What is gravity?

US, WASHINGTON (ORDO NEWS) — Of the fundamental forces of the universe, only one dominates at every moment of our conscious experience: gravity. It holds us close to the Earth, raises and lowers soccer and basketballs and gives our muscles strength. From a cosmic point of view, gravity is just as consistent. From the collapse of hydrogen clouds into stars to the confluence of galaxies, gravity is one of the few players that define a broad view of the evolution of the universe.

In a sense, the history of gravity is the history of physics, but even after more than 400 years of study, mysterious power still lies at the heart of some of the greatest secrets of this discipline.

People pondered for a long time about gravity, which affects everything from raindrops to cannonballs. Ancient Greek and Indian philosophers noticed that objects naturally move toward the Earth, but it would take a flash of insight from Isaac Newton to raise gravity from the incomprehensible tendency of objects to a measurable and predictable phenomenon.

Newton’s leap, made public in his treatise “Philosophia Naturalis Principia Mathematica” of 1687, was the realization that every object in the Universe – from a grain of sand to the largest stars – attracts all other objects. This concept combined events that seemed completely unrelated, from apples falling to Earth (although this probably did not inspire him to break through, but Newton really worked next to the apple tree) to planets orbiting the Sun. He also attributed to the attraction of the number: doubling the mass of one object makes its attraction twice as strong, he determined, and the convergence of two objects doubles their mutual attraction fourfold. Newton packed these ideas into his universal law of gravity.

Newton’s description of gravity was accurate enough to understand the existence of Neptune in the mid-1800s before anyone could see it, but Newton’s law is not perfect. In the 1800s, astronomers noticed that the ellipse traced in the orbit of Mercury moves around the Sun faster than Newton’s theory predicted, suggesting a slight discrepancy between its law and the laws of nature. This riddle was ultimately solved by Albert Einstein’s general theory of relativity, published in 1915.

Before Einstein published his groundbreaking theory, physicists knew how to calculate the planet’s gravitational pull, but their understanding of why gravity behaved this way did not go much further than ancient philosophers. These scientists understood that all objects were attracted to each other with instant and infinitely far-reaching power, as Newton postulated, and many physicists of the Einstein era were ready to leave everything as it is. But, working on his special theory of relativity, Einstein came to the conclusion that nothing can move instantly, and the gravitational pull should not be an exception.

For centuries, physicists have considered space as an empty structure, against which events are played out. It was absolute, unchanging and in the physical sense did not exist. The general theory of relativity contributed to the transformation of space and time from a static background into a substance, somewhat reminiscent of the air in a room. Einstein believed that space and time together constitute the fabric of the Universe, and that this “spatio-temporal” material can stretch, contract, twist and rotate, dragging everything in it.

Einstein suggested that the form of spacetime is what generates a force that we perceive as gravity. The concentration of mass (or energy), such as the Earth or the Sun, bends the space around them, like a stone bends the course of a river. When other objects move side by side, they follow the bend of space, since the leaf can follow the whirlwind around the stone (although this metaphor is not ideal, because at least in the case of planets orbiting the Sun, space-time does not “flow”) . We see the orbits of planets and apples fall because they follow paths through the distorted shape of the universe. In everyday situations, these trajectories correspond to the force predicted by Newton’s law.

Einstein’s field equations of relativity, a set of formulas that illustrate how matter and energy deform space-time, were recognized when they successfully predicted changes in the orbit of Mercury, as well as the bending of starlight around the Sun during a 1919 solar eclipse.

The modern description of gravity so accurately predicts the interaction of masses that it has become a guide for cosmic discoveries.

American astronomers Vera Rubin and Kent Ford noticed in the 1960s that galaxies rotate fast enough to push off from the stars as a dog shakes off a drop of water from itself. But since the galaxies they studied did not fly away from each other in different directions, something seemed to help them stick together. Careful observations by Rubin and Ford provided convincing evidence confirming the earlier theory of the Swiss astronomer Fritz Zwicky, proposed in the 1930s, that a certain invisible mass species accelerates galaxies in a nearby cluster. Most physicists now suspect that this mysterious “dark matter” bends space-time to keep intact galaxies and clusters of galaxies. However, others are wondering if gravity itself can be magnified throughout the galaxy,

Changes in the general theory of relativity should be really delicate, as researchers recently began to discover one of the most subtle predictions of the theory: the existence of gravitational waves or ripples in space-time caused by the acceleration of masses in space. Since 2016, as a result of joint research work with three detectors in the United States and Europe, many gravitational waves passing through the Earth have been measured. More and more detectors are appearing, opening a new era of astronomy in which researchers study distant black holes and neutron stars – not by the light that they emit, but by the way they vibrate the fabric of space when they collide.

However, a number of experimental successes in the general theory of relativity are overshadowed by what many physicists consider a fatal theoretical failure: the theory describes classical space-time, but the Universe ultimately turns out to be quantum or consists of particles (or “quanta”), such as quarks and electrons.

The classical concept of space (and gravity) as a single smooth fabric contradicts the quantum picture of the universe as a collection of sharp small pieces. How to expand the prevailing standard model of particle physics, which covers all known particles, as well as three other fundamental forces (electromagnetism, weak and strong interactions) to cover space and gravity at the particle level, remains one of the deepest mysteries in modern physics.


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