A team of physicists is about to recreate the jets of matter that erupt from gamma-ray bursts—the most powerful explosions in the Universe—inside a laboratory at CERN. These jets, made of electrons and their antimatter counterparts (positrons), race through space at nearly the speed of light. Astrophysicists have long suspected that the jets generate magnetic fields and accelerate particles to ultra-high energies, producing the cosmic rays that strike Earth. But no telescope has ever directly observed these processes. The problem is that the extreme conditions inside a real gamma-ray burst cannot be simulated with current computer models, which fail to capture the chaotic, long-term behaviour of the plasma. The researchers will fire a high-power laser at a target inside CERN's accelerator complex, generating a dense beam of electron-positron pairs that mimics an astrophysical jet. Instruments placed directly in the beam will measure how the jet becomes unstable, how turbulence develops, and how magnetic fields and particles exchange energy—measurements impossible from Earth-bound telescopes. This is fundamental science. It will not produce a new battery or a faster computer. But understanding how magnetic fields emerge from pure energy and how particles are accelerated to near-light speeds could, over decades, inform future plasma-based technologies or reveal why the Universe is filled with high-energy radiation.
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Gamma-ray bursts (GRBs) are among the most energetic events in the Universe. They occur at cosmological distances and are the result of the collapse of massive stars or neutron stars mergers, with emission of relativistic 'fireballs' of electron-positron pairs. From astrophysical observations, a wealth of information has been gleaned about the mechanism that leads to such strong emission of radiation, with leading models predicting that this is due to the disruption of the beam as it blasts through the surrounding plasma. This produces shocks and hydromagnetic turbulence that generate synchrotron emission, potentially accelerating to ultra-high energies the protons which are observed on Earth as cosmic rays. However, there is no direct evidence of the generation of either magnetic fields or cosmic rays by GRBs. Estimates are often based on crude energy equipartition arguments or idealized numerical simulations that struggle to capture the extreme plasma conditions. We propose to address this lacuna by conducting laboratory experiments at CERN to mimic the jet propagation through its surrounding plasma. Such experiments will enable in situ measurement of the plasma properties, with exquisite details that cannot be achieved elsewhere. The experiments also complement numerical simulations by providing long measurement times extending into the non-linear regime where numerical simulations are not possible today. The proposed experiments will study fundamental physics processes, unveil the microphysics of GRBs, and address the following, yet answered, key questions: 1) What mechanisms drive the energetic beams unstable and their long-term evolution? 2) What is the role of turbulence in the acceleration of particles to the highest energies? 3) What is the interplay between magnetic fields and particles and how does this affect the observed electromagnetic emission? We will provide a new window in high energy astrophysics using novel Earth-based laboratory experiments.
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