Gravity: the story on HearLore

Gravity

Gravity is the word used to describe a physical law, a fundamental physical interaction that derives primarily from mass, and the observed consequences of that interaction on objects. It is the invisible hand that draws material objects towards each other, operating with an infinite range even though its effects become weaker as objects get farther away. Without this force, the clouds of primordial hydrogen and clumps of dark matter in the early universe would never have coalesced to form stars, galaxies, or clusters. It is the primary driver for the large-scale structures in the universe, determining the motion of satellites, planets, stars, and even light. On Earth, gravity gives weight to physical objects and is essential to understanding the mechanisms responsible for surface water waves, lunar tides, and weather patterns. It also has biological functions, guiding the growth of plants through gravitropism and influencing the circulation of fluids in multicellular organisms. Despite being one of the four fundamental interactions, gravity is the weakest, with the ratio of gravitational attraction of two electrons to their electrical repulsion being 1 to 10^36, yet it dominates the cosmos because it acts over infinite distances and is always attractive.

Ancient Minds and Falling Stones

The nature and mechanism of gravity were explored by a wide range of ancient scholars long before the scientific revolution. In Ancient Greece, Aristotle believed that each of the classical elements had a natural place in the universe, with earth at the center, and that the speed of a falling object should increase with its weight, a conclusion later shown to be false. While Aristotle's view was widely accepted, other thinkers like Plutarch correctly predicted that the attraction of gravity was not unique to the Earth. The ancient Greek philosopher Archimedes discovered the center of gravity of a triangle, postulating that if two equal weights did not have the same center of gravity, the center of gravity of the two weights together would be in the middle of the line that joins their centers of gravity. Two centuries later, the Roman engineer Vitruvius contended that gravity is not dependent on a substance's weight but rather on its nature. In the 6th century CE, the Byzantine Alexandrian scholar John Philoponus proposed the theory of impetus, modifying Aristotle's theory by incorporating a causative force that diminishes over time. In 628 CE, the Indian mathematician and astronomer Brahmagupta proposed the idea that gravity is an attractive force that draws objects to the Earth, using the term gurutvākarshana to describe it. In the ancient Middle East, the Persian intellectual Al-Biruni believed that the force of gravity was not unique to the Earth, correctly assuming that other heavenly bodies should exert a gravitational attraction as well, while Al-Khazini held the same position as Aristotle that all matter in the Universe is attracted to the center of the Earth.

Common questions

What is the definition of gravity according to the script?

Gravity is the word used to describe a physical law and a fundamental physical interaction that derives primarily from mass. It is the invisible hand that draws material objects towards each other, operating with an infinite range even though its effects become weaker as objects get farther away.

Who was the first person to propose that gravity is an attractive force that draws objects to the Earth?

The Indian mathematician and astronomer Brahmagupta proposed the idea that gravity is an attractive force that draws objects to the Earth in 628 CE. He used the term gurutvākarshana to describe this force.

When did Isaac Newton publish his groundbreaking book on gravity?

Isaac Newton published his groundbreaking book called Philosophiæ Naturalis Principia Mathematica a few years after sending a manuscript to Edmond Halley in 1684. The book provided a physical justification for Kepler's laws of planetary motion and established the inverse-square law.

What year did Albert Einstein develop the theory of general relativity?

Albert Einstein developed a theory of general relativity in 1915 which was able to accurately model Mercury's orbit. This theory brought the principle of relativity and non-Euclidean geometry into the physical theories of gravity.

When was the first direct evidence for gravitational radiation measured by the LIGO detectors?

The first direct evidence for gravitational radiation was measured on the 14th of September 2015 by the LIGO detectors. The gravitational waves emitted during the collision of two black holes 1.3 billion light years from Earth were measured.

What year was the phenomenon of gravitational lensing first confirmed by observation?

The phenomenon of gravitational lensing was first confirmed by observation in 1979 using the 2.1 meter telescope at Kitt Peak National Observatory in Arizona. The telescope saw two mirror images of the same quasar whose light had been bent around the galaxy YGKOW G1.

Newton's Universal Law of Attraction

In 1666, Giovanni Alfonso Borelli avoided the key problems that limited Kepler by developing the idea of mechanical equilibrium, a balance between inertia and gravity. Newton cited Borelli's influence on his theory. In 1657, Robert Hooke published his Micrographia, in which he hypothesized that the Moon must have its own gravity. In a communication to the Royal Society in 1666 and his 1674 Gresham lecture, Hooke took the important step of combining related hypothesis and then forming predictions based on the hypothesis. He wrote that the Moon must have its own gravity, and in a letter to Newton in 1679, outlined a model of planetary motion in a void or vacuum due to attractive action at a distance. This letter likely turned Newton's thinking in a new direction leading to his revolutionary work on gravity. Before 1684, scientists including Christopher Wren, Robert Hooke and Edmund Halley determined that Kepler's third law would prove the inverse square law if the orbits were circles. At Halley's suggestion, Newton tackled the problem and was able to prove that ellipses also proved the inverse square relation from Kepler's observations. In 1684, Isaac Newton sent a manuscript to Edmond Halley titled De motu corporum in gyrum, which provided a physical justification for Kepler's laws of planetary motion. Halley was impressed by the manuscript and urged Newton to expand on it, and a few years later Newton published a groundbreaking book called Philosophiæ Naturalis Principia Mathematica. The revolutionary aspect of Newton's theory was the unification of Earth-bound observations of acceleration with celestial mechanics, claiming that it operated on objects according to the quantity of solid matter which they contain and propagates on all sides to immense distances always at the inverse square of the distances. Newton's formulation was later condensed into the inverse-square law, where the force is proportional to the product of the masses and inversely proportional to the square of the distance between them. The value of the gravitational constant was eventually measured by Henry Cavendish in 1797. More than a century later, in 1821, his theory of gravitation rose to even greater prominence when it was used to predict the existence of Neptune. In 1846, the astronomers John Couch Adams and Urbain Le Verrier independently used Newton's law to predict Neptune's location in the night sky, and the planet was discovered there within a day.

