Cosmic black holes, first predicted by Einstein's general relativity, represent the universe's most extreme environments, where matter, space, and time are stretched to their limits. The area around a black hole’s event horizon - the boundary beyond which nothing can escape - conceals the mysterious interior from outside observers. This region is crucial for understanding the fundamental laws of physics that govern the universe.
Studying black holes is a key focus in modern astrophysics, and recent years have seen groundbreaking achievements. Scientists have detected gravitational waves from colliding black holes and captured the first images of black hole "shadows," created by the intense bending of light near their event horizons.
However, it's possible that other compact objects, which lack event horizons, might mimic black holes. These objects could produce similar gravitational waves and shadows, making it challenging to differentiate them from traditional black holes. To distinguish between these possibilities, we need significant improvements in the sensitivity and accuracy of gravitational wave detectors and black hole imaging. These upgrades, though complex and expensive, are in progress and should be completed over the next decade.
Given this context, it might be worth exploring some additional indicators to distinguish black holes from their horizonless counterparts. One promising factor is the properties of the magnetic field near the event horizon. For horizonless objects, the magnetic field strength is expected to be four or more magnitudes higher. This difference is significant enough to be detectable from about 10,000 gravitational radii away - that's about 10,000 times the event horizon's scale. These measurements can be made using current technology, specifically radio interferometric imaging of polarized light from the area surrounding these compact objects.
The M2FINDERS project is taking advantage of this opportunity by using Very Long Baseline Interferometry (VLBI) to conduct detailed measurements of the magnetic field's total strength and three-dimensional distribution near supermassive black holes in active galaxies. These measurements are among the most efficient and reliable methods to investigate the presence of event horizons in black holes. The approach chosen by M2FINDERS will help us distinguish traditional black holes from alternative models, including exotic horizonless objects like boson stars, gravastars, and wormholes.
The impact of the activities of the M2FINDERS project is manifold. Black hole research continues to capture the public imagination, and we regularly share research news with the general public, such as last year's first image of a jet and black hole shadow together in Messier 87. We also engage the academic community through the four M2FINDERS training schools we have organised, covering astronomical imaging, polarisation calibration and magnetohydrodynamical simulations. In addition, the astronomical community benefits from our efforts through presentations at national and international conferences, with dozens of talks and posters so far, and regular publications in scientific journals. As of the end of June, there are fifty peer-reviewed papers from the programme, including twenty in the European open access journal Astronomy & Astrophysics, fifteen in the US open access journal The Astrophysical Journal, and others in prestigious journals such as Nature and Nature Astronomy.
As well as pushing the boundaries of our understanding of the Universe, the M2FINDERS project is inspiring a global audience and training the next generation of astrophysicists, making a lasting impact on both science and society.
