Is Gravity Time-reversible?

I am lead to believe that the laws of physics are time reversible, except for the asymmetries of the weak force, 2nd law of thermodynamics and quantum noninvasive measurements. Source wikipedia :)I am confused, because in my simple mind , gravity is time irreversible. If I have two bodies at rest, with no other forces acting upon them, they will get closer as time increases. What am I not understanding?
- Nick Deguillaume (age 34)
Bristol, United Kingdom

Think of a planet orbiting a sun. Although they're always falling toward each other they stay in a mutual orbit forever, at least in the Newtonian approximation. The time reversed version is just the same orbit running backward, following the same law of gravity. If the orbit is strongly elliptical, they'll fall toward each other, just miss, then whiz way past each other, and the whole process will keep repeating.

What you're thinking of is what happens if they don't have enough sideways velocity to avoid crashing into each other. That irreversible crash involves all sorts of shorter-range non-gravitational effects. There's still a mystery, because on a microscopic scale those other effects (mainly electromagnetism) are also reversible, but that's another question.

If you include General Relativity, the orbiting ones do radiate some gravitational wave energy, and the orbit gradually shrinks. Even that is time-reversible, however, in that it could run backwards with the gravitational waves coming in from far away.

This is one of my favorite topics, so feel free to follow up with more questions.

Mike W.

(published on 11/08/2015)