According to the Oxford Dictionary of Physics, physics is “the study of the laws that determine the structure of the universe with reference to the matter and energy of which it consists. It is concerned with the forces that exist between objects, and the interrelationship between matter and energy. Until the early 20th century, physics was divided into six diverse areas of study: heat, light, magnetism, sound, and electricity. Since then quantum mechanics and Einstein’s theory of relativity have become separate fields of inquiry.” Modern physics also includes other subdivisions that are useful. Matter and energy are manifest in the interactions of subatomic particles and the nuclei of atoms as well as in the materials (condensed matter) that make up solids, liquids, and other forms of matter, leading to the branches of study known respectively as particle physics, nuclear physics, and condensed-matter physics. Relativity theory also predicted the expansion of the universe, linking physics for the first time to cosmology, the study of the universe as a whole. Recently the links between physics, astronomy, and cosmology have become tighter; nuclear and particle physics are needed to explain the stars and galaxies. At the same time, physicists who study advanced concepts such as string theory, which replaces particles with strings, now look to astronomy to validate their work.
Physics in Antiquity and the Middle Ages
The ancient Greeks tried, sometimes successfully, to explain materials and motion on the basis of observation and reasoning. One successful explanation, advocated by Democritus of Abdera (ca. 470–380 B.C.) and others, is that all matter is composed of small particles called atoms. Their theories proposed that different atoms have different shapes and that all materials can be based on atoms of fire, air, water, and earth. Today scientists recognize that most matter is made from combinations of nearly 100 different atoms, usually joined to form larger particles called molecules. (Traditionally there are 92 elements, although only 88 or so are normally found on Earth; there is hardly any astatine, francium, or protactinium, and no promethium or technitium. Some transuranic elements are manufactured in relatively large amounts, notably plutonium, americium, and californium. Neptunium, although considered artificial, may exist in greater supply on Earth than astatine, owing to some natural creation. Other synthetic elements are not stable enough to be counted.)
Aristotle The most complete Greek theory of physics, incorporating the ideas of earlier writers as well as his own, is that of Aristotle (384–322 B.C.). Most of what Aristotle thought about physics is now recognized as incorrect. Aristotle rejected atoms because any space between atoms must be empty, but he had based his theories on the idea that a vacuum cannot exist. He thought that an object in motion requires a continuing force to keep it in motion. If an object is moving fast enough, he conjectured, air rushing to prevent a vacuum behind the object could provide the necessary force for a time, but that force would gradually diminish. So a thrown object could travel through the air, but eventually would drop to the ground. The object slows and falls, in Aristotle’s view, because the natural place for material objects containing earth or water is toward the center of Earth.
Aristotle remained the main influence on physics for the next 2,000 years. Although some Chinese philosophers developed ideas of motion similar to those we use today, they were unknown in the Arab world or in Europe. Arabic scholars followed the ideas of Aristotle and other Greek philosophers, but they also advanced beyond Greek concepts in some areas of physics, notably optics (the science of light). Aristotle had believed that light travels from an object to the eye, but other influential writers of antiquity, such as Euclid (fl. ca. 300 B.C.) and Ptolemy (ca. 100–170), thought that light proceeds from the observer to the object. The decisive arguments in favor of Aristotle’s view were made by Alhazen (Al-Haytham,
965–ca. 1040) around 1020. Alhazen extended the laws of reflection from those applying to flat mirrors, which had been known to Ptolemy, to cover curved mirrors and lenses.
In the Middle Ages, physics began to free itself from some of Aristotle’s inaccurate ideas about motion. The French philosopher Jean Buridan (ca. 1295–ca. 1358) was the first to propose that a body in motion contained a mysterious inner force, called impetus, that maintains motion for a time. As a body moves, the impetus dissipates, especially if some force opposes the motion, speeding dissipation.
The Scientific Revolution
Near the end of the 16th century, Galileo Galilei (1564–1642) accepted Aristotelian ideas and such modifications as impetus at first, but he soon brought a radical concept to studies of motion. Instead of trying to explain why objects move as they do, he experimented and then described exactly how they move. He also used experiment to determine how forces affect objects that do not move. Although he made some errors, his basic conclusion in 1590—that an object in motion continues to move in a straight line until stopped by a force—is still accepted. His other famous conclusion—that light bodies and heavy bodies fall through the same distance in the same amount of time—is also true. He had established it by experiment, and announced the correct mathematical law governing falling (distance increases with the square of time) in 1638.
Other scientists continued in the same vein as Galileo during the 17th century. The German astronomer Johannes Kepler (1571–1630) advanced optics and showed in 1604 that the intensity of light diminishes as the square of the distance from its source. In 1643 the Italian physicist Evangelista Torricelli (1608–47), with the invention of the barometer, showed that Aristotle had been wrong about the vacuum, since a vacuum forms above the mercury column in the original barometer. Blaise Pascal experimented with the vacuum and with fluids, establishing that in a fluid, force is transmitted in all directions and always acts perpendicular to the surface of the container (1654, published 1662). Isaac Newton experimented with breaking light into its components (1665) and reported that white light is the combination of the colored lights of the rainbow (1675).
This period when experiments began to dominate physics is known as the scientific revolution. It culminated in 1687 when Newton’s Principia emerged. Newton improved on Galileo’s laws of motion and combined them with a mathematical law of gravity. The combination was sufficient to explain not only the motions of objects on Earth, but also the motions of the heavenly bodies (see Law of Gravity and Newton’s Laws of Motion). Newton (and independently Leibniz) had also invented a new mathematical tool, the calculus. Throughout the 18th century, Newtonian physics and calculus were combined to develop systematically a wide range of topics in physics, ranging from acoustics to detailed orbits of the planets. Some scientists believed that if the exact position and momentum of every point in space were known, the future of the universe could be predicted exactly as well.
Electromagnetism Although Newton’s work explained how gravity functioned, it did not explain why material objects attract each other with this force. There were also other forces that were unexplained, and less was known of their rules. As early as 1600 William Gilbert applied the experimental method to two of these forces, identifying and differentiating between magnetism and static electricity. A hundred years later, scientists began to attempt to tease from nature the secrets of these forces. Weak electric charges were made by rubbing glass tubes with silk or by similar means at first. In 1729 another English experimenter, Stephen Gray (1666–1736), was the first to recognize that these weak charges could travel from one material to another through substances that were later called conductors. He soon showed that when conductors do not carry away the charge, almost anything—even a human being—could be charged with electricity. A French experimenter, Charles Du Fay (1698–1739), was the first to recognize that there are two kinds of charge and that like charges repel, whereas different charges attract, each other (1733).
The English and French experimenters and their assistants also began to build up charges strong enough to produce the first recognized shocks. In 1746 the invention of the way to store static electricity (called a Leiden jar after the site of its discovery) permitted experiments with much more powerful charges. In 1751 Benjamin Franklin connected the small shocks from Leiden jars with the powerful shock of lightning, proving his theory by flying a kite in a thunderstorm and conducting the charge down the wet string. In 1769 a Scottish scientist, John Robison (1739–1805), showed that the repulsive force caused by charge obeys an inverse-square law like the law for loss of intensity of light over distance.
A new source of electric charge began to be developed in Italy during the 1770s and 1780s when Luigi Galvani (1737–1798) investigated charge produced in the muscles of animals, which Alessando Volta (1745–1827) recognized as the result of chemical interactions. Volta in 1800 built a chemical device (similar to a modern automobile battery) that produced the first current electricity.
Meanwhile a parallel set of experiments with magnets began in 1749 when the English experimenters John Canton (1718-72) and John Michell (1724–93) developed stronger magnets than occur in nature. Michell immediately used his magnets to derive the mathematical laws of attraction and repulsion. In 1751 Benjamin Franklin showed that electric charge can produce magnetism. In 1785 the French physicist Charles Coulomb (1736–1806) carefully measured both electric and magnetic forces and also found that both obey exactly the same inverse-square laws. As early as 1807 the Danish physicist Hans Christian Oersted (1777–1851) began to search for a deeper connection between electricity and magnetism, which he found in 1820 when he observed that an electric current affects a magnetized needle. The recognition that electricity and magnetism are closely connected quickly led to the discovery of the laws governing electromagnetism (see Laws of Current Electricity) as well as to devices that combined the two forces to produce motion (electric motors), powerful electric currents (generators, or dynamos), and powerful electromagnets.
