[HN Gopher] Anomalous contribution to galactic rotation curves d...
___________________________________________________________________
Anomalous contribution to galactic rotation curves due to
stochastic spacetime
Author : kergonath
Score : 125 points
Date : 2024-03-09 09:13 UTC (13 hours ago)
(HTM) web link (arxiv.org)
(TXT) w3m dump (arxiv.org)
| bloopernova wrote:
| Related article from The Guardian:
| https://www.theguardian.com/science/2024/mar/09/controversia...
| dbacar wrote:
| Thank you, this is much more accessible for me.
| boxed wrote:
| That sounds very unconvincing. Afaik there are several found
| galaxies with more than normal amounts of dark matter, and
| less. How would these fit into ANY modified gravity theory?
|
| This type of theory seems to only handle the common case, but
| the universe is full of edge cases...
| scotty79 wrote:
| If the base of this theory is some form of chaos you can
| always fit the distribution of the chaos to explain outliers
| with the probability they actually occur at.
| BlueTemplar wrote:
| Can you ? How is this different from, say, anything
| involving pressure and temperature, which under current
| theories, are statistical phenomena arising from quantum
| behaviour, fundamentally based on our lack of information
| about what is going on (aka entropy) ?
| scotty79 wrote:
| I think technically you can. Although it's just moving
| the question to "why is the chaos distributed like
| that?".
|
| Current explanation for existence of galaxies is those
| are quantum fluctuations that grew large. So apparently
| fluctuations can explain everything if you do enough of
| hand waving in between. I don't think any particular
| quality of galaxies as we see them today can be traced to
| specific quantum property. But that doesn't stop us from
| believing in that explanation.
| data_maan wrote:
| Isn't that already enough to know that it is some form
| chaos that underlies many of the current phenomena?
|
| For any explanation X there will always be the question,
| "now what explains X"?
| layer8 wrote:
| The Guardian headline stands in contradiction to this statement
| from the paper's abstract: "We caution that a greater
| understanding of this effect is needed before conclusions can
| be drawn".
| quantum_mcts wrote:
| Good. Now explain the Bullet Cluster.
| naasking wrote:
| No one can explain the Bullet Cluster.
| quantum_mcts wrote:
| Cold dark matter made of weakly interacting massive particles
| can.
| naasking wrote:
| No, LCDM can't explain the observed velocities.
| quantum_mcts wrote:
| Yes it can. https://arxiv.org/abs/1410.7438
| raattgift wrote:
| > Good.
|
| Is it, or isn't it, and why? You don't say.
|
| The authors asked themselves how their quantum gravity theory
| differs from General Relativity, and whether the successes of
| General Relativity in astrophysical settings would be fatal if
| their theory has strong differences, and that's the basis for
| this paper. The tl;dr is that their theory predicts different
| trajectories outside of large central masses, but that _might_
| not conflict with evidence from galactic-dynamics astronomy.
|
| This is the second paper released in the past few days by the
| University College London Oppenheim group. It's a preliminary
| investigation of the longer length scale features of their
| classical stochastic theory. The central question is how its
| version of Schwarzschild-de Sitter (SdS) differs from standard
| General Relativity.
|
| The first paper, and I think the more interesting one, is about
| the short length scale aspects of their asymptotically free
| theory, in which the gravitational interaction weakens as
| distances between interacting sources decreases. The asymptotic
| freedom means the theory is amenable to renormalization, unlike
| perturbative quantum gravity and a number of other approaches.
| That paper is at <https://arxiv.org/abs/2402.17844>. Note that
| they do not know how to make the gravitational part quantum
| mechanical without introducing problems (i.e., it is haunted by
| "bad" ghosts in the sense of
| <https://en.wikipedia.org/wiki/Ghost_(physics)>); their
| classical and stochastic gravitational sector is ghost-free (a
| point also made at the end of Appendix A in the large-scale
| paper), and it is reasonable for them to believe that could be
| good enough that it's worth continuing to investigate what the
| theory predicts and how its parameters are set.
|
| The second paper was motivated by the first: "The theory was
| not developed to explain dark matter, but rather, to reconcile
| quantum theory with gravity. However, it was [noted] that
| diffusion in the metric could result in stronger gravitational
| fields when one might otherwise expect none to be present, and
| that this raised the possibility that gravitational diffusion
| may explain galactic rotation curves".
|
| That MOND-like effects might arise in their approach to the
| problem of small-scale quantum gravity is at least interesting.
