Gravity and spacetime. Another question.

[ZenBeam]

@Hari_Seldon

Docks attract boats? Sure they do, how often do you see boats at docks? Seriously, though they do and it is a well known phenomenon that you could demonstrate with a toy boat in a bathtub. The reason is that a boat in open water is being bombarded constantly by water molecules but the effects cancel. (Ultimately it is the failure to cancel on a tiny scale and over a short time that results in Brownian motion, but that is another story.) Now imagine that same boat just one cm from the dock (pretend the water is very smooth). It will still be bombarded on the outside by water molecules, but much less on the side adjacent to the dock. The result is a slow motion towards the dock and it thus appears that a dock attracts boats. Too bad it doesn’t work; it is such a pretty idea.

The number of molecules hitting the boat from even 1 cm of water will be the same as from an indefinitely large expanse. It sounds like it’s more the impact of waves in the water that would be different. This effect would be similar to that of the Casimir Effect. In the image at the top, imagine the left plate is the dock, and the right plate is the boat.

@jackdavinci

The reason the warped space analogy doesn’t work for me is the issue of stillness. I can see that if an object otherwise moving in a straight line encounters distorted space, that what for it would seem to be a straight line would look from the outside like a curved line (remapping of sorts). However this wouldn’t cause attraction in the case of two bodies that started out motionless unless the ‘waterfall’ idea was in effect and there not only distortion of coordinates but also movement of them through time. So that’s something I can’t wrap my head around.

It’s not just space that is curved, it’s spacetime. Everything is “moving” through time, so there’s no issue of being still.

Here’s a different analogy that may help. Consider a typical lampshade, conical and larger at the bottom than the top. Imagine time being the direction around the shade, and space being the direction along the shade. if you start at a point near the top of the shade, moving horizontally, that corresponds to being stationary in space with time passing. As you move forward in “time” in a straight line, your path will “curve” down, analogous to a gravitational attraction from the bottom of the shade. This is a purely geometrical “attraction”, no external gravity needed.