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Could Modified Gravity Kill Planet Nine?

nautil.us

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Re: Could Modified Gravity Kill Planet Nine?

#41

Post by isthatafact »

throwawaymaths wrote: post #30
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.

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".

Re: Could Modified Gravity Kill Planet Nine?

#42

Post by tsunamifury »

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…

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.

Re: Could Modified Gravity Kill Planet Nine?

#43

Post by pfdietz »

gray_-_wolf wrote: post #40
Qem wrote: post #33

Earlier quoted context omitted.

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.

Radar faces the same inverse fourth power effect.

Re: Could Modified Gravity Kill Planet Nine?

#44

Post by the__alchemist »

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.

Re: Could Modified Gravity Kill Planet Nine?

#45

Post by Qem »

gray_-_wolf wrote: post #40
Qem wrote: post #33

Earlier quoted context omitted.

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.

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

Re: Could Modified Gravity Kill Planet Nine?

#46

Post by vikingerik »

lolinder wrote: post #22

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.wikiped…

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.

Re: Could Modified Gravity Kill Planet Nine?

#47

Post by pwdisswordfishc »

FrustratedMonky wrote: post #12
nubinetwork wrote: post #4

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.

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

Re: Could Modified Gravity Kill Planet Nine?

#48

Post by AnimalMuppet »

tsunamifury wrote: post #42
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…

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.

Re: Could Modified Gravity Kill Planet Nine?

#49

Post by antonioevans »

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.

[flagged]

Re: Could Modified Gravity Kill Planet Nine?

#50

Post by bastawhiz »

gray_-_wolf wrote: post #40
Qem wrote: post #33

Earlier quoted context omitted.

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.

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