Representation of an early Chinese compass

The Fundamental Forces and History of Electromagnetism

Forces in NatureEvery process in the universe that we understand can be explained in terms of the four fundamental forces (strong nuclear, electromagnetic, weak nuclear and gravitational). Each force is defined by the way it interacts with particles to build up composite form of matter: protons, neutrons, nuclei, atoms, molecules, planets, star, and so on.

Force (Strongest to weakest) Range Characteristics

Strong Nuclear distances Attracts neutrons and protons to each other

Electromagnetic All distances (Inverse Square Law) Attraction between positive protons and negative electrons and repulsion of like charges

Weak Nuclear distances Responsible for radioactive (beta) decayGravity All distances (Inverse Square Law) Attractive force between two pieces of matter

Electromagnetic ForceThe electromagnetic force affects everything in the universe because (like gravity) it has an infinite range. It has the ability to attract and repel charges. It is responsible for giving matter strength, shape, and hardness. The electromagnetic force is generated by fields and works over a distance.

Electrons are bound to the nucleus by electromagnetic force. The force is the attraction between protons (positive) and electrons (negative). Electrons in the outer part of an atom are attracted to proton in the nucleus. The momentum of the electron causes it to move around the nucleus rather than falling straight in.

Early observations of electricity and magnetismElectromagnetism holds electrons to the nucleus of atoms and allows atoms to bond together to form chemical compounds. All solids and liquids are held together by electromagnetic forces. Light is an example of an electromagnetic wave. Humans have harnessed electromagnetism to power an enormous variety of technology, from compasses to computers and cell phones.

In spite of its importance, electromagnetism is not always easy to observe. Nevertheless, ancient peoples did observe several examples of electricity including lightning, and many also noted the powerful electric shocks that occurred when they touched certain animals such as electric catfish and electric eels. The ancient Greeks were the first to report on static electricity: Around 600 BCE Thales of Miletus noted that rubbing amber, a form of fossilized tree resin, with fur caused small objects such as feathers or hair to be attracted to the amber.

The Chinese are credited with the invention of the magnetic compass, around 200 BCE. (It is possible that the Olmec culture of Mexico may have developed a compass 800 years earlier, but evidence is not conclusive.) The earliest Chinese compasses consisted of a spoon-shaped piece of lodestone balanced on a square tray. The Chinese were also the first to use compasses as a navigational aid.

The first scientific experiments in electricity and magnetism were performed by William Gilbert, an English doctor, in the late 16th century. Gilbert discovered many substances capable of holding an electrical charge. He also discovered new ways of producing magnets. The first electrostatic generators were built in 17th century. These machines worked by friction—a leather pad was placed against a rotating glass globe, generating static electricity.

The 17th century saw several major advances in electricity and magnetism. A major advance was the accidental invention of the Leyden jar by Ewald Georg von Kleist in 1744. Von Kleist was experimenting with an electrostatic generator while holding a jar with an iron nail in his hand. When he touched the nail, he received a nasty shock. The ability of the jar to store electrical charge was later improved by adding layers of foil to the inside and outside of the jar.

History from www.explorelearning.com Four Fundamental from Foundations of Physics, Tom Hsu, Ph.D., CPO Sciencehttp://emandpplabs.nscee.edu

Benjamin Franklin flies a kite

Benjamin Franklin (1706–1790) made two major contributions to the study of electricity. First, Franklin is generally credited as the first to describe electrical charge as “positive” and “negative,” a convention that lasts to this day. Second, Franklin was able to establish the link between electricity and lightning. Franklin did this famously by flying a kite in an electrical storm. As the kite floated near the storm clouds, a key tied near the bottom of the string became charged. Franklin observed a spark when he placed his hand near the key.

Maturity of electromagnetism: 1780–present In the 19th century, the study of electricity and magnetism developed from a curiosity to a central pillar of physics. Scientific discoveries were followed by commercial applications as people discovered the usefulness of these forms of energy in emerging technologies. The timelines below summarize some of the major scientific and technological advances of this period.

