Could Modified Gravity Kill Planet Nine? | Hacker News Reader

Could Modified Gravity Kill Planet Nine?

nautil.us

116 pointsdnetesn152 comments

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)

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.

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.

Yes, this is correct. The orbital precession occurs even for a point mass.

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’

Just think about it. If relativistic mass (a term that indeed tends to be avoided by actual physicists for good reasons, because it confuses concepts of mass and energy which are clearly defined in relativity) was the explanation, then the shape of the planet's orbit itself would influence the anomalous precession component from General Relativity. And you would see no relativistic effect at all for perfectly circular orbits with zero eccentricity. That is simply not the case.

The explanation of grandparents for the perihelium effect of Mercury tried to explain the 'overshoot'. And yes, in a perfectly circular orbit there would be no overshoot??

I do not think grandparent tried to say that all relativistic effects were caused by the increase of the 'relativistic' mass, just that the overshoot was.

So I am sorry: I cannot see how this argument disproves the explanation of grandparent. And just to be clear: I do not think in the way of grandparents argument myself.

Kind regards,

Roel

>And yes, in a perfectly circular orbit there would be no overshoot??

That's where you and the other comment are wrong. The only way to get a stable circular two body orbit is for newtonian potentials which are exactly 1/r shaped. Add any other term (like 1/r^3 for General Relativity) and you essentially get more or less chaotic movement over time, similar to the three body problem in newtonian gravity. See Betrand's theorem [1].

[1] https://en.wikipedia.org/wiki/Bertrand%27s_theorem

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

Wow, even for a Kirk McDonald "pedagogic diversion", there is a lot in footnote 4.

There are scattered typos, but it's an excellent annotated bibliography. I was unaware of some of the 21st century papers it lists.

A small thing, and it could just be me. I'm not sure where McDonald is going with respect to his point in footnote 7 about his reference [54] (Einstein 1911, https://onlinelibrary.wiley.com/doi/abs/10.1002/andp.1911340... (German), https://einsteinpapers.press.princeton.edu/vol3-trans/393 (English)) and unfortunately his ...kirkmcd.princeton.edu/examples/GR/... links all want passwords. It's obvious that the footnote is about §2 of [54], but to me it does not really engage with the \gamma m in the text referring to footnote 7 (or indeed the content of your own comment's parent comment). Instead the (spring-balance-indicated, i.e. "contact"[1]) change of weight when light is beamed from one weighed system to another is diagnostic, with the focus of thought being on how that beamed light is adapted when acceleration or gravitation is relevant. This is further developed into gravitational redshift in [54]§3 (which, frankly, is an amazing bit of Einstein's early thinking). So I don't see how footnote 7 fits the perihelion precession topic, where the (rest) masses are defined as constant and radiation pressure is ignored.

[The sodium quasi-cometary tail of Mercury is cool, though. <https://www.nature.com/articles/s41467-020-18220-2>]

[1] https://math.ucr.edu/home/baez/physics/General/Weight/whatIs...

If that’s true then it should be possible to predict the precession from special relativity alone.

I don't think there's a way to meaningfully connect your comment to general relativity (GR). "The answer" in GR is that Mercury is in geodesic motion ("free fall"), with its geodesic picked out by the mass of the solar system and principally its central mass (the sun). For all practical purposes, nothing about Mercury itself selects which geodesic its on.

The weak equivalence principle (WEP) says that in curved spacetime a small freely falling object's orbit around a large central pointlike mass is completely determined by the former's initial position in spacetime and its initial velocity.

The strong equivalence principle (SEP) says that this remains true for the smaller body even if that body is bound by its own self-gravitation: the orbit is determined fully by the initial position and velocity and not by the small body's internal composition.

We can of course replace the large-mass pointlike generator of a (exterior) Schwarzschild-like spacetime with an extended body that generates some perturbation of a more general central-mass spacetime (like Kerr or Kerr-Newman).

The WEP has been tested extensively in terrestrial labs (torsion balances) and satellites in Earth orbit (e.g. MICROSCOPE).

The SEP has been tested extensively using satellite and lunar laser ranging, and is supported by astrophysical observations including the triple-relativistic-star system comprising an inner white dwarf and PSR J0337+1715 orbited by an outer white dwarf.

