[HN Gopher] Could Modified Gravity Kill Planet Nine?
___________________________________________________________________
Could Modified Gravity Kill Planet Nine?
Author : dnetesn
Score : 106 points
Date : 2024-03-02 12:02 UTC (10 hours ago)
(HTM) web link (nautil.us)
(TXT) w3m dump (nautil.us)
| nerbert wrote:
| Fascinating. So this MOND theory works in this case. If I
| understand correctly, scientists still prefer the dark matter
| theory because it applies in more cases at galactic scale, and
| this is just one occurrence of the MOND theory working fine?
| bena wrote:
| I think dark matter is more appealing to them because the first
| person to discover a dark matter object is going to get
| something named after them.
| Ar-Curunir wrote:
| First, most modern research in physics (especially
| experimental physics) have a ton of coauthors; we're well
| past the stage of things being named for a single person.
|
| Second, what makes you think there isn't great fame in
| disproving dark matter?
| mr_mitm wrote:
| That doesn't make any sense at all unless they simultaneously
| believe that DM is actually the right answer.
| bena wrote:
| I'm sorry, I can see where I was ambiguous.
|
| I meant I can see why dark matter is appealing to more
| researchers in general and why MOND is in the minority.
|
| If dark matter is right, we can find it.
|
| If MOND is right, we've just been doing incomplete math
| this whole time.
|
| One of those is way more exciting.
| kadoban wrote:
| The last person who figured out we were doing incomplete
| math is the single most famous scientist in the history
| of the world. Being the next one isn't more exciting than
| getting some particle named after yourself?
| cdelsolar wrote:
| Yeah I interpreted that person's post as saying "we've
| been doing incomplete math this whole time" is way more
| exciting, which I completely agree with.
| mr_mitm wrote:
| They prefer it because it fits the data best.
|
| The biggest piece of evidence for DM is the BAO patterns in the
| CMB. Forget all the other numerous mountains of evidence, that
| is the biggest one. MOND has no good explanation for this
| without introducing something that's effectively DM.
| beautifulfreak wrote:
| Perhaps you meant...
|
| DM = Dark Matter, BAO = Baryon Acoustic Oscillations, CMB =
| Cosmic Background Radiation, MOND = Modified Newtonian
| Dynamics
| moffkalast wrote:
| To put some sources to that unsubstantiated "data", it's
| about the Bullet Cluster specifically.
|
| https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to.
| ..
| throwawaymaths wrote:
| DM can't explain renzo's rule, or the tully-fisher
| relationship, or why the milky way has a keplerian return
| (efe from the magellanic clouds), or why elliptical and
| lenticular galaxies don't seem to have dark matter. All these
| are explainable by MOND.
|
| MOND also predicted early galaxies, and a group seeking to
| disprove MOND by disproving EFE changed their mind because
| they found evidence of EFE.
|
| > it fits the data best.
|
| It's easy to fit the data when you can conjure a parameter to
| explain anything. What if I told you that GR is wrong and
| there's a ball of dark matter orbiting the sun that distorts
| Mercury's orbit.
|
| You wouldn't be able to prove me wrong.
| mr_mitm wrote:
| I wouldn't have to prove you wrong. If your stance is that
| Mercury's perihelion shift is explained by DM, then I'll
| counter that GR explains that, gravitational lensing,
| gravitational waves, black holes, the CMB, the helium
| abundance, and then some. By Occam's razor, GR would be the
| preferable theory.
|
| The claim that DM requires conjuring up parameters is
| completely baseless. There are one or two parameters
| (besides a handful other parameters from LambdaCDM) that
| determine the statistics of the DM distribution, and
| observations match well with simulations based on those
| parameters. There are small deviations called the dwarf
| galaxy problem and no one is inventing parameters to
| explain those, so what are you talking about?
| throwawaymaths wrote:
| There is a parameter per galaxy, and it can be wildly
| different. Some galaxies have no dm, some galaxies are
| "only dm". Hardly "small deviations"
| mr_mitm wrote:
| Those are measurements, not parameters. Just like the
| exact baryonic matter distribution is not a parameter of
| GR. You have an initial matter distribution, which is a
| random sample of a probability distribution (that is a
| part of the model) and then it starts clumping together
| over time.
| throwawaymaths wrote:
| You're wrong. They are parameters if you're using them to
| bestfit another value (rotation curves). This is basic
| high school science/stats.
|
| And yes, baryonic distribution is absolutely a parameter,
| but it's not a free parameter (or it's a less free
| parameter) because it's value is constrained to a
| measurement that is orthogonal to the quantity inferred
| (light vs rotation curve). Meanwhile, dm density is a
| _free_ parameter. It could be zero, or, 10x the baryonic
| mass, or anything in between.
| throwawaymaths wrote:
| > then I'll counter that GR explains that, gravitational
| lensing, gravitational waves, black holes,
|
| That interpretation of GR assumes that Mercury isnt
| perturbed by some form of dark matter. Go back and redo
| all the equations with a dark matter that obeys the right
| rules before claiming that GR is a better explanation.
