In Beta decay, a neutron decays into a proton, and "throws out" an electron at high speed. However, this, to me, suggests that the law of conservation of mass is not being kept here.
Neutrons have a mass of "1",
Protons have a Mass of "1",
Electrons have a mass of "1/1840"
This means that, before the decay, the total mass is "1", but after the decay has occurred, the total mass in the system is "1841/1840", we have gained an electron's worth of mass from somewhere.
DISCLAIMER: I have been taught at a GCSE level, so much dumbing down has occurred in terms of what I have been taught. If I say something wrong, its because I've been taught that at school. Sorry!
This means that, either energy is being converted to mass here, or the "mass" values I have been taught are wrong, or the reason could be completely different. Which is it?
TL;DR: In beta decay, we seemingly gain one electron's worth of mass. Where has it come from?
Best Answer
The mass of a free neutron is 939.566 MeV/c$^2$ (almost 1 GeV/c$^2$, so that's probably where your instructor got the "1" value), and the mass of a free proton is 938.272 MeV/c$^2$. A free neutron will decay into a free proton, free electron ($\beta^-$), and an anti-neutrino, $\bar{\nu}$. The mass of the electron is 0.511 MeV/c$^2$, and of the anti-neutrino, practically zero.
In the center-of-mass (CoM) reference frame (the rest-frame of the neutron), the total energy to start with is the mass energy of the neutron: 939.566 MeV.
After the decay, the mass energy of the products is 938.783 Mev, so there remains 0.783 MeV of energy to be shared as kinetic energy between the proton and the anti-neutrino. The net momentum in the CoM frame must be zero, but with three particles involved, the energy is not split uniquely among the three. The $\beta^-$ and $\bar{\nu}$ carry most of the kinetic energy, but again, not uniquely split.
The non-unique energy and momentum of the $\beta^-$ is what led physicists to consider the existence of the third particle in the decay.