What is gravity

(ORDO NEWS) — Of the fundamental forces of the universe, only one dominates every moment of our conscious experience: gravity. It keeps us close to the ground, raises and lowers footballs 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 merger of galaxies, gravity is one of the few players that defines a broad view of the evolution of the universe.

In a sense, the history of gravity is the history of physics, but even after over 400 years of study, a mysterious force still lies at the heart of some of the discipline’s greatest mysteries.

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

Newton’s leap, made public in his 1687 treatise Philosophia Naturalis Principia Mathematica, was the realization that every object in the universe, from a grain of sand to the largest stars, attracts every other object to itself.

This concept unified events that seemed completely unrelated, from apples falling to the Earth (although this probably did not inspire him to break through, but Newton did work near the apple tree) to planets orbiting the Sun.

He also attributed the attraction of number: doubling the mass of one object makes its attraction twice as strong, he determined, and the approach of two objects doubles their mutual attraction four times. Newton packaged 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 was moving around the Sun faster than Newton’s theory predicted, suggesting a slight discrepancy between his law and the laws of nature. This riddle was eventually solved by Albert Einstein’s general theory of relativity, published in 1915.

Before Einstein published his groundbreaking theory, physicists knew how to calculate the gravitational pull of a planet, but their understanding of why gravity behaves this way was little further than the ancient philosophers.

These scientists understood that all objects are attracted to each other with an instantaneous and infinitely far-reaching force, as Newton postulated, and many physicists of the Einstein era were ready to leave it at that.

But while working on his special theory of relativity, Einstein came to the conclusion that nothing can move instantly, and the attraction of gravity should be no exception.

For centuries, physicists have viewed space as an empty structure against which events play out. It was absolute, immutable, and physically non-existent.

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 make up the fabric of the universe, and that this “spatio-temporal” material can stretch, shrink, twist and rotate, dragging everything in it with it.

Einstein suggested that the shape of spacetime is what gives rise to the force we perceive as gravity. A 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 curve of space, since a leaf can follow a swirl around a rock (although this metaphor is not ideal because, at least in the case of planets orbiting the Sun, space-time doesn’t “flow”).

We see planetary orbits and apples fall as 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 warp spacetime, gained recognition when they successfully predicted changes in Mercury’s orbit, as well as the bending of starlight around the Sun during the 1919 solar eclipse.

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

American astronomers Vera Rubin and Kent Ford noticed in the 1960s that galaxies spin fast enough to bounce off stars like a dog shakes off water drops. But since the galaxies they studied didn’t fly away from each other, something seemed to keep them together.

Careful observations by Rubin and Ford provided strong evidence to support Swiss astronomer Fritz Zwicky’s earlier theory, proposed in the 1930s, that some invisible kind of mass is accelerating galaxies in a nearby cluster.

Most physicists now suspect that this mysterious “dark matter” is bending spacetime to keep galaxies and clusters of galaxies intact. However, others are wondering if gravity itself can increase on a galaxy-wide scale,

Changes in general relativity must be really delicate, as researchers have recently begun to discover one of the theory’s most subtle predictions: the existence of gravitational waves or ripples in space-time caused by the acceleration of masses in space.

Starting in 2016, a collaborative research effort with three detectors in the United States and Europe measured many gravitational waves passing through the Earth.

More and more detectors are coming, ushering in a new era of astronomy in which researchers study distant black holes and neutron stars – not by the light they emit, but by how they wobble the fabric of space when they collide.

However, a series of experimental successes in general relativity overshadows what many physicists see as a fatal theoretical failure: the theory describes classical spacetime, but the universe is ultimately quantum or composed of particles (or “quanta”) such as quarks and electrons.

The classical notion of space (and gravity) as a single smooth fabric conflicts with the quantum picture of the universe as a collection of sharp little pieces.

How to extend the prevailing standard model of particle physics, which encompasses all known particles as well as the other three fundamental forces (electromagnetism, the weak force and the strong force), to encompass space and particle-level gravity remains one of the deepest mysteries in modern physics.


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