According to Einstein’s general theory of relativity, gravitation has the same effect on all objects – regardless of their mass or composition. Astronomers have now checked whether this strong principle of equivalence also applies to very large masses. To do this, they tested whether the movements of a pulsar in a triple system with two white dwarfs correspond to Einstein’s predictions. Long-term measurements using a French radio telescope showed that possible deviations must be less than two millionths. This confirms the validity of the strong principle of equivalence with the highest accuracy to date, even for large masses.
Galileo Galilei suspected it around 400 years ago, Isaac Newton formulated it for the first time and Albert Einstein made it a cornerstone of his general theory of relativity: the principle of equivalence. According to this, gravitation has the same effect on all objects – regardless of their mass or composition. If there were no air resistance, a lead ball would fall to the ground just as quickly as a spring. One therefore speaks of the “universality of free fall”. The simple yet profound knowledge of this universality gave Einstein an important impetus to consider gravitation as a result of the curved space-time. He later called this sudden inspiration “the happiest thought of my life.” Since then, Einstein’s strong equivalence principle has passed all tests with flying colors.
A pulsar and two white dwarfs as an Einstein test
However, it remained unclear whether the universality of free fall still applies when extremely high masses are involved. Because according to some alternative theories, there could be deviations due to the strong gravity effect of such objects. In this case, astronomical objects in a gravitational field would have to be accelerated depending on the extent of the curvature of space-time that they themselves created – and that would have to lead to differences. According to Einstein, the earth and moon should be accelerated equally strongly by the influence of gravity from the sun, according to alternative theories there could be deviations between them, because the earth itself bends space more due to its larger mass. The problem, however, was that there was little opportunity to check these assumptions due to the lack of suitable test objects.
That changed when astronomers discovered the triple system PSR J0337 + 171 some 4200 light years away a few years ago. It consists of two white dwarfs and a neutron star that emits radio waves at regular intervals – it is a pulsar. The neutron star orbits one of the white dwarfs in 1.6 days, the second white dwarf orbits both partners in a 327-day orbit. Because of the large, compact mass of the pulsar and its two lighter partners, this triple system is an ideal laboratory for checking the universality of free fall for larger masses. Because if Einstein is right, then the pulsar and his white partner would have to react equally strongly to the gravity of the external third party in the group. The pulsating radio signals from the neutron star make it possible to precisely track its movement – and thus detect possible deviations.
Deviations are less likely
Astronomers around Guillaume Voisin from the Jodrell Bank Center for Astrophysics in Manchester have now carried out this test with the greatest accuracy so far. To do this, they analyzed the arrival times of the radio pulses from PSR J0337 + 1715 over a period of eight years with the Nançay radio telescope in France. They subjected the data obtained to a whole battery of statistical tests in order to reduce the uncertainty as much as possible. As a result, they were given a value according to which the deviation in the acceleration of the neutron star and white dwarf – if it exists at all – must be less than 2.05 millionths. “This means that our results are fully in line with the general theory of relativity,” the researchers state. At the same time, this result improves the accuracy of a previous test with the same pulsar by 30 percent. “Confirmation with this accuracy is one of the most convincing tests ever for Einstein’s theory – and the theory passes the test with flying colors,” says Voisin.
For the alternative theories to Einstein’s theory, this means that they become less likely because the area where their effects could occur has narrowed further. “There are now narrow limits for alternative theories of gravity that are proposed in competition with general relativity, for example to explain dark energy,” explains Voisin. Einstein is right again for the time being.
Source: Astronomy & Astrophysics, doi: 10.1051 / 0004-6361 / 202038104