There are many challenging problems and unanswered mysteries in the history of physics, and among these unanswered mysteries, gravity is the greatest.
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 Earth without drifting away, allows the Moon to orbit Earth, and maintains the stability of the Solar System.
Gravitational force may seem simple and natural, yet it is one of the most perplexing and difficult forces to understand and describe. Physicists have proposed various theories to explain the nature and origin of gravitational force, but a perfect answer has yet to emerge.
The Standard Model of Physics recognizes four fundamental interactions: gravity, electromagnetic force, strong force, and weak force. (Image: NBC).
We know that gravitational force has two characteristics: first, it is universal, meaning any object with mass will experience gravitational attraction; at the same time, it is extremely weak, weaker than the other fundamental forces – the Standard Model of Physics recognizes four fundamental interactions: gravity, electromagnetic force, strong force, and weak force.
Why is this the case? This is one of the greatest unresolved mysteries in physics, also known as the hierarchy problem. The hierarchy problem refers to the reason behind the significant difference in strength among the four fundamental forces. For example, the electromagnetic force is approximately 10^36 times stronger than gravity, the strong force is about 10^38 times stronger than gravity, and the weak force is roughly 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 attraction. These numbers make it hard for us to imagine why gravity is so weak. Does this imply that gravitational force has some special or hidden properties? Or does it mean that there is something fundamentally wrong in our understanding of gravity?
The first to propose the concept of gravitational force was Isaac Newton, a 17th-century English physicist and mathematician. (Image: ZME).
To answer these questions, we must look back at the history of gravitational force. The first to propose the concept of gravitational force was Isaac Newton, a 17th-century English physicist and mathematician. While observing an apple falling from a tree, he put forth a bold hypothesis: There might be a force acting between Earth and the apple that causes the apple to move closer to Earth. And perhaps this force exists not only between Earth and the apple but also between any two objects with mass. This is the law of universal gravitation proposed by Newton.
The law of universal gravitation tells us that the gravitational force is directly proportional to the masses of the two objects and inversely proportional to the square of the distance between them. This means that if the masses of the two objects increase, the gravitational force between them will also increase; if the distance between the two objects increases, the gravitational force will decrease. The law of universal gravitation can clearly explain astronomical phenomena such as the motion of planets and tidal phenomena, and it can also be used to calculate the trajectory and speed of spacecraft such as satellites and rockets.
The law of universal gravitation made humanity the first to realize that there is a universal force in nature and helped humans to use mathematical language to describe and predict the laws of nature for the first time.
Newton himself acknowledged that he did not know how gravity is generated and propagated; he only described its effects. (Image: Zhihu)
However, the law of universal gravitation also has its limitations; it cannot explain effects such as light bending in strong gravitational fields or the precession of Mercury’s perihelion. Furthermore, Newton’s law of universal gravitation does not provide answers about 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 phenomena that could not be explained by Newton’s theory and to explore deeper principles behind gravity, in the early 20th century, another great physicist and mathematician, Albert Einstein, proposed a new and revolutionary theory: the theory of special relativity.
The theory of special relativity could explain some electromagnetic phenomena and atomic phenomena, but it could not reconcile with Newton’s law of universal gravitation. This is 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 gravitational force 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 proposed a surprising and wonderful hypothesis in this theory: gravitational force is not a force but the curvature of spacetime.
Einstein believed that gravitational force is the curvature of spacetime. (Image: Allthatsinteresting).
He believed that matter would warp spacetime and that spacetime would influence the motion of matter. The general theory of relativity could explain phenomena that Newton’s theory could not, such as the bending of light in strong gravitational fields or the precession of Mercury’s perihelion.
Moreover, the general theory of relativity also predicted several fascinating and astonishing phenomena such as black holes, gravitational waves, spacetime singularities, gravitational lensing, and gravitational redshift, among others. These phenomena have been confirmed or demonstrated by experiments or observations, thus verifying the correctness and accuracy of the general theory of relativity.
However, the general theory of relativity also has its limitations; it cannot reconcile with quantum mechanics, nor can it describe gravitational force in extreme conditions, such as singularities or extremely small particles. Furthermore, the general theory of relativity does not answer the question of what gravitational force is or why it exists.
Einstein himself acknowledged that he did not know whether the equations he proposed truly revealed the deepest truths about nature. To address phenomena that could not be explained by the general theory of relativity and to explore the behavior of gravitational force at a microscopic scale, in the early 20th century, a group of outstanding physicists and mathematicians developed a new and revolutionary theory: quantum mechanics.
Einstein himself acknowledged that he did not know whether the equations he proposed truly revealed the deepest truths about nature. (Image: 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 inherent uncertainty principle. Quantum mechanics can clearly explain electromagnetic force, strong force, and weak force, all of which can be attributed to the exchange between different types of particles.
For example, the electromagnetic force is generated by the exchange of photons, the strong force is generated by the exchange of gluons, and the weak force is generated by the exchange of W and Z bosons. What about gravity? Quantum mechanics suggests that gravitational force must also have a corresponding particle to mediate it, and this particle is called the graviton.
The graviton is a hypothetical particle, considered to be a boson with zero mass and spin 2. It could mediate gravitational force between any objects with mass, just as photons can mediate electromagnetic force between any charged objects. If gravitons exist, we could use quantum field theory to describe the unification of gravitational force with the other three fundamental forces. This is the quantum gravity theory that physicists have long anticipated.
Is it possible to unify general relativity and quantum mechanics? (Illustration: Zhihu).
However, quantum gravity theory is still not fully developed and realized, as there are currently no experimental observations of the existence of gravitons, nor is there a complete theory that describes the interaction between gravitons and other particles.
So, is it possible to unify general relativity and quantum mechanics? This is the grand unified theory that physicists aspire to achieve. Currently, there are several candidate theories attempting to reach this goal, such as string theory, loop quantum gravity, and twisted extra dimension theory, among others. Each of these theories has its own strengths and weaknesses, but none have been verified or disproven by experiments.
With the development of physics up to this point, both general relativity and quantum mechanics cannot propel humanity into the next era.
