Despite the doubts of quantum physicists: Einstein’s theory of

Joint press release by Hannover’s Leibniz University and ZARM from the University of Bremen

One of the most fundamental assumptions of fundamental physics is that the different properties of mass – weight, inertia and gravity – always remain the same in relation to each other. Without this equivalence, Einstein’s theory of relativity would be challenged and our current physics textbooks would have to be rewritten. Although all measurements to date confirm the equivalence principle, quantum theory postulates that there should be a violation. The inconsistency between Einstein’s theory of gravity and modern quantum theory is why more precise testing of the equivalence principle is so important. A team from the Center of Applied Space Technology and Microgravity (ZARM) at the University of Bremen, in collaboration with the Institute of Geodesy (IfE) at Leibniz University Hannover, have now managed to prove with 100 times greater accuracy that a passive gravitational mass and an active gravitational mass are always equivalent – regardless of the specific composition of the individual masses. This research was conducted within the framework of the “Quantum Frontiers” Cluster of Excellence. Today, the team published their findings as a lead article in the scientific journal “Physical Review Letters”.

Physical context
The inertial mass resists acceleration. For example, causing you to be pushed back into your seat when the car starts. Passive gravitational mass reacts to gravity and produces our weight on Earth. Active gravitational mass refers to the gravitational force exerted by an object, or more precisely, the size of its gravitational field. The equality of these properties is the basis of general relativity. Therefore, the equivalence of inertial and passive gravitational masses and the equivalence of passive and active gravitational masses are being tested with increasing precision.

What is the research about?
If we assume that the passive and active gravitational masses are not the same – that the ratio depends on matter – then objects made of different materials with different centers of mass will accelerate themselves. Since the Moon is composed of an aluminum shell and an iron core, with their centers of mass offsetting each other, the Moon will accelerate. This hypothetical change in speed can be measured with great precision, via “Lunar Laser Ranging”. This involved pointing a laser from Earth to reflectors on the Moon placed there by the Apollo missions and the Soviet Luna program. Since then, the round trip time of the laser beam is recorded. The research team analyzed “Lunar Laser Ranging” data collected over 50 years, from 1970 to 2022, and investigated the effect of the difference in mass. Since no effect was found, this means that the passive and active gravitational masses are approximately equal to 14 decimal places. This estimate is a hundred times more accurate than the best previous study, dating back to 1986.

Unique skill
The LUH Geodesy Institute – one of only four centers in the world to analyze laser distance measurements to the Moon – has unique expertise in assessing data, particularly for testing general relativity. In the current study, the institute analyzed measurements of Lunar Laser Range, including error analysis and interpretation of results.

Vishwa Vijay Singh, Jürgen Müller and Liliane Biskupek from the Institute of Geodesy at Leibniz University Hannover, and Eva Hackmann and Claus Lämmerzahl from the Center of Applied Space Technology and Microgravity (ZARM) at the University of Bremen published their findings in the journal Physical Review Letterwhere the paper is highlighted in the “editor’s suggestions” category.