| isthatafact wrote:
| >> It's easy to fit the data when you can conjure a
| parameter to explain anything.
|
| I know you are already aware, but that is literally the
| entire premise of MOND -- the M is for "MOdified Newtonian
| Dynamics".
| AnimalMuppet wrote:
| Not the same. MOND adds a parameter - the non-
| newtonianness which is purely a function of the masses
| and the distances. DM lets you add a new parameter (the
| DM density) at each point in space. That's effectively an
| infinite number of parameters, whereas MOND has very few.
| mr_mitm wrote:
| > DM lets you add a new parameter (the DM density) at
| each point in space. That's effectively an infinite
| number of parameters, whereas MOND has very few.
|
| No one is doing that, though. What cosmologists do is
| parameterize the statistics of the DM distribution.
| That's one or two parameters. Then we compare
| observations to simulations to determine how likely the
| observed distribution is given the statistical
| properties. For example, a few galaxies with almost no
| dark matter would be expected due to the dynamics of
| clusters and galaxies. You could in principle calculate
| how often that should be the case, and if we were to
| observe it much more often than we should then there
| would be a problem with DM. No one is suggesting that the
| DM distribution can assume any arbitrary shape.
| AnimalMuppet wrote:
| That's at the universe-sized level. At the galaxy level,
| as you state, we say "oh, that galaxy has almost no dark
| matter". That's a per-galaxy parameter. At the Bullet
| Cluster, we say "the dark matter must be here and here".
| That's a point-by-point distribution.
| isthatafact wrote:
| > "That's a per-galaxy parameter."
|
| No, variation in galaxy properties is an output, not an
| input, of the model.
|
| You could decide to quantify and catalog different
| galaxies with one or more parameters that describe their
| properties. You could then compare whether that catalog
| is statistically consistent with the output of the model
| (and must take into account all uncertainties in the
| model and the observations).
|
| By analogy, you can measure that different people have
| different heights, but it does not mean that the specific
| height of each individual person is a unique input
| parameter in any fundamental model of biology.
| AnimalMuppet wrote:
| Let me change your analogy. You take each person, and
| measure their height. You also "measure" how tall they
| "should be". You then show that the differences between
| their actual height and the height they should have had
| fits a model. That's nice, but for each person, you still
| assigned a value for the difference between how tall they
| are and how tall they should have been.
|
| _That 's_ what I mean by "it's a per-galaxy parameter".
| For each galaxy, to explain the behavior of _that_
| galaxy, you 're saying "it must have X amount of dark
| matter".
| isthatafact wrote:
| There is no DNA for galaxies, so how could you know what
| the properties of a particular galaxy "should be"?
|
| The focus on "per-galaxy parameters" is like expecting to
| be able to predict how tall Tom Cruise should be after
| reading a textbook on the theory of evolution.
| MattPalmer1086 wrote:
| It fits the data best because they tweak a lot of different
| parameters to obtain the fit. MOND only has one parameter.
| mr_mitm wrote:
| LCDM has six parameters, only a few of them are related to
| the DM distribution. Is that a lot to you or what
| specifically are you referring to?
| MattPalmer1086 wrote:
| The DM distribution itself is effectively a huge number
| of parameters. You have to have just the right amount of
| dark matter distributed differently in each case to
| explain observations and get those best fits.
|
| For example, see:
|
| https://en.m.wikipedia.org/wiki/Cuspy_halo_problem
| mr_mitm wrote:
| Yes, the cuspy halo problem wouldn't be a problem at all
| if you were to simply adjust the infinite parameters that
| you suggest that DM has. The fact that there are
| statistical discrepancies proves my point that no one is
| adjusting a huge number of parameters.
| MattPalmer1086 wrote:
| The point is that all kinds of dark matter distribution
| models are proposed and evaluated to see which one fits
| best. Again, for example:
|
| https://arxiv.org/abs/2001.10538
|
| And they still fail to explain other observations which
| require additional arguments as to why the dark matter is
| once again distributed in _just_ the right way to give
| the rotation curve... that is already successfully
| predicted by MOND in most cases with just one universal
| parameter.
| rocqua wrote:
| We have found galaxies with differing amounts of dark matter
| (as measurer by rotational speed) and have confirmed these
| measurements with gravitational lensing being more or less
| effective.
| pfdietz wrote:
| If MOND were a single theory such ideas would be more
| interesting.
| adastra22 wrote:
| You could say the same about dark matter.
| nubinetwork wrote:
| Pluto is a planet ( /s but not really)
| LoganDark wrote:
| Wasn't this a Rick and Morty episode?
| aquova wrote:
| Fine, but then so is Eris
| moffkalast wrote:
| Ceres to beltalowda!
| FrustratedMonky wrote:
| Pluto should have had some grand-father clause. Keep it a
| planet for historical sake, but with an asterisk. So we
| wouldn't keep arguing about the new objects that are same size
| or bigger, yes, Pluto isn't a planet. But lets just go ahead
| and acknowledge that we are going to call Pluto a planet in
| 'name-only' for the memories.
