What is the force of gravity?

Gravity is an invisible force we encounter every day, but it remains mysterious. It's what keeps us on the ground here on Earth and causes things to fall when dropped. While many theories have tried to explain gravity, scientists still don’t fully grasp its nature.

In this article, we’ll explore the question, "What is gravity?" by diving into Newton’s and Einstein’s theories, looking at modern perspectives, and examining how gravity plays a crucial role in both our solar system and daily lives.

Newton's Theory of Gravity

In the 1600s, a physicist and mathematician from England, Isaac Newton, is said to have been sitting beneath an apple tree when an apple fell on his head — sparking his curiosity about why the apple fell toward the ground in the first place.

In the 1680s, Newton unveiled his Theory of Universal Gravitation. This theory proposed that gravity is a predictable force acting on all matter in the universe, dependent on both mass and distance. It states that every particle of matter attracts every other particle (such as the particles of "Earth" and the particles of "you") with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.

Therefore, the greater the distance between particles, or the smaller the particles' masses, the weaker the gravitational force.

The commonly used formula for the law of gravitation is as follows [source: UT]:

Gravitational force = (G x m1 x m2) / (d)

In this equation, G represents the gravitational constant, m1 and m2 are the masses of the two objects for which the force is being calculated, and d is the distance between the centers of mass of both objects.

The value of G is 6.67 x 10 dyne * cm/gm. This means that if two 1-gram objects are placed 1 centimeter apart, they will attract each other with a force of 6.67 x 10 dyne. A dyne is roughly equivalent to 0.001 gram weight, meaning that a dyne of force could lift 0.001 grams in Earth's gravity. Thus, 6.67 x 10 dyne represents an incredibly small force.

However, when dealing with much larger objects like Earth, which has a mass of 6 x 10 kilograms (see How much does planet Earth weigh?), the gravitational force becomes immense. This is why you're not floating in space right now.

The force of gravity acting on an object is also known as its weight. When you step on a scale, it measures the gravitational force exerted on your body. The formula to calculate weight is [source: Kurtus]:

weight = m x g

In this equation, m represents an object's mass, and g refers to the acceleration due to gravity. On Earth, the acceleration due to gravity is 9.8 m/s² — this value remains constant, no matter how heavy or light the object is. That’s why a pebble, a book, and a couch would all fall at the same rate if dropped from a roof (unless the roof is really high, in which case terminal velocity becomes a factor).

For centuries, Newton's theory of gravity was the predominant explanation in the scientific world. However, this began to change in the early 1900s.

Einstein's Theory of Gravity

Albert Einstein, who received the Nobel Prize in Physics in 1921, presented a different theory of gravity in the early 1900s. This theory was part of his renowned General Theory of Relativity and provided a radically different viewpoint compared to Newton's Law of Universal Gravitation.

Einstein rejected the idea of gravity as a force. Instead, he proposed that gravity was a curvature in the shape of space-time, often referred to as "the fourth dimension" (for more on space-time, see How Special Relativity Works).

According to basic physics, if no external forces are acting on an object, it will always travel in the straightest path. As a result, two objects moving along parallel trajectories will never intersect, as they will always remain parallel.

But in reality, they do collide. Particles that begin along parallel trajectories sometimes end up intersecting. Newton's theory explains this by suggesting gravity — a force that draws objects together or toward a third object. Einstein also agrees gravity is responsible for this phenomenon, but in his theory, gravity isn’t a force. Instead, it's a curvature in space-time.

According to Einstein, the objects are still traveling along the straightest possible path. However, due to the warping of space-time, the straightest path now takes the form of a spherical curve. So, two objects that once moved in parallel on a flat plane are now following curved, spherical paths. And two straight lines on that sphere eventually meet at the same point.

Gravitational Waves and Other Theories

More contemporary gravity theories explain the phenomenon in terms of particles and waves. One perspective proposes that particles called gravitons cause objects to attract each other. However, gravitons have never been directly detected.

Another theory suggests the existence of gravitational waves or gravitational radiation, created when an object is accelerated by an external force. While gravitational waves haven’t been directly observed, their existence has been validated through indirect evidence.

Gravity in Our Solar System

Gravity is essential in shaping and maintaining the stability of our solar system. It played a key role in the formation of the universe, keeps the moon in orbit around Earth, and prevents our planet from spiraling into the sun.

The sun’s gravitational force keeps the planets in their respective orbits, while the gravitational pull between planets and their moons ensures that the moons stay in orbit.

Gravity: Did You Know?

Gravity Changes Across Earth's Surface

The strength of gravity varies across Earth’s surface. It is slightly weaker at the equator because of the centrifugal force caused by Earth's rotation, as well as the fact that Earth is not a perfect sphere but rather an oblate spheroid. Gravity is stronger at the poles and weaker at higher altitudes. This variation explains why your weight can fluctuate slightly as you travel from one location to another on Earth's surface.

Gravity and Black Holes

A black hole is a region in space where the gravitational pull is so immense that not even light can escape it. The boundary beyond which nothing can return is called the event horizon. Once an object crosses this threshold, it is inevitably drawn into the black hole. The central point of a black hole, where all its mass is concentrated, is known as the singularity.

Gravitational Constant

The gravitational constant (G) is a key physical constant that quantifies the strength of the gravitational attraction between two objects. Its value is approximately 6.67430(15) x 10 Newtons (m/kg).

Gravity and Everyday Objects

The gravitational pull between everyday items, such as a book and a pen, is extremely weak due to their small masses. Nevertheless, the collective force of Earth's gravity ensures that everything stays firmly on the ground.

Fundamental Forces

Gravity is one of the four fundamental forces of nature, alongside electromagnetic forces, the strong nuclear force, and the weak nuclear force. It is the weakest of the four, yet its influence extends infinitely and is responsible for the large-scale structure of the universe.

Unraveling the Mystery

Although we've made great strides in unraveling the mysteries of gravity, it continues to be an intriguing and somewhat enigmatic force. Whether it's the force that holds us to the Earth, governs the orbits of planets around the sun, or fuels revolutionary scientific theories, gravity is an essential force that influences both the vast universe and our everyday experiences.