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

Summary

This video explores the evolution of our understanding of gravity, from Newton's theory of mass attraction to Einstein's revolutionary concept of space-time curvature. It begins with Newton's explanation of gravitational force and its relationship with mass and distance. The narrative then shifts to Einstein's theory, which redefines gravity not as a force, but as the bending of space-time. Through thought experiments, Einstein demonstrates how this curvature affects objects, including light. The script also highlights the groundbreaking experiment by Arthur Eddington, which confirmed Einstein's theory, marking a pivotal moment in our comprehension of the universe.

Takeaways

• 😀 Newton’s theory of gravity states that every mass attracts every other mass in the universe.
• 😀 The strength of gravitational force depends on the mass of the objects and the distance between them.
• 😀 Gravitational force decreases as the distance between two objects increases, following an inverse square law.
• 😀 Newton's laws successfully explain many phenomena, such as the Earth orbiting the Sun and the Moon orbiting the Earth.
• 😀 The confusion about gravity arose when scientists questioned how such an attraction works over vast distances, like between Earth and the Sun.
• 😀 Einstein proposed that gravity is not a force, but the result of the curvature of space-time caused by mass.
• 😀 Space-time is a fabric that bends around massive objects, influencing the motion of other objects, like Earth orbiting the Sun.
• 😀 Einstein’s thought experiment with a falling man inside an elevator led him to the idea that free fall and weightlessness are the same as being in space.
• 😀 The acceleration of an object can simulate gravity, as seen in an elevator accelerating in space.
• 😀 Light can bend in the presence of strong gravity, a prediction made by Einstein and confirmed by Arthur Eddington’s 1919 experiment during a solar eclipse.
• 😀 The curvature of space-time also affects the path of light, showing that the shortest path in space can be curved, which confirmed Einstein's theory of General Relativity.

Q & A

What is gravity, according to Isaac Newton?

Isaac Newton explained gravity as a force where every mass attracts every other mass. The strength of this attraction depends on the masses involved and the distance between them. The force is stronger for larger masses and weaker with increased distance.

How does the gravitational force change with distance?

The gravitational force decreases as the distance between two masses increases. This relationship is inverse to the square of the distance, meaning if the distance doubles, the force becomes four times weaker.

Why doesn’t the Earth fall into the Sun, despite the Sun’s strong gravity?

The Earth doesn’t fall into the Sun due to the combination of distance and angular momentum. The Earth's orbit is stable because it is constantly moving sideways, preventing it from being pulled directly into the Sun.

How did Einstein’s theory of gravity differ from Newton’s?

Einstein’s theory, unlike Newton’s, suggests that gravity is not a force. Instead, it is the result of space-time curvature caused by massive objects. The Sun’s gravity, for example, bends space-time, and this curvature causes Earth to orbit the Sun.

What does space-time refer to, according to Einstein?

Space-time is a combination of the three dimensions of space and the fourth dimension of time. It forms a fabric that can bend or curve due to the presence of mass, and this curvature is what causes the phenomenon we perceive as gravity.

How does the fabric of space-time work in Einstein’s analogy?

Einstein used the analogy of a fabric mat to explain space-time. When a heavy object (like a fat man) sits on the mat, it causes the mat to bend. A lighter object (like a skinny man) is then drawn towards the heavier object, similar to how Earth orbits the Sun due to the curvature of space-time.

What thought experiment did Einstein use to understand gravity?

Einstein’s famous thought experiment involved imagining a person falling in an elevator. He realized that, in free fall, a person would not feel gravity, much like astronauts floating in space, leading to his insight that acceleration and gravity are indistinguishable.

Can light be bent by gravity, and if so, how?

Yes, according to Einstein’s theory, light can be bent by gravity. Light travels along the curved paths in space-time, which means its path can bend around massive objects like stars or the Sun.

What experiment confirmed Einstein’s prediction about light bending?

In 1919, during a total solar eclipse, astronomer Arthur Eddington’s team observed stars that appeared displaced from their usual positions due to the Sun's gravity bending the light. This confirmed Einstein's prediction about light bending due to space-time curvature.

How does the bending of light relate to the shortest path in space?

Even though light always travels in a straight line, in a curved space-time, the shortest path for light can also be curved. This bending is due to the warping of space-time around massive objects like the Sun.