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Is the "mass means rest mass" (MMRM) school of thought within relativity theory still "Einstein's theory of relativity"?

I have read that both MMRM and "mass depends on velocity" relativity predict experimental results equally well.

I think Karl Popper said that when two theories both predict experimental results equally well you should choose the simpler theory. Is the "MMRM" theory simpler than the old "mass increases with speed" theory, and adopted by some/all for that reason, or is it just easier, mathematically?

I have read that MMRM allows use of a unified system based on tensors instead of different math for different tranformations, making the math easier.

Matthew Christopher Bartsh
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    I don't know this question is supposed to be about - there are no different "theories" here, just different conventions for what, exactly, one means by the term "mass" when not adding any extra qualifiers. See also https://physics.stackexchange.com/q/133376/50583 and its linked question. – ACuriousMind Dec 10 '21 at 19:05
  • I agree... the disagreement is purely about what the term "mass" should mean. There is no actual physics in dispute, both groups of physicists agree on what the predictions of relativity are, they just use the word "mass" to mean different things. – Eric Smith Dec 11 '21 at 02:32

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The mass means rest mass "school of thought" is just an arbitrary convention regarding how a particular term is defined. You could call rest mass "gorble" and you could call relativistic mass "bornk" and the physics wouldn't change.

It is no more consequential than the different East Coast and West Coast sign conventions, or the convention of using base 10 numbers in equations, in general relativity. Everyone is using the same physical and mathematical theory.

There aren't two theories that are being distinguished, just two sets of arbitrary word choices. So, the better theory is simply the one that people find more convenient and comfortable to work with that best avoids misunderstandings.

ohwilleke
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  • Could you please clarify/expand on what kind of "misunderstandings" one should be avoiding? Please say which kinds of people (that should not be misunderstanding) you have in mind. – Matthew Christopher Bartsh Jul 04 '22 at 06:42
  • @MatthewChristopherBartsh "Please say which kinds of people (that should not be misunderstanding) you have in mind." Misunderstandings by people reading things written about mass in a particular convention. You don't want a reader to think that you are talking about rest mass when you are not talking about rest mass. – ohwilleke Jul 04 '22 at 18:58