Why does light bend near a large mass?

The culprit is gravity.

Gravity is defined as the attraction generated by any body or object with mass on another object.

The more massive a body is, the more gravitational pull it will exert.

Although it is not known if it is really true, it is said that the English physicist and mathematician Isaac Newton (1643-1727) had a “eureka moment” when he saw an apple fall from a tree (Another version is that the poor apple fell in the head!).

But behind that fall, true or not, there was a question: Why does the apple always fall perpendicular to the ground?

From there, Newton deduced that the Earth must exert an attractive force, called it gravity and consolidated its definition.

With this seed he formulated the "Theory of Universal Gravitation" (1687), which precisely describes the way in which objects move within a gravitational field, explaining not only the fall of objects to the ground, but also the movement of objects. planets and that is why he called it universal.

A whole scientific revolution!

More information

About the invisible, the strange and the unknown

These laws remained intact until the beginning of the 20th century, when the German physicist Albert Einstein (1879-1955) published his “General Theory of Relativity,” which describes gravity as a geometric effect due to a deformation of space caused by the mere presence of objects with mass, similar to what happens when we place a heavy ball on an elastic cloth or mesh.

If we apply this, for example, to our Solar System (SS), the deformation produced by the Sun (which has 98% of the mass of the SS) in the mesh will tell the planets how they should move through space.

This concept of a deformable mesh due to the presence of massive bodies is what is known as the space-time continuum.

As the planets move, the mesh deformation changes.

What does all this nonsense have to do with the initial question?

We have only talked about massive objects.

Light has no mass, but it is an electromagnetic wave that moves precisely through space-time, just as sound is a mechanical wave that propagates through the air (in a vacuum we would not be able to hear).

If space-time is curved in the presence of a large mass, light propagating through it is forced to follow a path that is also curved.

German theoretical physicist John Archibald Wheeler (1911-2008) summed it up brilliantly: “Matter curves space and space curves material trajectories, therefore space-time tells matter how to move;

“matter tells space-time how to curve.”

Beatriz Agís González

is a doctor in Astrophysics, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC).

Question sent via email by 

Ricardo Gomez Kenny

Coordination and writing: 

Victoria Toro

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