this post was submitted on 31 Oct 2023
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Showerthoughts
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A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. The best ones are thoughts that many people can relate to and they find something funny or interesting in regular stuff.
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If the spacetime is distorted, and the light no longer appears to travel in a straight line, does that not mean that spacetime itself and the light that travels in are no longer straight?
How can a straight thing be distorted but still be straight?
It is straight in reference to the distorted spacetime.
Conventionally, a straight line is defined as the shortest path between two points, but if you take a plane that is not flat, say the surface of a ball, the shortest path between two points will be curved. But from the perspective of the two-dimensional man who lives in surface, the line is straight because it moves perfectly along his world.
Is this 2D-3D comparison supposed to be like a human-understandable analogy for a 3D-4D relationship?
I saw an explanation once about how time is the 4th dimension. They drew a line on the edge of a book. From the perspective of a single page (2D) it just looks like a dot, but because we can see many instances of that 2D representation it appears to us as a line. An individual page represents how we experience time.
Is your ball example supposed to be kind of like that because I just can't imagine how spacetime could be a 2D thing in a 3D universe.
Well, kind of. The time dimension is a bit tricky, though -- in Minkowsky space, a common way to think about spacetime, it is hyperbolic in relation to the other dimensions. In a nutshell, this means that distance is not the square root of the sum of the squares of the distances in specific dimensions, but rather of the difference. This makes it especially tricky to visualise. (I do recommend you check out this series by minutephysics, he does a great job at making it intuitive)
My analogy, therefore, doesn't translate directly to spacetime, but it does provide a very simple 2d explanation for why straight things can act curved (even in 4d).
I think a better way of explaining it is with the idea of a shortest path, and not nessesarily a straight line. With two points in space the shortest path between them will be a straight line. If there’s a large amount of gravity tugging on space time the shortest path will be curved.
That's called General Relativity and Reference Frames.
Start watching PBS Spacetime if you actually want to get into it.
You're also entirely missing the point of the comment that was a joke entirely re-explaining the "reason" for ghosts ending up at the center of the Earth, which is implied by the original comment.