What is Gravity?
Introduction: Gravity is one of the four fundamental forces of nature, and it plays a crucial role in shaping the universe. Ever wonder why objects are not falling away while the Earth rotates? Gravity holds some secret, and here we delve into its mystery.
What is Gravity?
Gravity is the force of attraction between two masses. It’s responsible for keeping planets in orbit around stars, causing objects to fall to the ground, and forming the structure of the universe.
Newton’s Law of Universal Gravitation
Isaac Newton formulated the law of universal gravitation in the 17th century, stating that every mass attracts every other mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres. Newton's Law of Universal Gravitation states that every mass attracts every other mass with a force that is directly proportional to the product of
There are a few reasons why this law might seem different in different situations:
1. The strength of the gravitational force is very weak: Compared to other forces like electromagnetism or the strong nuclear force, gravity is extremely weak. This means that it is often difficult to observe its effects on a small scale.
2. The gravitational force is always attractive: Unlike other forces that can be both attractive and repulsive, gravity is always attractive. This means that objects with mass will always be pulled towards each other.
3. The gravitational force is proportional to the product of the masses: This means that the gravitational force between two objects is not just dependent on the mass of one object, but also on the mass of the other object. This can make it seem like the gravitational force is not consistent, as it will vary depending on the combination of objects involved.
4. The gravitational force is inversely proportional to the square of the distance: This means that the gravitational force between two objects decreases rapidly as the distance between them increases. This can make it seem like the gravitational force is not as strong over long distances.
5. The gravitational force is affected by other factors: In some situations, the gravitational force between two objects can be affected by other factors, such as the presence of other objects or the curvature of spacetime. These factors can make the gravitational force seem different from what is predicted by Newton's law.
Einstein’s General Theory of Relativity
Albert Einstein revolutionized our understanding of gravity in the 20th century with his general theory of relativity. He proposed that gravity is not a force but the curvature of spacetime caused by mass. Massive objects like planets and stars bend the fabric of spacetime, and this curvature directs the motion of objects.
Einstein’s Field Equations
Visualization
Rubber Sheet Analogy: Imagine spacetime as a stretched rubber sheet. Placing a heavy ball in the centre causes the sheet to dip. Smaller balls placed on the sheet will roll towards the dip, not because of a force pulling them but because of the curved surface guiding their motion.
Orbits and Curves: This analogy helps explain planetary orbits. Planets orbit the sun because they are moving along the curves in spacetime created by the sun’s mass.If unaffected by the attraction of another planet, a planet's orbit is elliptical; some elliptical orbits are very nearly circles, while others are much elongated. Some bodies may follow parabolic or hyperbolic paths (open-ended curves).
Experimental Evidence
Gravitational Lensing: Light from distant stars bends around massive objects, such as galaxies, because of spacetime curvature, leading to observable effects like gravitational lensing.
Mercury’s Orbit: The precession of Mercury’s orbit around the Sun aligns with predictions made by general relativity, confirming the curvature of spacetime.
Gravitational Waves: Ripples in spacetime caused by accelerating masses, predicted by Einstein and confirmed by LIGO in 2015.
LIGO, or the Laser Interferometer Gravitational-Wave Observatory, is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and develop gravitational-wave observations as an astronomical tool.
Why It Matters
Gravity shapes the cosmos, influencing everything from galaxies' formation to light's behaviour. Understanding gravity is key to unlocking the mysteries of the universe and advancing our knowledge in astrophysics and cosmology.
