The everyday puzzle
Drop something and it falls. For most of history that seemed too obvious to need an explanation, and early thinkers treated it as an object’s natural tendency to move toward the center of the world. The modern story begins when people started to measure falling motion and ask how it works in numbers.
Falling objects
Galileo Galilei, working in the late sixteenth and early seventeenth centuries, studied how objects move down inclined planes and showed that falling bodies speed up at a steady rate. He also argued that, once air resistance is set aside, heavy and light objects fall together. A famous story has him dropping weights from the Leaning Tower of Pisa, though historians doubt that it happened as told; his careful ramp experiments are better documented.
A dramatic demonstration of the principle came centuries later, when an astronaut on the Moon dropped a hammer and a feather together in the near-vacuum and they landed at the same time. On Earth, a feather falls slowly mostly because of air resistance, not because gravity pulls it less.
Near Earth’s surface, falling objects gain speed at about 9.8 meters per second every second, a value written as g. After one second of free fall, an object is moving at roughly 9.8 meters per second, and after two seconds, roughly 19.6 meters per second, ignoring air resistance.
Newton’s universal gravitation
Isaac Newton, in the late seventeenth century, made a bold step: the same force that pulls an apple to the ground also holds the Moon in orbit and the planets around the Sun. Gravity, in his view, acts between every pair of objects with mass.
His law says the attractive force is proportional to the product of the two masses and inversely proportional to the square of the distance between their centers. Double the distance and the force falls to one quarter; triple it and the force falls to one ninth. A constant, written G, sets the scale. Because G is very small, gravity between everyday objects is far too weak to notice. We feel it only when one of the masses is as large as a planet.
An orbit, in this picture, is a kind of continuous fall. A satellite is moving sideways so fast that as it falls toward Earth, the curved surface drops away beneath it. It keeps missing the ground. This is why astronauts aboard the International Space Station float: they are not beyond gravity, which is still strong at that height, but are in free fall together with their spacecraft.
Newton’s law predicted the motions of planets and comets with impressive accuracy and later helped lead to the discovery of Neptune, which was predicted from irregularities in the orbit of Uranus.
Mass and weight
Mass is how much matter an object has, and it stays the same everywhere. Weight is the force of gravity on that mass, so it depends on where you are. On the Moon, surface gravity is roughly one sixth of Earth’s, so a person weighs about one sixth as much there but has the same mass. A bathroom scale measures weight but is calibrated to display mass.
Einstein and curved spacetime
Newton’s law works very well, but it does not say how gravity works, and it assumes the force acts instantly across any distance. In 1915, Albert Einstein published the general theory of relativity, which describes gravity differently. Mass and energy bend spacetime, the combined fabric of three space dimensions and time, and objects follow the straightest possible paths through that curved geometry. A common picture is a heavy ball resting on a stretched rubber sheet, with smaller balls rolling toward it. The picture is a rough analogy, since the real curvature involves time as well as space.
In this view, planets orbit the Sun because the Sun curves the spacetime around it, not because it exerts a pull in the old sense. For weak gravity and slow speeds, Einstein’s equations give nearly the same answers as Newton’s, which is why Newton’s law remains in everyday use for engineering and spaceflight.
Evidence for general relativity
General relativity makes predictions that differ from Newton’s, and several have been confirmed.
- Bending of light. Starlight passing near the Sun is deflected. Measurements made during a solar eclipse in 1919 are often credited with giving early support to Einstein’s theory.
- Orbit of Mercury. The slow shift in Mercury’s orbit includes a small part that Newton’s law did not explain, and general relativity accounts for it.
- Gravitational time dilation. Clocks run slightly slower in stronger gravity. This effect is real and measurable, and GPS positioning has to correct for it, along with effects from motion, for satellites to stay accurate.
- Gravitational waves. Ripples in spacetime from violent events, such as merging black holes, were directly detected for the first time in 2015. This is the subject of gravitational waves research.
Gravity on the grandest scales
Gravity is by far the weakest of the fundamental forces, yet it dominates the universe at large scales because it is always attractive and acts over unlimited distance. It pulled gas into stars and planets and arranges galaxies into clusters. Observations suggest that galaxies contain more mass than we can see, an effect attributed to dark matter, whose nature is still unknown.
Gravity is also the force that squeezes massive stars so strongly that they can collapse. Black holes are regions where gravity is so strong that nothing, not even light, can escape. A full theory that merges gravity with quantum physics remains one of the open problems of science.
Key points
- Near Earth, objects accelerate downward at about 9.8 meters per second every second.
- Newton described gravity as a force depending on masses and distance.
- Einstein described it as the curvature of spacetime, which refines Newton’s picture in extreme conditions.
- Mass stays the same everywhere, while weight varies with gravity.