# General Relativity Theory: If light has no mass, why is it affected by gravity?

## Introduction

Can you explain what General Relativity Theory is? As a professional engineer passionate about science and physics, I have stumbled upon a fascinating exploration of the universe’s secrets, made accessible through a compelling YouTube video that delves into the intricacies of gravity and its surprising relationship with light. Allow me to share with you the key takeaways from this video and how it can broaden our understanding of our cosmic neighborhood.

## Understanding Gravity: Newton’s Legacy

Our journey begins with the legendary figure, Isaac Newton, who theorized that every object with mass attracts every other object. Newton’s gravitational theory succinctly summarized:

• Every mass attracts every other mass.
• The gravitational force between two objects is inversely proportional to the square of the distance between them.

This law flawlessly explains why Earth orbits the Sun and why the Moon orbits the Earth. However, Newton’s conception of gravity brought about some confusion among scientists. The invisible force binding celestial bodies, despite the vast distances separating them, raised questions that Newton’s framework couldn’t entirely address.

## Einstein’s Game-changing Insight: Space-Time Curvature

In came Albert Einstein, offering a revolutionary perspective. According to Einstein, gravity isn’t a force. Instead, he proposed the existence of a four-dimensional space-time fabric that governs the movement of masses. To visualize this:

• Imagine our universe as a fabric mat.
• Heavier objects (e.g., the Sun) create more significant curvatures on this mat, attracting lighter objects (e.g., Earth) towards them.

This interplay between space, time, and mass opened up new pathways for understanding gravity and how objects move in our universe.

## Einstein’s Thought Experiments: Equivalence Principle and Light Bending

One of the major thought experiments that helped Einstein develop his theory was centered around the concept of a freely falling elevator. This mental exercise elucidated the equivalence principle:

• Free fall on Earth = Weightlessness in space
• Acceleration in space = Gravitational pull on Earth

But the truly groundbreaking insight came when Einstein applied this thought experiment to light. Despite having no mass, light could be influenced by gravity – it could bend! To illustrate:

• Imagine an accelerating elevator in space.
• A beam of light shone inside this elevator would follow a curved path due to the acceleration, effectively demonstrating how light can bend under gravity.

Although bending light within an elevator accelerating at Earth’s gravitational pull (9.8 m/s^2) would be minuscule, increasing the acceleration could result in a more noticeable bending.

## Validation of Theory: The Sun’s Gravitational Lensing

Einstein’s theory had a grand claim: even light can be influenced by gravity and take a curved path. This prediction was put to test in 1919 by the British astronomer Arthur Eddington during a total solar eclipse. Eddington observed that:

• Light from stars behind the Sun appeared to be displaced from their usual position.
• The apparent shift of the star’s position was due to the bending of light as it passed close to the Sun, a phenomenon known as gravitational lensing.

This experimental validation of Einstein’s predictions made him one of the greatest scientists in history, and his theory, known as the General Theory of Relativity, became a cornerstone of modern physics.

## Insights and Implications

In this roller-coaster journey from Newton’s understanding of gravity to Einstein’s radical shift in perspective, we can take away several key lessons:

• The laws of physics that we perceive may merely be a curvature in the cosmic fabric of space and time.
• Light, although massless, can be influenced by gravity due to this space-time curvature.
• Gravitational lensing serves as a testament to the profound reality of the universe.

By sharing this video and the insights it contains, I aim to inspire curiosity and a love for the mysteries of the universe. Remember, as future scientists, gaining a profound understanding of the theories shaping our universe today is an exciting and vital part of our journey. Happy exploring!

## Reference

YouTube Video: If light has no mass, why is it affected by gravity? | General Relativity Theory