Understanding Gravity : From Antiquity to Edge of Modern World

Let us dive into the world of one of the fascinating concepts of physics : Gravity

Once a child asked me , “ If the earth is round, then why don’t the people who live on the bottom fall ?” I also used to have curiosity like that when I was young and I assume that many of you have wondered about the same question. So, did you find your answer ? It is gravity. 

Figure: Gravitational force of Earth 

Everything we throw up, falls down. We are attached to earth. The moon revolves the earth and the earth revolves the sun. All of this is due to gravity. So, what is this gravity and why does this work? If you are wondering about it then let’s go on a journey where we will understand what gravity really is.

1. Part one : Introduction

1.1 Definition

Gravity is a force of attraction that pulls objects with mass towards each other. It is the force by which earth or other heavenly bodies attract anything towards their center.

 Figure : Astronomer on the space ( out of Earth’s gravitational field)

1.2 Why does it matter ?

Gravity is a force that shapes our universe and affects every object whether it is big or small. So, gravity matters and it is very important for the existence of the universe. Some of the application of gravity on our daily life are: 

• Gravity is the force that keeps us anchored to the Earth’s surface and allows us to perform daily activities like moving, walking and running.
• Gravity holds the Earth’s atmosphere which is crucial for sustaining life.
• Even our bodily functions like blood circulation, muscle function, bone density, etc are affected by gravity.
• Gravity allows us to measure weight.
• Gravity keeps satellites in orbit around the earth, which is essential for GPS, telecommunication, weather forecasting and other scientific observations.
• Gravity stabilizes Earth’s orbit and tilt.

2. Part Two: The Evolution of Theory of Gravity

The theory of gravity did not develop because of one person only. There were many scientists and physicists who contributed to the field of physics and the theory of gravity was formed.  

Here, we will discuss the impact of major physicists due to which we now know what gravity is:

2.1 Ancient ideas : Aristotle’s Theory of Gravity ( The Beginning )

Aristotle (384-322 BCE) was an ancient Greek philosopher and scientist, whose work laid the foundational principle in the field like logic, biology and ethics.

Some concept and terminologies :

1. The Realm

According to Aristotle, the universe is divided into two parts: the Realm of Earth and the Celestial realm (the realm of the Sun, the Stars and other heavenly bodies); where the Earth is the center of the universe. The Moon is the border between the Realm of Earth and the Celestial realm.

2. The Elements 

In Aristotle’s view, in the Realm of Earth all the bodies are made up of four basic elements- earth, fire, air and water. Whereas the Celestial realm is made up of a fifth element known as quintessence. All the things in the universe were made up of the elements and their combination. 

For instance, rocks are made up of element earth, and the air we breathe is made up of the elements-fire and air.

Figure : Aristotle’s elements

3. Natural Place

In Aristotle’s concept, the Earth is the center of the universe and at the center of earth is the element earth, then above is the concentric shell of the elements water, air and fire respectively. Beyond that is the Moon and the Celestial realm.

So to understand simply, the above mentioned order is the ideal position for the elements and the objects made up of them, which is called the natural place.

Figure : Aristotle concept of Earth

Theory of Gravity :

Aristotle’s theory of Gravity is that : Every object in the universe has its own natural place and they naturally move towards their respective natural place seeking rest unless stopped by an impenetrable surface like the ground.

Theory of motion :

Aristotle stated that : The speed at which two identically shaped objects sink or fall is directly proportional to their weights and inversely proportional to the density of the medium through which they move.

These theories explained the falling of stones, circulation of air and rising of flames and gave a proper logic to the motion of objects in the earth. These ideas of gravity and motion were widely accepted for nearly 2,000 years from around the 4th century BCE until the 16th century.

Criticism: What is wrong with the theory?

• Aristotle theories were based on philosophy rather than mathematical calculations and proofs.
• His theory of motion can not explain the motion of projectiles. 
• There was not any strong evidence of his claims.
• The basics of his theory was wrong.

Contribution:

Although we now know that Aristotle theories were wrong, he was the first to talk about motion and explain the ways of nature. Due to this, many people later studied about motion and gravity and the theory of gravity was devised.

2.2 Galileo’s observation :  The famous experiment

Galileo Galilei (1564 AD-1642 AD)also known as the father of modern science, was a great scientist born in Pisa, Italy. He contributed to the field of astronomy, physics, mathematics, cosmology and philosophy. 

Galileo experiment : 

Between 1589 and 1592, the Italian scientist Galileo Galilei is said to have dropped “unequal weights of the same material” from the Leaning tower of Pisa to demonstrate that their time of descent was independent of their mass, according to a biography by Galileo’s pupil Vincenzo Viviani, composed in 1654 and published in 1717.

