Physics – Highest precision in space

In physics, things can never be too precise. In the history of natural science, fundamental advances and scientific breakthroughs often only occurred when new measurement technology made it possible to test traditional theories in more detail. If, thanks to the greater precision, a discrepancy arose from the previously considered valid theory, the time was ripe for the next scientific revolution.

The hunt for ever more precise measurement methods is therefore almost an end in itself in physics. An international research team has carried out an experiment on the International Space Station (ISS) that could have a groundbreaking character. The process makes even better tests of the basics of Einstein’s theory of relativity possible, as well as more efficient sensors for geodesy, i.e. measuring the earth.

“The experiment on the ISS revolved around the creation of a mixture of two different ultracold quantum gases that form a so-called Bose-Einstein condensate,” says Naceur Gaaloul, who works as a theoretical physicist at the Leibniz University in Hanover and is involved in the experiments contributed to their evaluation.

It is extremely difficult to achieve such a mixture. “Such a mixture has already been successful on Earth, but never in space,” explains Gaaloul. Such experiments are quite complex. That’s why the team consisted of many researchers from several American and German institutes and one French institute. “You need an optical system with certain lasers, a vacuum and cooling system and a lot more,” says the researcher. The so-called “Cold Atom Laboratory” has been operating on the ISS for a few years now and is tailored precisely to such experiments. The team has now also demonstrated the mixture of two quantum gases.

Theoretically predicted effect

First, potassium and rubidium atoms were cooled very deeply, to fractions of a degree above absolute zero temperature. At such low temperatures, a strange quantum physical effect occurs that was theoretically predicted by Satyendranath Bose and Albert Einstein: Since atoms are waves and particles at the same time, they also have a wavelength. The colder the particles are, the longer this wavelength is. At very low temperatures, these atomic waves overlap, so that the atoms enter a common quantum state and behave like a large collective. This special aggregate state is called Bose-Einstein condensate. From a technological point of view, this is very interesting: Because the atoms are all in the same state, such condensates are ideal as highly sensitive quantum sensors.

The feather falls in a vacuum just as quickly as the hammer.

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But why do you need condensates from two different types of atoms? In the future, such mixtures will be used to test one of the basic principles of Einstein’s theory of relativity. The so-called equivalence principle states that heavy and inert mass are ultimately one and the same. This principle also means that in free fall all bodies are accelerated to the same extent – because the inert mass is what offers resistance to the gravitational, heavy mass. If both are identical, they cancel each other out of the gravity equation and what is left is a mass-independent acceleration. The feather falls in a vacuum just as quickly as the hammer.

If the principle is violated, one should be able to observe at least a tiny discrepancy – that is, for example, that different types of matter react to gravity to different degrees. Previous tests have always confirmed the equivalence principle with an extremely high level of accuracy. “But Bose-Einstein condensates are so sensitive that we could increase the accuracy when testing the equivalence principle by around two orders of magnitude,” explains Gaaloul, explaining the aim of this research project.

After the first successful experiments, the scientists want to examine the condensate mixture in detail to see whether the two types of atoms of different weights really behave in exactly the same way in weightlessness. Such tests cannot be carried out on Earth because gravity separates the two types of atoms.

Accuracy to 15 decimal places

Earth tests of the equivalence principle have already achieved an accuracy of twelve decimal places. That’s surprisingly good. “However, there are various theories that go beyond the theory of relativity and violate this principle at least very slightly,” says Gaaloul. A recently completed experiment on the Microscope satellite provided the best test – with an accuracy of 15 decimal places. The aim of the experiments now started on the ISS is to test the equivalence principle to probably 17 decimal places. This should also be possible in the future with a satellite mission.

Such satellite experiments with Bose-Einstein condensates could also be used for completely different purposes and not just for basic physical research. The extremely high accuracy that is possible with such measurements will be used in the future, particularly in geodesy. So far, the Earth’s gravity field has been measured using satellite duos such as “Grace Follow-On”. These are two satellites that fly over the earth in quick succession. The distance between satellites changes as one of the satellites flies over an area of ​​the Earth that has a higher or lower density. In this way, among other things, it is possible to determine how much groundwater a region has lost during a drought. Thanks to their precision, quantum sensors with Bose-Einstein condensates could provide valuable services in such missions in the future and complement conventional measurement technology. Better observation options are extremely valuable, especially for researching climate change and the Earth’s changing water balance.