In classical physics, it is well understood that when hot air is mixed with cold air, a state of equilibrium is reached, where the velocities of the air molecules depend solely on the temperature of the final mixture. This principle has been established since the late 19th century. However, new findings from scientists at Instituto Superior Técnico suggest that this principle does not hold in relativistic systems (those involving particles moving at speeds close to that of light) subjected to extremely strong magnetic fields. The surprising conclusions were published in the Science Advances journal.
According to their research, when electrons cool in these relativistic conditions, their entropy, a measure of the system’s disorder, decreases. In order to comply with the second law of thermodynamics – which states that the entropy of a system cannot decrease in a spontaneous process – it was observed that the reduction in electron entropy is balanced by the emission of light, which increases entropy.
These findings provide insights into the light emissions (including radio waves) from neutron stars with extreme magnetic fields. As noted in the publication, the distribution of velocities of charged particles (like electrons) in these conditions diverges from expectations, resulting in coherent light emissions—a highly organised form of light, similar to a laser beam. “Some of this light occurs at wavelengths that can account for the radiation observed in neutron stars with colossal magnetic fields”, the authors explain.
According to Luís Oliveira e Silva, a Técnico professor and co-author of the paper, ‘this result opens the way to exploring a vast area of plasma physics in extreme conditions, both in the laboratory using high-intensity lasers and in astrophysical environments, such as around neutron stars and black holes’. According to the IPFN researcher, these environments represent ‘strongly non-linear systems [(i.e., highly complex and challenging to understand)], governed by multiple scales and diverse physical phenomena, making them both challenging and fascinating.’
Pablo Bilbao, a PhD student at Técnico, emphasises the astrophysical impact of the discovery. ‘It was a surprise; we kept noticing an unusual effect in our simulations and only after careful research did we realise its importance’, he explains. ‘What we found reveals a new way for plasmas to convert energy into coherent radiation, opening an unexplored path,’ he says.
Thales Silva, another researcher at IPFN, stresses that ‘simulations of this magnitude have only become feasible very recently’ due to computational requirements, which were met by the Deucalion supercomputer in Guimarães, part of the National Advanced Computing Network.
This discovery underscores the importance of studying the physics of relativistic plasmas, bridging high-performance computing, fundamental theory, and astrophysical observations, with potential applications for understanding high-energy phenomena throughout the universe.
