According to Wikipedia thorium is probably created from neutron stars or supernovae making it one of the rarest elements in the galaxy. But, it’s quite abundant on Earth, maybe 3x as abundant as Uranium on earths crust. Because of this abundance it is, along with Radon, the primary constituent of earths natural background radiation.
Iron is the last element that can form through a sustaining fusion process. All elements above iron are formed in super nova or neutron star collisions. Specifically gold and platinum are thought to be formed primarily in neutron star collisions
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.
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.
Will we at Some point start slamming two stars into eachother to make gold pressed platinum bars? Or will the energy expenditure required for that be sufficient to create gold and platinum straight?
That's sort of the question that all nuclear fusion projects are trying to answer: can you replicate the processes that happen in a star without having a star?
Potassium is everywhere, including in every living cell.
So the exposure to potassium radiation is constant for any living being on Earth.
Exposure to thorium and radon varies greatly depending on the location, i.e. it can be significant in places with granitic rocks or with sediments whose origin is in the erosion of granitic rocks.
So some people, animals etc. may be exposed to more thorium/radon radiation than from other sources, especially if inhaling dust or radon gas, but for most the greatest exposure is from the content of potassium 40 and carbon 14 that is inside their bodies (which produce only weak beta radiation).
Yes that is true, but most of potassium and carbon are not radio isotopes. All of thorium is radioactive. As such it is likely true that all life carries some quantity of radioactive potassium and radioactive carbon but that does not mean radioactive carbon or potassium are more abundant or available than thorium everywhere life exists.
In sea water the amount of thorium is negligible, because thorium minerals have very low solubility. So there radioactive potassium and carbon are much more abundant.
Averaged over the entire Earth, thorium is about one hundred thousand times less abundant than potassium, while potassium 40 is only ten thousand times less abundant than all potassium, so there is more potassium 40 than thorium. When the Earth condensed, there was much more K40 than today, so in the early Earth potassium produced more heat than thorium and uranium.
In the continental crust, more of the thorium of the entire Earth has been concentrated than the corresponding fraction of potassium, so thorium is only about 2300 times less than potassium by mass, so in the continental crust there is about 4 times more thorium than potassium 40 by mass.
However, rocks with potassium, like feldspars, are everywhere, while phosphates, which are the main minerals with abundant thorium, are more rarely encountered at the surface. So there are plenty of places where there is much more K40 than thorium or uranium.
Moreover, at equal mass potassium 40 is a few times more radioactive than either thorium or natural uranium, due to having both a higher probability to decay and to having more atoms per unit of mass. So even if potassium 40 is 4 times less abundant by mass than thorium in the continental crust, it generates several times more radioactive decay events per second. The energy of the radiation from K40 is lower, so overall thorium produces somewhat more heat in the crust.
While potassium generates more radiation than thorium, it is dispersed, there will never be in nature a place with concentrated K40, without being diluted with non-radioactive potassium. So any living being or any potassium mineral will never be as radioactive as a piece of rock that contains thorium minerals.
Moreover, if thorium remains enclosed in some mineral for thousands of years, that mineral accumulates the decay products of thorium, e.g. radium, polonium etc., becoming several times more radioactive than pure thorium. If such a thorium mineral is broken, the dust from it would be many times more radioactive than pure thorium.
So there is a risk that one may encounter a piece of something containing thorium with relatively high radioactivity, while there is no such risk with potassium 40, outside of some laboratory that would try to separate it from natural potassium (for which no good reason exists and such a purified K40 would be highly dangerous, because unlike with thorium or uranium, any living cell would attempt to incorporate such potassium).
In conclusion, while some animals, or more likely some humans, might encounter some thorium minerals and be irradiated by them, the radiation doses averaged over all living beings consist overwhelmingly of radiation from K40 and C14, augmented for marine living beings with a lower dose from the uranium dissolved in sea water. The radiation dose coming from thorium or from other weakly radioactive elements that are abundant, e.g. calcium and vanadium, is pretty negligible in comparison with those.
Potassium is an element absolutely essential for any life form on Earth.
Any cell of any living being, including humans, contains potassium ions, which are required to neutralize the excess negative charge of proteins, otherwise the interior of the cells would become acidic and it would self-destroy.
