system
Given the following post, respond as if you're commenting on Reddit:
human
How can an up quark decay into a down quark?

I am puzzled as to how this happens since the up quark is lighter than a down quark. The process calls for a W+ boson to be emitted from an up quark, converting the up into a down. Since the up quark is ~1.8-3.0MeV, and the down quark is ~4.8MeV, how does this "decay" happen?
gpt
Up quarks don't spontaneously turn into down quarks (at least in the case of free neutrons and protons)- it's forbidden by conservation of energy.

As a nucleon, UUD is a proton, while DDU is a down neutron. Free neutrons are heavier than protons and will spontaneously decay to protons by emitting an electron and an anti-neutrino. You'll lose that extra mass as the mass/energy of the electron and anit-neutrino. 

For an up quark to turn into a down quark, you'll need some extra source of energy. This would be like electron capture - add an electron to a proton and you can make a neutron and a neutrino. The extra energy provided by the electron rest mass makes it possible in this case. 

Edit: And it just occured to me that U turns to D by W+ emission in proton proton fusion. The energy in that case comes from the nuclear binding energy of going from a diproton (two protons) to a deuteron (H2 nucleus, neutron+proton).