Philipp Haslinger: Measuring gravitational attraction in a vacuum
“In school, I was always looking for alternative solutions for mathematical or physical problems,” Philipp Haslinger (Recipient of the 2016 ASCINA Young Scientist Award) says, adding: “My teachers were not always very amused!” It’s likely that his childhood teachers in Großkrut, Lower Austria, would be impressed by his current pursuit of alternative solutions, as Haslinger applies his ingenuity to improving the measurement of tiny forces using atomic interferometry.
“Atomic interferometry is still a very young research field, and it uses the particle/wave duality of Quantum mechanics,” Haslinger explains. As a measuring tool, the technique is highly sensitive to accelerations – that is, changes in velocity. Some of the measurements he performed have been attributed to a force that hasn’t even been discovered yet: Called the “fifth force,” it may be a manifestation of Dark Energy, a concept that was proposed to explain the accelerated expansion of the Universe. Although no one has seen Dark Energy, it is thought to make up 68.3% of the universe, while Dark Matter comprises 26.8%, and Ordinary Matter, which we are made of, a paltry 4.9%.
Haslinger’s research with the Müller group at UC Berkeley has developed an atom interferometer in an optical cavity – this improved atom interferometer can measure forces one billionth as strong as the Earth’s gravity! Inside a hollow spherical vacuum chamber, billions of cesium (Cs) atoms are cooled by laser fields very close to a temperature of absolute zero, then are launched upwards and fall freely in the direction of a 2.5 cm-sized tungsten cylinder test mass at the center of the vacuum chamber. Resonant laser pulses split and recombine the wavefunction of each falling cesium atom, forming a so-called Mach-Zehnder interferometer: The phase that the atomic wavefunction acquires during this splitting and recombining is highly sensitive to all forces acting on the atoms. The device can detect the minuscule gravitational force between a falling cesium atom and the 0.19 kg tungsten test mass, which is more than a billion times weaker than Earth’s gravity.
With the atom interferometer now 120X more sensitive, the group has developed experiments to test models of Dark Energy, whose properties are the opposite of gravity on large length scales: e.g., pushing matter away. The “fifth force” could be mediated by a so-called “chameleon field” and is screened by dense objects – and on cosmological scales, even air is a dense object! This means that the force barely exists near large or dense objects like the Earth, but is powerful enough in the void of space to accelerate the expansion of the Universe! The ultra-high vacuum chamber of the atomic interferometer attempts to recreate the vacuum of outer space: The cesium atoms are acted upon by Earth’s gravity and, if the Dark Energy is strong enough, its force will also influence their fall. Although no force other than gravity was detected in this experiment, there are plans to repeat it on the International Space Station, where gravity is weaker and other interactions are less suppressed than in regions with greater matter density.
This search for chameleon dark energy excites physicists for being the first measurement with this kind of high sensitivity. But even for those who aren’t experimental physicists, atom interferometry has “big potential,” says Haslinger. “It can also be used to search for hidden water, gas, and oil resources” due to slight changes in Earth’s gravity in the vicinity of those deposits. Clearly, the pursuit of alternative solutions still motivates this Erwin Schrödinger Fellowship recipient. His childhood teachers should be proud!
The preceding article is part of a series featuring the scientific work of 20 young Austrian researchers, all who are active members of the OSTA's Research and Innovation Network Austria. The initial presentation of their work took place at the ASCINA poster session under the auspices of the "Austrian Research and Innovation Talk" in Toronto on October 21, 2016. Three of these scientists were subsequently awarded the ASCINA award the same evening, honoring their outstanding scientific work.