Question: Does the influence of gravity extend out forever?

Zsolt Keszthelyi answered on 18 Nov 2020: last edited 18 Nov 2020 4:07 pm

In principle, yes. In fact, gravity (unlike other forces) is the most effective on large scales, and for large masses. But the laws of gravity are universal, so every object which has mass, has a gravitational force. For example, there is a gravitational force between a smartphone and an orange. Any object really. The reason why a smartphone and an orange will not move by this gravitational force is because their masses are so small that they lead to an incredibly small, basically unnoticeably small force. So you need two things to see the influence of gravity: large mass and small distance. We know that this force depends directly on the mass. The larger the mass, the larger the gravitational force. But it depends inversely on the square of the distance. So if a planet is 3 times farther from its star than another planet with the same mass, then that planet will only “feel” 9 times less of the gravitational force. So going very far from the source of strong gravity (that is, an object with a lot of mass, for example a star) means that the influence will become neglibily small at some point.