Lecture #4 - Two Models of Gravity

Newton’s Law of Gravity

The force of gravity between two objects is equal to the product of their masses and the gravitational constant, divided by the square of the distance between them.

Newton’s First Law

An object in motion remains in motion with constant speed and direction unless acted upon by an external force. If no force acts on an object, its speed and direction will remain unchanged, forever.

Newton’s Second Law

The change in speed (acceleration) (a) is equal to the applied force (F) divided by the mass of the object, m. Means that the second an object loses contact with another, it will stop experiencing a force — accelerating and changing speed.

• a = F/m

• F = ma

Four Fundamental Forces

• Strong nuclear force

• Electromagnetism

• Weak nuclear force

• Gravity

Strong Nuclear Force

• Strongest force in the universe, with a smaller range

• Holds atomic nuclei together

• Principally involved in the nuclear bomb

Electromagnetism

• Second strongest and most common force experienced in daily life

• Infinite range

Weak Nuclear Force

• Second weakest force, extremely small range

• Primarily involved in radioactive decay

Gravity

• Extremely weak strength, infinite range

• Makes masses attract one another.

Fields

Transmit forces, convey information about the charge and position of each particle.

Quantization

A subset of quantum mechanics that tells us that fields are made up of quantum particles, which ‘carry’ the forces.

Gluons

Carrier particle of the strong nuclear force.

Photons

Carrier particle of the electromagnetic force

W and Z bosons

Carrier particles of the weak nuclear force.

Gravitons

Theoretical carrier particle of gravity — never been found.

Equivalence Principle

Tells us that gravity and acceleration cannot be distinguished from one another. Being in a gravitational field with a downward acceleration is the same as being accelerated upward in the absence of gravity — a dropped ball in each of those scenarios will behave the same way, fall to the floor at the same time.

Einstein’s Theory of Gravity

Imagines gravity as a curved spacetime, wherein objects attempt to follow the straightest possible path. Conceptualizes gravity as a “stage” that the other forces, like actors, stand on/act on. Mass deforms this curved stage, forcing the actors to take alternative, curved trajectories.

Precession

The concept of planetary orbit being warped/shifted upward. Most significant in our solar system in terms of Mercury, which orbits closest to the sun.

Gravitational Lensing

When massive objects bend the space around them and force light to follow its curved contours.