Expert: Philip A. Stahl - 8/21/2008

Question
Whats the reason for so much high gravity in black hole that light can not escap from black hole.and which effect the photons.can earth attract photon.

Answer
Hello,


The reason for the extremely high gravity arises from the incredible high density of the matter that has collapsed to form a black hole. As you may know, black holes are the result of a massive (> 3 solar mass) star collapsing.

The German astrophysicist Karl Schwarzschild actually calculated the critical radius (called the Schwarzschild radius) for any given size object of known mass to become a black hole. For example, an object the size (and mass) of the Sun (1.4 million km diameter, or 700,000 km radius) would have to have all its mass collapse to a volume with a radius of 3 km to become a black hole. At that point, the gravity would be so strong that not even light could escape.  (In other words, we would say the escape velocity of the black hole is actually greater than the speed of light)

In the case of photons, the gravitational field of a black hole is so strong that the light rays are bent around the collapsed object and cannot escape.  Before the final limit is reached the light rays undergo increasing bending, until the totally wrapped light path is achieved around the object.

To have photons behave in this way obviously an extremely powerful gravitational field must be available. The Earth's field is simply too small to attract photons - which have a vanishingly small mass (rest mass).