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Last night I posted this question: Since light has inertia and experiences gravity, what does it mean for photons to be massless?, which I now think was overly wordy and didn't properly express what I actually want to know. But I don't want to change the meaning of that post since it's already gotten answers, so this is my second attempt to ask the right question.

Is there a unified conceptual definition of mass that's valid in classical physics, SR, GR, and particle physics/QFT? To clarify, I'm thinking of how, in mathematics education and the history of math, we often start with a special case of some concept and treat it as the definition, then later we look at a more generalized definition that encompasses a larger set of objects. An example is vectors sometimes being defined in introductory physics and/or linear algebra classes as "objects with both magnitude and direction", which is just a clunky way of referring to the special case of members of \$R^n\$, before later defining vectors more broadly as members of vector spaces.

Is there something analogous with the concept of mass? As I mentioned in the linked question, there seem to be various ways of thinking about what mass is, depending on the context, from the simple "takes up space and has inertia" from introductory physics courses, to the more abstract like "confined energy" (as PBS Spacetime describes it in their video, On the Nature of Matter and Mass). Is there an overarching concept here that can give one conceptual definition (i.e. not simply that mass is the quantity related to energy and momentum in Einstein's equation or another equation. I get that might work as a mathematical definition, but it doesn't satisfy my curiosity to understand the underlying concept(s))?

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    There isn't even a single unique definition of "mass" in GR alone, cf. ADM mass, Bondi mass, etc.! It's perfectly normal for some physics concepts to not have some fully general definition that covers all cases at once, or to have different possible generalizations from a more limited use case where the different generalizations happen to coincide. Words have meaning only in context, this applies to "mass" as well as any other word. – ACuriousMind Dec 28 '23 at 23:26
  • @ACuriousMind, OK maybe "definition" was the wrong word. I suppose what I'm really asking is if there's some unifying underlying CONCEPT behind the various ways to think about mass? Or is this just an issue of the same word being used to refer to entirely different things? – Mikayla Eckel Cifrese Dec 28 '23 at 23:32
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    The CONCEPT is “invariant length of energy-momentum four-vector”, as Dale explained. You are not going to find a better way to think about mass IMHO. – Ghoster Dec 28 '23 at 23:45
  • I agree with @Ghoster. The best definition up to my knowledge is that mass is the "charge" associated to translations (charge is improper but it is useful to give the idea). You take what describe translations and calculate the invariant quantity associated to this: the invariant lenght in energy-momentum 4 vector for SR and the Casimir eigenvalue of $\hat{P}^2$ in QFT – LolloBoldo Dec 28 '23 at 23:53
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    I don’t agree with the closure. Asking for a unified concept across mainstream physics disciplines is mainstream science, even if the answer is that such a unified concept doesn’t exist. – Dale Dec 29 '23 at 02:06
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    @Dale I second this. The comments (especially by @ACuriousMind) have already provided really good content. The possible answer that a unified definition of mass (across all disciplines) is currently not mainstream is itself an answer within the realm of mainstream science. – Maximal Ideal Dec 29 '23 at 02:21
  • @MikaylaEckelCifrese To answer to your response to ACuriousmind comment, rather to your question itself : I think it is indeed an issue of the same word used to refer to different things. There are many concepts. In GR, ADM mass and Bondi mass are indeed different concepts, and have different values. And in SR, the obsolete notion of "relativistic mass" was, IMHO, a concept that deserved the name "mass" more than the "invariant mass" concept, which is now the only one to be called "mass". – Alfred Dec 29 '23 at 03:11
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    @Alfred, there is no possible place in which that obsolete concept is of any use. In particular, whenever somebody thinks that relativistic mass is useful, one has to reply to them about which of transverse mass or longitudinal mass is the correct relativistic mass. Instead, things are much clearer with the invariant rest mass. The essence of MEC's question is tackled in my new answer just uploaded. – naturallyInconsistent Dec 29 '23 at 03:49

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I wanted to give an answer to your original question from last night, but I was too late for that; it was already unfairly closed and dunked upon. Not to mention that they closed your question as duplicate on something that is not the heart of your question; and there are so many correctly downvoted answers. So I am going to give you an answer that goes to the very essence of your question.


