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

TLDRThe video script delves into the concept of gravity, highlighting Newton's law of universal gravitation which posits that every mass attracts every other mass, with the force being proportional to mass and inversely proportional to the square of the distance between them. It then transitions to Einstein's revolutionary perspective, explaining gravity not as a force but as a curvature of space-time caused by mass. The script uses the analogy of a fabric sheet to illustrate how mass bends space-time, affecting the motion of objects. It also discusses Einstein's thought experiments and the empirical evidence supporting the bending of light due to gravity, as demonstrated during a solar eclipse. The script concludes by emphasizing the significance of Einstein's general theory of relativity in understanding gravity and the workings of the universe.

###### Takeaways

- π Gravity is a fundamental force in the universe that attracts all objects with mass towards each other.
- π Newton's research on gravity revealed that every mass attracts every other mass and that gravitational force is directly proportional to mass.
- π The gravitational force decreases with the square of the distance between two masses, meaning that distance has an inverse relationship with gravitational force.
- π Newton's laws of gravity explain phenomena such as the Earth's orbit around the Sun and the Moon's orbit around the Earth.
- π€ The mystery of why Earth doesn't smash into the Sun despite its strong gravitational pull is explained by the combination of distance and angular momentum.
- π§ Einstein introduced a revolutionary concept of gravity, stating that it is not a force but rather a curvature of space-time caused by mass.
- π The universe is likened to a fabric sheet where mass bends this 'fabric', causing objects to move towards each other due to the curvature.
- π Einstein's thought experiments, including imagining the effects of acceleration and free fall, led to the understanding that acceleration causes the sensation of gravity.
- π‘ The bending of light by gravity was experimentally confirmed during a solar eclipse, demonstrating the curvature of space-time around massive objects like the Sun.
- π Time dilation is a concept related to space-time curvature, explained by Einstein's Special Theory of Relativity, which will be detailed in a subsequent video.
- π Einstein's General Theory of Relativity, which encompasses both space and time, is considered one of the greatest scientific achievements, explaining the workings of the entire universe.

###### Q & A

### What is gravity according to Newton?

-Gravity, as explained by Newton, is the universal force of attraction between all objects with mass. Every mass attracts every other mass, and the gravitational force is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

### How does the distance between two masses affect their gravitational force?

-The gravitational force between two masses decreases as the distance between them increases. Specifically, if the distance is doubled, the force decreases by a factor of four, and if the distance is tripled, the force decreases by a factor of nine, following an inverse-square law.

### Why doesn't Earth smash into the Sun despite its strong gravitational pull?

-Earth does not smash into the Sun due to its distance from the Sun and its angular momentum. The combination of these factors allows Earth to orbit the Sun without crashing into it.

### What was Einstein's revolutionary idea about gravity?

-Einstein proposed that gravity is not a force but rather a curvature of space-time caused by the presence of mass. He suggested that objects move along the curved paths in space-time due to this curvature, which we perceive as the force of gravity.

### How did Einstein explain the phenomenon of gravity without the concept of a force?

-Einstein used the analogy of a fabric sheet to explain gravity. He described the universe as a fabric sheet combined with time to form space-time. When a massive object, like the Sun, sits on this sheet, it bends the fabric, causing nearby objects to move towards it. This bending of space-time is what we experience as gravity.

### What thought experiment led Einstein to his realization about gravity?

-Einstein's thought experiment involved imagining a man falling from a building and then locking that man inside an elevator. He realized that the man would not feel any force during free fall, just as he wouldn't feel gravity if he were floating in space. This led Einstein to conclude that there is no difference between free fall on Earth and floating in space, and that gravity is equivalent to acceleration.

### How did Einstein demonstrate that light can bend due to gravity?

-Einstein predicted that light would bend in the presence of a strong gravitational field. This was confirmed during the 1919 solar eclipse when Arthur Eddington's team observed that the light from certain stars was bent around the Sun, causing the stars to appear in different positions than expected.

### What is the significance of the General Theory of Relativity in explaining gravity?

-The General Theory of Relativity, proposed by Einstein, revolutionized our understanding of gravity. It explained gravity not as a force but as a curvature of space-time caused by mass. This theory has been fundamental in our understanding of the universe, black holes, and the expansion of the universe.

### How does the concept of space-time curvature relate to the movement of objects in the universe?

-According to Einstein's theory, the mass of an object tells space-time how to curve, and the curvature of space-time tells objects how to move. This interplay between mass and space-time is responsible for the motion of objects, including the orbits of planets around stars and the overall structure of the universe.

