Rydberg constant: There's a fundamental quantity in physics that's getting an update
The true size of the proton has been a headache for researchers for some time. Different experiments produce different results. A more precise definition of the Rydberg constant should now help determine the true value.
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Every atomic nucleus consists of at least one proton. This makes the particle one of the most common objects in the universe. However, there are still a number of unresolved questions.
The proton is one of the most common particles in the universe. There is at least one of these elements in every atomic nucleus. However, it causes a lot of trouble. For example, there is disagreement about the true size of a proton; when experts measure it, they get very different results depending on the method. This unpleasant situation has been bothering experts since 2010, when a group from the Max Planck Institute for Quantum Optics in Garching “shrinked the size of the proton” (as stated in the associated report). Editorial summary in Nature) – by a full five percent, from 0.887 femtometres to only 0.842 femtometres. Since then, almost every research group has had its own proton radius, and no one knows why.
There are now a whole host of possible explanations for this circumstance: unknown structures in the proton itself, mysterious new particles, or a previously unknown source of error. The problem is further exacerbated by known uncertainties, but they are no less disturbing. For example, the size of a proton can be calculated from the wavelength of radiation absorbed by the electrons of a hydrogen atom when it moves to a higher energy level. Unfortunately, this requires knowing the size of the proton very precisely.
A team led by Simon Scheidegger and Frederic Merkt of the Swiss Federal Institute of Technology in Zurich succeeded in avoiding precisely this chicken-and-egg problem. The researchers measured the difference between two specific energy states in the hydrogen atom. This consists only of a proton and an electron. The trick to the procedure is now In the specialized journal “Physical Review Letters”., is that one of the selected states is completely independent of the size of the proton. This means that the team was able to obtain a measurement value from which the proton size could be calculated using only one additional assumption.
To do this, Scheidiger and Merkt brought the electron into the so-called Rydberg state. In this excited state, the electron is very far from the nucleus and is somewhat unstable. By controlling the atom with electric fields, the researchers were able to increase the stability of the electron and precisely measure the energy absorbed by the electron during this jump. The energy required to completely remove an electron from the shell is also called ionization energy and is related to the radius of the proton via the Rydberg constant.
However, the chosen starting point for the electron at the so-called 2S energy level near the nucleus depends on the size of the proton. To eliminate this effect, Scheidegger and Merkt used the value of the so-called Lamb transition, which is from the 2S energy level to the 2P energy level.1/2 It was measured by a group in Canada in 2019. By adding this value to their measurements, they were effectively able to determine the transition from 2P1/2state to the highly excited Rydberg state – this is independent of the size of the proton. Through their method, the researchers obtained a value for the Rydberg constant of 3,289,841,960,194(40) kHz and a proton radius of 0.822 femtometer.
However, the reliability of the Scheidigger and Merkt measurement depends on how accurately the value of the Lamb shift was determined in the past. This indicates that there is still a lot of work to be done before the true value of the Rydberg constant can be determined, and thus the actual size of the proton can be determined without any doubt.
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