This paper introduces in detail the unique role and far-reaching influence of gravity in the universe from the basic concept of gravity, the role of gravity in the universe, and Einstein's general theory of relativity. Gravity not only determines the motion of celestial bodies, but also shapes the large-scale structure of the universe, and is a mysterious force that maintains the stability of the universe.
1. The basic concept of gravity.
What is gravity?Why is gravity present?What is the nature of gravity?These questions are probably the earliest and most basic scientific explorations of mankind. From Newton's classical mechanics to Einstein's theory of relativity, from gravitational fields to gravitational waves, from gravitational constants to gravitons, the nature and laws of gravity are still one of the most important and difficult mysteries in physics.
Gravity is the mutual attraction between matter, it is a universal natural phenomenon where any object with mass generates gravity and is subjected to the gravitational force of other objects. Gravity is a non-contact force that does not require a medium substance and can propagate in a vacuum. Gravity is a weak force that is much smaller than the electromagnetic, strong and weak interaction forces, but it is an infinite force that has no limit in its range of action and can span the entire universe.
Newton was the first scientist to use mathematical formulas to describe gravity, in the autumn of 1665 or possibly 1666 one day, Newton was thinking about life in the courtyard of a large villa under his hometown, and happened to be sitting under an apple tree, suddenly, a big porcelain apple fell from the tree, just smashed on his head, Newton held it in his hand, just about to scold, suddenly a flash of inspiration, why did the apple specifically hit him, is there any guidance?After more than 10 years of retreat, he proposed the famous law of gravitation in his Principia Mathematica of Natural Philosophy, which states that the gravitational force between two objects is proportional to the product of their masses and inversely proportional to the square of their distances, acting in the direction of their connection. Newton's law of gravitation, which can explain the phenomenon of gravity on Earth, as well as the motion of the planets in the solar system, is the basis of classical mechanics and the first recognition of the existence of a universal force in nature.
Although Newton's law of gravity is successful, it also has some unexplained problems, such as Mercury's perihelion precession, the deflection of light, the propagation speed of gravity, etc.
These questions led Einstein to propose the general theory of relativity in 1915, that gravity is not a force, but a curvature of space-time. Einstein believed that matter affects the geometry of space-time, making space-time uneven, and the curvature of space-time affects the trajectory of matter, causing matter to move along the shortest path of space-time (geodesic).
General relativity can explain phenomena that cannot be explained by Newton's laws of gravity, and also predicts some new phenomena, such as black holes, gravitational waves, gravitational lensing, etc., which is an important pillar of modern physics, and it is also the first time that mankind has recognized the dynamics and relativity of space-time.
Although gravity is a weak force, it is the most important force in the universe, which determines the motion of celestial bodies and shapes the large-scale structure of the universe.
Gravity is the main driving force for the movement of celestial bodies, which causes the formation of stable orbital motion between celestial bodies and also keeps the interior of celestial bodies in a state of equilibrium. For example, the movement of the earth around the sun and the movement of the moon around the earth are due to the action of gravity, and the internal structure of the sun and the earth is also formed by the balance of gravity and other forces.
Gravity can also cause tidal action between celestial bodies, causing changes in the shape, rotation, and orbit of celestial bodies. For example, the tidal action of the moon on the earth causes the earth's sea water and crust to fluctuate, and also makes the earth's rotation speed gradually slow down, and the earth's tidal action on the moon makes the moon's rotation speed and revolution speed synchronize, that is, the back side of the moon is always facing the earth.
Gravity not only controls the movement of celestial bodies, but also the formation and evolution of celestial bodies. In the early days of the universe, matter was evenly distributed, but due to small density fluctuations, gravity caused matter to gradually gather and form structures of different scales, such as stars, planets, star clusters, galaxies, and galaxy clusters. Gravity can also lead to dynamic processes such as collisions, mergers, and mergers of celestial bodies, making the structure of the universe constantly changing.
