Question

The Theory of Gravity. How does the fact of gravity-for example, that things really do fall down-differ from what we think of as the theory of gravity? Briefly explain how and why Einstein's theory of gravity supplanted Newton's theory of gravity, and why we expect that we'll eventually find a theory that is even more general than Einstein's.

Short answer

Gravity is an observable fact; Newton's theory describes it as a force, while Einstein's theory describes it as spacetime curvature. Einstein's theory succeeded Newton's as it explains phenomena that Newton's couldn't, but a more general theory is expected to integrate quantum mechanics.

Step-by-step solution

Understanding the Fact of Gravity

The fact of gravity is the observation that objects fall toward each other, such as an apple falling to the ground. This is a consistent, observable phenomenon that can be experienced in everyday life.

Understanding the Theory of Gravity

A theory of gravity attempts to explain why and how this phenomenon occurs. Newton's theory described gravity as a force between two masses, governed by his law of universal gravitation. Einstein's theory, however, explains gravity as the curvature of spacetime caused by mass and energy.

Transition from Newton's to Einstein's Theory

Einstein's theory of general relativity replaced Newton's theory by providing a more comprehensive description of gravitational phenomena. It explained anomalies that Newton's theory could not, such as the precession of Mercury's orbit. It also offered a new framework where gravity was not a force but the result of curved spacetime.

Looking Beyond Einstein's Theory

While Einstein's theory has been successful in many ways, physicists believe it is incomplete because it does not account for quantum effects. A more complete theory, often referred to as quantum gravity, would integrate general relativity with quantum mechanics to explain phenomena at both large and small scales.

Key concepts

Newton's Law of Universal Gravitation

Newton's Law of Universal Gravitation was a groundbreaking idea introduced by Sir Isaac Newton in the 17th century. This theory describes gravity as a force that acts between two masses.
Newton's formula for this gravitational force is: \[ F = G \frac{m_1 m_2}{r^2} \] where:

• \( F \) is the gravitational force between the objects,
• \( G \) is the universal gravitational constant,
• \( m_1 \) and \( m_2 \) are the masses of the objects, and
• \( r \) is the distance between the centers of the two masses.

Newton's law was revolutionary because it gave a simple yet powerful formula that could predict the motion of planets and falling apples alike.
However, it treats gravity as an instantaneous force acting at a distance and can't completely explain all gravitational phenomena, especially those on an astronomical scale.

General Relativity

Albert Einstein introduced the theory of General Relativity in the early 20th century. It revolutionized our understanding by describing gravity not as a force but as the curvature of spacetime around massive objects.
According to this theory, massive objects like planets and stars warp the fabric of spacetime, much like a heavy ball placed on a stretched rubber sheet causes it to sag.
Einstein's theory explained unsolved mysteries from Newton's time, such as the precise orbit of Mercury, and predicted phenomena like the bending of light by gravity (gravitational lensing), which have been verified by experiments.
Key highlights include:

• Gravity affects the passage of time; clocks in strong gravitational fields tick slower compared to those in weaker fields, known as gravitational time dilation.
• General relativity predicts the existence of black holes, a region of such strong gravitational pull that nothing, not even light, can escape from it.

While General Relativity beautifully describes large scale structures and phenomena, it doesn't fully integrate with quantum mechanics, pointing towards a need for a more comprehensive theory.

Quantum Gravity

Quantum Gravity is a theoretical framework that seeks to blend the principles of quantum mechanics with general relativity.
The standard theories we have today work well on their own: quantum mechanics explains the smallest particles and forces, while general relativity explains the universe at astronomical distances and massive bodies. Yet they don't pair successfully.
Physicists believe a unified theory could solve this puzzle. Potential theories of quantum gravity include:

• String Theory: Suggests that the basic building blocks of the universe are one-dimensional "strings" rather than point-like particles. It attempts to describe all particles and forces, including gravity, on a quantum level.
• Loop Quantum Gravity: Proposes that spacetime itself is made up of discrete chunks, or "loops," rather than being continuous, which may help explain gravity at a quantum level.

Once a successful theory of quantum gravity is developed, it could provide a more complete picture of the universe by explaining phenomena across all scales, from the subatomic to the cosmic.