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 Mar 12 comment Why does matter exist in 3 states (liquids, solid, gas)? Except, maybe for solid and fluid (good for @Agent_L) all the other states mentioned are actually a continuum (e.g., there are no well defined boundaries to distinguish a gas from a liquid). Therefore I would say that there is nothing fundamental about the number 3 (or 4) except to what (how many) states our mind can imagine. (The fundamental concept is symmetry breaking --and even that, can tricky to define precisely--). Dec 27 awarded Scholar Dec 27 accepted With radian as a unit, should action and angular momentum have the different units? Dec 27 comment With radian as a unit, should action and angular momentum have the different units? Thanks for the answer. Reading many different opinions elsewhere and doing some algebra I got to the conclusion that radian in some sense tags pseudo-quantities (like pseudo-vectors and pseudo-scalars), which I think makes clear what would be the role. The problem is that opens another Pandora's box because that also means that to be consistent $\mathrm{rad^2} = 1$ which still makes it special if we force it as a dimensions. So, for different reasons, I start to agree with your statement "However, that distinction cannot be usefully maintained in more complicated calculations" Dec 27 awarded Commentator Dec 27 comment Unit of torque with radians? ...although I am getting to the conclusion that in a consistent radian system $\mathrm{rad^2} = 1$ and $\mathrm{rad} = 1/\math{rad}$. In the same way that the product of two pseudovectors is a vector. Dec 27 comment Unit of torque with radians? Actually, the result in your example is $m/s$ no matter what. The reason is that $\vec v = \vec\omega \times \vec r$ and the $\times$ operation is the one that eliminates the radian units. Dec 27 comment Unit of torque with radians? Actually the unit of angular momentum, using radias is $\mathrm{kg\cdot m^2/s/rad}$. Dec 27 comment Unit of torque with radians? In some sense the unit "rad" gives the information that the quantity is a pseudovector. It is "generated" by the $\times$ operation. Dec 26 comment With radian as a unit, should action and angular momentum have the different units? There is an opinion about this from a units library: boost.org/doc/libs/1_56_0/doc/html/boost_units/… I agree with it but I also see that to be consistent also requires hbar to have different units than h. Dec 25 asked With radian as a unit, should action and angular momentum have the different units? Nov 10 awarded Critic Nov 25 comment How to rebut denials of the existence of photons? A more concrete question to pose would be whether the EM fields are quantized or not. (Let alone, that a theory with quantized electrons and not quantized EM fields would be difficult to state consistently) Aug 4 comment Is a Perfect/Lossless Mirror possible? Note that even if intensity is preserved, there will be loss of phase-information. An essential quality of a mirror is that it preserves phase so you can do for example a diffraction experiment. Jun 25 comment Will a relativistic rocket be destroyed by incident photons? Fast atoms will not necessary do any damage. Since they go fast the damage that they can produce in a given material thickness will be negligible. Am I missing something? For example, en.wikipedia.org/wiki/Stopping_power_%28particle_radiation%29 Apr 3 revised Phonon carries zero spin. Why? added 77 characters in body Mar 31 answered Phonon carries zero spin. Why? Dec 17 awarded Student Dec 17 comment Name for thermodynamic derivative $dP/dT|_V$? @NickKidman, thank you, yes simply related quantities, including dimensionless, with given names, are also welcomed. I added that to the question. Dec 17 awarded Editor