Perilaku Aman dan Nyaman ber-Internet

Waktu itu aku sedang asyik membuka e-mail di sebuah Internet Cafe, tanpa melakukan kesalahan apa-apa, browser yang saya gunakan tiba-tiba Hang, dan tidak bisa dijalankan.
Aku tekan Alt+Tab untuk berpindah ke halaman lain juga ngga ngaruh..., Aku sungkan mau pakai task manager, ntar dikira mau menutup program billingnya, , trus aku minta tolong Operator untuk membetulkannya. Ternyata yang dia lakukan adalah menekan Ctrl+Alt+Del, dan Task Manager terbuka, sang operator langsung pilih Aplikasi browser untuk dieksekusi mati (End Task). Kemudian operatornya membuka lagi browser yang sama, 
Tapi anehnya setelah aku buka halaman situs tempat aku pakai e-mail, masih ada account-ku login di situ, tanpa pikir panjang dan tanpa menuliskan nama dan password, aku langsung klik tombol Sign-In... , dan mengulangi menulis e-mail balasan buat temenku, yang tadi sempat terjadi hang.
Setelah selesai, aku tutup billingnya dan pulang tanpa pikiran apa-apa, seperti orang yang tidak punya salah ... hehehehe.
Dua minggu kemudian aku jadi inget dan mikir-mikir ...”Lhoooo, kalau begitu... semua orang yang pakai komputer kemaren, bisa langsung buka e-mail ku juga dong..” ,
Lha kenapa harus pakai password segala ?

Makanya buat kamu yang terbiasa menggunakan “data pribadi sungguhan” untuk mendaftar di situs tertentu, hati-hati ... bisa-bisa tanggal lahir kamu, alamat kamu, bahkan nomor PIN rekening ATM kamu bisa dicuri orang.
Ada tiga hal yang ingin saya bagi dengan kamu..untuk berhati-hati


A. Apa yang perlu diingat ?
Paling tidak ada 3 hal yang perlu kita ingat saat browsing internet. Yaitu :
Cookies
Password saved (Auto complete)
Remember Me / Remember my Password
Sebenarnya ada beberapa tips, saya akan menambahkannya lain waktu...

B. Kenapa Begitu ?
Halaman web yang dinamis biasanya menggunakan code PHP, java ASP dan sebagianya. Jenis kode ini memiliki beberapa fungsi yang dapat digunakan untuk menyimpan data sementara, yaitu fungsi session dan cookies. Data ini akan disimpan dalam sebuah variabel dengan jangka waktu tertentu selama user membuka halaman web atau selama user belum menutup aplikasi internet browsernya. Fungsi dari session dan cookies berhubungan dengan akses database dari masing-masing user yang terdaftar pada registrasi suatu web.
Pada dasarnya cookies hampir sama dengan session. Akan tetapi, dengan fungsi cookies dapat diatur berapa lama waktu untuk menyimpan variabel data tersebut pada browser user hingga data cookies tersebut terhapus secara otomatis.

COOKIES
Cookies adalah bagian terkecil informasi yang tersimpan dalam harddisk user. Cookies berisi bit teks yang dapat dibaca oleh web server. Dengan cookies, anda dapat melakukan pelacakan dan pengidentifikasian user yang berkunjung ke website anda. Cookies ini umumnya merupakan bagian dari HTTP header dan disimpan dalam folder tertentu. Untuk Windows XP, disimpan dalam folder Cookies seperti gambar berikut.


















Saved Password
Setiap aplikasi browser menyediakan fasilitas untuk mengijinkan browser mengingat (menyimpan) form ataupun password yang pernah kita ketik







































sebenarnya tujuan para pengembang aplikasi adalah baik, yaitu kita dimanjakan dengan fasilitas pengingat password (jika password kita sepanjang kereta api ... ) siapa tahu kita lupa buatan kita sendiri.... Namun bagi para pelanggan Internet Cafe atau Warnet, hal ini sangat riskan.


Remember me ..???
Kebanyakan situs-situs terkenal menyediakan pilihan bagi membernya untuk mengingat password log-in mereka di halaman Web. Bagus juga apabila kita menggunakan Konputer atau Laptop sendiri. Namun jika kita menggunakannya di Internet cafe atau Warnet ... waaahhhh.





















C. Bagaimana Caranya ?
Sebelum menuliskan alamat web pada address bar, sebaiknya kita cek dulu apakah browser kita aman untuk digunakan.
Pada dasarnya semua aplikasi browser hampir sama setting-nya..
Pilih menu Tool | Option atau Tool | internet Option
Lalu kosongkan pilihan untuk Remember Passwords



















Untuk Internet Explorer, Menu ini ada di tab Content


Kalau perlu tutup (restart) aplikasi browser kamu.
Sedangkan untuk fasilitas Keep Me Sign in atau Remember me on this Computer .... Sebaiknya jangan tertarik dengan hal itu ... cukup ingat password kamu di kepala saja...  ,

Tips membuat password :
- Gunakan sebuah kalimat (tidak sekedar kata)
- Gunakan semua karakter yang diperbolehkan
- Hindari penggunaan angka urut atau huruf yang sama berurutan
- Sibisa mungkin, ganti/update password kamu tiap 8-10 bulan

Selamat berinternet dengan aman dan nyaman.

Keep looking around :-)

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Istilah dalam sinyal digital

Istilah Dalam DSP

1/f noise: Salah satu jenis noise acak yang meningkatkan amplitudo pada frekuensi rendah. Ini sering teramati pada sistem fisik (physical system), namun belum diketahui dengan pasti. Lihat White noise.

Aliasing: Proses dimana gelombang sinusoidal berubah dari frekuensi satu, ke frekuensi lain akibat dari proses sampling atau proses nonlinier lain. Biasanya ditandai dengan hilangnya informasi pada suatu sinyal.

Amplitudo Modulation: metode yang digunakan pada komunikasi radio untuk menggabungkan informasi yang dibawa sinyal dengan gelombang pembawa. Biasanya dibawa dengan mengalikan 2 buah sinyal.

Antialiasing filter: Filter analog Low-pass, ditempatkan sebelum ADC converter. Membuang frekuensi diatas satu-setengah dari sampling rate.

ASCII: metode untuk merepresentasikan huruf dan angka dalam bentuk biner. Setiap karakter tersusun atas angka antara 0 – 127.

Autokorelasi: Suatu sinyal dikorelasikan dengan sinyal itu sendiri. Berguna, karena transformasi fourier dari autocorrelasi adalah power spectrum dari sinyal asal.

Cepstrum: susunan ulang dari SPECTRUM, digunakan pada proses homomorphic untuk mendeskripsikan spectrum saat domain waktu dan frekuensi ditukar.

Continous signal: Sinyal yang terbentuk dari variabel continu, lawan dari diskrit. Contoh: voltase yang berubah thp waktu. Kadang dihubungkan dengan sinyal analog.

Cooley and Tukey: J.W. Cooley dan J.W Tukey telah memberikan sumbangan tentang FFT pada dunia lewat jurnal mereka di tahun 1965.

Correlation: Operasi matematika yang mirip dengan konvolusi, namun dengan urutan proses dari kiri ke kanan. Ini adalah cara optimal untuk mendeteksi gelombang tertentu pada suatu sinyal .

Cross-correlation: Sinyal baru akan terbentuk apabila suatu sinyal dikorelasi dengan sinyal lain. Tingginya amplitudo menandakan kesamaan antara dua sinyal tersebut.

DFT : family dari FFT untuk sinyal domain waktu yang diskrit dan periodik

Discrete signal: Sinyal yang menggunakan variabel quantisasi, misalnya sinyal digital yang ada pada komputer.

DTFT : family dari FFT untuk sinyal domain waktu yang diskrit dan aperiodik

Domain: variabel bebas dri suatu sinyal.

Dynamic range: Amplitudo tertinggi yang bisa dicapai sistem dalam membandingkan noise pada sistem

Frequency Domain: Sinyal yang menggunakan frekuensi debagai variabel bebasnya.

Frequency response: Perubahan magnitude dan phase yang dimiliki gelombang sinusoidal ketika melewati sistem linier. Seringkali didapat dengan mentransformasi fourier suatu respons impulse.

Fundamental Frequency: Frekuensi yang mana gelombang periodik selalu berulang.

Harmonics: Komponen frekuensi dari gelombang periodik, selalu terdiri dari perkalian integer dari frekuensi fundamental. Fundamental adalah harmonik pertama, duakalinya adalah harmonik kedua dst.

Impulse: Sinyal yang dibuat dari nilai nol semua, kecuali pulse yang sangat singkat.

Impulse response: Output dari sistem apabila inputnya adalah impulse normal.

IIR: response impulse yang memiliki angka bukan nol takterbatas, misalnya eksponen yang menurun. Serinykali digunakan sebagai indikasi bahwa filter menggunakan proses recursiv daripada kovolusi.

Intereger : semua angka. ..., -2, -1, 0, 1, 2, ...

Linier system: Dari definisi, sistem yang memiliki properties additivity dan homogenity.

Moving-average filter: setiap sampel pada sinyal output adalah rata-rata dari berbagai sampel pada sinyal input. Dapat dilakukan dengan konvolusi dan recursiv

Natural frequency: Frekuensi diekspresikan dalam radian /sekon, sebagai pembanding cycle/sekon (hertz). Untuk mengubah frekuensi (Hz) ke natural frekuensi, kalikan dengan .

Sampling theorema: Jika sinyal kontinu terbentuk dengan frekuensi lebih kecil dari , disampling dengan frekuensi , maka semua informasi dalam sinyal kontinu akan dapat ditampilkan pada sinyal sampel. Sering juga disebut dengan teorema shanon, atau Nyquist theorema.

Signal: Diskripsi tentang bagaimana suatu variabel mempengaruhi variabel yag lain.

Sinc Function: Secara formal didefinisikan dengan hubungan . Istilah kadang disembunyikan dalam variabel lain. Membuatanya menjadi bentuk umum: . Penting, karena ini adalah transfom fourier pulsa rectangular.

Spectral Analysis: Memahami suatu sinyal dengan menguji/mempelajari amplitudo, frekuensi, dan phase sinusiodal pada komponennya.

System: semua proses yang menghasilkan sinyal output sebagai respon dari sinyal input

Time domain: sinyal yang menggunakan waktu sebagai variabel bebasnya.

Transfer function: Sinyal output dibagi dengan sinyal input. Dapat berasal dari berbgai bentuk, tergantung dari bagaimana sinyal direpresentsaikan.

Transform: Prosedure, persamaan atau algoritma untuk mengubah sekelompok data menjadi kelompok data yang lain.

White noise: Noise acak yang memiliki spektruk frekuensi datar. Terjadi jika setiap sample pada domain waktu tidak mengandung informasi tenatng sinyal yang lain.