That the SEP holds up so well means that we can replace Mercury with any mass much smaller than the sun (it does not have to be as small or smaller than Mercury) at any density and with any internal configuration as long as it is self-bound gravitationally and/or electromagnetically. So we could replace Mercury with a small black hole or a "hot Jupiter" and the orbit would be identical.

The sun generates a perturbed Kerr metric that must take into account not just its rotation but the sun's non-sphericity (the "solar bulge" makes it slightly oblate). Given the metric (or a good enough approximation) we can solve the geodesic equation, and see that Mercury's orbit follows one of the generated geodesics to high precision.

In <http://dx.doi.org/10.1103/PhysRevLett.120.191101> Clifford Will writes: "Finally, at a purely pedagogical level, it is often stated that the relativistic perihelion advance of Mercury is really only a test of the vacuum Schwarzschild solution (or of the slow rotation limit of the vacuum Kerr solution, if one wishes to include the frame-dragging effect), since all the relativistic effects can be derived simply from those metrics." Note that Will's paper uses post-Newtonian corrections to the 2PN level, which is perfectly reasonable given how small Mercury's v/c^2 is, and the ease with which the approach deals with perturbations from the other planets. See also Will's 1986 book, chapter 5 of which is devoted to Mercury's orbit.

> Mercury is heavier when it is at perihelion ... because it's moving faster

"Moving faster" is not a frame-independent statement. We can always use a freely falling coordinate system where Mercury is always at the origin, or a freely-falling coordinate system where in the neighbourhood of a point Mercury experiences no acceleration against those coordinates. There are of course an infinite number of systems of coordinates in which Mercury, the Sun, or both accelerate(s) against that set of coordinates over the course of an Earth year. The point of relativity is that physics do not depend on a choice of coordinates.

"Heavier" is at best ambiguous, requires a lot of care in stating it covariantly (compare the stress-energy tensor), and is in any event irrelevant if the Strong Equivalence Principle holds. More technically, the backreaction of Mercury on the metric generated by the sun (or sun + other planets) is negligible.

If you explain what you mean by Mercury's "relativistic mass" and roughly the magnitude you think its change should be through Mercury's orbit, someone might be help clear up what is probably a misconception. Bear in mind that Mercury moves very slowly compared to c.

Finally, with respect to your last paragraph:

Mercury orbital eccentricity: 0.205630. Pluto orbital eccentricity: 0.2488. If Earth has no perihelion precession because of the low eccentricity (0.01671) of Eearth's orbit, and Mercury's perihelion precession is driven by its higher eccentricity, what do you think Pluto's perihelion precession should be: higher or lower than Mercury's? And why?

ETA: huh, sorry, this seems to have turned into something more like a blog entry than a brief self-correction.

Obvious editing errors in the last paragraph - fixes italicized: "If Earth has an order of magnitude smaller perihelion precession ... Earth's orbit". The first fix was meant to echo the parent's wording:

  "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"

With regard to that, here is a table of exactly and approximately solved perihelion advances of all eight planets and Pluto: <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9635960/table/t...>. [Sourced from an interesting (but as yet uncited, and weirdly published in TSWJ which let through some minor typos like "elliptik") paper in the category of "the expansion of the universe does not affect solar system orbits": <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9635960/> [*]].

Worth noting that Mars's orbit is highly eccentric at 0.0934, and its eccentricity is growing (as is Mercury's). Venus's orbit is much less eccentric at 0.0068. (Greater precision and eccentricities of the other planets can be found at <https://calgary.rasc.ca/orbits.htm>).

[*] Taking the geometric mean across significantly different calculated values of the cosmological constant using their method (which I have no problem with) is to me a mathematical trick too far. That averaging does arrive at the conventional order of magnitude of \Lambda in m^-s, but I wonder if that's coincidence. The first author is essentially a mathematician <https://orcid.org/0000-0001-9343-6062>, and whether the metric for the solar system must have a \Lambda term is much more a physical argument.

On that front, perhaps the authors should have cited another Iorio paper along with their [10], namely "Can solar system observations tell us something about the cosmological constant?" (2006) < https://scholar.google.com/citations?view_op=view_citation&h...> although tbf Iorio cites his earlier and more popular paper in [10].

