Hayley and I were having a fascinating conversation at the beach yesterday. I don’t recall how it came up, but at some point she mentioned shrinking the moon with the Shrink Ray from Despicable Me so she could bring it down to the surface (for a reason I’m still unaware, because that’s where the conversation got interesting).
When I asked her what would happen she immediately noted the affect on the tides here on Earth. We were, after all, at the beach and had watched over the course of a few hours the tide go out substantially. My intuition suggested that in fact there would be no affect on the tides because the moon’s gravity well is based on mass, not diameter. For the same reason, its orbit would be unaffected.
This is where it got interesting for me, and where my knowledge of physics was insufficient. Dramatically changing the diameter of the moon without changing its mass would have an affect, for sure. Would the increased density create new elements, heat up the moon, turn it into a star?
Randall Monroe, creator of the popular xkcd comic and two of my favorite books, How To: Absurd Scientific Advice for Common Real-World Problems and What If?: Serious Scientific Answers to Absurd Hypothetical Questions asks and answers these exact types of questions. If strange physics is interesting to you, or simply the absurd, I encourage you to check out his books.
So, Randall, what would happen if the moon were shrunken to the size of a grapefruit without changing its mass?