(diambil dari buku "Digital Signal Processing"

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Pendahuluan Teori Getaran

Sinyal Getaran ditandai dengan perubahan secara periodik dari suatu besaran. Sinyal getaran dapat dilihat dimana-mana, misalnya sinar matahari yang berubah dengan periode 24 jam, ayunan piston motor bakar yang bergerak maju mundur dll.
Besaran yang menyatakan sinyal dapay berupa suhu, simpangan, sudut, tekanan, tegangan listrik, kecepatan dll.
Fungsi Periodik
Secara umum, gerak getaran merupakan suatu fungsi periodik. Dan dapat dinyatakan sebagai
X(t) = x( t + T )
Dimana, t adalah waktu dan T merupakan konstanta yang bersatuan waktu dan disebut sebagai periode. Contoh fungsi periodik digambarkan dalam gambar berikut.

Figure 1. Fungsi Periodik

Nilai kebalikan dati T disebut frekuensi, yaitu

F=1/T

Frekuensi menyatakan jumlah getaran per satuan waktu. Satuan frekuensi adalah Hertz (Hz). Getaran dengan frekuensi 10 Hz, misalnya, berarti getaran tersebut bergetar 10 siklus dalam 1 sekon.
Disamping frekuensi f, dikenal juga frekuensi sudut yang diberi lambang ω. Satuan frekuensi sudut ini adalah rad/s. Hubungan antara f dengan ω dapat dituiskan sebagai:

ω = 2πf = 2 π / T

Fungsi Harmonik
Fungsi harmonik sederhana dapat ditulisan sebagai

X = A cos ωt

Dari persamaan itu, A menyatakan amplitudo dan ω merupakan frekuensi sudut. Fungsi harmonik, digambarkan sebagai berikut:



Figure 2. Fungsi harmonik sederhana
Periode atau waktu getar T dapat dinyatakan sebagai

T = 2 π / ω

Dan frekuensi nya

f = 1/ T = ω / 2 π

Jika fungsi harmonik diatas menyatakan simpangan, maka kecepatan nya adalah:
Dimana, adalah turunan pertama dari X. Dan percepatannya adalah;
atau

Analisa frekuensi (Spectrum analyser)
Sinyal getaran pada cacat bantalan sangat lemah, sehingga sangat sulit untuk dideteksi. Salah satu keunggulan domain frekuensi adalah kemampuannya untuk “menggali” sinyal lemah dari bayangan sinyal 1000 kali lebih kuat. Hal ini dapat dilukiskan dalam domain waktu dan doamin frekuensi yang tertera pada gb. 1.11. Pada gambar tersebut tampak sinyal lemah yang nampak pada domain frekuensi tetapi tidak nampak pada domain waktu. Hal ini dimungkinkan dengan tayangan amplitudo dalam skala logaritma, yang dinyatakan dalam dB. Walaupun skala linier terasa lebih alamiah bagi kebanyakan orang, namu skala logaritma lebih detail.
Spektrum frekuensi gelombang sinus murni berupa satu garis spektral. Bila periode getaran T detik maka garis spektra terletak pada 1/T Hz.
Gelombang segiempat (square wave), dihasilkan oleh tutup bantalan kendor, terdiri dari harmonik ganjil sebanyak tak hingga. Harmonik adalah komponen getaran dengan frekuensi kelipatan frekuensi dasar. Dalam hal analisis getaran mesin, frekuensi dasar ini adalah frekuensi putaran mesin. Dalam kenyatannya gelomnbang segiempat tidak sama persis dengan teori, sehingga kelipatan harmonik berupa ganjil dan genap.
Cacat pada bantalan dan roda f\gigi biasanya menghasilkan sinyal impuls, yang dalam domain frekuensi akan diwakili oleh harmonik.
Bermacam sinyal yang dihasilkan oleh cacat dimodulir oleh sinyal masa tak balans. Spektrum frekuensi suatu sinyal yang dimodulir oleh sinyal yang termodulir akan terdiri atas frekuensi sinyal pembawa dan sidebands dikedua sisi dengan jaraj sebesar frekuensi sinyal yang dimodulir.

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Logis juga

Ada seorang pemuda yang lama sekolah di negeri paman Sam kembali ke tanah
air. Sesampainya dirumah ia meminta kepada orang tuanya untuk mencari
seorang Guru agama, kiai atau siapapun yang bisa menjawab 3 pertanyaannya.
Akhirnya Orang tua pemuda itu mendapatkan orang tersebut.

Pemuda: Anda siapa? Dan apakah bisa menjawab pertanyaan-pertanyaan saya?
Kyai : Saya hamba Allah dan dengan izin-Nya saya akan menjawab pertanyaan
anda

Pemuda: Anda yakin? sedang Profesor dan banyak orang pintar saja tidak
mampu menjawab pertanyaan saya.
Kyai : Saya akan mencoba sejauh kemampuan saya

Pemuda: Saya punya 3 buah pertanyaan
1. Kalau memang Tuhan itu ada, tunjukan wujud Tuhan kepada saya
2. Apakah yang dinamakan takdir
3. Kalau syetan diciptakan dari api kenapa dimasukan ke neraka yang
dibuat dari api,tentu tidak menyakitkan buat syetan Sebab mereka memiliki
unsur yang sama. Apakah Tuhan tidak pernah berfikir sejauh itu?

Tiba-tiba Kyai tersebut menampar pipi si Pemuda dengan keras.
Pemuda (sambil menahan sakit): Kenapa anda marah kepada saya?
Kyai : Saya tidak marah...Tamparan itu adalah jawaban saya atas 3 buah
pertanyaan yang anda ajukan kepada saya

Pemuda: Saya sungguh-sungguh tidak mengerti
Kyai : Bagaimana rasanya tamparan saya?
Pemuda: Tentu saja saya merasakan sakit
Kyai : Jadi anda percaya bahwa sakit itu ada?
Pemuda: Ya
Kyai : Tunjukan pada saya wujud sakit itu !
Pemuda: Saya tidak bisa
Kyai : Itulah jawaban pertanyaan pertama: kita semua merasakan keberadaan
Tuhan tanpa mampu
melihat wujudnya.
Kyai : Apakah tadi malam anda bermimpi akan ditampar oleh saya?
Pemuda: Tidak
Kyai : Apakah pernah terpikir oleh anda akan menerima sebuah tamparan dari
saya hari ini?
Pemuda: Tidak
Kyai : Itulah yang dinamakan Takdir

Kyai : Terbuat dari apa tangan yang saya gunakan untuk menampar anda?
Pemuda: kulit
Kyai : Terbuat dari apa pipi anda?
Pemuda: kulit
Kyai : Bagaimana rasanya tamparan saya?
Pemuda: sakit
Kyai : Walaupun Syeitan terbuat dari api dan Neraka terbuat dari api, Jika
Tuhan berkehendak
maka Neraka akan Menjadi tempat menyakitkan untuk syeitan.

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Physics

I INTRODUCTION
Physics, major science, dealing with the fundamental constituents of the universe, the forces they exert on one another, and the results produced by these forces. Sometimes in modern physics a more sophisticated approach is taken that incorporates elements of the three areas listed above; it relates to the laws of symmetry and conservation, such as those pertaining to energy, momentum, charge, and parity. See Atom; Energy.
See also separate articles on the different aspects of physics and the various sciences mentioned in this article.

Physics

II SCOPE OF PHYSICS
Physics is closely related to the other natural sciences and, in a sense, encompasses them. Chemistry, for example, deals with the interaction of atoms to form molecules; much of modern geology is largely a study of the physics of the earth and is known as geophysics; and astronomy deals with the physics of the stars and outer space. Even living systems are made up of fundamental particles and, as studied in biophysics and biochemistry, they follow the same types of laws as the simpler particles traditionally studied by a physicist.
The emphasis on the interaction between particles in modern physics, known as the microscopic approach, must often be supplemented by a macroscopic approach that deals with larger elements or systems of particles. This macroscopic approach is indispensable to the application of physics to much of modern technology. Thermodynamics, for example, a branch of physics developed during the 19th century, deals with the elucidation and measurement of properties of a system as a whole and remains useful in other fields of physics; it also forms the basis of much of chemical and mechanical engineering. Such properties as the temperature, pressure, and volume of a gas have no meaning for an individual atom or molecule; these thermodynamic concepts can only be applied directly to a very large system of such particles. A bridge exists, however, between the microscopic and macroscopic approach; another branch of physics, known as statistical mechanics, indicates how pressure and temperature can be related to the motion of atoms and molecules on a statistical basis (see Statistics).
Physics emerged as a separate science only in the early 19th century; until that time a physicist was often also a mathematician, philosopher, chemist, biologist, engineer, or even primarily a political leader or artist. Today the field has grown to such an extent that with few exceptions modern physicists have to limit their attention to one or two branches of the science. Once the fundamental aspects of a new field are discovered and understood, they become the domain of engineers and other applied scientists. The 19th-century discoveries in electricity and magnetism, for example, are now the province of electrical and communication engineers; the properties of matter discovered at the beginning of the 20th century have been applied in electronics; and the discoveries of nuclear physics, most of them not yet 40 years old, have passed into the hands of nuclear engineers for applications to peaceful or military uses.

III EARLY HISTORY OF PHYSICS
Although ideas about the physical world date from antiquity, physics did not emerge as a well-defined field of study until early in the 19th century.

