Testing Einstein's relativity from space using quantum physics

Although we suffer its consequences every day, from the tragic phone drop on the tile to the movement of the planets around the sun, gravity remains a mysterious force. Who will fall faster into the void, the hammer or the feather? This question actually hides an ancient principle, formulated in the time of Galileo, that became the cornerstone of Albert Einstein's theory of general relativity: the equivalence principle. Both physicists agree that whatever their nature and mass, all objects fall in the same way!

However, in the search for new theories to understand our universe, this principle can come into question. The challenge is for experimental physics: to test this principle with increasing precision, using objects of varying masses and compositions. The development of quantum technologies now allows us to imagine testing this principle in space, aboard a satellite, using clouds of atoms cooled to temperatures close to absolute zero. to'Ice experiment Serves as a ground-based demonstration of these future space missions.

The principle of equivalence from Galileo to Einstein

At the beginning of the seventeenth century^H In the twentieth century, the scientist Galileo imagined two objects with different masses falling into a frictionless environment. The prevailing idea at that time was that heavy objects were attracted to the Earth more than light objects. However, he points out a paradox: if we tie these two objects with a rope, the heavier object should accelerate the fall of the lighter one. Therefore, the whole must fall faster than the light body alone. We can also assume that the lighter object slows the fall of the second, making the system slower. Will the fall therefore be accelerated or slowed down?

The only solution that seems reasonable is a tie. Although we like to imagine the Italian physicist testing his principle from the top of the famous Tower of Pisa, his first tests of the equivalence principle were actually conducted on inclined planes in his laboratory.

More than 400 years later, Albert Einstein exploited this principle in his theory of general relativity, concluding that all frames of reference were equivalent. This equivalence makes it impossible for an observer in an accelerating elevator to distinguish between a fall due to gravity and an acceleration of another nature. General relativity's predictions have since been successfully verified experimentally, most notably through direct observation of gravitational waves in 2015.

However, general relativity is not the only theory to triumph in the last century. Quantum mechanics, which explores the world at the scale of atoms, has revolutionized our understanding of the universe. Although these theories are not in competition, their formality is fundamentally different, and the idea of ​​unifying them within a common framework still exists. However, some of the models envisaged predict a violation of the equivalence principle, underscoring the importance of more precise experiments to confirm – or refute! – This possibility.

Quantum techniques for testing the principle of equivalence

To accurately test this principle, very long fall times and the pristine environment that only space can provide are necessary. By comparing the acceleration experienced by titanium and platinum test blocks in free fall in a satellite for several months, the space mission microscope (2016-2018) made it possible to verify the universality of free fall with a standard precision of 15 decimal places. After these “classical” blocks, the ICE experiment team performs this test using clouds of atoms, by comparing their acceleration. The use of these atoms makes it possible to increase the sensitivity of the measurement, in particular by taking advantage of the absolute nature of the quantum measurement: the atoms allow measurement without acceleration deviation over time.

To do this, a very high vacuum (less than a millionth of atmospheric pressure!) is created in the experiment chamber. Rubidium and potassium atoms are released into this environment isolated from external disturbances and are trapped using laser light whose wavelength is controlled with great precision. Thanks to knowledge of the interaction between light and atoms, the thermal strain within the cloud of trapped atoms can be reduced to temperatures equivalent to only a few tens of nanokelvins, very close to absolute zero. Once launched, their fall can be studied by accelerometers using methods called atomic interferometry.

At a low temperature, matter behaves like a wave: we can visualize this as waves on the surface of the ocean. When two wave packets meet, they interfere and produce a periodic pattern, in which the waves cancel or add. The properties of these patterns depend largely on the path the two waves take before they meet, making it a very accurate measurement method. Atoms thus provide us with an accurate and absolute way to measure acceleration.

CARIOQA: A quantum space mission to study gravity

To demonstrate the feasibility of this type of experimental device in space, measurement campaigns are being carried out on board a zero-gravity aircraft: a real flying laboratory that makes it possible to produce 22 seconds of weightlessness by carrying out parabolic trajectories in the sky. This unique platform in Europe has enabled the development of such atomic accelerometers with simultaneous measurement of both types of atoms and Prove the space embedding of these techniques.

A long-term goal is emerging: Take these quantum sensors aboard a satellite To carry out a quantum version of the microscope experiment. As a first step, the first atomic accelerometer using a single type must be developed and tested in orbit. This is the goal of the mission Carioca Which aims to prove the feasibility of the technology. This new quantum toolkit also enables the visualization of more precise gravity mapping tasks, valuable data for climate scientists. In fact, the latter uses gravity data to study the distribution of water masses on Earth and monitor sea levels, melting glaciers, hydrological flows, etc.

These tools resulting from quantum revolutions are not limited to revealing the secrets of the basic laws of the universe. They are also essential allies in meeting the enormous challenges of the coming decades.