However, the Aharonov Bohm effect is a quantum mechanical effect which illustrates that non-zero electromagnetic potentials ( even when the and fields are zero ) lead to changes in the phase of the wave function of an electrically charged particle, so the potentials appear to have measurable significance.
22.
If I am holding a book on earth, and you define the system to be me + the book + earth, there is no energy being added to the system when I raise the book higher off the ground, I'm simply converting the electromagnetic potential energy stored in my muscles into another form of energy.
23.
Poincar?( 1905 ) removed this objection by showing that the non-electromagnetic potential ( " Poincar?stress " ) holding the electron together can be formulated in a Lorentz covariant way, and showed that in principle it is possible to create a Lorentz covariant model for gravitation which he considered non-electromagnetic in nature as well.
