I'm reading "Zee A. - Quantum Field Theory, as Simply as Possible", where near beginning of explanation of QFT he gives what appears to be path integral formulation, he states: The corresponding probability equals the absolute value squared of the probability amplitude, that is, the square of th...

Having a gap in the spectrum doesn't exclude massive free quarks. It only tells us that there can't be free gluons or other massless excitations in QCD. I saw this statement in the opening paragraph of this paper arXiv:1502.04135. I'm not sure whether we have a rigorous argument for this statement.

Before coming to my actual question, let me write some assumptions that I believe we both agree on:-(1) Normal force can do work(for example in an elevator)(2) Normal force is always perpendicular to the contact area. questions says My doubt arises in part(b)The general solution for this part th...

If the force (from electrical field) pushing the charges is always the same, surely increasing the length (of circuit) they travel should increase the total energy transferred due to $W=Fd$, or is this force not constant across the circuit? If not constant, what causes the force to vary? Intuitio...