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Could Modified Gravity Kill Planet Nine?

nautil.us

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Re: Could Modified Gravity Kill Planet Nine?

#151

Post by throwawaymaths »

isthatafact wrote: post #132
AnimalMuppet wrote: post #127

Earlier quoted context omitted.

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 pa…

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.

Re: Could Modified Gravity Kill Planet Nine?

#152

Post by bastawhiz »

catlifeonmars wrote: post #146
bastawhiz wrote: post #136

Earlier quoted context omitted.

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).

Re: Could Modified Gravity Kill Planet Nine?

#153

Post by isthatafact »

throwawaymaths wrote: post #150
isthatafact wrote: post #148

Earlier quoted context omitted.

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…

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

Re: Could Modified Gravity Kill Planet Nine?

#154

Post by plank »

sigmoid10 wrote: post #142
plank wrote: post #126

Earlier quoted context omitted.

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 gra…

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 circula…

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

Re: Could Modified Gravity Kill Planet Nine?

#155

Post by sigmoid10 »

plank wrote: post #154
sigmoid10 wrote: post #142

Earlier quoted context omitted.

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 circula…

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 j…

>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

Re: Could Modified Gravity Kill Planet Nine?

#156

Post by lamontcg »

Izkata wrote: post #125
lamontcg wrote: post #119

Earlier quoted context omitted.

"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_...

Re: Could Modified Gravity Kill Planet Nine?

#157

Post by isthatafact »

throwawaymaths wrote: post #151
isthatafact wrote: post #132

Earlier quoted context omitted.

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.
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