No other positive ion can replace potassium, because any other abundant positive ion has a much greater tendency of forming solid precipitates, which would also destroy the cell. That is why any living cell expels the abundant ions of sodium and calcium outside it, while pumping inside any potassium ions from the environment. The moment when a living cell stops pumping potassium and magnesium inside and sodium and calcium outside, is when the cell dies.
Among the primordial chemical elements, which already existed at the formation of the Solar System, there are many which are weakly radioactive, i.e. they have some isotopes with half lives that are of at least many hundred million years, but of many billion years for most of them.
Among these weakly radioactive elements, the human body contains not only potassium 40, but also calcium 48, but the latter is much more weakly radioactive than potassium. Besides primordial radioactive isotopes, there are also radioactive isotopes that are formed continuously by the cosmic radiation, like carbon 14, which is also present in the body of any living being.
Among the primordial radioactive isotopes, the most radioactive are uranium 235 and potassium 40, followed by uranium 238, thorium 232 and platinum 190. The existence of weakly radioactive isotopes is not random, but it is determined by a set of rules of nuclear stability. For instance, potassium 40 is radioactive because any isotope with an even atomic mass of a chemical element with an odd atomic number and heavier than nitrogen is radioactive. Potassium 40 just happens to have an unusually long half life, so it has not decayed yet (the long half life is because potassium 40, like Buridan's ass, cannot decide whether it should decay into argon or into calcium, so it stays in limbo).
All living beings have mechanisms for repairing damages caused by radiation to their nucleic acids, so the very low levels of ubiquitous natural radiation are not worrisome, except in certain locations where they are much higher than normal.
> All living beings have mechanisms for repairing damages caused by radiation to their nucleic acids, so the very low levels of ubiquitous natural radiation are not worrisome, except in certain locations where they are much higher than normal.
It is quite hard to say, how much radiation (natural, or man-made) is worrisome, because people have been living in locations with high level of background radiation for centuries, or even longer.
Ramsar (in Iran), Guarapari (in Brazil), Orissa and Kerala (in India) and Yangjiang (in China)
Yes, there is a great uncertainty about which is the level where radiation becomes measurably harmful.
Nonetheless, I think that I have seen recently right here on HN comments about an article showing that there is a higher incidence of cancer for the flight personnel of airlines, which is likely to be caused by the long time spent during their careers at high altitudes with increased radiation level.
In the case of high altitude, exactly like for the weakly radioactive elements that stay inside our body, the exposure to radiation is certain and permanent for everyone.
For the people who live in geographic areas with higher radiation levels, the difference in individual radiation exposures can be very great, because most of the extra radiation exposure would come from ingestion or inhalation of dust, as otherwise the alpha and beta radiation from the soil or rocks would not penetrate the body.
Individual radiation exposures are calculated for different pathways (external radiation, internal radiation), different kinds of ionizing radiation (UV, X-ray, alpha, beta, gamma, neutron, cosmic rays - high energy protons or atomic nuclei), energy of the ionizing radiation, different weighting for different kinds of tissue.
In the end you get the stochastic health risk to the whole body, which is the probability of cancer induction and genetic effects in Sieverts. 1 sievert (Sv) corresponds to a 5.5% chance of developing cancer.
In most cases you deal with mili Sievert amounts of dose, which correspond to very small increases in probability of cancer and they are hard to prove in medical studies because the base rate of cancer in humans is large and you have many factors that affect cancer rate: sex, weight, age, smoking, life style, environmental factors, food.
I read the article about higher incidence of cancer for the flight personnel of airlines, it would be interesting to compare it to risks of thermal spa workers. Or miners, there can be a lot of radon underground, especially with insufficient ventilation.
How do they know the accuracy of the clock? Eg: 2s of drift in 65M years.
Naively, measuring that seems more difficult than the clock. Wouldn’t you need a perfect clock as a baseline? There must be something really clever here.
By adding up the uncertainties of all intermediate measurements they made.
As a parallel, for a regular clock, you'd measure and compute the uncertainties of:
- The distance between the axis of rotation and the center of mass of the pendulum,
- The local acceleration of gravity.
And deduce the pendulum period and its uncertainty by calculus.
