One of the most basic ideas in physics is that the different properties of mass – weight, inertia and gravitation – always stay the same in relation to each other. This is really important because if it wasn’t true, Albert Einstein’s theory of relativity would be wrong and we’d have to rewrite our physics textbooks. While all the measurements we’ve done so far show that this idea is true, quantum theory says that it might not be. This is a problem because it means there’s a conflict between Einstein’s theory and modern quantum theory. That’s why it’s important to keep testing this idea to make sure it’s still true. A team from the Center of Applied Space Technology and Microgravity (ZARM) at University of Bremen, working with the Institute of Geodesy (IfE) at Leibniz University Hannover, has proved that passive gravitational mass and active gravitational mass are always equal – no matter what the masses are made of – with 100 times greater accuracy than before. They did this research as part of the Cluster of Excellence “QuantumFrontiers”. They published their findings in the scientific journal “Physical Review Letters” today.
Inertial mass resists speeding up. This is why you get pushed back into your seat when a car starts. Passive gravitational mass is what determines our weight on Earth. Active gravitational mass is the size of an object’s gravitational field and how much gravity it exerts. These properties are equally important in general relativity. Scientists are testing the equivalence of inertial and passive gravitational mass, as well as the equivalence of passive and active gravitational mass, with increasing accuracy.
If passive and active gravitational mass were unequal and depended on the material, then objects made of different materials with a different center of mass would move at different speeds. The Moon is made of an aluminum shell and an iron core, which have centers of mass that are not in the same place. This means the Moon should move at different speeds. To measure this, lasers are pointed from Earth at reflectors on the Moon placed there by the Apollo missions and the Soviet Luna program. The time it takes for the lasers to make the round trip is recorded. Researchers have analyzed this data for 50 years (from 1970 to 2022) to study the difference in mass. They found no difference, which means that the passive and active gravitational masses are equal up to 14 decimal places, which is a significant improvement over the previous estimate from 1986.