While it's correct that fusion stops producing energy once the fusion product is iron, it is not true that all elements beyond iron are only produced in cataclysmic events (supernovae / neutron star collisions).

Stars can function as "breeder reactors", where the natural flow of neutrons produced as intermediate / side effect of the various fusion reactions going on in a later-stage-life star get absorbed by nuclei and then (by beta decay) produce beyond-iron elements. This is called the "s-Process" (slow), and responsible for a large range of elements into the Lanthanides or so. And stellar winds, or the planetary nebula stage at the end, will return some of this to the interstellar medium.

It's correct that the heaviest "naturally present" elements require the so-called "r-Process", heavy overabundance of neutrons / extremely high neutron flow as in supernovae, or direct fusion of beyond-iron nuclei as in neutron star collisions.

There is rather active research happening here, both astronomical (trying to detect various nuclei from x ray spectroscopy of cataclysmic events) and theoretical (because r-Process cannot be simulated in labs since the neutron fluxes needed are not within our reach).

That said, it remains true that many "transition groups" elements are bred via s-Process in relatively-ordinary stars.

It is kind of astonishing (well, not really if you think about it - more violent stuff actually happens faster with stars, and energy density was higher in the early universe) that every stable element exists on earth even if some are very rare. That means that earth's materials have had quite a lot of stuff happening to them before forming earth.

This is only approximately correct.

Iron 56 is the isotope with the highest nuclear binding energy per nucleon, so it can form from the fusion of any lighter nuclei.

After iron 56, the binding energy decreases slowly, so the next heavier nuclei can still form from the fusion of certain lighter nuclei, but not from any of them, but only from pairs with a lower average binding energy.

So the following heavier nuclei after iron 56 can still form through fusion, but with increasing atomic mass the probability of their formation decreases quickly, until it becomes negligible.

Relatively large amounts of cobalt, nickel, copper, zinc, gallium and germanium still form through the fusion of lighter elements, but after germanium the amount of chemical elements formed through fusion becomes extremely low. Already the amount of germanium formed through fusion is almost ten thousand times less than the amount of iron.

The binding energy per nucleon decreases very slowly, so even uranium has a higher binding energy per nucleon than helium, so energetically it could form through the fusion of hydrogen or helium, but such an event has a completely negligible probability (because there is a negligible chance for so many hydrogen or helium nuclei to collide simultaneously and if they fuse into nuclei of intermediate mass those block the propagation of the fusion reaction by having higher binding energies than the heavier nuclei).

The elements heavier than germanium form almost only through neutron capture, with the exception of some proton-rich isotopes, which form through collisions with protons. There are several kinds of environments with abundant neutrons where heavy elements can form, where the concentrations of neutrons and their energy distributions are different, so in any of these environments there are different classes of isotopes that form preferentially there.

In a relatively young stellar system like ours, the matter from which the star and the planets have condensed is a mixture of chemical elements coming from different sources.

In environments with extremely high neutron abundances (which include the nuclear explosions on Earth, not only supernova explosions, neutron star collisions and the like), all chemical elements up to fermium (Z = 100) are formed. Nonetheless, while the matter composed of these elements travels through space until the formation of a new stellar system, most of the trans-uranium elements, except the plutonium, decay. When the Solar System was formed, it still contained relatively large amounts of plutonium, not only thorium and uranium, as the heaviest elements, but since then until now the plutonium has decayed, like also most of the uranium 235 that is used now in nuclear reactors.

This is why I love gold so much. Two of the most extreme stars in existence slam into each other and create and earth-sized pile of gold (and other elements). Very extreme. Very cool.

Yes, it is produced artificially in nuclear reactors, thus it is much more expensive than any natural isotope.

The amount used in a nuclear clock would be very small, but even so, its availability would be a problem.

This article says it’s all about the thorium and radon. https://en.wikipedia.org/wiki/Background_radiation

I have no idea which of these articles is more correct.

Where does the potassium come from?

There are already many years since the best optical clocks have exceeded by many orders of magnitude the precision of those on which the SI definition is based.

There are 2 reasons why the redefinition has been postponed by a couple of decades.

One reason is that the best optical clocks are very complex and it can be difficult to make them operate continuously for a long time. Until more reliable clocks will be made, the only solution for maintaining a continuous time scale would be to keep in each laboratory a relatively large number of optical clocks in operation, so that every time when one or more are halted for maintenance there would remain enough clocks still running. This is feasible, but expensive.

Besides waiting for more reliable or cheaper optical clocks, there is one more fundamental reason why the redefinition has been postponed.

When the cesium clock was chosen for the SI definition, there was no alternative with comparable precision, so the choice was obvious. Now there are about a dozen of different kinds of optical clocks with similar performances and it is not clear which will become eventually the best.

Therefore there exists a proposal with which I agree, that the future SI definition should no longer be based on a single frequency standard, but the unit of time should be derived from a frequency that would be the weighted geometric mean of the frequencies of many kinds of optical clocks, perhaps 5 or more of them.

Such a definition would also give the possibility that depending on the evolution of clock technology some standard frequencies could be added to or deleted from the set of frequencies used to compute the mean, or their weights could be altered.

The fact that thorium 229 can be used to make a nuclear clock has been discovered many years ago.

Nonetheless, this has become possible only today, after many years of researches. There is no doubt that future thorium 229 clocks will be better by several orders of magnitude than these first 2 prototypes, so it is likely that the corresponding clock frequency will be added to the set of frequencies on which the future SI definition will be based.

I would in general agree with you, but having read through the journal paper: I think the fraction of general audience newspaper readers who could actually read that paper is very low. I come from a science background, spent time dabbling in time and frequency measurement enough to understand some of the concepts, and still found it a painful slog. (I am very much not a physicist!)

I still think it's the right practice for newspapers to always link to the primary source, but in this particular case I understand why they might make an editorial decision not to.

Is there evidence that is the actual reasoning, because I find it ridiculous. People who don’t care about the primary source are not going to follow a link to, e.g., a court ruling, government report, fed minutes, a leaked document, a research paper, etc; but those who are interested are going to immediately leave the site and likely not come back for a long while.

Am I missing something?

This is actually very rare for the New York Times - they're the only major publication that regularly links to the original sources.

The Langoliers of course.

"atomic" and "nuclear" have been used interchangeably at various times. In the past almost everything was "atomic". But more recently and more formally atomic deals with nucleus and it's electron shells, and nuclear deals just with the nucleus.

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