Gravity is one of the fundamental forces of the universe and dominates every moment of our conscious experience. It keeps us low to the ground, rips baseballs and basketballs out of the air, and gives our muscles something to fight against. On a cosmic level, gravity is just as important.
From collapsing hydrogen clouds into stars to collapsing galaxies, gravity is one of the few players that determines the broad outlines of the evolution of the universe.
In a way, the history of gravity is also the history of physics, with some of the biggest names in this field having found fame defining the force that governed their lives. But even after more than 400 years of study, this enigmatic force remains at the heart of some of the discipline’s greatest mysteries.
Four fundamental forces act on us every day. The strong force and the weak force only work in the center of the atoms. The electromagnetic force governs objects with excessive charge (like electrons, protons, and socks lying around on fluffy carpet), and gravity directs objects with mass.
The first three forces largely escaped the notice of mankind until recent centuries, but people have long speculated about gravity, which acts on everything from raindrops to cannonballs.
Ancient Greek and Indian philosophers observed that objects moved naturally toward the ground, but it took a flash of Isaac Newton’s lucidity to transform gravity from an inscrutable tendency of objects to a measurable and predictable.
Newton’s description of gravity was accurate enough to detect Neptune’s existence in the mid-1800s before anyone could see it, but Newton’s law isn’t perfect. In the 1800s, astronomers noticed that the ellipse traced by Mercury’s orbit moved faster around the sun than Newton’s theory predicted, suggesting a slight discrepancy between his law and the laws of nature. The riddle was finally solved by Albert Einstein’s theory of general relativity, published in 1915.
Before Einstein published his groundbreaking theory, physicists knew how to calculate a planet’s gravitational pull, but their understanding of why gravity behaved the way it did had hardly surpassed that of philosophers of the Earth. Antiquity. These scientists understood that all objects attract each other with an instantaneous force of infinite range, as Newton postulated, and many physicists of Einstein’s day were content to leave it at that. . But while working on his special theory of relativity, Einstein determined that nothing could travel instantaneously, and gravitational pull was no exception.
For centuries, physicists treated space as an empty frame in which events unfolded, according to Stanford’s Encyclopedia of Philosophy. He was absolute, immutable and did not actually exist, in any physical sense. General relativity transformed space, as well as time, from a static backdrop to a somewhat air-like substance in a room. According to Einstein, space and time together form the fabric of the universe and this “space-time” matter can stretch, compress, twist and turn, dragging along everything in it.
Einstein suggested that the shape of spacetime is the source of the force we know as gravity. A concentration of mass (or energy), like the Earth or the Sun, distorts the space around it like a rock distorts the course of a river. When other objects move nearby, they follow the curvature of space, like a leaf might follow a whirlwind around the rock (although this metaphor isn’t perfect because, at least in the case of planets orbiting around of the sun, space-time does not “flow”. We see planets spinning around the sun and apples falling 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 general relativity field equations, a set of formulas that illustrate how matter and energy warp spacetime, gained acceptance when they predicted changes in Mercury’s orbit, as well as the curvature of starlight around the sun during a solar eclipse in 1919.
The modern description of gravity so accurately predicts the interaction of masses that it has become a guide to cosmic discoveries.
American astronomers Vera Rubin and Kent Ford noticed in the 1960s that galaxies seem to spin fast enough to spin stars like a dog shaking off water droplets. But since the galaxies they studied didn’t swirl apart, something seemed to help keep them together. Rubin and Ford’s extensive observations provided strong evidence in support of Swiss astronomer Fritz Zwicky’s theory, proposed in the 1930s, that an invisible variety of mass accelerates galaxies in a nearby cluster. Most physicists today suspect that this mysterious “dark matter” distorts space-time enough to keep galaxies and clusters of galaxies intact. Others, however, wonder if gravity itself isn’t pulling more at the scale of galaxies, in which case Newton’s and Einstein’s equations would have to be adjusted.
Changes to general relativity will have to be tricky, as researchers have recently begun to detect one of the theory’s more subtle predictions: The existence of gravitational waves, or ripples in spacetime, caused by the acceleration of masses in space. Since 2016, a research collaboration operating three detectors in the United States and Europe has measured multiple gravitational waves traversing the Earth. More detectors are on the way, ushering in a new era of astronomy in which researchers study black holes and distant neutron stars — not by the light they emit, but by the way they rumble the fabric of space when they collide.
Yet the string of experimental successes in general relativity glosses over what many physicists consider a fatal theoretical failure: It describes classical spacetime, but the universe ultimately appears to be quantum, that is, that is, made up of particles (or “quanta”) such as quarks and electrons.
The classical notion of space (and gravity) as a smooth fabric clashes with the quantum image of the universe as a collection of sharp little bits. How to extend the Standard Model of particle physics, which encompasses all known particles plus the other three fundamental forces (electromagnetism, weak force, and strong force), to cover space and gravity at the particle level remains one of the greatest mysteries of modern physics.
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Gravity: What is it?
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