There are several reasons why we are unable to see black holes directly with the human eye. Here are some of the main factors:
Lack of visible light: Black holes, by definition, are objects with such strong gravitational pull that nothing, including light, can escape their event horizon. The event horizon is the boundary beyond which no information can be received from the black hole. Therefore, black holes do not emit or reflect visible light, making them invisible to the human eye.
Absence of contrast: Black holes do not have a solid surface or a distinct boundary that we can visually perceive. They are regions of spacetime with extremely high gravitational curvature. Since we primarily rely on detecting objects based on the light they emit or reflect, the absence of light from the black hole itself makes it difficult for us to distinguish it from the background of space.
Distance and size: Black holes are typically located very far away from Earth. The closest known black hole, named V616 Monocerotis or A0620-00, is about 3,000 light-years away. The immense distances involved make it challenging to observe and resolve black holes with the human eye, even if they were emitting visible light.
Interstellar dust and gas: The space between black holes and Earth often contains interstellar dust and gas, which can absorb and scatter light. This further hinders our ability to observe black holes directly, as the intervening material can block or distort the light coming from distant objects.
Despite these limitations, scientists have developed indirect methods to detect and study black holes. These methods involve observing the effects of black holes on nearby matter and the surrounding environment. For instance, the accretion disks, jets, and gravitational lensing caused by black holes can be detected and studied using various telescopes and instruments that operate in different regions of the electromagnetic spectrum, such as X-rays and radio waves.
Advancements in observational techniques, including the use of sophisticated telescopes and the collaboration of international research teams, have allowed scientists to gather substantial evidence for the existence of black holes and study their properties indirectly. The recent direct imaging of the supermassive black hole at the center of the M87 galaxy by the Event Horizon Telescope (EHT) is a groundbreaking achievement, but it required an array of telescopes working together to capture the radio waves emitted by the surrounding material, rather than the black hole itself.