Could Modified Gravity Kill Planet Nine? • Hacker News Board

Re: Could Modified Gravity Kill Planet Nine?

#21

Post by lisper » Sat, Mar 02, 2024, 1:36 PM UTC

GordonS wrote: post #8

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?

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

Re: Could Modified Gravity Kill Planet Nine?

#22

Post by lolinder » Sat, Mar 02, 2024, 1:39 PM UTC

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)

Re: Could Modified Gravity Kill Planet Nine?

#23

Post by beautifulfreak » Sat, Mar 02, 2024, 1:40 PM UTC

mr_mitm wrote: post #11

nerbert wrote: post #2

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.

Perhaps you meant...

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

Re: Could Modified Gravity Kill Planet Nine?

#24

Post by lupire » Sat, Mar 02, 2024, 1:43 PM UTC

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?

Re: Could Modified Gravity Kill Planet Nine?

#25

Post by eigenspace » Sat, Mar 02, 2024, 1:44 PM UTC

Blows my mind that MOND still gets attention.

Re: Could Modified Gravity Kill Planet Nine?

#26

Post by moffkalast » Sat, Mar 02, 2024, 1:45 PM UTC

mr_mitm wrote: post #11

nerbert wrote: post #2

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.

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

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

Re: Could Modified Gravity Kill Planet Nine?

#27

Post by dudeinjapan » Sat, Mar 02, 2024, 1:47 PM UTC

[flagged]

Re: Could Modified Gravity Kill Planet Nine?

#28

Post by Tuna-Fish » Sat, Mar 02, 2024, 1:48 PM UTC

lupire wrote: post #24

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.

Re: Could Modified Gravity Kill Planet Nine?

#29

Post by sspiff » Sat, Mar 02, 2024, 1:49 PM UTC

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.

Re: Could Modified Gravity Kill Planet Nine?

#30

Post by throwawaymaths » Sat, Mar 02, 2024, 1:50 PM UTC

mr_mitm wrote: post #11

nerbert wrote: post #2

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.