# Why do people say that mass is too fundamental to know exactly what it is?

sodium.dioxid said:
This is what I don't get. Why are we literally equating [a measurement of mass] to the object's [resistance in motion]? They are proportional. But why equal?
If you are referring to the equivalence principle, then the proportionality is implied when it is not stated. Also, the equivalence principle does not involve force. That's because $F = m_i a$. However, $F \neq m_g a$.

Why are we literally equating [a measurement of mass] to the object's [resistance in motion]?
Sorry, guys. What I actually meant to say was:

Why are we literally equating [a measurement of matter] to the object's [resistance in motion]?

... the problem remains in that even if you describe mass as an interaction with the higgs field, the question of "what is mass" still hasn't been answered to some people.
I agree that this explanation, if eventually experimentally verified, should lay to rest the problem of mass. The difficulty for some people might still be the underlying nature of energy which is responsible for mass, but as we have seen from the other thread I think that is a discussion that can create quite a commotion :D

Inertia is proportional to mass, not equal to it.
How would one differentiate between the two? For example, in trying to define mass how would you go without involving inertia or resistance in motion. If you say mass is the amount of matter in a something, then what is matter?..........
....That's because $F = m_i a$. However, $F \neq m_g a$.
but doesn't equivalence principle say $m_g = m_i$ ?

but doesn't equivalence principle say $m_g = m_i$ ?
Yes, they are proportionally equal. That only means that you cannot change one without proportionally changing the other. It does not necessarily have anything to do with force. The magnitude of gravitational force of an object is not related to the objects resistance to change in motion. Or in other words, if you could change the active gravitational mass of an object without changing it's inertial mass, it would have no effect on it's resistance to change in motion (theoretically).

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Inertia is proportional to mass, not equal to it. They cannot be equal because they are defined and measured differently.
Thanks a lot for clearing that up. My AP Physics teacher told me wrong last year when he said that m represents the inertia in F=ma. And I went on for a year thinking this way. By the way, how is inertia measured? You say that it can be done.

As far as mass goes, mass is simply a systematic quantification of matter as I have tried to explain. You guys are telling me it is something more as if it is a ghost. It is an amount, not a property.

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Drakkith
Staff Emeritus
Thanks a lot for clearing that up. My AP Physics teacher told me wrong last year when he said that m represents the inertia in F=ma. And I went on for a year thinking this way. By the way, how is inertia measured? You say that it can be done.

As far as mass goes, mass is simply a systematic quantification of matter as I have tried to explain. You guys are telling me it is something more as if it is a ghost. It is an amount, not a property.
Hrmm. I think I was incorrect. The way inertia and mass are related, i believe it would be m that is the inertia in that formula. Honestly, after a bit more reading, it looks like mass and inertia are almost the same thing. It just depends on what you define as what. As wikiepedia put it, you could define mass as : "the quantitative or numerical measure of body’s inertia, that is of its resistance to being accelerated". Changing mass always results in a change of inertia, and changing the inertia requires changing the amount of mass.

But mass is defined differently under GR and such. So one could say that the inertia of an object is the measure of it's mass. Which makes sense, as measuring how fast a force will accelerate an object will give you its mass.

Is there anyone reading this with more knowledge that could elaborate?

"My message was meant to convey that mass should have been defined in terms of distance and time only."

Yes yes yes!

One thing I think we can all agree upon is that the "masses" of the particles are characteristic, as in eigenvalues. If we can express mass interms of space/time, don't you think this hints at the wave-metric structure of these particles? That the particles are localized eigenmetric solitons?

Has anyone looked at localized time-harmonic eigenmetric solutions to equations that describe space (more specifically, the relationship between metric perturbations and energy), such as the Einstein field equations?

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How can anything be too ''fundamental''. It either is fundmental or isn't. And mass by the way, as fundmental as it is, it is not fully understood why photons can gain a mass, but it not because it is fundamental which makes it almost incomprehensible. If you don't know exactly what mass is, then that is a strict statement saying we will never know what mass is. That is demonstratably false, especially for anyone with any background in spontaneous symmetry breaking.