A hypothetical universe.
In it the only two things that exist are a planet say the size of Earth (spining on it’s axis) and a Hydrogen atom.
Is there any amount of distance I can put between the planet and the atom where the atom wouldn’t be affected by the planets gravitational pull?
Basicaly, what I’m trying to say is: How far extending is it’s gravitational pull? And what sort of crazy equation do you come up with to figure that out?
Newton’s law for gravitational attraction is F = G((M1*M2)/r^2).
The gravitational force falls off as the square of the distance, but never reaches zero.
More info here: Randy Kobe, University of Winnipeg
If the atom is outside the observable universe*, then it is separating from us by a speed faster than the speed of light. The generally accepted view is that gravity waves move at the speed of light.
Ergo, the atom no longer interacts with the Earth in any way.
*Assuming the universe is at least that large.
To expand on what ftg wrote, the universe is about 14,000,000,000 years old. So the masses of any two objects more than 14,000,000,000 light years apart have not yet had time to interact.
But you’re referring to our expanding universe. The universe given by the OP is not ours and is not said to be expanding. The objects are apparently not moving away from one another at all.
If it’s not and they’re not, then it’s correct that there is never a time when the objects do not have a non-zero gravitational effect on one another.
Waiting for the Total Perspective Vortex to enter in to this:
True only in a Newtonian universe.
Which might explain a lot about Adams’ worlds. 
Is the idea here that “our” universe is somehow emptied of everything except one big planet-sized rock and one atom of hydrogen, placed REALLY far apart?
IANAphysicist, but as I understand it, the expansion of the universe is accelerating, which suggests there is some force working against gravitational attraction, anti-gravity if you will. One hypothesis of how this works involves involves anti-gravity increasing relative to the distance between objects.
So it is conceivable there exists two points, where you could place two objects, and they would remain motionless in relation to each other.
But gravity WOULD still be affecting the two objects – only its force would be counterbalanced by this other mystery force.
Two errors here. 1. See the link in my post about why the 14BLY number doesn’t apply here. 2. Due to expansion, it’s actually the opposite direction. Things once in causal connection become unconnected.
I’m not sure what your point is. I used the fourteen billion light-year figure because it’s a limit - any objects that far apart have not had a gravitational effect on each other. You can lower the figure down however to a local situation - two objects that are only 700,000,000 miles apart have not had any gravitational effect on each other if the two objects have existed less than a hour. The point I was making was that gravitational effects cannot occur faster than the speed of light, which is the same point you were making I believe.
I’d also point out that when I wrote about objects being “more than 14,000,000,000 light years apart” I meant that they had always been that far apart not just currently.
Did you actually look at the link? Did you notice the section entitled “Misconceptions”?