Why is light affected by gravity?

By Nishtha Hari
10 January 2022

If you are a science enthusiast and science fiction lover, then you may know that even light cannot escape from a black hole. But have you ever wondered how light, with its constant speed of travel and absence of mass, can be affected by gravity?

In fact, light should always travel in the same, straight line and should not be affected by gravity. Why, then, does this not happen?

Let us jump into history, to understand why…

The great physicist and mathematician Newton was famous for elucidating the scientific reasoning behind many natural phenomena, including gravitational potential energy, escape velocity, the orbits of planets around stars and even the trajectory of a rocket launch. Pivotally, Newton discovered that every object in the universe is attracted to one other, providing mathematical equations to justify his laws. In other words, Newton discovered gravity and devised the Universal Gravitational constant to explain this breakthrough.

However, Newton was unable to explain many other phenomena including acceleration, space, time, and light. Indeed, whilst Newton’s laws can explain what gravity is, do they inform us as to how gravity is created?

Almost 200 years after Newton, a clerk working at a patent office theorized a concept encompassing the very phenomena that Newton’s gravitational law was lacking. This clerk was none other than Sir Albert Einstein, one of the most renowned physicists in history.

Einstein published his paper, the General theory of Relativity, to explain space and time and thus all other phenomena. To understand why light is affected by gravity, we need to comprehend this theory.

According to Einstein, space and time are not individual concepts – instead, they are one: Space-time. His theory denoted that gravity is not a force alone, but a consequence of Space-time curvature in 4-dimensional space.

To picture this, let us consider a long cloth tightly held by four people, each situated at one of the four corners. The people and the cloth exist in three-dimensional space. One person throws a metallic ball onto the cloth. Due to the mass or energy of the metallic ball, the cloth will become curved. Similarly, Einstein described the space-time of our universe in four dimensions: three dimensions of space and one dimension of time.

In Einstein’s eyes, time was an illusion. Perhaps time is the universe’s greatest magic trick…

Nishtha Hari is a third-year student at Imperial College London studying Physics.