The proton has been measured and found lacking in weight. The difference between the expected weight and the measured weight was a full three standard deviations, which means that the proton was substantially lighter than previously thought.
At first glance, this may seem like little more than a curiosity. However, the implications of this when it comes to theories related to nuclear fusion and stars are significant.
According to the standard solar model, stars are fueled by hydrogen fusion in a super dense and super hot core. Mass of single protons are turned into heat through a process in which four protons and two electrons are fused together to produce helium.
Atomic nuclei of hydrogen, deuterium, helium, lithium and beryllium
This is a two step process, in which energy is spent in order to fuse two and two protons together to form deuterium, followed by a release of energy when two and two deuterium atoms are fused together to form helium.
The overall process is believed to be exothermic.
However, with the proton substantially smaller in mass than originally thought, the energy that can be released through fusion is smaller as well.
This follows directly from the famous energy equation E = mc². If the mass of the proton is smaller, the energy associated with it is correspondingly smaller.
This means that the process of fusing two protons together to form deuterium is likely to require a lot more energy than originally thought.
The overall process of fusion, thought to take place in the core of our Sun, is suddenly lacking enormously in potential energy to draw on. It may even be the case that fusion is not an exothermic process as a whole, and that it is only the second part in which deuterium is fused together that produces an excess of energy.
This in turn opens for the possibility that we may be completely wrong about nuclear fusion and fission. Iron may not be the element above which fission is exothermic and fusion is endothermic. It may be that all fission is exothermic and all fusion is endothermic, with the only exceptions being deuterium and other rare isotopes.
The fact that single protons are much lighter than thought is a very big deal. It puts the standard model of stars into doubt, and it makes controlled hydrogen fusion a lot less likely to ever deliver on its promise of limitless free energy.