Before Newton
The concept of gravity was explored by several Indian scholars long before Isaac Newton formulated his famous law of universal gravitation. One prominent figure is Bhaskaracharya (Bhaskara II), a 12th-century mathematician and astronomer from Karnataka. In his work Siddhanta Shiromani, he described the attractive power of the Earth, which draws objects toward itself. Ref to my previous blog post on Bhaskara II
Another notable figure is Brahmagupta, a 7th-century mathematician and astronomer from Rajasthan. He also discussed gravitational principles in his work, Brahmasphutasiddhanta. In his work, Brahmasphutasiddhanta (628 CE) approx 1,396 years ago, Brahmagupta described gravity as an attractive force. He used the Sanskrit term "gurutvākarṣaṇam" (गुरुत्वाकर्षणम्) to explain this concept. This was one of the earliest known descriptions of gravity as an attractive force, predating Newton's formulation by several centuries. Brahmagupta's contributions to mathematics and astronomy were significant, and his work had a lasting impact on both Indian and Islamic science.
What is Gravity?
Gravity is the force of attraction between two masses. It’s responsible for keeping planets in orbit around stars, causing objects to fall to the ground, and forming the structure of the universe.
*(Refer to a previous post on "Time" for related understanding. Future posts will delve deeper into time mechanics.)
Newton’s Law of Universal Gravitation
Isaac Newton formulated the law of universal gravitation in the 17th century, stating that every mass attracts every other mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres. Newton's Law of Universal Gravitation states that every mass attracts every other mass with a force that is directly proportional to the product of
There are a few reasons why this law might seem different in different situations:
1. The strength of the gravitational force is very weak: Compared to other forces like electromagnetism or the strong nuclear force, gravity is extremely weak. This means that it is often difficult to observe its effects on a small scale.
2. The gravitational force is always attractive: Unlike other forces that can be both attractive and repulsive, gravity is always attractive. This means that objects with mass will always be pulled towards each other.
3. The gravitational force is proportional to the product of the masses: This means that the gravitational force between two objects is not just dependent on the mass of one object, but also on the mass of the other object. This can make it seem like the gravitational force is not consistent, as it will vary depending on the combination of objects involved.
4. The gravitational force is inversely proportional to the square of the distance: This means that the gravitational force between two objects decreases rapidly as the distance between them increases. This can make it seem like the gravitational force is not as strong over long distances.
5. The gravitational force is affected by other factors: In some situations, the gravitational force between two objects can be affected by other factors, such as the presence of other objects or the curvature of spacetime. These factors can make the gravitational force seem different from what is predicted by Newton's law.
Einstein’s General Theory of Relativity
Albert Einstein revolutionized our understanding of gravity in the 20th century with his general theory of relativity. He proposed that gravity is not a force but the curvature of spacetime caused by mass. Massive objects like planets and stars bend the fabric of spacetime, and this curvature directs the motion of objects.
- Ricci Curvature Tensor (Rμν): Defines the difference in the volume of a geodesic ball in a Riemann manifold compared to a Euclidean manifold. It determines spacetime curvature and represents matter content in Einstein’s field equations.
- Riemann Manifold: Space defined by the Riemann tensor, enabling calculations of volume, arc length, gradient, etc., and determining surface curvature.
- Euclidean Space: A two or more-dimensional space represented by Cartesian coordinates in a set of real numbers.
- Scalar Curvature (R): Represents the small volume of a geodesic object affecting the Riemann manifolds, used in the Einstein-Hilbert action.
- Metric Tensor (gμν): Defines the distance between objects in geometry and characterizes geometrical properties of space in Riemann manifolds.
- Stress-Energy Tensor (Tμν): Describes the flux of energy density and momentum in spacetime, acting as the source of gravitation in general relativity.
- Universal Gravitational Constant (G), Speed of Light (c), Cosmological Constant (Λ): Constants used in field equations to balance spacetime contraction and determine expansion or contraction possibilities.
Einstein’s Field Equations
- Field Equations: Solutions for the metric tensor determining object movements and spacetime curvature. Non-linear and complex to solve without assumptions.
- Experimental Verification: Perihelion of Mercury, bending of light during solar eclipse, and the application of linearization assuming weak gravitational fields. Which means gravity can bend light.
