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I'm talking about http://what-if.xkcd.com/

One one hand, the FAQ stands against them:

To prevent your question from being flagged and possibly removed, avoid asking subjective questions where … we are being asked an open-ended, hypothetical question: “What if ______ happened?”

On the other hand, there is precedent, such as my own Would a submarine float in the atmosphere of Jupiter?

Bonus question: Why wasn't my submarine question closed and/or deleted?

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    $\begingroup$ "Would a submarine float in the atmosphere of Jupiter" is outlandish, but not outside of current physics knowledge. Whereas "if I got into a time machine and went back and killed my grandfather" cannot be answered by anything in our current understanding of physics. I think this should be the operational difference. $\endgroup$ – Peter Shor Oct 22 '12 at 15:58
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If you literally just ask "What if X happened?", that's too open ended - it's not asking for something specific, it's just inviting speculation. And if X is something that isn't physically possible, it'd also be too hypothetical. But your submarine question is not of that type. It's looking for a specific answer based on a physical argument, which makes it okay.

Naturally, the XKCD blog has looser standards than we do for what is acceptable, but many of the questions addressed on there would probably be okay here, as long as they are phrased properly. For example, a question phrased as "What if everyone on Earth pointed a laser at the moon?" would be inappropriate here, but asking "If everyone on Earth pointed a laser at the moon, would it look different?" is fine. (And "How much light would we need to shine at the moon to make it visibly brighter?" would probably be even better.)

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I think the difference is pretty well definable. The xkcd comics are a good case-in-point, because they are fun to read precisely because the author doesn't just answer the question. Most of what is written is, in fact, a tangent. Let's look at examples.

How long would the Sun last if a giant water hose were focused upon it?

This would be a perfectly valid question for Physics SE (and nice that the intuition is wrong).

What if you exploded a nuclear bomb (say, the Tsar Bomba) at the bottom of the Marianas Trench?

This is not really a valid question. The cynical obvious answer is "a large bomb goes off in the Marianas Trench". More to the point, there is no clear answer.

What if a rainstorm dropped all of its water in a single giant drop?

This is a good case where both the question and the answer aren't really appropriate for Physics SE. There are physics peppered in there, but at one point, I think it was talking about the lore and terror people were living in after the great kinematic destruction of a town by the giant raindrop.

That's not the only problem. Perhaps the biggest problem is that the author had to fill in so much about the question. Physics requires good definition. We are better off demanding solid definition (and thus quality) in the questions with our format.

If someone was crystal clear that they were talking about a giant spherical ball of water with volume equal to a rainstorm materializing instantly above suburban America, then... I suppose we would have to allow it.

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