| pwdisswordfishc wrote:
| That's what the "dwarf planet" term is.
| iraqmtpizza wrote:
| they prefer the term "planets with dwarfism"
| vundercind wrote:
| Colloquial language doesn't have to follow scientific
| language. No facts about the physical universe changed with
| the name change. We can just keep calling Pluto the ninth
| planet, which lets the hypothesized distant planet be "Planet
| X" which is way the hell cooler than "planet 9". There's no
| need for any permission to do this.
| juped wrote:
| Also the IAU only has as much authority to define "planet"
| as you give it, and there's not much reason to give it any,
| especially when the geologists make so much more sense.
| Izkata wrote:
| NASA still has a page where that's the primary term:
| https://science.nasa.gov/solar-system/planet-x/
| greggsy wrote:
| That's only in the context of 'x' being unknown. They're
| still referring to it as 'planet nine' in the article.
| juped wrote:
| True, but people who make sense and people who have effective
| PR machines are not always the same.
| adastra22 wrote:
| Especially when you consider that whatever planet is found
| won't have cleared its orbital path (it's in the Kuiper Belt)
| and so wouldn't be Planet 9 by the IAU's ridiculous definition.
| The headline is doubly wrong.
| antonioevans wrote:
| Dr Becky explaining the MOND theory and why it's invalid -->
| https://www.youtube.com/watch?v=HlNSvrYygRc
| lolinder wrote:
| For those of us who don't already know her, can you explain who
| Dr Becky is and why we should trust her perspective? I can't
| watch it just yet, but judging from the thumbnail this feels
| like just another educational YouTube video, and while I watch
| more than my fair share of that genre I don't tend to
| implicitly trust it on complicated scientific topics.
| boesboes wrote:
| She has a PhD in astrophysics and is a specialist on
| blackholes. In her video's such as this one she goes into
| what the paper claims and then looks at the evidence for and
| against, based on other research and theory.
|
| She knows her stuff and approaches things rationally.
| Jleagle wrote:
| Wikipedia: `Rebecca Smethurst, also known as Dr. Becky, is a
| British astrophysicist, author, and YouTuber who is a junior
| research fellow at the University of Oxford. She was the
| recipient of the 2020 Caroline Herschel Prize Lectureship,
| awarded by the Royal Astronomical Society, as well as the
| 2020 Mary Somerville Medal and Prize, awarded by the
| Institute of Physics. In 2022, she won the Royal Astronomical
| Society's Winton Award "for research by a post-doctoral
| fellow in Astronomy whose career has shown the most promising
| development".`
| sapiogram wrote:
| She has a PhD in astrophysics, with her academic work
| centered around the co-evolution of supermassive black holes
| and their host galaxies. As far as I can tell, she's still an
| active researcher, even though she clearly spends a lot of
| her time on her Youtube channel.
| weego wrote:
| AFAIK spreading interest in the field through social media
| was/is part of her post doc job goals
| lupire wrote:
| The YouTube channel doesn't have hard verification, but
| claims that "Dr Becky Smethurst, an astrophysicist at the
| University of Oxford."
|
| and Wikipedia editors seem to agree, with further references
| https://en.m.wikipedia.org/wiki/Becky_Smethurst
|
| The video cites two papers, one for MOND and one against, so
| you can look up the papers.
|
| It's better than the OP article, which ignores the
| counterarguments against MOND.
| GuB-42 wrote:
| From her bio, she is a legit astrophysicist, who publishes in
| peer-reviewed journals.
|
| But more important than that, if you want more, you can read
| the paper her entire video is based on [1]
|
| Also be aware that Sabine Hossenfelder, also a popular
| science YouTuber and published physicist released a video
| supportive of that paper, even though she was somewhat in
| favor of MOND before [2]. She even co-authored a paper about
| it [3] which she presented in a video [4]
|
| [1] https://doi.org/10.1093/mnras/stad3393
|
| [2] https://www.youtube.com/watch?v=i4lu9AxRtqA
|
| [3] https://arxiv.org/abs/2401.10202
|
| [3] https://www.youtube.com/watch?v=J7MVl1cSmYE
| tsunamifury wrote:
| Let's turn science into commentator sports! That way an
| already needlessly polarized field full of petty
| researchers who promote poorly founded theories to the
| public in order to gain notoriety can continue to
| completely screw up physics in the public eye, and frankly
| in academia, too. Looking at you string theory.
| AnimalMuppet wrote:
| It's been that way for a long time. Einstein published a
| paper in support of Bose's statistics for integer-spin
| particles, explaining that no, that's not an off-by-one
| error, and now they're called Bose-Einstein statistics.
| tsunamifury wrote:
| Yea but YouTube personalities were never appealed to to
| find validation amount public opinion.
| adrian_b wrote:
| Such papers only prove that the simplest formula proposed
| for MOND may be too simple.
|
| While it matches well many experimental facts, there also
| other experimental facts that appear to contradict it.
|
| There is an essential difference between a MOND-like theory
| and a dark matter based theory.
|
| For a MOND-like theory one has to choose some mathematical
| relationship that determines the gravitational forces,
| given the observed distribution of matter in the Universe.
