Q: What is mass?

Physicist: Short questions are real killers!

“Mass”, like practically everything in physics, is defined in terms of its properties, like:

1) Mass has inertia.

2) Mass creates gravity (energy does this too).

3) Mass travels slower than the speed of light.

Anything with these properties has mass.  But as for what mass is: no idea.

I’ve heard theories about mass being “knots” of spacetime, but the most promising theory today involves the Higgs field.  Essentially it describes “a field” that imparts the properties of mass to most types of particles (not photons, for example).

But that’s not much closer to an answer.

I suppose it’s best to say that there are a lot of things in this universe with a certain set of properties in common, and we say that those things “have mass”.

I do recognize how deeply disappointing that answer is.  I mean “mass is something with the properties of mass”?  What the hell are you supposed to do with that?

You could make the argument that it’s impossible to really understand what anything is, and that what we consider “intuitive understanding” is nothing more than a familiarity with a some set of familiar properties.  If you wanted to make that argument anyway.

Even electrons, which today we know are some kind of especially small particle thing, were named long before their size or particleness were known.  They were named for the fact that when you rub amber with fur, the amber acquires some strange properties (a negative electric charge), and the thing that causes those properties came to be known as “electron”, the Greek name for amber.

You don't have to understand exactly what something is to know it when you see it.

Point is, the name was given to a set of properties, and at the end of the day that’s probably the case for damn near every thing worth naming.

This kind of crappy answer isn’t restricted to physics, by the way.  You want to waste an evening; get a biologist or zoologist drunk and ask them, “Hey, what’s a species?”.

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4 Responses to Q: What is mass?

  1. In Philosophy of Science it is usually understood that science (any science) isn’t about describing “things” (or, more technically, “objects”) themselves, but rather how they relate to each other. Every single physical concept always involves at least two objects and an explanation on how one influences (or not) the other, usually (but not necessarily) in a mathematical way. This is why questions of what this or that “is” can rarely, if ever, be answered.

    An example, taking from your 3 characteristics of mass:

    1) To understand inertia one has to take into account force, hence, a certain way in which one something acts over another something.

    2) Gravity is the same: a certain way in which one something acts over another. Classically it’s an attractive force between at least two somethings. Relativistically it’s a geometric relationship between things and the space around them. Quantically it’s about an exchange of somethings between somethings. Even Aristotelically it’s about how somethings feel the need to move into somewhere specific.

    3) And speed also revolves around the change in position of something in relation to another something, or at least to some specific frame of reference.

    One can go about asking what any of the above terms “is”, but the result in the same: you always find some other relationship between two other somethings that, each, can also have its “is-ness” questioned. But, for the same reason what asking what “part” is cannot be answered without making reference to “whole”, doing so won’t get anywhere. Both terms are necessary for an understanding to arise.

  2. Sanal Kumar MS says:

    mass is the amount of matter in an object..;-)

  3. Joman says:

    The most satisfying answer I ever got was back in high school when it was jokingly asked to define the nature of matter and a brainiac classmate answered by saying it was “highly condensed energy”. Thoughts?

  4. Xerenarcy says:

    in a sense mass is “mostly” condensed energy yes. for one, the mass of an atom is for the most part the mass of the nucleus, which itself is the masses of the neutrons and protons and some binding energy, and the masses of neutrons and protons is largely the binding energy that keeps them intact.

    getting deeper into things, you will find that the only way to ‘nicely’ describe matter is loops of energy – every description i have seen of what matter / mass is comes down to ‘something’ in a loop or in a knot, including time itself.

    the reason for this is not immediately obvious… if you take some massless form of energy, (bad example but nothing else to work with) for instance a photon, and force it to travel in a small loop (somehow of its own accord or due to some force / field; not terribly important); while the photon does not itself have mass, the loop structure effectively confines the photon’s energy to a set region of space, which causes the loop for all intents and purposes to have inertial mass.

    the loop then has no specific handedness (though it can gain it if the loop path itself can have handedness), and is trivially orientable at the least. it takes energy to displace the loop and the loop can (with some hand-waving) exhibit momentum / inertial-like behavior, and then gains an inertial reference frame as a result. gravitation is self-explanatory here, since we are still dealing with energy in some form as a given. it is also possible the energy of rest mass is much larger for this reason too – the energy that keeps the loop in place i would expect contributes to rest mass as well.

    however, there is little meaning today to try to ask for a definite answer as to what that loop is made of – some will say strings / string theory / extra tiny dimensions, others say (per my example) a literal loop of energy in some form, and another group will go with the ‘matter = knots of spacetime’ approach. personally i’m on the temporal karnot engine bandwagon (explains the arrow of time within 3 time dimensions).

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