| It was not the starting point.
|
| Moreover, they did not start with the idea of modifying General
| Relativity to get rid of the need for (some or all) cold dark
| matter. As they say: "While this study demonstrates that
| galactic rotation curves can undergo modification due to
| stochastic fluctuations, a phenomenon attributed to dark
| matter, it is important to acknowledge the existence of
| separate, independent evidence supporting LCDM. In particular,
| in the CMB power spectrum, in gravitational lensing, in the
| necessity of dark matter for structure formation, and in a
| varied collection of other methods used to estimate the mass in
| galaxies."
|
| > Now explain the Bullet Cluster
|
| This paper does not seek to do so. "To make it tractable
| analytically, we have restricted ourselves to spherically
| symmetric and static spacetimes, with metrics of the form of
| Eqs. (17)." Eqn 17 describes out an adapted Schwarzschild-de
| Sitter spacetime and leans on an argument that Birkhoff's
| theorem applies (in particular that their model spacetime is
| stable against certain perturbations, notably those concentric
| upon the source mass). There is further detail in Appendix B.
|
| Studying this restricted model, the de Sitter expansion of the
| spacetime and MOND-like anomalous Kepler orbits at some remove
| from the Schwarzschild central mass are in their theory driven
| by entropic forces generated by the fluctuations in the
| gravitational field of the central mass (and they do a good job
| in Appendix D explaining this).
|
| In GR's Schwarzschild-de Sitter the free-fall trajectories of
| test particles around the central mass are totally determined
| by the mass; the gravitational field doesn't fluctuate. The
| (Boltzmann) gravitational entropy of the region outside the
| central mass is everywhere very high.
|
| In GR-SdS we can consider adaptations where with M=const. we
| turn the pointlike central mass into a spherically symmetric
| shell, or a concentric set of such shells, or even a ball of
| fluid, or a ball of dust, or a ball of stars and other galactic
| matter. None of these symmetry-preserving adaptations changes
| the free-fall trajectories of test particles outside the outer
| surface, or the gravitational entropy at any outside point.
|
| In the author's theory, the spacetime is stochastic. It
| fluctuates. Close to the central mass fluctuations are
| unnoticeably small; the gravitational entropy is very low. Far
| from the central mass the gravitational entropy is very high,
| and gravitational fluctuations are noticeable. A sort of
| thermodynamics leads to a diffusive flow outwards from the
| central mass, from the low entropy near there to the high
| entropy at increasing radial distance. This diffusion is
| carefully constructed so that the outwards flow is only really
| appreciable at large-scale distances. The effect is that large-
| radius orbits are statistically pulled inwards by something
| describable as stronger gravity at larger radiuses (see around
| Eqn (21)). This is an "entropic force", very roughly analogous
| to squashing a sponge ball in your hand then releasing the
| pressure and watching the sponge ball expand, where the
| material of the sponge represents the gravitational field.
|
| Their stochastic fluctuations are still generated by the
| spherically-symmetric central mass. These fluctuations break
| the spherical symmetry of the outside metric. Consequently they
| have to do some work to make the outside metric look
| appropriately Schwarzschild-like in their "diffusion regime",
| and to keep that stable against the stochastic perturbations.
|
| The authors contend that with reasonable choices of parameters,
| and restricted to static spherical symmetry of the central mass
| (and no additional dynamics), this effect comes close to
| duplicating MOND's low-acceleration regime.
|
| They don't go into anything like a backreaction upon the
| Schwarzschild metric by large fluctuations.
|
| (They do have an idea about how to get the de Sitter
| trajectories though, but that doesn't fit very naturally into
| this comment, which is already long.)
|
| > Bullet cluster
|
| The authors know full well that the metric for a
| gravitationally bound cluster of galaxies isn't well-
| represented by their choice of SdS-like metric. A galaxy
| cluster is too lumpy for the Schwarzschild part.
|
| Two gravitationally bound galaxy clusters having passed through
| each other (trailing collided gas and dust, and tidally
| stripped stars and other matter) is even less like
| Schwarzschild. This is because SdS solutions of the Einstein
| Field Equations do not linearly superpose. So their metric is a
| poor description of any sort of "close call" interaction
| between galaxies or galaxy clusters, even if the individual
| components are "close enough" to Schwarzschild from the
| perspective of an observer sufficiently large (as in
| cosmologically large) distances. They do not (and within this
| initial paper should not really be expected to) offer a more
| suitable metric. I'm sure they'd love to look into things like
| that though.