A Antiquity

The Babylonians, Egyptians, and early Mesoamericans observed the motions of the planets and succeeded in predicting eclipses, but they failed to find an underlying system governing planetary motion. Little was added by the Greek civilization, partly because the uncritical acceptance of the ideas of the major philosophers Plato and Aristotle discouraged experimentation.
Some progress was made, however, notably in Alexandria, the scientific center of Greek civilization. There, the Greek mathematician and inventor Archimedes designed various practical mechanical devices, such as levers and screws, and measured the density of solid bodies by submerging them in a liquid. Other important Greek scientists were the astronomer Aristarchus of Sámos, who measured the ratio of the distances from the earth to the sun and the moon; the mathematician, astronomer, and geographer Eratosthenes, who determined the circumference of the earth and drew up a catalog of stars; the astronomer Hipparchus, who discovered the precession of the equinoxes (see Ecliptic); and the astronomer, mathematician, and geographer Ptolemy, who proposed the system of planetary motion that was named after him, in which the earth was the center and the sun, moon, and stars moved around it in circular orbits (see Ptolemaic System).
B Middle Ages
Little advance was made in physics, or in any other science, during the Middle Ages, other than the preservation of the classical Greek treatises, for which the Arab scholars such as Averroës and Al-Quarashi, the latter also known as Ibn al-Nafīs, deserve much credit. The founding of the great medieval universities by monastic orders in Europe, starting in the 13th century, generally failed to advance physics or any experimental investigations. The Italian Scholastic philosopher and theologian Saint Thomas Aquinas, for instance, attempted to demonstrate that the works of Plato and Aristotle were consistent with the Scriptures. The English Scholastic philosopher and scientist Roger Bacon was one of the few philosophers who advocated the experimental method as the true foundation of scientific knowledge and who also did some work in astronomy, chemistry, optics, and machine design.
C 16th and 17th Centuries

The advent of modern science followed the Renaissance and was ushered in by the highly successful attempt by four outstanding individuals to interpret the behavior of the heavenly bodies during the 16th and early 17th centuries. The Polish natural philosopher Nicolaus Copernicus propounded the heliocentric system that the planets move around the sun. He was convinced, however, that the planetary orbits were circular, and therefore his system required almost as many complicated elaborations as the Ptolemaic system it was intended to replace (see Copernican System). The Danish astronomer Tycho Brahe, believing in the Ptolemaic system, tried to confirm it by a series of remarkably accurate measurements. These provided his assistant, the German astronomer Johannes Kepler, with the data to overthrow the Ptolemaic system and led to the enunciation of three laws that conformed with a modified heliocentric theory. Galileo, having heard of the invention of the telescope, constructed one of his own and, starting in 1609, was able to confirm the heliocentric system by observing the phases of the planet Venus. He also discovered the surface irregularities of the moon, the four brightest satellites of Jupiter, sunspots, and many stars in the Milky Way. Galileo's interests were not limited to astronomy; by using inclined planes and an improved water clock, he had earlier demonstrated that bodies of different weight fall at the same rate (thus overturning Aristotle's dictums), and that their speed increases uniformly with the time of fall. Galileo's astronomical discoveries and his work in mechanics foreshadowed the work of the 17th-century English mathematician and physicist Sir Isaac Newton, one of the greatest scientists who ever lived.
IV NEWTON AND MECHANICS

Starting about 1665, at the age of 23, Newton enunciated the principles of mechanics, formulated the law of universal gravitation, separated white light into colors, proposed a theory for the propagation of light, and invented differential and integral calculus. Newton's contributions covered an enormous range of natural phenomena: He was thus able to show that not only Kepler's laws of planetary motion but also Galileo's discoveries of falling bodies follow a combination of his own second law of motion and the law of gravitation, and to predict the appearance of comets, explain the effect of the moon in producing the tides, and explain the precession of the equinoxes.
A The Development of Mechanics
The subsequent development of physics owes much to Newton's laws of motion (see Mechanics), notably the second, which states that the force needed to accelerate an object will be proportional to its mass times the acceleration. If the force and the initial position and velocity of a body are given, subsequent positions and velocities can be computed, although the force may vary with time or position; in the latter case, Newton's calculus must be applied. This simple law contained another important aspect: Each body has an inherent property, its inertial mass, which influences its motion. The greater this mass, the slower the change of velocity when a given force is impressed. Even today, the law retains its practical utility, as long as the body is not very small, not very massive, and not moving extremely rapidly. Newton's third law, expressed simply as “for every action there is an equal and opposite reaction,” recognizes, in more sophisticated modern terms, that all forces between particles come in oppositely directed pairs, although not necessarily along the line joining the particles.
B Gravity
Newton's more specific contribution to the description of the forces in nature was the elucidation of the force of gravity. Today scientists know that in addition to gravity only three other fundamental forces give rise to all observed properties and activities in the universe: those of electromagnetism, the so-called strong nuclear interactions that bind together the neutrons and protons within atomic nuclei, and the weak interactions between some of the elementary particles that account for the phenomenon of radioactivity. Understanding of the force concept, however, dates from the universal law of gravitation, which recognizes that all material particles, and the bodies that are composed of them, have a property called gravitational mass. This property causes any two particles to exert attractive forces on each other (along the line joining them) that are directly proportional to the product of the masses, and inversely proportional to the square of the distance between the particles. This force of gravity governs the motion of the planets about the sun and the earth's own gravitational field, and it may also be responsible for the possible gravitational collapse, the final stage in the life cycle of stars. See Black Hole; Gravitation; Star.
One of the most important observations of physics is that the gravitational mass of a body (which is the source of one of the forces existing between it and another particle), is effectively the same as its inertial mass, the property that determines the motional response to any force exerted on it (see Inertia). This equivalence, now confirmed experimentally to within one part in 1013, holds in the sense of proportionality—that is, when one body has twice the gravitational mass of another, it also has twice the inertial mass. Thus, Galileo's demonstrations, which antedate Newton's laws, that bodies fall to the ground with the same acceleration and hence with the same motion, can be explained by the fact that the gravitational mass of a body, which determines the forces exerted on it, and the inertial mass, which determines the response to that force, cancel out.
The full significance of this equivalence between gravitational and inertial masses, however, was not appreciated until Albert Einstein, the theoretical physicist who enunciated the theory of relativity, saw that it led to a further implication: the inability to distinguish between a gravitational field and an accelerated frame of reference (see the Modern Physics: Relativity section of this article).
The force of gravity is the weakest of the four forces of nature when elementary particles are considered. The gravitational force between two protons, for example, which are among the heaviest elementary particles, is at any given distance only 10-36 the magnitude of the electrostatic forces between them, and for two such protons in the nucleus of an atom, this force in turn is many times smaller than the strong nuclear interaction. The dominance of gravity on a macroscopic scale is due to two reasons: (1) Only one type of mass is known, which leads to only one kind of gravitational force, which is attractive. The many elementary particles that make up a large body, such as the earth, therefore exhibit an additive effect of their gravitational forces in line with the addition of their masses, which thus become very large. (2) The gravitational forces act over a large range, and decrease only as the square of the distance between two bodies.
By contrast, the electric charges of elementary particles, which give rise to electrostatic and magnetic forces, are either positive or negative, or absent altogether. Only particles with opposite charges attract one another, and large composite bodies therefore tend to be electrically neutral and inactive. On the other hand, the nuclear forces, both strong and weak, are extremely short range and become hardly noticeable at distances of the order of 1 million-millionth of an inch.
Despite its macroscopic importance, the force of gravity remains so weak that a body must be very massive before its influence is noticed by another. Thus, the law of universal gravitation was deduced from observations of the motions of the planets long before it could be checked experimentally. Not until 1771 did the British physicist and chemist Henry Cavendish confirm it by using large spheres of lead to attract small masses attached to a torsion pendulum, and from these measurements also deduced the density of the earth.
In the two centuries after Newton, although mechanics was analyzed, reformulated, and applied to complex systems, no new physical ideas were added. The Swiss mathematician Leonhard Euler first formulated the equations of motion for rigid bodies, while Newton had dealt only with masses concentrated at a point, which thus acted like particles. Various mathematical physicists, among them Joseph Louis Lagrange of France and Sir William Rowan Hamilton of Ireland extended Newton's second law in more sophisticated and elegant reformulations. Over the same period, Euler, the Dutch-born scientist Daniel Bernoulli, and other scientists also extended Newtonian mechanics to lay the foundation of fluid mechanics.
C Electricity and Magnetism

Although the ancient Greeks were aware of the electrostatic properties of amber, and the Chinese as early as 2700 BC made crude magnets from lodestone, experimentation with and the understanding and use of electric and magnetic phenomena did not occur until the end of the 18th century. In 1785 the French physicist Charles Augustin de Coulomb first confirmed experimentally that electrical charges attract or repel one another according to an inverse square law, similar to that of gravitation. A powerful theory to calculate the effect of any number of static electric charges arbitrarily distributed was subsequently developed by the French mathematician Siméon-Denis Poisson and the German mathematician Carl Friedrich Gauss.
A positively charged particle attracts a negatively charged particle, tending to accelerate one toward the other. If the medium through which the particle moves offers resistance to that motion, this may be reduced to a constant-velocity (rather than accelerated) motion, and the medium will be heated up and may also be otherwise affected. The ability to maintain an electromotive force that could continue to drive electrically charged particles had to await the development of the chemical battery by the Italian physicist Alessandro Volta in 1800. The classical theory of a simple electric circuit assumes that the two terminals of a battery are maintained positively and negatively charged as a result of its internal properties. When the terminals are connected by a wire, negatively charged particles will be simultaneously pushed away from the negative terminal and attracted to the positive one, and in the process heat up the wire that offers resistance to the motion. Upon their arrival at the positive terminal, the battery will force the particles toward the negative terminal, overcoming the opposing forces of Coulomb's law. The German physicist Georg Simon Ohm first discovered the existence of a simple proportionality constant between the current flowing and the electromotive force supplied by a battery, known as the resistance of the circuit. Ohm's law, which states that the resistance is equal to the electromotive force, or voltage, divided by the current, is not a fundamental and universally applicable law of physics, but rather describes the behavior of a limited class of solid materials. See Electric Circuit.

The historical concepts of magnetism, based on the existence of pairs of oppositely charged poles, had started in the 17th century and owe much to the work of Coulomb. The first connection between magnetism and electricity, however, was made through the pioneering experiments of the Danish physicist and chemist Hans Christian Oersted, who in 1819 discovered that a magnetic needle could be deflected by a wire nearby carrying an electric current. Within one week after learning of Oersted's discovery, the French scientist André Marie Ampère showed experimentally that two current-carrying wires would affect each other like poles of magnets. In 1831 the British physicist and chemist Michael Faraday discovered that an electric current could be induced (made to flow) in a wire without connection to a battery, either by moving a magnet or by placing another current-carrying wire with an unsteady—that is, rising and falling—current nearby. The intimate connection between electricity and magnetism, now established, can best be stated in terms of electric or magnetic fields, or forces that will act at a particular point on a unit charge or unit current, respectively, placed at that point. Stationary electric charges produce electric fields; currents—that is, moving electric charges—produce magnetic fields. Electric fields are also produced by changing magnetic fields, and vice versa. Electric fields exert forces on charged particles as a function of their charge alone; magnetic fields will exert an additional force only if the charges are in motion.
These qualitative findings were finally put into a precise mathematical form by the British physicist James Clerk Maxwell who, in developing the partial differential equations that bear his name, related the space and time changes of electric and magnetic fields at a point with the charge and current densities at that point. In principle, they permit the calculation of the fields everywhere and any time from a knowledge of the charges and currents. An unexpected result arising from the solution of these equations was the prediction of a new kind of electromagnetic field, one that was produced by accelerating charges, that was propagated through space with the speed of light in the form of an electromagnetic wave, and that decreased with the inverse square of the distance from the source. In 1887 the German physicist Heinrich Rudolf Hertz succeeded in actually generating such waves by electrical means, thereby laying the foundations for radio, radar, television, and other forms of telecommunications. See Electromagnetic Radiation.
The behavior of electric and magnetic fields in these waves is quite similar to that of a very long taut string, one end of which is rapidly moved up and down in a periodic fashion. Any point along the string will be observed to move up and down, or oscillate, with the same period or with the same frequency as the source. Points along the string at different distances from the source will reach the maximum vertical displacements at different times, or at a different phase. Each point along the string will do what its neighbor did, but a little later, if it is further removed from the vibrating source (see Oscillation). The speed with which the disturbance, or the message to oscillate, is transmitted along the string is called the wave velocity (see Wave Motion). This is a function of the medium, its mass, and the tension in the case of a string. An instantaneous snapshot of the string (after it has been in motion for a while) would show equispaced points having the same displacement and motion, separated by a distance known as the wavelength, which is equal to the wave velocity divided by the frequency. In the case of the electromagnetic field one can think of the electric-field strength as taking the place of the up-and-down motion of each piece of the string, with the magnetic field acting similarly at a direction at right angles to that of the electric field. The electromagnetic-wave velocity away from the source is the speed of light.

