# If I could turn back time

Newton's law states $$F = m\ddot{r}(t)$$. 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 $$\displaystyle F = -G\frac{Mm}{r(t)^2}\hat{r}$$
• Electric $$\displaystyle F = q\vec{E}$$
• Magnetic $$\displaystyle F = q\vec{v}(t)\times\vec{B}$$

Assumptions

• Take the fields in question to be constant, and time-independent.
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