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Could Modified Gravity Kill Planet Nine?

nautil.us

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Re: Could Modified Gravity Kill Planet Nine?

#81

Post by MattPalmer1086 »

karmakaze wrote: post #63
MattPalmer1086 wrote: post #52

Earlier quoted context omitted.

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…

It's simpler to say Newtonian gravity and visible matter alone is sufficient to explain the need for dark matter. Adding GR to the picture doesn't take away the need for dark matter to account for observations.

True, even without GR the same issue would exist with Newtonian gravity.

Re: Could Modified Gravity Kill Planet Nine?

#82

Post by Gooblebrai »

perihelions wrote: post #80
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…

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

The so called Rogue Planets.

There's a nice video from Kurzgesagt on the topic: https://youtu.be/M7CkdB5z9PY?si=0BwFwotoHi8PxL1f

Re: Could Modified Gravity Kill Planet Nine?

#84

Post by perihelions »

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.

Microlensing events! (to be slightly more precise)

https://en.wikipedia.org/wiki/Rogue_planet#Microlensing

- "They found 474 incidents of microlensing, ten of which were brief enough to be planets of around Jupiter's size with no associated star in the immediate vicinity. The researchers estimated from their observations that there are nearly two Jupiter-mass rogue planets for every star in the Milky Way.[26][27][28] One study suggested a much larger number, up to 100,000 times more rogue planets than stars in the Milky Way, though this study encompassed hypothetical objects much smaller than Jupiter.[29]"

Re: Could Modified Gravity Kill Planet Nine?

#85

Post by mannykannot »

perihelions wrote: post #80
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…

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.

Re: Could Modified Gravity Kill Planet Nine?

#86

Post by mannykannot »

A seemingly obvious question is whether dark matter could provide an equivalent result. I'm not really sure, but the paper's authors seem to be saying 'no' in this passage:

Equations (B2), (B5), and (B7) are the main results of this section. They provide an expression for the phantom mass that sources anomalous effects in the inner solar system. As discussed above, these effects are absent in Newtonian gravity and hence absent in any dark matter model.

Perhaps someone more knowledgeable could comment on this?

Re: Could Modified Gravity Kill Planet Nine?

#87

Post by mr_mitm »

nicktelford wrote: post #69
pfdietz wrote: post #60

Earlier quoted context omitted.

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.

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 essentially the same problem with Dark Matter though? They keep looking for it, not finding it and proclaiming "well, it must be somewhere else!".

No (unless you mean "where" in parameter space), we have a pretty good idea where it is thanks to gravitational lensing surveys. We don't what it is.

Re: Could Modified Gravity Kill Planet Nine?

#88

Post by kmm »

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…

That doesn't sound right. Relativistic mass as a concept is avoided in modern physics because it doesn't yield much insight and is hard to keep consistent. Involving gravity only makes it worse, because it's very hard to consistently include gravity in special relativity. For one, does the relativistic mass gravitate or not?

And if you actually do go through the calculations, you find that you do not get the observed result. This paper[0] sums up a few of them.

0 (PDF): http://kirkmcd.princeton.edu/examples/perihelion.pdf

Re: Could Modified Gravity Kill Planet Nine?

#89

Post by AnimalMuppet »

mr_mitm wrote: post #54
AnimalMuppet wrote: post #51

Earlier quoted context omitted.

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…

That's at the universe-sized level. At the galaxy level, as you state, we say "oh, that galaxy has almost no dark matter". That's a per-galaxy parameter. At the Bullet Cluster, we say "the dark matter must be here and here". That's a point-by-point distribution.

Re: Could Modified Gravity Kill Planet Nine?

#90

Post by _petronius »

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…

The gravitational effect of a lot of spread out mass outside the Oort cloud (or indeed, a lot more mass in the Oort cloud) would have a different effect on the orbits of the planets than a single planet that we had not found yet.

So for some known configuration of orbits of the known planets, there are a limited number of solutions (in terms of mass, inclination, eccentricity, etc.) that you could add to the gravitational interactions of the solar system and still have the current orbits we observe. That gives a reasonable guess about what (another planet) and where (in the sky) to look for to explain anomalies.

But with distant objects that don't emit their own light, even having a good guess of a bunch of the orbital parameters doesn't mean it is easy to find, because you don't have much sunlight reflecting off of it, and the chances it will occlude something else brighter (like a background star) are needle-in-a-haystack level.

Even _if_ you knew the _exact_ orbit you were looking for, there are 360 degrees of sky to search for a tiny, dark object, because you don't necessarily know where in the orbit the object currently is.

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