I asked this question on the basis that it's a question about physics of a fictional universe, but is entirely grounded in real physics based on the following rules.

Questions about physics of fictional worlds which are not sufficiently grounded in real physics are off topic here.

The top voted answer here suggests it should be on-topic

But it was closed as 'not mainstream physics.' I thought orbital mechanics were very much mainstream physics so I don't understand this closure; Does this rule preclude me from asking any questions about hypothetical scenarios within our universe such as this?

If I re-word it to "Could an object in a 1 AU orbit from the sun reach mars with only 460m/s of thrust available in a single near-instantaneous burst?" would it be re-opened? That doesn't fundamentally change the current answer, but I don't know enough to know if it is actually an equivalent question, or does it oversimplify it?


3 Answers 3


I will say that I don't think your fictional scenario itself is sufficient for closure, but I do think in this specific instance there is still some troubles with it.

The other meta post you link to seems more concerned with actual published works of fiction that still has well defined rules and things that happened that other people can go look at. The physics in the questions discussed there are assumed to be the same physics we all know and love, it's just in the context of fiction.

In your case you have made up your own fictional scenario that starts off with something completely unphysical. In general, the issue is that this can often involve assumptions that aren't clear, are wrong, or open the door for nonsensical answers. Assumptions might also not be complete, thus forcing answerers to fill in the gaps can causing the question to not have a unique answer. To quote Stanley Hudson:

How 'bout make believe land has anything you want.

In your case some of the fiction is confusing. You said the Earth has disappeared, but you are talking about Earth escape velocities, which wouldn't be relevant if the Earth disappeared. Are you wanting to consider the alterations to the orbit of Mars due to the disappearance of the Earth? Or are you assuming in this scenario that also Mars continues on it's same path?

If you really are interested in real physics, then I would try to find an actual real example to set up your question. This is because you don't need fiction if your question didn't start in fiction (like the other meta post describes). When possible, just stick with what is real so that the question can be as clear as possible and everyone is working with the same physics.

Why not just ask about a possible scenario of an astronaut on a spaceship in a sun-planet system that is wanting to launch objects onto the planet? Then you don't need to bring in disappearing planets.

Although, in my opinion the question is really just asking for a calculation to be done, as evident by the current single answer. So part of me feels like the question would need to stay closed anyway. It seems like you know the physics already, you just want a calculation to be done. You aren't asking about how gravity, escape velocity, orbits, etc. work in the context of physics.


I think this SE has an overly trigger-happy closing policy for questions like this, that does have interesting physics content even though they start out by setting up a crazy scenario.

The concern that making unphysical initial assumptions will make answers arbitrary depends strongly on what the initial assumptions and questions are. The perennial "what if the sun disappeared, would Earth keep on orbiting for 8 minutes?" question really deals with issues of the speed of propagation of disturbances in gravitational fields. Asking why FTL implies time travel is a way of understanding special relativity on a deeper level. The details of how the sun disappeared or the FTL drive are unimportant.

"Mainstream physics" involves not just idealized cases (frictionless planes, free particles) and toy models (various Lagrangians) that in many cases behave quite differently from reality (a solitary hydrogen atom at finite temperature is not stable due to a divergent partition function: stability requires a less idealized case) but also many established theoretical models (MWI, superstring theory, MOND, nonlinear QM) often have parameters weakly constrained by evidence and wild implications. Indeed, tracing out the wild implications and using them as arguments for or against the theories (e.g. nonlinear QM is disfavored because the radical consequences). There is no clear line between "serious mainstream theory" and a "too wild theory"; consider the gradual journey of black holes from argument against GR to mainstream theory (and then empirical fact), or the iffy status of objects like neutrino stars.

I think the closing criterion needs to be that possible answers depend too much on scenario assumptions to be of general interest (and answerable). And we need to help people asking questions to reframe their questions into an answerable form rather than sniffly close the door to curiosity.

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    $\begingroup$ Yeah, a lot of users on this site seem to get unnecessarily "upset" when a question brings up disappearing planets, despite the question being about what happens after the disappearance. $\endgroup$ Commented Aug 18, 2020 at 13:04

Could an object in a 1 AU orbit from the sun reach mars with only 460m/s of thrust available in a single near-instantaneous burst?

Yes, that would be an acceptable, well-posed question. It is extremely important that you be clear exactly what velocity your object starts with (is it travelling at the standard orbital speed of the Earth, 29.78 km/s? or is it stationary, i.e., at the top of a parabolic arc that starts and ends in the Sun?) and what the conditions are for the thrust. But if you do that, it's an on-topic question.

It is also acceptable to include in the question, either at the start or at the end, a section describing the motivation for why you're interested in that calculation. This is where the "Earth vanishes" scenario can be mentioned. (But note that your description of the scenario is currently too vague to be helpful.)

However, it is crucial that the two parts (non-mainstream motivation, and mainstream actual question) be clearly delineated. If you mix the two then that's where the non-mainstream closures come in.

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    $\begingroup$ IMO the proposed fix just asks for someone to do a calculation and is off topic. What do you think difference is between the proposed question and an off topic "do this calculation" question? $\endgroup$ Commented Aug 17, 2020 at 13:14

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