Light At the same time as charge and magnetism were being analyzed, there were apparently unrelated studies concerning light. As early as 1678, the Dutch physicist Christiaan Huygens (1625–95) had proposed a theory of light based on waves. But in 1704 Newton published Opticks, which summarized his view that light consists of small particles. About a hundred years later the study of light experienced several rapid advances. In 1800 and 1801 two forms of invisible light were discovered: infrared by William Herschel and ultraviolet by Johann Ritter (1776–1810). Also in 1801, the English scientist Thomas Young (1773-1829) conducted experiments that convinced scientists everywhere that light must be a wave phenomenon, a view reinforced in 1808 when the French physicist Etienne Malus (1775–1812) discovered polarized light, a form of light in which waves are confined to a plane.
It was already known that electric charge could in some circumstances produce light (in lightning, for example). In 1839 the French physicist Edmond Becquerel (1820–91) determined that the opposite also occurs in some circumstances; light produces electric current, known as the photovoltaic effect. A few years later Michael Faraday showed that a magnetic field changes the polarization of light (1845). With these discoveries in mind James Clerk Maxwell concluded that light consists of waves incorporating both electricity and magnetism—that is, electromagnetic waves. He predicted that electromagnetic waves exist in the electromagnetic spectrum below infrared and above ultraviolet radiation. In 1873 Maxwell published a complete mathematical theory of electromagnetism.
There were still mysteries. While setting up the equipment to produce and detect radio waves (the long electromagnetic waves predicted by Maxwell) in 1887, the German physicist Heinrich Hertz (1857–94) observed that light shining on the apparatus affects the size of an electric spark. Further investigation with more energetic electromagnetic radiation revealed that the amount of charge released by the metal depends on the frequency rather than the intensity of the radiation, a finding which made no sense at first. The problem was resolved in 1905 when Albert Einstein proved that light, as Newton had proposed, behaves in this case as a particle instead of as a wave.
Heat As early as 1724 scientists tried to explain heat and cold with the idea that heat is an unusual component of matter, similar to a liquid, which they called caloric. Caloric persisted throughout the 18th century until a decisive experiment by Benjamin Thompson (Count Rumford, 1753–1814) showed that heat is closely connected to motion. Scientists since have believed heat to be an effect of the motion of molecules in any substance (cold is simply the absence of heat, or slower molecular motions), but it was not until 1860 that James Clerk Maxwell and, independently, the Austrian physicist Ludwig Boltzmann (1844—1906) worked out the mathematical theory of such particles.
Meanwhile, physicists were discovering the general laws of heat. The French physicist Sadi Carnot (1796–1832), after studying the still new steam engines, established mathematically in 1824 that work is done as heat passed from a high temperature to a lower one and that the maximum amount of work possible depends only on the temperature. Heat was recognized as a form of energy, along with motion, electricity, light, and stored, or potential, energy. Several English and German physicists measured exactly the amount of heat produced by motion, work that led to the laws of thermodynamics (“movement of heat”). With the new understanding of heat the British
physicist William Thomson (Baron Kelvin, 1824–1907) recognized in 1851 that the total absence of heat would produce a specific coldest temperature, absolute zero.
Experimentalists used various methods to lower temperatures nearly to absolute zero, liquefying air in 1878, hydrogen in 1895, and helium, the element that has the coldest known transition from a gas to a liquid, in 1908.
With liquid helium near absolute zero, strange new forms of matter could be created. One of the most important is matter that superconducts—an electric current started in a ring of a superconducting material will continue around the ring as long as the temperature is maintained at a few degrees above absolute zero. Since the Dutch physicist Heike Kamerlingh-Onnes (1853–1926) discovered the first form of superconductivity in 1911, other materials, called high-temperature superconductors, have been found (starting in 1986), although none are superconducting at temperatures above –200°F (–130°C). Liquid helium itself was found to have unusual properties similar to those of superconductors, such as superfluidity. Like some very cold gases, first produced in 1995, liquid helium is a Bose-Einstein condensate (BEC), matter in which the atoms merge into a single superatom, first predicted by Albert Einstein in 1924.
Relativity Maxwell’s theory of electromagnetism (proposed in 1873) assumed that electromagnetic waves must be motions in some all-pervasive but undetectable substance, which was called ether. Various attempts were made to define the properties of ether and, in a famous failed experiment of 1888, to determine Earth’s motion through the ether. The Polish-American physicist Albert Michelson (1852–1931) and the American physicist Edward Morley (1838–1923) used a sensitive device invented by Michelson to measure the speed of light in the direction of Earth’s motion through space and perpendicular to that motion, but failed to find any difference, suggesting that ether was a flawed concept. When Einstein developed the special theory of relativity (1905), however, he concluded that electromagnetic waves do not need ether to explain their properties. He took as a postulate that light travels through a vacuum at the same speed under all conditions; thus you cannot determine how Earth is moving by looking for variations in the speed of light that such motion would cause. He also observed that physical laws as measured should be the same for two entities moving with respect to each other with no change in velocity. From these ideas he concluded that the universe can be described in terms of four-dimensional space-time and that matter and energy are related by the famous equation E = mc2, where E is energy, m is mass, and c is the speed of light in a vacuum. Relativity theory also showed that time can be viewed as a dimension related to the dimensions of space. A definition of modern physics, then, might be that it is the study of matter-energy in space-time.
Next Einstein considered what happens if one entity is accelerated with relation to the other. He based this theory, the general theory of relativity, on the idea that no test can determine a difference between gravitational force and the force produced by acceleration, called inertia.
The general theory of relativity, which resulted from this postulate in 1915, is a description of gravity in terms of the curvature of space-time. Einstein’s theory explained previously observed, but unexplained, changes in the orbit of Mercury and in 1919 described how light from a star was bent by the sun’s gravitational field. Almost as soon as the general theory was published, it became clear that the theory as originally formulated predicted an expanding universe and also predicted the existence of what we now call black holes (1917), stars that have collapsed into points with such a strong gravitational force that light cannot escape. In 1979 another effect predicted by the theory, the lensing effect caused by the gravitational force of an entire galaxy, was observed for the first time; since then, gravitational lenses have become one of the principal tools astronomers use to observe the early universe.
Einstein thought in 1917 that the universe should be static, but assumed that gravity would cause the universe to be contracting. He interpreted his original equations as showing a universe that is slowly collapsing. To resolve this, he added a “cosmological constant” to the equations for general relativity to provide a small force opposing gravity. Einstein was clearly wrong about the possibility of gravitational collapse, for the Dutch physicist Willem de Sitter (1872–1934) showed in 1919 that Einstein’s equations without the cosmological constant actually predicted an expanding universe. When expansion of the universe was observed by astronomers in the 1920’s, Einstein abandoned the cosmological constant. In recent years, however, astronomers have detected an acceleration of the expansion of the universe. The mysterious force that causes this expansion is called “dark energy.” Some physicists think that dark energy is evidence suggesting
that Einstein’s cosmological constant was correct and should be reinstated.
Particles and Quantum Theory Several Greek and Roman writers had a theory that matter is made from small, indivisible particles; this theory was revived in 1803 as atomic theory by the British chemist John Dalton (1766–1844). During the 19th century, the idea of indivisible atoms came to be accepted, but near the end of the century evidence emerged that atoms themselves are made from even smaller particles. The electron was discovered by the English physicist J. J. Thomson (1856–1940) in 1897 and measured to be smaller by far than the smallest atom. Two years later, Thomson showed that the electron is a part of the atom.
Because electrons have a negative charge, but atoms are electrically neutral, it was apparent that there must be some particle (or other entity) in the atom with a positive charge toto neutralize the charge of the electron. By 1911 the New Zealand-born British physicist Ernest Rutherford (1871–1937) had established that the positive charge is carried by a particle much heavier than the electron; he named this new particle the proton. The Danish physicist Niels Bohr (1885–1962) developed the mathematical theory of hydrogen, which has the simplest atom, in 1913. He found that the theory was correct in terms of experiment only if he used the idea that electrons can travel in only a few orbits and that they must be able to change from one orbit to another instantly (giving off or absorbing light in the process).