D Light
The apparent linear propagation of light was known since antiquity, and the ancient Greeks believed that light consisted of a stream of corpuscles. They were, however, quite confused as to whether these corpuscles originated in the eye or in the object viewed. Any satisfactory theory of light must explain its origin and disappearance and its changes in speed and direction while it passes through various media. Partial answers to these questions were proposed in the 17th century by Newton, who based them on the assumptions of a corpuscular theory, and by the English scientist Robert Hooke and the Dutch astronomer, mathematician, and physicist Christiaan Huygens, who proposed a wave theory. No experiment could be performed that distinguished between the two theories until the demonstration of interference in the early 19th century by the British physicist and physician Thomas Young. The French physicist Augustin Jean Fresnel decisively favored the wave theory.
Interference can be demonstrated by placing a thin slit in front of a light source, stationing a double slit farther away, and looking at a screen spaced some distance behind the double slit. Instead of showing a uniformly illuminated image of the slits, the screen will show equispaced light and dark bands. Particles coming from the same source and arriving at the screen via the two slits could not produce different light intensities at different points and could certainly not cancel each other to yield dark spots. Light waves, however, can produce such an effect. Assuming, as did Huygens, that each of the double slits acts as a new source, emitting light in all directions, the two wave trains arriving at the screen at the same point will not generally arrive in phase, though they will have left the two slits in phase. Depending on the difference in their paths, “positive” displacements arriving at the same time as “negative” displacements of the other will tend to cancel out and produce darkness, while the simultaneous arrival of either positive or negative displacements from both sources will lead to reinforcement or brightness. Each apparent bright spot undergoes a timewise variation as successive in-phase waves go from maximum positive through zero to maximum negative displacement and back. Neither the eye nor any classical instrument, however, can determine this rapid “flicker,” which in the visible-light range has a frequency from 4 × 1014 to 7.5 × 1014 Hz, or cycles per second. Although it cannot be measured directly, the frequency can be inferred from wavelength and velocity measurements. The wavelength can be determined from a simple measurement of the distance between the two slits, and the distance between adjacent bright bands on the screen; it ranges from 4 × 10-5 cm (1.6 × 10-5 in) for violet light to 7.5 × 10-5 cm (3 × 10-5 in) for red light with intermediate wavelengths for the other colors.
The first measurement of the velocity of light was carried out by the Danish astronomer Olaus Roemer in 1676. He noted an apparent time variation between successive eclipses of Jupiter's moons, which he ascribed to the intervening change in the distance between Earth and Jupiter, and to the corresponding difference in the time required for the light to reach the earth. His measurement was in fair agreement with the improved 19th-century observations of the French physicist Armand Hippolyte Louis Fizeau, and with the work of the American physicist Albert Abraham Michelson and his coworkers, which extended into the 20th century. Today the velocity of light is known very accurately as 299,292.6 km (185,971.8 mi sec) in vacuum. In matter, the velocity is less and varies with frequency, giving rise to a phenomenon known as dispersion. See also Optics; Spectrum; Vacuum.
Maxwell's work contributed several important results to the understanding of light by showing that it was electromagnetic in origin and that electric and magnetic fields oscillated in a light wave. His work predicted the existence of nonvisible light, and today electromagnetic waves or radiations are known to cover the spectrum from gamma rays (see Radioactivity), with wavelengths of 10-12 cm (4 × 10-11 in), through X rays, visible light, microwaves, and radio waves, to long waves of hundreds of kilometers in length (see X Ray). It also related the velocity of light in vacuum and through media to other observed properties of space and matter on which electrical and magnetic effects depend. Maxwell's discoveries, however, did not provide any insight into the mysterious medium, corresponding to the string, through which light and electromagnetic waves had to travel (see the Electricity and Magnetism section above). Based on the experience with water, sound, and elastic waves, scientists assumed a similar medium to exist, a “luminiferous ether” without mass, which was all-pervasive (because light could obviously travel through a massless vacuum), and had to act like a solid (because electromagnetic waves were known to be transverse and the oscillations took place in a plane perpendicular to the direction of propagation, and gases and liquids could only sustain longitudinal waves, such as sound waves). The search for this mysterious ether occupied physicists' attention for much of the last part of the 19th century.
The problem was further compounded by an extension of a simple problem. A person walking forward with a speed of 3.2 km/h (2 mph) in a train traveling at 64.4 km/h (40 mph) appears to move at 67.6 km/h (42 mph), to an observer on the ground. In terms of the velocity of light the question that now arose was: If light travels at about 300,000 km/sec (about 186,000 mi/sec) through the ether, at what velocity should it travel relative to an observer on earth while the earth also moves through the ether? Or, alternately, what is the earth's velocity through the ether? The famous Michelson-Morley experiment, first performed in 1887 by Michelson and the American chemist Edward Williams Morley using an interferometer, was an attempt to measure this velocity; if the earth were traveling through a stationary ether, a difference should be apparent in the time taken by light to traverse a given distance, depending on whether it travels in the direction of or perpendicular to the earth's motion. The experiment was sensitive enough to detect even a very slight difference by interference; the results were negative. Physics was now in a profound quandary from which it was not rescued until Einstein formulated his theory of relativity in 1905.

E Thermodynamics
A branch of physics that assumed major stature during the 19th century was thermodynamics. It began by disentangling the previously confused concepts of heat and temperature, by arriving at meaningful definitions, and by showing how they could be related to the heretofore purely mechanical concepts of work and energy. See also Heat Transfer.

E1 Heat and Temperature
A different sensation is experienced when a hot or a cold body is touched, leading to the qualitative and subjective concept of temperature. The addition of heat to a body leads to an increase in temperature (as long as no melting or boiling occurs), and in the case of two bodies at different temperatures brought into contact, heat flows from one to the other until their temperatures become the same and thermal equilibrium is reached. To arrive at a scientific measure of temperature, scientists used the observation that the addition or subtraction of heat produced a change in at least one well-defined property of a body. The addition of heat, for example, to a column of liquid maintained at constant pressure increased the length of the column, while the heating of a gas confined in a container raised its pressure. Temperature, therefore, can invariably be measured by one other physical property, as in the length of the mercury column in an ordinary thermometer, provided the other relevant properties remain unchanged. The mathematical relationship between the relevant physical properties of a body or system and its temperature is known as the equation of state. Thus, for an ideal gas, a simple relationship exists between the pressure, p, volume V, number of moles n, and the absolute temperature T, given by pV = nRT, where R is the same constant for all ideal gases. Boyle's law, named after the British physicist and chemist Robert Boyle, and Gay-Lussac's law or Charles's law, named after the French physicists and chemists Joseph Louis Gay-Lussac and Jacques Alexandre César Charles, are both contained in this equation of state (see Gases).
Until well into the 19th century, heat was considered a massless fluid called caloric, contained in matter and capable of being squeezed out of or into it. Although the so-called caloric theory answered most early questions on thermometry and calorimetry, it failed to provide a sound explanation of many early 19th-century observations. The first true connection between heat and other forms of energy was observed in 1798 by the Anglo-American physicist and statesman Benjamin Thompson who noted that the heat produced in the boring of cannon was roughly proportional to the amount of work done. In mechanics, work is the product of a force on a body and the distance through which the body moves during its application.

E2 The First Law of Thermodynamics
The equivalence of heat and work was explained by the German physicist Hermann Ludwig Ferdinand von Helmholtz and the British mathematician and physicist William Thomson, 1st Baron Kelvin, by the middle of the 19th century. Equivalence means that doing work on a system can produce exactly the same effect as adding heat; thus the same temperature rise can be achieved in a gas contained in a vessel by adding heat or by doing an appropriate amount of work through a paddle wheel sticking into the container where the paddle is actuated by falling weights. The numerical value of this equivalent was first demonstrated by the British physicist James Prescott Joule in several heating and paddle-wheel experiments between 1840 and 1849.
That performing work or adding heat to a system were both means of transferring energy to it was thus recognized. Therefore, the amount of energy added by heat or work had to increase the internal energy of the system, which in turn determined the temperature. If the internal energy remains unchanged, the amount of work done on a system must equal the heat given up by it. This is the first law of thermodynamics, a statement of the conservation of energy. Not until the action of molecules in a system was better understood by the development of the kinetic theory could this internal energy be related to the sum of the kinetic energies of all the molecules making up the system.

E3 The Second Law of Thermodynamics
While the first law indicates that energy must be conserved in any interactions between a system and its surroundings, it gives no indication whether all forms of mechanical and thermal energy exchange are possible. That overall changes in energy proceed in one direction was first formulated by the French physicist and military engineer Nicolas Léonard Sadi Carnot, who in 1824 pointed out that a heat engine (a device that can produce work continuously while only exchanging heat with its surroundings) requires both a hot body as a source of heat and a cold body to absorb heat that must be discharged. When the engine performs work, heat must be transferred from the hotter to the colder body; to have the inverse take place requires the expenditure of mechanical (or electrical) work. Thus, in a continuously working refrigerator, the absorption of heat from the low temperature source (the cold space) requires the addition of work (usually as electrical power), and the discharge of heat (usually via fanned coils in the rear) to the surroundings (see Refrigeration). These ideas, based on Carnot's concepts, were eventually formulated rigorously as the second law of thermodynamics by the German mathematical physicist Rudolf Julius Emanuel Clausius and by Lord Kelvin in various alternate, although equivalent, ways. One such formulation is that heat cannot flow from a colder to a hotter body without the expenditure of work.
From the second law, it follows that in an isolated system (one that has no interactions with the surroundings) internal portions at different temperatures will always adjust to a single uniform temperature and thus produce equilibrium. This can also be applied to other internal properties that may be different initially. If milk is poured into a cup of coffee, for example, the two substances will continue to mix until they are inseparable and can no longer be differentiated. Thus, an initial separate or ordered state is turned into a mixed or disordered state. These ideas can be expressed by a thermodynamic property, called the entropy (first formulated by Clausius), which serves as a measure of how close a system is to equilibrium—that is, to perfect internal disorder. The entropy of an isolated system, and of the universe as a whole, can only increase, and when equilibrium is eventually reached, no more internal change of any form is possible. Applied to the universe as a whole, this principle suggests that eventually all temperature in space becomes uniform, resulting in the so-called heat death of the universe.
Locally, the entropy can be lowered by external action. This applies to machines, such as a refrigerator, where the entropy in the cold chamber is being reduced, and to living organisms. This local increase in order is, however, only possible at the expense of an entropy increase in the surroundings; here more disorder must be created.
This continued increase in entropy is related to the observed nonreversibility of macroscopic processes. If a process were spontaneously reversible—that is, if, after undergoing a process, both it and all the surroundings could be brought back to their initial state—the entropy would remain constant in violation of the second law. While this is true for macroscopic processes, and therefore corresponds to daily experience, it does not apply to microscopic processes, which are believed to be reversible. Thus, chemical reactions between individual molecules are not governed by the second law, which applies only to macroscopic ensembles.
From the promulgation of the second law, thermodynamics went on to other advances and applications in physics, chemistry, and engineering. Most chemical engineering, all power-plant engineering, and air-conditioning and low-temperature physics are just a few of the fields that owe their theoretical basis to thermodynamics and to the subsequent achievements of such scientists as Maxwell, the American physicist J. Willard Gibbs, the German physical chemist Walther Hermann Nernst, and the Norwegian-born American chemist Lars Onsager.

