Comment by JumpCrisscross
4 days ago
> Strong and weak force don't come into it in either case. Fusion requires overcoming electrostatic repulsion, that's about it
You're wrong and right. Electrostatic repulsion is the barrier, and at its limit, defines electron degeneracy pressure. But the strong force is the ultimate source of energy of the reaction, and the weak force is important in stellar reactions.
The weak force initiates proton-proton fusion [1]. (We still struggle to empirically measure its cross section because it's so low. Weak force be weak.) DT fusion, on other hand, has to crack open the energy in those delicious gluons with raw temperature. This is why PP fusion occurs around 4 MK while DT fusion needs over 1,000 MK.
[1] Anthony Phillips' The Physics of Stars
I think that it is inaccurate to say that weak force "initiates" fusion.
Fusion is initiated by bringing nuclei very close one of another, overcoming the electrostatic repulsion.
When fusion is successful, the output energy is a consequence of the strong forces, i.e. it is the difference between the binding energies caused by strong forces in the output and input reactants.
The role of the weak force is that it can determine the probability of success of the fusion.
When the input nuclei have enough neutrons, the nuclei that have collided may remain fused. Otherwise, even after being fused for an extremely short time, the compound nucleus will break again, regenerating the input nuclei which are repulsed, so fusion fails.
In cases when fusion would fail due to a bad proton/neutron ratio in the fused nucleus, e.g. for the case of proton-proton fusion, during the very short time when the input nuclei are fused, weak forces may transform a proton into a neutron, preventing the separation of the fused nuclei and allowing fusion to succeed.
So overcoming the electromagnetic forces initiates fusion, strong forces determine the amount of energy obtained per fusion event and weak forces can determine the probability for fusion to succeed when nuclei collide.
> Fusion is initiated by bringing nuclei very close one of another, overcoming the electrostatic repulsion
Protons overcoming their electrostatic repulsion doesn't mean fusion--formation of a deuteron does [1]. Protons overcoming their repulsion creates the initial conditions for fusion, but in most cases no fusion occurs. The weak force "chooses" whether fusion occurs or two protons come unusually close and fly apart.
This is a bit of a pedantic line. But nuclear physisist say the weak force initiates fusion because if we take something with as low a cross section as proton-proton interaction to be the starting point of fusion, we might as well extend it to protons being in a star at all. (A greater fraction of protons in a star will fuse than proton-proton interactions graduate to fusion.)
Without the weak force, we have no stellar fusion. Without the weak force, artificial fusion is still possible. That's both a blessing and a curse, since the weak force permits lower-temperature fusion.
> When the input nuclei have enough neutrons
Irrelevant for proton-proton fusion.
[1] https://physics.stackexchange.com/questions/526471/why-is-th...