The idea that light energy has only separate (discrete) levels had first been used in 1900 to explain the spectrum of light emitted as a body is heated. The discrete levels were called quanta by the German physicist Max Planck (1858–1947), who had developed this theory. In 1905 Einstein used the same idea to explain the phenomenon discovered by Hertz in 1887, showing that light behaves like particles (quanta of light). Bohr showed that electron orbits are also quanta. Thus the theory of particle behavior is called the quantum theory.
Quantum theory advanced rapidly in the 1920s, beginning with the idea of the French physicist Louis de Broglie (1892–1987) that particles such as the electron have a wave aspect. The following year the Pauli exclusion principle (see “Two Basic Laws of Quantum Physics”) and the matrix theory of the electron were established, along with the concept of particle spin. In 1926 the German physicist Erwin Schrödinger developed the equation of the electron wave. In 1927 the Heisenberg uncertainty principle was introduced. During this period, the only known particles were the photon, electron, and proton, but in 1930 Wolfgang Pauli (1900–58) proposed the neutrino, which was followed by dozens of other particles (see Subatomic Particles). Quantum theory was cast into the more precise form called quantum electrodynamics in 1947, when several physicists developed mathematical techniques to resolve problems with the original quantum theory.
Nuclear Physics Radioactivity, which was discovered in 1896 by the French physicist Henri Becquerel (1852–1908), was the key to discovery of the proton and the concept that each atom has a positive nucleus surrounded by negative electrons. The study of the nucleus could not advance much until the discovery, in 1932, of the neutral particle the neutron, which is part of the nucleus in all atoms but the simplest hydrogen atom. Different forms of the same element, called isotopes, have the same number of protons in the nucleus, but different numbers of neutrons.
The French wife-and-husband team Irène Joliot-Curie (1897-1956) and Frédéric Joliot-Curie (1900–58) showed in 1934 that an element can be changed to a radioactive isotope by bombarding the atoms with neutrons. In 1937 the Italian-American physicist Emilio Segrè used the same idea to produce a previously unknown artificial element, technetium. In 1940 the first artificial element with an atomic number higher than that of uranium was created and named neptunium, element 93. The following year element 94, plutonium, joined the list. Today there are artificial elements through element 116.
In 1938 the German physicist Otto Hahn (1879–1968) and the Austrian physicist Lise Meitner (1878–1968) discovered that the large uranium atom could break into pieces (fission) when stuck with a neutron, releasing additional neutrons and other forms of energy in the process. This discovery led to the atomic, or nuclear fission, bomb and nuclear power (see “Technology”). Also in 1938 two physicists, Hans Bethe and Carl von Weizsäcker, proposed that in the intense heat and pressure of the interior of a star, hydrogen nuclei combine with each other to form helium (fusion), releasing energy in the process. This process also led to the development of a fusion bomb (the hydrogen bomb, 1952).
In the last decades of the 20th century physicists developed the standard model of elementary particles. This model incorporates three of the four fundamental forces in nature: the strong and weak nuclear forces and electromagnetic force (the other force is gravity). The standard model has so far stod up to testing, thought the goal for scientists is to incorporate gravity into the standard model, creating a “theory of everything” (see “String Theory and Supersymmetry” and “Subatomic Particles”).
Times focus
The Large Hadron Collider and the Higgs Particle
From NYTimes.com, “Times Topics”
Call it the Hubble Telescope of Inner Space.
The Large Hadron Collider, located 300 feet underneath the French-Swiss border outside Geneva, is the world’s biggest and most expensive particle accelerator. It is designed to accelerate the subatomic particles known as protons to energies of 7 trillion electron volts apiece and then smash them together to create tiny fireballs, recreating conditions that last prevailed when the universe was less than a trillionth of a second old.
Whatever forms of matter and whatever laws and forces held sway Back Then—relics not seen in this part of space since the universe cooled 14 billion years ago—will spring fleetingly to life. If all goes well, they will leave their footprints in four mountains of hardware and computer memory that international armies of physicists have erected in the cavern.
After 16 years and $10 billion, on March 30, 2010, the collider finally began its work of smashing subatomic particles. The day was a milestone—delayed a year and a half by an assortment of technical problems—and brings closer a moment of truth for CERN and for the world’s physicists, who have staked their credibility and their careers, not to mention all those billions of dollars, on the conviction that they are within touching distance of fundamental discoveries about the universe. If they fail to see something new, experts agree, it could be a long time, if ever, before giant particle accelerators are built on Earth again, ringing down the curtain on at least one aspect of the age-old quest to understand what the world is made of and how it works.
“If you see nothing,” said John Ellis, a theoretical physicist at CERN, “in some sense then, we theorists have been talking rubbish for the last 35 years.”
Machines like CERN’s new collider get their magic from Einstein’s equation of mass and energy. The more energy that these machines can pack into their little fireballs, in effect the farther back in time they can go, closer and closer to the Big Bang, the smaller and smaller things they can see.
The new hadron collider, scientists say, will take physics into a realm of energy and time where the current reigning theories simply do not apply, corresponding to an era when cosmologists think that the universe was still differentiating itself, evolving from a primordial blandness and endless potential into the forces and particles that constitute modern reality.
One prime target is a mysterious particle called the Higgs that is thought to endow other particles with mass, according to the reigning theory of particle physics, known as the Standard Model. That theory will now face its most severe test. Other theories go beyond this model to predict new forms of matter that explain the mysterious dark matter waddling the cosmos and even new dimensions of space-time.
The guts of the collider are some 1,232 electromagnets, thick as tree trunks, long as boxcars, weighing in at 35 tons apiece, strung together like an endless train stretching around the gentle curve of the CERN tunnel.
In order to bend 7-trillion-electron-volt protons around in such a tight circle these magnets, known as dipoles, have to produce magnetic fields of 8.36 Tesla, more than 100,000 times the Earth’s field, requiring in turn a current of 13,000 amperes through the magnet’s coils. To make this possible the entire ring is bathed in 128 tons of liquid helium to keep it cooled to 1.9 degrees Kelvin, at which temperature the niobium-titanium cables are superconducting and pass the current without resistance.
Running through the core of this train, surrounded by magnets and cold, are two vacuum pipes, one for protons going clockwise, the other counterclockwise. Traveling in tight bunches along the twin beams, the protons will cross each other at four points around the ring, 30 million times a second. During each of these violent crossings, physicists expect that about 20 protons, or the parts thereof—quarks or gluons—will actually collide and spit fire. It is in vast caverns at those intersection points that the detectors, or “sunken cathedrals” in the words of a CERN theorist, Alvaro de Rujula, are placed to capture the holy fire.
The payoff for this investment, physicists say, could be a new understanding of one of the most fundamental of aspects of reality, namely the nature of mass. This is where the shadowy particle known as the Higgs boson, a.k.a. the God particle, comes in.
In the Standard Model, a suite of equations describing all the forces but gravity, which has held sway as the
law of the cosmos for the last 35 years, elementary particles are born in the Big Bang without mass.
Some of the particles acquire their heft, so the story goes, by wading through a sort of molasses that pervades all of space. The Higgs process, named after Peter Higgs, a Scottish physicist who first showed how this could work in 1964, has been compared to a cocktail party where particles gather their masses by interaction. The more they interact, the more mass they gain.
The Higgs idea is crucial to a theory that electromagnetism and the weak force are separate manifestations of a single so-called electroweak force. It shows how the massless bits of light called photons could be long-lost brothers to the heavy W and Z bosons, which would gain large masses from such cocktail party interactions as the universe cooled.
The confirmation of the theory by the Nobel-winning work at CERN 20 years ago ignited hopes among physicists that they could eventually unite the rest of the forces of nature.
Moreover, Higgs-like fields have been proposed as the source of an enormous burst of expansion, known as inflation, early in the universe, and, possibly, as the secret of the dark energy that now seems to be speeding up the expansion of the universe. So it is important to know whether the theory works and, if not, to find out what does endow the universe with mass.
