重力とは何か？

What is gravity? | Space Skip to main content Jump to: Gravity: A universal force Gravity Q&A with an expert Gravity: A tool of discovery Additional resources Don't miss these Black Holes James Webb Space Telescope's strange little red dots may really be 'black hole stars', X-ray data suggests Astronomy What is quantum gravity? Scientists think it could explain the beginning of our universe Black Holes Did decaying dark matter help create the universe's first supermassive black holes? Galaxies Help scientists find spacetime warps in these Euclid Space Telescope images Stars Why do some starburst galaxies mysteriously shut down? New study provides clues Stars Scientists learn how much baby stars in Orion weigh — by watching their dance moves The Sun What will happen when our sun starts dying? These 'stellar archaeologists' may have found a clue Dark Universe Does dark matter actually exist? 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Read a sci-fi short story every month and join a virtual community of fellow science fiction fans! Signup + An account already exists for this email address, please log in. Click for next article NASA used a Lunar Landing Walking Simulator to study astronauts' ability to perform tasks whilst experiencing one-sixth of normal gravity in preparation for the Apollo moon landings. (Image credit: NASA) Jump to: Gravity: A universal force Gravity Q&A with an expert Gravity: A tool of discovery Additional resources Share Copy link Facebook X Whatsapp Reddit Pinterest Flipboard Email Share this article Join the conversation Follow us Add us as a preferred source on Google Newsletter Subscribe to our newsletter Gravity is one of the universe's fundamental forces and dominates every moment of our conscious experience. It keeps us close to the ground, drags baseballs and basketballs out of the air and gives our muscles something to struggle against. Cosmically, gravity is just as consequential. From collapsing hydrogen clouds into stars to gluing galaxies together, gravity represents one of just a few players that determine the broad strokes of the universe 's evolution. In some ways, 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. But even after more than 400 years of study, the enigmatic force still lies at the heart of some of the discipline's greatest mysteries. You may like What is quantum gravity? Scientists think it could explain the beginning of our universe Do you suffer from IBS? This doctor says 'gravity intolerance' may be to blame Does dark matter actually exist? New theory says it could be gravity behaving strangely Related: How do you create lunar gravity in a plane? A veteran zero-G pilot explains Gravity: A universal force Four fundamental forces act upon us every day. The strong force and the weak force operate only inside the centers of atoms . The electromagnetic force rules objects with excess charge (like electrons , protons , and socks shuffling over a fuzzy carpet), and gravity steers objects with mass. The first three forces largely escaped humanity's notice 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 naturally moved toward the ground, but it would take a flash of insight from Isaac Newton to elevate gravity from an inscrutable tendency of objects to a measurable and predictable phenomenon. Signup to our newsletter Follow us on Google Newton's leap, which became public in his 1687 treatise " Philosophiae Naturalis Principia Mathematica ", was to realize that every object in the universe — from a grain of sand to the largest stars — pulled on every other object. This notion unified events that appeared unrelated, from apples falling on Earth to the planets orbiting the sun . He also put numbers to the attraction: Doubling the mass of one object makes its pull twice as strong, he determined and bringing two objects twice as close quadruples their mutual tug. Newton packaged these ideas into his universal law of gravitation. Gravity Q&A with an expert Massive bodies warp the fabric of space and time around them, leading to nearby objects following a curved path. This artist's illustration shows gravity bending the fabric spacetime around the Earth and sun. (Image credit: vchal via Getty Images) We asked Glenn Starkman, a Distinguished University Professor and co-Chair of Physics and Professor of Astronomy at Case Western Reserve University, a few frequently asked questions about gravity. Glenn Starkman Distinguished University Professor and Co-chair Department of Physics Distinguished University Professor and co-Chair of Physics and Professor of Astronomy at Case Western Reserve University, Director of the Institute for the Science of Origins, Director of the Center for Education and Research in Cosmology and Astrophysics. How does gravity work? Now that's a straightforward question with a deep answer. Newton did an awfully good job at giving us an answer — the Law of Universal Gravitation that I quoted above. So good that we call the constant of proportionality, Newton's Gravitational Constant, and write it GN , or just G . In equation form I would write the gravitational force F between two objects as F = Gm1m2/r^2 , where m1 and m2 are the two masses, and r is the distance between their centers. Unlike g (lower case), which as I said varies with your location, G appears to be a constant of nature — the same in every place and at every time. People spend a lot of time trying to get very accurate measurements of G , but it is the most poorly measured constant of nature, known to only about 20 parts per million. (In contrast, the comparable constant for the electromagnetic force, called the fine-structure constant, alpha, is measured to about one part in 10 billion.) So Newton's Law of gravitation is a very very good description of how gravity works in most circumstances. Without getting into all the controversies around who came up with what first, let's say that Newton discovered (or at least first published) this law in 1686. It is good enough for almost everything we do in everyday life. What is amazing is that it was also good enough to explain everything that astronomers were learning about the orbits of planets and moons. However, by the late 1850s, it was recognized that Mercury's orbit was ever so slightly wrong. (To be specific the orientation of the elliptical orbit was off by about 43 arcseconds — an arcsecond is 1/3600 of a degree — per century!) This was the observational motivation that led to Einstein's Theory of General Relativity (GR), which he published in 1915. GR has a completely different take on the nature of gravity. Indeed, it has a completely different take on the nature of space and time. Or at least that is the most common interpretation of GR, called the geometric interpretation. (An interpretation is the story we tell in ordinary language about the mathematical equations of a theory. It is the mathematical equations that we compare with the measurements, but we use the story to explain the equations to fellow humans. The story also helps us think about the theory.) I will just say "according to GR," when what I mean is "according to the geometrical interpretation of GR." According to GR's precursor, Special Relativity (published by Einstein in 1905), space and time are not separate in the way we normally think about them and in the way scientists treated them until then. They are part of a combined object/concept called spacetime. The division of spacetime into space and time depends on the person doing it, in particular, two people moving relative to one another will make that separation differently. Yeah, whoa. That's hard to wrap your head around. According to GR, space (and time) are also not "static." In our normal life, we think of space as kind of like a giant "stage" that the actors — us, the planets, the stars — move around on. So according to our normal picture space doesn't change. GR upends that completely. It says that actually space(time) changes in res