We’ve clarified the key physics term energy previously. Mass is another often confused word in the wider scientific conversation. For instance, it is often misused to signify the amount of material in an object.



This is the common conception, and one generally taught in academic textbooks. It absolutely works that way most of the time in our everyday worlds. However, the “mass as quantity of material” definition starts to break down when we look at events occurring at the atomic scale.

Recall that some quarks have more mass than atoms .

On the other hand, the proton is much, much more massive than the quarks that comprise it. That is unless you include virtual quarks, a somewhat debatable addition, in which case the top quark is more than 100 times heavier than the proton it’s integral to! What’s going on? If mass does not mean the quantity of material, what is the unambiguous and consistent meaning of the word?



The development and confirmation of the Higgs process in recent decades has successfully detailed the quantitative affairs which terminate in the subatomic world’s apparent mass. But perhaps mass is best comprehended more generally through the oppositional notions of inertia and gravity. Congruence of these concepts provided the pivotal equivalence that initially allowed Einstein to formalize his General Theory of Relativity.



Einstein, in recognizing ideas by his contemporary Ernst Mach, realized that inertial mass had to be the product of bodies being held in place by their surroundings. Gravity, on the other hand seems to depend upon the distance between bodies. Clearly mass refers to some action being done between materials that can affect both inertial and gravitational experiences. But what exactly are the atoms doing to each other during these phenomena?



Well, atoms behave as if they are pulling on one another.



One way to visualize mass is to imagine atoms connected by an imaginary tensile structure.

Consider a tiny three atom universe in this context. As pictured below, imagine each atom pulling on the others ceaselessly with some fixed effort called mass, m. Atom B in the center must split its effort to pull against each of the opposite ends. The end atoms A and C can put all of their effort in one direction but in doing so they directly oppose one another.