|
| The non-linear superposeability of useful solutions of General
| Relativity is a problem for asking how astrophysics differ in
| most theories that preserve the equivalence principle (this one
| does, it's a metric theory of gravitation). As the replacement
| for the Einstein Field Equations lose symmetries (sphericity,
| staticity) they tend to become analytically intractable and
| non-numerical approximations become unreliable.
|
| The authors -- imho in a strikingly principled way -- call
| attention to various difficulties in using this work to
| describe astrophysical systems, particulary from the middle of
| the fifth page of the PDF.
|
| They are not obviously worse off than the Verlinde programme of
| emergent-entropic gravity, where the gravitational field is
| generated by entropic forces rather than vice-versa.
| indeyets wrote:
| But... WHY is it wobbly? Is there a particle which represents
| this wobbliness? :)
| SideburnsOfDoom wrote:
| Is that the "postquantum" bit? i.e. not that spacetime is
| quantized as such, but that it's not a uniform continuous
| medium at all scales, that classical mechanics would assume.
| scotty79 wrote:
| I can imagine that wobbliness comes from chaotically passing
| strong gravity waves of very high wavelengths coming from
| outside of the observable universe and from before times of Big
| Bang.
|
| There's no specific reason that forces us to believe that Big
| Bang was the actual beginning of everything. It might have been
| "local" high energetic event happening in much larger
| structure.
|
| For example try to imagine how the collision of two dense
| clusters of trillions of black holes each traveling at nearly
| light speed would look like from the point of view of the dust
| swirling in-between and around them. I can't imagine it would
| be very different from what we observe in our universe.
| MauranKilom wrote:
| Don't you think that we would be able to observe some
| anisotropy in the data if that were the case? (Note: I have
| no clue, but it seems unlikely...)
| scotty79 wrote:
| Maybe? I don't know. I imagine we don't have any good ways
| to observe gravitational waves, even fairly strong ones
| with periods of millenia or even centuries which are still
| very short times on a galactic scale.
|
| And apart from that, I think that we can only look at CMB
| and conclude that it's a bit wobbly. Must be quantum
| fluctuations, sure, why not, but is it only the quantum
| fluctuations? Or maybe the spacetime between us and CMB
| source is a bit wobbly too?
|
| Another thing is large scale structure of our universe.
| Visualizations look like foam. Planty of chaos that
| wobblines can safely compose into.
|
| I think the work of those scientists is very important
| because it might allowed us to pinpoint how strong
| distortions of the spacetime would need to be to explain
| what we see and maybe we could narrow down the range of
| frequencies which might gives us ideas how to look for
| them.
|
| Simplistic and grandiose assumptions make our current best
| model of the universe a bit restrictive. To the point that
| we are starting to find direct counterexamples for our
| theories derived from it. Mature galaxies way too old.
| Megastructures way too large. CMB fluctuation not exactly
| fitting best theoretical models. Unreconcilable differences
| between Hubble constant measured from CMB and that measured
| from galaxies.
|
| I think accepting a bit of chaos beyond what we currently
| believe is an inevitable way out.
| oneshtein wrote:
| Finally, someone is at right path.
| A_D_E_P_T wrote:
| There's something distinctly unsatisfying about this sort of
| paper, because the number of potential solutions to the problem
| they're trying to solve is in principle vast, and you can arrive
| at any number of them if you laboriously work backwards and fit
| your equations to the data. Without very rigorous efforts towards
| empirical validation, in novel ways if required, this sort of
| thing is just another wholly speculative theory to add to the
| large and growing pile.
|
| The interesting thing, as far as I'm concerned, is the size and
| shape of the answer space. How many theoretical solutions can be
| coaxed to fit the cosmological data? It can't be infinite, but it
| seems as though it's a rather large number.
| szundi wrote:
| This is exactly what Sabine Hossenfelder is talking about for
| years now.
| naasking wrote:
| Indeed, and even she thinks this is at least a fairly novel
| approach in this space which hasn't seen much new thinking
| recently (20+ years).
| hnaccount_rng wrote:
| To be fair, everything that astrophysics comes up with will be
| massively under constrained. For all intents and purposes we
| work with a single datapoint and try to infer dynamics.
| sebzim4500 wrote:
| I'm far from an expert, and I'm still struggling my way through
| this paper, but from what I can see they are only intruducing
| one new free parameter, the same number as lambda CDM.
