Rules and regularities
This new research shows that remarkable rules and regularities can be found in the seeming chaos of these non-equilibrium processes. This points towards the possibility that these processes can be classifiedin the form of universality classes. In many aspects, systems belonging to the same universality class behave exactly the same,which means that experiments can be conducted with experimentally more easily accessible quantum systems in order to learn exact information about other systems (within the same universality class), which cannot be studied directly in experiments.
Dr Sebastian Erne from the school of mathematics at the University of Nottingham explains why this discovery is important for providing new ways to study quantum systems; “Conducting experiments to reproduce the Big Bang in a laboratory is impossible, but if we know which universality class it belongs to, we can study quantum systems within the same universality class to indirectly investigate the physics of the Big Bang.”
Dr Silke Weinfurter head of the Analogue Gravity lab at the University of Nottingham, where Dr. Erne currently works, added; "The idea to quantum simulate cosmological systems through laboratory analogues is an exciting and timely field of research. The experiment carried out by Erne et al gives insight to fundamental non-equilibrium behaviour expected to arise for example in the Early Universe that are otherwise not accessible through observations."
Jörg Schmiedmayer from the Atominstitut at the TU Vienna undertook the experimental stage and explains: “Understanding quantum many-body systems far from equilibrium is considered one of the most pressing questions in physics. Even the most powerful supercomputers can’t calculate these processes exactly. Therefore, our universality classes are an important opportunity to learn something new.
“Universality classes are known from other areas of physics”, he continues “When studying phase transitions, for example materials close to their melting point, certain properties can be described with equations that are very universal, as for example the relationshipbetween thespecific heat and the temperature.” For these kind of connections, the microscopic details of the melting process do not matter. Therefore very different materials obeythe same equations.
“It is certainly very astonishing, that one can also find this universality in quantum systems far from equilibrium.”, says Jörg Schmiedmayer. “On first sight this appearsunexpected. Why should a quantum system consisting of many particles, which is rapidly changing obey any universal laws? Nonetheless, this is exactly what theoretical work in the group of Jürgen Berges and Thomas Gasenzer from Heidelberg University predicts.” These remarkable properties were now proven experimentally twice at the same time – at the TU Vienna and Heidelberg University.
The fast and the slow direction
The research group used a very special Atom-trap: On a special Atom-Chip thousands of atoms can be trapped and cooled by electromagnetic fields. “We thereby produce an atom-cloud with a short and a long direction – similar to a cigar”, explains Sebastian Erne, the first author of the study.
Initially the atoms move with the same velocity in both directions. However, one can open the atom-trap along the short (transverse) direction in such a way, that all atoms that move very fast in this direction evaporate. The remaining atoms have only a relatively small velocity in the transversal direction.
Dr Erne continues:“The experiment showed that the velocity distribution along this direction is changed so rapidly, that the velocity distribution along the other direction – along the elongated direction of the cigar – remains practically unchanged. So, we produce an initial state far from (thermal) equilibrium.” Collisions and interactions disperse the energy between the atoms, the system is said to “thermalise”.
“Our experiment demonstrates that this thermalisation process follows a very universal law and does not depend on any details (of the initial state)”, says Jörg Schmiedmayer. “No matter how we start the thermalisation, the transition is always described by the same equation.”
The researchers in Heidelberg proceeded in a similar fashion: There the system also starts with an elongated cloud of atoms. In Heidelberghowever, the researchers didn’t study the velocity but instead the spin of the particles. By initially controlling the direction of the spin, they were able to observe its change in direction over time, caused by the interactions between the atoms. This change in time can be described by exactly the same equation used in the other experiment (at the TU Vienna) “We have found a process that also obeys the universality but belongs to a different universality class. This is great because it confirms our theories very convincingly and suggests that we are really on to something – a new, fundamental law, “ says Markus Oberthaler from the University Heidelberg.
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