Gravitation is a natural phenomenon by which all objects with mass attract each other, and is one of the fundamental forces of physics. In everyday life, gravitation is most commonly thought of as the agency that gives objects weight. It is responsible for keeping the Earth and the other planets in their orbits around the Sun; for keeping the Moon in its orbit around the Earth, for the formation of tides; for convection (by which hot fluids rise); for heating the interiors of forming stars and planets to very high temperatures; and for various other phenomena that we observe. Gravitation is also the reason for the very existence of the Earth, the Sun, and most macroscopic objects in the universe; without it, matter would not have coalesced into these large masses and life, as we know it, would not exist.
Modern physics describes gravitation using the general theory of relativity, but the much simpler Newton’s law of universal gravitation provides an excellent approximation in most cases.
The terms gravitation and gravity are mostly interchangeable in everyday use, but in scientific usage a distinction may be made. “Gravitation” is a general term describing the attractive influence that all objects with mass exert on each other, while “gravity” specifically refers to a force that is supposed in some theories (such as Newton’s) to be the cause of this attraction. By contrast, in general relativity gravitation is due to spacetime curvatures that cause inertially moving objects to accelerate towards each other.
Gravitation keeps the planets in orbit about the Sun
Gravitation is the force of attraction that acts between all objects because of their mass. An object’s mass is its amount of matter. Because of gravitation, an object that is near Earth falls toward the surface of the planet. An object that is already on the surface experiences a downward force due to gravitation. We experience this force on our bodies as our weight. Gravitation holds together the hot gases that make up the sun, and it keeps the planets in their orbits around the sun. Another term for gravitation is the force of gravity.
People misunderstood gravitation for centuries. In the 300’s B.C., the Greek philosopher and scientist Aristotle taught the incorrect idea that heavy objects fall faster than light objects. People accepted that idea until the early 1600’s, when the Italian scientist Galileo corrected it. Galileo said that all objects fall with the same acceleration unless air resistance or some other force acts on them. An object’s acceleration is the rate of change of its velocity (speed in a particular direction). Thus, a heavy object and a light object that are dropped from the same height will reach the ground at the same time.
Newton’s law of gravitation
Ancient astronomers measured the movements of the moon and planets across the sky. However, no one correctly explained those motions until the late 1600’s. At that time, the English scientist Isaac Newton described a connection between the movements of the celestial bodies and the gravitation that attracts objects to Earth.
In 1665, when Newton was 23 years old, a falling apple caused him to question how far the force of gravity reaches. Newton explained his discovery in 1687 in a work called Philosophiae naturalis principia mathematica (Mathematical Principles of Natural Philosophy). Using laws of planetary motion discovered by the German astronomer Johannes Kepler, Newton showed how the sun’s force of gravity must decrease with the distance from the sun. He then assumed that Earth’s gravitation decreases in the same way with the distance from Earth. Newton knew that Earth’s gravitation holds the moon in its orbit around Earth, and he calculated the strength of Earth’s gravitation at the distance of the moon. Using his assumption, he calculated what the strength of that gravitation would be at Earth’s surface. The calculated result was the same as the strength of the gravitation that would accelerate an apple.
Newton’s law of gravitation says that the gravitational force between two objects is directly proportional to their masses. That is, the larger either mass is, the larger is the force between the two objects. The law also says that the gravitational force between two objects is inversely (oppositely) proportional to the distance between the two objects squared (multiplied by itself). For example, if the distance between the two objects doubles, the force between them becomes one-fourth of its original strength. Newton’s law is given by the equation F = m1m2 / d 2, where F is the gravitational force between two objects, m1 and m2 are the masses of the objects, and d2 is the distance between them squared.
Until the early 1900’s, scientists had observed only one movement that could not be described mathematically using Newton’s law — a tiny variation in the orbit of the planet Mercury around the sun. Mercury’s orbit — like the orbits of the other planets — is an ellipse, a geometric figure with the shape of a flattened hoop. The sun is not at the exact center of the ellipse. So one point in each orbit is closer to the sun than all other points in that orbit. But the location of the closest point changes slightly each time Mercury revolves around the sun. Astronomers refer to that variation as a precession.
