Even in some of the most extreme areas in the universe, Albert Einstein’s theory of general relativity seems to hold up. A test of a key tenet of general relativity using three stars has shown that all objects fall the with the same acceleration regardless of their composition.
This follows the strong equivalence principle, which states that any two objects in the same gravitational field fall with the same acceleration regardless of their mass or their make-up. This was famously shown by Galileo’s apocryphal test, in which he is said to have dropped two spheres of different masses off the Leaning Tower of Pisa and found that they hit the ground at the same time.
Guillaume Voisin at the University of Manchester in the UK, and his colleagues tested this principle by measuring the movement of a white dwarf star and a pulsar – a type of fast-spinning neutron star – orbiting around a second white dwarf. “It’s basically Galileo dropping things from the Tower of Pisa, but on a much more massive scale,” says Voisin. “It’s a test that two objects are reacting in the same way to the third one.”
This test came with a particular twist: under some formulations of gravity, but not general relativity, a pulsar would be expected to behave differently than other stars, planets, or even balls dropped from a tower in Italy because it is so much more massive and compact.
The researchers found that the pulsar and white dwarf orbited exactly the same as one another with the highest precision of any test of the equivalence principle performed with such massive objects. “It’s about 1000 times better than anything that was done with neutron stars before,” says Voisin.
“The only theory of gravity that strictly follows the equivalence principle is general relativity – every other hypothesis breaks it at some level,” he says. Once again, Einstein’s general relativity has stood the test of time and gravity.