It is often said that the decay rates used in radio dating may have varied over time. Let’s see that.
The nucleus of an atom is determined solely by the number of protons and neutrons it contains. Around the nucleus gyrate some electrons, equal in number to the number of protons in the nucleus. For quantum mechanical reasons, these electrons are arranged in shells, pretty much like an onion (or Shrek).
(Very) schematic representation of an atom. There may be more than 3 shells, and more than one electron per shell. The total number of electrons is equal to the number of protons in the nucleus. During a disintegration, the ejected particle (black point) first meets the electrons of the inner shell.
Some nuclei, like our good old carbon-12 (6 protons, 6 neutrons) are stable. Put a billion in a box. Go for a walk and then take a look: the billion is still there.
Others, like the famous carbon-14 (6 protons, but 8 neutrons) are unstable. Put a billion in a box. Go for a walk and then take a look: there is no longer one billion. Some have changed to nitrogen-14 (7 protons, 7 neutrons). If your walk lasted about 5,730 years, you will find that half of your C14 nuclei have changed to nitrogen. This duration, 5 730 years, is the half-life of C14.
Radioactivity is therefore a completely natural phenomenon (don’t tell Greenpeace), changing one nucleus into another. Out of the 3,000 or so known nuclei, about 250 are stable. The others are unstable, that is, radioactive.
When a radioactive nucleus decays, it ejects something. The most common decay modes are:
- Ejection of two neutrons and two protons. This is “alpha” decay.
- Ejection of an electron. This is “beta minus” decay.
- Ejection of a positron (the antiparticle of the electron). This is the “beta plus” decay.
The decay rate sets the pace at which the process occurs. The half-life of a radioactive atom is the time it takes for half of a sample to be transmuted. It is the fruit of the laws of physics. There are therefore 2 possibilities for half-lives to change:
- The laws themselves change, or
- Within the limits of these laws, something alters the half-lives.
Have the laws of nuclear physics changed?
Let me here recycle this article on the speed of light. There are literally millions of observational evidences that the laws of physics, nuclear physics included, have been the same for billions of years. Let me emphasize the observations of decay events million light years away, which rates are the ones we observe here and now (this is in the article).
Let me also remind that changing the laws of nature implies losing the fine-tuning argument, as well as energy conservation (this is also in the article).
The laws of nuclear physics have not changed in the last billion years. This comes from observation. Let’s move on to the second possibility.
Can decay rates change… even in the laws of physics don’t?
Yes, they can. Let’s see that.
Radioactivity is a purely quantum phenomenon. Half-lifes depend on the ejection probability of the particle leaving the nucleus. Doing so, this particle will first meet the electrons of the inner shell of the atom. The ones closest to the nucleus.
We can imagine that if we remove these internal electrons by completely ionizing the atom (that is, removing all of its electrons), the ejected particle runs into a modified environment as it leaves the nucleus, which could alter the probability it had to get out in the first place. Quite like it may be easier, or more difficult, to leave a room depending on whether the next room is crowded or not. And indeed, it can happen.
The record in this respect belongs to Rhenium-187 (75 protons, 112 neutrons). It undergoes beta-minus decay and turns to Osmium-187. Its half-life is 42 billion years. In 1987, theorists computed the half-life of the same atom, but stripped of its 75 electrons. They found… 14 years!! The experiment, very difficult to perform, was made in 1996 and found 33 years. An excellent theory/experiment agreement, considering the variation of the half-life at stake (tens of billions of years -> ten years) and the challenges presented by this kind of experiments.
For the curious mind, this dramatic change is explained, in part only, by the fact that the electron ejected during the beta-minus decay is more easily released from the nucleus if it does not meet other electrons when exiting (electrons repel each other). The rest is a matter of quantum mechanics.
Can we then trust radioactive dating?
If, then, the laws of nature allow for large variations of half-lives, how can we consider them constant when dating?
Simply because the conditions required to change the half-lives are extreme. At first glance, one could think, “Removing all the electrons? It doesn’t seem very complicated”. In fact, it is. To deprive hydrogen atoms from their unique electron, for example, a temperature of some 10,000 degrees Kelvin is required. To do the same with the 6 electrons of carbon, about 400,000 degrees. And for Rhenium-187, it will take more than 65 million degrees to remove its 75 electrons and divide its half-life by about 1 billion . And these are lower temperature limits.
We could draw a parallel with tree dating through rings counting. Trees can burn, right? So how can rings counting be a reliable dating method? Simply because if I have a tree trunk before me, it means it did not burn.
So if the bone I want to date with C14 is in my hands, it means it has never been heated to more than 400,000 degrees. The electrons of the inner layers have been quiet, together with the half-life.
Let’s conclude commenting on some alleged variations of decay rates with the Earth-Sun distance. Two comments about it:
- The claimed variation is only 0.1%. Such error would translate to the dating, adding a 0.1% uncertainty to the final result. Considering that C14 already has error bars of the order of 10%, we see that plus or minus 0.1% will not change much. It’s as if you were told that the length of a bridge might have changed by one millimeter, while you can only measure it to within 1-meter accuracy anyway. You would politely acknowledge, while thinking that you will worry about it the day you’ll have millimeter precision.
- When people make an important observation like this one, others try to reproduce it. If 1 team, then 2, then 3 independently confirm the result, no doubt something is happening. But if other teams fail to reproduce the observation, there is a problem.
In this case, those who tried to repeat the measurements did not detect any change of the decay rates. The reader can check it by having a look at the articles citing the original work.
A seemingly more established oddity (yet, seldom, if ever, mentioned in radio dating discussions) is the co-called GSI anomaly. It has to do with unexpected variations of quite exotic decay rates, of highly ionized atoms (so, exotic decay of exotic atoms). Several options have been proposed to explain the observations, but to date, none of them has been considered to settle the issue. Indeed, people would be happy if known physics could not explain the thing, for it would mean we have, at last, an experiment breaking the “Standard Model”.
At any rate, such enigmas imply by no means that we don’t understand mundane decay rates like C14, in the same way that our current inability to explain high temperature superconductivity does not mean we don’t understand how the processor of my laptop works.
Summary: Observations tell the laws of nature have been the same for billions of years. These laws do allow decay rates to vary, but in such extreme conditions that if the object I want to date had been through them, it would be a pile of ashes.
We can count on the temporal constancy of half-lives for radiometric dating.
 These temperatures are what it takes to ionize a macroscopic number of atoms, that is, the typical amount of atoms you find in a real life object (10^23, Avogadro’s number). In the Re187 experiment, scientists observed far less atoms than that (only 10^8), and used other techniques to ionize them.