In that case I'm not sure you can build (and measure) a pendulum that's more precise than a previous realization of the second (because the meter is defined from the speed of light and the second, and gravimeters probably depend on the definition of the second too).
That's why the definition of base units is carefully chosen to not depend on previous realization. IIRC time is the most fundamental of base units, because other base units depend on it.
They measured the stability by comparing the nuclear clocks with more stable conventional atomic clocks. In Vienna, they used a commercial single-ion optical clock [1], and in Beijing they used a hydrogen maser (a type of microwave atomic clocks) that was calibrated to the International Atomic Time scale via GNSS satellites.
Your intuition is not completely wrong. We are currently in process of redefining the fundamental SI second because our new clocks are more precise than the definition. It is quite mind-bending stuff.
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 may be completely ignorant, but aren't these strictly inferior to optical lattice atomic clocks? Maybe they have a better form factor/are more transportable?
While both teams have used fluorite crystals doped with Th229, which was the most obvious choice for the active medium, they have used different kinds of optical frequency multipliers in order to generate the ultraviolet light required to probe the thorium nuclei.
For now, the frequency multiplication method used by the Chinese team is superior, allowing a greater output power, which ensures a better signal-to-noise ratio.
The team from Austria intends to use a better frequency multiplier in the future, to increase their output power, and they are evaluating several methods, one of which is the 4-wave mixing in cadmium vapor, already demonstrated by the Chinese team.
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.
You're looking for evidence that providing external links might lead to users visiting those external links? I'm not sure what you're doubting here, it's a pretty typical "web user behavior", if you've ran a website before. Any exit points you provide leads to more users leaving at those points.
The KPIs these websites use to measure "how good an article is" is (among other things) how long time people "spend" on the article. Remove all external links, and it'll at least look like people engage for longer with the content. Does it make sense? No, but won't stop the people who run the business to believe so.
To address some of the most pressing open questions in fundamental physics, such as the origin and properties of dark matter...
Owing to unknown nuclear parameters, precise values of these sensitivity factors cannot at present be calculated, but it is predicted that they will exceed those of the most sensitive atomic clock transitions by several orders of magnitude.
So when you achieve FTL, the Vulcans show up.
Who (or what) shows up when you solve the riddle of dark matter?
"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.
The hope is that with nuclear clocks there will be no need for the very complex single-ion traps or optical lattices required for clocks that use ionic or atomic energy levels.
Even with the atoms of thorium contained in a small crystal, the frequency of their clock transitions should not be influenced by the environment.
Unfortunately, for now no other suitable nucleus has been found, because the frequency corresponding to nuclear energy levels is normally too high for the current or near future technology.
With thorium 229, the potential simplicity of the clock is counterbalanced by the great expense required for producing this artificial isotope.
According to Wikipedia thorium is probably created from neutron stars or supernovae making it one of the rarest elements in the galaxy. But, it’s quite abundant on Earth, maybe 3x as abundant as Uranium on earths crust. Because of this abundance it is, along with Radon, the primary constituent of earths natural background radiation.
Iron is the last element that can form through a sustaining fusion process. All elements above iron are formed in super nova or neutron star collisions. Specifically gold and platinum are thought to be formed primarily in neutron star collisions
Reality is amazing!!
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’s true - sustaining was the key word I picked :-) but thanks for the more complete explanation
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.
Will we at Some point start slamming two stars into eachother to make gold pressed platinum bars? Or will the energy expenditure required for that be sufficient to create gold and platinum straight?
That's sort of the question that all nuclear fusion projects are trying to answer: can you replicate the processes that happen in a star without having a star?
The nuclear clock uses 229Th though which has an 8000 year half life and does not occur in nature at all.
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.
"We are star stuff", Carl Sagan [1]
[1] https://www.youtube.com/watch?v=LNqqFhFa4dI
Wikipedia says that potassium-40 has the largest contribution to background radiation, about 10x as much as thorium: https://en.wikipedia.org/wiki/Environmental_radioactivity
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.
Potassium is everywhere, including in every living cell.
So the exposure to potassium radiation is constant for any living being on Earth.
Exposure to thorium and radon varies greatly depending on the location, i.e. it can be significant in places with granitic rocks or with sediments whose origin is in the erosion of granitic rocks.
