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But to say this, you should give a distance where gravity will no longer affect an object X lightyears away, more precise X miles away.
Two bodies of matter attract each other proportional to their masses and inversely proportional to the square of distance between them. If the mass of one body is designated as M, the mass of the other as m, and the distance between them is r, then the force of attraction between the two bodies is:
F = G ((Mm)/r^2)
where G is the universal gravitational constant G = 6.67 x 10-11 N(m2/kg2) (Newton-meter squared per kilogram squared).
Since force = mass times acceleration, the universal gravity equation implies that as objects are attracted and get closer together, the force increases and the acceleration between them also increases.
Feel free to plug in the numbers that you want. Once you get this result, all you need to do is figure out the force required to move a given body X (you can find both this equation and the gravitational one all over the internet. It's physics 101). Once you have the amount of force required to move X, all you need to do is compare it to the amount of gravitational force available. If it takes more energy to move X than there is available from gravitational force, then X is going nowhere.
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If light can be seen from one object to another, why couldn't gravity also be influenced? Expecially if gravity travels about the same speed as light, which I heard mentioned in another thread around here.
Light is a projected energy, whereas gravity is a property of mass. Once light leaves the source, it remains a specific unit and form of energy, even if the light source disappears. Gravity exists only as the product of another object. If that object disappears, the force of its gravity does as well.