Gravity

• 1. Introduction

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  Gravity is the reason an apple falls and the reason the Moon does not fly off into the dark. Isaac Newton's startling claim was that these are the same thing. The force pulling a piece of fruit toward the ground is the very force holding the Moon in orbit around the Earth. From clouds of primordial hydrogen, gravity drew matter together until it condensed and fused into the first stars. At larger scales it gathered galaxies and clusters, shaping the structure of the entire universe. Its range is infinite, though its grip weakens with distance. Yet for all its reach, gravity is strangely feeble. Compare the gravitational attraction of two electrons to their electrical repulsion and the ratio is just 1 to 4.17. So how did a force this weak come to govern planets, stars, and light itself? Why did it take humanity thousands of years to describe it, and why is it still the one force that resists being written in the language of quantum mechanics? This is the story of how we learned to read the hidden law that binds the cosmos.

• 2. What Gravity Is

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  Every object with mass attracts every other object in the universe. The pull grows with each mass and shrinks with the square of the distance between them. That single sentence is the law, and it places gravity among the four fundamental interactions of physics. The electromagnetic force law resembles it closely. Both depend on the inverse square of the distance between objects in typical interactions. The difference is one of scale. Because gravity is so weak, it can generally be neglected at the level of subatomic particles. At the scale of astronomical bodies it becomes the dominant interaction, steering satellites, planets, stars, galaxies, and even the path of light. There is a deeper fact hiding inside the law. The inertial mass that appears in Newton's second law is the same as the gravitational mass. This is the equivalence principle, a hypothesis tested experimentally to more than one part in a trillion. On the surface of a body like the Earth, gravity points toward the center and is modified by the centrifugal effects of the planet's rotation. It gives weight to objects, drives lunar tides and surface water waves, and contributes to weather patterns. It even reaches into biology, guiding the growth of plants through gravitropism and shaping how fluids circulate in multicellular organisms. The next puzzle was simpler to state and harder to solve: do heavier objects really fall faster?

• 3. Falling Bodies

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  Aristotle thought the speed of a falling object should increase with its weight, a conclusion later shown to be false. He also held that each classical element had a natural place: earth at the center of the universe, then water, air, fire, and aether in concentric shells. Not everyone in Ancient Greece agreed with his picture. Plutarch correctly predicted that the attraction of gravity was not unique to the Earth. Archimedes, though he did not understand gravity as a force, discovered the center of gravity of a triangle and reasoned about how two equal weights combine. Centuries later the debate spread far beyond Greece. The Roman engineer Vitruvius contended in his De architectura that gravity depends not on a substance's weight but on its nature. In the 6th century CE the Byzantine Alexandrian scholar John Philoponus proposed the theory of impetus, modifying Aristotle by adding a causative force that diminishes over time. In 628 CE the Indian mathematician and astronomer Brahmagupta described gravity as an attractive force drawing objects to the Earth, using the term gurutvākarṣaṇ. In the ancient Middle East the question turned fierce. The Persian intellectual Al-Biruni believed gravity was not unique to the Earth and that other heavenly bodies should exert their own attraction. Al-Khazini took Aristotle's side, holding that all matter is attracted to the center of the Earth. The experimental refutation came in the mid-16th century. The Spanish Dominican priest Domingo de Soto wrote in 1551 that bodies in free fall uniformly accelerate. With the 1586 Delft tower experiment, the Flemish physicist Simon Stevin watched two cannonballs of differing sizes and weights fall at the same rate. Galileo Galilei then measured balls rolling down inclines and firmly established that gravitational acceleration is the same for all objects. In his 1638 work Two New Sciences he proved that the distance a falling object travels is proportional to the square of the time elapsed.

• 4. The Apple And The Moon

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  Robert Hooke wrote in 1657 that the Moon must have its own gravity, and he kept building on the idea. In a 1674 Gresham lecture titled An Attempt to prove the Annual Motion of the Earth, he declared, "I will explain a system of the world very different from any yet received." His three positions held that all heavenly bodies attract each other, that bodies move in straight lines unless deflected, and that attraction grows stronger as bodies draw nearer. But he confessed his limit: "As to the proportion in which those forces diminish by an increase of distance, I own I have not discovered it." Hooke worked as the Royal Society's curator of experiments for 40 years and helped reformulate the scientific enterprise itself. For the mathematics he could not finish, he turned to Newton, writing him a letter in 1679 that likely turned Newton's thinking in a new direction. Before 1684, Christopher Wren, Hooke, and Edmund Halley had determined that Kepler's third law would prove the inverse square law if orbits were circles. The trouble was that orbits were known to be ellipses. At Halley's suggestion, Isaac Newton proved that ellipses also yielded the inverse square relation. In 1684 Newton sent Halley a manuscript titled De motu corporum in gyrum, On the motion of bodies in an orbit. Halley was impressed and urged him to expand it, and a few years later Newton published Philosophiæ Naturalis Principia Mathematica. Its revolutionary move was to unify Earth-bound acceleration with celestial mechanics. Newton wrote that gravitation operates on objects according to the quantity of solid matter they contain and propagates to immense distances at the inverse square of those distances. He reached an astounding conclusion we now take for granted: the gravity of the Earth on the Moon is the same as the gravity of the Earth on an apple. He could not at first prove that the Earth's gravity acts as if all its mass were concentrated at its center. That proof took him twenty years.