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?

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

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.

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

The so called Rogue Planets.

There's a nice video from Kurzgesagt on the topic: https://youtu.be/M7CkdB5z9PY?si=0BwFwotoHi8PxL1f

If there is so many of these materials not necessarily planet, can we have this partially dark matter?

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.

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]"

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...

...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?

What if dark matter is primordial bowling balls!

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://iopscience...

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.”

Really dark bowling balls would still exhibit blackbody radiation

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.

> 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.

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.

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.

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.

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.

The first step is to propose a model that explains stuff. Then you test it on new predictions.

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.

Sometimes it's nontrivial to show that a theory predicts something, even if the thing is already observed.

MOND was there years before Batygin (co-author of planet Nine hypothesis) was born.

> 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.

"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.

The general theory was called planet X before pluto was demoted; X just means 10.

https://science.nasa.gov/solar-system/planet-x/

Except you and that article are quite wrong and the naming of the "Planet X" theory happened in 1894 long before the discovery of Pluto in 1930, back when astronomers also thought there was only 8 planets:

> In 1894, with the help of William Pickering, Percival Lowell (a wealthy Bostonian) founded the Lowell Observatory in Flagstaff, Arizona. In 1906, convinced he could resolve the conundrum of Uranus's orbit, he began an extensive project to search for a trans-Neptunian planet,[18] which he named Planet X, a name previously used by Gabriel Dallet.[11] The X in the name represents an unknown and is pronounced as the letter, as opposed to the Roman numeral for 10 (at the time, Planet X would have been the ninth planet).

https://en.wikipedia.org/wiki/Planets_beyond_Neptune#Planet_...

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.

MOND has successfully predicted:

- "no dark matter" in dense ellipticals and lenticular

- external field effect (including keplerian descent for the milky way)

- early galaxies

> 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.

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?

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.

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.

>> 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".

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.

> 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.

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.

> "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.

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".

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.

Ok so you are admitting that just as evolution is a poor model for human height, DM is a poor model for galaxy rotation curves.

Maybe my metaphor was too sloppy to illustrate the point, but I do not follow your logic here at all.

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?

Process is (laymans understanding):

1. you observe a galaxy doing something strange

2. whatever it does, you add just enough dark matter to account for that behaviour.

Such process seems fishy, because it can explain ANY observation.

If we would go back 130 years ago where GM was not a thing yet, and GM would be competing with DM theory, occams razor would point to DM, because it is simpler one - it fits with newton nicely.

(not an expert though, just repeating stuff I heard on youtube; I’m happy that experts work on all kinds of angles)

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"

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.

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.

They're not measurements. The measurements are rotation rates at various distances from the galactic center, then you plug that into a model and the model tells you where the DM is in that galaxy, then you say "DM explains it all for this galaxy!", but no, the amount and distribution of the DM is an output of the model and that cannot prove anything. There is never an explanation for why DM amounts and distribution vary so much. DM theory needs to make predictions we can then test, not produce model outputs.

What you are calling a model is simply a way of quantifying a measurement of a galaxy to describe (or model) its mass distribution. There is no physics involved.

> "DM theory needs to make predictions we can then test"

That is what scientists do: Start from the hypothesis of a dark matter dominated universe, from the beginning (or soon after the big bang), then turn on time and physics (gravity plus gas physics in a computer simulation), and galaxies form as gravity causes matter to clump together. The properties of those theoretical galaxies are testable predictions.

You're confusing the broad scale cosmological predictions of DM with explaining rotational parameters of individual galaxies.

Even so, there are predictions low-parameter DM models seemingly can't make, like: what percentage of galaxies have zero dark matter?

If you were to create any model of the universe, you would require it to have the correct (observed) percentage of galaxies without dark matter, right?

You assert that it is impossible for a model of a dark matter dominated universe to "predict" galaxies without dark matter, but that is exactly what people are looking at here in a large scale cosmological simulation: https://arxiv.org/abs/2202.05836

> 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.

Perhaps you meant...

