What if the Constants of the Universe Aren’t Truly Constant? Researchers Say the Key Lies in the Nuclear Clock

Atomic clocks have revolutionized the ability to measure time with unprecedented accuracy. Some of the most precise atomic clocks are so reliable that they would be off by less than a second over the entire lifetime of the universe. However, despite their remarkable precision, these clocks are still not accurate enough to address some of the major unknowns in physics.

Closer to nuclear clocks than ever. The scientific community is now closer to achieving a significant milestone that could help resolve these uncertainties: nuclear clocks. These clocks promise to advance time measurement technology considerably, allowing for the development of ultra-precise clocks that can investigate new areas of physics.

From atomic to nuclear. It’s essential to clarify the difference between atomic and nuclear clocks, given that the terminology can be confusing. Atomic clocks operate based on the excitation states of electrons, while nuclear clocks depend on the particles within an atom’s nucleus. As their name implies, nuclear clocks focus on the behavior of subatomic particles in the nucleus.

Atomic clocks rely on electron transitions. When electrons absorb energy, they can “jump” to higher energy states. These “jumps” can then be reversed when they release energy in electromagnetic radiation.

In contrast, the processes occurring in an atom’s nucleus are even more isolated from external physical interactions. As a result, transitions of subatomic particles in the nucleus are expected to be more precise and reliable than the transitions that take place within the atomic “shell” formed by electrons.

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Thorium-229. Scientists need to transfer energy to the atom’s nucleus to make a nuclear clock operational. They can alter the atom’s energy state by directing electromagnetic radiation at a specific frequency toward the nucleus, similar to flipping a switch. Like atomic clocks, nuclear clocks would function by counting these energy changes.

However, inducing these “jumps” in the atomic nucleus is quite challenging. The primary difficulty lies in exciting the atomic nuclei sufficiently to trigger these transitions. This requires coherent X-rays, which are high-frequency and high-energy, typically beyond current technological capabilities.

Notably, researchers discovered nearly 50 years ago that the atomic nuclei of the isotopes of thorium-229 undergo transitions that require energy equivalent to ultraviolet light. Because this requires less energy, it’s become feasible to build a laser that can effectively transfer energy to the nucleus.

Half a century of research. The nuclear transition of thorium was first discovered in 1976. However, it wasn’t until 2016 that scientists were able to observe and measure it. This measurement is crucial because, in order to induce the transition, they must know the exact frequency needed to “bombard” the atomic nucleus of this isotope.

How close are scientists to achieving this? A group of researchers recently tested some of the critical components needed for this technology. They provided insight into the scientific community’s progress toward creating a nuclear clock based on thorium-229.

The team tested an ultraviolet laser capable of generating the precise energy required to induce “jumps” in the nucleus’ state. Researchers also examined a “frequency ratio” to measure these transitions directly and revisited the thorium-229 transition itself.

From dark matter to universal constants. What is all this for? Does our society truly need clocks that exceed the precision of atomic clocks? This new technology could offer significant benefits not only for the scientific community but also for the people.

These advanced clocks could enhance technologies such as GPS and other navigation systems. They could also improve the synchronization of the global Internet, leading to faster connections and more secure communications.

Moreover, they could pave the way for more precise measurements that help unravel mysteries in physics, including dark matter. Perhaps most importantly, these clocks could aid in experiments addressing one of physics’ fundamental questions: Are universal constants constant? Do they change depending on factors such as the age of the universe or the frame of reference?

Images | Steven Burrows/Ye group | NIST

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