A French satellite challenges Einstein’s General Relativity
21st April 2016
Tristan Vey | Le Figaro
Launched on Friday [15th April], the experiment will attempt to falsify the “principle of equivalence”, which is the foundation of the theory of General Relativity.
Einstein had better watch out. Built by Cnes, the French space agency, a small satellite, Microscope, is expected to lift off on Friday evening [15th April] from Kourou in French Guiana, on board a Soyuz rocket, with the unbridled ambition of questioning the foundation on which General Relativity is built: the principle of equivalence. What is this, exactly?
Imagine that you’ve released a lead ball and a sheet of paper from the top of a tower. What happens? The ball falls quickly to the ground, while the paper flutters in the wind. So far so good. Now place yourself on the moon, which has virtually no atmosphere. The two objects then fall in precisely the same manner, and hit the ground simultaneously. Counter-intuitive, but this is what happens. As a matter of fact, an astronaut on the Apollo 11 mission was able to check this on the spot, with the aid of a hammer and a feather. (A simple evacuated bell-jar is enough to reproduce this experiment ‘at home’.)
Conclusion: in a vacuum, any two objects fall in identical fashion, irrespective of their mass or composition. For the uninitiated, the fact that a pellet of lead weighing one gramme falls at the same rate as a lead ball of one kilo, can appear surprising. But it’s not so hard to understand. Divide the ball into a thousand small pellets, each of one gramme, and it’s no longer so absurd that the thousand-piece ensemble falls at the same speed as each of its parts.
Universality of free fall
For the specialist, the most surprising thing is that a kilo of lead and a pumpkin of one kilo also fall in exactly the same way. “The atoms that compose the two objects are different. For a given mass, and therefore energy content (mass and energy being related by the famous equation, E=mc²), the breakdown of the total energy into its different components (nuclear, electric, etc.) is not the same for the lead as for the pumpkin. It is therefore not trivial that they move in the same way in a gravitational field”, explains Thibault Damour, tenured professor at the Institute of Advanced Scientific Studies.
This universality of free fall, on which Einstein founded his theory of General Relativity, is known as “the principle of equivalence” because it implies that the inertial mass of an object (the force required to accelerate it) is proportional to its gravitational mass (the force required to support it in a given gravitational field against an equal acceleration).
In practical terms, Microscope carries two pairs of cylinders. The first pair, which serves as reference, comprises two concentric platinum cylinders. The second pair, which is expected to test the principle of equivalence, consists of a platinum cylinder surrounded by a cylinder of titanium.
A principle never disproved
“The cylinders are held in pairs in relation to each other, by electrostatic fields”, explains Pierre Touboul, director of the department of physical measurement at Onera [the French aerospace laboratory], and responsible for the experiment. “If it is necessary to apply different forces to the titanium and platinum cylinders in order to keep them in a constant spatial relationship, then that would mean that the principle of equivalence had been violated.” The big difficulty, amongst others, is to compensate for the drag experienced by the satellite (residual air friction at altitude 700km, and the pressure of solar wind photons), in order to keep the cylinders in perfect free-fall.
To this day, the principle has never been falsified. Neither the torsion balances nor the lunar laser range-finder has revealed the least anomaly. We are certain that, to 13 decimal places, inertial and gravitational mass are equal. Microscope will allow this to be pushed out to 15 decimal places. The first results are expected from the end of the year. But the tremendously refined processes of verification indispensable to this kind of experiment will not allow a conclusion to be reached before the end of 2017, at the earliest.
“The least difference would constitute a clap of thunder for fundamental physics”, comments Thibault Damour. It would mean that a fifth fundamental force existed [in addition to electro-magnetism, the strong and weak nuclear forces, and classical gravity -Ed.] — a force gravitational in nature, but which acts on atoms in a manner different from classical gravity. String Theory, which attempts to unify quantum and relativistic mechanics in a single framework, predicts a violation of the principle of equivalence at one instant or another. Its confirmation would provide the first experimental support for String Theory.”
In the case of non-violation of the principle, it would probably be necessary to go further. A more ambitious space experiment, Step, could see as far as 18 decimal places. Planned originally by the United States in collaboration with Europe, it is currently on hold. “To increase the precision, we would need to conduct the experiment under cryogenic conditions, which would incur much higher costs”, comments Gilles Metris, scientific co-investigator on the Microscope mission at the Observatory of the Côte d’Azur.
Cnes estimates the cost of Microscope at €140m, including launch (the small 300kg satellite was launched in tandem with Sentinel 1-B, a far more imposing European earth-observation satellite). The Step mission would probably pass the half-billion mark.