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Refer to this question: How would the rotational and orbital mechanics differ on a torus-shaped planet as compared to a spherical planet?

This is asking about torus-shaped planet rotational and orbital mechanics.

To me this is a question can be answered completely using Newtonian physics, and is as mainstream as the mechanics of spherical planet, however it was closed because "we deal with mainstream physics here", implying that somehow, Newtonian physics doesn't apply to torus shaped planet.

I don't see the logic of the closure. Care to explain?

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  • $\begingroup$ In my opinion, that's a misuse of the close reason (which unfortunately happens a lot around here). It should maybe have been closed as too vague, because it's asking for a poorly defined list of differences. $\endgroup$ – DanielSank Feb 6 at 23:20
  • $\begingroup$ Newtonian physics surely apply, but asking the question as "how would it differ" is dishonest because Newtonian physics tells me that such an object cannot exist. Pay attention here: not just does not exist in nature, but cannot exist in nature! $\endgroup$ – Kostas Feb 8 at 17:28
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Demanding that questions only deal with situations that unequivocally can occur in the real world rules out objects on infinite planes, point masses, Einsteinian trains, Maxwell's demon, supersymmetry, superstring theory, and any of the other standard thought experiments that form the underpinnings of a standard physics education. The ability of applying physics rigorously to unusual or hypothetical scenarios is one of the most rewarding and valuable aspects of physics.

I think there is a serious risk that the site will lose readers and intellectual vigour if it takes a too narrow view of what constitutes "mainstream physics".

The proper distinction should not be the subject of the question but whether it can be answered using mainstream physics. The physics of what happens if Earth turns into blueberries is totally mainstream, whether it could turn to blueberries is non-mainstream. We can all agree on that torus-shaped planets are unlikely, yet analyse their gravitational fields and orbital trajectories with the same means we do with any other mass distribution.

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  • $\begingroup$ For the record, Maxwell's demon unequivocally can be implemented in the real world, see e.g. science.sciencemag.org/content/354/6315/1021.abstract (although this group was the first to call what was essentially the same technique an implementation of Maxwell's demon nature.com/articles/s41586-018-0458-7) $\endgroup$ – aquirdturtle Jan 30 at 5:23
  • $\begingroup$ Exactly, thanks for your clear answer. You are less emotional than me on this $\endgroup$ – lurscher Feb 2 at 5:25
  • $\begingroup$ Would you allow Everyone Dies™ scenarios? $\endgroup$ – Peter Mortensen Feb 2 at 7:35
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    $\begingroup$ Although I agree that this is how the mainstream policy is applied, I believe this doesn't address other issues with the question, such as how unspecific it is. $\endgroup$ – JMac Feb 2 at 17:44
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    $\begingroup$ @JMac - I don't think the question is particularly unspecific. It just asks to characterise how motion near such a mass distribution behaves. Would asking to characterise motion in the 2-body problem be out of place here? Surely not. We might prefer questions that are by their nature a bit more closed, but many important physics questions are fairly open-ended. $\endgroup$ – Anders Sandberg Feb 2 at 19:33
  • $\begingroup$ @PeterMortensen - Everyone Dies™ is a final state of a physics scenario; the appropriateness of the question is about whether it can be answered, not how survivable the answer would be. Although I would recommend against giving people ideas for superweapons. $\endgroup$ – Anders Sandberg Feb 2 at 19:35
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    $\begingroup$ @anders many important physics questions are fairly open-ended. True to some degree, but that does not mean that such questions could be asked in this venue. We have a limited scope compared to the scope of all of physics. This particular question (which you haphazardly answer with "yes orbits are different") is too broad of a question for the Q&A format that we have on this site. $\endgroup$ – Kyle Kanos Feb 3 at 21:29
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I agree that it's not non-mainstream, but I think the question is overly broad, asking what the "rotational motion" would be in such a place. This is not or less going to require a very lengthy response, probably chapter-of-a-textbook kind of answer to address the question.

I'm also of the opinion that only OP should be the one make changes to reduce the broadness, as such a reduction of topicality significantly alters the question asked, so we can only request in the comments the change.

Note also that the given answer basically says, "The projectile motion does change. Here, check out my blog where I talk about this topic!!" And the blog post is not terribly short either, indicating much can be said on the topic. Hence, fairly broad request.