So some people, animals etc. may be exposed to more thorium/radon radiation than from other sources, especially if inhaling dust or radon gas, but for most the greatest exposure is from the content of potassium 40 and carbon 14 that is inside their bodies (which produce only weak beta radiation).
Yes that is true, but most of potassium and carbon are not radio isotopes. All of thorium is radioactive. As such it is likely true that all life carries some quantity of radioactive potassium and radioactive carbon but that does not mean radioactive carbon or potassium are more abundant or available than thorium everywhere life exists.
In sea water the amount of thorium is negligible, because thorium minerals have very low solubility. So there radioactive potassium and carbon are much more abundant.
Averaged over the entire Earth, thorium is about one hundred thousand times less abundant than potassium, while potassium 40 is only ten thousand times less abundant than all potassium, so there is more potassium 40 than thorium. When the Earth condensed, there was much more K40 than today, so in the early Earth potassium produced more heat than thorium and uranium.
In the continental crust, more of the thorium of the entire Earth has been concentrated than the corresponding fraction of potassium, so thorium is only about 2300 times less than potassium by mass, so in the continental crust there is about 4 times more thorium than potassium 40 by mass.
However, rocks with potassium, like feldspars, are everywhere, while phosphates, which are the main minerals with abundant thorium, are more rarely encountered at the surface. So there are plenty of places where there is much more K40 than thorium or uranium.
Moreover, at equal mass potassium 40 is a few times more radioactive than either thorium or natural uranium, due to having both a higher probability to decay and to having more atoms per unit of mass. So even if potassium 40 is 4 times less abundant by mass than thorium in the continental crust, it generates several times more radioactive decay events per second. The energy of the radiation from K40 is lower, so overall thorium produces somewhat more heat in the crust.
While potassium generates more radiation than thorium, it is dispersed, there will never be in nature a place with concentrated K40, without being diluted with non-radioactive potassium. So any living being or any potassium mineral will never be as radioactive as a piece of rock that contains thorium minerals.
Moreover, if thorium remains enclosed in some mineral for thousands of years, that mineral accumulates the decay products of thorium, e.g. radium, polonium etc., becoming several times more radioactive than pure thorium. If such a thorium mineral is broken, the dust from it would be many times more radioactive than pure thorium.
So there is a risk that one may encounter a piece of something containing thorium with relatively high radioactivity, while there is no such risk with potassium 40, outside of some laboratory that would try to separate it from natural potassium (for which no good reason exists and such a purified K40 would be highly dangerous, because unlike with thorium or uranium, any living cell would attempt to incorporate such potassium).
In conclusion, while some animals, or more likely some humans, might encounter some thorium minerals and be irradiated by them, the radiation doses averaged over all living beings consist overwhelmingly of radiation from K40 and C14, augmented for marine living beings with a lower dose from the uranium dissolved in sea water. The radiation dose coming from thorium or from other weakly radioactive elements that are abundant, e.g. calcium and vanadium, is pretty negligible in comparison with those.
Where does the potassium come from?
Potassium is an element absolutely essential for any life form on Earth.
Any cell of any living being, including humans, contains potassium ions, which are required to neutralize the excess negative charge of proteins, otherwise the interior of the cells would become acidic and it would self-destroy.
No other positive ion can replace potassium, because any other abundant positive ion has a much greater tendency of forming solid precipitates, which would also destroy the cell. That is why any living cell expels the abundant ions of sodium and calcium outside it, while pumping inside any potassium ions from the environment. The moment when a living cell stops pumping potassium and magnesium inside and sodium and calcium outside, is when the cell dies.
Among the primordial chemical elements, which already existed at the formation of the Solar System, there are many which are weakly radioactive, i.e. they have some isotopes with half lives that are of at least many hundred million years, but of many billion years for most of them.
Among these weakly radioactive elements, the human body contains not only potassium 40, but also calcium 48, but the latter is much more weakly radioactive than potassium. Besides primordial radioactive isotopes, there are also radioactive isotopes that are formed continuously by the cosmic radiation, like carbon 14, which is also present in the body of any living being.
