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LIMITATIONS OF GAUGE INVARIANCE

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These equations can be combined to give:<br />

x<br />

qE ,<br />

m<br />

0<br />

This is Newton’s equation: (mass)(acceleration) = force<br />

The solution is elementary; it is quadratic in time.<br />

Now a simple gauge transformation will be applied.<br />

What has been done above corresponds to what is often called the<br />

length gauge, because the interaction term in the Hamiltonian has the<br />

form – E 0 x . The gauge transformation to be applied eliminates the<br />

scalar potential and introduces a one-dimensional version of a vector<br />

potential. This is called the velocity gauge.<br />

The gauge transformation has the generating function<br />

= - cE 0 x t<br />

11

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