I am trying to understand something. Let me pitch a scenario: You have two asteroids in a vacuum. Each is large enough to be round by gravity. They are separated by roughly the distance from the earth to the moon. Relative to each other, they are moving at 0 in all directions (aka, they are immobile). Time starts. Gravity causes the two objects to begin to move toward each other. Time stops when they have traversed half the gap between them. The distance is now 1/2 Earth to Moon. In this situation, the "action" is the two bodies moving toward each other. What is the equal and opposite reaction? [Mentor's note: Some speculation not allowed under the forum rules has been removed] Thank you for your replies.