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Comment by tejtm | Hacker News Reader

space is pretty big, but I would bet eternity wins this one.

As a layperson, I always wondered this about gravity. Does it attract over infinite distance or does it have a range?

Thinking in continuous terms, it would be infinite and inversely proportional to distance squared. But thinking in terms of a distortion in space-time and also considering that it may be quantized, perhaps there is some limit. Also for points that are separating faster than the speed of light due to inflation gravity couldn't alter that space.

> for points that are separating faster than the speed of light due to inflation gravity couldn't alter that space.

This is not correct. The effects of inflation on spacetime geometry are a form of "gravity" as far as GR is concerned. They're just not a form of "gravity" that you could ever get out of the Newtonian approximation.

I don't see how this could work. Say there's a significant increase in mass at a point in space A. And there's another point B that's where the distance A-B is separating faster than the speed of light. How could any changes in mass at point A alter the spacetime/gravity at point B? Are we presuming a means of propagating changes faster than light via graviton?

In General Relativity (which is the theory we have to use if we're talking about black holes), gravity is not a force, it's spacetime curvature. The spacetime curvature at any point in spacetime depends only on the matter in the past light cone of that point. Heuristically, "gravity" travels at a finite speed in GR. But there is matter and energy everywhere in the universe, so there is spacetime curvature everywhere; there is no place you can go to "escape" from its effects.

So thinking of gravity as a Newtonian "attraction over infinite distance" doesn't really work in GR, but neither does thinking of it as a "force" with a finite range. It works differently from either of those.

Of course, nothing can have infinite range in an infinite Universe, because the infinite sum of gravitation fields of an infinite number of objects can create infinite force at any point of the Universe.

At some range, any field will decrease to below thermal noise of medium.

Not in an expanding universe.

This has to be a question someone here might be able to answer.

Is there some mass-distance(-velocity?) function that defines the alternative fates? Where below some threshold, things are fated to merge and beyond which ...not so much.

I am pretty confident all the mass on the earth would be expected to end up together, and would be comfortable imagining that theoretical planets on opposite sides of the observable universe may never merge but what is the transition point? the solar system? the milky way? the local group?

Systems that are gravitationally bound will stay that way, yes. (At least as long as a "big rip" scenario does not happen; our best current observations say it won't, but the error bars don't completely rule it out.) Generally speaking, the boundary in our universe seems to be around the galaxy cluster level: galaxy clusters are the largest systems that are generally gravitationally bound (although often quite loosely, so that objects near the outer edge of the cluster might fairly easily acquire escape velocity through random encounters with other objects). That corresponds roughly to a distance scale of a few tens of millions of light-years.

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