Thursday, April 25, 2019

quantum field theory - Time-ordering and Dyson series




  1. In Dyson series we use a time-ordered exponential by arguing that Hamiltonians at two different instants of time do not commute. Why is that so?





  2. Can anyone explain with an example why should the same Hamiltonians at two different times not commute?






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classical mechanics - Moment of a force about a given axis (Torque) - Scalar or vectorial?

I am studying Statics and saw that: The moment of a force about a given axis (or Torque) is defined by the equation: $M_X = (\vec r \times \...