The First Nuclear Clock Will Test if Fundamental Constants Change

Physicists have developed equations to characterize the forces that bind the universe, and these equations are fitted with some 26 numbers called fundamental constants. These numbers, such as the speed of light or the gravitational constant, define how everything works in our universe. But lots of physicists think the numbers might not actually be constant.

Theoretical ideas like string theory that try to build a deeper, more complete understanding of where forces come from often predict that these numbers, even the speed of light, change ever so slightly over time. In other words, the constants may result from underlying phenomena or processes that are themselves dynamic. This is also predicted by one of the most popular theories of dark matter, the invisible substance that floats in and around galaxies. If dark matter is made up of wavelike particles called axions, then the varying density of axions from place to place should cause the strength of some of the forces to wiggle up and down.

These small tweaks to nature’s laws could slightly disrupt the delicate balancing act that takes place inside every atom’s nucleus, altering the energies of its states. The energies of nuclear states come from adding and subtracting the huge electromagnetic and strong forces acting on all the protons and neutrons. Even a relatively small change in the strength of one of these forces would result in a substantial shift in energy. The shift would be especially noticeable when applied to the thorium-229 transition’s remarkably tiny energy.

Over the 2000s and 2010s, several teams entered the race to build the first nuclear clock. To win, they needed to figure out the exact energy a laser would need to excite the nuclear state in question, now called the nuclear clock transition.

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The existing estimate of the energy required for the nuclear clock transition was a thousand times less precise than the wavelengths of the lasers that researchers were trying to probe it with. So there were thousands of laser wavelengths to rule out. After tuning a laser to one of these wavelengths, researchers had to trap a few thorium-229 atoms, hit them with the laser, then wait for photons showing they’d excited the state. This process of elimination was simply going to take too long.

Following Hudson’s lead, groups began building solid crystal compounds with the thorium embedded inside — an approach mentioned in Peik and Tamm’s original proposal. The crystals can hold quadrillions of atoms instead of just a few, so a laser could rule out wavelengths at a rapid clip.

A breakthrough last year at CERN kicked the race into overdrive. As in the older Idaho studies, the CERN team produced excited thorium-229 through radioactive decay, then looked at the photons coming out. But they found a way to do so in a much quieter environment, which enabled them to directly measure the faint rays of ultraviolet light coming from the nuclear clock transition and put a tighter estimate on the transition energy.

The CERN team’s updated estimate narrowed the wavelength hunters’ search from an entire forest to a small copse of trees, which they immediately began scouring. In April of this year, a European team became the first to report that they had probed the state with a laser. Peik contributed his laser expertise, and the collaboration made use of a crystal-growing powerhouse built by the physicist Thorsten Schumm at the University of Vienna.

Hudson’s group was right on their heels — a paper reporting their discovery ran in Physical Review Letters in July.

Ye’s group at JILA had also obtained one of Schumm’s crystals and was racing to excite the thorium-229 transition as well. For years, the group has been using its clock-building acumen to engineer a special ultraviolet laser with the sole purpose of turning thorium-229 into a nuclear clock. The laser allows Ye and his group to test many wavelengths at once to close in on any transition he seeks. His team’s new paper caps this trio of parallel discoveries with what will likely be the most precise measurement of the state’s energy for years to come.

“These results have all come out in a very short period of time,” Williams said, “so that is very exciting as to what they’re going to do next.”

The result starts the clock on thorium’s test of nature’s forces. “Now the fun starts,” Hudson said, excited to put the new tool to use studying fundamental constants. “We can actually do this stuff.”

The thorium nuclear state’s energy is far more sensitive to variations in the fundamental constants than that of any atomic state. But scientists will need to improve the precision of their measurements even further to notice changes more subtle than those already ruled out by conventional atomic clocks. Currently, Ye can measure the nuclear clock transition with a precision of one part in a trillion, but possible variations would be as small as one part in 10 trillion. “It’s many years down the road,” he said.

Eventually, though, some old Cold War byproducts could yield the first evidence for deeper, still undiscovered physics that underlies the universe we see. “We call them constants, but why?” Hudson asked. “Nothing is ever that simple when you zoom in and look at it.”