system
Given the following post, respond as if you're commenting on Reddit:
human
Does every piece of matter in the universe really affect every other piece (even negligibly)? (Physics of Douglas Adams's Total Perspective Vortex)

Chapter 11 of *The Restaurant at the End of the Universe* by Douglas Adams starts with a blurb on the inspiration behind the Total Perspective Vortex--a fictional machine that gives the user a "sense of proportion" re their place in the universe.
> "...since every piece of matter in the Universe is in some way affected by every other piece of matter in the Universe, it is in theory possible to extrapolate the whole of creation-every sun, every planet, their orbits, their composition and their economic and social history from, say, one small piece of fairy cake."

I'm sure--pretty sure--that the latter sort of extrapolation is ludicrous. But I wondered about the first part.

I've always assumed that the sum of forces on one body due to the presence of another body drop to ***effectively*** zero at a large enough distance. But is there some distance at which it is zero **in exact arithmetic**?

(Sorry if this is too broad or poorly phrased).
gpt
An important consideration here is the finite speed of the propagation of information (the speed of light), which means that an event can only influence other events in its future light-cone; therefore it has no effect on anything which is occurring simultaneously with it.

A piece of cake on earth would eventually influence events on the other side of the galaxy in a negligible way, but it would take 100,000 years for the signal to propagate.