| Then one must compute the expected movements and compare
| them with the observed movements, to verify or falsify the
| postulated mathematical model.
|
| On the other hand, any theory based on dark matter does not
| have any predictive power or any usefulness. Because the
| observed movements of the bodies in the Universe cannot be
| explained by the conventional mathematical model, one adds
| arbitrarily dark matter wherever it is necessary to remove
| the discrepancies in the observed movements.
|
| When one is free to add dark matter, then all mathematical
| models for gravitation become equivalent and none can be
| used to predict what we observe.
|
| Unless an alternative method for observing dark matter
| would be discovered, using it is just an euphemism for
| avoiding to recognize that the current model of gravitation
| is not accurate enough.
| pfdietz wrote:
| But you're just demonstrating the annoying aspect of
| MOND: it's not one theory, so it fails that first
| requirement of being science, that it be falsifiable.
|
| We see this _all the time_ when MOND is tested. It ends
| up failing, but then the proponents say "oh, well, it's
| not _that_ MOND we 're talking about". It's like some
| sort of pseudoscientific cockroach that keeps escaping
| after you crush it with a shoe.
| jcranmer wrote:
| ... so just like supersymmetry and string theory?
| pfdietz wrote:
| Yes, just like that.
| adrian_b wrote:
| Any specific variant of MOND, with definite formulae for
| the gravitational quantities, is falsifiable. (Though it
| may not be very easy to verify or falsify it, because
| besides the hypothetical "dark matter" there also exists
| true dark matter, i.e. interstellar gas and dust clouds.)
|
| Any theory based on dark matter is not falsifiable,
| regardless what model is used for gravitation, because
| for now there is no constraint on the distribution of the
| dark matter.
|
| The only way for the theory of dark matter to become a
| scientific theory is to discover an alternative way to
| determine where the dark matter is located, besides
| placing it wherever necessary to remove the discrepancies
| between the observations of the movements of the
| celestial bodies and the predictions of the current model
| of gravitation.
| nicktelford wrote:
| Isn't this essentially the same problem with Dark Matter
| though? They keep looking for it, not finding it and
| proclaiming "well, it must be somewhere else!".
|
| I always got the impression that when Dark Matter was
| initially labelled as such, it was just a name for the
| discrepancy between theoretical models and observations;
| and that the name itself seems to have driven this idea
| that it's the observations that are wrong and not the
| models.
|
| Personally, when discussing Dark Matter vs. MOND, I think
| neither should be treated as a concrete "theory", but
| simply a different perspective on where the problem lies.
| "Dark Matter" is the idea that our observations are
| incomplete, and MOND is the idea that our theoretical
| models are wrong.
|
| Hopefully this conundrum is resolved within my lifetime,
| because I'd love to know what the answer is. It would be
| absolutely wild if they're _both_ right i.e. that our
| observations are incomplete _and_ our models are wrong.
| mr_mitm wrote:
| > that our observations are incomplete and our models are
| wrong.
|
| I'd say that's a given regardless of the DM mystery.
|
| It's consensus that QM and GR are incompatible and that
| we need a new theory out of which both of these come out
| as a special case. String theory was considered a hopeful
| contender for that for a while.
|
| And that we haven't observed everything to a satisfying
| degree yet should be obvious.
|
| > Isn't this essentially the same problem with Dark
| Matter though? They keep looking for it, not finding it
| and proclaiming "well, it must be somewhere else!".
|
| No (unless you mean "where" in parameter space), we have
| a pretty good idea _where_ it is thanks to gravitational
| lensing surveys. We don 't _what_ it is.
| nicktelford wrote:
| Yes, I meant in "parameter space" :-)
| greggsy wrote:
| She includes references in the description, in case you need
| to personally double check the content.
| the__alchemist wrote:
| Question, regarding 4:00 in that video. Dr. Becky states that
| GR, our best theory of gravity, at large scale, predicts dark
| matter. Is this correct? My understanding is that the models
| that predict dark matter use Newtonian physics. And more, the
| problem with GR is that its calculations are complicated, in a
| partial-differential-equations sense.
| MattPalmer1086 wrote:
| No GR does not predict dark matter.
|
| But when we look at galaxies and so on, things don't seem to
| add up if you only look at the visible matter.
|
| So, there could be a lot of dark matter we can't see. Or GR
| might not be correct and MOND (or a relativistic formulation
| like AQUAL) might be the right answer.
|
| It's true I guess that if GR is correct, then we need some
| kind of dark matter to explain observations. But it might not
| be correct.
| karmakaze wrote:
| It's simpler to say Newtonian gravity and visible matter
| alone is sufficient to explain the need for dark matter.
|
| Adding GR to the picture doesn't take away the need for
| dark matter to account for observations.
| MattPalmer1086 wrote:
| True, even without GR the same issue would exist with
| Newtonian gravity.
| adrian_b wrote:
| She does not express this fact in the right way.
|
| Whenever the facts predicted by a theory do not match the
| experimental facts, one could say that the theory has
| predicted the existence of an unknown factor that has
| affected the experiments.