| eigenket wrote:
| I agree with your point in general, but in this case
| specifically I strongly disagree.
|
| The idea worked out by Oppenheim and his colleagues/students is
| a fairly concrete theory which aims to marry quantum mechanics
| and general relativity. They're working on a bunch of
| approaches to turn their model into testable predictions in
| different regimes.
|
| They have a bunch of different papers with different ways their
| model could be falsified. None of them are at the level where
| you could falsify their model with current technology but
| they're seriously working on the theory required to make their
| ideas potentially testable.
|
| I went to a talk a couple of years ago at a conference by
| Oppenheim and he is very clear that what they're looking for is
| to make concrete predictions which can be tested.
|
| Edit: you can see here for another paper where they try to push
| their theory towards producing some testable predictions
|
| https://arxiv.org/abs/2203.01982
| BlueTemplar wrote:
| Correct me if I am missing something, but it seems like the
| speculative bit was Milgrom noticing that coincidence in 1983,
| and now they are trying to explore the consequences of assuming
| that it's not just a coincidence... hopefully with testable
| predictions coming out of this ?
|
| How _else_ should they go about it ?
| zikzak wrote:
| I forget who said it but asking "hm, what's causing this
| anomaly in my experimental results" is how science
| progresses.
| webmaven wrote:
| ISTR someone saying that the most common words spoken prior
| to a breakthrough aren't "Eureka!" or "Aha!" but instead
| "That's strange..."
| mkl wrote:
| Often atrributed to Asimov, but it doesn't seem to have
| been him:
| https://quoteinvestigator.com/2015/03/02/eureka-
| funny/?amp=1
| keepamovin wrote:
| I think you're missing the point: you can say this about _any_
| theory. At least ones that attempt to deal with something
| significant and difficult.
|
| It's therefore not a very perceptive critique to make to say
| the problem with this specific theory is that it is one of many
| possible ones. It's more meaningful to compare such wide-
| aperture theories like this to their actual alternatives,
| rather than distrust them on principle because the space of
| potential hypotheses in which they carve out a specific model
| is so large.
|
| Funnily, your observation could be said to be an instance of
| itself: in that it is an overly general appraisal unrooted in
| pertinent specifics, therefore mostly divorced from explanatory
| power - akin to what you're misjudging this theory to be.
|
| I don't know if this theory will pan out, but it seems more
| interesting than you say. While different to the paper, I have
| the following related thoughts to offer: I've often thought
| gravity makes sense, not as a fundamental force, but as an
| emergent metric, rooted in randomness (perhaps the
| informational content of matter, for which mass is, normally
| but not always, useful proxy?), similar to how temperature is
| merely an emergent property that we can measure that arises
| from (and reflects) the microstates/ensembles of gigantic
| numbers of particle interactions, owing to their energy
| content.
|
| I wonder if a "time" or "age" factor could be needed in revised
| calculations of gravity to account for the plausible way in
| which information increases over time (if we assume, without
| trying to understand, that the history is recorded somewhere,
| somehow), and if that's why these very old and very distant
| galaxies bork our normal (non dark matter fudged) calculations
| of gravity, because they are so old that the time accrual of
| information is an effect which begins to come into play.
| A_D_E_P_T wrote:
| > _I think you 're missing the point: you can say this about
| any theory. At least ones that attempt to deal with something
| significant and difficult._
|
| Hardly. In fact, just the opposite is true: You can say it
| about virtually no theories outside a niche within modern
| physics.
|
| The great theories of 20th century physics were empirically
| validated almost immediately. Even their thought experiments
| were subjected to intense scrutiny and debate. Now those
| theories are fodder for engineers who tinker with material
| devices like MRI machines.
|
| In other disciplines, e.g. biology or chemistry, it's
| effectively forbidden to work backwards from data and come up
| with a "solution" that fits but can't be validated in any
| way. Chemical space is vast; if you have a complex molecule,
| there are any number of ways retrosynthesis will work;
| finding one such way, without empirically validating it, is
| of near-zero value.
|
| > _I 've often thought gravity makes sense, not as a
| fundamental force, but as an emergent metric, rooted in
| randomness (perhaps the informational content of matter, for
| which mass is, normally but not always, useful proxy?)_
|
| The randomness of what now?
|
| How do you reconcile this with the frame-dragging effect?
