An ‘Atomic Fountain’ Has Measured the Curvature of Spacetime for the First Time

The atom interferometry technique exploits time dilation effects to show small variations in gravity’s intensity.

Henry Cavendish, an English scientist, used a device composed of lead spheres, wooden rods, and wire to measure the strength of gravity in 1797. Scientists in the twenty-first century are doing something quite similar, but with far more advanced tools: atoms.

Even though gravity is a basic concept in beginning physics lectures, scientists are still seeking to quantify it with ever-increasing accuracy. Now, utilising the effects of time dilation — the slowing of time produced by increasing velocity or gravitational force — on atoms, a group of scientists has done it. The researchers report that they have been able to detect the curvature of space-time in a publication published online today (Jan. 13) in the journal Science.

The experiment falls within the category of atom interferometry. It makes use of a quantum physics principle: a particle (such as an atom) may be represented as a “wave packet,” just as a light wave can be represented as a particle. Matter-wave packets can overlap and cause interference in the same way as light waves can.

If the wave packet of an atom is divided in two, given time to accomplish anything, and then recombined, the waves may no longer line up — in other words, their phases may have altered.

Albert Roura, a physicist at the Institute of Quantum Technologies in Ulm, Germany, who was not involved in the new study, told Space.com, “One attempts to extract meaningful information from this phase change.” Roura authored an essay for Science’s “Perspectives” section about the new findings, which was published online today in the same issue.

A similar concept governs the operation of gravitational wave detectors. Scientists can fine-tune the mathematics underlying some of the universe’s essential workings, such as how electrons behave and how strong gravity actually is — and how it slowly varies over even short distances — by examining particles in this way.

Chris Overstreet of Stanford University and his colleagues assessed this last impact in their current study. To accomplish so, they built an “atomic fountain,” which consisted of a 33-foot (10-meter) tall vacuum tube with a ring around the top.

By firing laser pulses into the atomic fountain, the researchers were able to regulate it. They blasted two atoms from the bottom with a single pulse. Before a second pulse blasted them back down, the two atoms had reached different heights. The atoms at the bottom were captured by a third pulse, which recombined their wave packets.

The researchers discovered that the two wave packets were out of phase, indicating that the atomic fountain’s gravity field was not uniform.

“That may be interpreted, basically, as the consequence of space-time curvature in general relativity,” Roura told Space.com, alluding to one of Albert Einstein’s most renowned ideas.

The atom that went higher suffered more acceleration due to the ring’s gravity because it was closer to it. Such effects would balance out in a completely homogeneous gravitational field. That wasn’t the case here; the atoms’ wave packets were out of phase, and due to the effects of time dilation, the atom that experienced more acceleration was just a fraction of a second behind its counterpart.

Although the change is tiny, atom interferometry is sensitive enough to detect it. “They can quantify and investigate these impacts” since the scientists can regulate the ring’s positioning and mass, Roura told Space.com.

Although the technique behind this discovery — atom interferometry — may appear obscure at first, experts believe it will one day be used to detect gravitational waves and aid navigation better than GPS.

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