Newton's law states F=mr¨(t). Solving this equation for a given force field yields a curve r(t) that governs the motion of particles in that field.
The physics in a field is said to obey time-reversal symmetry if whenever r(t) is a permissible motion, r(−t) is as well.
Of the following forces, which does not obey time-reversal symmetry?
- Gravitational F=−Gr(t)2Mmr^
- Electric F=qE
- Magnetic F=qv(t)×B
Assumptions
- Take the fields in question to be constant, and time-independent.