From help-center:

Questions that are purely hypothetical, for example a question such as 'Could a black hole destroy the universe' or 'What if our solar system had two suns' (however, feel free to visit World Building) [are off-topic]

From my understanding, a "hypothetical question" is a question discussing events/observation not possible in real universe or an imaginary event (suppose gravity stops for 1 sec, what will happen? or suppose Moon vanishes, what will happen to Earth?), imaginary scenarios (stepping on sun). Many such question starts with "What if", "What would happen" or "Suppose" and they should be closed based on the above rule (however not all question are technically hypothetical). But, there is an inconsistency in closing "hypothetical question". While some are closed as "opinion-based" or "not within the scope", some are open and has answers. I don't mind answers on those question but shouldn't it count as "hypothetical questions" and thus should be closed?

I did some look-up and listed some of the questions:

  1. What would happen if (imaginary tennis ball colliding with sun- closed)
  2. Would an object falling into a black hole ever collide with the surface of the black hole? (elephants falling in black hole - open)
  3. What would happen if a medium -sized black hole passed the Earth? (open)
  4. What would happen if we stepped on the Sun? (open)
  5. What if the Sun turned into a red giant tomorrow? (closed)
  6. What would happen if a spaceship goes faster than the speed of light? (closed)
  7. What if one teleportes to a star that is some light years away? (closed)
  8. What would happen if the Sun, Moon and Earth were isolated? (open)
  9. What would happen if a planet is removed from the solar system? (open)
  10. What would happen to the Galilean moons and Titan if Jupiter and Saturn disappeared? (open)

...and there are many more

Now there are posts discussing about scenarios that are not technically "hypothetical" and can be possible if done in the near future. For e.g. "In the near future, a space probe is launched and is tasked to release ice cubes and water in space, what will happen?" (post 1, post 2).

Question: What exactly counts as a "hypothetical question"?

Related but different: Thought experiment questions that aren't completely scientific?

  • $\begingroup$ This is a great question! I think that many thought experiment questions can be useful and should stay open. Einstein certainly found such exercises helpful! This will be hard to figure out but it's probably worth it. $\endgroup$
    – uhoh
    Aug 14, 2021 at 14:38

1 Answer 1


In my view it is a question about something that could not happen, but I would not vote-to-close purely on that basis.

From your list, that applies to (I think) 1, 4, 5, 6, 7, 8, 9, 10.

Some of them, particularly 8, 9, 10 are salvageable by changing the question to be more general rather than about our Solar System (since there are so many other "solar systems" out there, that particular configurations almost certainly occur somewhere).

2 is just a misapprehension that requires a simple answer, 3 is hypothetical but perfectly possible (so I would allow it).

There is though another batch of "hypothetical questions" that I really like - these are the ones that ask how a particular physical situation would develop if you change the initial conditions or alter some particular aspect of the (astro)physics. Some of these are possible, but some aren't or are exceedingly unlikely. Neverthless the reason I like them, and would be sad to see them disallowed, is that they encourage a much deeper understanding of what is going on and for that reason I often like to set these as problems for students or even as examination questions.

Examples that I have answered would include:

Would we have more than 8 minutes of light, if the Sun "went out"?

How large can a ball of water be without fusion starting?

If Alpha Centauri A's solar system exactly mirrored our own, what would we be able to detect?

What would the night sky look like if the Milky Way were the only galaxy in the universe?

Would a tablespoon of a neutron star remain intact?

What would the night sky look like if the interstellar medium didn't exist to absorb or block light?

Would stars have formed in the Universe if atomic hydrogen couldn't make molecular hydrogen?

How much larger would a star have to be to cause thermonuclear reactions if it was made out of mostly rock like Earth, instead of gases?

... and you get the idea.


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