# Is the speed of Magnetic field infinite?

My real question is: From Amperes law we know that there is no magnetic field outside of the coaxial cable because the magnetic field generated by the inner wire and outer shell are equal but in opposite directions, so they cancel each other. However, if we consider that the magnetic field has to travel in a certain speed (close or equal to light speed), there will always be a time delta between the two magnetic fields that are generated by inner wire (travelling distance A) and outer shell (travelling distance B), so if I place an extremely precise magnetic field detector outside of the coaxial cable (at the detecting point), I should detect a magnetic field pulse generated by the outer shell and it quickly goes to zero at the time when the magnetic field generated by the inner wire arrived, am I thinking right? If so, there will always be some tiny amount of EMI out side of a perfect coaxial cable when transmitting high frequency ac signal, is it?

• Based on your edit and clarification, I've deleted my post, but the answer is still no. The (electro)magnetic field "consists" of photons (virtual or real depending on the case, but regardless). The field outside is zero not because one photon compensates another, but because no photons go there in the first place. And since they don't go, they can't arrive at different times. – safesphere May 7 at 5:01
• How about a transmission line composed of two parallel wire then? The electromagnetic field outside them is not zero, will the photon go there? – Vincent_CHEN May 7 at 5:06
• Yes. Just note one thing. While the field "consists" of many photons, it is not a result of different photons doing different things. They all do the same thing and their number simply magnifies the same effect. Two photons cannot cancel each other, because both have a positive energy that cannot disappear. Oh, and the most weird thing is that each photon is emitted by both "wires", so again, it's not like a photon from one wire competing with a photon from another. – safesphere May 7 at 5:14
• I see. Thanks for your answers! May I ask you where can I learn this stuff? I did my undergraduates in Physics but I've never heard this concept during my study... – Vincent_CHEN May 7 at 5:24
• Paul Dirac: "the wave function gives information about the probability of one photon being in a particular place and not the probable number of photons in that place" - Read that paragraph here: informationphilosopher.com/solutions/scientists/dirac/… - Also note that too many people disagree, because they don't understand that a single photon can be emitted by two synchronized sources (e.g. two lasers) simultaneously. – safesphere May 7 at 5:49