Einstein's Curved Spacetime

Eventually, astronomers noticed an eccentricity in the orbit of the planet Mercury which could not be explained by Newton's theory: the perihelion of the orbit was increasing by about 42.98 arcseconds per century. The most obvious explanation for this discrepancy was an as-yet-undiscovered celestial body, such as a planet orbiting the Sun even closer than Mercury, but all efforts to find such a body turned out to be fruitless. In 1915, Albert Einstein developed a theory of general relativity which was able to accurately model Mercury's orbit. Einstein's theory brought two other ideas with independent histories into the physical theories of gravity: the principle of relativity and non-Euclidean geometry. In 1907, Einstein took his first step by using special relativity to create a new form of the equivalence principle. In what he later described as the happiest thought of his life, Einstein realized that in free-fall, an accelerated coordinate system exists with no local gravitational field. Einstein's description of gravity was accepted by the majority of physicists for two reasons: first, by 1910 his special relativity was accepted in German physics and was spreading to other countries, and second, his theory explained experimental results like the perihelion of Mercury and the bending of light around the Sun better than Newton's theory. In 1919, the British astrophysicist Arthur Eddington was able to confirm the predicted deflection of light during that year's solar eclipse. Eddington measured starlight deflections twice those predicted by Newtonian corpuscular theory, in accordance with the predictions of general relativity. Although Eddington's analysis was later disputed, this experiment made Einstein famous almost overnight and caused general relativity to become widely accepted in the scientific community. In 1959, American physicists Robert Pound and Glen Rebka performed an experiment in which they used gamma rays to confirm the prediction of gravitational time dilation. By sending the rays down a 74-foot tower and measuring their frequency at the bottom, the scientists confirmed that light is Doppler shifted as it moves towards a source of gravity. The time delay of light passing close to a massive object was first identified by Irwin I. Shapiro in 1964 in interplanetary spacecraft signals.

Black Holes and Gravitational Waves

In 1971, scientists made the first-ever discovery of a black hole, in the galaxy Cygnus. The black hole was detected because it was emitting bursts of x-rays as it consumed a smaller star, and it came to be known as Cygnus X-1. This discovery confirmed yet another prediction of general relativity, because Einstein's equations implied that light could not escape from a sufficiently large and compact object. Frame dragging, the idea that a rotating massive object should twist spacetime around it, was confirmed by Gravity Probe B results in 2011. In 2015, the LIGO observatory detected faint gravitational waves, the existence of which had been predicted by general relativity. Scientists believe that the waves emanated from a black hole merger that occurred 1.5 billion light-years away. The first indirect evidence for gravitational radiation was through measurements of the Hulse, Taylor binary in 1973. This system consists of a pulsar and neutron star in orbit around one another. Its orbital period has decreased since its initial discovery due to a loss of energy, which is consistent for the amount of energy loss due to gravitational radiation. This research was awarded the Nobel Prize in Physics in 1993. The first direct evidence for gravitational radiation was measured on the 14th of September 2015 by the LIGO detectors. The gravitational waves emitted during the collision of two black holes 1.3 billion light years from Earth were measured. This observation confirms the theoretical predictions of Einstein and others that such waves exist. It also opens the way for practical observation and understanding of the nature of gravity and events in the Universe including the Big Bang. Neutron star and black hole formation also create detectable amounts of gravitational radiation. This research was awarded the Nobel Prize in Physics in 2017. In October 2017, the LIGO and Virgo interferometer detectors received gravitational wave signals 2 seconds before gamma ray satellites and optical telescopes seeing signals from the same direction, from a source about 130 million light-years away. This confirmed that the speed of gravitational waves was the same as the speed of light.

The Mystery of Dark Matter and Energy

At the cosmological scale, gravity is a dominant player. About 5/6 of the total mass in the universe consists of dark matter which interacts through gravity but not through electromagnetic interactions. The gravitation of clumps of dark matter known as dark matter halos attract hydrogen gas leading to stars and galaxies. Galaxy rotation curves show that stars in galaxies follow a distribution of velocities where stars on the outskirts are moving faster than they should according to the observed distributions of luminous matter. Galaxies within galaxy clusters show a similar pattern. The pattern is considered strong evidence for dark matter, which would interact through gravitation but not electromagnetically; various modifications to Newtonian dynamics have also been proposed. The expansion of the universe seems to be accelerating. Dark energy has been proposed to explain this. Gravity acts on light and matter equally, meaning that a sufficiently massive object could warp light around it and create a gravitational lens. This phenomenon was first confirmed by observation in 1979 using the 2.1 meter telescope at Kitt Peak National Observatory in Arizona, which saw two mirror images of the same quasar whose light had been bent around the galaxy YGKOW G1. Many subsequent observations of gravitational lensing provide additional evidence for substantial amounts of dark matter around galaxies. Gravitational lenses do not focus like eyeglass lenses, but rather lead to annular shapes called Einstein rings. Despite its success in predicting the effects of gravity at large scales, general relativity is ultimately incompatible with quantum mechanics. This is because general relativity describes gravity as a smooth, continuous distortion of spacetime, while quantum mechanics holds that all forces arise from the exchange of discrete particles known as quanta. This contradiction is especially vexing to physicists because the other three fundamental forces were reconciled with a quantum framework decades ago. As a result, researchers have begun to search for a theory that could unite both gravity and quantum mechanics under a more general framework.