Scientific AdvancesDate Discoverer(s) Discovery Description

1784 Charles-Augustin de Coulomb Coulomb’s law Using a torsion balance, Coulomb finds the relationship between

electrical force (F), charge (q), and distance (r).

1800 William Nicholson and Johann Wilhelm Ritter Electrolysis Water is first decomposed into hydrogen and oxygen by

electrolysis. Electroplating is discovered soon after.

1820 Hans Ørsted and André-Marie Ampere Electromagnetism Ørsted finds that an electrical current produces a magnetic force.

Ampere expresses these results mathematically.

1827 Georg Ohm Ohm’s law Ohm discovers that the current in a circuit is proportional to voltage divided by resistance.

1831 to 1835

Michael Faraday and Joseph Henry

Electromagnetic induction

Henry and Faraday independently observe that a changing magnetic field can induce a current.

1861 James Clerk Maxwell Maxwell’s equations Maxwell unites all knowledge of electromagnetism with four equations.

1864 James Clerk Maxwell Electromagnetic theory of light Maxwell proposes that light is an electromagnetic wave.

1887 Heinrich Hertz Radio waves Hertz discovers radio waves and proves the existence of electromagnetic waves.

1896 J.J. Thomson Electron is discovered Thomson’s cathode ray tube experiments confirm the existence of the electron.

1905 Albert Einstein Photoelectric effect Einstein explains the photoelectric effect by stating that light is composed of “quanta.”

Advances in TechnologyDat

eDiscoverer(s) Discovery Description

1800 Alessandro Volta First battery The first battery is a voltaic pile. It consists of alternating layers of zinc and

copper separated by pieces of cardboard soaked in electrolyte.1821 Michael Faraday Electrical motor Faraday invents the first motor that uses electricity to cause a wire to rotate

in a magnetic field. 1837 Samuel Morse Telegraph Morse develops a practical system for instantly transmitting messages across

great distances.1876 Alexander Graham Bell Telephone Bell receives the first patent for the telephone.

1879 Thomas Edison Incandescent light

bulbEdison files a patent for an incandescent light bulb.

1888 Nikola Tesla Alternating current Tesla popularizes the use of alternating current (AC) over the much less

efficient direct current (DC). 1925 Julius Lilienfeld Transistor The transistor is an essential component of most electronic devices.

1926 John Logie Baird Television Baird, a Scottish inventor, demonstrates a primitive television that displays

shadowy images.1937 Alan Turing Turing machine Although only a theoretical device, the Turing machine provided the

foundation of all modern computers.1969

Willard Boyle and George E. Smith CCD/Image sensor The image sensor in digital cameras collects photons of light to form a

digital image.

1991 CERN World Wide Web The Internet is launched, revolutionizing the way information is transmitted

and shared.

The discovery of radiation: ghostly images on a photographic plate produced by uranium salts.

The Fundamental Forces and the Weak Force

Forces in NatureEvery process in the universe that we understand can be explained in terms of the four fundamental forces (strong nuclear, electromagnetic, weak nuclear and gravitational). Each force is defined by the way it interacts with particles to build up composite form of matter: protons, neutrons, nuclei, atoms, molecules, planets, star, and so on.

Force (Strongest to weakest) Range Characteristics

Strong Nuclear distances Attracts neutrons and protons to each other

Electromagnetic All distances (Inverse Square Law)

Attraction between positive protons and negative electrons and repulsion of like charges

Weak Nuclear distances Responsible for radioactive (beta) decay

Gravity All distances (Inverse Square Law) Attractive force between two pieces of matter

Weak Nuclear ForceThe weak nuclear force is the least observed of the four fundamental forces. It is responsible for things like beta decay. It only occurs over small distances (1x10-16m).

RadioactivityIn 1896, a French physicist named Henri Becquerel was conducting a series of experiments with phosphorescent substances, or substances that will glow in the dark after heating or exposure to sunlight. To study the phosphorescence of uranium salts, Becquerel prepared a photographic plate, wrapped the plate in opaque paper, placed uranium salts on top of the paper, and exposed it to sunlight for several hours. When he developed the plate, Becquerel discovered a blurry image of the salts on the plate. Becquerel concluded that the uranium salts must emit some sort of energy or particle that could penetrate paper and fog the plate.