Among the primordial radioactive isotopes, the most radioactive are uranium 235 and potassium 40, followed by uranium 238, thorium 232 and platinum 190. The existence of weakly radioactive isotopes is not random, but it is determined by a set of rules of nuclear stability. For instance, potassium 40 is radioactive because any isotope with an even atomic mass of a chemical element with an odd atomic number and heavier than nitrogen is radioactive. Potassium 40 just happens to have an unusually long half life, so it has not decayed yet (the long half life is because potassium 40, like Buridan's ass, cannot decide whether it should decay into argon or into calcium, so it stays in limbo).
All living beings have mechanisms for repairing damages caused by radiation to their nucleic acids, so the very low levels of ubiquitous natural radiation are not worrisome, except in certain locations where they are much higher than normal.
> All living beings have mechanisms for repairing damages caused by radiation to their nucleic acids, so the very low levels of ubiquitous natural radiation are not worrisome, except in certain locations where they are much higher than normal.
It is quite hard to say, how much radiation (natural, or man-made) is worrisome, because people have been living in locations with high level of background radiation for centuries, or even longer.
Ramsar (in Iran), Guarapari (in Brazil), Orissa and Kerala (in India) and Yangjiang (in China)
https://www.sciencedirect.com/science/article/abs/pii/S13504...
Alsa indoor radon levels in thermal spas, is quite high.
https://www.researchgate.net/publication/319898073_Indoor_ra...
Yes, there is a great uncertainty about which is the level where radiation becomes measurably harmful.
Nonetheless, I think that I have seen recently right here on HN comments about an article showing that there is a higher incidence of cancer for the flight personnel of airlines, which is likely to be caused by the long time spent during their careers at high altitudes with increased radiation level.
In the case of high altitude, exactly like for the weakly radioactive elements that stay inside our body, the exposure to radiation is certain and permanent for everyone.
For the people who live in geographic areas with higher radiation levels, the difference in individual radiation exposures can be very great, because most of the extra radiation exposure would come from ingestion or inhalation of dust, as otherwise the alpha and beta radiation from the soil or rocks would not penetrate the body.
Individual radiation exposures are calculated for different pathways (external radiation, internal radiation), different kinds of ionizing radiation (UV, X-ray, alpha, beta, gamma, neutron, cosmic rays - high energy protons or atomic nuclei), energy of the ionizing radiation, different weighting for different kinds of tissue.
https://en.wikipedia.org/wiki/Effective_dose_(radiation)
In the end you get the stochastic health risk to the whole body, which is the probability of cancer induction and genetic effects in Sieverts. 1 sievert (Sv) corresponds to a 5.5% chance of developing cancer.
In most cases you deal with mili Sievert amounts of dose, which correspond to very small increases in probability of cancer and they are hard to prove in medical studies because the base rate of cancer in humans is large and you have many factors that affect cancer rate: sex, weight, age, smoking, life style, environmental factors, food.
I read the article about higher incidence of cancer for the flight personnel of airlines, it would be interesting to compare it to risks of thermal spa workers. Or miners, there can be a lot of radon underground, especially with insufficient ventilation.
Bananas.
Kazakhstan, according to one popular source.
[dead]
> No one had ever made the necessary type of laser with a frequency as high as 148 nanometers.
No one had ever made the Kessel Run in under 12 parsecs until Han Solo, either.
How do they know the accuracy of the clock? Eg: 2s of drift in 65M years.
Naively, measuring that seems more difficult than the clock. Wouldn’t you need a perfect clock as a baseline? There must be something really clever here.
By adding up the uncertainties of all intermediate measurements they made.
As a parallel, for a regular clock, you'd measure and compute the uncertainties of:
- The distance between the axis of rotation and the center of mass of the pendulum,
- The local acceleration of gravity.
And deduce the pendulum period and its uncertainty by calculus.
In that case I'm not sure you can build (and measure) a pendulum that's more precise than a previous realization of the second (because the meter is defined from the speed of light and the second, and gravimeters probably depend on the definition of the second too).
That's why the definition of base units is carefully chosen to not depend on previous realization. IIRC time is the most fundamental of base units, because other base units depend on it.
They measured the stability by comparing the nuclear clocks with more stable conventional atomic clocks. In Vienna, they used a commercial single-ion optical clock [1], and in Beijing they used a hydrogen maser (a type of microwave atomic clocks) that was calibrated to the International Atomic Time scale via GNSS satellites.