F Kinetic Theory and Statistical Mechanics
The modern concept of the atom was first proposed by the British chemist and physicist John Dalton in 1808 and was based on his studies that showed that chemical elements enter into combinations based on fixed ratios of their weights. The existence of molecules as the smallest particles of a substance that can exist in the free—that is, gaseous—state and have the properties of any larger amount of the substance, was first hypothesized by the Italian physicist and chemist Amedeo Avogadro in 1811, but did not find general acceptance until about 50 years later, when it also formed the basis of the kinetic theory of gases (see Avogadro's Law). Developed by Maxwell, the Austrian physicist Ludwig Boltzmann, and other physicists, it applied the laws of mechanics and probability to the behavior of individual molecules, and drew statistical inferences about the properties of the gas as a whole.
A typical but important problem solved in this manner was the determination of the range of speeds of molecules in the gas, and from this the average kinetic energy of the molecules. The kinetic energy of a body, as a simple consequence of Newton's second law, is mv2, where m is the mass of the body and v its velocity. One of the achievements of kinetic theory was to show that temperature, the macroscopic thermodynamic property describing the system as a whole, was directly related to the average kinetic energy of the molecules. Another was the identification of the entropy of a system with the logarithm of the statistical probability of the energy distribution. This led to the demonstration that the state of thermodynamic equilibrium corresponding to that of highest probability is also the state of maximum entropy. Following the success in the case of gases, kinetic theory and statistical mechanics were subsequently applied to other systems, a process that is still continuing.

G Early Atomic and Molecular Theories
The development of Dalton's atomic theory and Avogadro's molecular law had overriding influence on the development of chemistry, in addition to their importance in physics.

G1 Avogadro's Law
Avogadro's law, which was easily proved by kinetic theory, indicated that a specified volume of a gas at a given temperature and pressure always contained the same number of molecules, irrespective of the gas selected. This number, however, could not be accurately determined, and the 19th-century physicists therefore had no sound knowledge of molecular or atomic mass and size until the turn of the 20th century, when subsequent to the discovery of the electron, the American physicist Robert Andrews Millikan carefully determined its charge. This finally permitted accurate determination of the so-called Avogadro's number, which is the number of molecules in that amount of material exactly equal to its molecular weight.
Besides the mass, another quantity of interest was the size of an atom. Various and only partly successful attempts at finding the size of an atom were made during the latter part of the 19th century; the most successful applied the results of kinetic theory to nonideal gases—that is, gases the behavior of which depended on the fact that molecules were not points but had finite volumes. Only later experiments involving the scattering of X rays, alpha particles, and other atomic and subatomic particles by atoms led to more precise measurements of their size as being between 10-8 and 10-7 cm (4 × 10-7 and 4 × 10-6 in) in diameter. A precise statement about the size of an atom, however, requires some explicit definition of what is meant by size, since most atoms are not exactly spherical and can exist in various states that change the distance between the nucleus and the electrons within the atom.

G2 Spectroscopy
One of the most important developments leading to the exploration of the interior of the atom, and to the eventual overthrow of the classical theories of physics, was spectroscopy; the other was the discovery of the subatomic particles themselves.
In 1823 the British astronomer and chemist Sir John Frederick William Herschel suggested that a chemical substance might be identified by examining its spectrum—that is, the discrete wavelength pattern in which light from a gaseous substance is emitted. In the years that followed, the spectra of a great many substances were cataloged by two Germans, the chemist Robert Wilhelm Bunsen and the physicist Gustav Robert Kirchhoff. Helium was first discovered as a new element following the discovery of an unexplained spectral line in the sun's spectrum by the British astronomer Sir Joseph Norman Lockyer in 1868. From the standpoint of atomic theory, however, the most important contributions were made by the study of the spectra of simple atoms, such as hydrogen, which showed few spectral lines. See Chemical Analysis.
Discrete line spectra originate from gaseous substances where, in terms of modern knowledge, the electrons have been excited by heat or by bombardment with subatomic particles. In contrast, a heated solid has a continuous spectrum over the full visible range and into the infrared and ultraviolet regions. The total amount of energy emitted depends strongly on the temperature, as does the relative intensity of the different wavelength components. As a piece of iron is heated, for example, its radiation is first in the infrared spectrum and cannot be seen; it then extends into the visible spectrum where the glow shifts from red to white as the peak of its radiant spectrum shifts toward the middle of the visible range. Attempts to explain the radiation characteristics of solids, using the tools of theoretical physics available at the end of the 19th century, led to the prediction that at any given temperature the amount of radiation increased with frequency and without limit. This calculation, in which no error was found, was in disagreement with experiment and also led to an absurd conclusion: A body at a finite temperature could radiate an infinite amount of energy. This required a new way of thinking about radiation and, indirectly, about the atom. See Infrared Radiation; Ultraviolet Radiation.

H The Breakdown of Classical Physics
By about 1880 physics was serene; most phenomena could be explained by Newtonian mechanics, Maxwell's electromagnetic theory, thermodynamics, and Boltzmann's statistical mechanics. Only a few problems, such as the determination of the properties of the ether and the explanation of the radiation spectra from solids and gases, appeared unsolved. These unexplained phenomena, however, formed the seeds of revolution, a revolution that was augmented by a series of remarkable discoveries within the last decade of the 19th century: the discovery of X rays by Wilhelm Conrad Roentgen of Germany in 1895; of the electron by Sir Joseph John Thomson of Great Britain in 1895; of radioactivity by Antoine Henri Becquerel of France in 1896; and of the photoelectric effect by Hertz, Wilhelm Hallwachs, and Philipp Eduard Anton Lenard of Germany during the period from 1887 to 1899 (see Photoelectric Cell). Coupled with the disturbing results of the Michelson-Morley experiments and the discovery of cathode rays, or electron stream, the experimental evidence in physics outstripped all available theories to explain it.

V MODERN PHYSICS
Two major new developments during the first third of the 20th century, the quantum theory and the theory of relativity, explained these findings, yielded new discoveries, and changed the understanding of physics as it is known today.

A Relativity
To extend the example of relative velocity introduced with the Michelson-Morley experiment, two situations can be compared. One consists of a person, A, walking forward with a velocity v in a train moving at velocity u. The velocity of A with regard to an observer B stationary on the ground is then simply V = u + v. If, however, the train were at rest in the station and A was moving forward with velocity v while observer B walked backward with velocity u, the relative speed between A and B would be exactly the same as in the first case. In more general terms, if two frames of reference are moving relative to each other at constant velocity, observations of any phenomena made by observers in either frame will be physically equivalent. As already mentioned, the Michelson-Morley experiment failed to confirm the concept of adding velocities, and two observers, one at rest and the other moving toward a light source with velocity u, both observe the same light velocity V, commonly denoted by the symbol c.
Einstein incorporated the invariance of c into his theory of relativity. He also demanded a very careful rethinking of the concepts of space and time, showing the imperfection of intuitive notions about them. As a consequence of his theory, it is known that two clocks that keep identical time when at rest relative to each other must run at different speeds when they are in relative motion, and two rods that are identical in length (at rest) will become different in length when they are in relative motion. Space and time must be closely linked in a four-dimensional continuum where the normal three-space dimensions must be augmented by an interrelated time dimension.
Two important consequences of Einstein's relativity theory are the equivalence of mass and energy and the limiting velocity of the speed of light for material objects. Relativistic mechanics describes the motion of objects with velocities that are appreciable fractions of the speed of light, while Newtonian mechanics remains useful for velocities typical of the macroscopic motion of objects on earth. No material object, however, can have a speed equal to or greater than the speed of light.
Even more important is the relation between the mass m and energy E. They are coupled by the relation E = mc2, and because c is very large, the energy equivalence of a given mass is enormous. The change of mass giving an energy change is significant in nuclear reactions, as in reactors or nuclear weapons, and in the stars, where a significant loss of mass accompanies the huge energy release.
Einstein's original theory, formulated in 1905 and known as the special theory of relativity, was limited to frames of reference moving at constant velocity relative to each other. In 1915, he generalized his hypothesis to formulate the general theory of relativity that applied to systems that accelerate with reference to each other. This extension showed gravitation to be a consequence of the geometry of space-time and predicted the bending of light in its passage close to a massive body like a star, an effect first observed in 1919. General relativity, although less firmly established than the special theory, has deep significance for an understanding of the structure of the universe and its evolution. See also Cosmology.

B Quantum Theory
The quandary posed by the observed spectra emitted by solid bodies was first explained by the German physicist Max Planck. According to classical physics, all molecules in a solid can vibrate with the amplitude of the vibrations directly related to the temperature. All vibration frequencies should be possible and the thermal energy of the solid should be continuously convertible into electromagnetic radiation as long as energy is supplied. Planck made a radical assumption by postulating that the molecular oscillator could emit electromagnetic waves only in discrete bundles, now called quanta, or photons. See Photon; Quantum Theory. Each photon has a characteristic wavelength in the spectrum and an energy E given by E = hf, where f is the frequency of the wave. The wavelength λ related to the frequency by λf = c, where c is the speed of light. With the frequency specified in hertz (Hz), or cycles per second, h, now known as Planck's constant, is extremely small (6.626 × 10-27 erg-sec). With his theory, Planck again introduced a partial duality into the theory of light, which for nearly a century had been considered to be wavelike only.