But nobody has ever seen a Higgs boson, the particle that personifies this molasses. It should be producible in particle accelerators, but nature has given confusing clues about where to look for it. Measurements of other exotic particles suggest that the Higgs’s mass should be around 90 billion electron volts, the unit of choice in particle physics. But other results, from the LEP collider before it shut down in 2000, indicate that the Higgs must weigh more than 114 billion electron volts. By comparison, an electron is half a million electron volts, and a proton is about 2,000 times heavier.
The new collider was specifically designed to hunt for the Higgs particle, which is key to the Standard Model and to any greater theory that would supersede it. The Tevatron is also searching for the Higgs.
Theorists say the Higgs or something like it has to show up simply because the Standard Model breaks down and calculations using it go kerflooey at energies exceeding one trillion electron volts. If you try to predict what happens when two particles collide, it gives nonsense, explained Dr. Ellis.
Condensed Matter Nuclear and particle physics apply to what occurs within atoms and in isolated subatomic particles but do not explain the behavior of surface interactions, of clusters of small numbers of atoms or molecules, of complex molecular structures such as colloidal solutions or foams, or of electromagnetic phenomena in solids or liquids. Physicists have come to refer to the branch of the science that is concerned with the collective behavior of many particles as “condensed matter” physics. Today condensed-matter physics is one of the most active areas of the science.
Although scientific studies of magnetism and static electricity began in 1600, the first accurate theory of the cause of magnetism was that of the French physicist André-Marie Ampère (1775-1836) in 1825. Michael Faraday (1791–1867) recognized in 1845 that there are several magnetic effects, including diamagnetism (opposition to a magnetic field), paramagnetism (which disappears when a magnetic field is removed), and ferromagnetism (the familiar “permanent” magnetism that can be induced in iron and some other metals). Another major advance occurred in 1907 when the French physicist Pierre-Ernest Weiss (1865–1940) explained ferromagnetism as the effect produced when many small regions, called domains, become aligned by a magnetic field.
Early experimenters with static electricity observed that some substances—notably metals—conduct electricity and others are insulators. But not until 1900 did the German physicist Paul Drude (1863–1906) establish that in conductors some electrons are free to move away from their atoms, carrying negative charge with them. When quantum theory was developed, the Russian-German physicist Arnold Sommerfeld (1868–1951) developed in detail the theory of how electrons behave in a conductor. But there were still mysteries unsolved, for superconductivity was not explained until 1957, and high-temperature superconductivity still lacks a satisfactory explanation.
Understanding how conductors and insulators work led to a better understanding of semiconductors. This provided the background for the development in 1947 of the transistor and for subsequent applications of semiconductors, including some types of lasers and light-emitting diodes. Today condensed-matter physicists are applying the concept of spin to produce the effective disk drives in modern computers and look forward to using the electronics of spin, called spintronics, to develop improved devices that accomplish the tasks of transistors and their variants better and faster.
Physics and Other Disciplines Physics is a fundamental underpinning of most science other than the studies of human beings and some theories concerning living organisms, and sometimes physics becomes completely combined with parts of other sciences. Three notable examples are combinations of physics with astronomy, earth science, and biology.
Astrophysics is the study of stars, gas clouds, and other astronomical bodies, based on the application of the laws of physics, including energy production, composition, and evolution. While a broad view of astrophysics would include virtually all of astronomy, the disciple was originally concerned primarily with energy production and the development of stars from gas clouds through several stages such as red giants or white dwarfs to concluding explosions as supernovas or collapse into burned-out cinders or black holes. In recent years, the evolution of the universe as a whole (cosmology) has become a central focus of many astrophysicists; cosmology includes the development of subatomic particles in the early universe and the possible roles of subatomic particles and physical forces in such concepts as dark matter or the unknown energy that is accelerating the expansion of the universe.
Geophysics is the study of the structure of Earth based on the application of physical laws to Earth’s shape, seismology, electromagnetic properties, oceans, and atmosphere. The methods of geophysics have revealed Earth’s layered structure, consisting of inner and outer cores, mantle, and crust, and have provided the theoretical basis of plate tectonics. In recent years, the definition of geophysics has been stretched to include the physical properties of planets other than Earth as well as of the satellites of planets.
Biophysics is the study of such physical processes as transport of materials in living organisms, growth of such organisms, and their structural stability in terms of the laws of physics. Of particular concern are transport of ions across cell membranes and the mechanisms of protein folding along with the physics of such imaging techniques as CT, MRI, and PET scans.
String Theory and Supersymmetry Although quantum theories of particle physics explain many phenomena and allow interactions to be calculated to a high degree of accuracy, some of the mathematics involved has been viewed as questionable. Positive and negative infinities are added in such a way that their difference nearly cancels, but leaves a tiny amount that is exactly the amount measured by experiment. Also, physicists since Einstein have hoped to develop a unified theory that would include relativity and quantum mechanics as the logical outgrowth. Several developments since 1970 have attempted to resolve the mathematics and unify the various theories. The first was string theory, which replaced the concept of particles with one-dimensional strings whose properties are mathematically tractable, but only in spaces with more than four dimensions. In 1974 this was joined with a theory that every particle has a partner—if one particle represents matter, then the other represents force, and vice versa. This symmetry, called supersymmetry, called for a wide range of new particles that had not been previously observed. Two years later the recognition that certain strings behave like the graviton, a particle predicted by general relativity theory, led to combining relativity with string theory in a theory called supergravity. By 1984 string theory and supersymmetry had also been combined to create superstring theory—strings instead of particles, very massive and unknown partners for every known string, and all in ten- or eleven-dimensional space. The dimensions above the three known dimensions of space and the dimension of time are also unobserved and thought to be curled so tightly that they are too small to observe. In 1995 the American physicist Edward Witten (b. 1951) extended the symmetric theory of supergravity to a theory in which the fundamental entities are membranes in eleven-dimensional space. Variations on this concept, known as M theory or brane theory, remain the most popular concept of the underlying reality of the universe, called the “theory of everything,” for today’s theoretical physicists, although these theories are hampered by the inability of experimenters to prove or disprove them.
Key Terms
Mass is a measure of the amount of matter; it is proportional to weight. Near the surface of Earth it is roughly equivalent to weight.
Velocity measures how an object changes position with time.
Acceleration is how an object changes velocity with time.
Momentum is the product of mass and velocity.
Energy is the ability to do work.
Law of Gravity
The gravitational force between any two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them. If F is the force, G is the number that represents the ratio (the gravitational constant), m and M are the two masses, and r is the distance between the objects:
In metric measure, the gravitational constant is 0.0000000000667390 (6.67390 × 10–11) newton m2/kg2, so another way of writing the basic law of gravity is
This law implies that objects falling near the surface of Earth will fall with the same rate of acceleration (ignoring drag caused by air). This rate is 32.174 feet per second per second (ft/sec2), or 9.8 m/sec2, and is conventionally labeled g. Applying this rate to falling objects gives the velocity, v, and distance, d, after any amount of time, t, in seconds. If the object starts at rest and 32 ft/sec2 is used as an approximation for g,
v = 32t
d = 16t2
d = 16t2
For example, after 3 seconds, a dropped object that is still falling will have a velocity of 32 × 3 = 96 feet per second and will have fallen a distance of 16 × 32 = 144 feet.
If the object has an initial velocity vo and an initial height above the ground of a, the equations describing the velocity and the distance, d, above the ground (a positive velocity is up and a negative velocity is down) become
v = vo–32t
and
d =–16t2 + vo t + a
After 3 seconds, an object tossed in the air from a height of 6 feet with a velocity of 88 feet per second will reach a speed of 88 – 96 = –8 feet per second, meaning that it has begun to descend, and will have a height of (–16 × 9) + (88 × 3) + 6 =–144 + 264 + 6 = 126 feet above the ground.
The maximum height, H, reached by the object with an initial velocity vo and initial height a is
For the object tossed upward at 88 feet per second from a height of 6 feet, the maximum height reached would be 6 + 882/64 = 6 + 121 = 127 feet. Therefore, after 3 seconds, the object has just reached its peak and has fallen back only 1 foot.
