A Quantum Field Theory can be obtained by quantising a Classical Field Theory through to a procedure. The procedure involves starting with a set of equations of motion that describe a Classical Field Theory, then find a Lagrangian that produces that set of equations of motion, then use that Lagr...

A magnetic field describes the influence a charge (in motion) experiences. In other words, it is essentially a vector field that describes the force that a particle will feel at a given location. However, a moving charge experiences a magnetic force that is perpendicular the direction of its magn...

In introductory time dependent perturbation theory, I have read that given a system with a Hamiltonian of the form: $H = H^{(0)} + \delta H(t) \tag{0} $ where the $H^{(0)}$ is independent of time and has known eigenstates given by $|n \rangle $. Now, if the perturbation is 'small' then the wave f...

The spin of a photon has a counterpart in classical physics, it's polarization, right? And if you spin a polarized light wave by 180°, (or pi radians), it is now the same as before, correct? So why isn't the spin of a photon said to be two, rather than one?

I've read many times that the fact that gravitational mass is equal to inertial mass (as far as we can tell) used to be a puzzle. I believe that Einstein explained this by showing that gravity is itself just an inertial force. When I first encountered this concept, I thought "isn't there just one...