|
| Nevertheless, until there is an alternative way to determine
| the existence of that unknown factor, the right way is to
| simply say that there is a mismatch between predictions and
| observations and the reason for this mismatch must be
| determined in the future. For now, the current theory is not
| accurate enough.
|
| For instance, when some planetary movements did not coincide
| with the predictions, it was supposed that perhaps there
| exists an extra planet which explains the discrepancies
| between predictions and observations.
|
| This supposition was confirmed only when Neptune was also
| observed with a telescope. If Neptune had never been
| observed, perhaps it would have been discovered that the
| mathematical model of gravitation must be improved.
|
| For now, there are discrepancies between observations and the
| predictions of the current mathematical model of gravitation.
| Like in the cases of Neptune and Pluto, there is a
| supposition that perhaps there exists some kind of dark
| matter that would be the cause of the discrepancies.
|
| Until the moment when an alternative way to determine the
| existence of dark matter will be discovered, like the optical
| observations of Neptune and Pluto, the existence of dark
| matter remains just a hypothesis that cannot be used for any
| practical purpose, because it cannot predict anything. Dark
| matter can be added arbitrarily in any place and this can
| make any theory of gravity match the observations.
|
| Therefore now we have galaxies that are supposed to be rich
| in dark matter and galaxies that are supposed to be poor in
| dark matter, in order to fit the observations, but without
| any a priori rule that could be used to predict this.
| GordonS wrote:
| If I understand correctly, MOND is suggesting that gravity
| behaves differently at sufficiently large scale (galactic),
| similar to how we observe different physics at a quantum level?
| lisper wrote:
| The analogy is a teensy bit strained but you are substantively
| correct.
| samus wrote:
| MOND and DM try to explain the same phenomena, and the debate
| can probably be settled on way or the other by looking at
| enough interesting galaxies, or by finding dark matter or
| definitely ruling out that it exists.
|
| GR and Quantum Mechanics are not even really talking about the
| same thing and use incompatible mathematical frameworks.
| Another problem is also that there are actually not so many
| phenomena that we would _need_ Quantum Gravity for. Particle
| physicists are yearning to find experimental evidence that
| would force us to retire the Standard Model to the history
| books. And their event horizon makes it kinda hard to study
| what is going on inside black holes.
| catlifeonmars wrote:
| As a layperson, it's hard to tell from the debate of evidence
| I read elsewhere in the comments: what are the falsifiable
| predictions of e.g. DM and MOND?
| jfengel wrote:
| I wouldn't have put it that way, since the quantum/classical
| transition is an emergent phenomenon without additional
| parameters, while MOND proposes a new term being a added to the
| physics.
|
| Still, at a broad level there are similarities, yes. In QM
| scale practically has a life of its own -- made explicit in the
| Copenhagen interpretation.
| adastra22 wrote:
| What? Quantum physics definitely adds new terms...
| lolinder wrote:
| Whether or not they're right in their answer, it's interesting to
| me that this debate is a modern repeat of the discussion about
| the oddities of Mercury's orbit that some attempted to explain
| with a hypothetical planet Vulcan [0]. One of the early evidences
| for Einstein's general relativity was that it accounted for those
| oddities with only the planetary bodies that had already been
| observed.
|
| [0] https://en.m.wikipedia.org/wiki/Vulcan_(hypothetical_planet)
| vikingerik wrote:
| And since readers may find this interesting: The various
| writeups all say that relativity explained the precession of
| Mercury's orbit, but they never say exactly what factor was
| accounted for by relativity.
|
| The answer: Mercury is heavier when it is at perihelion in its
| elliptical orbit, because it's moving faster. The increased
| relativistic mass makes for increased momentum at perihelion,
| which carries the planet a little farther than expected before
| it starts to swing back up out of the gravity well, so the
| perihelion precesses more.
|
| And the effect exists and is known for the other planets now;
| the mass increase is proportional to velocity-squared and the
| velocity difference depends on the eccentricity of the orbit,
| so it's an order of magnitude smaller for Earth, but still
| measurable and now known. It's also known for other objects:
| one example is stars in elliptical orbits around the galaxy's
| central black hole, which undergo extra precession in the same
| way.
| sigmoid10 wrote:
| This might sound true if you have a superficial understanding
| of relativity (and I would not be surprised to see people
| repeating this stuff on the internet), but it's completely
| false. The correction to Mercury's orbit precession from
| General Relativity has nothing to do with Mercury itself (its
| mass, speed or otherwise). It is purely based on the sun's
| gravitational field. If you want to interpret it as a
| potential field in the classical Newtonian sense, it
| basically picks up an additional 1/r^3 term. That's what
| disturbs the orbit and it's also the reason why this effect
| is negligible for other planets apart from Mercury, because
| it drops off much faster than the normal 1/r potential. Any
| object very close to a large mass like the sun would feel
| this distortion, irrespective of how fast it travels.
| antognini wrote:
| Yes, this is correct. The orbital precession occurs even
| for a point mass.