| Namely, in that it shows that the distribution and movement
| of mass (not just the presence of mass/entropy) can influence
| the curvature of spacetime and thereby gravitational effects.
|
| > _I wonder if a "time" or "age" factor could be needed in
| revised calculations of gravity to account for the plausible
| way in which information increases over time_
|
| Information is degraded over time; the universe will
| eventually contain zero useful information content ("heat
| death") and will then be modellable as a homogeneous space
| subject to statistical fluctuation.
| keepamovin wrote:
| > Hardly. In fact, just the opposite is true: You can say
| it about virtually no theories outside a niche within
| modern physics.
|
| Again I think you're missing the point. There's many ways
| in which you can construct theories on any topic. They're
| all just models: which can be seen by how these models
| often evolve over time, and yet were at each time seen as
| fairly valid. This is true in all the sciences, and is
| inherent to science. You make like that's a criticism of
| modern physics only, but I think it's more an effect of the
| point below about measurement.
|
| > In other disciplines, e.g. biology or chemistry, it's
| effectively forbidden to work backwards from data and come
| up with a "solution" that fits but can't be validated in
| any way.
|
| _in any way_ : I think this is a mischaracterization.
| Fitting with the data _is_ a validation in some way. You
| seek orthogonal validation via new testable predictions,
| which is fair enough, but it 's also fair to consider the
| measurement problem again (mentioned below), and the
| process of development wherein advances may be made before
| testing is figured out, if the domain is of sufficient
| complexity.
|
| > Chemical space is vast; if you have a complex molecule,
| there are any number of ways retrosynthesis will work;
| finding one such way, without empirically validating it, is
| of near-zero value.
|
| That's the thing. These synths _look like_ they work, on
| paper. The groups and charges move around right, but they
| don 't actually work, which contradicts your point that you
| can't devise a large number of plausible alternatives in
| chem. The same is true of bio and metabolic pathways.
|
| In practice, often we don't know how to test something at
| the time we sketch it out. I guess that's the consequence
| of being at the frontier, there's much unkown. So the
| analogy between these disciplines and physics fits, but
| perhaps not in the way you intended.
|
| But on another level, the analogy doesn't work, as finding
| synths is more an eng problem. You don't have to understand
| why a synth works, for it to work...which is often the
| case. And the theories about why that was allowed by
| enthalpy or catalysis or whatever are often evolved over
| time. Whereas theory building is more focused on having a
| why that might bring insight.
|
| I think your main problem here is the false dichotomy you
| see with "work backwards from data to solve the
| constraints" and "this doesn't validate it in any way". In
| fact, in the world of theories, fitting the data, is a
| pretty fucking great validation hahaha! :) But I do get the
| sense you are expressing about the futility, which I think
| is real, but just not the whole picture.
|
| > The great theories of 20th century physics were
| empirically validated almost immediately. Even their
| thought experiments were subjected to intense scrutiny and
| debate. Now those theories are fodder for engineers who
| tinker with material devices like MRI machines.
|
| _almost immediately_ : is inaccurate. Theories can make
| many predictions, and some of GR and SR were only validated
| recently.
|
| I guess you can say that our measuring ability has not
| caught up to our theorizing or mathematical ability. Which
| is regrettable, but not a condemnation of the theories, as
| you seem to think.
|
| I get the impression you advance that modern physics is in
| the business of producing time wasting untestable theories.
| Which is a fair enough take. I just think there's more
| nuance there, and you risk maligning a good theory, with
| this broad stroke.
|
| > How do you reconcile this with the frame-dragging effect?
| Namely, in that it shows that the distribution and movement
| of mass (not just the presence of mass/entropy) can
| influence the curvature of spacetime and thereby
| gravitational effects.
|
| I don't see that it contradicts it, so it may not be the
| best counter example. The movement or spin of the mass that
| induces frame-dragging can be considered information as
| well.
|
| > Information is degraded over time; the universe will
| eventually contain zero useful information content ("heat
| death") and will then be modellable as a homogeneous space
| subject to statistical fluctuation.
|
| I know that's the model, but what if it's inaccurate/only a
| part of the picture? What if the information is elsewhere
| and we don't know how to measure it? There's precedent for
| that: there's information in quantum states that was not
| known to exist or be measurable before quantum theory.
| AnimalMuppet wrote:
| Without some specific idea of "elsewhere", that idea is
| in the "not even wrong" category.
| keepamovin wrote:
| > Without some specific idea of "elsewhere", that idea is
| in the "not even wrong" category.