Scientists used Newton’s law to calculate the precession. The calculated amount differed slightly from the observed amount.
Einstein’s theory of gravitation
In 1915, the German-born physicist Albert Einstein announced his theory of space, time, and gravitation, the general theory of relativity. Einstein’s theory completely changed scientists’ way of thinking about gravitation. However, it expanded upon Newton’s law, rather than contradicting it.
In many cases, Einstein’s theory produced results that differed only slightly from results based on Newton’s law. For example, when Einstein used his theory to calculate the precession of Mercury’s orbit, the result agreed exactly with the observed motion. That agreement was the first confirmation of Einstein’s theory.
Einstein based his theory on two assumptions. The first is related to an entity known as space-time, and the second is a rule known as the principle of equivalence.
In the complex mathematics of relativity, time and space are not absolutely separate. Instead, physicists refer to space-time, a combination of time and the three dimensions of space — length, width, and height. Einstein assumed that matter and energy can distort (change the shape of) space-time, curving it; and that gravitation is an effect of the curvature.
The principle of equivalence states that the effects of gravity are equivalent to the effects of acceleration. To understand this principle, suppose you were in a rocket ship so far from any planet, star, or other celestial object that the ship experienced virtually no gravitation. Imagine that the ship was moving forward, but not accelerating — in other words, that the ship was traveling at a constant speed and in a constant direction. If you held out a ball and released it, the ball would not fall. Instead, it would hover beside you.
But suppose the rocket accelerated by increasing its speed. The ball would appear to fall toward the rear of the ship exactly as if gravity had acted upon it.
Predictions of general relativity
In the years since the calculation of Mercury’s precession confirmed Einstein’s theory, several observations have verified predictions made with the theory. Some examples include predictions of the bending of light rays and radio waves, the existence of gravity waves and black holes, and the expansion of the universe.
Bending of light rays
Einstein’s theory predicts that gravity will bend the path of a light ray as the ray passes near a massive body. The bending will occur because the body will curve space-time. The sun is massive enough to bend rays by an observable amount, and scientists first confirmed this prediction during a total eclipse of the sun in 1919.
Bending and slowing of radio waves
The theory also predicts that the sun will bend radio waves and slow them down. Scientists have measured the sun’s bending of radio waves emitted (sent out) by quasars, extremely powerful objects at the centers of some galaxies. The measurements agree well with the prediction.
Researchers measured a delay of radio waves that pass near the sun by sending signals between Earth and the Viking space probes that reached Mars in 1976. Those measurements still represent one of the most precise confirmations of general relativity.
General relativity also indicates that massive bodies in orbit around each other will emit waves of energy known as gravitational waves. Since 1974, scientists have confirmed the existence of gravitational waves indirectly by observing an object known as a binary pulsar. A binary pulsar is a rapidly rotating neutron star that orbits a similar, but unobserved, companion star. A neutron star consists mostly of tightly packed neutrons, particles that ordinarily occur only in the nuclei of atoms.
A pulsar emits two steady beams of radio waves that flow away in opposite directions. As the star rotates on its own axis, the beams sweep around in space like searchlight beams. If one of the radio beams periodically sweeps over Earth, a radio telescope can detect the beam as a series of pulses. By closely observing changes in the pulse rate of a binary pulsar, scientists can determine the pulsar’s orbital period — the time it takes the two stars to completely orbit each other.
Observations of the binary pulsar called PSR 1913 + 16 indicate that its orbital period is decreasing, and astronomers have measured the amount of the decrease. Scientists have also used equations of general relativity to calculate the amount by which the orbital period would decrease if the binary pulsar was radiating away energy as gravitational waves. The calculated amount agrees with the measured amount.
In addition, the pulsar’s orbit precesses as the pulsar revolves around the companion star. General relativity predicts the precession rate, and measurements match the prediction with great precision.