### What is the difference between Newton's and Einstein's explanations of gravity?

-Newton explained gravity as a force of attraction between masses, while Einstein described it as a curvature of space-time caused by mass. Newton's law of gravitation is an approximation that works well for everyday experiences and celestial mechanics, but Einstein's General Theory of Relativity provides a more comprehensive and accurate description, especially in extreme gravitational fields.

### How did the observation of light bending during a solar eclipse support Einstein's theory?

-The 1919 solar eclipse observation by Arthur Eddington's team showed that light from stars was bent around the Sun, which was a direct observation of the curvature of space-time as predicted by Einstein's theory. This empirical evidence confirmed that massive objects like the Sun can bend the path of light, supporting the idea that gravity is not a force but a result of space-time curvature.

###### Outlines

##### π Newton's Understanding of Gravity

This paragraph delves into the fundamental concepts of gravity as explained by Sir Isaac Newton. It highlights that every mass in the universe attracts every other mass due to their inherent mass, with the force of attraction being directly proportional to the product of their masses and inversely proportional to the square of the distance between them. The explanation extends to the motion of celestial bodies, such as the Earth orbiting the Sun and the Moon orbiting the Earth, as a direct consequence of Newton's laws of gravitation. The paragraph also touches upon the mystery of how gravity operates over vast distances without physical contact, setting the stage for Einstein's revolutionary insights into the nature of gravity.

##### π Einstein's Revolution of Gravity and Space-Time

The second paragraph shifts focus to Albert Einstein's groundbreaking theories on gravity and space-time. It begins by challenging the traditional view of gravity as a force and introduces the concept of space-time curvature as the cause of gravitational effects. Einstein's thought experiments, including the equivalence of free fall and floating in space, lead to the conclusion that acceleration and gravity are fundamentally linked. The paragraph further explores the idea that light, despite having no mass, can also be bent by gravity due to the curvature of space-time. It describes the historical experiment conducted by Arthur Eddington during a solar eclipse, which provided empirical evidence for Einstein's theory of general relativity, confirming that massive objects like the Sun can bend light. The narrative concludes with a nod to the complexity of understanding space-time curvature in three dimensions and the profound impact of Einstein's theories on our comprehension of the universe.

###### Mindmap

###### Keywords

##### π‘Gravity

##### π‘Mass

##### π‘Newton

##### π‘Space-time

##### π‘Einstein

##### π‘Thought Experiment

##### π‘Curvature

##### π‘Free Fall

##### π‘Light Bending

##### π‘Acceleration

##### π‘General Theory of Relativity

###### Highlights

Gravity is a fundamental force that governs the attraction between all objects with mass in the universe.

Newton's research on gravity revealed that every mass attracts every other mass, with the force of attraction depending on their masses.

The gravitational force between two objects decreases as the square of the distance between them, meaning if the distance doubles, the force reduces by a factor of four.

Newton's laws of gravity explain the orbits of celestial bodies, such as the Earth around the Sun and the Moon around the Earth.

Despite the Sun's strong gravitational pull, the Earth does not smash into it due to distance and angular momentum.

Einstein introduced a revolutionary concept of gravity, stating that it is not a force but rather a curvature of space-time caused by mass.

Einstein's theory suggests that the fabric of space-time is bent by massive objects, causing objects to move along curved paths, which we perceive as gravitational attraction.

Einstein's thought experiments, including imagining the effects of acceleration and free fall, led to the development of his General Theory of Relativity.

Einstein proposed that even light, which has no mass, would be affected by gravity due to the curvature of space-time.

The bending of light by gravity was confirmed during the 1919 solar eclipse, when star positions shifted due to the Sun's gravitational influence.

Einstein's theories of Special and General Relativity fundamentally changed our understanding of space, time, and gravity.

The concept of space-time curvature explains not only gravity but also time dilation, which is a key aspect of Einstein's Special Theory of Relativity.

Einstein's thought experiments and the mathematical confirmation of his theories have been hailed as some of the greatest in human history.

The idea that empty space is not truly empty and that space-time is a dynamic entity has profound implications for our understanding of the universe.

The mass of an object dictates how it curves space-time, and this curvature, in turn, dictates how other objects move within it.

Einstein's theories have been tested and confirmed through various experiments and observations, solidifying his status as one of the greatest scientists.

The General Theory of Relativity not only explains gravity but also provides a framework for understanding the large-scale structure and behavior of the universe.

The concept of space-time curvature has practical applications in modern physics, including the study of black holes, gravitational waves, and the expansion of the universe.

###### Transcripts

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