ISS Space clocks accurate to a second in 300 million years allowing scientists to test fundamental theories

ESA has welcomed the arrival of Pharao, an important part of ESA’s atomic clock experiment that will be attached to the International Space Station in 2018. Delivered by France’s CNES space agency, Pharao is accurate to a second in 300 million years, which will allow scientists to test fundamental theories proposed by Albert Einstein with a precision that is impossible in laboratories on Earth. Time is linked to gravity and, for example, passes faster at the top of Mount Everest than at sea level. These effects have been proven in experiments on Earth but the Atomic Clock Ensemble in Space, ACES, will make more precise measurements as it flies 400 km high on humanity’s weightless laboratory. Comparing clocks under different gravity levels allows researchers to test Einstein’s theories on space–time and other theories in fundamental physics. To achieve its accurate timekeeping, the Pharao space clock uses lasers to cool caesium atoms down to –273°C, close to absolute zero. Moreover there is the possibility of including Pharao-type clocks in next-generation Galileo systems.