Albert Einstein’s general theory of relativity introduced laws of gravity more accurate than those just given, which were discovered by Sir Isaac Newton. Newton’s gravitational theory is extremely accurate for most practical situations, however. For example, Newton’s theory is used to determine how to launch satellites into proper orbits.
Newton’s Laws of Motion
Newton’s laws of motion apply to objects in a vacuum and are not easily observed in the real world, where forces such as friction tend to overwhelm the natural motion of objects. To obtain realistic solutions to problems, however, physicists and engineers begin with Newton’s laws and then add in the various forces that also affect motion.
1. Any object at rest tends to stay at rest. A body in motion moves at the same velocity in a straight line unless acted upon by a force. This is also known as the law of inertia. Note that this law implies that an object will travel in a curved path only so long as a force is acting on it. When the force is released, the object will travel in a straight line. A weight on a string swung in a circle will travel in a straight line when the string is released, for the string was supplying the force that caused circular motion.
2. The acceleration of an object is directly proportional to the force acting on it and inversely proportional to the mass of the object. This law, for an acceleration a, a force F, and a mass m, is more commonly expressed in terms of finding the force
when you know the mass and the acceleration. In this form it is written as
F = ma
The implication of this law is that a constant force will produce acceleration, which is an increase in velocity. Thus a rocket, which is propelled by a constant force as long as its fuel is burning, constantly increases in velocity. Even with an infinite supply of fuel, the rocket would eventually cease to increase in velocity, however, because Einstein’s other relativity theory, the special theory of relativity, states that no object can exceed the speed of light in a vacuum (see “Conservation of Mass-Energy”). Nevertheless, even a small force, constantly applied, can cause a large mass to reach velocities near the speed of light if enough time is allowed.
3. For every action there is an equal and opposite reaction. (See “Conservation of Momentum.”)
Conservation Laws
Many results in physics come from various conservation laws. A conservation law is a rule that a certain entity must not change in amount during a certain class of operations. All such conservation laws treat closed systems. Anything added from outside the system could affect the amount of the entity being conserved.
Conservation of Momentum In a closed system, momentum stays the same. This law is equivalent to Newton’s third law of motion. Since momentum is the product of mass and velocity, if the mass of a system changes, then the velocity must change. For example, consider a person holding a heavy anchor in a stationary rowboat in the water. The momentum of the system is o, since the masses have no velocity. Now the person in the rowboat tosses the anchor toward the shore. The momentum of the anchor is now a positive number if velocity toward the shore is measured as positive. To conserve momentum, the rowboat is accelerated in the opposite direction, away from the shore. The positive momentum of the anchor is balanced by the negative momentum of the rowboat and its cargo. In terms of two masses, m and M, and matching velocities v and V,
mv = MV
Conservation of Angular Momentum An object moving in a circle has a special kind of momentum, called angular momentum. As noted above, motion in a circle requires some force. Angular momentum combines mass and velocity with distance from the point the object is spinning around. For a body moving in a circle, the acceleration depends on both the speed of the body in its path and the radius of the circle. The product of this speed, the mass, and the square of the radius is the angular momentum of the mass.
In a closed system, angular momentum is conserved. This effect is used by skaters to change their velocity of spinning. Angular momentum is partly determined by the masses of a skater’s arms combined with the rate of rotation and the square of the radius to the center of mass of each arm (the point that can represent the total mass of the arm). When skaters bring their arms close to their body, this would tend to reduce the angular momentum, because the center of mass is closer to the body. But, since angular momentum is conserved, the rate of rotation has to increase to compensate for the decreased radius. Because the rate depends on the square of the radius, the rate increases exponentially.
Conservation of Mass In a closed system, the total amount of mass appears to be conserved in all but nuclear reactions and other extreme conditions.
Conservation of Energy In a closed system, energy appears to be conserved in all but nuclear reactions and other extreme conditions. Energy comes in very many forms: mechanical, chemical, electrical, heat, and so forth. As one form is changed into another (excepting nuclear reactions and extreme conditions), this law guarantees that the total amount remains the same. Thus, when you change the chemical energy of a dry cell into electrical energy and use that to turn a motor, the total amount of energy does not change (although some becomes heat energy—see “Laws of Thermodynamics”).
Conservation of Mass-Energy Einstein discovered that his special theory of relativity implies that energy and mass are related. Consequently, mass and energy by themselves are not conserved, since one can be converted into the other. Mass and energy appear to be conserved in ordinary situations because the effect of Einstein’s discovery is very small most of the time. The more general law, then, is the law of conservation of mass-energy: The total amount of mass and energy must be conserved. Einstein found the following equation that links mass and energy:
E = mc2
In this equation, E is the amount of energy, m is the mass, and c is the speed of light in a vacuum.
One instance of energy changing to mass occurs in Einstein’s equation for how the mass increases with velocity. If mo is the mass of the object when it is not moving, v is the velocity of the object in relation to an observer who is considered to be at rest, and c is the speed of light in a vacuum, then the mass, m, is given by the equation
This accounts for the rule that no object can exceed the speed of light in a vacuum. As the object approaches this speed, so much of the energy is converted to mass that it cannot continue to accelerate.
In both nuclear fission (splitting of the atomic nucleus) and nuclear fusion (the joining of atomic nuclei, producing the energy of a hydrogen bomb), mass is converted into energy.
Conservation for Particles Many properties associated with atoms and subatomic particles are also conserved. Among them are charge, spin, isospin, and a combination known as CPT: charge conjugation, parity, and time.
First and Second Laws of Thermodynamics
First Law This is the same as the law of conservation of energy. It is a law of thermodynamics, or the movement of heat, because heat must be treated as a form of energy to keep the total amount of energy constant. All bodies contain heat as energy no matter how cold they are, although there is not much heat at temperatures close to absolute zero.
Second Law Heat in a closed system can never travel from a low-temperature region to one of higher temperature in a self-sustaining process. Self-sustaining in this case describes a process that does not need energy from outside the system to keep it going. In a refrigerator, heat from the cold inside of the refrigerator is transferred to a warmer room, but energy from outside is required to make the transfer happen, so the process is not self-sustaining.
The second law has many implications. One of them is that no perpetual motion machine can be constructed. Another is that all energy in a closed system eventually becomes heat that is diffused equally throughout the system, so that one can no longer obtain work from the system.
The equations that describe the behavior of heat also can be applied to order and therefore to information. The word entropy refers to diffuse heat, disorder, or lack of information. Another form of the second law of thermodynamics is that in a closed system, entropy always increases.
Laws of Current Electricity
Key Terms When electrons flow in a conductor, the result is electric current. The amount of current is based on an amount of electric charge called the coulomb, which is the charge of about 6.25 quintillion (6.25 × 1018) electrons. When 1 coulomb of charge moves past a point in 1 second, it creates a current of 1 ampere. Just as a stream can carry the same amount of water swiftly through a narrow channel or slowly through a broad channel, the energy of an electric current varies depending on the difference in charge between places along the conductor. This is called potential difference and is measured in volts. The voltage is affected by the nature of the conductors. Some substances conduct an electric current much more easily than others. This resistance to the current is measured in ohms. Electric power is the rate at which electricity is used.
Ohm’s Law Electric current is directly proportional to the potential difference and inversely proportional to resistance. If you measure current, I, in amperes, potential difference, V, in volts, and resistance, R, in ohms, then the current is equal to the potential difference divided by the resistance.
Law of Electric Power If electric power, P, is measured in watts, then the power is equal to the product of the current measured in amperes and the potential difference measured in volts.
P = IV
Laws of Light and Electromagnetic Radiation
Key Terms Light is a part of a general form of radiation known as electromagnetic waves, or, when thought of as particles, photons. Here, electromagnetic radiation is considered as a wave phenomenon for the most part. The velocity of a wave is how fast the wave travels as a whole. The wavelength is the distance between one crest of the wave and the next crest. The frequency is how many crests pass a particular location in a unit of time. One crest passing each second is called a hertz.