C Photoelectricity
If electromagnetic radiation of appropriate wavelength falls upon suitable metals, negative electric charges, later identified as electrons, are ejected from the metal surface. The important aspects of this phenomenon are the following: (1) the energy of each photoelectron depends only on the frequency of the illumination and not on its intensity; (2) the rate of electron emission depends only on the illuminating intensity and not on the frequency (provided that the minimum frequency to cause emission is exceeded); and (3) the photoelectrons emerge as soon as the illumination hits the surface. These observations, which could not be explained by Maxwell's electromagnetic theory of light, led Einstein to assume in 1905 that light can be absorbed only in quanta or photons, and that the photon completely vanishes in the absorption process, with all of its energy E (=hf) going to one electron in the metal. With this simple assumption Einstein extended Planck's quantum theory to the absorption of electromagnetic radiation, giving additional importance to the wave-particle duality of light. It was for this work that Einstein was awarded the 1921 Nobel Prize in physics.

D X Rays
These very penetrating rays, first discovered by Roentgen, were shown to be electromagnetic radiation of very short wavelength in 1912 by the German physicist Max Theodor Felix von Laue and his coworkers. The precise mechanism of X-ray production was shown to be a quantum effect, and in 1914 the British physicist Henry Gwyn Jeffreys Moseley used his X-ray spectrograms to prove that the atomic number of an element, and hence the number of positive charges in an atom, is the same as its position in the periodic table (see Periodic Law). The photon theory of electromagnetic radiation was further strengthened and developed by the prediction and observation of the so-called Compton effect by the American physicist Arthur Holly Compton in 1923.

E Electron Physics
That electric charges were carried by extremely small particles had already been suspected in the 19th century and, as indicated by electrochemical experiments, the charge of these elementary particles was a definite, invariant quantity. Experiments on the conduction of electricity through low-pressure gases led to the discovery of two kinds of rays: cathode rays, coming from the negative electrode in a gas discharge tube, and positive or canal rays from the positive electrode. Sir Joseph John Thomson's 1895 experiment measured the ratio of the charge q to the mass m of the cathode-ray particles. Lenard in 1899 confirmed that the ratio of q to m for photoelectric particles was identical to that of cathode rays. The American inventor Thomas Alva Edison had noted in 1883 that very hot wires emit electricity, called thermionic emission (now called the Edison effect), and in 1899 Thomson showed that this form of electricity also consisted of particles with the same q to m ratio as the others. About 1911 Millikan finally determined that electric charge always arises in multiples of a basic unit e, and measured the value of e, now known to be 1.602 × 10-19 coulombs. From the measured value of q to m ratio, with q set equal to e, the mass of the carrier, called electron, could now be determined as 9.110 × 10-31 kg.
Finally, Thomson and others showed that the positive rays also consisted of particles, each carrying a charge e, but of the positive variety. These particles, however, now recognized as positive ions resulting from the removal of an electron from a neutral atom, are much more massive than the electron. The smallest, the hydrogen ion, is a single proton with a mass of 1.673 × 10-27 kg, about 1837 times more massive than the electron (see Ion; Ionization). The “quantized” nature of electric charge was now firmly established and, at the same time, two of the fundamental subatomic particles identified.

F Atomic Models
In 1913 the New Zealand-born British physicist Ernest Rutherford, making use of the newly discovered radiations from radioactive nuclei, found Thomson's earlier model of an atom with uniformly distributed positive and negative charged particles to be untenable. The very fast, massive, positively charged alpha particles he employed were found to deflect sharply in their passage through matter. This effect required an atomic model with a heavy positive scattering center. Rutherford then suggested that the positive charge of an atom was concentrated in a massive stationary nucleus, with the negative electron moving in orbits about it, and positioned by the electric attraction between opposite charges. This solar-system-like atomic model, however, could not persist according to Maxwell's theory, where the revolving electrons should emit electromagnetic radiation and force a total collapse of the system in a very short time.
Another sharp break with classical physics was required at this point. It was provided by the Danish physicist Niels Henrik David Bohr, who postulated the existence within atoms of certain specified orbits in which electrons could revolve without electromagnetic radiation emission. These allowed orbits, or so-called stationary states, are determined by the condition that the angular momentum J of the orbiting electron must be a positive multiple integral of Planck's constant, divided by 2 , that is, J = nh/2p, where the quantum number n may have any positive integer value. This extended “quantization” to dynamics, fixed the possible orbits, and allowed Bohr to calculate their radii and the corresponding energy levels. Also in 1913 the model was confirmed experimentally by the German-born American physicist James Franck and the German physicist Gustav Hertz.
Bohr developed his model much further. He explained how atoms radiate light and other electromagnetic waves, and also proposed that an electron “lifted” by a sufficient disturbance of the atom from the orbit of smallest radius and least energy (the ground state) into another orbit, would soon “fall” back to the ground state. This falling back is accompanied by the emission of a single photon of energy E = hf, where E is the difference in energy between the higher and lower orbits. Each orbit shift emits a characteristic photon of sharply defined frequency and wavelength; thus one photon would be emitted in a direct shift from the n = 3 to the n = 1 orbit, which will be quite different from the two photons emitted in a sequential shift from the n = 3 to n = 2 orbit, and then from there to the n = 1 orbit. This model now allowed Bohr to account with great accuracy for the simplest atomic spectrum, that of hydrogen, which had defied classical physics.
Although Bohr's model was extended and refined, it could not explain observations for atoms with more than one electron. It could not even account for the intensity of the spectral colors of the simple hydrogen atom. Because it had no more than a limited ability to predict experimental results, it remained unsatisfactory for theoretical physicists.

G Quantum Mechanics
Within a few years, roughly between 1924 and 1930, an entirely new theoretical approach to dynamics was developed to account for subatomic behavior. Named quantum mechanics or wave mechanics, it started with the suggestion in 1923 by the French physicist Louis Victor, Prince de Broglie, that not only electromagnetic radiation but matter could also have wave as well as particle aspects. The wavelength of the so-called matter waves associated with a particle is given by the equation λ = h/mv, where m is the particle mass and v its velocity. Matter waves were conceived of as pilot waves guiding the particle motion, a property that should result in diffraction under suitable conditions. This was confirmed in 1927 by the experiments on electron-crystal interactions by the American physicists Clinton Joseph Davisson and Lester Halbert Germer and the British physicist Sir George Paget Thomson. Subsequently, Werner Heisenberg, Max Born, and Ernst Pascual Jordan of Germany and the Austrian physicist Erwin Schrödinger developed Broglie's idea into a mathematical form capable of dealing with a number of physical phenomena and with problems that could not be handled by classical physics. In addition to confirming Bohr's postulate regarding the quantization of energy levels in atoms, quantum mechanics now provides an understanding of the most complex atoms, and has also been a guiding spirit in nuclear physics. Although quantum mechanics is usually needed only on the microscopic level (with Newtonian mechanics still satisfactory for macroscopic systems), certain macroscopic effects, such as the properties of crystalline solids, also exist that can only be satisfactorily explained by principles of quantum mechanics.
Going beyond Broglie's notion of the wave-particle duality of matter, additional important concepts have since been incorporated into the quantum-mechanical picture. These include the discovery that electrons must have some permanent magnetism and, with it, an intrinsic angular momentum, or spin, as a fundamental property. Spin was subsequently found in almost all other elementary particles. In 1925 the Austrian physicist Wolfgang Pauli expounded the exclusion principle, which states that in an atom no two electrons can have precisely the same set of quantum numbers. Four quantum numbers are needed to specify completely the state of an electron in an atom. The exclusion principle is vital for an understanding of the structure of the elements and of the periodic table. Heisenberg in 1927 put forth the uncertainty principle, which asserted the existence of a natural limit to the precision with which certain pairs of physical quantities can be known simultaneously.
Finally, a synthesis of quantum mechanics and relativity was made in 1928 by the British mathematical physicist Paul Adrien Maurice Dirac, leading to the prediction of the existence of the positron and bringing the development of quantum mechanics to a culmination.
Largely as a result of Bohr's ideas, a different and statistical approach developed in modern physics. The fully deterministic cause-effect relations produced by Newtonian mechanics were supplanted by predictions of future events in terms of statistical probability only. Thus, the wave property of matter also implies that, in accordance with the uncertainty principle, the motion of the particles can never be predicted with absolute certainty even if the forces are known completely. Although this statistical aspect plays no detectable role in macroscopic motions, it is dominant on the molecular, atomic, and subatomic scale.

H Nuclear Physics
The understanding of atomic structure was also facilitated by Becquerel's discovery in 1896 of radioactivity in uranium ore (see Uranium). Within a few years radioactive radiation was found to consist of three types of emissions: alpha rays, later found by Rutherford to be the nuclei of helium atoms; beta rays, shown by Becquerel to be very fast electrons; and gamma rays, identified later as very short wavelength electromagnetic radiation. In 1898 the French physicists Marie and Pierre Curie separated two highly radioactive elements, radium and polonium, from uranium ore, thus showing that radiations could be identified with particular elements. By 1903 Rutherford and the British physical chemist Frederick Soddy had shown that the emission of alpha or beta rays resulted in the transmutation of the emitting element into a different one. Radioactive processes were shortly thereafter found to be completely statistical; no method exists that could indicate which atom in a radioactive material will decay at any one time. These developments, in addition to leading to Rutherford's and Bohr's model of the atom, also suggested that alpha, beta, and gamma rays could only come from the nuclei of very heavy atoms. In 1919 Rutherford bombarded nitrogen with alpha particles and converted it to hydrogen and oxygen, thus producing the first artificial transmutation of elements.
Meanwhile, a knowledge of the nature and abundance of isotopes was growing, largely through the development of the mass spectrograph. A model emerged in which the nucleus contained all the positive charge and almost all the mass of the atom. The nuclear-charge carriers were identified as protons, but except for hydrogen, the nuclear mass could be accounted for only if some additional uncharged particles were present. In 1932 the British physicist Sir James Chadwick discovered the neutron, an electrically neutral particle of mass 1.675 × 10-27 kg, slightly more than that of the proton. Now nuclei could be understood as consisting of protons and neutrons, collectively called nucleons, and the atomic number of the element was simply the number of protons in the nucleus. On the other hand, the isotope number, also called the atomic mass number, was the sum of the neutrons and protons present. Thus, all atoms of oxygen (atomic no. 8) have eight protons, but the three isotopes of oxygen, O16, O 17, and O18, also contain within their respective nuclei eight, nine, or ten neutrons.
Positive electric charges repel each other, and because atomic nuclei (except for hydrogen) have more than one proton, they would fly apart except for a strong attractive force, called the nuclear force, or strong interaction that binds the nucleons to each other. The energy associated with this strong force is very great, millions of times greater than the energies characteristic of electrons in their orbits or chemical binding energies. An escaping alpha particle (consisting of two protons and two neutrons), therefore, will have to overcome this strong interaction force to escape from a radioactive nucleus such as uranium. This apparent paradox was explained by the American physicists Edward U. Condon, George Gamow, and Ronald Wilfred Gurney, who applied quantum mechanics to the problem of alpha emission in 1928 and showed that the statistical nature of nuclear processes allowed alpha particles to “leak” out of radioactive nuclei, even though their average energy was insufficient to overcome the nuclear force. Beta decay was explained as a result of a neutron disruption within the nucleus, the neutron changing into an electron (the beta particle), which is promptly ejected, and a residual proton. The proton left behind leaves the “daughter” nucleus with one more proton than its “parent” and thus increases the atomic number and the position in the periodic table. Alpha or beta emission usually leaves the nucleus with excess energy, which it unloads by emitting a gamma-ray photon.
In all these nuclear processes a large amount of energy, given by Einstein's E = mc2 equation, is released. After the process is over, the total mass of the product is less than that of the parent, with the mass difference appearing as energy. See Nuclear Energy.

