Why Can't Light Escape through Black Holes?
6:33 PM Science 0 Comments

A Blackhole

Black holes started to grab the attention of astronomers since 20^th century. Black holes are sources of tremendous gravitational forces in space-time continuum. They have such a powerful gravitational field that they don’t allow any object to escape through them including light. There arises a question that “If light is composed of photons which are charge less and whose rest mass is zero, why light cannot escape through black holes?”

This can be explained by quantum physics. Einstein’s theory of relativity is used to prove this. Classical mechanics do not accept the existence of blackholes. According to Newton, gravity is a force which attracts the matter. But, Einstein’s theory of relativity defines gravity as a force which warps the space-time. Our eye can see 3 dimensions, but time is another dimension which cannot be seen but can be perceived.

Einstein’s mass-energy equivalence relation E=mc^2 is an approximate relation but it is not the exact one. We cannot apply it for a photon whose rest mass is zero. Einstein found that energy of the light is not due to mass but due to its frequency. This shows that gravity is not affected by mass but it is affected by energy.

There might be a possibility that photons may have negligible mass. Even though the rest mass of a photon is considered as zero, the moving mass of a photon cannot be determined. The mass-energy equivalence relation can orient mass as the amount of energy which is concentrated in a space and the converse is also true i.e., energy can have some mass (light is a form of energy).

It was observed that when light from distant stars approach Sun, there was a slight bend in its path. This was later confirmed during eclipses. During an eclipse, when Moon covers the glare of Sun, the light was found to be bent. This confirms that light is affected by gravity. We learnt that light always travels in a straight line or a wave but its direction remains same. But what is this bending? This bending of light doesn’t mean the bending of light in Euclidean geometry. The bending of light is possible in space-time continuum.

Image showing the bending of light during eclipses

The fact that light cannot escape through blackholes could be proved mathematically. According to Galileo’s equation of acceleration due to gravity,

g= GM/R^2

Centripetal acceleration is v^2/R

When light orbits around a blackhole and this acceleration due to gravity is equated to centripetal acceleration,

R=GM/v^2 (v is orbital velocity)

Hence, the escape velocity can be calculated as

V (escape) = 2GM/R^2

R is called radius of black hole.

If blackhole has this radius or smaller (called Schwarzschild radius), any object cannot escape through blackhole unless it has velocity more than that of light. This shows that even light can’t escape through blackholes. So, a blackhole absorbs everything in its path including light.