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    $\begingroup$ You beat me to it: the OP is basically asking someone else to do much of the work and answer an overly broad question. BTW the answer is about a ring-shaped planet, which is not what comes to my mind when I think of a torus. $\endgroup$ – ZeroTheHero Jan 28 at 14:10
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    $\begingroup$ Yeah, I don't understand why it's closed for its current reason; but just one look at it and it's clear they are looking for way too much. Basically "explain everything about orbital dynamics that would change with a torus compared to a sphere". The scope of that is insane. $\endgroup$ – JMac Jan 28 at 14:54
  • $\begingroup$ I've removed a comment discussion which contained inappropriate comments from multiple users. $\endgroup$ – rob Feb 4 at 20:58
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I think it's an acceptable question and I've cast a reopen vote, since it appears to be asking what mainstream descriptions would make of a hypothetical idea to understand how the dynamics of such a body would be.

For reference, similar questions are also a good fit for Worldbuilding SE (see this question and the linked posts); they have a 'nonspherical-planet' tag too.

That being said, I don't think "answered completely using Newtonian physics", the phrase used in the question, is appropriate justification for a claim that a question is not non-mainstream. For example, someone could pitch a theory of their own which completely violates Newtonian mechanics (at orders of magnitude where Newtonian mechanics is correct), in which case an 'answer' would be a description of Newtonian mechanics; such a question would be off-topic.

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    $\begingroup$ "Someone could pitch a theory of their own"...then that would be automatically non-mainstream as we don't deal with personal theories here (at least not those which haven't been published in reputable peer-reviewed journals). On the other hand, Newtonian mechanics is completely mainstream and asking for an explanation of a physical situation (whether real or imaginary), without pitching for a personal theory, within the domain of Newtonian mechanics, sounds perfectly on-topic to me. $\endgroup$ – Blue Feb 2 at 7:15
  • $\begingroup$ @Blue Yep, that's why i cast a reopen vote in this case. Does the answer somehow imply otherwise? $\endgroup$ – user191954 Feb 2 at 9:19
  • $\begingroup$ Nope, just that at the first reading your first sentence seems to contradict the rest of your post. You might want to change the language a bit. Otherwise, it's fine. :) $\endgroup$ – Blue Feb 2 at 11:04
  • $\begingroup$ @Blue Sure, I've tried reorganising it and emphasizing that I'm discussing a counter example in that section. $\endgroup$ – user191954 Feb 2 at 12:30
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Torus-shaped planets do not exist. A planet-sized object has sufficient self-gravity to pull itself into an approximately spherical shape. So you are asking about a situation that is known to be contrary to the laws of physics.

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    $\begingroup$ I'm not too sure whether you read the comments here: "And there is a biggish literature in astrophysics on orbits related to massive torii in AGNs, so torus shaped mass distributions are also mainstream. " $\endgroup$ – Graviton Jan 28 at 10:49
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    $\begingroup$ Periodic solutions of the $N$-body problem also do not exist in the real world, but that does not preclude us from studying the properties of the problem within the standard frameworks of newtonian physics. Regardless of whether torus-shaped mass distributions could arise in the real world, there's plenty of on-topic physics to ask regarding the gravitational field that they produce. (Admittedly the question phrasing is sub-optimal, but the completley-on-topic answer shows how it can go.) $\endgroup$ – Emilio Pisanty Jan 28 at 11:23
  • $\begingroup$ @EmilioPisanty right but given there is $0$ known planets or exoplanets of toroidal shape one can reasonably expect a better motivated question, especially given that such a toroidal shape would be quite unexpected for any sizeable object. IMO borderline as "non-mainstream" but plenty of valid reasons to ask for improvements through edits. $\endgroup$ – ZeroTheHero Jan 28 at 14:39
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    $\begingroup$ @ZeroTheHero There's plenty of on-topic stuff to be said, starting with "what are newtonian orbits like in the gravitational field of a toroidal mass distribution", and I don't see how the configuration is any more unreasonable than e.g. the electric field of an infinite line charge. (The last time I checked with the lab, they didn't have any of those available for testing.) Close it as Too Broad, if you want to, but it's a perfectly mainstream question. $\endgroup$ – Emilio Pisanty Jan 28 at 14:46
  • $\begingroup$ @EmilioPisanty I see the system remembers my vote from way back when, and its "too broad". Anyways there is currently one VTC, it's mine and it's because the question is too broad. $\endgroup$ – ZeroTheHero Jan 28 at 19:40

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