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Could Modified Gravity Kill Planet Nine?

nautil.us

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Re: Could Modified Gravity Kill Planet Nine?

#101

Post by throwawaymaths »

mr_mitm wrote: post #96
throwawaymaths wrote: post #93

Earlier quoted context omitted.

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.

Re: Could Modified Gravity Kill Planet Nine?

#102

Post by catlifeonmars »

samus wrote: post #74
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?

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…

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?

Re: Could Modified Gravity Kill Planet Nine?

#103

Post by catlifeonmars »

Qem wrote: post #33
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?

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.

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

Re: Could Modified Gravity Kill Planet Nine?

#104

Post by throwawaymaths »

mr_mitm wrote: post #32
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…

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…

> 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.

Re: Could Modified Gravity Kill Planet Nine?

#105

Post by catlifeonmars »

canjobear wrote: post #100
sspiff wrote: post #29

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.

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.

Re: Could Modified Gravity Kill Planet Nine?

#106

Post by iraqmtpizza »

pwdisswordfishc wrote: post #47
FrustratedMonky wrote: post #12

Earlier quoted context omitted.

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.

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

they prefer the term "planets with dwarfism"

Re: Could Modified Gravity Kill Planet Nine?

#107

Post by semi-extrinsic »

mike_hock wrote: post #98
ryandrake wrote: post #94

Earlier quoted context omitted.

...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...

Re: Could Modified Gravity Kill Planet Nine?

#108

Post by Izkata »

Arech wrote: post #37
sspiff wrote: post #29

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.

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.

Re: Could Modified Gravity Kill Planet Nine?

#110

Post by juped »

vundercind wrote: post #97
FrustratedMonky wrote: post #12

Earlier quoted context omitted.

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.

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.
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