[1] https://www.toptica.com/products/optical-quantum-clocks/topt...
Your intuition is not completely wrong. We are currently in process of redefining the fundamental SI second because our new clocks are more precise than the definition. It is quite mind-bending stuff.
See for example previous discussion at https://news.ycombinator.com/item?id=44264481
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.
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https://unwall.app/www.nytimes.com/2026/10/07/science/first-...
archived no paywall: https://nonograph.com/in-vienna-and-beijing-the-first-nuclea...
Much better use than for toothpaste.
https://en.wikipedia.org/wiki/Doramad_Radioactive_Toothpaste
https://archive.ph/1PBHj
I may be completely ignorant, but aren't these strictly inferior to optical lattice atomic clocks? Maybe they have a better form factor/are more transportable?
The actual journal article, since NYT didn't bother linking to it: https://www.nature.com/articles/s41586-026-11084-4
There are 2 Nature articles, for the 2 independent teams who have built such nuclear clocks with thorium 229.
This is the other article, from the Chinese team:
https://www.nature.com/articles/s41586-026-11122-1
While both teams have used fluorite crystals doped with Th229, which was the most obvious choice for the active medium, they have used different kinds of optical frequency multipliers in order to generate the ultraviolet light required to probe the thorium nuclei.
For now, the frequency multiplication method used by the Chinese team is superior, allowing a greater output power, which ensures a better signal-to-noise ratio.
The team from Austria intends to use a better frequency multiplier in the future, to increase their output power, and they are evaluating several methods, one of which is the 4-wave mixing in cadmium vapor, already demonstrated by the Chinese team.
It's unfortunately not a case of bothering but rather intent: Keep the user on the site even at cost of quality
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?
You're looking for evidence that providing external links might lead to users visiting those external links? I'm not sure what you're doubting here, it's a pretty typical "web user behavior", if you've ran a website before. Any exit points you provide leads to more users leaving at those points.
The KPIs these websites use to measure "how good an article is" is (among other things) how long time people "spend" on the article. Remove all external links, and it'll at least look like people engage for longer with the content. Does it make sense? No, but won't stop the people who run the business to believe so.
> The KPIs these websites use to measure "how good an article is" is (among other things) how long time people "spend" on the article
The Times earns most of its revenue from subscriptions. And knowing a few folks on their tech team, I can tell you dwell is far from their North Star.
And how many people could actually understand that paper? Anyone who could would find it easily enough.
That's not a reason for omitting the link to the original source. If anything, it would help illustrate the value of science journalism/reporting.
This is actually very rare for the New York Times - they're the only major publication that regularly links to the original sources.
To address some of the most pressing open questions in fundamental physics, such as the origin and properties of dark matter...
Owing to unknown nuclear parameters, precise values of these sensitivity factors cannot at present be calculated, but it is predicted that they will exceed those of the most sensitive atomic clock transitions by several orders of magnitude.
So when you achieve FTL, the Vulcans show up.
Who (or what) shows up when you solve the riddle of dark matter?
The Langoliers of course.
since NYT didn't bother linking to it
The solution to your complaint is right at the end of the article:
"We acknowledge mistakes in our reporting with corrections. If you spot an error, please let us know at corrections@nytimes.com."
That's one of the big differences between journalism and mere blogging.
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TIL an atomic clock is not a nuclear clock.
the former brings (cesium) atoms, including their electrons, into resonance.
the latter brings (thorium) _nuclei_ into resonance.
both then "just like any clock" measure that oscillation. "simples" ;-)
and I learned "environment" creating drift is temperature, magnetism, anything that influences electrons.
"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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The hope is that with nuclear clocks there will be no need for the very complex single-ion traps or optical lattices required for clocks that use ionic or atomic energy levels.
Even with the atoms of thorium contained in a small crystal, the frequency of their clock transitions should not be influenced by the environment.
Unfortunately, for now no other suitable nucleus has been found, because the frequency corresponding to nuclear energy levels is normally too high for the current or near future technology.
With thorium 229, the potential simplicity of the clock is counterbalanced by the great expense required for producing this artificial isotope.