Becquerel’s discovery led to a new field of physics, the study of radioactive substances. Subsequent research by such pioneers as Ernest Rutherford, Paul Villard, and the Curies (Pierre and Marie) established other characteristics of radiation:

Radiation is the release of particles and/or energy from unstable atomic nuclei. Nuclei emit particles in a process called nuclear decay. There are several forms of nuclear decay, such as:

o In alpha decay, the nucleus emits an alpha particle, which consists of two protons and two neutrons.o In beta decay, the nucleus emits a beta particle, a high-energy electron or positron. (A positron is the

antimatter equivalent of an electron.) Nuclear decay can change atoms of one element to another.

o Alpha decay reduces the atomic number by two (because two protons are emitted) and reduces the mass number by four (two protons and two neutrons are emitted).

o When an electron is emitted in beta decay, a neutron is transformed into a proton. This increases the atomic number by one (a proton is added) while leaving the mass number unchanged.

o When a positron is emitted in beta decay, a proton is transformed into a neutron. This reduces the atomic number by one (a proton becomes a neutron) while leaving the mass number unchanged.

Alpha decayWithin the nucleus of an atom, two opposing forces act on protons and neutrons: the strong nuclear force and electromagnetism. The strong nuclear force is an attractive force between protons and neutrons that bind them together inside the nucleus. As its name implies, the strong force is very powerful. However, it only operates at short distances. Electromagnetism is an attractive force between opposite charges and a repulsive force between similar charges. Inside the nucleus, the positively-charged protons are repelled from one another by electromagnetism but held in place by the strong force, which at that distance is more powerful than electromagnetism. For most atoms, this results in a stable nucleus. However, in some cases, the geometry of the nucleus and relative numbers of protons and neutrons cause a difference between the strength of electromagnetism and the

Quantum tunneling: An object tunnels through a potential energy barrier to achieve a more stable (lower energy) state.

Protons (left) and a neutrons (right)

Astronomy connection

Although the weak force is the least known of all of the fundamental forces, it plays a vital role in the universe. Inside a star, hydrogen atoms are fused into helium atoms, producing vast quantities of energy. The formation of helium occurs over several steps; summarized below:

Notice that step two requires a proton to transform into a neutron, releasing a W+ boson. Without this step, helium could not be produced from hydrogen and significant energy could not be produced. The

Annihilation occurs when a positron collides with an electron.

strong nuclear force. In this case the atom may have the potential for alpha decay.The actual occurrence of alpha decay depends on a bizarre process called quantum tunneling.

Alpha particles are very massive compared to beta particles but do not penetrate materials very easily: they can be blocked by a layer of clothing or even a layer of dead skin cells. Alpha radiation is usually only harmful if radioactive materials, such as radon gas, are inhaled and become lodged in the lungs.

Beta decayThe second common type of nuclear decay is beta decay. Beta decay occurs if there are too many neutrons or too many protons in the nucleus. If there are too many neutrons, a neutron will transform into a proton. If there are too many protons, a proton will change into a neutron.

Positrons are the antimatter equivalents of electrons. Like electrons, positrons have almost no mass. When a positron meets an electron, the particles annihilate one another in a burst of energy.

Beta particles (electrons and positrons) are much smaller and lighter than alpha particles. Individual beta particles have less energy than alpha particles, but are also able to travel farther and can penetrate thin materials such as clothing or skin.

Beta decay occurs because of the weak force, one of the four fundamental forces in nature (the others are the strong nuclear force, electromagnetism, and gravity).

The weak forceTo understand how protons and neutrons turn into one another, it helps to know the structure of these particles. Each nucleon, or nuclear particle, is composed of three smaller particles called quarks.

For a neutron to turn into a proton, a down quark must turn into an up quark. The down quark does this by emitting a particle called a W– boson. The W– boson is an odd particle. It is very heavy, about 80 times more massive than a proton, but only exists for a tiny period of time. After about a 3 × 10–25 seconds, the W– boson decays into a high-energy electron and an electron antineutrino. The electron antineutrino is a neutral particle that has about the same mass as an electron. This event is caused by the weak force, which is also known as the weak interaction.

