Quantum mechanics put to the test
23 September 2022
For almost 100 years, quantum mechanics has been one of the pillars of modern physics. It has made it possible for the first time to describe processes in the microscopic subatomic world, for example inside atoms, very precisely and accurately. And yet quantum mechanics is incomplete – for example in that it basically treats atoms as isolated systems and is further incompatible with general relativity, which can correctly describe processes on macroscopic scales. Nobel laureate Steven Weinberg, who died in 2021, tried to address the shortcomings of quantum mechanics to make it a consistent theory. To do this, he conceived of various possibilities and proposed a principled idea of how to test them. This idea involves a very precise and separate measurement of three specific, interrelated energy transitions in atoms – atomic clocks are also based on the precise measurement of such transitions in atoms. Now, three scientists – Professor Dmitry Budker of the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU) and of the Helmholtz Institute Mainz (HIM), Professor Mark G. Raizen of the University of Texas, Austin, and Dr. Gerald Gilbert of the MITRE Quantum Information Science Program located on the campus of Princeton University – have reanalyzed Weinberg's hypothesis in detail and proposed a concrete experiment with ytterbium atoms to test it, now published in Physical Review A. After thoroughly studying the properties of the ytterbium atom with mass number 171, the researchers conclude that three matching transitions exist that can be measured extremely accurately – similar to certain transitions in other atoms that form the basis of the most accurate atomic clocks.
"According to currently accepted quantum mechanics, the sum of both transitions 1 and 2 should yield transition 3," explained experimental physicist Professor Mark G. Raizen. "If Weinberg's idea is correct, it should manifest itself in the fact that the sum of transitions 1 and 2 is just not equal to transition 3. And with the ytterbium system, for the first time, we should be able to achieve the accuracy needed to detect a minimal deviation."
In addition, because Steven Weinberg's theory makes no claims about how large the predicted effect is, tests in different systems are useful – also to systematically set limits on the effect. In the literature, the researchers found two other systems with suitable transitions. Based on radium and calcium, appropriate measurements have already been made, but not always in the context of the Weinberg hypothesis. Therefore, in addition to their new proposed ytterbium system, the researchers would like to improve measurements on these known systems to systematically constrain the effect they are looking for.
Experimental physicist Professor Dmitry Budker described the big goal as follows: "In general, I believe that we should always test our assumptions at the limits of technology. The best case is when there is a self-consistent theory, like the one by Weinberg to guide such tests. If we could prove experimentally that predictions of such a theory are actually realized in nature, it could be a more general, comprehensive theory having the 'normal' quantum mechanics as one of its limits, just as Galileo's mechanics is a limit of Einstein's relativistic mechanics."