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New limit placed on physics constant

Analysis could help scientists determine whether quantum mechanics parameter varies with time

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One of physics’ most fundamental constants just got a little more constant.

Physicists have placed the strictest limit yet on how much the fine structure constant — which determines how strongly electrically charged objects interact — could change with the density of nearby matter. The team's method for measuring the constant may help scientists probe whether the value has remained constant over the lifetime of the universe.

The fine structure constant, also known as alpha, has long been both essential and confounding to physicists. It was introduced in 1916 to describe the strength of the electromagnetic force, which governs how charged objects interact and how molecules form. From there, the constant worked its way into important quantum mechanical equations. "It pops up all of the time in all of our theories," says Michael Tarbutt, an Imperial College London physicist who led the study.

However, physicists know of no fundamental reason why alpha has the value it does. They just know that if it were much larger or smaller, complex molecules could not form. And alpha may not be a true constant. Some recent theories about the origins of dark energy — the mysterious repulsive energy field that seems to pervade the universe — suggest that alpha’s value might vary depending on how much matter is nearby.

To probe this possibility, Tarbutt and his team measured alpha both on Earth and in interstellar space, where the density of matter is far lower. To do this, they measured the frequency of light needed to change an electron’s energy in a particular way in a molecule called CH, which is composed of one carbon atom and one hydrogen atom. The researchers chose to measure this frequency because it should vary sensitively with any change in alpha.

Existing astronomical data on interstellar gas clouds in the Milky Way, where CH is abundant, provided the frequency in deep space. For their Earthbound measurement, the scientists produced the normally unstable CH in the lab at ultracold temperatures. They then put the CH molecules in a cavity and bombarded them with microwave pulses to force some of the molecules' electrons to jump to a higher energy. The scientists then measured the frequency of light emitted as the electrons returned to a lower energy.

The scientists report October 15 in Nature Communications that based on their measurements, alpha cannot vary between Earth and interstellar space by more than 1.4 parts in 10 million. Though not proof that alpha is the same everywhere, the result is the most stringent limit yet on the constant's dependence on local matter density.

With its precise lab measurements, Tarbutt's team has achieved a "remarkable result," says Wim Ubachs, a physicist at Vrije Universiteit Amsterdam who studies fundamental constants. However, Ubachs says, to probe whether alpha has changed since the universe's early days, which some theories suggest, researchers need similar data from gas molecules far outside our own galaxy, preferably on the other side of the universe.

Such data should become accessible with new telescopes, such as the Very Large Array in New Mexico and the Square Kilometre Array to be built in South Africa and Australia, Tarbutt says. "That's one of the things that we're quite excited about." 

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