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OK, I do not understand prof.Dyson's argument at all. "This does not mean that an electric field-strength can be measured with the square-root of a calorimeter
by Mikhail_K 3y ago
OK, I do not understand prof.Dyson's argument at all.
"This does not mean that an electric field-strength can be measured with the square-root of a calorimeter. It means that an electric field-strength is an abstract quantity, incommensurable with any quantities that we can measure directly."
Electric field-strength is measurable no less directly than energy, it is a force experienced by a unit charge placed within the electric field.
- ThePhysicist 3y agoYou can make the electric field disappear by choosing the right gauge. Same goes for the magnetic field (can't make both disappear together though). The vector potential, in that sense, can be regarded as a more fundamental description of the electromagnetic field. It can't be observed directly though, but electric and magnetic field strengths are manifestations of the vector potential, they are not fundamental in that sense. Not sure if it's that what he's getting at, though.
- toth 3y ago>You can make the electric field disappear by choosing the right gauge. Same goes for the magnetic field (can't make both disappear together though). What? No you can't. The fields are invariant under gauge transformations.
- ThePhysicist 3y agoYou're right, sorry I was thinking of a Lorentz transformation that would make either the magnetic or electric field disappear under certain conditions.
- wch4999 3y ago"transform into each other" would be more appropriate. The gauge choice you mentioned is not totally wrong. The gauge freedom can be used to set the electric field to zero, but only once at a single point.
- toth 3y agoSorry, but gauge transformations do not (by construction) affect the physical fields at all. You cannot set E to 0, even at a point, with a gauge transformation.
- Mikhail_K 3y ago> You can make the electric field disappear by choosing the right gauge. That, had it been true, would have made the gauge observable.
- wch4999 3y agoYou need to read the paragraph prior to the quote. He is talking about which one of field and mechanical stress is more "fundamental" or less "direct". If one measure the force exerted by electric field using a unit charge, one measures the field by measuring the mechanical stress first. Of course, the context matters. Often if one compares potential and field, field would be the one directly measured. It is just semantics really.
- Mikhail_K 3y agoI have read the prior paragraph. It does not clarify how measurement of energy is more direct than measuring the field.