Why are Black Holes black? (Gravitational Red Shift)
Why are Black Holes black? We all know: black holes are black. No light escapes a black hole. But, why? We come across the answer for this while studying gravitational red shift. Gravitational Red Shift Gravitational red shift is similar to the concept of Doppler red shift, but different. Doppler red shift is a change (lowering) in frequency of a wave due to its velocity. But, gravitational red shift is the lowering of frequency of a wave due to gravitational force in the space-time fabric. The derivation that explains this shift happens o be simple, but we shall leave that for now. At the end of the derivation we conclude that the relative frequency change of a wave (due to gravitational force) equals to GM/c 2 R; where G and c are our familiar constants - Newton's 'G' and the light's 'c', and M and R are the mass and radius of the star respectively. The star here could be the star of any phase - a main sequence star, a super giant or even a