| plank wrote:
| Former physicist myself: Not sure I understand your (and
| parents) argument.
|
| Given the Suns gravitational field, and the orbit that
| Mercurius has, the speed of the planet can be determined.
| So the argument that a point mass would have the same
| precession does not dispute the argument that it is the
| relativistic mass of the point mass/planet that
| determines the precession.
|
| I will not venture into arguments* whether grandparents
| explanation is 'correct', 'best' or 'useful', there is an
| equivalence between 'gravitational field + orbit' being
| the reason and '(relativistic) speed + orbit' being the
| reason. * A sybling comment states that relativistic mass
| is avoided in modern physics. As someone who did physics
| 30 years ago, I can not deny or corroborate this
| statement. And indeed, the explanation of grandparent is
| not the way I myself think about the perihelium
| precession. But that does not make me certain enough to
| say it is 'wrong'.
|
| Edit: typo 'of' --> 'or'
| kmm wrote:
| That doesn't sound right. Relativistic mass as a concept is
| avoided in modern physics because it doesn't yield much
| insight and is hard to keep consistent. Involving gravity
| only makes it worse, because it's very hard to consistently
| include gravity in special relativity. For one, does the
| relativistic mass gravitate or not?
|
| And if you actually do go through the calculations, you find
| that you do not get the observed result. This paper[0] sums
| up a few of them.
|
| 0 (PDF): http://kirkmcd.princeton.edu/examples/perihelion.pdf
| canjobear wrote:
| If that's true then it should be possible to predict the
| precession from special relativity alone.
| lupire wrote:
| I don't know MOND at all, but is it still highly plausible that
| decades of modern astronomy haven't found better more direct
| evidence for Planet Nine existing?
| Tuna-Fish wrote:
| The evidence from clustered orbits points towards planet nine
| likely being right in the middle of the busiest part of the
| night sky, nestled between millions of stars of the milky way.
| The assumption is that we already have pictures of it, it's
| just that when you photograph that part of the sky you are not
| likely to notice one very faint point of light in the middle of
| the ten million brighter points of light on the same plate.
| Qem wrote:
| Detecting planets or other cold objects is hard, when they are
| far from the sun. Illumination from the sun falls with inverse
| square law (~1/r^2). When the reflected light does the return
| trip, it also falls with (~1/r^2). The effect compounds on both
| legs of the trip, and overall the brightness of a planet falls
| proportional to ~1/r^4, quickly getting lost amidst all the
| noise in the night sky as distance increases.
| gray_-_wolf wrote:
| Could you build an uber radar for detecting the planet,
| possibly working together with the radiotelescopes we already
| have? Given the article, it should be ~150 light hours from
| sun, which round trip time wise sounds doable.
| pfdietz wrote:
| Radar faces the same inverse fourth power effect.
| Qem wrote:
| Not sure. But to map polar ice in mercury it was necessary
| to use the Arecibo radio telescope as radar[1], with the
| benefit we knew previously where to point the instrument,
| exactly. Given planet nine is hundreds of times more
| distant, it would probably take a humungous radio telescope
| and a lot of power to conduct the search.
|
| [1] https://www.nature.com/articles/369213a0
| bastawhiz wrote:
| My suspicion is that you'd have a really hard time doing
| that. For one, if so little of the light from the sun
| (notably the brightest thing in our solar system) is being
| reflected, we'd be hard pressed to build something capable
| of spraying the whole search area and getting back enough
| particles to detect and count.
|
| But even more than that, the power requirements would be
| immense. We have retro reflectors on the moon, and even
| with knowing where the moon is and where they are on the
| moon, we get _almost no_ photons back. From Wikipedia:
|
| > Out of a pulse of 3x1017 photons[25] aimed at the
| reflector, only about 1-5 are received back on Earth, even
| under good conditions.
|
| That's with a dedicated reflector positioned well on the
| moon's surface, about 0.002au away. You're talking about
| hitting a rocky lump in front of a bright background
| 400-800au away. The power requirements wouldn't be five
| orders of magnitude more, it would be perhaps hundreds of
| orders of magnitude more.
| catlifeonmars wrote:
| Is it fair to say it is easier to detect planets in other
| solar systems?
| bastawhiz wrote:
| Yes! We often detect planets in other solar systems because
| they pass in front of their stars, causing the brightness
| to dip. You can also look for Doppler shifts in the light
| you see from stars, which is caused by the planet pulling
| the star slightly. You can also look for the sorts of
| brightness changes you'd expect as a result of gravity
| lensing.
|
| In almost all cases, you're looking for changes in
| something you can already see. The problem with a planet 9
| is that so little light is getting to it that it's going to
| be very very dim.
| greggsy wrote:
| Wouldn't it still be passing in front of something, or is
| it likely to be comparatively 'stationary' against a
| similarly stationary backdrop of stars and other objects?
| Arech wrote:
| With our current, but especially decades ago capabilities, it's
| (was) similar to Russell's teapot problem.
| api wrote:
| It would be incredibly dark. The sun would just be a bright
| star out there.