Sadly, we have to first come to grips with an observation that is made famous in Feynman Lectures, where he talked about lego blocks as an analogy to energy. In particular, the idea is that physics might well never provide us with an explanation of what exactly energy is (and momentum too), that their essence might well forever be mysterious and abstract for humanity for the rest of the universe's lifespan, but we will still be able to reason with and work things out in physics relying on them. They are just these abstract conserved quantities, and you can understand a lot of physics phenomena just keeping track of conserved quantities.


With that out of the way, then we can give a definition of mass, valid in the SR limit, whereby the spatial extent of the system of particles is not so big as to require GR corrections, as $$\tag1m_0=+c^{-2}\sqrt{(\sum E)^2-(\sum\vec pc)^2}$$ You will find that this definition immediately settles a lot of the deeper confusions.

First of all, this definition means that rest mass is really just the amount of energy a system of particles localised at a place has when this system is not moving in the overall sense. Note that system can be just system of one particle. It immediately asserts that, say, if you provided heat to this system, then it will have greater rest mass. That means that the ground state would have a different mass than excited states of the same particle; it is not just the rest mass + kinetic energy that contributes to the above sum, but rather also energy stored in the electromagnetic field, as is necessary for considering atoms in the Coulombic model; this energy is spatially extended away from just the nucleus and where the electron precisely is. As a consequence of all the above considerations, it is thus that the law of conservation of mass is only an approximation, valid only because it is difficult to convert between rest mass energy and other forms of energy, giving us the illusion of conservation. (And the reason why it is difficult, is because most of the rest mass energy is in the nuclei, so that the chemical conservation of mass is really just accounting for the numbers of protons and neutrons.)

In introductory physics and chemistry, students are told that massive objects are those that are made of matter and take up space. But then matter is defined as anything that takes up space and has mass, which is circular.

The chemical concept of matter is just a mess. The EM wave also takes up space. The wording is just ridiculous and there is no way to fix this lie. In the above, I have already given an account that is completely settling all chemical insights into the nature of mass, so that I think there is no need to consider any more questions from chemistry on this topic.

OK, so if I look deeper I find that, in particle physics, mass is supposedly just the confinement of energy -- the Higgs field somehow "confines" massive fundamental particles and composite particles, like protons, gain most of their mass from the confinement of the fundamental particles that make them up.

This is a bizarre mix of simultaneously correct and extremely wrong. It is much more fruitful and simple to tackle the mistake upfront:

Earlier, I invoked the fact of life that most energy that we care about are locked up tightly in rest mass energy. After all, it is the basis of the chemical law of conservation of mass. It just is the case that

  1. Chemical games with electrons keep nuclei far apart
  2. This means that the nuclei cannot interact with other nuclei (as in, strong interaction effects are negligible between nuclei in usual chemical settings)
  3. Rest mass energy is mostly tightly confined in nuclei So, as long as you are not sending these stuff to relativistic speeds, you will get the illusion of conservation of mass. Minor chemical and thermal energy changes would not change the rest mass energy enough to be easily detected.

This means, fortunately for us, that if we want to explain most of the masses we see in everyday life, we just need to explain the masses of nuclei. This is a much reduced problem scope, and where a huge mistaken idea afflicted you. Around 2007, teams of researchers, e.g. KEK and others, made a computer simulation of nuclear material, at the quarks and gluons level, and discovered that if you made the quarks massless (and gluons are massless either way), then you can explain about 97% of the masses of protons. You might also be interested in some of the beautiful visual animations of quark-gluon plasma from that period.


That is, if you want to talk about the Higgs, you need to know that you are only going to explain 3% of the masses we usually think about. Do not make it the huge deal that you think you have to master just to understand this.

What the Higgs is really important for, is initially to give the W and Z bosons their masses. Otherwise, the ElectroWeak theory would fail to have the $U(1)\times SU(2)$ symmetry (and breaking) that we really wanted the theory to have. We could have, say, assumed that the Higgs only worked on W and Z bosons, and continued having, say, electrons to have the standard rest masses. It just so happens, however, that, since we have already invented the Higgs, the theory will just be so much easier and nicer and more symmetric, if we sent all the rest mass terms of the SM interactions to the Higgs. It is just about swapping the parameter of rest mass for each particle, to the parameter of interaction strength between each (bare) particle recast as massless, and the Higgs. There is really nothing conceptually clarifying that comes from this. Pure mathematical trickery.