Einstein’s theory predicts the existence of objects called black holes. A black hole is a region of space whose gravitational force is so strong that not even light can escape from it. Researchers have found strong evidence that most very massive stars eventually evolve into black holes, and that most large galaxies have a gigantic black hole at their centers.
Expansion of the universe
In a paper published in 1917, Einstein applied general relativity to cosmology, the study of the universe as a whole. The theory showed that the universe must either expand or contract. In 1917, however, scientists had not yet found any evidence of expansion or contraction. To prevent his theory from disagreeing with the available evidence, Einstein added a term, the cosmological constant, to the theory. That term represented a repulsion (pushing away) of every point in space by the surrounding points, preventing contraction.
But in 1929, the American astronomer Edwin Hubble discovered that distant galaxies are moving away from Earth and that, the more distant a galaxy, the more rapidly it is moving away. Hubble’s discovery indicated that the universe is expanding. In response to Hubble’s discovery and confirming observations by other astronomers, Einstein abandoned the cosmological constant, calling it his greatest blunder.
The discovery of the expansion of the universe, together with other observations, led to the development of the big bang theory of the origin of the universe. According to that theory, the universe began with a hot, explosive event — a “big bang.” At the beginning of the event, all the matter in the part of the universe we can see was smaller than a marble. Matter then expanded rapidly, and it is still expanding.
Although Einstein called the cosmological constant his greatest blunder, it may turn out to be one of his greatest achievements. Measurements reported in 1998 suggest that the universe is expanding more and more rapidly. Furthermore, the rate of expansion has been increasing as predicted by general relativity with a cosmological constant.
Until the measurements were reported, astronomers generally thought that the rate of expansion was decreasing due to the gravitational attraction of galaxies for one another. The measurements showed that exploding stars known as supernovae in distant galaxies were dimmer than expected and that the galaxies therefore were farther away then expected. But the galaxies could be so far away only if the rate of expansion had begun to increase in the past.
Astronomers have concluded that the increase in the expansion rate is due to an entity that presently opposes gravitation. That entity could be a cosmological constant or something much like it called dark energy. Scientists have not yet developed theories to account for the existence of dark energy, but they know how much of it probably exists. The amount of dark energy in the universe is about twice as much as the amount of matter.
The matter in the universe includes both visible matter and a mysterious substance known as dark matter. Scientists do not know the composition of dark matter. But measurements of the motion of stars and gas clouds in galaxies have led scientists to believe that it exists. Those measurements show that the masses of galaxies are many times larger than the masses of the visible objects in them. These and other observations suggest that the universe has at least 30 times as much dark matter as visible matter.
Gravitation and the age of the universe
Other observations have helped show that the theory of general relativity applies to the whole universe. Cosmologists have calculated the age of the universe using equations of general relativity, the measured rate of expansion of the universe, and estimates of the amounts of dark energy and dark matter. The calculated age, about 14 billion years, agrees well with results determined by two methods that do not involve general relativity: (1) calculations based on the evolution of stars and (2) the radioactive dating of old stars.
As a star evolves, its surface temperature and its brightness change in a well-understood way. Astronomers can determine the ages of certain stars by measuring their temperature and brightness, then performing calculations based on their knowledge of stellar evolution. By means of such techniques, astronomers have found stars that may be about 13 billion years old — but no stars that are clearly older than that.
Radioactive dating of stars is based on the fact that certain chemical elements undergo radioactive decay. In radioactive decay, an isotope (form) of an element turns into an isotope of another element. Radioactive isotopes decay at known rates.
In 2001, scientists working with the European Southern Observatory’s Very Large Telescope in Chile applied the radioactive dating technique to an old star in our galaxy, the Milky Way. The researchers studied the isotope uranium 238, whose nucleus contains 92 protons and 146 neutrons. The scientists knew how much uranium the star must have had when it formed, and they measured how much it has now. They then applied their knowledge of decay rates to calculate the age of the star. The most likely age of the star is 12.5 billion years, so the universe is probably older than that. Measurements of the ages of many old stars using another element, thorium, gave similar results.