Law of Electromagnetic Energy The energy of an electromagnetic wave depends on a small number known as Planck’s constant. Measured in joules per hertz (energy per frequency), Planck’s constant is 6.672 59 × 10–34. The energy is equal to the product of Planck’s constant and the frequency. Using E for energy, h for Planck’s constant, and f for frequency,
E = hf
When thought of in terms of the particles called photons, the energy of a photon obeys the same law. The law of wave motion and the law of electromagnetic energy can be combined with the speed of light in a vacuum (c) to give
The energy of a photon is the product of Planck’s constant and the speed of light, divided by the wavelength (l) of the photon.
Inverse-Square Law All radiation obeys an inverse-square law, which is similar to the law of gravity. The intensity of the radiation decreases as the inverse of the square of the distance from a point source of radiation.
Two Basic Laws of Quantum Physics
When one considers effects on very small masses and at very small distances, it is necessary to recognize that objects behave differently from their action at sizes and distances one can observe directly. Since these effects occur in discrete steps based upon Planck’s constant times the frequency, called the quantum—which is the size by which energy changes in steps (instead of continuously) —the science of such effects is called quantum physics. Small masses act sometimes like particles and sometimes like waves. Two laws in particular that describe the behavior of small masses are basic and easily stated.
Heisenberg’s Uncertainty Principle It is impossible to specify completely the position and momentum of a particle, such as an electron, at the same time.
Pauli’s Exclusion Principle Two particles of matter cannot be in the same exact state. Particles of matter include the electron, neutron, and proton. Bosons, particles of force, do not obey Pauli’s exclusion principle. (see “Subatomic Particles.”)
The idea of an atom goes back to the ancient Greek philosophers, who thought that matter was composed of tiny indivisible particles. The concept was put on a scientific basis by John Dalton (1766–1844) in 1803 and became the foundation of chemistry. Nearly a hundred years later, experiments by J.J. Thomson (1856—1940) in 1899 were the first to show that atoms are not indivisible after all. In the past hundred years, physics at almost all levels has been completely revolutionized by the study of the particles that make up atoms or that are smaller than atoms. In the last decades of the 20th century physicists developed the standard model of elementary particles. This model incorporates three of the four fundamental forces in nature: the strong and weak nuclear forces and electromagnetic force (the other force is gravity). In the model, bosons mediate the forces: gluons for the strong nuclear force, the photon for electromagnetism, and W and Z particles for the weak nuclear force. Within this model, the weak and electromagnetic forces have been combined into electroweak theory. The standard model has thus far met all experimental challenges, but it has some gaps in addition to the omission of gravity: in particular, the strong and electroweak forces are not completely unified. Theories that attempt to unify the strong and electroweak forces are called grand unified theories (GUTs). Beyond grand unified theories, a great challenge for physicists is a “theory of everything” (TOE) that would account for all of the fundamental forces in nature. Below is a list of all the most important subatomic particles, in the chronological order of their discovery.
1897 Electron The first subatomic particle to be identified, also by J. J. Thomson, was the electron, a low-mass particle that can be found in the outer reaches of the atom. One property of the electron is charge, the response to electric or magnetic fields. The charge of a single electron is always the same, and is identified as–1 (negative one). Each atom consists of a cloud of electrons around a center of positive charge, which is called the nucleus.
1905 Photon The photon is the particle that carries the electromagnetic force. This concept began with Albert Einstein (1879–1955) in 1905, when he established that
light acts sometimes as a particle instead of as waves. Although we usually think of the photon as the particle of light, it is also the particle form of radio waves, X rays, or gamma rays. The mass of the photon is 0.
1911 Proton At least one proton is always found in the nucleus of every atom. The proton has a charge that is the same in size as that of the electron, but responds in the opposite direction to an electric or magnetic field. This charge is +1 (positive one). Each proton is almost 2,000 times as heavy as an electron, or about the same as the mass of a single hydrogen atom.
1924 Bosons While matter is made from subatomic particles, the forces that act on matter are also produced by subatomic particles. The particles that create these forces are collectively called bosons because the mathematics of the behavior of this type of particle was worked out originally by Satyendranath Bose (1894–1974) in 1924, although it was put into final form by Einstein. The observed bosons are the photon, pions, gluons, W particles, and Z particles. Bosons that are predicted, but that have not been observed, include the Higgs particle and the graviton.
1925–26 Quantum Mechanics The basic theory of subatomic particles, called quantum mechanics, was developed in two different forms, in 1925 by Werner Heisenberg (1901-76) and in 1926 by Erwin Schrödinger (1887–1961). Although the two forms appear very different, they produce the same results.
1926 Fermions All the particles that make up matter are called fermions, as opposed to the bosons that create forces. The fermions include all the leptons and quarks as well as the particles made from quarks (see below). Fermions are named for Enrico Fermi (Italian-American, 1901–54), who first worked out the mathematics of their interactions in 1926. Fermions all obey the Pauli exclusion principle (see Basic Laws of Physics, earlier in this chapter); that is, they occupy a definite space. Two fermions cannot be in the same place at the same time.
1930 Antiparticles When Paul A. M. Dirac (1902–84) completed his mathematical version of the theory of the electron in 1930, he observed that one solution to his equations predicted a particle that would be a mirror image of the electron, exactly the same as the electron but with a positive instead of a negative charge. The particle, discovered two years later in 1932, was named the positron. The same equations predicted mirror images for all subatomic particles. These particles are called antiparticles, so another name for positron is antielectron.
1932 Neutron The neutron is very much like a neutral proton, with just slightly more mass. Neutrons are stable when they are found in atoms, but decay into other particles when left to themselves.
1935 Muon The muon is now recognized as a high-energy analogue to the electron with a mass about 200 times that of the electron.
1947 Pion (Predicted in 1935.) A pion carries the strong force that holds the nucleus of atoms together, but since each pion appears and disappears almost instantly, the pions are not usually counted as part of the nucleus. In the same year that the pion was found, theoreticians were able to work out a comprehensive theory of the electron, called quantum electrodynamics (QED).
1950 Strange Particles Starting in 1950 experimenters observed a number of previously undetected particles that did not behave as particles were expected to. Because these particles have masses greater than that of the proton and neutron, they were called hyperons. Other unexpected particles, about the size of the pion, were classed as mesons. A classification scheme for the hyperons, developed in 1961, helped physicists understand them better, but their essential difference was already labeled “strangeness.”
1955 Neutrinos Neutrinos are thought to be among the most common particles in the universe, but they interact with ordinary matter so weakly that they are very difficult to observe. Predicted in 1930, neutrinos were thought to have no rest mass, but Canadian experiments in 2001 indicate that they have a very small mass equal to less than about 10–7 of the mass of an electron. Different neutrinos are associated with electrons, muons, and tauons.
1964-95 Quarks Murray Gell-Mann (b. 1928) and several other physicists determined that a way to explain the properties of protons, neutrons, mesons, and hyperons is to think of the heavy particles as made from combinations of light ones, just as the atom is made from combinations of electrons, protons, and neutrons. The smaller particles are quarks; there are six of them in all. Two quarks, known as up and down, form protons and neutrons. The top quark is the most massive—about as heavy as an atom of gold—and was the last to be detected. (First version of theory in 1964, evidence for top quark in 1995.)
1965–73 Gluons The eight different bosons that produce a force between quarks known as the color force are called gluons. The color force is also the basis of the strong force that holds the nucleus together. Because of the color force, the study of quarks and gluons is today called quantum chromodynamics (QCD).
1974 J/psi Particle Like the strange hyperons, the J/psi particle is a heavy particle that appears at high energies. It also is produced by a different kind of quark, the charm quark. The odd name J/psi comes from the particle’s having been discovered independently by two investigators, of whom one called it J and the other named it psi.
1983 W and Z Particles The particles that produce the weak force are called W and Z. At high energies, however, the weak force merges with the electromagnetic force, so that W and Z are to some extent analogues to the photon, although they could not be more different, since the photon has a 0 rest mass and both W particles and the single Z particle are very massive.
1995 Antiatoms Since antiparticles have all the properties of ordinary particles except for being mirror images, it is possible to create an antiatom by combining subatomic antiparticles. This was accomplished in 1995 with the production of a few antiatoms of antihydrogen made by causing an antielectron (positron) to orbit an antiproton.
