1. is not very intuitive/useful to have mass that varies on the direction (which is what this implies)
2. is somewhat tautological to define a new mass m_rel = E/c^2 and say that it satisfies the equation when this is not what most people understand mass to be. most people understand photons to be massless particles.
at minimum, relativistic mass should always be specified as m_rel to distinguish from what is typically referred to as mass.
but i don’t think relativistic mass is a wrong concept any more than any other mathematical convenience like virtual particles. the main question is how useful is it and should it be described using the word “mass” or is this confusing. there is value in having shared language, even if you can construct an alternate system of symbols and rules that can yield the same answer to every question. to the extent to which intent of the author matters at all (probably doesn’t), Einstein agreed that relativistic mass was not a useful concept.
i'll concede that the arguments in the thread you linked are not good
> most people understand photons to be massless particles.
I suspect that most people have no opinion at all and are generally unaware of the properties of light.
That being said, a (perfectly reflective or otherwise steady state, e.g. black body at equilibrium) box of photons has mass due to those photons. You can stick it on a scale or a pendulum and measure it. It attracts other nearby objects according to GR or, in the appropriate limit, Newtonian gravity, in accordance with the relativistic mass of those photons.
1. is not very intuitive/useful to have mass that varies on the direction (which is what this implies)
2. is somewhat tautological to define a new mass m_rel = E/c^2 and say that it satisfies the equation when this is not what most people understand mass to be. most people understand photons to be massless particles.
at minimum, relativistic mass should always be specified as m_rel to distinguish from what is typically referred to as mass.
but i don’t think relativistic mass is a wrong concept any more than any other mathematical convenience like virtual particles. the main question is how useful is it and should it be described using the word “mass” or is this confusing. there is value in having shared language, even if you can construct an alternate system of symbols and rules that can yield the same answer to every question. to the extent to which intent of the author matters at all (probably doesn’t), Einstein agreed that relativistic mass was not a useful concept.
i'll concede that the arguments in the thread you linked are not good