DM = Dark Matter, BAO = Baryon Acoustic Oscillations, CMB = Cosmic Background Radiation, MOND = Modified Newtonian Dynamics

To put some sources to that unsubstantiated "data", it's about the Bullet Cluster specifically.

https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to...

It fits the data best because they tweak a lot of different parameters to obtain the fit. MOND only has one parameter.

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?

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

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.

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.

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.

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.

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?

That doesn't make any sense at all unless they simultaneously believe that DM is actually the right answer.

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.

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?

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.

The real issue is that you need a lot of money to test DM for real, but not so much to test MOND.

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?

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.

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?

The analogy is a teensy bit strained but you are substantively correct.

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.

What? Quantum physics definitely adds new terms…

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?

If MOND were a single theory such ideas would be more interesting.

You could say the same about dark matter.

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.

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?

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.

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.

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.

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 3×1017 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.

Radar faces the same inverse fourth power effect.

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

Is it fair to say it is easier to detect planets in other solar systems?

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.

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?

When a star is occluded by a planet, it'll dip in brightness at a pretty regular interval (because of the orbit). So you can look look for a series of dips, which gives you a candidate. That's then looked at more closely.

Since a planet 9 wouldn't transit a star at any regular interval, it wouldn't show up as a candidate for that search technique.

So we would need to potentially deliver the telescope far outside of our solar system and have it look back at us, kind of like a very expensive selfie stick?

That's not possible. The furthest things we've ever sent out are the voyager probes, and they're 135 and 162au away, each. We'd have to get twice that distance to even be at the distance that we expect a planet 9 to be.

Even if we managed to get many multiples of the distance planet 9 is from the sun to take pictures backward and send them home to earth, it's _so far_ from the sun that a) the amount it would occlude the sun is ridiculously small, if it ever even lined up. At that distance, the sun just looks like another star in the sky. That's like waiting for one particular asteroid to block out the North Star. And b) the orbit period would still need to be short enough that you could see it transit a few times to use the technique that scientists use to detect planets elsewhere. Just seeing something block the sun doesn't tell you what it was that blocked it, especially at 400-800au distance. Plus, then you're talking about latencies of about a light month, which makes it super hard to even know to look for the thing you saw (a month ago).

With our current, but especially decades ago capabilities, it's (was) similar to Russell's teapot problem.

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.

Need some context like evidence. The article mentioned but I think you need more elaboration. Only recall the guy killing Pluto said as a consolation he gave us another planet.

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.

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.

And that's good: anything that has a theoretical basis is definitely not new physics. And we are talking about new physics here. The same could be said about special relativity or quantum mechanics in the past.

Could Barnard's Star Kill Planet Nine?

https://www.listennotes.com/podcasts/the-demystifysci/great-...

Pluto is a planet ( /s but not really)

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.

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.

NASA still has a page where that's the primary term: https://science.nasa.gov/solar-system/planet-x/

That’s only in the context of ‘x’ being unknown. They’re still referring to it as ‘planet nine’ in the article.

> Batygin and Brown nicknamed their predicted object "Planet Nine," but the actual naming rights of an object go to the person who actually discovers it. The name used during previous hunts for the long suspected giant, undiscovered object beyond Neptune is "Planet X."

It's the same thing. These researchers just chose a different name. And the article does use "planet X" more than "planet 9"/"planet nine".

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.

That’s what the "dwarf planet" term is.

they prefer the term "planets with dwarfism"

Fine, but then so is Eris

Ceres to beltalowda!

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.

True, but people who make sense and people who have effective PR machines are not always the same.

Wasn't this a Rick and Morty episode?

Dr Becky explaining the MOND theory and why it's invalid --> https://www.youtube.com/watch?v=HlNSvrYygRc

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.

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

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.

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.

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.

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.

> 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.

Yes, I meant in "parameter space" :-)

... so just like supersymmetry and string theory?

Yes, just like that.

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.

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.

Yea but YouTube personalities were never appealed to to find validation amount public opinion.

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.

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".`

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.

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.

AFAIK spreading interest in the field through social media was/is part of her post doc job goals

She includes references in the description, in case you need to personally double check the content.

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.

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.

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.

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.

True, even without GR the same issue would exist with Newtonian gravity.

Blows my mind that MOND still gets attention.

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.

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