|
| There's also some interesting speculation that planet nine
| could be a primordial black hole, in which case it could only
| be located by its indirect effects. It likely wouldn't radiate
| much at all, making it basically invisible.
| eigenspace wrote:
| Blows my mind that MOND still gets attention.
| russdill wrote:
| There's still plenty of room to study MOND. There's a lot of
| people outside the scientific community that seem to misjudge
| the general scientific consensus on it. But doing publishing
| research on things like MOND is still important.
| sspiff wrote:
| I've always thought that a good model or theory predicts
| observations (like a new particle, or an unseen planet) and is
| confirmed to be accurate by those observations.
|
| A new model that is designed to fit existing observations that
| don't match our expectations based on the old model, without it
| correctly predicting new unexpected observations, seems like a
| rather weak proposition to me.
| Wowfunhappy wrote:
| This new model was designed to fit existing observations
| without the presence of dark matter, and turns out to also
| predict the orbits of planets without planet nine.
| Arech wrote:
| MOND was there years before Batygin (co-author of planet Nine
| hypothesis) was born.
| Izkata wrote:
| > co-author of planet Nine hypothesis
|
| Are you referring to what he published in 2016? Because it's
| much older than that, we used to call it Planet X.
| lamontcg wrote:
| "Planet Nine" is a term of art that refers specifically to
| the 2016 theory.
|
| "Planet X" refers specifically to a theory proposed by
| Percival Lowell in 1906 about the orbit of Uranus which was
| disproven in 1993 by Voyager 2 flyby data.
|
| They are both more specific theories than just "a trans-
| Neptunian planet we haven't discovered" which is overly
| broad.
| dragonwriter wrote:
| Somewhat related are "Nemesis", the 1984 hypothesis of a
| distant companion red or brown dwarf to the Sun, and
| "Tyche", the 1999 hypothesis of a gas giant in the Oort
| Cloud.
|
| On "Planet X" specifically, the effect it was offered to
| explain was determined to have been measurement error
| after recalculation of Neptune's mass and consequently
| effect on Uranus's orbit in 1993 based on _Voyager 2_
| data.
| Izkata wrote:
| The general theory was called planet X before pluto was
| demoted; X just means 10.
|
| https://science.nasa.gov/solar-system/planet-x/
| gorkish wrote:
| > A new model that is designed to fit existing observations
| that don't match our expectations based on the old model,
| without it correctly predicting new unexpected observations,
| seems like a rather weak proposition to me.
|
| This description roughly applies to General Relativity; there
| are clearly merits to both approaches.
| canjobear wrote:
| The first step is to propose a model that explains stuff. Then
| you test it on new predictions.
| catlifeonmars wrote:
| God did it.
|
| In all seriousness, I think the bar is higher than just "here
| is an explanation that makes sense". The new model has to
| explain stuff better than existing established models.
| canjobear wrote:
| Sometimes it's nontrivial to show that a theory predicts
| something, even if the thing is already observed.
| throwawaymaths wrote:
| MOND has successfully predicted:
|
| - "no dark matter" in dense ellipticals and lenticular
|
| - external field effect (including keplerian descent for the
| milky way)
|
| - early galaxies
| denton-scratch wrote:
| I thought MOND only kicked-in in regions of a galaxy where
| acceleration was much lower than in the Sun's region.
|
| These guys appear to be reasoning that because the Oort Cloud is
| far from the Sun (i.e. relatively low acceleration) then MOND
| might apply. But isn't part of the reasoning for MOND that its
| effects can't be tested anywhere near Earth, because this whole
| galactic neighbourhood experiences much too much acceleration?
|
| The article was very thin on explaining how MOND could explain
| anomalous orbits of Oort Cloud objects, or what kinds of anomaly
| they are trying to address.
| mannykannot wrote:
| The relevant distance here is not that to the sun, but that to
| the galactic center, which is within the range over which MOND
| is proposed to be noticeable. The argument is that, according
| to MOND, the galactic center would perturb orbits in the Kuiper
| belt, creating the alignment which has heretofore been taken as
| evidence for Planet Nine.
| vld_chk wrote:
| Whenever I read about Planet Nine search, I have a very naive (as
| a non-physicist) childish question: if we detect anomaly in our
| Solar System just less than a decade ago and can't find anything
| visual which explains it, assuming there is giant planet/piece of
| ice floating somewhere at the edge of Solar System, how we are
| sure that there are not a lot of such objects and real space is
| not as "empty" as we think it is? Simply, what is the probability
| that let's say space between us and Alpha Centaurs is not filled
| with objects like this? Invisible and leaving a tiny
| gravitational trace at the edge of our ability to detect it?
| ano-ther wrote:
| Very good question. As a non-astronomer, my guess is that we
| don't observe a lot of transient black outs for example of the
| milky way. Which would happen if there's a lot of these around.
|
| That gives you an upper bound of how many objects like that
| exist.