Figure : Galileo Experiment

According to the story, Galileo discovered through this experiment that the objects fell with the same acceleration, proving his prediction true, while at the same time disproving Aristotle’s theory of gravity (which states that objects fall at speed proportional to their mass). Though Viviani wrote that Galileo conducted “repeated experiments made from the height of the Leaning Tower of Pisa in the presence of other professors and all the students,” most historians consider it to have been a thought experiment rather than a physical test.

Whether the experiment was real or a thought experiment, the theory of Galileo was correct and he also created a mathematical formula for the falling object.

Theory :

Galileo stated that :  All the objects fall at the same rate in the absence of air resistance, regardless of their masses.

Mathematical formulation :

Galileo deduced the equation s=1/2 gt2 in his work geometrically, using the Merton rule, now known as a special case of one of the equations of kinematics.

Contribution :

Galileo’s theories were based on mathematical solutions and he was the first to state that the laws of nature are mathematical. Galileo was the first to show that the path of a projectile is a parabola. Galileo had an understanding of centrifugal force and gave a correct definition of momentum. This emphasis of momentum as a fundamental quantity in dynamics is of prime importance. He measured momentum by the product of velocity and weight; mass is a later concept, developed by Huygens and Newton. His analysis on projectiles indicates that Galileo had grasped the first law and the second law of motion. He did not generalize and make them applicable to bodies not subject to the earth’s gravitation. That step was Newton’s contribution.

2.3 Newtonian Gravity : The universal law of gravity

Isaac Newton (1643 AD – 1727 AD ) was an English polymath active as a mathematician, physicist, astronomer, alchemist, theologian and author. He is very famous and still known for his contribution to the field of gravity and motion.

Newton’s inspiration :

You may have heard about the famous apple anecdote. What happened was, when Newton was sitting below an apple tree, an apple fell on his head and got curious about the fall of apple and people believe that due to this he created the theory of gravity. However, according to experts it was not the apple but the question Newton asked after the incident , looking at the moon, “ Will the moon fall?” This inspired him to create his famous laws and formulas. 

Theory of Gravity:

Newton stated that : Every object in the universe attracts each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of distance between them.

So, Newton defined gravity as an attractive force between two bodies.

Derivation:

      Fig Gravitational force between two bodies

If F be the magnitude of the force between two bodies of mass m₁ and m₂ separated by a distance r then, according to above statements 

F ∝ m₁ . m₂  …………….(i)

F ∝ 1/r² …………………(ii)

Combining equation (i) and (ii)

F= G m1.m2 / r2        

Where G is a proportionality constant called universal gravitational constant.

Impact:

• Newton’s law of gravitation holds true for every planetary motions, motions of parabola and almost everything. 
• Today also, the rockets we build are based on the law of gravitation by Newton. This law explained every motion and effects of gravity. 
• From the motion of falling objects on Earth to all the orbits of planets in the solar system, everything can be explained by Newton’s law.
• It helped to discover planets like Neptune of the solar system.

However, there was a flaw in Newton’s law and it could not explain the orbit of mercury, which was later explained by Einstein.

2.4 Einstein’s General theory of Relativity: A new view of Gravity

Albert Einstein (14 March 1879 – 18 April 1955) was a German-born theoretical physicist who is widely held as one of the most influential scientists. Best known for developing the theory of relativity, Einstein also made important contributions to quantum mechanics.

The shift in perspective: 

Newton had defined gravity as a force, however, Einstein’s perspective was different. According to Einstein gravity is the curvature of space and time fabric. 

Spacetime : Normally, in geometry and coordinates, in our normal life, we use three dimensions ( representing forward – backward, left – right and up – down) known as dimensions of space. So, in physics spacetime is a concept where an additional dimension (time) is added in three space dimensions. 

Theory: 

According to Einstein : Gravity is a curvature created in spacetime fabric, caused by any mass or energy. 

Einstein Field Equations :

Einstein’s Field Equations describe how mass and energy warp spacetime, creating what we experience as gravity. These equations are often written as:

Let’s break down what each part of this equation means.

•Gμν : Known as the Einstein tensor, this term represents the curvature of spacetime caused by gravity. It describes how spacetime bends in response to the presence of mass and energy.

• gμνΛ: This term includes the cosmological constant (Λ), which Einstein originally introduced to allow for a static universe. Today, it represents dark energy, an unknown force that’s causing the universe to expand at an accelerating rate. The metric tensor g mu*nu defines the structure of spacetime itself.

• Tμν: This is the stress-energy tensor. It represents the density and flow of energy and momentum in spacetime, including all matter, radiation, and energy.

• G: Newton’s gravitational constant, which defines the strength of gravitational interaction.

• c: The speed of light in a vacuum. Since space and time are interwoven in relativity, c acts as a conversion factor between mass-energy and spacetime curvature.

