Einstein’s Theory of General Relativity She has done it again: she has once again successfully passed one of the many tests to which she has been exposed . This theory is one of the most robust in modern physics, as it has been able to overcome the countless tests it has faced. Shortly after being developed by Einstein, it successfully predicted the deflection of light of distant stars as they passed close to the Sun during the May 1919 eclipse. Later, the existence of black holes and gravitational waves was predicted , whose experimental verification had to wait several years but ended up arriving. Recently, the “weak equivalence principle” has been put under the microscope to a precision never before achieved, obtaining the results that General Relativity predicted.
This recent test has been carried out as part of the MICROSCOPE mission , designed by the French space agency (CNES) and launched in 2016. This mission consisted of a satellite of about 300 kilograms whose objective was to measure the acceleration of different falling masses free inside the satellite as they orbited the Earth. The experiment determined that the acceleration of these masses was identical to a precision of one part in a thousand trillion (that is, in a billion million, or 1 in 10 15 ), ruling out any violation of the principle of weak equivalence at that level.
General Relativity is the theory we currently use to describe gravity and was formulated by Albert Einstein in 1915 . This theory basically tells us that energy tells spacetime how it should warp and the warping of spacetime tells energy how it should move . Well, as part of this theory, Einstein introduced the equivalence principle, which has two versions: the strong and the weak. This equivalence principle refers to the fact that, under General Relativity (and from our experimental experience), it is impossible to distinguish between a gravitational field and an accelerating frame of reference . In other words, there is no possible experiment that allows us to distinguish between being on the surface of the Earth and being inside a spaceship that is constantly accelerating at 9.8 meters per second squared.
The weak version of the equivalence principle speaks only of bodies in free fall, saying that this free fall will not depend on the composition or structure of the body . The fort itself is more restrictive and refers to any other type of experience or experiment that we can carry out submerged in a gravitational field.
This is therefore what they have tried to verify with the MICROSCOPE experiment, that the free fall of two objects does not depend on what material they are made of or on their internal structure. To do this, they have measured the acceleration of different masses of alloys with platinum and titanium as the main elements, while experiencing free fall on board the satellite. By obtaining a negative result, since no differences have been measured within the sensitivity levels of the instruments on board, General Relativity becomes a little more robust than it already was.
These experiments are important because we firmly believe that General Relativity should not be the definitive theory when it comes to gravity. This is so because this theory is “classical” in the sense that it does not incorporate the fundamentals of quantum physics.
The rest of the fundamental interactions, which are the electromagnetic, the weak nuclear and the strong nuclear, are described by quantum theories. This is why we think we should be able to come up with a theory that describes gravity down to the quantum scale. There are already some proposals, but none of them have managed to make predictions that we can test in a laboratory (be it on the ground or in orbit). String theory or loop quantum gravity are two of the most popular.
However, whatever theory finally manages to explain gravity with a quantum formalism, this theory will have to be at least as powerful and as robust as General Relativity. It must explain absolutely everything that General Relativity already explains and at its level of precision and also give us new predictions, the result of that quantum nature. It is hoped that a quantum theory of gravity can explain what happens inside the event horizon of a black hole, as this is something that current physics is completely unaware of.
One of the greatest difficulties in the design of this experiment was to check if the systems would work before taking off , since this experimental setup does not work on Earth and needs microgravity to be tested. The results obtained by MICROSCOPE exceed by more than 100 times the best results we had to date and open the way to improvements in the experimental setup to, in the future, carry out an even more precise test of General Relativity. However, the research team does not believe that this will be possible in the next 10 or 20 years.
Pierre Touboul et al, MICROSCOPE Mission: Final Results of the Test of the Equivalence Principle. Physical Review Letters, 2022; 129 (12) DOI: 10.1103/PhysRevLett.129.121102
Pierre Touboul et al, The MICROSCOPE space mission: the first test of the equivalence principle in a space laboratory. Classical and Quantum Gravity, 2022; 39 (20): 200401 DOI: 10.1088/1361-6382/ac5acd