(Not Yet Observed) Graviton and Higgs Particle A particle that produces gravitational force by its exchange between all kinds of particles is known as the graviton, but so far it is known only in theory. The Laser Interferometer Gravitational Wave Observatory (LIGO) that began operations in 2000 seeks to observe gravity waves, the wave version of the graviton. The Higgs particle is the main undetected particle of the standard model of subatomic particles. Physicists believe that the Higgs, named after Peter Higgs (b. 1919), who predicted it in 1964, confers mass on all other particles.
alternating current electric current in which an electric field flows first in one direction and then the opposite way at a constant frequency (in the United States, 60 cycles per second). It is produced by switching current back and forth at the source. Current supplied over power lines is alternating current because AC voltages can be stepped up for transmission and down for distribution to users, allowing current to travel farther with less loss.
anode positive electrode. In an electron tube the anode attracts electrons from the cathode.
atom electrically neutral particle that is the smallest part of a chemical element. Atoms are formed by a nucleus of at least one proton and, except for the hydrogen atom, one or more neutrons, surrounded by a cloud of exactly as many electrons as protons.
boiling point temperature at which the vapor pressure of a liquid equals that of the surrounding gas or vapor. At this temperature, a phase change begins, as the liquid boils and changes to a vapor (gas) state at a given pressure.
Bose-Einstein condensate phase of matter occurring at extremely low temperatures in systems consisting of large numbers of atoms in the same quantum state, so that they act like a single entity or superatom. The atoms must be bosons (particles with integral spin, which can apply to either atomic nuclei or atoms). The first Bose-Einstein condensate was created in 1995 by physicists at the Joint Institute of Laboratory Astrophysics, in Boulder, Colo.
capillary action process by which liquid in a very narrow tube rises against the pull of gravity. When the surface of a liquid is in contact with a solid, the liquid is elevated or depressed depending upon the relative attraction of the molecules of the liquid for each other or for those of the solid.
cathode negative electrode. In a vacuum tube, electrons flow from the cathode to the anode.
centripetal and centrifugal force centripetal force acts on a body to cause it to move in a circular path. A satellite circling the earth is held by the centripetal force of the Earth’s gravity. Inertia tends to keep the body moving straight, and this is referred to as centrifugal force.
Cerenkov radiation any charged particle traveling faster than light moves in a liquid or solid medium produces a
wake of electromagnetic radiation called Cerenkov radiation or Cerenkov light. The phenomenon was discovered in 1934 by Pavel A. Cerenkov (Russian, 1904—90).
charge property of matter that gives rise to electrical phenomena. The unit of charge is that of the proton or the electron; the proton is designated as positive (+1) and the electron as negative (–1). All other charged elementary particles have charges equal to +1 or–1, except quarks, whose charge can be 1/3 or 2/3. Every charged particle is surrounded by an electric force field so that it attracts any charge of opposite sign brought near it and repels any charge of the same sign. This force is responsible for holding protons and electrons together in atoms and for chemical bonding.
colloid combination of two materials in which one is in a gas, liquid, or solid phase (called the medium) and the other is dispersed through the first in tiny clusters of atoms or molecules or in very large molecules. A colloid in which liquid or solid particles are dispersed in a gas is also called an aerosol; one with gas in a liquid or solid is a foam; one with liquid particles in another liquid is an emulsion; and solid particles in a liquid form a sol. Gels are colloids in which both elements have a three-dimensional structure throughout the material. Examples of colloids include the aerosols fog (water in air) and smoke (soot in air); the foams whipped cream (air in milk) and Styrofoam (air in styrene); the emulsion mayonnaise (oil in egg); the gel gelatin (protein in water); and the sol ruby glass (metal in glass).
convection process in which heat is transported by the movement of a fluid. Responding to gravitational force, parts of a fluid such as air that are denser than surrounding parts sink. The sinking fluids displace hotter, less dense material and cause it to rise, transporting heat. From the point of view of someone surrounded by the fluid, as we are by air, it is more obvious that the less dense fluid is rising than that the denser fluid is sinking. Thus people say “hot air rises,” although that occurs only because cold air sinks. When a fluid is heated from the bottom, the unheated fluid sinks and the heated fluid is pushed away—but then the previously unheated fluid becomes heated.
crystal solid with a regular geometric shape, with a defined internal structure, and enclosed by symmetrically arranged plane surfaces that intersect at definite and characteristic angles. The particles (atoms, ions, or molecules) in a crystal have a regular three-dimensional repeating arrangement in space, called the crystal structure.
decay spontaneous disintegration of the nucleus of an atom, such as an isotope of uranium, by the emission of particles, usually with the emission of electromagnetic radiation. The half-life of a radioactive substance is the time that is required for half of the quantity of the substance to decay.
density mass of a substance per unit of volume, which can be measured in units such as pounds per cubic inch or kg/m3. It is commonly confused with weight; lead is denser than water, not heavier. Density is often compared with that of water. Such a comparison, made with the densest water—at 39.2°F (4°C)—is called relative density (formerly specific gravity). Relative density of lead is about 11 (it is 11 times as dense as water); the lightest metal, lithium, has a relative density of about 0.5, so it floats on water.
diffraction bending or spreading of waves (such as light) when a wave encounters either an object or an opening; some of the wave near the edges of the object or opening is bent. The diffracted waves interfere with each other, producing reinforcement or weakening.
direct current electric current in which the net flow of charge is in one direction.
Doppler effect apparent change in the frequency of a wave with the relative motion between source and observer (after Christian Doppler, 1803–53). For example, sound from an approaching vehicle, such as a siren, is perceived as having a higher pitch because more sound waves per second are perceived by the human ear; the apparent pitch falls as the vehicle passes and moves away from the observer. Light from distant galaxies is affected by a related phenomenon. As the universe expands, electromagnetic waves emitted in the distant past become longer in wavelength, shifting frequencies lower, which for visible light is toward the red.
efficiency measure that applies to any transformation of energy by an engine, machine, etc., from one form to another. It is the ratio of the amount of work done to the amount of energy used to produce the work. Since work and energy are measured in the same unit (in scientific notation, the joule), this ratio is a pure number, most commonly expressed as a percent. No transformations in the real world have 100 percent efficiency.
energy measure of a system’s ability to do work (measured in joules).
entropy measure of the unavailability of a system’s energy to do work. In a closed system entropy tends to increase (the second law of thermodynamics), resulting in less energy available to do work. In a more general sense, all ordered systems tend to become less ordered—that is,to increase in entropy: solids crumble; liquids and gases diffuse.
field in physics a field assigns to every point in space an amount—often the size of a force—and a direction, for example the direction in which a force acts. Typical fields include gravitational, electric, and magnetic fields.
fluorescence light produced by a material that was induced by incident radiation. A fluorescent light is one in which a gas in a glass tube coated with a fluorescent substance is excited by electrons, resulting in the emission of photons of ultraviolet radiation converted to visible light by the coating in the tube.
force any push or pull; that is, a quantity that changes the motion of a body if it is free to move. Force has both magnitude and direction; the magnitude is measured in pounds or newtons. A force acting on a mass produces acceleration proportional to the force unless balanced by a force in the opposite direction. Physicists generally recognize four fundamental forces: gravity, electromagnetism, and two atomic-level forces: the strong force, which holds the atomic nucleus together; and the weak force, which is associated with beta particle emission and particle decay.
frequency number of waves per second; it is measured as hertz (Hz): 10 Hz is 10 cycles per second. Frequency is determined by wavelength and the speed at which a wave travels.
friction resistance to the movement of one object past another with which it is in contact. Friction is dependent on the size of the force holding the objects together. For an object sliding on top of another, this is the weight (or force of gravity). An increase in the weight causes an increase in resistance. Friction is also dependent to some degree on the smoothness of the surfaces; generally, rougher surfaces have more friction, i.e., require a greater force to move one object past another.
half-life time in which one-half of the original quantity of a radioactive element will decay. Radioactive elements decay atom by atom in random events. Which individual atom decays is inherently unpredictable, but statistically the change follows a specific decay function for each isotope of an element.
incandescence light emitted by heating a material to a high temperature. In the common lightbulb a tungsten filament, usually in an inert gas, is heated to a high temperature to produce light.