It comes time to tackle inertia. Yes, you are correct in that a lot of introductory texts say something like

Later on, we learn that mass is related to inertia, or the ability to resist changes in motion and that mass is proportional to gravity and I've read multiple times about Einstein unifying those definitions. OK, that works well enough in classical physics, but then we learn that photons are massless -- logically, that must mean they don't have inertia and/or aren't affected by gravity.

This is just irresponsible. Anybody who has learnt any bit of the General Theory of Relativity will understand that inertia had never been about masses; What was masses really were disguised full Einsteinian energies. $$\tag2F_G=G\frac{Mm}{r^2}=G\frac{(E_1/c^2)(E_2/c^2)}{r^2}$$ That almost all the appearances of masses in classical physics and chemical physics is really just references that cared about the total amount of energy, whereas old classical physics's idea of energy is really just differences in energy from the ground state rest energy.

It is only if you understand that GR says that it is energy-momentum that gives rise to both inertia and gravitation, that it makes sense that photons can have zero rest mass and yet cause gravitation and be affected by gravitation. In particular, when you have two colliding photons so that their momenta cancel, then invoking Equation (1), we see that that system of two colliding photons has a rest mass energy that causes the gravitation that we are familiar with. Causes the curvature of spacetime due to a concentration of energy that is overall at rest. And then photons that are moving, have a well-defined energy-momentum 4-vector (or better, the spatially extended beam has a stress-energy-momentum 4-tensor density) that can then be "deflected" by such curvature.

In a sense, it is masslessness that is a weirdness that needed explanation, but it is easy to understand under this scheme, and clearly tangential to the actual essence of your question. It would be much less confusing on students if people rigorously defaulted to this scheme of talking about masses. And note that this all about rest mass, never the accursed talk about relativistic mass, transverse mass, longitudinal mass that they love to bring in. Alas, the people who are interested in the pedagogy of physics are just too few, and researchers and educators simply would not learn.


Yes, in GR, the concept of mass, which, mind you, is defined as the rest energy, is altogether a tremendous mess, extremely difficult to both conceptualise nor get technically correct. This lends credence to the claims that the generalisation of the mass concept is that which is impossible in the GR sense. They are technically correct, but that is such a cop-out. It is clear that the viewpoint that I just gave you above, is logically internally consistent and supported by the evidence accrued thus far, only needing the nuance that the GR complications bring in. It is not the same as saying that it cannot be done and thus we should do absolutely nothing about it.

  • you should make clear that the first formula above has c=1. has to do with the units too not only size. there is a $c^2$ there. – anna v Dec 29 '23 at 05:29
  • @annav between suggesting that everybody should just get used to working with c=1 and just changing Equation (1), I have given up and chosen the latter. – naturallyInconsistent Dec 29 '23 at 06:02
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Instead of asking whether there is a unified paradigm-spanning characterization of mass, you should first be asking what one would look like, if there were.

In Newton's original formulation - indeed in his very first two definitions - he defined the mass $m$ of a body to be what we in modern language express as the integral of the body's density over the region $V$ space covered by the body, and the momentum $$ of the body as the integral of the product of the density of each part of the body with the velocity which this part is moving at: $$m = \int_V ρ() d^3,\quad = \int_V ρ() () d^3.$$ Provided that the density is non-negative everywhere $ρ() ≥ 0$ (and positive in the region, i.e. $ρ() > 0$ if $ ∈ V$), then you could also define the body's center of mass motion by assigning to it a velocity: $$ ≡ \frac{\int_V ρ() () d^3}{\int_V ρ() d^3},$$ expressed as a density-weighted average of the localized velocity of its parts. With this, one can express the body's momentum in terms of its center of mass velocity as: $$ = m .$$

The key properties of these quantities relate to how they transform under a change to a moving frame, i.e. under a "boost" transform. Suppose you change to a frame that is moving at a velocity $Δ$. Then, in the moving frame, the values of the density and localized velocities would respectively be transformed as: $$ρ → ρ,\quad () → () - Δ(),$$ which yields the corresponding transforms for the mass, center of mass velocity and momentum: $$m → m,\quad → - Δ,\quad → - m Δ.$$

The key take-home is the combined transform: $$(m,) → (m, - m Δ).$$ Mass and momentum transform together, in a self-contained way, as a 4-vector.