The existence of W– and W+ bosons was predicted in 1968 by Sheldon Glashow, Abdus Salam, and Steven Weinberg. They were discovered at the 1983 at CERN, the world’s most powerful particle accelerator at the time. This discovery was a major triumph for particle physics and has encouraged scientists to explore how each of the fundamental forces is related to one another. In 2008 an even larger particle accelerator, the Large Hadron Collider (LHC) opened at CERN. The main goal of the LHC is to discover the Higgs boson, a particle that will confirm that the electromagnetic force and the weak force are the same force at extremely high energies.

The Rutherford-Geiger-Marsden experiment

The planetary atom: Protons are purple, neutrons are white, and electrons are blue.

The Fundamental Forces and History of Strong Nuclear ForceForces in NatureEvery process in the universe that we understand can be explained in terms of the four fundamental forces (strong nuclear, electromagnetic, weak nuclear and gravitational). Each force is defined by the way it interacts with particles to build up composite form of matter: protons, neutrons, nuclei, atoms, molecules, planets, star, and so on.

Force (Strongest to weakest) Range Characteristics

Strong Nuclear distances Attracts neutrons and protons to each other

Electromagnetic All distances (Inverse Square Law)

Attraction between positive protons and negative electrons and repulsion of like charges

Weak Nuclear distances Responsible for radioactive (beta) decay

Gravity All distances (Inverse Square Law) Attractive force between two pieces of matter

Strong ForceThe strong force is the strongest of the four fundamental forces. It is an attractive force that keeps the repelling protons together in a nucleus. It only works over a very small distance (1x10-15m).

The structure of the atomAt the end of the 19th century, the prevailing model of the atom was one proposed by J.J. Thomson. Thomson, who had recently discovered the electron, hypothesized a “plum pudding” model in which negatively charged electrons were embedded within a positively charged mass. In 1909, Ernest Rutherford, Hans Geiger, and Ernest Marsden devised an ingenious experiment to test Thomson’s model.

Rutherford had previously done a series of experiments on alpha particles, tiny particles that were emitted by certain radioactive materials. (An alpha particle is equivalent to the nucleus of a helium atom, consisting of two protons and two neutrons.) Rutherford and his colleagues set up an experiment in which a stream of alpha particles would be “shot” at a sheet of gold foil that was only a few atoms thick. A screen coated in zinc sulfide surrounded the foil. When an alpha particle hit the screen, a tiny glowing dot could be observed under a microscope.

Assuming Thomson’s atom model was correct, Rutherford expected all the alpha particles to pass directly through the gold foil with little or no

deflection. What they discovered was that, while the vast majority of the particles did pass directly through the foil, some particles were deflected to the side. A few others were even bounced backwards!

Rutherford concluded that most of an atom’s mass was concentrated in a tiny, positively-charged nucleus. Most of the alpha particles passed through the empty space that made up most of the atom, but once in a while an alpha particle collided with the nucleus and was bounced backward or deflected to the side. Rutherford supposed that the electrons orbited the nucleus like planets around the Sun, as shown at right.

Rutherford went on to discover the proton in 1919. Rutherford then noticed that the mass of most nuclei was greater than the mass of the protons, leading him to predict the existence of a neutral particle in the nucleus. This was confirmed with the discovery of the neutron by James Chadwick in 1932.

A multitude of particlesThe discovery of the nucleus was a tremendous step in the history of science, but it created a mystery that would go unsolved for nearly 60 years. Because of the electromagnetic force, each proton is repelled from other protons by a force that increases

Fermilab particle accelerator

The Standard Model

The “standard model” of particle physics includes three classes of fundamental particles—force carrier particles, quarks, and leptons—and at least 16 fundamental particles. As ungainly as this model is, it has been tremendously successful in predicting the existence of new particles years before their actual discovery.

Today many physicists are looking for a theory of everything (TOE): a single theory that explains the existence of all of the particles and forces and unites

greatly as the distance between the protons is reduced. If the nucleus was composed of positively-charged protons and neutral neutrons, what kept it from flying apart?

