Gravity: From Newton to Quantum Gravity

 Gravity is one of the fundamental forces in the universe, governing the motion of planets, stars, and galaxies. For centuries, scientists have sought to understand its nature and describe it with mathematical models. In this article, we will explore the evolution of our understanding of gravity, starting with Newton's classical theory, moving through Einstein's theories of relativity, and arriving at the search for a quantum theory of gravity.

Newton's Theory of Gravitation

In 1687, Isaac Newton published his law of universal gravitation, which states that every particle in the universe attracts every other particle with a force proportional to the product of their masses and inversely proportional to the square of the distance between them . This law is extremely successful in describing the motion of planets in the solar system and forms the basis of classical mechanics.   

Newton described gravity as a force acting at a distance, without explaining how this force is transmitted . He knew that the force that causes an apple to fall depends on the mass of the apple and the mass of the Earth . According to him, gravity is a universal property of matter, and all objects with mass attract each other . Interestingly, on the surface of the Earth, the gravitational force of the Moon is about 2.2 times greater than that of the Sun . The tidal actions of the Moon are due to variations in its gravitational field over the Earth's surface compared to its strength at the Earth's center.   

Special Relativity

At the beginning of the 20th century, Albert Einstein revolutionized physics with his special theory of relativity . It is based on two postulates:   

1. The laws of physics are the same for all observers in inertial frames of reference.
2. The speed of light in a vacuum is the same for all observers, regardless of their relative motion or the motion of the light source .   

Special relativity shows that space and time are not absolute but relative to the observer . It introduces concepts such as:   

• Four-dimensional spacetime, unifying space and time.
• Relativity of simultaneity.
• Kinematic and gravitational time dilation.
• Length contraction .   

For example, if one person moves at high speed relative to another, time for the moving person will flow more slowly than time for the stationary observer . Also, the moving object will appear shorter in the direction of its motion.   

Special relativity is not limited to objects moving uniformly relative to each other. It also considers how acceleration affects measurements of time, space, and mass . It is important to note that special relativity is the basis of the famous equation E=mc², which expresses the fact that mass and energy are the same physical phenomenon and can be converted into each other . This equation has enormous significance for nuclear energy and the creation of nuclear weapons.   

General Relativity

In 1915, Einstein published his general theory of relativity, which describes gravity as a curvature of spacetime caused by mass and energy . According to this theory, objects are not attracted directly but move along geodesic lines in curved spacetime. One of the main principles of general relativity is the equivalence principle, which states that at the local level, it is impossible to distinguish between physical effects due to gravity and those due to acceleration . This principle connects the experiences of weightless observers with those in inertial frames.   

General relativity predicts several phenomena not explained by Newton's theory, such as:

• Gravitational time dilation: Clocks run slower in stronger gravitational fields.
• Precession of orbits: The orbits of planets precess in a way not predicted by Newton's theory.
• Deflection of light: Light rays are deflected in the presence of a gravitational field.
• Frame-dragging: Rotating masses "drag" spacetime around them.
• Expansion of the universe: The universe is expanding, and some components within it can accelerate the expansion .   

General relativity is very successful in describing gravity on a macroscopic scale, but it is not compatible with quantum mechanics .   

Differences Between the Theories

Theory	Description of Gravity	Space and Time
Newton	Force acting at a distance	Absolute
Special Relativity	Does not account for gravity	Relative, but not affected by gravity
General Relativity	Curvature of spacetime	Relative and affected by gravity

   

Key difference: While Newton describes gravity as a force acting at a distance, Einstein presents it as a geometric property of spacetime itself .   

Quantum Gravity

Both special and general relativity have their limitations. Special relativity does not account for gravity, and general relativity is not compatible with quantum mechanics . This necessitates the search for a new, more general theory that unifies these two fundamental areas of physics.   

Quantum gravity is a hypothetical theory that attempts to unify general relativity with quantum mechanics . It is needed to describe phenomena where both gravitational and quantum effects are significant, such as:   

• The behavior of black holes.
• The origin of the universe .   
• The interaction of gravity with a superposition of different mass distributions .   

Understanding quantum gravity would help us unravel some of the greatest mysteries of the universe, such as how it began and what happens inside black holes .   

One of the main challenges facing quantum gravity is that it requires the quantization of spacetime itself . This is difficult because quantum theory is usually formulated against the background of classical spacetime. Quantum gravity could change our notions of space and time, making them dynamic and interconnected with matter .   

Within quantum cosmology, which is the application of quantum gravity to the early universe, the concept of the "Big Bounce" emerges . According to this concept, the Big Bang is not the beginning of the universe but rather a transition from a phase of contraction to a phase of expansion.   

Approaches to Quantum Gravity

There are different approaches to quantum gravity, such as:

• String theory: According to this theory, all particles are made up of tiny, vibrating strings . One of the vibrational states of the strings corresponds to the graviton, a hypothetical particle that carries the gravitational interaction .   
• Loop quantum gravity: This approach attempts to quantize the gravitational field, similar to other fundamental fields .   

Challenges Facing Quantum Gravity

The creation of a complete theory of quantum gravity faces several challenges:

• Lack of experimental data: Quantum gravitational effects manifest at very high energies, which are inaccessible to modern experiments . To test theories of quantum gravity, a particle accelerator larger than our galaxy would be needed .   
• Conceptual difficulties: It is difficult to reconcile the principles of general relativity with the principles of quantum mechanics .   

Our understanding of gravity has evolved significantly over the centuries. From Newton's classical theory, through Einstein's theories of relativity, to the search for a quantum theory of gravity, we have come closer to unraveling the secrets of this fundamental force. Each new stage in the development of physics has expanded our knowledge of gravity, but at the same time has revealed new questions and challenges. The search for a quantum theory of gravity is a key stage in this journey, which could lead us to a deeper understanding not only of gravity but also of the very nature of space, time, and the universe. Although we do not yet have a complete theory of quantum gravity, scientists continue to work on this problem, which is crucial to understanding the universe.