Do we age faster in space than on Earth? A cesium atom clock could provide the answer

On April 22, the International Space Station has welcomed a new instrument, the high-precision Pharao clock (for Project for an Atomic Clock with Cooling of Atoms in Orbit), as part of the European Space Agency's ACES (Atomic Clock Ensemble in Space) mission. Thanks to quasi-immobile atoms whose properties are being probed by lasers, Pharao's ticking deviates by only one second every 300 million years (less than a minute since the Big Bang). This allows for an unprecedented test of the predictions of the theory of general relativity, according to which we age more quickly far from a large mass like Earth than on the ground. This prediction has already been confirmed, but ACES must improve its precision twenty times. With the hope of finding a clock tempo different from that predicted, a sign of new forces at work in nature.

The ignition of Pharao has been underway since early May, and several weeks of testing and calibration are planned before the experiment itself begins. It was about time. The ideas and initial tests carried out by the Kastler-Brossel Laboratory and the Paris Observatory are already almost thirty years old. The project has been repeatedly delayed by financial and technical difficulties—such as the construction of an ultra-high vacuum chamber to protect the atoms from any collision, or the fabrication of an optical system that meets the constraints of space. It has even been twice on the verge of abandonment. But, with more than ten years of delay, scientists are ready to use this technological gem developed by French companies, notably Thales and Sodern. "Fortunately, this delay has not jeopardized the relevance of the experiment," reassures Didier Massonnet, the mission's project manager at the French National Center for Space Studies (CNES). No measurement of time acceleration at altitude has reached the precision targeted by Pharao. Much more precise clocks have been developed, but have remained on Earth. The record is a deviation of a quarter of a second over 10 billion years. The ACES mission will have to last thirty months, a sufficient period of time to meet its objectives.

Pharao isn't alone in space. It's connected to Earth by radio and laser links to allow for comparisons. Importantly, another clock, called a "maser," made of hydrogen, is used to measure and correct Pharao's imperfections.

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