In years following Newton, it was gradually realized that the kinetic (+ internal) energy, which will be denoted here as $H$, played in important role in dynamics. Expressed as: $$H = ½ m ||^2 + U,$$ it consists of a part $½ m ||^2$ that is dependent on the body's center of mass motion, and a part $U$, that expresses the total (kinetic + internal) energy of the body's parts. The latter accounts for the motions of the body's parts relative to the body's center of mass, and for the internal energies in each of the parts. Being fixed to the body's center of mass, it is independent of the observer's frame of reference, so that one has the following transform under a boost: $$H → ½ m | - Δ|^2 + U = H - ·Δ + ½ m |Δ|^2.$$

Combined with the mass and momentum, yields the following as a self-contained transform: $$(m,,H) → \left(m, - m Δ, H - ·Δ + ½ m |Δ|^2\right).$$

This can be expressed more succinctly as an infinitesimal transform with an infinitesimal boost velocity $$ in place of $Δ$, keeping only quantities to the first order in $$, as the following set of infinitesimal changes: $$δm = 0,\quad δ = -m ,\quad δH = -·.$$

It's actually easier to use transforms in infinitesimal form since they are linear. There's no real loss in doing so, since the finite version of the transform can be obtained by way of a Taylor series: $$X + ΔX = X + s δX + \frac{s^2}{2!} δ^2X + \frac{s^3}{3!} δ^3X + ⋯,$$ with: $$\begin{align} δ(m,,H) &= (0,-m ,-·),\\ δ^2(m,,H) &= (0,-δm ,-δ·) = (0,,m||^2),\\ δ^3(m,,H) &= (0,,δm||^2) = (0,,0),\\ δ^n(m,,H) &= (0,,0)\quad (n > 3). \end{align}$$ by setting $Δ = s$. Thus, $$\begin{align} (m,,H) + Δ(m,,H) &= (m,,H) + s (0,-m ,-·) + \frac{s^2}{2} (0,,m||^2)\\ &= \left(m, - ms, H - ·s + ½m||^2s^2\right)\\ &= \left(m, - m Δ, H - ·Δ + ½m|Δ|^2\right). \end{align}$$

Now ... we get to Relativity, in which a new talking point is added: mass-energy conversion. If we use a similar expression for mass versus momentum, $ = M $, distinguishing it from the invariant mass $m$, then according to Relativity, there is an additional contribution to $M$ arising from the mass equivalent of the body's kinetic energy, $H$, so that one has the following decomposition: $$M = m + \frac{H}{c^2}.$$

If we continue to assume $m$ is invariant (i.e. $δm = 0$), and that $H$ has the same infinitesimal transform ($δH = -·$), and that the same ratio $M$ applies both to the finite form of the expression for momentum ($ = M $) as does to the infinitesimal transform ($δ = -M $), then this entails the following set of infinitesimal transforms: $$δ(M,,H) = \left(-\frac{·}{c^2}, -M, -·\right),\quad δm = 0,\quad δ = - + \frac{·}{c^2}.$$

The extra term in the transform of $$ is needed to ensure the consistency between $ = M $ and $δ = -M $, but it can also be connected to the Lorentz transform. If we write the differential equation for the velocity $ = d/dt$ in differential form and apply the transform separately to the coordinate differentials $d$ and $dt$, we get: $$\begin{align} &d = dt\\ &⇒\quad δ(d) = \left(- + \frac{·}{c^2}\right)dt + δ(dt)\\ &⇒\quad δ(d) + dt = \left(δ(dt) + \frac{·dt}{c^2}\right)\\ &⇒\quad δ(d) + dt = \left(δ(dt) + \frac{·d}{c^2}\right) \end{align}$$ and assume that this holds for arbitrary $$, then it follows that: $$ = δ(d) + dt\quad⇒\quad δ(d) = - dt,\\ 0 = δ(dt) + \frac{·d}{c^2}\quad⇒\quad δ(dt) = -\frac{·d}{c^2}. $$ This is the infinitesimal version of the Lorentz transform. You can run this sequence in reverse to get the transform for $$ and - from this - the connection between $ = M $ and $δ = -M $ that they both be with the same ratio $M$.

