Talk:Mass
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Mass is NOT the quantity of matter of a body
I have edited the first paragraph of the article, providing a more scientifically accepted definition of mass.
The previous definition of mass as "the quantity of matter of a body" is a wrong and misleading pseudo-scientific conception. It also lacked references: the provided dictionary definitions https://www.dictionary.com/browse/mass, do not corroborate the definition of mass as "quantity of matter". It should be noted, still, that the proper definition should come from a standard, serious, widely accepted university-level physics textbook and not from a dictionary.
I foresee that someone might be able to provide a textbook reference supporting the "mass as a quantity of matter" claim. This is because centuries ago it was indeed believed by some mainstream physicists (such as Newton) that mass was "quantity of matter". Likewise with phlogiston, this once somewhat mainstream belief was discarded experimentally with the advent of particle physics. Because of that, some school textbooks still present the erroneous notion of mass as "quantity of matter", which is wrong and misleading for children. But no university-level, serious physics textbook, written in the past century by a high caliber physicist would use that definition, since it has been ruled out as a scientific hypothesis over 100 years ago.
Elementary particles, which all have the same "quantity of matter" (i.e., 1), have widely different masses. More than that: systems composed of massless particles have positive mass, when the massless particles interact and form bounded states. If you bring together two particles, the mass of the system is greater than the sum of the mass of the particles. Two bodies orbiting one another have more mass than the sum of the masses of the individual bodies. The same body, if it heated and increases it's temperature, also increases it's mass.
So it is as misleading to say that mass is "quantity of matter" as it is to say that electric charge is "quantity of electrons". Or even more, since, while massive macroscopic bodies in everyday live are composed of only one particle which causes electric displacement (the electron), they are composed of three different particles (protons, electrons and neutrons) which cause measurable mass, and the difference between the mass of the proton and the neutron is easily measurable: matter formed by atoms which differ only in that one proton is replaced by a neutron (thus conserving the "amount of matter") in each atom have noticeable difference in mass.
Probably the most proper experimental definition of mass is via inertia, and relativistically then mass is the internal energy of a system (meaning the total energy of that system in a frame of reference where the system has 0 total momentum), apart from the unit of measurement. The experimental and the relativistic definition are equivalent as long as relativity holds. So, as far as science goes, the relativistic is the correct explanation for mass, and it is also corroborated experimentally. For example, this definition correctly explains the mass of a proton (which is greater than the masses of the composing quarks) or the mass of a nucleus (which is greater than the sum of the masses of the protons and neutrons composing it). It is the definition every working physicist uses in order to calculate the mass of a body. Not "quantity of matter".
The sources for that definition are the same that you find in the page https://en.wikipedia.org/wiki/Invariant_mass. I think it is interesting that all the proper definitions and sources are listed on that page, while here you have kind of a completely different physics.
To be clear, it is incorrect to state that the relativistic explanation would be the "concept of mass in special relativity", as implied by this link: https://en.wikipedia.org/wiki/Mass_in_special_relativity. This would be like saying that the concept of heat as caused by microscopic movement (instead of phlogiston) is just the "thermodynamic concept of heat", while the main wikipedia article for "heat" still accepted the old phlogiston explanation. The relativity theory was the first (and only) theory to explain mass, just as the thermodynamic theory was the first to correctly explain heat. The other theories (such as the "quantity of matter" theory) have been experimentally discarded, while the relativistic theory is the only one that correctly calculates the masses of physical systems.
Notice also that this is NOT a situation where the old theory is an approximation of the modern one, like Newton's theory is an approximation of relativity which is still valid for practical purposes. This is not the case. Macroscopic bodies which have the same amount of atoms have widely different masses, depending on the mass of the atoms. Now someone might say: "but these atoms have more mass because they have more particles in it", and, as explained before, that would be wrong: atoms with the same amount of particles often have measurably different masses.
This is NOT original research as it is the standard understanding among physicists. I hope people can help me with sources that might discuss this more directly.
Minor mistake
Redefinition of 1 kg
Hey guys, the redefinition of 1 kilogram has been done and Wikipedia has an article for this. Kindly update the page as it says that the redefinition is expected on may 20, 2019. Thanks. clickheretogototheuserpageofAggarwala2727 (talk) 07:16, 21 September 2019 (UTC)
