Einstein’s Theory of Relativity Put to the Test

Albert Einstein’s general theory of relativity predicts that time does not pass at the same speed everywhere. Rather, it depends on gravity: time passes more slowly in places with higher gravity than in places with lower gravity.

On Earth, this means that time passes more quickly on top of high mountains than at sea level. The effect is minuscule, but it can be detected with extremely accurate clocks on Earth. To measure it, a difference in altitude of just a few centimeters is sufficient, as UZH physics professor Philippe Jetzer explains. The phenomenon could therefore be used on Earth to predict volcanic eruptions or, with even more accurate clocks, possibly even earthquakes.

However, it has not yet been proven that the differences in the speed of time correspond exactly to what is described in the theory of relativity. “We don't know if there is a deviation between the measurements and what the theory predicts,” says Jetzer.

ACES (Atomic Clock Ensemble in Space), a project run by the European Space Agency (ESA), aims to shed light on this unresolved question. It consists of an ensemble comprising a cesium atomic clock and a hydrogen maser, a type of laser that emits microwaves instead of light, designed and built by the Swiss company Safran Time Technologies.

The two are based on different methods of accurately measuring time using atomic vibrations. “The maser provides stability and the cesium atomic clock provides precision,” Jetzer explains. Together, they produce the most accurate time signal ever generated in space. It is so accurate that it deviates by no more than of one second over a period of 300 million years.

On Easter Monday, ACES was launched from NASA's Kennedy Space Center to the International Space Station (ISS) aboard a Falcon 9 rocket and, last Friday, attached to the outside of the ISS using a robotic arm. After several months of commissioning and calibration, ACES will transmit a time signal to a network of atomic clocks at various locations on Earth over a period of 30 months. The time signals on Earth and from space can then be compared.

Jetzer has been working on the idea of sending atomic clocks into space to test the general theory of relativity for more than ten years. However, the first ESA project he was involved in, the STE-QUEST mission, was too technically demanding at the time. Now, with ACES, a set of high-precision atomic clocks have been sent into space after all, albeit in a slightly modified and simplified form.

As head of ESA’s ACES Topical Team for General Relativity, Jetzer is jointly responsible for the theoretical foundations of the project and is eagerly awaiting the results of the measurements. He does not expect them to show any deviation from the predictions of the theory of relativity. “If they do, it would be a sensation,” he explains. Even if potential deviations were in an infinitesimally small range, they would make a huge difference to the theory. “They would mean that one of the foundations of the theory of relativity is not entirely correct.”

In this case, an alternative theory would have to explain this deviation. A window would be opened for new fundamental explanations in physics. “However, there is no alternative to the theory of relativity in sight yet,” says Jetzer. This is because it would have to take into account all parts of general relativity that have been confirmed experimentally to date. “At present, considerations on this topic are therefore largely speculative.”

Together with the ACES topical team, however, Jetzer intends to explore alternative theories in the coming years. One possibility would be to explain the deviations in terms of quantum effects. “This would be a first step towards reconciling the two great fundamental theories of modern physics, which have not yet been brought into harmony with each other,” says Jetzer.