Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Could Modified Gravity Kill Planet Nine?

nautil.us

Post reply on HN
Print view View original

51–60 of 157 posts

Re: Could Modified Gravity Kill Planet Nine?

#51

Post by AnimalMuppet »

isthatafact wrote: post #41
throwawaymaths wrote: post #30

Earlier quoted context omitted.

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 bes…

>> 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.

Re: Could Modified Gravity Kill Planet Nine?

#52

Post by MattPalmer1086 »

the__alchemist wrote: post #44
antonioevans wrote: post #7

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

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.

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.

Re: Could Modified Gravity Kill Planet Nine?

#53

Post by MattPalmer1086 »

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.

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

Re: Could Modified Gravity Kill Planet Nine?

#54

Post by mr_mitm »

AnimalMuppet wrote: post #51
isthatafact wrote: post #41

Earlier quoted context omitted.

>> 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.

Re: Could Modified Gravity Kill Planet Nine?

#55

Post by mr_mitm »

MattPalmer1086 wrote: post #53
mr_mitm wrote: post #11

Earlier quoted context omitted.

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.

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?

Re: Could Modified Gravity Kill Planet Nine?

#56

Post by bena »

mr_mitm wrote: post #13
bena wrote: post #6

Earlier quoted context omitted.

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.

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.

Re: Could Modified Gravity Kill Planet Nine?

#57

Post by adrian_b »

GuB-42 wrote: post #36
lolinder wrote: post #16

Earlier quoted context omitted.

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 abou…

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.

Re: Could Modified Gravity Kill Planet Nine?

#58

Post by MattPalmer1086 »

mr_mitm wrote: post #55
MattPalmer1086 wrote: post #53

Earlier quoted context omitted.

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

Re: Could Modified Gravity Kill Planet Nine?

#59

Post by tsunamifury »

AnimalMuppet wrote: post #48
tsunamifury wrote: post #42

Earlier quoted context omitted.

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.

Re: Could Modified Gravity Kill Planet Nine?

#60

Post by pfdietz »

adrian_b wrote: post #57
GuB-42 wrote: post #36

Earlier quoted context omitted.

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 abou…

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 obser…

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.

Post reply on HN