Demystifying the Higgs Boson with Leonard Susskind

> no one really knows what mass is. We only know that mass bends space-time curve, hence gravity. Mass is much better understood by its role in inertia. Basically mass is the amount of energy you need to exchange with a thing to change its current speed. This observation works from Newtonian mechanics to QM and GR as well. Now, why do things have mass? The famous E=mc² explains this for most things: they have mass because something inside them has potential or kinetic energy. This works all the way down to the atomic level - the mass of a proton for example is almost entirely explained by the potential energy of the quarks being held together in a small volume; the total mass of the quarks themselves is only a small fraction of that. Now, the mass of the elementary particles is somewhat more complicated, but the Standard Model does have explanations for those - symmetry breaking for fermions, and the Higgs mechanism for the massive bosons. The next mystery is: why is inertial mass equal to gravitational mass? GR has essentially explained this, by showing that acceleration is equivalent to gravitational attraction depending on your frame of reference. So overall, I'm not sure what Brian Greene means by that - mass is at least as well understood as other basic properties of particles (charge, spin, color charge). This lecture by Leonard Susskind explains most of these things about mass in a way I found easy to follow: [this video]

The quarks that are bound in a way we call a proton are held together by the strong force at a certain distance from each other - that distance is what we consider the volume of the proton, with the quarks being "inside that volume". This system of 3 quarks has some amount of potential energy, and/or some amount of kinetic energy from the relative movement of the 3 quarks. The mass of this system is then given essentially by E=mc^2, with E being this kinetic energy. There is a very good video of a lecture by Leonard Susskind that explains why energy and mass are interchangeable in this way if you want a more in-depth explanation: [this video]