Discovery That Changed Physics! Gravity is NOT a Force!

Understanding Gravity: From Newton to Einstein

The Basics of Gravity

• Gravity is commonly perceived as a force between objects with mass, easily demonstrated by weighing oneself on a scale, which reflects Earth's gravitational pull.

• While gravity seems straightforward, it is part of a more complex phenomenon explained by the general theory of relativity.

Newton's Contribution

• Isaac Newton theorized that gravity is a force pulling two masses together, proportional to their mass and inversely proportional to the square of the distance between them.

• He believed that an apple falls straight down due to gravitational acceleration towards Earth but felt his theory was incomplete since he viewed gravity as a push rather than a pull.

Einstein's Revolution

• Albert Einstein challenged Newton's ideas in 1905 with his general theory of relativity, which initially faced skepticism due to its radical nature.

• The key insight from Einstein’s theory is that all objects in a gravitational field fall at the same rate, regardless of their mass—a principle first established by Galileo.

The Equivalence Principle

• An experiment conducted by Apollo 15 astronaut David Scott demonstrated that a hammer and feather dropped simultaneously on the Moon hit the ground at the same time, illustrating Galileo's principle.

• According to Einstein, objects are not pulled by gravity; instead, they follow curved paths in space-time created by massive bodies.

Space-Time and Gravity Wells

• In Einstein’s view, gravity results from the warping of space-time around massive objects rather than being an inherent force acting between them.

• This concept combines three dimensions of space with time into what is known as space-time; larger masses create deeper "gravity wells" affecting nearby objects' motion.

Practical Implications of Gravity Wells

• Visualizing Earth on a grid shows how its mass creates a significant gravity well that attracts other bodies like satellites and moons.

• Spacecraft utilize these gravity wells for slingshot maneuvers—using planets' warped space-time to gain speed when changing direction.

Gravitational Fields Explained

• A gravitational field exists around every object with mass; for example, Earth has a stronger field than the Moon due to its greater mass.

• Even in orbit (like aboard the International Space Station), effective gravity remains nearly equal to surface levels—about 90%—demonstrating pervasive gravitational influence.

Understanding Gravity and Microgravity

The Weight of Objects in Space

• An object weighing 100 kilograms on Earth would weigh approximately 90 kilograms at the International Space Station due to reduced gravitational effects.

The Concept of Microgravity

• Astronauts appear to float in space because both they and the International Space Station are in a state of free fall, creating an environment known as microgravity.

Falling Towards Earth

• The International Space Station is constantly falling towards Earth while simultaneously moving at a high speed (about 28,000 kilometers per hour), which contributes to its orbit.

Warping of Space-Time by Mass

• All objects with mass warp the fabric of space-time; gravity is essentially this curvature that pulls objects toward one another.

Journey to Earth's Center and Gravity's Effects

• If one were to travel to the center of the Earth, they would experience weightlessness due to being away from gravitational pull until returning closer to the surface where gravity increases again.

Proving Einstein's Theory: Gravitational Lensing

Understanding Gravitational Lensing

• Gravitational lensing occurs when massive celestial bodies bend light around them, allowing us to observe distant galaxies that would otherwise be hidden.

Example: Einstein's Cross

• A notable instance of gravitational lensing is Einstein's Cross, where four images of a quasar appear due to strong gravitational effects from a galaxy positioned between it and observers on Earth.

Challenges with General Relativity

Incompatibility with Quantum Mechanics

• Despite substantial evidence supporting general relativity, it currently conflicts with quantum mechanics, leading researchers to explore theories like quantum gravity that remain unproven.

Observations Near Black Holes

• Scientists have observed stars near black holes (e.g., SO2 near Sagittarius A*) confirming predictions made by Einstein regarding light wavelength shifts caused by intense gravitational fields.

Future Directions in Physics

• Researchers anticipate discovering extreme curvatures in space-time that challenge general relativity within the next decade, potentially leading to new understandings beyond Einstein’s framework.