How Gravity Affects Time: Galileo Satellites Prove Einstein Was Right


The European Space Agency (ESA) has announced that its Galileo global satellite navigation satellite system has proven – albeit by accident – a key element of Einstein’s theory of general relativity by enabling the most accurate measurement to date of how shifts in gravity change the passing of time.


Two teams of European physicists – at the SYRTE Observatoire de Paris in France and the ZARM Center of Applied Space Technology and Microgravity in Germany -- have worked independently, and as a result, achieved a fivefold improvement in the accuracy of measuring the so-called gravitational redshift – a gravity-driven effect on time dilation. The results, published in the Physical Review Letters journal, were based on thousands of days worth of data from the Galileo satellites.


“It is hugely satisfying for ESA to see that our original expectation that such results might be theoretically possible have now been borne out in practical terms, providing the first reported improvement of the gravitational redshift test for more than 40 years,” said Javier Ventura-Traveset, head of ESA’s Galileo Navigation Science Office.


“These extraordinary results have been made possible thanks to the unique features of the Galileo satellites, notably the very high stabilities of their onboard atomic clocks, the accuracies attainable in their orbit determination and the presence of laser-retroreflectors, which allow for the performance of independent and very precise orbit measurements from the ground, key to disentangle clock and orbit errors.”


Ironically, this scientific breakthrough comes as a result of an accident. In 2014, two of Galileo’s satellites were blocked from navigation use by a malfunction on a Soyuz upper stage. ESA flight controllers were able to raise the low point of the satellites’ orbits, making them more circular. Fully functional, the eccentric satellites are used today as part of Galileo search and rescue services. Because their orbits are elliptical, each satellite climbs and falls approximately 8,500 kilometers twice a day. The result of the regular shift in height is the gravity level changes that allowed the researchers to conduct their observations.


This experiment is one of many, and perhaps the most impressive, that has supported Albert Einstein’s 1915 general theory of relativity, which is still used to describe gravitation in modern physics. In essence, Einstein combined his special relativity theory with Newton’s law of universal gravitation, describing gravity as a geometric property that depends on space and time, collectively named spacetime. He stated that time passes more slowly close to a massive object. The Einstein field equations provide a way to calculate exactly how much the time slows down.


The theory has been verified by a number of scientific experiments, including the hydrogen maser atomic clock was launched 10,000 kilometers into space in 1976. Atomic clocks on navigation satellites must take into account that they run faster in orbit then on the ground. In the recent studies, the researchers also used the passive hydrogen maser (PHM) clocks on the Galileo satellites.  


“The fact that the Galileo satellites carry passive hydrogen maser clocks, was essential for the attainable accuracy of these tests,” said Sven Hermann at the University of Bremen’s ZARM Center of Applied Space Technology and Microgravity. “While every Galileo satellite carries two rubidium and two hydrogen maser clocks, only one of them is the active transmission clock. During our period of observation, we focus then on the periods of time when the satellites were transmitting with PHM clocks and assess the quality of these precious data very carefully. Ongoing improvements in the processing and in particular in the modelling of the clocks, might lead to tightened results in the future.”


The studies are just the beginning, say the researchers, as challenges in gravitational redshift measurements remain. Clock error, orbital drift, temperature variations and solar radiation pressure are cited as the ongoing challenges in this type of research.


“Careful and conservative modelling and control of these systematic errors has been essential, with stabilities down to four picoseconds over the 13 hours orbital period of the satellites; this is four millionth of one millionth of a second,” said Pacôme Delva of SYRTE Observatoire de Paris. “This required the support of many experts, with notably the expertise of ESA thanks to their knowledge of the Galileo system.”


Photo Credit: ESA