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 Sep 20 comment What axiomatizations exist for special relativity? Nowadays, we'd say there is a universal speed limit c that light happens to travel at, making its measurement convenient. Sep 18 comment How can (in Dirac's terminology) the product of two “real” linear operators be “not real”? This answers my question. It would help if you could put $\alpha = \bar\alpha$ in brackets after Hermitian. Sep 3 accepted How does Dirac show that $\langle B|\bar{\bar{\alpha}}|P\rangle\;=\; \overline{\langle P|{\bar{\alpha}}|B\rangle}\;=\; \langle B|{\alpha}|P\rangle$? Sep 3 comment How does Dirac show that $\langle B|\bar{\bar{\alpha}}|P\rangle\;=\; \overline{\langle P|{\bar{\alpha}}|B\rangle}\;=\; \langle B|{\alpha}|P\rangle$? Indeed; I assumed that the adjoint of an adjoint cancelled like the inverse operator. It seems obvious now that the adjoint of an adjoint should initially be assumed to give a different linear operator since its still just a linear operator. Sep 2 asked How does Dirac show that $\langle B|\bar{\bar{\alpha}}|P\rangle\;=\; \overline{\langle P|{\bar{\alpha}}|B\rangle}\;=\; \langle B|{\alpha}|P\rangle$? Aug 26 accepted Can relativistic kinetic energy be derived from Newtonian kinetic energy? Aug 25 comment Explanation for $E~$ not falling off at $1/r^2$ for infinite line and sheet charges? @RonMaimon now that you've had a long rest, does this answer still make sense to you honestly? Aug 24 comment Is there a “forwards” and “backwards” in one dimension? So a magnitude can be negative? Aug 24 accepted Is there a “forwards” and “backwards” in one dimension? Aug 24 comment Is there a “forwards” and “backwards” in one dimension? @HDE226868 sure you can. But maybe that isn't a true one dimensional space as defined by a physicist or mathematician, and a direction paramaterized by two discrete symbols +,- has been added. Aug 24 asked Is there a “forwards” and “backwards” in one dimension? Aug 24 comment Physical reason for Lorentz Transformation You're only looking at a light-like interval; you need to show how invariance of this implies invariance of space-like and time-like intervals also. Aug 20 comment Rotation axis of a rigid body +1 all this work, and just one lousy up-vote? Aug 19 comment Why should multiplication of a ket vector by a complex number change only its “direction”? @ACuriousMind A vector multiplied by a negative real number changes its direction, yes? How are we to interpret multiplying it by a complex number? Aug 19 revised Why should multiplication of a ket vector by a complex number change only its “direction”? improved context Aug 19 comment Why should multiplication of a ket vector by a complex number change only its “direction”? @Nathan I'm looking for an answer in the spirit of Dirac talking vaguely about vectors in an infinite dimensional space up to this point. I guess this will become clearer later on in the book, but Dirac seems to suggest this statement is obvious for physical/mathematical reasons. Aug 19 asked Why should multiplication of a ket vector by a complex number change only its “direction”? Jul 13 awarded Yearling Jul 11 comment How does the speed of electrons change around a circuit? The drift velocity is typically around a few centimeters/hour. Jul 2 awarded Curious