# What Goes Up Must Come Down: Fundamentals of Gravity

During our childhood, we used to throw a ball into the air, and after some time we tried to catch it as it fell down. We lift up the carry bags from the grocery stores and realize that it is heavy. All these and many other daily tasks we do involve the role of force of Earth or “Gravity”. Gravity is attraction of one mass containing body to another mass containing body or vice versa. It is the reason that we don’t float away into space when we jump, the moon stays in orbit of the earth and many other physical phenomena.  As simple as it may sound, what we perceive it to be is substantially lower than what it actually is. As strange as it is there are several theories, each having their own significance,  which try to explain what gravity is. We all know what gravity is, the trouble arises when we are asked “why does gravity exist?” or “what causes gravity?” Let us look into the main theories of gravity and analyse them. We’ll start from the beginning at Newton’s theory and then go onto a more complicated theory by Albert Einstein and then have a glance at what modern scientists have to say about gravity through the field of quantum physics. Base Jumping: An adventure sport utilizing gravity (Image source: Wikipedia)

##### Newton’s Theory of Gravity

Sir Isaac Newton was a British mathematician and physicist belonging to the 17th century. The story of how Newton discovered gravity is quite popular. However let’s take a recap at how it happened. In the early 1600s, Newton was sitting under an apple tree, coincidentally; an apple fell on his head. After this event had occurred, he wondered why the apple was attracted towards the ground. Later in the 1680s, he published his theory of “Universal Gravitation”.

In this theory he explains gravity as a force of attraction between any two bodies that have mass. This force depends on the amount of mass of the two bodies and the distance between their centres of gravity. If ‘m1 and ‘m2 are the masses of these bodies and ‘r’ is the distance between their centres of gravity, then Newton’s Universal law gravitation formulates as:

F = (G*m1*m2)/r2

Here G is a proportionality constant known as the universal gravitational constant. Since the mass of the earth is extremely large when compared to ourselves (6,000,000,000,000,000,000,000,000 kilograms! – if you are feeling lazy to count, there are 24 zeros in that figure) the earth exerts a large gravitational force on us and therefore allows us stay on the ground, rather than float in space. The weight of the moon being comparatively closer to the mass of the earth also feels a portion the earth’s gravitational pull but at a lower magnitude and so it remains orbiting around the earth.

Whenever we throw anything into the air, it reaches a certain height, stops, and falls back with acceleration. This acceleration is constant in a particular gravitational field. In the earth’s gravitational field this acceleration with which a body is attracted towards it is equal to 9.8 metres per second squared (represented by ‘g’, not to be confused with universal gravitational constant ‘G’). Therefore the force, with which a body is attracted to the earth, is known as its weight and is equal to the product of the body’s mass and the acceleration due to gravity (F = m*g). When we stand on a weighing scale, the reading actually shows the weight but not the mass. Since the value of ‘g’ varies on every planet, we would weigh different on every planet.

Example: A person who weighs 60kg on earth would weigh 10kg on the moon (gmoon = 1.62 m/s2).

Even you go up with certain acceleration against gravity; you tend to feel heavier. This is called the G force. Fighter pilots and astronauts are trained to withstand the G force when they are accelerated at twice and thrice the supersonic speed (speed of sound). The opposite case is the case of weightlessness. When the effective acceleration is significantly lower than the value of ‘g’, the person will experience weightlessness. Astronauts, who have to work for long periods in space stations, are trained to work in weightless environments in swimming pools. Weightlessness environments created by special zero gravity planes (Image source: NASA)

##### Albert Einstein’s Approach to Gravitation

After Newton, no one touched the topic of gravity for two centuries, after which in 1905, Einstein submitted his “Theory of Relativity”. In this he mentioned space and time to be related and referred it to as the space-time, this is referred to as the fourth dimension. This space-time is like a piece of fabric. If a body having mass is placed on this space-time fabric, the space time fabric bends and attracts the object which is placed on the fabric. To avoid confusion let us understand this concept through a practical approach. Take a piece of cloth and keep it stretched, and place a football in the centre. The mass of the football creates an impression on the fabric by stretching it. Now place a marble anywhere on the cloth. The marble rolls towards the football, in this context the football can be compared to the sun and the marble to the earth. To avoid colliding with the sun the earth revolves around the sun. The centrifugal force caused by the revolution will cancel the gravitational force and so the earth remains in orbit with the sun. The orbit is elliptical in shape.

One important feature of Einstein’s Theory of relativity is that the never treated gravity as a “Force” like Newton, but merely a curve in space time caused due to the mass of the body. This is the main difference between Newton’s gravity and Einstein’s gravity. (Also read Is Time Travel Possible?)

##### Modern Gravitational Theories

Newton and Einstein studied gravity at a more large scale level, but recently as developments have been made nuclear research and quantum physics, gravity is being studied at the sub atomic level. The “Atom” is made up sub atomic particles like neutrons, protons and electrons. There are many more sub atomic particles such as bosons, positrons, neutrinos etc. The particles of concern here are “Gravitons” give matter the property of mass. These particles have a property of attracting each other by having a gravitational field around them. Another theory is the emission of gravitational waves, more the mass of the matter, further the gravitational field will flow. Both of these theories have not been proved yet, but can explain why gravity exists or from where it originates.

##### Dark Energy

When the universe was created, it started spreading out. Since gravity exists between particles the spreading is expected to slow down after some time. But recently it was observed that the spreading out is actually speeding up. To understand this practically, consider a car moving in a certain speed, the driver releases the gas pedal, the car will be expected to slow down because of friction between the road and the tyres (here gravity is analogous to friction), but it is observed that what is happening in the universe is similar to the speed of the car is increasing rather than decreasing when the driver removes his foot from the pedal. They named this as “dark energy”. So, scientists are predicting that the galaxies will spread out so far that the other galaxies will be practically millions of light years away from ours. As we can see, there is much more to the humble “gravity” than we expect. Gravitational energy is one the main forms of energy and used as a mechanism in some machines as well. Gravity is as destructive as it is helpful. Black Holes are formed when stars die; they are known to have practically infinite gravity. The gravity is so strong that even light cannot escape it. Gravity is therefore simple property which can have vast applications.

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