Proton mass + Electron mass != Neutron mass?

[Themohawkninja]

I'm not a particle physicist, so I'm sure I'm missing something here, but as far as I know, a neutron should have the mass of a proton plus an electron, since that is how the neutrons of a neutron star come about (the electrons contact the protons and combine to make neutrons).

However, when you add the mass (in atomic mass units) of an electron (0.000544366) and a proton (1.007276466812) you get 1.00782, which is less than the mass of a neutron (1.00866491600).

Where is that little bit of extra mass coming from?

[Kryten]

It's a result of the energy expended to cause the proton and electron to fuse, which becomes part of the binding energy of the neutron.

[Themohawkninja]

It's a result of the energy expended to cause the proton and electron to fuse, which becomes part of the binding energy of the neutron.

Does that mean that all the neutrons in the universe are the result of this sub-atomic fusion?

[Tex_NL]

...

Where is that little bit of extra mass coming from?

Energy. E = mc²

[Kryten]

Does that mean that all the neutrons in the universe are the result of this sub-atomic fusion?

No, it's just a function of the binding energies involved. Any neutron will show the same excess mass, regardless of how it was produced.

[jwenting]

Does that mean that all the neutrons in the universe are the result of this sub-atomic fusion?

if you go back far enough, every single sub atomic particle larger than a quark (and they're not splitting those I think) is.

[Palladium Corp]

You need energy to turn an up quark into a more unstable down quark.

[K^2]

It's a result of the energy expended to cause the proton and electron to fuse, which becomes part of the binding energy of the neutron.

That's actually completely wrong, because neutron is heavier than proton and electron combined. Hence, neutrons aren't stable, but protons are. If this wasn't the case, the only kind of matter in the universe would be neutron matter.

You need energy to turn an up quark into a more unstable down quark.

That's a gross oversimplification. By this logic, neutral pions should be uu-bar particle. But they are not. They are uu-bar - dd-bar mix. Difference in current masses of up and down quarks is almost insignificant compared to energies of nucleons. It's important in the dynamics, which makes neutrons and protons not quite identical, but to say that neutron is heavier simply because down quarks are heavier is an oversimplification.

[Idobox]

From wikipedia:

Free neutrons decay by emission of an electron and an electron antineutrino to become a proton, a process known as beta decay:[21]

n0 → p+ + e− + νe

The decay energy for this process (based on the masses of neutrino, proton, and electron) is 0.782343 MeV

That's your extra mass: the neutrino, and some extra binding energy energy (released as kinetic energy in beta decay)