I've read in some texts that we can't directly observe a black hole in space because not even light can escape from its gravity. Some of the indirect observational methods mentioned are, gravitational lensing and gamma ray outbursts created by swirling matter into the event horizon. My question is, if a black hole is surrounded by glowing matter, it should appear just like a star (except with a different spectral signature), and it should be fairly easy to detect. I don't understand the statement: "If you find an object orbiting around nothing, there's likely to be a black hole at the center". Though a black hole is invisible, the enveloping matter should reveal its identity... What am I missing here? Please answer!
Answer
The reason has to do with time dilation, and specifically, with the resulting red shift.
A black hole forms from a collapsing star, which is of course made of brightly glowing matter. The event horizon forms in the centre and moves outwards while the star-matter falls towards it. Because of gravitational time dilation, the infalling matter never crosses the event horizon from the outside perspective, and thus can technically still be "seen."
However, this time dilation also causes the light the matter emits to be redshifted. Essentially, every photon the matter emits is reduced in frequency due to the time dilation, and the time in between photons also reduces, asymptotically approaching infinity. This means that the black hole very rapidly converges to something that would appear completely black to an outside observer - the matter falling into it can only be seen by someone with the patience to collect many very low frequency photons over many billions of years.
That said, matter that falls into the black hole after it's been formed can certainly glow brightly enough to be detected - very much so in the case of an active galactic nucleus.
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