VI DEVELOPMENTS IN PHYSICS SINCE 1930
The rapid expansion of physics in the last few decades was made possible by the fundamental developments during the first third of the century, coupled with recent technological advances, particularly in computer technology, electronics, nuclear-energy applications, and high-energy particle accelerators.

A Accelerators
Rutherford and other early investigators of nuclear properties were limited to the use of high-energy emissions from naturally radioactive substances to probe the atom. The first artificial high-energy emissions were produced in 1932 by the British physicist Sir John Douglas Cockcroft and the Irish physicist Ernest Thomas Sinton Walton, who used high-voltage generators to accelerate protons to about 700,000 eV and to bombard lithium with them, transmuting it into helium. One electron volt is the energy gained by an electron when the accelerating voltage is 1 V; it is equivalent to about 1.6 × 10-19 joule (J). Modern accelerators produce energies measured in million electron volts (usually written mega-electron volts, or MeV), billion electron volts (giga-electron volts, or GeV), or trillion electron volts (tera-electron volts, or TeV). Higher-voltage sources were first made possible by the invention, also in 1932, of the Van de Graaff generator by the American physicist Robert van de Graaff.
This was followed almost immediately by the invention of the cyclotron by the American physicists Ernest Orlando Lawrence and Milton Stanley Livingston. The cyclotron uses a magnetic field to bend the trajectories of charged particles into circles, and during each half-revolution the particles are given a small electric “kick” until they accumulate the high energy level desired. Protons could be accelerated to about 10 MeV by a cyclotron, but higher energies had to await the development of the synchrotron after the end of World War II (1939-1945), based on the ideas of the American physicist Edwin Mattison McMillan and the Soviet physicist Vladimir I. Veksler. After World War II, accelerator design made rapid progress, and accelerators of many types were built, producing high-energy beams of electrons, protons, deuterons, heavier ions, and X rays. For example, the accelerator at the Stanford Linear Accelerator Center (SLAC) in Stanford, California, accelerates electrons down a straight “runway,” 3.2 km (2 mi) long, at the end of which they attain an energy of more than 20 GeV.
While lower-energy accelerators are used in various applications in industry and laboratories, the most powerful ones are used in studying the structure of elementary particles, the fundamental building blocks of nature. In such studies elementary particles are broken up by hitting them with beams of projectiles that are usually protons or electrons. The distribution of the fragments yields information on the structure of the elementary particles.
To obtain more detailed information in this manner, the use of more energetic projectiles is necessary. Since the acceleration of a projectile is achieved by “pushing” it from behind, to obtain more energetic projectiles it is necessary to keep pushing for a longer time. Thus, high-energy accelerators are generally larger in size. The highest beam energy reached at the end of World War II was less than 100 MeV. A bigger accelerator, reaching 3 GeV, was built in the early 1950s at the Brookhaven National Laboratory at Upton, New York. A breakthrough in accelerator design occurred with the introduction of the strong focusing principle in 1952 by the American physicists Ernest D. Courant, Livingston, and Hartland S. Snyder. Today the world's largest accelerators have been or are being built to produce beams of protons beyond 1 TeV. Two are located at the Fermi National Accelerator Laboratory, near Batavia, Illinois, and at the European Organization for Nuclear Research, known as CERN, in Geneva, Switzerland. See Particle Accelerators.

B Particle Detectors
Detection and analysis of elementary particles were first accomplished through the ability of these particles to affect photographic emulsions and to energize fluorescent materials. The actual paths of ionized particles were first observed by the British physicist Charles Thomson Rees Wilson in a cloud chamber, where water droplets condensed on the ions produced by the particles during their passage. Electric or magnetic fields can be used to bend the particle paths, yielding information about their momentum and electric charges. A significant advance on the cloud chamber was the construction of the bubble chamber by the American physicist Donald Arthur Glaser in 1952. It uses a liquid, usually hydrogen, instead of air, and the ions produced by a fast particle become centers of boiling, leaving an observable bubble track. Because the density of the liquid is much higher than that of air, more interactions take place in a bubble chamber than in a cloud chamber. Furthermore, the bubbles clear out faster than water droplets, allowing more frequent cycling of the bubble chamber. A third development, the spark chamber, evolved in the 1950s. In this device, many parallel plates are kept at a high voltage in a suitable gas atmosphere. An ionizing particle passing between the plates breaks down the gas, forming sparks that delineate its path.
A different type of detector, the discharge counter, was developed early during the 20th century, largely by the German physicist Hans Wilhelm Geiger, and later improved by the German American physicist Walther Müller. It is now commonly known as the Geiger-Müller counter, and although small and convenient, it has been largely replaced by faster and more convenient solid-state counting devices, such as the scintillation counter, developed about 1947 by the German American physicist Hartmut Paul Kallmann and others. It uses the ability of ionized particles to produce a flash of light as they pass through certain organic crystals and liquids. See Particle Detectors.

C Cosmic Rays
About 1911 the Austrian-American physicist Victor Franz Hess discovered that cosmic radiation, consisting of rays originating outside the earth's atmosphere, arrived in a pattern determined by the earth's magnetic field (see Cosmic Rays). The rays were found to be positively charged and to consist mostly of protons with energies ranging from about 1 GeV to 1011 GeV (compared to about 30 GeV for the fastest particles produced by artificial accelerators). Cosmic rays trapped into orbits around the earth account for the Van Allen radiation belts discovered during an artificial-satellite flight in 1959 (see Radiation Belts).
When a very energetic primary proton smashes into the atmosphere and collides with the nitrogen and oxygen nuclei present, it produces large numbers of different secondary particles that spread toward the earth as a cosmic-ray shower. The origin of the cosmic-ray protons is not yet fully understood; some undoubtedly come from the sun and the other stars. Except for the slowest rays, however, no mechanism can be found to account for their high energies and the likelihood is that weak galactic fields operate over very long periods to accelerate interstellar protons (see Galaxy; Milky Way).
D Elementary Particles
To the electron, proton, neutron, and photon have been added a number of fundamental particles. In 1932 the American physicist Carl David Anderson discovered the antielectron, or positron, predicted in 1928 by Dirac. Anderson found that the stopping of an energetic cosmic gamma ray near a heavy nucleus yielded an electron-positron pair out of pure energy. When a positron subsequently meets an electron, they annihilate each other with a burst of photons of energy.

D1 Discovery of the Muon
In 1935 the Japanese physicist Yukawa Hideki developed a theory explaining how a nucleus is held together, despite the mutual repulsion of its protons, by postulating the existence of a particle intermediate in mass between the electron and the proton. In 1936 Anderson and his coworkers discovered a new particle of 207 electron masses in secondary cosmic radiation; now called the mu-meson or muon, it was first thought to be Yukawa's nuclear “glue.” Subsequent experiments by the British physicist Cecil Frank Powell and others led to the discovery of a somewhat heavier particle of 270 electron masses, the pi-meson or pion (also obtained from secondary cosmic radiation), which was eventually identified as the missing link in Yukawa's theory.
Many additional particles have since been found in secondary cosmic radiation and through the use of large accelerators. They include numerous massive particles, classed as hadrons (particles that take part in the “strong” interaction, which binds atomic nuclei together), including hyperons and various heavy mesons with masses ranging from about one to three proton masses; and intermediate vector bosons such as the W and Z0 particles, the carriers of the “weak” nuclear force. They may be electrically neutral, positive, or negative, but never have more than one elementary electric charge e. Enduring from 10-8 to 10-14 sec, they decay into a variety of lighter particles. Each particle has its antiparticle and carries some angular momentum. They all obey certain conservation laws involving quantum numbers, such as baryon number, strangeness, and isotopic spin.
In 1931 Pauli, in order to explain the apparent failure of some conservation laws in certain radioactive processes, postulated the existence of electrically neutral particles of zero-rest mass that nevertheless could carry energy and momentum. This idea was further developed by the Italian-born American physicist Enrico Fermi, who named the missing particle the neutrino. Uncharged and tiny, it is elusive, easily able to penetrate the entire earth with only a small likelihood of capture. Nevertheless, it was eventually discovered in a difficult experiment performed by the Americans Frederick Reines and Clyde Lorrain Cowan, Jr. Understanding of the internal structure of protons and neutrons has also been derived from the experiments of the American physicist Robert Hofstadter, using fast electrons from linear accelerators.
In the late 1940s a number of experiments with cosmic rays revealed new types of particles, the existence of which had not been anticipated. They were called strange particles, and their properties were studied intensively in the 1950s. Then, in the 1960s, many new particles were found in experiments with the large accelerators. The electron, proton, neutron, photon, and all the particles discovered since 1932 are collectively called elementary particles. But the term is actually a misnomer, for most of the particles, such as the proton, have been found to have very complicated internal structure.
Elementary particle physics is concerned with (1) the internal structure of these building blocks and (2) how they interact with one another to form nuclei. The physical principles that explain how atoms and molecules are built from nuclei and electrons are already known. At present, vigorous research is being conducted on both fronts in order to learn the physical principles upon which all matter is built.
One popular theory about the internal structure of elementary particles is that they are made of so-called quarks (see Quark), which are subparticles of fractional charge; a proton, for example, is made up of three quarks. This theory was first proposed in 1964 by the American physicists Murray Gell-Mann and George Zweig. The theory explains a number of phenomena, and physicists have collected a great deal of evidence of quarks in combinations with each other. No individual quarks have been observed, however, and current theory suggests that quarks may never be released as separate entities except under such extreme conditions as those found during the very creation of the universe. The theory postulated three kinds of quarks, but later experiments, especially the discovery of the J/psi particle in 1974 by the American physicists Samuel C. C. Ting and Burton Richter, called for the introduction of three additional kinds.

