Physicists from Australia’s National University have created the most sensitive way of detecting an atom’s potential energy (within a hundredth of a decillionth of a joule — or 10-35 joule) and used it to prove one of physics’ most well-tested theories: quantum electrodynamics (QED).
The study, which was published in Science this week, relies on determining the colour of laser light in the presence of a helium atom and provides independent confirmation of prior methods for testing QED, which entailed detecting transitions from one atomic energy level to another.
“This invisibility is only for a single atom and a specific colour of light,” said lead author Bryce Henson, a PhD student at ANU Research School of Physics. “It couldn’t be utilised to build an invisibility cloak that Harry Potter would use to study dark places at Hogwarts.”
“However, we were able to use it to look into some of the more obscure aspects of QED theory.”
“We were expecting to catch QED off guard because there have been some prior inconsistencies between theory and experimentation, but it passed with flying colours.”
Quantum electrodynamics, or QED, was developed in the late 1940s and describes how light and matter interact in a way that has remained successful for almost eighty years, including both quantum mechanics and Einstein’s special theory of relativity.
Discrepancies in proton size measurements, which were substantially resolved in 2019, provided signals that QED theory needed some refinement.
He used unprecedented precision to quantify the frequency of the oscillations, discovering that interactions between the atoms and the laser light changed the frequency as the laser colour changed.
He realised that this phenomenon could be used to precisely determine the exact colour at which the atoms did not interact with the laser at all and the oscillation stayed steady — in other words when the atoms effectively became invisible to the laser.
The team achieved a sensitivity in their energy measurements that was 5 orders of magnitude less than the energy of the atoms, around 10-35 joules, or a temperature difference of about 10-13 of a degree Kelvin, using an extremely high-resolution laser and atoms cooled to 80 billionths of a degree above absolute zero (80 nanokelvins).
Mr Henson said, “That’s so little that I can’t think of any phenomenon to relate it to — it’s so far off the scale.”
The team was able to calculate very precise numbers for helium’s invisibility colour using these measurements. Professor Li-yan Tang of the Chinese Academy of Sciences in Wuhan and Professor Gordon Drake of the University of Windsor in Canada were consulted to compare their findings to theoretical predictions for QED.
Theoreticians had to rise to the challenge and improve their calculations because the new experimental technique improved accuracy by a factor of 20.
They were successful in this endeavour, reducing their uncertainty to a fraction of the current experimental uncertainty and identifying the QED contribution to the atom’s invisibility frequency, which was 30 times bigger than the experiment’s uncertainty.
By 1.7 times the experimental uncertainty, the theoretical value was only marginally lower than the observed value.
Professor Ken Baldwin of the Australian National University’s Research School of Physics, who led the multinational cooperation, stated that improving the experiment would not only assist clarify the disagreement but would also hone an extraordinary instrument that may elucidate QED and other theories.
“New precision measuring tools frequently lead to significant changes in theoretical understanding down the road,” Professor Baldwin stated.
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