inertia property that causes objects to resist any change in their motion. Objects at rest tend to stay at rest unless acted upon by an external force, and objects in motion continue in motion unless acted upon by an external force. This is a statement of Newton’s first law of motion.
ion atom or group of atoms that either gains one or more electrons, and thus becomes negatively charged (anion), or loses one or more electrons, becoming positively charged (cation).
isotope substance formed from atoms that each have the same number of protons and neutrons per atom. Elements usually exist in several different isotopes. The number of protons in each atom determines which element it is and most of the properties; the number of neutrons determines the specific isotope. The number that is the sum of the protons and neutrons per atom is combined with the element name to identify a specific isotope. For example, two isotopes of carbon are carbon 12, with six protons and six neutrons; and carbon 14, with six protons and eight neutrons.
kinetic energy energy of motion.
mass measure of an object’s inertia, that is, its resistance to acceleration. Inertial mass is exactly equivalent to gravitational mass, measurable by the force between two bodies separated by a given distance. The international standard of mass is a 1-kg platinum-iridium cylinder.
melting point temperature, usually measured at a pressure of 1 bar, at which a solid becomes liquid. The melting point is the same temperature as the freezing point, at which liquids become solid.
noise random changes in a signal being received at a detector. A mixture of sound or electromagnetic waves of random amplitude and frequency is called white noise.
osmosis process of diffusion of a solvent such as water through a semipermeable membrane that will transmit the solvent but impede most dissolved substances. Normally the flow of solvent is from the more dilute solution to the more concentrated solution; flow will stop when the solutions are of equal concentration. Movement
of water in plants and animals is determined to a large extent by osmosis.
period time that it takes for an oscillation or wave motion to repeat. Period (p) is the reciprocal of the frequency (f):p = 1/f. Any repeated motion, as for a pendulum or vibrating atom, similarly has a period—the amount of time for one repetition.
phase of matter traditionally matter exists in three states or phases—solid, liquid, and gas—but the modern view is that there are five phases of matter: the three traditional states plus plasma and Bose-Einstein condensates. For most elements and compounds, phases of matter change in response to heating.
phosphorescence light produced by causes other than increasing the heat of a substance that persists after the source of excitation has been removed.
photoelectric effects various electrical effects caused by light. The photoelectric (or photoemissive) effect occurs when electromagnetic waves strike a substance and liberate electrons from its surface In the photovoltaic effect a current flows across the junction of two dissimilar materials when light falls upon it. In the photoconductive effect, an increase in the electrical conductivity of a semi-conductor is caused by radiation.
piezoelectricity electric current caused by a mechanical force. When a mechanical force is applied to both sides of any of various nonconducting crystals such as quartz or Rochelle salt, positive charge builds on one face and negative charge on the opposite face, inducing a small electric current, in a circuit. The effect also works in reverse. Applying a current changes the shape of the crystal or other material as the faces repel or attract each other. These properties give rise to a variety of applications in acoustic and other devices such as microphones and quartz clocks.
pitch highness or lowness of a sound to an observer; higher pitches correspond to higher frequencies; pitch can also be affected by the loudness of a sound.
plasma one of the five phases of matter, consisting of a low-density, fully ionized gas with approximately equal numbers of positive and negative ions. Plasmas are electrically conductive and affected by magnetic fields. Interstellar gases, as well as the matter inside stars, is believed to be in the form of plasma. The study of plasma is important in the attempt to produce a controlled thermonuclear reaction as an energy source.
polarized light electromagnetic field confined to two dimensions, also called plane-polarized light. The electromagnetic field of unpolarized light vibrates in all directions perpendicular to the line of travel. One use of polarizing materials is in certain kinds of sunglasses, which absorb horizontally polarized light reflected off surfaces such as water.
potential energy energy stored in a system or body as a result of its position or shape; examples are gravitational, electrical, chemical, and nuclear energy. A rock on the edge of a cliff has potential energy because of its position in the Earth’s gravitational field.
power rate at which work is done or energy transferred. It is measured scientifically in watts (joules/second). In common usage, it is sometimes measured in horsepower (1 horsepower = 745.7 watts).
radiation emission and transmission of energy through space or a material medium; also the radiated energy. Generally the term is applied to the electromagnetic spectrum, which (from longest to shortest waves) includes radio, microwave, infrared, visible light, ultraviolet, X rays, and gamma rays; or to radioactivity, which includes streams of electrons or alpha particles as well as gamma rays.
radioactivity radiation produced by decay from one element to another or by nuclear fusion or fission. The disintegration of certain atomic nuclei is accompanied by the emission of alpha particles (helium nuclei), beta particles (electrons or positrons), or gamma radiation (short-wavelength electromagnetic radiation). Natural radioactivity is the disintegration of naturally occurring radioisotopes. Radioactivity can also be induced by bombarding the nuclei of normally stable elements in a particle accelerator to produce radioactive isotopes.
refraction deflection of a ray of light as it passes obliquely from one medium to another in which its speed is different. The incoming ray is the incident ray, and the deflected ray is the refracted ray. Other electromagnetic waves and sound waves can also be refracted.
spectrum rainbow or any of various related electromagnetic displays. A rainbow is just the spectrum of visible light. The complete electromagnetic spectrum includes (from longest to shortest waves) radio, microwave, infrared, visible light, ultraviolet, X-ray, and gamma-ray emissions. The emission (or “bright line”) spectrum of a body is the characteristic radiation pattern produced when the body is heated, bombarded by electrons or ions, or
absorbing photons; the lines in the spectrum can be used to identify elements with the spectroscope. The absorption (dark line) spectrum is the reverse of the emissions spectrum; it is produced by white light passing through a gas not hot enough to be incandescent.
specific heat amount of heat needed to raise the temperature of 1 gram of a substance, at a given pressure, by 1°C. For water at sea level, this is 4.187 joules (1 Calorie). Different substances have different specific heats.
spin quantum characteristic of particles best described as intrinsic angular momentum that is not associated with rotation. The quantum values of spin are restricted to integer or half-integer multiples of h/2, where h is Planck’s constant. Because of spin, particles also have their own intrinsic magnetic moments.
sublimation change of a substance directly from a solid to a gas without first becoming a liquid. This process is most familiar from frozen carbon dioxide, known as dry ice. Water ice also sublimes slowly at temperatures below 32°F (0°C).
surface tension property of liquids in which the liquid appears to be bounded by a thin elastic skin. Molecules in a liquid attract each other from all directions, but at the surface of a liquid there are attractive forces only from below. The unbalanced attraction creates the illusion of a skin at the surface.
suspension mixture in which finely divided particles of solid or liquid are suspended in a liquid or gas.
temperature measure of the average energy of motion in the atoms or molecules of a substance. As more heat is added to a system, its temperature rises. Temperature determines the direction of heat flow: heat transfers from a higher-temperature system to a lower one until the two systems are at the same temperature and thus in thermal equilibrium.
tunneling quantum-mechanical effect by which a particle can penetrate a barrier into a region of space that would be forbidden according to ordinary classic mechanics. Tunneling occurs because of the wavelike properties of particles; the wave associated with a particle can leak or decay through the barrier, and there is a finite probability of finding the particle on the other side. The theory of tunneling is the basis of the tunnel diode, used in electronic applications.
ultrasound sound beyond the range of human hearing, at frequencies greater than 20,000 cycles/second. Unlike audible sound, in which high intensity produces discomfort, ultrasound can increase in pressure (intensity) with little impact on human hearing. Ultrasound is widely used for medical and industrial imaging and testing: objects are scanned with ultrasound and the echoes recorded and analyzed.
vacuum region of space with very few atoms or other particles. A true or perfect vacuum would be a region of space that contains no matter, but in practice this is unattainable. The uncertainty principle permits subatomic particles to appear out of nothing and disappear before violating any physical laws. Thus any vacuum contains virtual particles.
viscosity property of a fluid’s resistance to flow; a higher viscosity means a slower flow. The cause of viscosity is internal molecular friction.
wave disturbance in space or a medium with a periodic form, such as electromagnetic waves and sound waves. The main characteristics of waves are speed of propagation, frequency, amplitude, and wavelength.
work a force that causes an object to move produces work, which is the product of the force and the distance moved. Work is measured in joules, the same unit used to measure energy.