| perihelions wrote:
| Microlensing events! (to be slightly more precise)
|
| https://en.wikipedia.org/wiki/Rogue_planet#Microlensing
|
| - _" They found 474 incidents of microlensing, ten of which
| were brief enough to be planets of around Jupiter's size with
| no associated star in the immediate vicinity. The researchers
| estimated from their observations that there are nearly two
| Jupiter-mass rogue planets for every star in the Milky
| Way.[26][27][28] One study suggested a much larger number, up
| to 100,000 times more rogue planets than stars in the Milky
| Way, though this study encompassed hypothetical objects much
| smaller than Jupiter.[29]"_
| lamontcg wrote:
| And "MACHOs":
|
| https://en.wikipedia.org/wiki/Massive_compact_halo_object
|
| In the 1990s I went to a talk which suggested that "WIMPs"
| (Weakly Interacting Massive Particles aka supersymmetric
| particles that would give rise to dark matter) was an
| acronym for "Well It Might be Physics" while MACHOs was
| "Maybe Astrophysics Can Help Out".
|
| The upper bound on MACHOs has looong since eliminated them
| as the explanation for all of dark matter though...
| ryandrake wrote:
| ...or an upper bound of how big they might be. For all we
| know, the galaxy might be full of really dark bowling ball
| sized objects. We'd have no way of observing them, right?
| mike_hock wrote:
| What if dark matter is primordial bowling balls!
| semi-extrinsic wrote:
| So these are called "snowballs" and are excluded as dark
| matter candidates because we would then see a much larger
| number of extra-solar asteroids/meteorites. Cf. e.g.
| section 2 of the reference below.
|
| https://web.archive.org/web/20180723032406/http://iopscie
| nce...
| jiggawatts wrote:
| I love how every time someone comes up with some random
| notion, a scientist can point at a chart and say "we've
| thought of that already and excluded it with experiment
| CoolName."
| throwawaymaths wrote:
| Really dark bowling balls would still exhibit blackbody
| radiation
| perihelions wrote:
| The simple answer is that there _are_ actually very large
| numbers of objects in the space between stars that aren 't
| detectable. Including, certainly, planet-sized objects that
| formed in stellar environments, escaped, and now fly freely in
| between.
|
| https://en.wikipedia.org/wiki/Rogue_planet
| Gooblebrai wrote:
| The so called Rogue Planets.
|
| There's a nice video from Kurzgesagt on the topic:
| https://youtu.be/M7CkdB5z9PY?si=0BwFwotoHi8PxL1f
| mannykannot wrote:
| Indeed - and the Oort cloud itself extends to about 1.5
| light-years, which is a substantial fraction of the ~ 4
| light-year distance to our nearest neighboring stars.
| ithkuil wrote:
| That's even more so substantial if the nearest neighboring
| star also has a comparable oort cloud. In that case a mere
| 1ly of interstellar space would separate the two systems
| _petronius wrote:
| The gravitational effect of a lot of spread out mass outside
| the Oort cloud (or indeed, a lot more mass in the Oort cloud)
| would have a different effect on the orbits of the planets than
| a single planet that we had not found yet.
|
| So for some known configuration of orbits of the known planets,
| there are a limited number of solutions (in terms of mass,
| inclination, eccentricity, etc.) that you could add to the
| gravitational interactions of the solar system and still have
| the current orbits we observe. That gives a reasonable guess
| about what (another planet) and where (in the sky) to look for
| to explain anomalies.
|
| But with distant objects that don't emit their own light, even
| having a good guess of a bunch of the orbital parameters
| doesn't mean it is easy to find, because you don't have much
| sunlight reflecting off of it, and the chances it will occlude
| something else brighter (like a background star) are needle-in-
| a-haystack level.
|
| Even _if_ you knew the _exact_ orbit you were looking for,
| there are 360 degrees of sky to search for a tiny, dark object,
| because you don't necessarily know where in the orbit the
| object currently is.
| dragonwriter wrote:
| > how we are sure that there are not a lot of such objects and
| real space is not as "empty" as we think it is?
|
| There could be lots of such objects without meaningfully
| denting how empty we think space is.
| cratermoon wrote:
| Milgrom's theory has a problem. There's no theoretical basis for
| it. It arises from observations, but nobody can say why or how it
| fits in cosmology.
| tgv wrote:
| TBF: doesn't the same apply to dark matter? It originated
| because something didn't fit, and it was an easy explanation,
| without further evidence.
| mannykannot wrote:
| A seemingly obvious question is whether dark matter could provide
| an equivalent result. I'm not really sure, but the paper's
| authors seem to be saying 'no' in this passage:
|
| _Equations (B2), (B5), and (B7) are the main results of this
| section. They provide an expression for the phantom mass that
| sources anomalous effects in the inner solar system. As discussed
| above, these effects are absent in Newtonian gravity and hence
| absent in any dark matter model._
|
| Perhaps someone more knowledgeable could comment on this?
| pmayrgundter wrote:
| Could Barnard's Star Kill Planet Nine?
|
| https://www.listennotes.com/podcasts/the-demystifysci/great-...
| bradley13 wrote:
| Dark matter is the modern ether. There is zero evidence that it
| exists. What we have are distant phenomena that we can't explain.
|
| If not MOND, then something like it.
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