text: Classical variables that time reversal negates include:
  :, the time when an event occurs
  :, velocity of a particle
  :, linear momentum of a particle
  :, angular momentum of a particle (both orbital and spin)
  :, electromagnetic vector potential
  :, magnetic field
  :, magnetic auxiliary field
  :, density of electric current
  :, magnetization
  :, Poynting vector
  :, power (rate of work done).