For example, the formation of the solar system may have been due to the collapse and rotation of a primordial nebula under the influence of gravity, forming a central star and the surrounding planetary disks, which in turn condensed into planets and small bodies of various sizes under the influence of gravity. The formation of the Milky Way may be due to the aggregation of tens of billions of stars under the action of gravity into a rotating planar disk, and the center of the disk has a huge black hole, and there are some globular clusters and satellite galaxies on the periphery of the disk.
Gravity affects not only matter, but also space-time itself. When there is material moving at an accelerated pace in the air, space-time fluctuations occur, and these fluctuations are gravitational waves. A gravitational wave is a wave that carries energy and information, and it travels through the universe at the speed of light and can travel through any matter. The existence of gravitational waves is an important prediction of general relativity, but because the effect of gravitational waves is very weak, it was not until 2015 that human beings detected gravitational wave signals for the first time, and this historic discovery opened a new window for human beings to observe the universe.
The generation of gravitational waves needs to meet two conditions: first, the motion of matter must be asymmetrical, that is, the center of mass of matter cannot move in a straight line;The second is that the motion of matter must be accelerating, i.e., the velocity or direction of matter cannot remain constant. Therefore, gravitational waves are mainly some extreme and violent celestial events in the universe, such as when two stars or two black holes or two neutron stars revolve around each other, they produce gravitational waves, and as the speed of rotation increases, the intensity and frequency of gravitational waves also increase, until the last two celestial bodies merge into one celestial body, releasing huge gravitational wave energy. This condition is known as binary merger and is the most intense source of gravitational waves ever detected by humans.
When a star's core runs out of fuel, it collapses and **, forming a neutron star or a black hole while releasing enormous amounts of energy and matter. If the process is asymmetrical, then it will also produce gravitational waves. This condition, known as a supernova explosion, is a potential source of gravitational waves, but it has not yet been detected.
In the early days of the universe, the universe may have gone through a period of extremely rapid expansion, a phase known as the cosmic inflation, which explains problems such as the flatness, uniformity, and density fluctuations of the universe. If the cosmic inflation is real, then it will also leave traces of gravitational waves in space-time. This gravitational wave, known as the primordial gravitational wave, is a very faint gravitational wave, but if detected, it would be important evidence of the origin of the universe.
The detection of gravitational waves is a very challenging task, because the effect of gravitational waves is very weak, and its perturbation of space-time is only of a very small order, for example, when a pair of black holes merge, the gravitational waves produced by the merger of a pair of black holes, when they reach the Earth, will only cause the distance on the Earth to change by a quadrillion of a meter, which is equivalent to the diameter of an atomic nucleus. In order to detect such small changes, humans have designed an instrument called the Laser Interferometer Gravitational-Wave Observatory (LIGO), which uses two long arms perpendicular to each other to emit and receive lasers in two directions respectively, and when the gravitational waves pass, the distance in both directions will change slightly, resulting in a change in the phase of the laser, which can be detected by sophisticated instruments. At present, humans have built multiple ligos, distributed in the United States, Europe, Japan and other places, they can cooperate with each other to improve the sensitivity and accuracy of detection, and can also determine the ** and direction of gravitational waves.
The detection of gravitational waves is a revolutionary observation of the universe by human beings, which provides a new window for human beings to observe the universe, which can reveal some celestial phenomena and physical processes that cannot be observed by electromagnetic waves, such as the nature of black holes, the state of neutron stars, and the origin of the universe. Gravitational waves can also verify the correctness of the general theory of relativity for mankind, test the limits of gravity, and explore the nature and laws of gravity. Gravitational waves can also bring some unexpected discoveries and surprises to mankind, such as new types of celestial bodies, new physical phenomena, new structures of the universe, etc. Gravitational waves are a new understanding of the universe and a new exploration of nature.
Gravity is the mysterious force that sustains the universe, it not only controls the movement of celestial bodies, but also reveals the mysteries of the universe, and is an important bridge between the macrocosm and the microcosm. But gravity is also the most difficult force to understand, and there are many unanswered questions, such as black holes and gravity.