To hold the nucleus together, scientists hypothesized that a binding force must exist between the protons and neutrons in the nucleus. This force, dubbed the strong nuclear force, would have to be stronger than the electromagnetic force but could only operate at extremely short distances.

The exact nature of this force was gradually revealed between 1945 and 1970. In this time period, a new branch of physics, particle physics, developed to discover and study the most fundamental particles in nature. To find these particles, physicists use particle accelerators, circular or linear tubes in which tiny particles are accelerated to nearly the speed of light. These particles are then smashed together with enormous energy, often producing new particles with exotic names such as muons, bosons, and neutrinos. There is also an antimatter particle for each particle: anti-protons, anti-electrons (also called positrons), and so forth.

While the leptons are currently considered fundamental particles because they have no internal structure and cannot be broken down, the hadrons are more massive particles that are thought to be made up of still smaller particles, called quarks. The existence of quarks was proposed by two scientists independently in 1964: Murray Gell-Mann and George Zweig.

Quarks are believed to be fundamental particles with very odd properties such as “color,” “flavor,” and “spin.” (Note: These words are used as analogies and should not be taken literally.) Quarks also have a fractional charge. Six flavors of quarks have been discovered so far.

Fundamental forcesIn addition to particles, particle physicists have studied the forces between particles, which they call “interactions.” Particles interact by exchanging special particles called force carrier particles. According to the theory, each of the four fundamental forces (gravity, electromagnetism, the strong nuclear force and the weak nuclear force) is mediated by its own particle: gravity by gravitons, electromagnetism by photons, the strong force by gluons and the weak force by bosons. (Note: Gravitons are purely hypothetical and have not been discovered yet.) These particles, their relative strength, and their relative range are described below:

Interaction Relative strength Range Force carrier particleGravity 1 infinite graviton (hypothetical)

Electromagnetism 1 × 1036 infinite photon (discovered ~1923)Strong force 2 × 1038 ≈1 × 10-15 m gluon (1979)Weak force 2 × 1033 ≈ 1 × 10-18 m W± and Z bosons (1983)

The strong nuclear force arises from the forces between quarks, called “color forces.” Quarks within the same baryon must represent all three colors—red, green, and blue. Every so often two quarks will exchange gluons, changing color at the same time. Unlike other forces, the color force increases in strength as the quarks move farther apart within the hadron. As a result, quarks can float quite freely within the confines of the hadron, but can never escape—no “free quark” could ever be observed. Likewise gluons are not free to leave the boundaries of the hadron and thus do not directly participate in strong nuclear interactions.

Thus, the strong nuclear force that holds baryons (protons and neutrons) together in nuclei is actually a faint residue of the color forces holding quarks together. Because both quarks and gluons are confined inside hadrons, the forces between baryons are mediated by mesons such as pions and rho mesons. A proton and a neutron must constantly exchange mesons to stick together. (Imagine the two particles playing a game of tennis, with mesons as the tennis balls.) The forces that arise from these interactions are only a fraction as great as those that occur between quarks, but are still tremendously strong. Unlike the color forces, the strength of the strong force drops off sharply with distance and is only significant within the nucleus.

The Fundamental Forces and History of Gravity

Forces in NatureEvery process in the universe that we understand can be explained in terms of the four fundamental forces (strong nuclear, electromagnetic, weak nuclear and gravitational). Each force is defined by the way it interacts with particles to build up composite form of matter: protons, neutrons, nuclei, atoms, molecules, planets, star, and so on.

Force (Strongest to

weakest)

Range Characteristics

Strong Nuclear distances

Attracts neutrons and protons to each other

Electromagnetic

All distances (Inverse Square Law)

Attraction between positive protons and negative electrons and repulsion of like charges

Weak Nuclear distances

Responsible for radioactive (beta) decay

GravityAll distances (Inverse Square Law)

Attractive force between two pieces of matter

Gravitational ForceGravity is the most commonly known and observed force. It is an attractive force that draws masses to each other. Gravity is everywhere because it acts between and among all different masses. Gravity’s range is infinite, but the strength of the attraction decreases rapidly (as the square of) increasing distance. For this reason there is no such thing as zero gravity, instead astronauts might experience microgravity.

