Two atomic watches in space

The European Space Agency is about to empirically verify one of the most fundamental theories of physics. To try this at home, you will only need an atomic clock and a space station.

Three decades of gestation. After 30 years in development, the launch of the ACES INSTALLATION (Atomic Clock Ensemble in Space) is just around the corner. This ambitious ESA project will send a set of atomic watches of very high precision to the International Space Station to measure how gravity affects the passage of time.

ACES not only hopes to validate, with unprecedented precision, the effect of the gravitational expansion of time, predicted 100 years ago by Albert Einstein in his theory of general relativity. He also hopes to open the door to new research on fundamental constants and dark matter, in addition to Improve time synchronization global.

A little context. According to Einstein’s general relativity, gravity deforms space-time, which has a direct consequence: time goes at different speed depending on the intensity of the gravitational field. Experiments on Earth have already shown that Watches on the top of the mountains (where gravity is slightly lower) they go infinitesimally faster than the watches located at sea level.

ACES will take this concept to the extreme. Orbiting 400 km on Earth aboard the ISS, the instrument of ESA will experience a remarkably weaker gravitational field than on the surface. The objective is to create an “atomic watches network” comparing the signals of ACES instruments with the most precise watches on Earth. It will be this comparison that allows measuring temporary dilation more accurately than ever.

In addition to ratifying general relativity, ACES will accurately measure the differences in gravitational potential between continents, will verify If the universal constants of physics are constant Over time, and will discover whether dark matter interacts with atomic watches.

Two watches better than one. The heart of ACES are two complementary atomic watches with each other. Pharao (Projet d’Onomique atomique French space agency CNES. AND SMH (Space Hydrogen Maser), produced in Switzerland by Safran Time Technologies.

Pharao is an atomic cesium clock. Use lasers to cool cesium atoms to almost absolute zero (-273 ° C). This extreme cooling allows incredibly precise time and frequency measurements. Pharao is much more compact than its terrestrial equivalents (which can measure several meters high), since in microgravity, the atoms “float” more time in the interaction zone, without having to launch vertically against gravity.

SMH uses hydrogen atoms as a frequency reference. It is an active masier, similar to those used in Galileo navigation satellites, but ten times more stable. It shines for its short -term stability, in periods of up to an hour. Combining this short -term stability with the long -term precision of the Pharao, ACES will offer a time signal with amazing precision, of only a second every 300 million years.

Launch by Spacex. After being assembled and subjected to tests at the Airbus facilities, ACES traveled in March 2025 to the NASA Kennedy Space Center, in Florida, where ESA, Airbus and NASA engineers completed the final preparations in a clean room before its launch.

The take -off of ACES to the ISS is scheduled for April 21 aboard the CRS-32 repayment missionin a Spacex dragon ship. Once at the orbital station, a robotic arm will install the instrument outside the European Columbus module. It will be operational for 30 months, collecting data continuously.

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