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samalkhaiat is offline
May9-08, 05:26 PM
Sci Advisor
P: 817
Prove that 4 vector potential does really a 4 vector?

The vector potential is not a 4-vector!

Under Lorentz transformation, the vector potential transforms according to

[tex]a_{\mu} \rightarrow \Lambda_{\mu}{}^{\nu} a_{\nu} + \partial_{\mu}\Omega[/tex]

This means that [itex]a_{\mu}[/itex] is a 4-vector, if and only if;

[tex]\partial_{\mu}\Omega = 0[/tex]

Since this is not compatible with the arbitrary nature of the gauge function, [itex]\Omega[/itex], we conclude that [itex]a_{\mu}[/itex] is not a 4-vector.

Deriving the transformation law of [itex]a_{\mu}[/itex] from the gauge-fixed Maxwell equation

[tex]\partial_{\mu}\partial^{\mu} a_{\nu} = J_{\nu}[/tex]

is a wrong practice. The gauge-invariant Maxwell equation is

[tex]\partial^{\nu}f_{\mu \nu} = J_{\mu}[/tex]

Lorentz covariance(of this gauge invariant equation) requires

[tex]a \rightarrow \Lambda a + \partial \Omega[/tex]

Well, this is not how a 4-vector transforms. Is it?

Genuine 4-vectors cannot describe the two polarization states of light. So, it is not a bad thing that the vector potential is not a 4-vector.

If this does not convince you, see Weinberg's book "Quantum Field Theory" Vol I, on page 251, he says:

"The fact that [itex]a_{0}[/itex] vanishes in all Lorentz frames shows vividly that [itex]a_{\mu}[/itex] cannot be a four-vector. ........
Although there is no ordinary four-vector field for massless particles of hilicity [itex]\pm 1[/itex], there is no problem in constructing an antisymmetric tensor ........
[tex]f_{\mu \nu} = \partial_{\mu}a_{\nu} - \partial_{\nu}a_{\mu}[/tex]

Note that this is a tensor even though [itex]a_{\mu}[/itex] is not a four-vector, ......"

See also Bjorken & Drell "Relativistic Quantum Fields", on page 73, they say this:

"Actually, under Lorentz transformation [itex]A_{\mu}[/itex] does not transform as a four-vector but is supplemented by an additional gauge term."

So, my friend, you should have asked the following instead;

"prove that the vector potential is not a 4-vector"