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Could Modified Gravity Kill Planet Nine?

nautil.us

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Re: Could Modified Gravity Kill Planet Nine?

#91

Post by nicktelford »

mr_mitm wrote: post #87
nicktelford wrote: post #69

Earlier quoted context omitted.

Isn't this essentially the same problem with Dark Matter though? They keep looking for it, not finding it and proclaiming "well, it must be somewhere else!". I always got the impression that when Dark Matter was initially labelled as such, it was just a name for the discrepancy between theoretical models and observations; and that the name itself seems to have driven this idea that it's the observations that are wron…

> that our observations are incomplete and our models are wrong. I'd say that's a given regardless of the DM mystery. It's consensus that QM and GR are incompatible and that we need a new theory out of which both of these come out as a special case. String theory was considered a hopeful contender for that for a while. And that we haven't observed everything to a satisfying degree yet should be obvious. > Isn't this…

Yes, I meant in "parameter space" :-)

Re: Could Modified Gravity Kill Planet Nine?

#93

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…

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"

Re: Could Modified Gravity Kill Planet Nine?

#94

Post by ryandrake »

ano-ther wrote: post #78
vld_chk wrote: post #73

Whenever I read about Planet Nine search, I have a very naive (as a non-physicist) childish question: if we detect anomaly in our Solar System just less than a decade ago and can’t find anything visual which explains it, assuming there is giant planet/piece of ice floating somewhere at the edge of Solar System, how we are sure that there are not a lot of such objects and real space is not as “empty” as we think it is…

Very good question. As a non-astronomer, my guess is that we don’t observe a lot of transient black outs for example of the milky way. Which would happen if there’s a lot of these around. That gives you an upper bound of how many objects like that exist.

...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?

Re: Could Modified Gravity Kill Planet Nine?

#95

Post by ithkuil »

mannykannot wrote: post #85
perihelions wrote: post #80

Earlier quoted context omitted.

The simple answer is that there are actually very large numbers of objects in the space between stars that aren't detectable. Including, certainly, planet-sized objects that formed in stellar environments, escaped, and now fly freely in between. https://en.wikipedia.org/wiki/Rogue_planet

Indeed - and the Oort cloud itself extends to about 1.5 light-years, which is a substantial fraction of the ~ 4 light-year distance to our nearest neighboring stars.

That's even more so substantial if the nearest neighboring star also has a comparable oort cloud. In that case a mere 1ly of interstellar space would separate the two systems

Re: Could Modified Gravity Kill Planet Nine?

#96

Post by mr_mitm »

throwawaymaths wrote: post #93
mr_mitm wrote: post #32

Earlier quoted context omitted.

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…

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.

Re: Could Modified Gravity Kill Planet Nine?

#97

Post by vundercind »

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.

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.

Re: Could Modified Gravity Kill Planet Nine?

#98

Post by mike_hock »

ryandrake wrote: post #94
ano-ther wrote: post #78

Earlier quoted context omitted.

Very good question. As a non-astronomer, my guess is that we don’t observe a lot of transient black outs for example of the milky way. Which would happen if there’s a lot of these around. That gives you an upper bound of how many objects like that exist.

...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!

Re: Could Modified Gravity Kill Planet Nine?

#99

Post by canjobear »

vikingerik wrote: post #46
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 l…

If that’s true then it should be possible to predict the precession from special relativity alone.

Re: Could Modified Gravity Kill Planet Nine?

#100

Post by canjobear »

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.
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