Penrose has raised comparable arguments. Our actual spacetime is not really generic on the whole, so we look at small pieces at a time and hope we are right in our guesses about how we can assemble multiple small pieces into an improved approximation of our universe. Those small pieces are calculated to be riddled with singularities for reasons related to Raychaudhuri, but that's an artifact of the construction of the small pieces as solutions of the EFEs, how we truncate those solutions, and/or how we stitch them together (e.g. Darmois-Israel).
Maybe what singularities arising in commonly-used black hole solutions of the Einstein Field Equations tell us is that our understanding of matter is off. Indeed, the whole article at the top is about a programme to study how quantum matter influences the gravitational field ("back-reaction"). For all we know, real matter that remains outside a black hole and does things like inverse Compton scattering (as well as matter that plunges inward) blocks the formation of a singularity inside. Part of the motivation for quantum gravity (CQ being a flavour thereof) is being able to extract observables that can answer that question.
> Physicists do not believe ...
I'm pretty sure many physicists hope that nature doesn't produce singularities mostly because that makes it harder to ask how parts of the universe evolve (solutions to the EFEs become infinite at a singularity, but we can sorta work around the supposed singularity with Bowen-York punctures, dynamical excisions, adaptive mesh refinement and other techniques). I'm also pretty sure that anyone with a background that supports the formation of such a belief (or its opposite, or some third choice) knows that today we just don't know how to prove much (even in principle) about the interior of black hole candidate objects in our sky.
Conversely, I do not believe there are singularities in those astronomically-observed objects, but that's because there isn't much evidence one way or another. In practice it really doesn't matter because the interior doesn't really matter much in practice, and if singularities are somehow observed we will cope with them. As to "somehow observed", quantum gravity phenomenology researchers will tell you that we would be lucky to catch the leading order quantum correction to GR with present-and-near-future observational ability, while theorists will then tell you that the singularity-or-not answer probably depends on the next-to-the-next-to-the-leading order. Consequently we have practically no way to distinguish among quantum gravity theories (including CQ) which are known to reproduce the successes of General Relativity in the neighbourhood immediately around our own planet.
(Indeed, the gravitational field of Earth barely needs General Relativity, let alone quantum corrections to that, and there are lots of things we don't know about the deep interior of our own planet. As far as anyone can tell using a post-Newtonian formalism one only needs the leading order (1PN) to fully describe all present measurements of gravitation around us. Earth's gravity essentially a barely-perturbed Kerr (exterior) metric (see e.g. Soffel & Frutos 2016). And that's only thousands of metres to thousands of kilometres away. So not knowing about the deep interior of things at minimum thousands of light-years away doesn't really disturb me, even as we go up to 3PN+ (see e.g. Clifford Will's 2011 "On the unreasonable effectiveness of the post-Newtonian approximation") based on our long-distance observation of the glowing and/or opaque stuff outside of them).
General relativity requires infinite time for singularities to form. Fully formed singularity is a mathematical construct which is assumed into existence, it doesn't correspond to anything physical.
Dogma and tradition are two very different concepts. Dogma is forced down from "above" and tradition is consensual. Tradition is "organic", dogma is proscribed.
Tradition: "We chase a wheel of cheese down Coopers hill in Gloucestershire and invite severe injury, for a laugh" - won by a somewhat concussed Canadian lass this year, well done her.
Skinner_ is saying that the description which nomel interpreted as describing "an alternative to the thing being called dogma" and as "tradition", was a description of the thing being described as "dogma", and not, as nomel interpreted, as an alternative thing to the thing that the quote was considering "dogma".
I would offer a slight alteration to your definition of "dogma". It's a Greek word and it was the name of wooden cubes that served as the markers of the edge of someone's property. It's more accurate to think of dogma as "boundaries" than as proscriptions.
I am saying this as an Orthodox Christian, so this is probably a different view than the view of the Roman Catholic church that you are probably more familiar with. They have definitely gone more towards the "top-down" approach than the Orthodox Church since the Great Schism in 1054.
What I mean by this is that the Roman Catholic church has a LOT more dogmas and they are much more specific than the Orthodox Church. The Orthodox Church has very few dogmatic statements and a much different view of the canons of the church.
Well we could go down a bit of an unproductive thread here! But let's not.
I've never heard of these wooden cubes and that needs looking into. I do know that in Germany, property boundaries are formally marked out or at least they were when I lived in West Germany on and off in the 70's-80's. In Britain we have red lines marked on really shit plans, registered with the Land Registry. Even so, it works.
I'm a Protestant (CofE). I was Confirmed by the Bishop in Jerusalem - we were stationed in Cyprus at the time (1996). Mind you it has suddenly occurred to me that there are probably several of them (denominations) and it was a long time ago ... (search) - https://en.wikipedia.org/wiki/Anglican_Diocese_of_Jerusalem I think it must have been this bloke: https://en.wikipedia.org/wiki/Samir_Kafity . Both he and his predecessor are/were Palestinian Arabs.
Well there's a thing. I recall a lovely man with a massive smile and his robes and mitre were coloured red, orange and yellow - really bright and very striking. I doubt our local lot could carry off that look!
My wife is a Roman Catholic. We married in a CofE church. Our Priest's wife very kindly sang Ave Maria for us. Neither of us suffered any calamity, you'll be glad to hear.
I'm not a fan of dogmatism of any sort. I will grant you that property demarcation is a great idea - it avoids arguments later on 8)
In General Relativity, matter curves spacetime, so instead of saying that all the “other fields” act within spacetime, perhaps “the metric [tensor] is determined by the matter and energy content of spacetime?” If geometry is described fully with a (tensor) field, perhaps we’re really just looking for a gauge theory but then that would be QM. Now I’m trying to imagine a tensor computing some infinite curvature supposedly necessary in GR.
Any reason you are specifically picking the Higgs field in your question? I am asking, because this sounds a bit like you are riffing on a common misconception that the Higgs field has something to do with gravity, which is not the case. The interaction with the Higgs field is the reason some (only some) of the particles have a mass, but explaining gravity does not need to have anything to do with the Higgs field.
But yeah, it is fair to ask why gravity should behave any differently than any other quantum field -- in the context of Quantum Field Theory (one of the two incredibly successful theories of physics) that is a great question. One handwavy reason it seems different is that in General Relativity (the other incredibly success theory of physics), gravity has to do with the geometry of space and time, not with what other fields exist in that space and time (and as such is explicitly different than the other fields).
My understanding is that the Higgs field should be simpler than gravity because it's just a static value whereas gravity is SU(1)? At least to me it seems logical that if the Higgs is quantized, surely more complicated fields would be as well?
SU(1) is the trivial group. SU(n) is the group of unitary nxn matrices with determinant 1. There is only one 1x1 unitary matrix with determinant 1, and in fact there is only one 1x1 matrix with determinant 1 period, namely, the 1x1 matrix whose only entry is the number 1.
So, to associated something with a symmetry group of SU(1) would be effectively the same as not associating it with a symmetry group.
I keep on citing Stephen Hawking here on HN, but it again seems very appropriate:
> It would be rather boring if this were the case. Gravity would be just like any other field. But I believe it is distinctively different, because it shapes the arena in which it acts, unlike other fields which act in a fixed spacetime background.[0]
[0]: https://arxiv.org/abs/hep-th/9409195v1