Theory of General Relativity: A century later, Einstein is still passing exams

In 1919, in a world fresh from war and scared by the biggest pandemic of the 20th century, astronomers Arthur Eddington and Frank Dyson presented an imaginary confrontation between two giants of science. Isaac Newton’s Law of Universal Gravitation, in force for more than two centuries, could be surpassed by Albert Einstein’s General Relativity , published in 1915. His observations of the total eclipse of that year, from the African island of Príncipe and the town Sobral, Brazil, determined that the light from several stars located behind the Sun was bent following the deformations of space-time caused by the mass of the star, as predicted by Einstein’s theory. “New theory of the Universe. “Newtonian ideas overthrown,” The Times published , perhaps forgetting that Newton’s ideas continued and continue to work with astonishing precision.

A century later, in another pandemic world, the scientist they aspire to surpass in gravitational matters is the then-crowned Albert Einstein . Since Eddington’s expedition, the conception of the world that the German scientist created in his mind, based on thought experiments and mathematics, has been tested numerous times by observing nature.

“The theory has been verified very precisely in the weak field regime, where the gravitational field is small, as on Earth or the Solar System,” explains José Luis Fernández Barbón, director of the Madrid Institute of Theoretical Physics. In addition to tests like Eddington’s, the creation of atomic clocks starting in the 1950s made it possible to measure a very unintuitive prediction of Relativity: that time does not pass at the same speed everywhere. According to the new theory, it does so more slowly in the presence of a strong gravitational field like that of a black hole than in the presence of a weaker one like that of the Earth. This consequence of the theory foreshadowed that time would pass more slowly at the base of a skyscraper than on its roof, and the exact atomic clocks confirmed that this was the case.

In experiments in these nearby regions where gravitational fields are weak, Relativity has been tested with an accuracy of one part in 10,000. Beyond “is the gap for new physics,” says Fernández Barbón. The new tests to go beyond Einstein require increasingly sophisticated devices that measure our immediate environment, but also that get closer to the gravitational monsters of the universe. Black holes are a perfect environment to test the limits of Relativity, but they are too far away. “In those strong fields, the precision of the tests of the theory is smaller. “This happens with the merger of black holes that are seen with gravitational wave detectors,” says the director of the IFT.

The possibility of measuring gravitational waves , those undulations of the space-time fabric caused by the merger of black holes or neutron stars, arrived in 2016 . Then, the LIGO terrestrial detector captured the waves generated by the collision of two black holes and began a new stage in the observation of the cosmos. It was one more test passed by Einstein, who with his theory was the first to advance the existence of these waves. However, these detectors “still do not provide enough data nor are they sufficiently precise,” says Fernández Barbón. These precise data may arrive in the next decade, when the launch of the LISA satellite constellation, designed to capture these waves with greater accuracy, is planned. Here we could begin to glimpse the limits of Relativity.

The result of one of the last tests passed by Einstein during the last century was presented last week in the journal Physical Review those of black holes. These are two neutron stars, objects that can concentrate the mass of the Sun, which is almost a million and a half kilometers in diameter, into one that is usually around 20. These stars, PSR J0737−3039 A and B, orbit one around the other at high speed and each time they rotate, in a regular manner, they emit radio pulses detectable from the Earth. Located 2,400 light years away, it is the only known system of this type of star in which both emit these pulses, something that makes the system especially interesting for scientists. “The pulsars work like two clocks that serve to measure the properties of that system with greater precision,” says Barbón.

An international team of scientists led by Michael Kramer of the Max Planck Institute for Radio Astronomy observed the two stars for 16 years using radio telescopes around the world. Following the photons emitted by these cosmic beacons, they were able to follow its movement and see, among other things, how it was curved by the presence of an object with a gravitational field as powerful as that of a neutron star. The experiment has similarities with Eddington’s, but the conditions are much more extreme.

In a statement titled Einstein Wins Again , Kramer assured that this natural laboratory for gravitational theories made it possible to measure the energy carried by gravitational waves with a precision “1000 times better than what is now possible with gravitational wave detectors.” In total, the scientists who participated in the project tested seven predictions of the theory of Relativity and even used the famous equation E=mc2 to calculate the effect of electromagnetic radiation from pulsars. According to Dick Manchester, from the Australian CSIRO, one of the entities that collaborated on the work, “this radiation represents a mass loss of eight million tons per second”, something that, “although it may seem like a lot, it is only a tiny fraction.” of the mass of the pulsar.”

Einstein, who some myth paints as a bad student, continues to pass the most demanding exams 100 years later and the time does not seem near when the cracks in his theory will begin to show. “For now there is no worthwhile modification of Einstein’s theory on paper,” summarizes Fernández Barbón. “Many alternatives have been tried, but they are all horrible.”