|
| Which idea?
|
| Also, based on _what_ is to-be-specified idea in the not
| even wrong category? [By which I guess you mean you think
| it 's so wrong that you cannot even dignify telling me
| why it's wrong? haha! :) Novel sly-arrogant/humble-brag
| appeal-to-authority, I'll give you that! Because your
| understanding is so above everyone else's you couldn't
| explain it or we wouldn't get it? Haha! :)]
|
| _Also_ , finally, pray tell what do you mean by
| "elsewhere" ? Hahahaha! :)
|
| I'll check back tomorrow, I'm off now
| AnimalMuppet wrote:
| Which idea? You seem to have heavily edited your post
| after I replied; I remember it as being much shorter. I
| specifically meant
|
| > I know that's the model, but what if it's
| inaccurate/only a part of the picture? What if the
| information is elsewhere and we don't know how to measure
| it?
|
| "What if the information is elsewhere"... well, there's a
| _lot_ of possible elsewheres. Without some kind of
| specifics, that 's "not even wrong", not because I'm so
| much smarter than you or so much more of an expert, but
| because there's nothing there to let anyone be able to
| determine whether it's right or wrong.
|
| By "elsewhere", I mean the word that you used. What did
| _you_ mean by that word? Without a specific, you 've got
| something that sounds like stoner physics: "Dude, what
| if, like, the information is, like, still there, man?
| What if it just, like, went elsewhere, man?" That's not
| something that we intelligently interact with. You
| introduced the word; what did _you_ mean?
|
| But... What if dark matter _is_ the "elsewhere"? (No, I
| don't have any idea how that would work. Nor do I
| necessarily think that this is a sane idea. But it's a
| candidate for "elsewhere" that kind of seems to fit with
| the course of the discussion.)
| A_D_E_P_T wrote:
| > _Again I think you 're missing the point. There's many
| ways in which you can construct theories on any topic.
| They're all just models: which can be seen by how these
| models often evolve over time, and yet were at each time
| seen as fairly valid. This is true in all the sciences,
| and is inherent to science. You make like that's a
| criticism of modern physics only, but I think it's more
| an effect of the point below about measurement._
|
| In science, as opposed to theology, the models in
| themselves are of no use until they're validated.
|
| Hence the motto of the Royal Society: _Nullius in verba._
| One expects more than words -- one expects experimental
| showings, or the reasonable expectation of an
| experimental showing in the very near future. 20th
| century physics, in the main, had this. It is the
| cornerstone of all other disciplines.
|
| > _in any way: I think this is a mischaracterization.
| Fitting with the data is a validation in some way._
|
| "In some way" is doing a lot of work there. How many
| potential fittings from cosmological data are there?
| 1000? 10^10? 10^500? Do you know?
|
| > _That 's the thing. These synths look like they work,
| on paper. The groups and charges move around right, but
| they don't actually work, which contradicts your point
| that you can't devise a large number of plausible
| alternatives in chem._
|
| What are you talking about? You can come up with any
| number of theoretical retrosyntheses that _do_ work, but
| are unwieldy, impractical, or can 't be validated for any
| number of reasons -- lack of reagents or intermediates,
| etc.
|
| You can derive any number of plausible processes. Nobody
| does that, though, because one is expected to do more --
| to come up with something that runs, and ideally to run
| it and report how it works, with yield rates and so
| forth.
|
| Similarly, I don't think that the paper in OP has
| constructed something that runs. It is mere backwards-
| fitting to cosmological data. The more interesting
| question, as I've noted, is _how many_ such things are
| possible.
| Kamq wrote:
| > I think your main problem here is the false dichotomy
| you see with "work backwards from data to solve the
| constraints" and "this doesn't validate it in any way".
| In fact, in the world of theories, fitting the data, is a
| pretty fucking great validation hahaha! :) But I do get
| the sense you are expressing about the futility, which I
| think is real, but just not the whole picture.
|
| I think your main problem is that you only read half of
| the comment you responded to.
|
| It has two parts:
|
| > There's something distinctly unsatisfying about this
| sort of paper, because the number of potential solutions
| to the problem they're trying to solve is in principle
| vast, and you can arrive at any number of them if you
| laboriously work backwards and fit your equations to the
| data.
|
| Which you've adequately responded to. But it also has:
|
| > Without very rigorous efforts towards empirical
| validation, in novel ways if required, this sort of thing
| is just another wholly speculative theory to add to the
| large and growing pile.
