The Most Mysterious Force in Physics: The Nature and Origin of Gravity!

There are many difficult problems and unsolved mysteries in the history of physics, among these unsolved mysteries, gravity is the greatest mystery.

Gravity is the most familiar physical phenomenon to us and it is also the force that we can feel every day, it keeps us standing on the Earth without drifting away, it allows the Moon to orbit the Earth and it keeps the Solar System stable.

Gravity seems simple and natural, but it is one of the most difficult forces to understand and describe. Physicists have proposed different theories to explain the nature and origin of gravity, but there is still no perfect answer.

The Standard Model of physics currently recognizes four fundamental interactions: gravity, electromagnetism, the strong interaction, and the weak interaction. (Photo: NBC).

We know that gravity has two properties: first, it is universal, meaning that any object with mass will experience gravity; and at the same time, it is extremely weak, meaning that it is weaker than the other fundamental forces - the Standard Model of physics currently recognizes four fundamental interactions: gravity, electromagnetism, the strong interaction, and the weak interaction.

Why is this? This is one of the greatest unsolved mysteries in physics, also known as the string problem.  The hierarchy problem deals with why there are such large differences in strength among the four fundamental forces. For example, the electromagnetic force is about 10^36 times stronger than gravity, the strong force is about 10^38 times stronger than gravity, and the weak force is about 10^25 times stronger than gravity.

This means that if two electrons are 1 meter apart, the electromagnetic repulsion between them is about 10^36 times greater than the gravitational force. These numbers make it hard to imagine why gravity is so small. Does this mean that gravity has some special or hidden properties? Or does it mean that there is something fundamentally wrong with our understanding of gravity?

The first person to propose the concept of gravity was Newton, a 17th-century English physicist and mathematician. (Photo: ZME).

To answer these questions, we must look back at the history of gravity . The first person to propose the concept of gravity was Newton, a 17th-century English physicist and mathematician. While observing an apple falling from a tree, he came up with a bold hypothesis: There might be a force acting between the Earth and the apple that causes the apple to move closer to Earth. And, perhaps this force exists not only between the Earth and the apple, but also between two objects with mass. This is Newton's law of universal gravitation.

The law of universal gravitation tells us that the force of gravity is directly proportional to the masses of two objects and inversely proportional to the square of the distance between them. That is, if the masses of two objects increase, the force of gravity between them will increase; if the distance between two objects increases, the force of gravity between them will decrease. The law of universal gravitation can clearly explain astronomical phenomena such as planetary motion and tides, and can also be used to calculate the orbits and speeds of spacecraft such as satellites and rockets.

The law of universal gravitation made humans realize for the first time that there is a universal force in nature and helped humans for the first time use mathematical language to describe and predict the laws of nature.

Newton himself admitted that he did not know how gravity was created and propagated; he only described its effects. (Photo: Zhihu)

However, the law of universal gravitation also has limitations, it cannot explain effects such as the deflection of light in strong gravitational fields or the precession of Mercury's perihelion. Furthermore, Newton's law of universal gravitation does not provide answers to what gravity is or why it exists.

Newton himself admitted that he did not know how gravity is created and propagated; he only described its effects. To address the phenomena that could not be explained by Newton's theory and to discover the deeper principles behind gravity, at the beginning of the 20th century, another great physicist and mathematician, Einstein, proposed a new and revolutionary theory: the special theory of relativity .

Special relativity can explain some electromagnetic and atomic phenomena, but it cannot be reconciled with Newton's law of universal gravitation. Because Newton's law of universal gravitation assumes that gravitational force propagates instantaneously, violating the principle of the constant speed of light.

Einstein realized that he had to rethink the nature of gravity in order to create a more complete and consistent theory. Einstein spent 10 years exploring and developing his new theory and finally completed the general theory of relativity in 1915. He made a surprising and wonderful hypothesis in this theory: gravity is not a force but a curvature of space-time.

Einstein argued that gravity is not a force but a curvature of space-time. (Photo: Allthatsinteresting).

He believed that matter would warp space-time and that space-time would influence the motion of matter. General relativity could explain phenomena that Newton's theory could not, such as the deflection of light in strong gravitational fields or the precession of Mercury's perihelion.

Furthermore, general relativity also predicts some amazing and fascinating phenomena, such as black holes, gravitational waves, space-time singularities, gravitational lensing, gravitational redshift, etc. These phenomena have been confirmed or demonstrated by experiments or observations, thus verifying the correctness and accuracy of general relativity.

However, general relativity also has limitations: it cannot be reconciled with quantum mechanics, nor can it describe gravity in extreme conditions, such as singularities or extremely small particles. Furthermore, general relativity does not provide answers to what gravity is or why it exists.

Einstein himself admitted that he did not know whether the equations he proposed really revealed the deepest truths about nature. To deal with phenomena that could not be explained by general relativity and to explore the behavior of gravity at the microscopic scale, at the beginning of the 20th century, a group of brilliant physicists and mathematicians developed a new and revolutionary theory: quantum mechanics .

Einstein himself admitted that he did not know whether the equations he proposed really revealed the deepest truths about nature. (Photo: Zhihu).

Quantum mechanics is a theory that describes the behavior and interactions of microscopic particles, it believes that particles have both wave properties and an uncertainty principle. Quantum mechanics can clearly explain the electromagnetic force, the strong force, and the weak force, all of which can be reduced to exchanges between different types of particles.

For example, the electromagnetic force is produced by the exchange of photons, the strong interaction is produced by the exchange of gluons, and the weak interaction is produced by the exchange of W and Z bosons. What about gravity? Quantum mechanics believes that gravity must also have a corresponding particle to transmit it, and this particle is called the graviton.

The graviton is a hypothetical particle, considered a boson with zero mass and spin 2. It can transmit the gravitational force between any massless object, just as the photon can transmit the electromagnetic force between any electrically charged object. If the graviton exists, then we can use quantum field theory to describe the unification of gravity with the other three fundamental forces. This is the quantum theory of gravity that physicists have been waiting for.

Is it possible to unify general relativity and quantum mechanics? (Illustration: Zhihu).

However, the theory of quantum gravity has not yet been completed and realized, because there is currently no experimental observation of the existence of gravitons, nor is there a complete theory describing the interactions between gravitons and other particles.

So is it possible to unify general relativity and quantum mechanics? This is the grand unified theory that physicists dream of. There are already a number of candidate theories that are trying to achieve this goal, such as superstring theory, loop quantum gravity, extra-dimensional twist theory, etc. Each of these theories has its own advantages and disadvantages, but none of them has been experimentally verified or disproved.

With the development of physics to this point, neither general relativity nor quantum mechanics can continue to bring humanity into the next era.