D2 Unified Field Theories
The interaction between elementary particles—and if quarks exist, between the quarks—is a more difficult area of research. The most successful theories, thus far, are called gauge theories. In these, the interaction between two kinds of particles is characterized by symmetry. The symmetry between neutrons and protons, for example, is such that if the identities of the particles are interchanged, nothing changes as far as the “strong” force is concerned. The first of the gauge theories applied to the electric and magnetic interactions between charged particles. Here, the symmetry consists in the fact that changes in the combination of electric and magnetic potentials have no effect on the results. A powerful gauge theory, which has since been verified, was that proposed independently by both the American physicist Steven Weinberg and the Pakistani physicist Abdus Salam in 1967 and 1968. Their model linked the intermediate vector boson with the photon, thus uniting the electromagnetic and weak interactions, although only for leptons. Later work by others (Sheldon Lee Glashow, J. Iliopolis, and L. Maiani) showed how the model could be applied to hadrons (the strongly interacting particles) as well.
Gauge theory, in principle, can be applied to any force field, holding out the possibility that all the interactions, or forces, can be brought together into a single unified field theory. Such efforts inevitably involve the concept of symmetry. Generalized symmetries extend to particle interchanges that vary from point to point in space and time. The difficulty for physicists is that such symmetries, while mathematically elegant, do not extend scientific understanding of the underlying nature of matter. For this reason, many physicists are exploring the possibilities of so-called supersymmetry theories, which would directly relate fermions and bosons to one another by postulating further particle “twins” to those now known, differing only in spin. Doubts have been expressed about such efforts, but another approach known as “superstring” theory is attracting a good deal of interest. In such theories, fundamental particles are considered not as dimensionless objects but as “strings” that extend one-dimensionally to lengths of no more than 10-35 meters. Such theories solve a number of problems for the physicists who are working on unified field theories, but they are still only highly theoretical constructs.

E Nuclear Physics
In 1931 the American physicist Harold Clayton Urey discovered the hydrogen isotope deuterium and made heavy water from it. The deuterium nucleus, or deuteron (one proton plus one neutron), makes an excellent bombarding particle for inducing nuclear reactions. The French physicists Irène and Frédéric Joliot-Curie produced the first artificially radioactive nucleus in 1933 and 1934, leading to the production of radioisotopes for use in archaeology, biology, medicine, chemistry, and other sciences.
Fermi and many collaborators attempted a series of experiments to produce elements beyond uranium by bombarding uranium with neutrons. They succeeded, and now at least a dozen such transuranium elements have been made. As their work continued, an even more important discovery was made. Irène Joliot-Curie, the German physicists Otto Hahn and Fritz Strassmann, the Austrian physicist Lise Meitner, and the British physicist Otto Robert Frisch found that some uranium nuclei broke into two parts, a phenomenon called nuclear fission. At the same time, a huge amount of energy was released by mass conversion, as well as some neutrons. These results suggested the possibility of a self-sustained chain reaction, and this was achieved by Fermi and his group in 1942, when the first nuclear reactor went into operation. Technological developments followed rapidly; the first atomic bomb was produced in 1945 as a result of a massive program under the direction of the American physicist J. Robert Oppenheimer, and the first nuclear power reactor for the production of electricity went into operation in England in 1956, yielding 78 million watts. See Nuclear Weapons.
Further developments were based on the investigation of the energy source of the stars, which the German American physicist Hans Albrecht Bethe showed to be a series of nuclear reactions occurring at temperatures of millions of degrees. In these reactions, four hydrogen nuclei are converted into a helium nucleus, with two positrons and massive amounts of energy forming the by-products. This nuclear-fusion process was adopted in modified form, largely based on ideas developed by the Hungarian-American physicist Edward Teller, as the basis of the fusion or hydrogen bomb. First detonated in 1952, it is a weapon much more powerful than the fission bomb. A small fission bomb provides the high temperature necessary to trigger fusion of hydrogen.
Much current research is devoted to producing a controlled, rather than an explosive, fusion device, which would be less radioactive than a fission reactor and would provide an almost limitless source of energy. In December 1993 significant progress was made toward this goal when researchers at Princeton University used the Tokamak Fusion Test Reactor to produce a controlled fusion reaction that output 5.6 million watts of power. However, the tokamak consumed more power than it produced during its operation.

F Solid-State Physics
In solids, the atoms are closely packed, leading to strong interactive forces and numerous interrelated effects that are not observed in gases, where the molecules largely act independently. Interaction effects lead to the mechanical, thermal, electrical, magnetic, and optical properties of solids, which is an area that remains difficult to handle theoretically, although much progress has been made.
A principal characteristic of most solids is their crystalline structure, with the atoms arranged in regular and geometrically repeating arrays (see Crystal). The specific arrangement of the atoms may arise from a variety of forces; thus, some solids, such as sodium chloride, or common salt, are held together by ionic bonds originating in the electric attraction between the ions of which the materials are composed. In others, such as diamond, atoms share electrons, giving rise to covalent bonding. Inert substances, such as neon, exhibit neither of these bonds. Their existence is a result of the so-called van der Waals forces, named after the Dutch physicist Johannes Diderik van der Waals. These forces exist between neutral molecules or atoms as a result of electric polarization. Metals, on the other hand, are bonded by a so-called electron gas, or electrons that are freed from the outer atomic shell and shared by all atoms, and that define most properties of the metal (see Metallography; Metals).
The sharp, discrete energy levels permitted to the electrons in individual atoms become broadened into energy bands when the atoms become closely packed in a solid. The width and separation of these bands define many properties, and thus the separation by a so-called forbidden band, where no electrons may exist, restricts their motion and results in a good electric and thermal insulator. Overlapping energy bands and their associated ease of electron motion results in their being good conductors of electricity and heat. If the forbidden band is narrow, a few fast electrons may be able to jump across, yielding a semiconductor. In this case the energy-band spacing may be greatly affected by minute amounts of impurities, such as arsenic in silicon. The lowering of a high-energy band by the impurity results in a so-called donor of electrons, or an n-type semiconductor. The raising of a low-energy band by an impurity like gallium results in an acceptor, where the vacancies or “holes” in the electron structure act like movable positive charges and are characteristic of p-type semiconductors. A number of modern electronic devices, notably the transistor, developed by the American physicists John Bardeen, Walter Houser Brattain, and William Bradford Shockley, are based on these semiconductor properties.
Magnetic properties in a solid arise from the electrons' acting like tiny magnetic dipoles. Electron spin plays a big role in magnetism, leading to spin waves that have been observed in some solids. Almost all solid properties depend on temperature. Thus, ferromagnetic materials, including iron and nickel, lose their normal strong residual magnetism at a characteristic high temperature, called the Curie temperature. Electrical resistance usually decreases with decreasing temperature, and for certain materials, called superconductors, it becomes extremely low, near absolute zero. These and many other phenomena observed in solids depend on energy quantization and can best be described in terms of effective “particles” such as phonons, polarons, and magnons.

G Cryogenics
At very low temperatures (near absolute zero), many materials exhibit strikingly different characteristics (see Cryogenics). At the beginning of the 20th century the Dutch physicist Heike Kamerlingh Onnes developed techniques for producing these low temperatures and discovered the superconductivity of mercury: It loses all electrical resistance at about 4 K. Many other elements, alloys, and compounds do the same at their characteristic near-zero temperature, with originally magnetic materials becoming magnetic insulators. The theory of superconductivity, developed largely by the American physicists John Bardeen, Leon N. Cooper, and John Robert Schrieffer, is extremely complicated, involving the pairing of electrons in the crystal lattice.
Another fascinating discovery was that helium does not freeze but changes at about 2 K from an ordinary liquid, He I, to the superfluid He II, which has no viscosity and has a thermal conductivity about 1000 times greater than silver. Films of He II can creep up the walls of their containing vessels and He II can readily permeate some materials like platinum. No fully satisfactory theory is yet available for this behavior.

H Plasma Physics
A plasma is any substance (usually a gas) whose atoms have one or more electrons detached and therefore become ionized. The detached electrons remain, however, in the gas volume that in an overall sense remains electrically neutral. The ionization can be effected by the introduction of large concentrations of energy, such as bombardment with fast external electrons, irradiation with laser light, or by heating the gas to very high temperatures (see Laser). The individually charged plasma particles respond to electric and magnetic fields and can therefore be manipulated and contained.
Plasmas are found in gas-filled light sources, such as a neon lamp, in interstellar space where residual hydrogen is ionized by radiation, and in stars whose great interior temperatures produce a high degree of ionization, a process closely connected with the nuclear fusion that supplies the energy of stars. For the hydrogen nuclei to fuse into heavier nuclei, they must be fast enough to overcome their mutual electric repulsion. This implies high temperature (millions of degrees) when the hydrogen ionizes into a plasma. In order to produce a controlled fusion, or thermonuclear reaction, it is necessary to generate and contain plasmas magnetically; this is an important but difficult problem that falls in the field of magnetohydrodynamics.

I Lasers
An important recent development is that of the laser, an acronym for light amplification by stimulated emission of radiation. In lasers, which may have gases, liquids, or solids as the working substance, a large number of atoms are raised to a high energy level and caused to release this energy simultaneously, producing coherent light where all waves are in phase. Similar techniques are used for producing microwave emissions by the use of masers. The coherence of the light allows for very high intensity, sharp wavelength light beams that remain narrow over tremendous distances; they are far more intense than light from any other source. Continuous lasers can deliver hundreds of watts of power, and pulsed lasers can produce millions of watts of power for very short periods. Developed during the 1950s and 1960s, largely by the American engineer and inventor Gordon Gould and the American physicists Charles Hard Townes, T. H. Maiman, Arthur Leonard Schawlow, and Ali Javan, the laser today has become an extremely powerful tool in research and technology, with applications in communications, medicine, navigation, metallurgy, fusion, and material cutting.

J Astrophysics
The construction of large and specially designed optical telescopes has led to the discovery of new stellar objects, including a number of quasars, which are billions of light-years away, and has led to a better understanding of the structure of the universe. Radio astronomy has yielded other important discoveries, such as pulsars and the cosmic background radiation, which probably dates from the origin of the universe. The evolutionary history of the stars is now well understood in terms of nuclear reactions. As a result of recent observations and theoretical calculations, the belief is now widely held that all matter was originally in one dense location and that about 14 billion years ago it exploded in one titanic event often called the big bang. The aftereffects of the explosion have led to a universe that appears to be still expanding. A puzzling aspect of this universe, recently revealed, is that the galaxies are not uniformly distributed. Instead, vast voids are bordered by galactic clusters shaped like filaments. The pattern of these voids and filaments lends itself to nonlinear mathematical analysis of the sort used in chaos theory. See also Inflationary Theory.
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