What is Gravity? Part 1
- kieronconway
- 6 days ago
- 6 min read
© 2026 Kieron Conway - All rights reserved.
This is the first of a two part article that asks the question, what is gravity?
The first part will look at Newton's version of gravity and the second will delve into Einstein's alternative theory.
The Facts
Gravity is the force that keeps you glued to the ground. Without it, we would all just drift away into space.
Gravity is what keeps the international space station going round the earth, locked into an orbit, just like the moon.
Newton's View of Gravity
Isaac Newton (1643 to 1727) was the first scientist to ponder the nature of gravity and he discovered some very important things;
The gravitational attraction between two bodies, like the earth and the moon, gets stronger as they get closer to each other and gets weaker, the further apart they are.
He believed that the physics that keeps us attached to the earth is the same as the physics that keeps the moon in orbit around the earth and the planets in orbit around the sun.
This second point was quite revolutionary. Up to the time when the apple fell from the tree and hit his head (the legend that is supposed to have helped Newton understand gravity), people thought that earth physics and space physics were quite separate.
Newton's Laws of Motion
Newton had already formulated a law that showed that a force F, acting on a body of mass m, created an acceleration of a or;
F = ma
He understood that the force that keeps a body of mass 'm' anchored to the earth involves an acceleration due to gravity, which is normally written as;
F = mg
Where g is the acceleration due to gravity on the earth's surface, something we refer to as 1g.
Newton Looked into Space
He studied what was known about space physics, the result of work done by Johannes Kepler (1571 to 1630), which showed the following;
Planets move around the sun in orbits shaped like ellipses (a rugby ball has an elliptical shape).
The speeds of the planets changed in predictable ways.
The time it took for a planet to orbit the sun was related to the distance of the planet from the sun.
So, Newton started to contemplate what kind of force would produce the elliptical orbits that Kepler had observed and studied?
Starting with the Moon
Newton began by analysing the near circular orbit of the moon and he realised that the force must weaken the further out the moon might orbit. So, he did the maths and came up with the following;
The gravitational force between two bodies, like the earth and the moon, decreases by 1 over the square of the distance between them.
This is known as the inverse square law of gravity. He then did a very fundamental calculation;
1. He calculated how fast objects fall to earth.
2. He calculated how far the moon is away from the earth.
3. He then calculated how much the moon falls towards earth as it orbits.
The results of these calculations showed that the numbers matched perfectly if the force of gravity fell off as 1 over the square of distance.
Newton was the first person to link earth physics and space physics.
He then went further and declared that if the earth pulled him towards it and the earth pulled the moon towards it and the planets were pulled towards the sun, then everything must pull everything else.
The Famous Equation
Newton realised that the size of two masses being pulled together must affect the magnitude of the force. This led to his famous equation describing the gravitational force of attraction;
F = Gm1m2/r²
F is the force between masses m1 and m2, which are a distance r apart. G is a constant that has become known as Newton's Gravitational constant.
This equation doesn't just link earth physics and space physics, it explained planetary motion with a single law.
The equation allowed predictions to be made, such as; when an eclipse would occur, or an alignment of the planets allowing his theory to be verified.
But what is Gravity?
None of this actually tells us what gravity is, but it does explain how it works.
On earth, a body with mass m1 is attracted to the centre of the earth by a force F=m1g.
And on the earth's surface, if m1 is your mass, standing on the earth and m2 is the total mass of the earth, then the two forces must be the same;
F = m1g = Gm1m2/r²
The force of gravity is always attractive between two bodies, no matter what they consist of.
Two bodies fall to earth with the same acceleration
If you release two weights, say 10Kg and 1,000Kg, from the same height above the earth at the same time, they reach the earth at exactly the same moment.
If you divide m1 into both sides of the equation above you get a single value for g, which is independent of m1, but does vary according to height above the earth 'r' and its mass m2.
g = Gm2/r²
This only works as long as air resistance has no effect on either object.
Interesting Fact
Astronauts from Apollo 15, on the moon, demonstrated to the TV cameras that a feather fell at the same rate as a heavy geological hammer under the influence of the moon's gravity in the absence of any atmosphere.
(You can find a YouTube video showing this demonstration by typing 'Apollo 15 hammer and feather' into Google).
A Fundamental Universal Force
The earth's gravitational field attracts your collection of atoms to the earth in the same way that the sun attracts a rocky planet like earth, or a gas planet like Jupiter. Gravity is involved in keeping galaxy clusters bound together, as well as the individual stars inside a galaxy.
The effects of this fundamental universal force are fully understood, but what actually causes gravity is a mystery.
Gravity is one of the four fundamental forces in nature.
What are the Other Fundamental Forces?
The other three fundamental forces are; strong nuclear, electromagnetic and weak nuclear. These forces can be explained by quantum theory as interactions between fields, involving force mediators.
The strong nuclear force is mediated by the gluon, the photon mediates the electromagnetic force and the W and Z bosons mediate the weak nuclear force.
At present, there is no quantum theory of gravity.
When this eventually manifests itself, then gravity will be explained by a field interaction involving a theoretical mediator, which already has a name: the graviton.
One day, quantum theory might give us a better understanding of what gravity really is.
The Two Flaws in Newtonian Gravity
Newton developed the mathematics (calculus) independently, that allowed him to analyse how gravity worked. At the same time, Gottfried Wilhelm Leibniz (1646 to 1716) also developed similar mathematics.
The calculus we use to day is that of Leibniz, but that does not undermine the astonishing achievements of Newton.
However, there were two flaws in Newton's theory of gravitational attraction.
1. The propagation of Changes in Gravity
Suppose the sun suddenly moved its position by a vast amount, the effect on the planets would have to be immediate according to the maths and Newton knew this couldn't be true. Put more scientifically;
A change in gravity would have to be propagated through space instantaneously.
Newton knew that this was wrong and felt very uncomfortable about it.
2. Mercury - The Rogue Planet
The second flaw wasn't discovered until much later and involved the prediction of the orbit of Mercury.
Using Newton's law of gravitational attraction, the orbits of all the planets were modelled with great accuracy.
Newtonian gravitation failed to predict with total accuracy - planet Mercury's orbit.
All the outer planets cause gravitational perturbations (small differences) to an inner planet's complete orbit around the sun and Newtonian gravity had this under control, except for Mercury.
Planets orbit in an ellipse and the two ends of the ellipse correspond to when the planet is closest to the sun and when it's furthermost from the sun.
Using Newton's gravitational theory, the exact point when mercury arrived at the closest point to the sun, known as the perihelion, could not be predicted precisely as it could for all the other planets.
Each year, the time when Mercury's perihelion occurred changed by a small amount.
This effect hinted at something much deeper than classical physics could explain.
One Final Question
If gravity pulls everything together, why doesn’t everything collapse instantly?
That's where the other three fundamental forces of nature come into their own.
(If you want to understand the workings of the three fundamental quantum interactions of nature, Part 1 of A Journey into Modern Physics is a good starting point).
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Coming soon - Part 2: What is Gravity?
In part 2, we go forward two hundred years to the beginning of the twentieth century when Albert Einstein (1879 to 1955) formulated his theories of relativity and banished both the flaws in Newtonian physics by taking a new approach that led to an alternative theory of gravity.
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