The force of gravity inside the atom is much weaker even than the weak force. It takes a relatively large mass to create enough gravity to make a significant force. We know particles inside an atom do not have enough mass for gravity to be an important force on the scale of atoms. Understanding how gravity works inside atoms is an unsolved mystery in physics.

Early concepts of gravityAlthough people have observed objects falling to Earth’s surface as long as they have existed, the first explanation of gravity is attributed to the Greek philosopher Aristotle (384–322 BCE). Aristotle believed in five elements, earth, water, air, fire, and aether, a crystalline substance that made up celestial objects. Each element occupied a sphere, with Earth in the center, followed by water, air, fire, and aether. Objects would move naturally to take their proper positions. Objects made of earth, such as rocks, tended to move toward their natural position at the center of the universe (towards Earth’s center, in other words.) Aristotle also believed that heavier objects would naturally fall more quickly than lighter objects and that a continuous force was required to maintain the motion of an object. Aristotle’s ideas about physics, all of which would eventually be proven wrong, were nevertheless tremendously influential.

Challenging Aristotle: Copernicus, Kepler, and GalileoA major challenge to Aristotle’s geocentric (Earth-centered) universe came from a Polish cleric named Nicholas Copernicus (1473–1543). Copernicus proposed a heliocentric, or Sun-centered system. Copernicus died shortly after publishing his book, De Revolutionibus Orbium Coelestium, in 1543. This heliocentric idea captured the imagination of two astronomers: Johannes Kepler and Galileo Galiei.

Johannes Kepler (1571–1630) was a German mathematician whose interest in astronomy sprung from a lifelong fascination with astrology. Unlike his mentor, Tycho Brahe, Kepler supported heliocentrism and dedicated his astronomical career to determining the laws that govern planetary motion.

Bust of Aristotle

Nicholas Copernicus, Johannes Kepler, and Galileo Galilei

Torsion balance: Gravitational attraction between the small weights (a and b) and the large weights (K) cause the string (cd) to twist.

An ellipse has two foci (F1 and F2). For any point on the ellipse, the sum of the distances to the focal points is constant: a1 + a2 = b1 + b2

After several years of analyzing Tycho’s observations of Mars, Kepler eventually determined that the orbit of Mars was an ellipse, a slightly flattened circle.

Kepler went on to formulate three laws of planetary motion:

The orbits of planets are elliptical with the Sun at one focus of the ellipse.

If you drew a line from the planet to the Sun, it would mark out equal areas in equal times.

The square of the period of a planet’s orbit is proportional to the cube of its average distance from the Sun.

Kepler was one of the first astronomers to hypothesize that the attractive force of the Moon caused the tides, a conjecture that has proven to be true. Kepler’s laws were essential in allowing Newton to develop his gravitational theory a century later.

Galileo Galilei (1564–1642) was an Italian mathematician and astronomer with a knack for showmanship and a nose for controversy. Galileo’s astronomical discoveries provided crucial evidence for heliocentrism. In 1609, Galileo began to observe the night skies with a telescope of his own design. He observed craters and plains on the Moon Jupiter’s moons, and tracked sunspots to measure the rotation of the Sun. One of Galileo’s Moon sketches is shown at right.

Galileo also observed Venus passing through a full range of phases, from crescent to full. These observations demonstrated conclusively that Venus orbited the Sun and suggested that Earth orbited the Sun as well. While Galileo was never able to conclusively prove that Earth orbited the Sun, his work helped to popularize heliocentric theory, especially among some of the leading scientists of Europe.

Universal gravitationFrench philosopher René Descartes (1596–1650). Descartes theory had two main tenants: 1) The universe is filled with particles, and 2) objects have a tendency to move away from the center of rotation. Objects fall to Earth’s surface because the particles surrounding Earth had a greater tendency to move outward than objects, so the objects were displaced downward.

