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Just before 10:15 he says we don't understand inertia. IDK exactly what he means there.
Inertia is defined as resistance to change in motion i.e. resistance to acceleration. We use the letter m to indicate this, for mass (but let's not get ahead of ourselves).
F = ma
is describing the relationship between a Force, F, acting on some object with property m, and the associated acceleration, a, of that object.
Ignoring for the moment whether or not we think we know what causes m, that relationship is simple enough to easily grasp the affect of m on the relationship between F and a. m is a constant of proportionality between them. I.e. F and a are proportional to each other and the constant multiple between them is F/a = m.
So mathematically, it's easy to understand "What is m?"
Physically, understanding "What is m?" leads us to put a label on it. We'll call it mass.
*brushes hands* My work is done here.
Seriously, though.
Via QM, energy conservation, and application of what parts of GR can be wed with QM, we were able to mathematically predict the masses of compound particles, specifically nucleons (protons and neutrons) to quite good precision. (I can't find the exact numbers on it right this second). We know that almost all of the mass of a proton or neutron comes from the binding energy of the quarks that make up those compound particles. Even without knowing the mass contribution of the quarks themselves, we can use E = mc2 and write m = E/c2, then plug in the binding energy due to the Strong Nuclear Force, divide by c2, and get almost exactly the empirical value.
The Higgs helps show how the quarks and electrons have "intrinsic" mass, even when not in a bound system. However, it's well worth noting that the mass of an atom is largely due to the mass of the nucleus (over 99%). An electron's mass is about 1/1800 that of a proton or neutron. For Hydrogen (no neutrons), that's 3 quarks and an electron not accounted for, so (pulling some numbers for quark and electron masses from a table) 19/1800 ~= 1% of the mass of Hydrogen needs the Higgs to explain. The rest is simply the binding energy of the quarks inside the proton, which is mathematically known. Far all larger atoms, that ratio is even smaller, due to the presence of neutrons in the nucleus of the atom contributing much more binding energy per quark than quark mass.
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Moving on... It sounds like he's saying, "We haven't studied the effects of accelerating at obscenely high rates up close, yet." I mean... IDK... the LHC at CERN is pulling some pretty extreme accelerations on objects moving a significant fraction of c.
Maybe he's postulating there's some kind of emergent property of inertia on bodies as large as stars, or maybe only very massive stars and up.
I didn't really follow that.
PBS Spacetime on Unruh Radiation
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