In Relativity, one normally combines the mass and kinetic energy into the total energy $E$, via the formula $E = M c^2$, as: $$E = \left(m + \frac{H}{c^2}\right)c^2 = m c^2 + H.$$ This allows you to rewrite the transform of the five-vector $(M,,H)$ in split form as self-contained transforms $$δ(E,) = \left(δm c^2 + δH, δ\right) = \left(-·, -M\right) = \left(-·, -\frac{E}{c^2}\right),\quad δm = 0,$$ split between the four-vector $(E,)$ and scalar $(m)$.

However...

there is no counterpart, in Relativity, to the "$U$" of the expression $H = ½m||^2 + U$, which means that the value of $H$ in the rest frame of the body is just 0, and correspondingly $m$ is just the rest frame value of $M$. So, it is no longer an independent quantity. Instead, one has the constraint: $$\frac{E^2}{c^2} = ||^2 + m^2 c^2,$$ which you can prove by (a) showing that the equation reduces to an identity in the body's rest frame and (b) showing that both sides have the same infinitesimal transform.

If we were to go beyond Relativity and actually attach a counterpart to $U$, then we would need to draw a distinction between $m$ as "rest mass" versus $μ = m$ as "invariant mass", renaming the latter $μ$. In that case, the decomposition for $M$ and expression for $m$ would be rewritten, respectively, as $$M = μ + \frac{H}{c^2},\quad m = μ + \frac{U}{c^2}$$ and with the transform $δμ = 0$.

Why the "beyond Relativity" part? Because it's only there that you will find the stitching together of the non-relativistic and Relativistic accounts for mass into the seamless unifying framework that you're asking for.

So, now the actual splitting of $(M,,H)$ is into $(E,)$ and $(μ)$, instead of into $(E,)$ and $(m)$. Instead, $m$ arises as the norm of $(E,)$: $$(mc)^2 = |(E,)|^2 = \frac{E^2}{c^2} - ||^2,$$ rather than as an independent quantity in its own right.

In Relativity, the finite version of the transform $δ(E,)$, when applied in reverse to the rest frame values $(mc^2,)$ for a body yield their values in a moving frame: $$(E,) = (Mc^2, M),\quad M^2 = m^2 + \frac{||^2}{c^2} = m^2 + M^2\frac{||^2}{c^2}\quad⇒\quad M = \frac{m}{\sqrt{1 - ||^2/c^2}}.$$

From this follows $$H = (M - μ)c^2 = \left(E - mc^2\right) + U = \frac{M||^2}{1 + \sqrt{1 - ||^2/c^2}} + U.$$

So, what are these quantities and what is the unified account for them?

In Relativity, you can verify that $E dt - ·d$ is an invariant as follows: $$\begin{align} δ(E dt - ·d) &= δE dt + E δ(dt) - δ·d - ·δ(d)\\ &= (-·) dt + E \left(-\frac{·d}{c^2}\right) - \left(-\frac{-E}{c^2}\right)·d - ·(-dt)\\ &= 0. \end{align}$$ So, $(E,)$ transform as $(∂/∂t,-∇)$.

However, if you try this make this work with the non-relativistic version $H dt - ·d$ and - specifically - with the kinetic energy, you'll instead get: $$\begin{align} δ(H dt - ·d) &= δH dt + H δ(dt) - δ·d - ·δ(d)\\ &= (-·) dt + H \left(-\frac{·d}{c^2}\right) - \left(-M\right)·d - ·(-dt)\\ &= \left(M - \frac{H}{c^2}\right)·d\\ &= μ·d. \end{align}$$ The non-relativistic version of this derivation - obtained by setting all of the $1/c^2$ terms to $0$ - yields the same result.

To make this invariant requiring slipping in an extra coordinate $u$ whose differential transforms as $$δu = ·d.$$ Then, we can write down the following invariant: $$H dt - ·d - μ du = H ds - ·d - M du,\quad s = t + \frac{u}{c^2}.$$ In the process, you'll also find another invariant emerge: $$δ(ds) = δ(dt) + \frac{δ(du)}{c^2} = -\frac{·d}{c^2} + \frac{·d}{c^2} = 0.$$ The coordinate $s$ is the "beyond" Relativistic version of the absolute time of non-relativistic theory, and the 5-vector $(H,,μ)$ transforms as $((∂/∂t)_u,-∇,-(∂/∂u)_t)$. Similarly, the 5-vector $(H,,M)$ transforms as $((∂/∂s)_u,-∇,-(∂/∂u)_s)$.

