A little science for Monday evening:
I understand the falling off of a complete parallel that is necessary to analogies; but, there is a point towards perpendicularity wherein an analogy becomes useless. Problem is, to understand whether an analogy is appropriately parallel, one must understand the substance of the two analogized items. I don’t, in the forthcoming instance; so any help in the matter is appreciated.
The following is the general analogy for gravity in general relativity, courtesy of Stanford’s Gravity Probe B project:
This is the core of Einstein’s theory of general relativity, which is often summed up in words as follows: “matter tells spacetime how to curve, and curved spacetime tells matter how to move”. A standard way to illustrate this idea is to place a bowling ball (representing a massive object such as the sun) onto a stretched rubber sheet (representing spacetime). If a marble is placed onto the rubber sheet, it will roll toward the bowling ball, and may even be put into “orbit” around the bowling ball. This occurs, not because the smaller mass is “attracted” by a force emanating from the larger one, but because it is traveling along a surface which has been deformed by the presence of the larger mass. In the same way gravitation in Einstein’s theory arises not as a force propagating through spacetime, but rather as a feature of spacetime itself.
I frequently see that analogy as the counter to Newton’s non-explanation for gravity; wherein gravity is simply a force that objects exert, increasing by mass. Rather, gravity is a result of objects bending spacetime.
My difficulty is that, for something to bend like a trampoline, it needs to have atoms bound together that hold up and stretch under the weight of an object. It must, in other words, by physical. Is spacetime a physical thing upon which the sun and planets sit? I suppose, if it is not, the analogy loses me.
Anyway – after googling that for a little while, I was amused that there exists a company that provides trampolines for parties – it’s called the Anti-Gravity Trampoline.
