The shadow is essentially an image of the event horizon, lensed by the strong gravitational field around the BH (which bends the path of light rays, acting as a lens and distorting the image) and superimposed over the background light. This shining material accreting onto the black hole crosses the event horizon, resulting in a dark area over a bright background: this is the so-called black hole “shadow”. This surrounding matter accreting onto the hole heats up through viscous dissipation and converts gravitational energy into radiation, radiating bright light at many frequencies, including radio waves that can be picked up by radio telescopes. Black holes tend to suck up surrounding matter in a process called ‘accretion’. Interestingly, BHs are completely ”black” only within the event horizon, from but outside the event horizon, light can escape. For these reasons, the event horizon has been defined as “the shell of points of no return”. The defining feature of a black hole (BH) is the event horizon, the boundary from within which a particle cannot escape.Īt the boundary of the event horizon, the gravitational pull becomes so large that nothing can escape, including light (that is why BHs are black!).
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