ESA tests Einstein's theory with atomic clocks launched to ISS

On April 21, 2025, the European Space Agency (ESA) successfully launched two advanced atomic clocks, known as PHARAO and SHM, aboard a SpaceX Falcon 9 rocket from NASA’s Kennedy Space Center in Florida. This mission aims to explore and test fundamental physics principles, including Albert Einstein’s theory of general relativity. The atomic clocks are set to be installed outside ESA's Columbus module on April 25, 2025, facing Earth. Once operational, they will begin a series of measurement sessions designed to redefine time measurement in space, transmitting highly accurate time signals back to Earth. The ACES clocks are equipped with a revolutionary time transfer system built by the German company TimeTech. This system utilizes both microwave and laser links to synchronize time between space and Earth, facilitating a new kind of 'internet of clocks'. Combining the precise operations of these clocks with measurements of gravitational phenomena is expected to shed light on the flow of time within the universe. This project signifies a major milestone for scientific research in the field of fundamental physics and could lead to groundbreaking discoveries regarding time and space. With these experiments, ESA joins international efforts to push the boundaries of our understanding of the universe. The launch of these atomic clocks represents a significant advancement in how we measure time and its interrelation with gravity and mass. Not only is the mission groundbreaking within the realm of theoretical physics, but it also underlines the spirit of international collaboration in scientific endeavors. Experts believe that successful data from the ACES clocks can enhance the precision of timekeeping on Earth and could in future benefit areas such as global synchronization efforts. Daniel Neuenschwander, the Director of Human & Robotic Exploration at ESA, emphasized the profound implications of this mission, referring to it as “a major milestone for European science and international cooperation in space.” As the clocks prepare to operate outside the International Space Station, they will conduct at least ten separate measurement sessions over 25 days, paving the way for future explorations of concepts related to dark matter and gravity through the lens of time measurement.