IPFN researchers challenge classical laws of statistical physics
IPFN Researchers have uncovered previously unknown properties in plasmas travelling near the speed of light when subjected to ultra-intense magnetic fields. The discovery, published in the scientific journal Science Advances, could help explain the origin of radio emissions observed in neutron stars.
When hot air is mixed with cold air, the system eventually reaches a state of equilibrium. At that point, the speed of the molecules depends solely on the final temperature of the mixture. This is a well-established result of classical physics, dating back to the late 19th century. However, this principle no longer applies to relativistic systems — those in which particles travel at speeds approaching that of light — when exposed to extremely strong magnetic fields. That’s according to researchers from IPFN. Their findings, which took the team by surprise, were published in Science Advances.
The study shows that when electrons cool down in these relativistic conditions, their entropy — a measure of disorder in a system — decreases. To comply with the second law of thermodynamics, which states that a system’s entropy cannot decrease during a spontaneous process, the researchers observed that the reduction in the electrons' entropy is balanced by an increase in the entropy of emitted light.
These results help shed light on how emission — including radio waves — occurs in neutron stars with extreme magnetic fields. Under these conditions, the paper reports that the velocity distribution of charged particles, such as electrons, differs from what is expected. This leads to the production of coherent light emissions — highly organised forms of light similar to laser beams. “Some of this light is emitted at wavelengths that help explain the radiation detected in neutron stars with colossal magnetic fields,” the authors state.
According to Luís Oliveira e Silva, a professor at Técnico and co-author of the paper, “this result opens the door to exploring a wide domain of plasma physics under extreme conditions, both in laboratories using high-intensity lasers and in astrophysical environments — for instance, around neutron stars and black holes.” These environments, he explains, “are strongly non-linear systems [that is, highly complex and difficult to grasp], governed by multiple scales and diverse physical phenomena, making them both challenging and fascinating.”
Pablo Bilbao, a PhD student at Técnico, highlighted the astrophysical impact of the discovery. “It came as a surprise. We kept seeing an unusual effect in our simulations, and it was only after careful investigation that we realised its significance,” he explains. “What we found reveals a new way for plasmas to convert energy into coherent radiation, opening up an unexplored path.”
Thales Silva, also a researcher at IPFN, added that “simulations on this scale only became feasible very recently,” due to the high computational demands — demands that, in this case, were met by the Deucalion supercomputer in Guimarães, part of Portugal’s National Advanced Computing Network.
The discovery reinforces the importance of studying relativistic plasma physics, which bridges high-performance computing, fundamental theoretical research and astrophysical observations. It holds promise for advancing our understanding of high-energy phenomena throughout the Universe.
