# Speed of the Coulomb Field Measured To Be “Infinite”

The paper "Measuring Propagation Speed of Coulomb Fields" by R. de Sangro et al points out that:

...the Lienard-Weichert retarded potential leads to the same formula as the one obtained assuming that the electric field propagates with infinite velocity

Moreover, they conduct an experiment which purportedly measures a "rigid" (aka infinite velocity) $-\nabla \phi$ ("Coulomb Field") and further claim:

The Feynman interpretation of the L.W. formula for uniformly moving charges does not show consistency with our experimental data.

This violates special relativity because information can be transmitted at infinite speed simply by changing the position of a charge and measuring, at a large distance, the associated changes in $-\nabla \phi$.

Where did they go wrong?

• I'm no expert, but I just read the paper and they definitely didn't provide evidence to support the second statement you quoted in the question. They collected some (very imprecise because of their lack of knowledge about the electron beam's shape) data which was consistent with all of the theories in question. It was a non-result, essentially. – Duncan Harris May 1 '16 at 23:31
• Also note how their sensor results are mostly oscillations at the natural frequency of their sensors. They got only a little bit of information about the shape of the electric field "wake" of the beam, and what little they got was made useless by their lack of knowledge of the shape of the beam itself. – Duncan Harris May 1 '16 at 23:36
• THe most important bit of the paper: – Duncan Harris May 1 '16 at 23:37
• "One issue common to all our measurements stands out clearly: in the same experimental conditions (sensor position, trigger timing, cable lengths, DAQ settings) the two distributions are different. We attribute this difference to less than perfect reliability in the beam delivery conditions (launch angles, total beam length, charge distribution in the beam pulse length, stray magnetic fields, etc.), over which we had little control." – Duncan Harris May 1 '16 at 23:38
• Why even publish this failure? – CuriousOne May 2 '16 at 0:20