• 5. A Force Without A Cause

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  Action at a distance troubled even those who used it. Newton's gravity ran counter to a key idea of science, both then and now: forces should not rely on instantaneous action at a distance. Newton was well aware of the problem, and his decision to continue anyway marked a shift away from philosophically sound but empirically flawed models. Gottfried Wilhelm Leibniz complained about exactly this aspect of the theory. The value of the gravitational constant was eventually measured by Henry Cavendish in 1797, long after Newton discussed only proportionality and never wrote the modern formula. The theory's power was undeniable. More than a century after the Principia, in 1821, the French astronomer Alexis Bouvard used Newton's law to model the orbit of Uranus, and his table differed significantly from the planet's actual path. Many astronomers speculated that a large object beyond Uranus was disrupting the orbit. In 1846 John Couch Adams and Urbain Le Verrier independently used Newton's law to predict the location of Neptune, and the planet was discovered there within a day. One discrepancy resisted every fix. The perihelion of Mercury's orbit was increasing by about 42.98 arcseconds per century, and no hidden planet closer to the Sun could be found to explain it. The issue would not be resolved until Einstein's work in the 20th century.

• 6. Curved Spacetime

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  In 1915 Albert Einstein developed general relativity, which accurately modeled Mercury's orbit where Newton had failed. His theory fused two ideas with separate histories: the principle of relativity and non-Euclidean geometry. Carl Gauss had discovered in the 1800s that surfaces could be characterized by a metric, a distance measured along the shortest path between two points. His student Bernhard Riemann developed this into a complete geometry by 1854, describing spaces that are locally flat but globally curved. In 1907 Einstein had what he later called "the happiest thought of my life." He realized that in free fall an accelerated coordinate system exists with no local gravitational field. Gravity, in this view, is the curvature of spacetime caused by the uneven distribution of mass. The most extreme example is a black hole, from which nothing, not even light, can escape once past the event horizon. The proofs arrived one by one. In 1919 the British astrophysicist Arthur Eddington measured starlight deflections during a solar eclipse twice those predicted by Newtonian corpuscular theory, and the result made Einstein famous almost overnight. In 1959 the American physicists Robert Pound and Glen Rebka sent gamma rays down a 74-foot tower and confirmed gravitational time dilation. In 1964 Irwin I. Shapiro identified the time delay of light passing close to a massive object in interplanetary spacecraft signals. In 1971 scientists discovered the first black hole, Cygnus X-1, detected by bursts of x-rays as it consumed a smaller star. Frame dragging, the twisting of spacetime by a rotating mass, was confirmed by Gravity Probe B in 2011.

• 7. Cosmic Architect

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  About 5/6 of the total mass in the universe is dark matter, which interacts through gravity but not through electromagnetism. Clumps of it called dark matter halos pull in hydrogen gas, seeding stars and galaxies. Inside those galaxies, gravity wrote the life stories of stars. During star formation, gravitational attraction in a hydrogen cloud competes with thermal gas pressure. If the available mass is low, the process yields a brown dwarf or a gas-giant planet. With more mass, the central region reaches pressures sufficient for nuclear fusion and a star ignites. The ending depends on total mass. Very low mass stars cool slowly into white dwarfs, balancing gravity against electron degeneracy pressure. Higher mass stars build complex cores that burn helium and heavier elements until they produce an iron core, turn unstable, and explode as a supernova. What remains can be a neutron star, where gravity balances neutron degeneracy pressure, or a black hole, where gravity operates alone so intensely that even light cannot escape. Gravity also bends light into images. This was first confirmed in 1979 using the 2.1 meter telescope at Kitt Peak National Observatory in Arizona, which saw two mirror images of one quasar whose light had been bent around the galaxy YGKOW G1. Such gravitational lenses do not focus like eyeglass lenses but produce annular shapes called Einstein rings, and they offer further evidence for dark matter around galaxies. Not everything fits the picture. Stars on the outskirts of galaxies move faster than the luminous matter should allow, the expansion of the universe seems to be accelerating, and dark energy has been proposed to explain it.

• 8. Listening For Gravity

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  On the 14th of September 2015, the LIGO detectors measured the first direct evidence for gravitational radiation. The waves came from the collision of two black holes 1.3 billion light years from Earth, confirming a prediction Einstein and others had made long before. The first indirect evidence had come earlier, through measurements of the Hulse, Taylor binary in 1973, a pulsar and neutron star whose orbital period shrank as they radiated energy away. That research won the Nobel Prize in Physics in 1993, and the direct LIGO detection won it again in 2017. The waves themselves travel at a measurable speed. In October 2017 the LIGO and Virgo detectors received gravitational wave signals 2 seconds before gamma ray and optical instruments saw the same event, from a source about 130 million light-years away, confirming that gravity travels at the speed of light. Yet the most successful theory of gravitation remains incomplete. General relativity describes gravity as a smooth, continuous distortion of spacetime, while quantum mechanics holds that forces arise from the exchange of discrete quanta. The other three forces, the strong force, the weak force, and electromagnetism, were reconciled with a quantum framework decades ago. Gravity has not. One path describes it through the exchange of virtual gravitons, reproducing general relativity in the classical limit, but the approach fails at the Planck length. There the search continues for a theory of quantum gravity, a single framework that would join gravity to the rest of physics and finish the work Newton began.