One of the fundamental forces of the universe is gravity. It keeps us close to the ground, drags baseballs and basketballs out of the air, and gives our muscles something to fight against. It's just as consequential as gravity.
One of only a few players that determine the broad strokes of the universe is gravity.
The story of gravity is also the story of physics, with some of the field's biggest names finding fame by defining the force that ruled their lives. The enigmatic force still lies at the center of some of the discipline's greatest mysteries after more than 400 years of study.
What is dark matter?
Every day, four fundamental forces act on us. The strong force and weak force are only found in the center of the atoms. The force of gravity steers objects with mass, and the force of electromagnetism rules objects with excess charge.
The first three forces largely escaped notice until recent centuries, but people have long speculated about gravity.
Ancient Greek and Indian philosophers observed that objects naturally move toward the ground, but it would take a glimpse of insight from Einstein to elevate gravity from an inscrutable tendency of objects to a measurable and predictable phenomenon.
The leap that was made public in his 1687 treatise was to realize that every object in the universe was a grain of sand. The events that appeared unrelated were apples falling on Earth and the planets circling the sun. He determined that doubling the mass of one object makes it pull twice as strong as if it were a single object. These ideas were put into the universal law of gravitation.
The existence of Neptune was discovered before anyone could see it, thanks to a description of gravity that was accurate. Astronomers noticed that the ellipse traced by Mercury was moving more quickly around the sun than they thought, suggesting a slight mismatch between his law and the laws of nature. Albert Einstein published a theory of general relativity in 1915.
Physicists knew how to calculate a planet's gravity before Einstein published his theory, but they didn't understand why gravity behaved in such a way. Many Einstein-era physicists were content to leave it at that, as they understood that all objects attract all others with an instantaneous and infinitely far-reaching force. While working on his theory of special relativity, Einstein decided that nothing could travel instantly, and the pull of gravity should be no exception.
According to the Encylopedia of Philosophy, physicists treated space as an empty framework against which events played out. It was unchanging and not really exist. Space and time were promoted from a static backdrop to a substance similar to the air in a room. The fabric of the universe was made up of space and time and Einstein was able to drag everything in it for the ride.
The force we experience as gravity is caused by the shape of spacetime, according to Einstein. A concentration of mass bends space around it like a rock. When other objects move nearby, they follow the curve of space, as a leaf might follow an eddy around the rock. The distorted shape of the universe causes planets and apples to fall. In everyday situations, those trajectory match the force of the law.
Einstein's field equations of general relativity, a collection of formulas that illustrate how matter and energy warp spacetime, gained acceptance when they successfully predicted the changes in Mercury's orbit, as well as the bending of starlight around the sun during a 1919 solar eclipse.
Einstein's 1919 solar eclipse experiment tested general relativity.
The description of gravity has become a guide for discovering new planets.
Astronomers Vera Rubin and Kent Ford noticed in the 1960s that the stars in the universe spin fast enough to shake off water droplets. Something seemed to be helping them stick together, because the galaxies they studied weren't whirling apart. Rubin and Ford's thorough observations provided strong evidence that some invisible mass was speeding up a nearby cluster of galaxies. Physicists suspect that dark matter warps spacetime to keep galaxies and clusters intact. Some wonder if gravity might pull harder at the scales, in which case both Einstein's andNewton's equations would need to be adjusted.
The existence of ripples in spacetime caused by the acceleration of mass in space is one of the theories most subtle predictions. Multiple gravitational waves have been measured through the United States and Europe. A new era of astronomy will be launched when more detectors are on the way, which will allow researchers to study black holes and neutron stars in a new way.
The string of experimental successesgloss over what many physicists see as a fatal theoretical failure: It describes a classical spacetime, but the universe ultimately appears to be quantum or made up of particles.
The quantum picture of the universe is a collection of sharp little pieces that clash with the classical notion of space and gravity. One of the deepest mysteries in modern physics is how to extend the Standard Model of particle physics to cover space and gravity at the particle level.
PBS Digital Media has a video explaining gravity. From the Australian Academy of Science, you can learn more about warps and ripples in space. Medium has an explanation of how gravity bends space.
Head, University of British Columbia.
The Cambridge University Press published "Gravity" by Eric and Clifford M. Will.
Why does gravity pull us down? The Conversation by Mario Borunda.
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