Generally speaking, if you play a movie backwards everything that happens is still physically possible. If I throw you a baseball and you catch it, reversing the video is just the equally plausible situation of you throwing the baseball followed by my catching it. If entropy is changing in the video – e.g., breaking an egg – the time-reversed video will not be especially *likely*. But it doesn’t break the laws of physics.

This is called time-reversal symmetry (sometimes T-symmetry for short). If you’re looking at planets orbiting in uniform circular motion about the sun, you know from your physics class that they have to satisfy a particular relationship between their velocity and the force F of gravity that the sun exerts on the planet:

The gravitational force doesn’t make any reference to time – if you have two masses, they attract. On the other hand, time reversal changes v to -v, since the planet is now moving in the opposite direction*. But the equation involves the square of v, and so the minus sign vanishes and the law retains the same form despite the time reversal.

This T-symmetry holds very generally in physics. both in Newtonian mechanics and Maxwellian electrodynamics. It came as a bit of a shock therefore when it was discovered that particle physics (the weak interaction, specifically) is not time-reversal invariant. It does hold to a more general CPT-symmetry where the laws are invariant if you flip the direction of time, flip left and right, and change matter into antimatter. Exactly why this is so is not so well understood.

Which is a long way of introducing an interesting APS article about a violation of T-symmetry in electromagnetism. This certainly raised my eyebrows when I read the headline, but as it turns out the symmetry is indeed broken in the device itself under the influence of a magnetic field. But magnetic fields do reverse direction under time reversal (because they’re generated by moving currents), and so the symmetry is preserved if you take the magnetic field into account. Still, it’s a pretty snazzy paper – both the summary and the paper itself are freely available online. Give ‘em a look!

*This is sort of handwaving since v in this notation is usually defined as a strictly positive magnitude in the first place. If this bothers you, define the quantity v^2 = **v**.**v** and it’s a little more clear.