Analogy: 

Imagine spacetime as a flexible fabric, like a trampoline. If we place a heavy object (like a bowling ball) in the middle, it creates a “dent” in the fabric. Smaller objects placed on the fabric (like marbles) will roll toward the bowling ball, not because of a direct pull but because of the curved surface. This analogy helps illustrate how spacetime curvature creates the effect of gravity.

Figure : Einstein’s spacetime fabric 

Impact 

1.  Solution of Mercury Orbit:

Mercury’s orbit around the Sun is elliptical, but over time, the closest point of Mercury’s orbit to the Sun shift due to precession. Newton’s laws of gravity predicted a certain amount of precession, but there was an unexplained extra precession of about 43 arcseconds per century. This discrepancy had puzzled astronomers for years.

Einstein’s General Relativity explained this extra precession by showing that spacetime is curved around the Sun. This curvature affects the movement of Mercury in a way that Newton’s laws, which treat gravity as a force acting at a distance, couldn’t account for.

( precession: the slow movement of the axis of a spinning body around another axis due to a torque – such as gravitational influence – acting to change the direction of the first axis. It is seen in the circle slowly traced out by the pole of a spinning gyroscope.)

2.  Revolutionized Cosmology: 

General Relativity transformed how we understand the universe’s structure and evolution, leading to the discovery of expanding space and concepts like black holes and the Big Bang.

3.  Prediction of Black Holes:

 Einstein’s equations showed that extremely dense objects could create such strong spacetime curvature that not even light could escape. This led to the concept of black holes, which were later observed and confirmed.

4.  Gravitational Waves: 

General Relativity predicted ripples in spacetime caused by massive accelerating objects, like colliding black holes. These gravitational waves were detected for the first time in 2015, opening a new way to observe cosmic events.

5.  Time Dilation and GPS Technology:

Einstein’s theory showed that time moves slower in stronger gravitational fields (like near Earth) compared to weaker fields (like satellites in orbit). This time dilation effect is critical to GPS technology, which corrects for these differences to provide accurate positioning.

6.  Redefining Gravity:

Einstein’s theory replaced Newton’s view of gravity as a “force” with the idea of spacetime curvature, changing how we think about motion, energy, and mass at cosmic scales.

7.  Inspirations in Science and Philosophy:

General Relativity sparked new questions in physics, inspiring research into quantum gravity and unification theories. It also influenced philosophical ideas about the nature of space, time, and reality itself.

3. Part Three : Quantum Gravity and Present condition of theory of gravity

Modern quantum physics describes the behavior of matter and energy on the smallest scales—such as atoms, subatomic particles, and quantum fields. It operates under principles that are radically different from classical physics, like wave-particle duality, uncertainty principles, and superposition. Quantum mechanics has been incredibly successful in explaining phenomena at microscopic scales, yet it faces challenges when trying to explain how gravity works at these scales.

Quantum gravity is an area of theoretical physics that attempts to reconcile quantum mechanics (which governs the microscopic world) with general relativity (which governs the macroscopic world, including gravity). Since quantum mechanics and general relativity are based on different mathematical frameworks, integrating them into a single, unified theory has been one of the biggest challenges in physics.

3.1 Current Theories Related to Quantum Gravity

Einstein Theory, perfectly explained all the classical concepts related to motions and gravity. After that scientists feel the need to unite quantum physics and general Relativity creating a unification theory ( theory of everything ) which would explain everything from quantum to large galaxies. However , the rules of general Relativity do not work in the quantum world and thus the search for the theory of quantum gravity begins. Several scientists created various theories, however , they do not have perfect mathematical proof and evidence. So, the search for quantum gravity is one of the major fields where physicists are working. Some of the theories for quantum physics and quantum gravity are :

1.  Quantum Field Theory (QFT):

In quantum field theory, forces like electromagnetism are mediated by particles (e.g., photons for electromagnetic force). But gravity is not yet part of the quantum field framework.

QFT works extremely well for describing three fundamental forces: electromagnetism, weak nuclear force, and strong nuclear force. However, it struggles with gravity due to its infinite nature at quantum scales (leading to infinities in calculations).

2.  String Theory:

String theory proposes that instead of point particles, the fundamental building blocks of nature are one-dimensional “strings” that vibrate in different modes. These strings can explain the fundamental forces and particles, including gravity.

One of the important aspects of string theory is that it naturally incorporates gravity into a quantum framework. String theory also suggests the existence of extra dimensions beyond the familiar three dimensions of space and one of time, though these extra dimensions are not yet observable.

3.  Loop Quantum Gravity (LQG):

Loop Quantum Gravity is a theory that attempts to quantize spacetime itself, breaking it down into discrete “chunks” or “loops” instead of treating spacetime as a continuous fabric, as in General Relativity.

