Newton's law of universal gravitation states that F= g (m1 x m2)/r², i.e. a particle attracts every other particle in the universe with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. In Newton's view, there need to be two masses present for gravitation to take effect.

It has been proven by general relativity that gravitation occurs with just one mass present, as it is not a force which acts upon objects per se, but rather on the space in between them. This all makes sense and Newton's law still holds true, as any two masses will behave in accordance to his equation.

However, I have just one problem with Einstein's depiction of gravity. (problem as in ''I don't understand it'', not as in ''I'm trying to refute it'', to be clear).

If it is true that Newton's inverse square law extends indefinitely and the universe is constantly expanding, then one of the two statements must be true:

1) The gravitational force of every object is weakening, because it needs to extend the same amount of force over a larger area. The overall amount of gravitational force exerted by a mass stays the same, but is decreased within any given distance.

2) The mass of every object grows proportionally to the rate of expansion of the universe. This is the only way an object could exert the same amount of force over a distance, but is bizzare to consider.

These are the only two options I can think of. Neither of these would refute Newton's law in reality, because as the universe expands, the objects get further and further apart, and so the weakening in gravitational attraction would simply be explained by the increase in r.

Actually, I am not sure how the second case would affect Newton's law. I am trying to think about it, but this option is far-fetched anyway.

This problem only occurs when you talk about general relativity's concept of gravity. What do you think about this? What am I missing here?

All replies are appreciated.

**Edited by Lord Antares, 6 January 2017 - 05:54 PM.**