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Old 31st December 2018, 10:22 AM   #67
Ziggurat
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Originally Posted by Roboramma View Post
The mass of an atom is very close to the mass of the protons + neutrons + electrons. The binding energy isn't that high compared to the masses of those particles, so while it's a measurable portion of the mass, it's certainly not 95% of it.
Quite so.
https://en.wikipedia.org/wiki/Nuclea...g_energy_curve
A proton has a mass on the order of 900 MeV, so a nucleon binding energy on the order of 9 MeV gives you only about 1% difference in mass.

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Actually, thinking about it the mass of, say, Helium is less than the mass of it's constituent particles, which is why you can get energy out of the fusion of hydrogen into helium.
That's the second fundamental mistake in that post: because you have to add energy to unbind them, the mass of separated constituents of a human would be larger, not smaller, than of a formed human.

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However, something like what he said is true, not of the atoms, but of the nucleons: if you were to disassemble the protons and neutrons (somehow?) into their constituent quarks, on the other hand, then you'd find that most of the mass was missing. Because the mass of a proton really is mostly in the form of the binding energy (the gluons I guess), and the quarks' mass makes up a small proportion of the total.
That's closer to it, but it's more complicated than just binding energy:
The bare mass of up quarks is so light, it cannot be straightforwardly calculated because relativistic effects have to be taken into account. Due to strong force mediated by gluons in the gluon field, the quarks move at roughly 99.995% of the speed of light, leading to Lorentz factor of roughly 100. As a result, the combined rest mass of quarks is barely 1% of proton or neutron mass.
This is further complicated by the fact that you cannot actually separate all the quarks, since they cannot exist on their own. Pull them apart hard enough, and you just end up generating new quarks so that the parts you pulled apart each still have at least 2 quarks.
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