Most people seem to think that science marches on inexorably, gradually uncovering more and more knowledge, going in a straight line towards “the final truth”. Actually, it is far from that, and it is really a lurch from one point to another. It is true science continues to make a lot of measurements, and these fill our banks of data. Thus in organic chemistry, over the odd century or so we have collected an enormous number of melting points. These were obtained so someone else could check whether something else he had could be the same material, so it was not pointless. However, our attempts to understand what is going on have been littered with arguments, false leads, wrong turns, debates, etc. Up until the mid twentieth century, such debates were common, but now much less so. The system has coalesced in acceptance of the major paradigms, until awkward information comes to light that is sufficiently important that it cannot be ignored.
As an example, currently, there is a debate going on relating to how elements like gold were formed. All elements heavier than helium, apart from traces of lithium, were formed in stars. The standard theory says we start with hydrogen, and in the centre of a star, where the temperatures and pressures are sufficient two hydrogen atoms combine to form, for a very brief instant, helium 2 (two protons). An electron is absorbed, and we get deuterium, which is a proton and a neutron combined. The formation of a neutron from a proton and an electron is difficult because it needs about 1.3 MeV of energy to force it in, which is about a third of a million times bigger than the energy of any chemical bond. The diproton is a bit easier because the doubling of the positive field provides some supplementary energy. Once we get deuterium, we can do more and eventually get to helium 4 (two protons, two neutrons) and then it stops because the energy produced prevents the pressure from rising. The inside of the sun is an equilibrium, and in any given volume, a surprisingly few fusion reactions take place. The huge amount of energy is simply because of size. However, when the centre starts to run out of hydrogen, the star collapses further, and if it is big enough, it can start burning helium to make carbon and oxygen. Once the supply of helium becomes insufficient, if the star is large enough, a greater collapse happens, but this refuses to form an equilibrium. Atoms fuse at a great rate and produce the enormous amount of energy in a supernova.
What has happened in the scientific community is that once the initial theory was made, it was noticed that iron is at an energy minimum, and making elements heavier than iron absorb energy, nevertheless we know there are elements like uranium, gold, etc, because we use them. So how did they form? The real short answer is, we don’t know, but scientists with computers like to form models and publish lots of papers. The obvious way was that in stars, we could add a sequence of helium nuclei, or protons, or even, maybe, neutrons, but these would be rare events. However, in the aftermath of a supernova, huge amounts of energy are released, and, it is calculated, a huge flux of neutrons. That 1.3 MeV is a bit of a squib to what is available in a supernova, and so the flux of neutrons could gradually add to nuclei, and when it contained too many neutrons it would decay by turning a neutron into a proton, and the next element up, and hence this would be available form further neutrons. The problem though, is there are only so many steps that can be carried out before the rapidly expanding neutron flux itself decays. At first sight, this does not produce enough elements like gold or uranium, but since we see them, it must have.
Or must it? In 2017, we detected gravitational wave from an event that we could observe and had to be attributed to the collision of two neutron stars. The problem for heavy elements from supernovae is, how do you get enough time to add all the protons and neutrons, more or less one at a time. That problem does not arise for a neutron star. Once it starts ejecting stuff into space, there is no shortage of neutrons, and these are in huge assemblies that simply decay and explode into fragments, which could be a shower of heavy elements. While fusion reactions favour forming lighter elements, this source will favour heavier ones. According to the scientific community, problem solved.
There is a problem: where did all the neutron stars come from? If the elements come from supernovae, all we need is big enough stars. However, neutron stars are a slightly different matter because to get the elements, the stars have to collide. Space is a rather big place. Let over all time the space density of supernovae be x, the density of neutron stars y, and the density of stars as z. All these are very small, but z is very much bigger than x and x is almost certainly bigger than y. The probability of two neutron stars colliding is proportional to y squared, while the probability of a collision of a neutron stars another star would be correspondingly proportional to yz. Given that y is extremely small, and z much bigger, but still small, most neutron stars will not collide with anything in a billion years, some will collide with a different star, while very few will collide with another neutron star. There have been not enough neutron stars to make our gold, or so the claims go.So what is it? I don’t know, but my feeling is that the most likely outcome is that both mechanisms will have occurred, together with possible mechanisms we have yet to consider. In this last respect, we have made elements by smashing nuclei together. These take a lot of energy and momentum, but anything we can make on Earth is fairly trivial compared with the heart of a supernova. Some supernovae are calculated to produce enormous pressure waves, and these could fuse any nuclei together, to subsequently decay, because the heavy ones would be too proton rich. This is a story that is unfolding. In twenty years, it may be quite different again.