|
| Which you haven't.
|
| Taken together, the parts are obviously talking about a
| class of theory that is fit to data AND does not bother
| checking the theory against other data or making
| predictions in reality.
|
| Your examples of theories that evolve don't seem
| relevant, as they're evolving because people made efforts
| to test the theory out and account for unexpected results
| (or, at the very least, look at datasets that weren't the
| ones that the theory was specifically designed to account
| for).
| gnramires wrote:
| > How do you reconcile this with the frame-dragging effect?
| Namely, in that it shows that the distribution and movement
| of mass (not just the presence of mass/entropy) can
| influence the curvature of spacetime and thereby
| gravitational effects.
|
| If gravity is somehow caused by something like information
| flow, then a rotating body has an associated rotating flow
| of information related to the rotating atoms/microscopic
| degrees of freedom observable at distance. This might have
| an effect making it distinct from a non-rotating body.
|
| I believe the gradient of a flow field is a tensor, giving
| gravity tensorial effects as expected.
| tigerlily wrote:
| > I wonder if a "time" or "age" factor could be needed in
| revised calculations of gravity ...
|
| There's a cosmological theory out there that does this,
| dubbed the _Timescape_ cosmology, and you can read all about
| it on the theorist 's university research webpage:
| http://www2.phys.canterbury.ac.nz/~dlw24/
|
| Clarkson-Bassett-Lu test of the Friedmann equation is
| proposed as a test of this model, using data from the
| recently launched Euclid satellite:
|
| http://www2.phys.canterbury.ac.nz/~dlw24/universe/wager.html
| josh-sematic wrote:
| Note that this isn't a "get out of dark matter free" card, as
| there are clear signatures of dark matter in the cosmic microwave
| background. https://physicsworld.com/a/dark-energy-spotted-in-
| the-cosmic...
| willis936 wrote:
| Only if we've constrained ourselves to take inflation as true.
| Vecr wrote:
| Inflation in what era? There has to have been some inflation
| at some point, right, otherwise how could the very hot period
| that's considered to be right after the start of the universe
| be low entropy? Sure, maybe you could say there's no big bang
| and time runs the other way on the other side of that low
| entropy state, but there has to be some sort of low entropy
| state.
| tsimionescu wrote:
| It could be that it is actually not low entropy, and there
| are some other processes that explain a recution in entropy
| at some scale. That is, maybe the second law of
| thermodynamics is not that universal.
| Vecr wrote:
| That would be really weird. I'd be more willing to buy
| galaxies just coming into existence somehow in the gaps
| as the universe expands, as that at least drives the
| arrow of time consistently in the right direction.
|
| The arrow of time _can_ survive local entropy reduction,
| but probably not what you 're talking about.
| chongli wrote:
| Yes and also in gravitational lensing. The most famous example
| of this is the Bullet Cluster [1], which displays a pattern of
| lensing that doesn't line up with the distribution of light
| (visible matter) but does line up with where we might expect
| dark matter to end up after a collision.
|
| [1] https://en.wikipedia.org/wiki/Bullet_Cluster
| a_gnostic wrote:
| Could we infer these effects to be from an average distribution
| of matter across multiversal spacetime, and how would we test
| for that?
| raattgift wrote:
| From the preprint link at the top's page 6, after Eqn (23):
| "While this study demonstrates that galactic rotation curves
| can undergo modification due to stochastic fluctuations, a
| phenomenon attributed to dark matter, it is important to
| acknowledge the existence of separate, independent evidence
| supporting LCDM. In particular, in the CMB power spectrum, in
| gravitational lensing, in the necessity of dark matter for
| structure formation, and in a varied collection of other
| methods used to estimate the mass in galaxies. These now form
| an important set of tools with which to test [our] theory".
|
| Other lines of evidence like the one at your link just increase
| the ability to test their theory (which is meant to solve some
| quantum gravity problems at very small length scales) at very
| large length scales.
|
| They are not motivated by the desire to prove MOND is correct
| or that cold dark matter doesn't exist. Rather they can show
| that in certain restricted circumstances their theory allows
| for nearly-MOND-like orbits. So their theory survives a small
| but important hurdle imposed by our observations of nature (we
| observe MOND-like orbits).
|
| Elsewhere in the comments someone asked the good question of
| (highly paraphrasing) whether this success is wiped out if a
| distribution of dark matter is added to their SdS-like universe
| as the generator of some of the observations normally taken as
| proof of \Lambda-CDM. That question is closely related to the
| final sentence in the quote above, and answers are hard to
| guess at.
