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Though we cannot "see" a black hole itself (since not even light can escape the hole's gravitational field), we may see the hole's effects on nearby matter. For example, if gas from a nearby star were sucked towards the black hole, the intense gravitation al energy would heat the gas to millions of degrees. The resulting X-ray emissions could point to the presence of the black hole.
Or, if a massive black hole were surrounded by large amounts of orbiting material -- gas, dust, even stars -- their rapid motion close to the hole could be observable via shifts in the energy of the radiation they emit. Evidence along these lines is mount ing, suggesting that black holes may not be that rare in the universe.
However, such evidence remains indirect and therefore inconclusive. To confirm that black holes actually exist, we'll need to be able to observe the gravitational waves they produce as they form or interact.
If scientists could build gravitational wave detectors of sufficient sensitivity, they should be able to measure the vibrations in spacetime generated by black holes as they form from a collapsing star, when they ingest large amoun ts of matter, or if they interact, even collide with a second black hole or another massive object, such as a neutron star. Certain patterns of gravitational waves emitted would reveal the "smoking gun."
So far, the wavelike disturbances in spacetime have eluded detection. In a relativistic universe, there should be no shortage of places in which to hunt for black holes. Much larger and more sensitive detectors are now under construction. With luck, soon gravitation scientists may be shouting "Eureka!"
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