The 5-dimensional geometry with coordinates $(t,u,=(x,y,z))$ - in the non-relativistic case - is known as Bargmann geometry. It is characterized by the coordinate invariants: $$|d|^2 + 2 dt du,\quad ds = dt.$$ The Relativistic version is a morphed version of Bargmann with the following as its coordinate invariants: $$|d|^2 + 2 dt du + \frac{(du)^2}{c^2},\quad ds = dt + \frac{du}{c^2}.$$

A unified account for mass that spans the paradigm divide between Relativity and non-relativistic theory can only be found by first providing a unified account of their respective geometries, and only within it will you find your answer. Just as the total energy $E$ and momentum $$ - in Relativity - can be expressed as conjugate to $t$ and $$ (up to sign), the kinetic energy $H$, momentum $$ and invariant mass $μ$ can - in both the "beyond" Relativistic and non-relativistic settings - be expressed (again, up to sign) as conjugates respectively to the coordinates $t$, $$ and $u$. Similarly, $H$, $$ and $M$ can be treated as conjugate to $s$, $$ and $u$, where $s$ is taken as a coordinate in place of $t$.

In the unified account you're looking for, mass (either as $μ$ or $M$) is conjugate to $u$.

In dynamics given by an action integral $S = \int L(,t,u) ds$, requiring independence of $u$ leads - by way of the Noether theorem - to a conservation law for $μ$. If you drop down from "beyond" Relativity to just ordinary Relativity, where $μ = m$, or down to the non-relativistic setting, that leads to a conservation law for $m$. For a single free body, you can directly write it as a Lagrange multiplier: $$S = \int \frac{m}{2} \left(\left|\frac{d}{ds}\right|^2 + 2\frac{dt}{ds}\frac{du}{ds} + α\left(\frac{du}{ds}\right)^2\right) ds = \int \frac{m}{2} \frac{\left|\frac{d}{dt}\right|^2 + 2\frac{du}{dt} + α\left(\frac{du}{dt}\right)^2}{1 + α\frac{du}{dt}}dt. $$

In the Relativistic case ($α = 1/c^2$) or non-Relativistic case ($α = 0$) that yields the correct values for the conjugate momentum, after eliminating the $u$ coordinate via the equation $$\left|\frac{d}{dt}\right|^2 + 2\frac{du}{dt} + α\left(\frac{du}{dt}\right)^2 = 0.$$

The Euler-Lagrange equation for $u$ will - after applying this constraint - just be $$\frac{dm}{dt} = 0,$$ while for $$, it will be $$\frac{d}{dt}\frac{m(d/dt)}{1 + α(d/dt)} = ,$$ or, since $$\left(1 + α\frac{du}{dt}\right)^2 = 1 - α\left|\frac{d}{dt}\right|^2,$$ it can be written equivalently as $$\frac{d}{dt}\frac{m(d/dt)}{\sqrt{1 - α|d/dt|^2}} = .$$

NinjaDarth
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    While this is a really nice and long treatise, and nicely formatted, it is only dealing with up to SR kinematics, and is of no use in explaining to OP's question about the nature of mass itself. It makes no mention of the Higgs and all, nor of GR complications. – naturallyInconsistent Dec 29 '23 at 05:57
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Mass of an object is object's inertia in the frame in which the object's center of mass does not move.

Or: Mass of an object is the inertia of the object in the frame where the inertia of the object is the smallest. (Works with massless particles)

stuffu
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  • Why restrict it to that frame? Easy enough, if unfashionable, to generalize. – John Doty Dec 29 '23 at 01:12
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    This doesn’t work very well for massless particles. – Ghoster Dec 29 '23 at 01:40
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    Your new edit also fails to cover massless particles, contrary to you thinking that it can. The bigger problem, however, is that your answer is a throwaway that the OP is not very much interested in. It adds no value to the conversation, and many other people have already stated the same. – naturallyInconsistent Dec 29 '23 at 05:58