LQG predicts that at the smallest scales, space is granular, not smooth. This theory is still being developed and is not yet as widely accepted as string theory but offers a promising approach to combining quantum mechanics with gravity.

4.  Causal Dynamical Triangulation (CDT):

Causal Dynamical Triangulation is a relatively new approach that tries to model the quantum nature of spacetime itself. Instead of using traditional smooth geometry, it breaks spacetime into small, discrete units and uses computer simulations to study the quantum behavior of gravity.

CDT provides insights into how spacetime might behave on the smallest scales, offering another avenue for understanding quantum gravity.

5.  The Holographic Principle:

The holographic principle suggests that the information contained within a volume of space can be represented on the boundary of that space, much like a hologram. It implies that our 3-dimensional universe might be a projection from 2-dimensional information encoded on a distant surface.

This concept, related to black hole thermodynamics and string theory, offers a new way of thinking about gravity and quantum mechanics.

3.2 Challenges and Open Questions

• Black Hole Singularities: Inside a black hole’s core, both quantum mechanics and general relativity give extreme results, leading to infinite densities and curvature—a situation that both theories fail to explain coherently.
• The Nature of Space and Time: The biggest challenge in quantum gravity is that the concept of spacetime itself may not be fundamental at the smallest scales. Quantum gravity may reveal that space and time themselves are emergent phenomena rather than basic building blocks.
• Quantum Fluctuations of Spacetime: At very high energies (near the Planck scale), spacetime may be subject to quantum fluctuations, causing the smooth fabric of spacetime to break down. This would have profound implications for our understanding of space, time, and gravity.

3.3 Future Perspectives in Gravity and Quantum Physics

1.  Experimental Tests: One of the major challenges in quantum gravity is the lack of experimental data. The effects of quantum gravity are thought to only be noticeable at extremely small scales (the Planck scale, about  meters) or extremely high energies. However, future technologies, such as more advanced particle colliders or gravitational wave observatories, might provide indirect evidence of quantum gravitational effects.

2.  Gravitational Waves: The detection of gravitational waves in 2015 provided a new way to study the universe. The study of gravitational waves from black hole mergers and other cosmic events might reveal insights into quantum gravity, especially in extreme conditions where quantum effects are most prominent.

3.  Quantum Gravity in Black Holes: Understanding gravity at the heart of black holes (especially the information paradox) is one of the most pressing questions. The idea that information that falls into a black hole may be destroyed contradicts quantum theory, which states that information is never lost. Solving this paradox could be a major breakthrough in understanding quantum gravity.

4.  String Theory and the Multiverse: String theory suggests that there may be many possible universes with different physical laws, forming a multiverse. Understanding gravity in such a context would dramatically reshape our understanding of physics, potentially leading to deeper insights into the nature of reality.

5.  New Insights into Space-Time: A future theory of quantum gravity might suggest that space-time itself is not smooth or continuous but rather made of discrete “atoms” or “quanta” at the Planck scale. This could provide new insights into the origin and fate of the universe, as well as its most mysterious phenomena, such as dark matter and dark energy.

4. Part Four: Conclusion and Comment

4.1 Conclusion:

So, the evolution of gravity was quite amazing. From Aristotle to Einstein, the meaning and cause of gravity have been changed and viewed with different  perspectives. Finally we have a theory of gravity, which perfectly explains everything about us and the universe. However, concepts like black holes and quantum gravity  are still to be discovered. 

Gravity is not just a fascinating concept of physics and it is not just a fantasy in someone’s mind. It is a reality and it has a huge impact in our daily life because gravity shapes our everyday life, from keeping us grounded on Earth to enabling space exploration, GPS navigation, and even the flow of liquids. It is essential for everything from the orbits of satellites to the functioning of our planet’s ecosystems.

4.2 My Opinion on gravity 

I find gravity , one of the most fascinating concepts in Physics. It seems impossible , but it is true. The far space that we had never seen can be explained by gravity. Einstein theory predicted black holes a century ago, and now the James Webb Space Telescope has captured a picture of black hole proving it is true. I think this is the beauty of physics and understanding gravity is both amazing and important. And , in future, when topics like quantum gravity get explored then the applications of gravity can be far greater. I myself would love to explore the concept of quantum gravity.

4.3 My comment:

So, whether you just enjoy reading physics or you want to be a physicist just like me, I think you should definitely dive into the world of physics and explore it with your own perspective. Like gravity, there are other topics such as time , quantum world , dimensions , in physics and I love to study those and share my knowledge. And I want all of you to explore amazing concepts of physics, and even if you do not want to be a physicist , just read the concepts. I would try to make them fun, easy and at the same time professional too.