Isaac Newton (1643–1727) was immediately attracted to the heliocentric ideas of Copernicus, Kepler, and Galileo. Newton began to think about the force that caused planets to orbit the Sun and the force that caused objects to fall to the ground. He sought a mathematical proof that a force of attraction was sufficient to produce circular and elliptical orbits of planets. He fleshed out the mathematics of this idea and published it in 1687. Although Newton is generally

given sole credit for the law of universal gravitation, there is some controversy about his claim as Newton’s contemporary Robert Hooke was working on similar ideas at the time. In 1666, Hooke discussed the gravitational attraction of the Sun on the planets in a lecture, and he proposed that the force of gravity is proportional to the inverse square of the distance in a letter to Newton in 1679. Whomever came up with the idea first, there is no doubt that Newton was the first to mathematically prove that this force results in elliptical orbits.

Although unknown when the law of universal gravitation was first published, the value of G was determined thanks to an ingenious experiment by Henry Cavendish in 1797. Cavendish used a delicate instrument called a torsion balance (right) to measure the minute force between two lead balls. Because the force of gravity is so small, it can only be felt when at least one of the objects involved is very massive, such as Earth. Newton’s law of universal gravitation allowed Newton to prove Kepler’s three laws of planetary motion and explained Galileo’s observation that objects with different mass accelerate at the same rate.

Isaac Newton in 1689

René Descartes

General relativityWhile Newton’s theory proved very successful, during the 19th century astronomers discovered small discrepancies in the orbit of Mercury that could not be explained by Newton’s theory. The problem was solved by Albert Einstein (1879–1955), one of the most original thinkers in the history of science.

Einstein’s theory of general relativity, first proposed in 1915, posits that all objects with mass caused a bending, or warping, of both space and time. To picture this in two dimensions, imagine a heavy bowling ball resting on a thin, elastic sheet, causing it to bend downward. If a smaller ball is then rolled across the sheet, its path will be bent as it enters the depression caused by the first ball. This is shown on the diagram.

The path of a small celestial body (green) is bent by the gravitation of a massive planet or star

One advantage of general relativity over Newton’s theory of gravity is that relativity actually provides a mechanism for gravity to work. If space itself is bent by gravity, the curved paths of objects in orbit are equivalent to straight paths in curved space. In other words, gravity is not a “force” that acts over a distance, but a warping of the fabric of space that causes objects to appear as if they are influenced by a force.

An interesting aspect of Einstein’s theory of general relativity is that, because space itself becomes curved, the path of light is bent by gravity. Thus light from distant stars located behind the Sun would be bent by the gravitation of the Sun. The solar eclipse of 1919 provided a way to test the predictions of general relativity and universal gravitation by allowing scientists to observe starlight from stars behind the Sun. Precise measurements made by Sir Arthur Eddington during the eclipse agreed with the predictions of general relativity and made Einstein an international sensation. Since then, numerous observations have confirmed general relativity and cemented its status as one of the seminal advances of 20th century physics.

Although the law of universal gravitation has been replaced by general relativity, universal gravitation still has practical value. In the vast majority of applications, Newton’s law is both accurate and much easier to use than the equations of general relativity. Thus Newton’s law is still used by scientists when calculating the trajectories of planets, moons, or spacecraft.

Gravity todayDespite the centuries of progress made by countless scientists, gravity remains one of the most mysterious phenomena in nature. One major problem is the seeming incompatibility of general relativity and quantum mechanics, the other great physics theory of the 20th century. Many scientists believe that the unification of gravity and quantum mechanics can be done using string theory, an idea that all fundamental particles are actually vibrating lines, or “strings.”

Another mystery of gravity is the observed acceleration of the expansion of the universe. After the Big Bang, astronomers believed that the gravity of all of the matter in the universe would cause the expansion of the universe to slow down. Instead, astronomers have discovered that the expansion of the universe is actually speeding up! Explanations for this acceleration have centered on such exotic concepts as “dark energy” and “dark matter.” Some scientists believe that these observations will cause future changes to our theories of gravity.

Albert Einstein in 1921

Some versions of string theory propose a universe with 11 dimensions.