| nabla9 wrote:
| >"explain galactic rotation curves without needing to evoke dark
| matter."
|
| I wonder if this makes it incompatible with dark matter? If
| stochastic spacetime explains the rotation curves exactly without
| black matter, then black matter can't exist at the same time.
|
| The evidence for dark matter is find in, galaxy rotation curves,
| gravitational lensing, cosmic microwave background, structure
| formation, bullet cluster and other galaxy cluster collisions,
| baryon acoustic oscillations, Lyman-alpha forest absorption
| lines, etc.
|
| There is plenty of evidence for dark matter and it's existence is
| widely accepted. Explaining just one piece of evidence can be
| evidence against the new explanation if it's incompatible.
| raattgift wrote:
| > Can dark matter and [the author's theory of gravitation] fit
| together...?
|
| Nobody knows yet. The authors aren't seeking to overthrow the
| concordance cosmology; this paper is essentially trying to see
| if their theory is quickly killed by being unable to model a
| _stable_ spherically symmetric galaxy in an expanding universe.
| Their theory predicts significant differences from the
| Schwarzschild-de Sitter (SdS) metric in General Relativity,
| with SdS serving as a proxy for an isolated galaxy. The paper
| in part investigates whether those differences can be "hidden"
| by removing the cold dark matter which otherwise would be the
| standard source of outer orbit anomalies.
|
| They don't go deeper than that, and at this stage just can't:
| "To make it tractable analytically, we have restricted
| ourselves to spherically symmetric and static spacetimes". They
| restrict themselves in other ways too.
|
| > The evidence for dark matter
|
| From just after Eqn (23) in the preprint:
|
| "While this study demonstrates that galactic rotation curves
| can undergo modification due to stochastic fluctuations, a
| phenomenon attributed to dark matter, it is important to
| acknowledge the existence of separate, independent evidence
| supporting LCDM. In particular, in the CMB power spectrum, in
| gravitational lensing, in the necessity of dark matter for
| structure formation, and in a varied collection of other
| methods used to estimate the mass in galaxies"
| nabla9 wrote:
| If stochastic spacetime explains the rotation curves exactly
| without black matter, then how black matter can't exist at
| the same time?
| raattgift wrote:
| It doesn't explain the rotation curves of galaxies at all,
| and doesn't try to.
|
| It takes a simple proxy for an isolated galaxy and sees if
| there are physically reasonable orbits around it. MOND-like
| orbits are physically reasonable (we observe them). The
| authors' model allows for _almost_ MOND-like orbits.
|
| The explanation for why these MOND-like orbits exist will
| vary. In the standard cosmology, it's because of a
| distribution of cold dark matter around the central mass.
| In MOND, it's because the strength of Newtonian gravitation
| falls off at great range from the central mass. In the
| theory discussed above it's because the central mass
| induces fluctuations in the gravitational field that grow
| in relevance with distance from the central mass, and in a
| particular range of radial distances those fluctuations are
| most likely to drop the orbiting body onto a closer orbit.
| _obviously wrote:
| Not only was Einstein wrong, he set us back over a hundred years.
| Special Relativity employs linear algebra for reference frames
| and Lorentz transformations. General Relativity relies on
| differential geometry and tensors to describe spacetime
| curvature. These areas of math have been rigorously tested and
| have shown his theories are wildly out of sync with reality.
| clysm wrote:
| The fuck... I can't believe what I've just read. That's like
| saying Newton set us back over a hundred years.
|
| There was no regression because of Einstein.
| _obviously wrote:
| Only in the same way a troll might bait someone into
| providing the real answer by being wrong. There's some
| relationship between energy and mass, why Einstein has any
| credit for that insight is beyond me.
| oneshtein wrote:
| Mainstream theories are so good that they dwarfed
| competition. Lack of competition caused lack of progress. It
| was easy to collect hundreds of downvotes on HN just by
| talking about alternative theories or about physical
| representation of abstract theories. Now such comments will
| collect mere dozens of downvotes. It also should be obvious
| that a new successful theory will account for more things, so
| it will be much more complex than previous theories.
| AnimalMuppet wrote:
| Yeah, um... if you're going to claim something like that, you
| need more than the claim. What evidence do you see for your
| position? What evidence can you give us so that _we_ can
| evaluate your position?
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