Visualization
Rubber Sheet Analogy: Imagine spacetime as a stretched rubber sheet. Placing a heavy ball in the centre causes the sheet to dip. Smaller balls placed on the sheet will roll towards the dip, not because of a force pulling them but because of the curved surface guiding their motion.
Orbits and Curves
Orbits and Curves: This analogy helps explain planetary orbits. Planets orbit the sun because they are moving along the curves in spacetime created by the sun’s mass.
Experimental Evidence
Gravitational Lensing
Gravitational Lensing: Light from distant stars bends around massive objects, such as galaxies, because of spacetime curvature, leading to observable effects like gravitational lensing.
Mercury’s Orbit: The precession of Mercury’s orbit around the Sun aligns with predictions made by general relativity, confirming the curvature of spacetime.
Implications and Phenomena
Orbital Mechanics: Gravity governs the motion of planets, moons, and satellites.
Black Holes: These are regions of spacetime where gravity is so strong that nothing, not even light, can escape.
Orbital Mechanics: Gravity governs the motion of planets, moons, and satellites.
Black Holes: These are regions of spacetime where gravity is so strong that nothing, not even light, can escape.
Gravitational Waves
Gravitational Waves: Ripples in spacetime caused by accelerating masses, predicted by Einstein and confirmed by LIGO in 2015.
What is LIGO?
Why It Matters
Gravity shapes the cosmos, influencing everything from galaxies' formation to light's behaviour. Understanding gravity is key to unlocking the mysteries of the universe and advancing our knowledge in astrophysics and cosmology.
The concept of gravity was explored by several Indian scholars long before Isaac Newton formulated his famous law of universal gravitation. One prominent figure is Bhaskaracharya (Bhaskara II), a 12th-century mathematician and astronomer from Karnataka. In his work Siddhanta Shiromani, he described the attractive power of the Earth, which draws objects toward itself. Ref to my previous blog post on Bhaskara II
Another notable figure is Brahmagupta, a 7th-century mathematician and astronomer from Rajasthan. He also discussed gravitational principles in his work, Brahmasphutasiddhanta. In his work, Brahmasphutasiddhanta (628 CE) approx 1,396 years ago, Brahmagupta described gravity as an attractive force. He used the Sanskrit term "gurutvākarṣaṇam" (गुरुत्वाकर्षणम्) to explain this concept. This was one of the earliest known descriptions of gravity as an attractive force, predating Newton's formulation by several centuries. Brahmagupta's contributions to mathematics and astronomy were significant, and his work had a lasting impact on both Indian and Islamic science.
Islamic science
Islamic scholars made significant contributions to the understanding of gravity and related concepts long before Newton's formalization of gravitational theory.
Al-Farabi (Alpharabius)
Al-Farabi (Alpharabius)
- Concept of Attraction: Al-Farabi discussed the idea of natural attraction between objects, which can be seen as an early understanding of gravitational principles.
Ibn Sina (Avicenna)
- Force and Inclination: Ibn Sina made distinctions between force and inclination (may), arguing that an object gains may when it opposes its natural motion. This concept is somewhat analogous to the idea of gravitational force.
Al-Biruni
- Earth's Radius: Al-Biruni was the first to obtain a simple formula for measuring the Earth's radius, which is crucial for understanding gravitational effects on a planetary scale.
Al-Khazini
- Balance of Wisdom: Al-Khazini wrote about the balance of wisdom and the forces acting on objects, contributing to the understanding of how gravity works.
Islamic Astronomy
- Celestial Mechanics: Islamic astronomers made detailed observations and calculations of celestial bodies, which indirectly contributed to the understanding of gravitational forces. These scholars laid the groundwork for later developments in physics and astronomy, showing a deep understanding of natural phenomena and the forces that govern them. Their work was part of a broader tradition of scientific inquiry in the Islamic Golden Age (Approx 1,000 to 1,200 years ago from today), which had a lasting impact on the development of science.
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