A black hole is an extreme celestial body that is so massive, very dense, and gravitational so strong that even light cannot escape. The existence of black holes is a prediction of general relativity and the main cause of gravitational waves, but the internal structure and properties of black holes remain a mystery. For example, the boundary of a black hole, called the event horizon, is a point from which there is no turning back, and once it crosses the event horizon, it cannot be returned, so what does the interior of the event horizon look like?The center of a black hole, called the singularity, is a point where the density is infinite, and space-time loses its meaning, so how is the singularity formed?Do black holes have entropy and temperature?Do black holes have a process of emission and evaporation?Do black holes have hair and memory?Are black holes multiplicity and complex?Do black holes have information and quantization?Does a black hole have an internal and external connection?Do black holes have other dimensions and universes?Do black holes have regularity and regulation?Do black holes have a meaning and purpose?Black holes are the ultimate manifestation of gravity and a great challenge to gravity, and they may hide the deepest secrets of the universe.
Gravitational waves are a manifestation of gravity and a probe of gravity, which can provide mankind with a new means of cosmological research, which can help mankind solve some cosmological problems, such as the origin of the universe: gravitational waves can be traced back to the earliest days of the universe, and may even be earlier than the cosmic microwave background radiation, therefore, gravitational waves can reveal the initial state of the universe for human beings, such as evidence of the universe's inflation, the geometry of the universe, the total energy of the universe, etc.
Gravitational waves can record various celestial events in the universe, such as the formation and death of stars, the merger and evaporation of black holes, the collision and fusion of galaxies, etc., therefore, gravitational waves can reveal the evolutionary history of the universe for human beings, such as the age of the universe, the expansion rate of the universe, the material composition of the universe, etc.
Gravitational waves can detect some hidden matter in the universe, such as black holes, neutron stars, dark matter, dark energy, etc., therefore, gravitational waves can reveal the structural characteristics of the universe for human beings, such as the large-scale uniformity of the universe, the small-scale inhomogeneity of the universe, and the density fluctuations of the universe.
Dark matter and dark energy are two major mysteries in the universe, they occupy the vast majority of the universe, but their nature and role are still unclear. Dark matter is a substance that does not emit and does not absorb electromagnetic waves, it can only be detected by gravity, it may be an unknown elementary particle, or it may be an unknown celestial body, its distribution and movement in the universe, affecting the formation and evolution of the universe. Dark energy is a kind of energy that accelerates the expansion of the universe, it may be a ubiquitous vacuum energy, or it may be a scalar field of change, and its strength and nature in the universe determine the fate and outcome of the universe. Gravity is the only known way of interaction between dark matter and dark energy, therefore, gravity is an important means to study dark matter and dark energy, and it is also an important clue to understand dark matter and dark energy.
The theory of quantum gravity is a dream in physics that attempts to unify general relativity and quantum mechanics to establish a complete theory that describes all matter and forces in the universe. The necessity of the theory of quantum gravity lies in the fact that general relativity and quantum mechanics are two very successful theories, but they will be contradictory and inconsistent in some extreme cases, such as the singularity of a black hole, the initial singularity of the universe, etc., so a new theory is needed to solve these problems.
The difficulty of the theory of quantum gravity lies in the fact that general relativity and quantum mechanics are two very different theories, they have different mathematical languages, different physical concepts, and different experimental verifications, so to integrate them, a new way of thinking and new experimental methods are needed. The significance of quantum gravity theory lies in the fact that it may be the ultimate understanding of nature by human beings, it may reveal the nature and laws of the universe, and it may open up new explorations and new creations of nature by human beings.
Gravity is the mysterious force that sustains the universe, it not only controls the movement of celestial bodies, but also reveals the mysteries of the universe, and is an important bridge between the macrocosm and the microcosm. A deeper understanding of gravity will be an important step in our exploration of the mysteries of the universe.