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The idea that matter consists of smaller particles and that there exists a limited number of sorts ofprimary, smallest particles innature has existed innatural philosophy at least since the 6th century BC. Such ideas gainedphysicalcredibility beginning in the 19th century, but the concept of "elementary particle" underwent somechanges in its meaning: notably, modern physics no longer deems elementary particles indestructible. Even elementary particles candecay or collide destructively; they can cease to exist and create (other) particles in result.
Increasingly small particles have been discovered and researched: they includemolecules, which are constructed ofatoms, that in turn consist ofsubatomic particles, namelyatomic nuclei andelectrons. Many more types of subatomic particles have been found. Most such particles (but not electrons) were eventually found to be composed of even smaller particles such asquarks.Particle physics studies these smallest particles;nuclear physics studies atomic nuclei and their (immediate) constituents:protons andneutrons.
The idea that allmatter is composed ofelementary particles dates to as far as the 6th century BCE.[1] TheJains in ancient India were the earliest to advocate the particular nature of material objects between 9th and 5th century BCE. According to Jain leaders likeParshvanatha andMahavira, theajiva (non living part of universe) consists of matter orpudgala, of definite or indefinite shape which is made up tiny uncountable and invisible particles calledpermanu.Permanu occupies space-point and eachpermanu has definite colour, smell, taste and texture. Infinite varieties ofpermanu unite and formpudgala.[2] The philosophical doctrine ofatomism and the nature of elementary particles were also studied byancient Greek philosophers such asLeucippus,Democritus, andEpicurus; ancientIndian philosophers such asKanada,Dignāga, andDharmakirti; Muslim scientists such asIbn al-Haytham,Ibn Sina, andMohammad al-Ghazali; and inearly modern Europe by physicists such asPierre Gassendi,Robert Boyle, andIsaac Newton. The particle theory oflight was also proposed byIbn al-Haytham,Ibn Sina, Gassendi, and Newton.
Those early ideas were founded throughabstract,philosophical reasoning rather thanexperimentation andempirical observation and represented only one line of thought among many. In contrast, certain ideas ofGottfried Wilhelm Leibniz (seeMonadology) contradict to almost everything known in modern physics.
In the 19th century,John Dalton, through his work onstoichiometry, concluded that each chemical element was composed of a single, unique type of particle. Dalton and his contemporaries believed those were the fundamental particles of nature and thus named them atoms, after the Greek wordatomos, meaning "indivisible"[3] or "uncut".However, near the end of 19th century, physicists discovered that Dalton's atoms are not, in fact, the fundamental particles of nature, but conglomerates of even smaller particles.
Throughout the 1800s scientists explored many phenomena of electricity and magnetism, culminating in an accurate theory byJames Clerk Maxwell.[4] This theory was a continuous field model developed around the ideas ofluminiferous aether. When no experiment could produce evidence of such an ether, and in view of the growing evidence supporting the atomic model,Hendrik Antoon Lorentz developed a theory of electromagnetism based on "ions" that reproduced Maxwell's model.[5]: 77
Theelectron was discovered between 1879 and 1897 in works ofWilliam Crookes,Arthur Schuster,J. J. Thomson, and other physicists; its charge was carefully measured byRobert Andrews Millikan andHarvey Fletcher in theiroil drop experiment of 1909. Physicists theorized thatnegatively charged electrons are constituent part of "atoms", along with some (yet unknown) positively charged substance, and it was later confirmed. Electron became the first elementary, truly fundamental particle discovered.
Studies of the "radioactivity", that soon revealed the phenomenon ofradioactive decay, provided another argument against consideringchemical elements as fundamental nature's elements. Despite these discoveries, the termatom stuck to Dalton's (chemical) atoms and now denotes the smallest particle of a chemical element, not something really indivisible.
Early 20th-century physicists knew only twofundamental forces:electromagnetism andgravitation, where the latter could not explain the structure of atoms. So, it was obvious to assume that unknown positively charged substance attracts electrons byCoulomb force.
In 1909Ernest Rutherford andThomas Royds demonstrated that analpha particle combines with two electrons and forms ahelium atom. In modern terms, alpha particles are doublyionized helium (more precisely,4
He) atoms. Speculation about the structure of atoms was severely constrained by Rutherford's 1907gold foil experiment, showing that the atom is mainly empty space, with almost all its mass concentrated in a tinyatomic nucleus.
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By 1914, experiments by Ernest Rutherford,Henry Moseley,James Franck andGustav Hertz had largely established the structure of an atom as a dense nucleus of positive charge surrounded by lower-mass electrons.[6]These discoveries shed a light to the nature ofradioactive decay and other forms oftransmutation of elements, as well as of elements themselves. It appeared thatatomic number is nothing else than (positive)electric charge of the atomic nucleus of a particular atom. Chemical transformations, governed byelectromagnetic interactions, do not change nuclei – that's why elements are chemically indestructible. But when the nucleus change its charge and/or mass (by emitting or capturing aparticle), the atom can become the one of another element.Special relativity explained how themass defect is related to theenergy produced or consumed in reactions. The branch of physics that studies transformations and the structure of nuclei is now callednuclear physics, contrasted toatomic physics that studies the structure and properties of atoms ignoring most nuclear aspects. The development in the nascentquantum physics, such asBohr model, led to the understanding ofchemistry in terms of the arrangement of electrons in the mostly empty volume of atoms.
In 1918, Rutherford confirmed that thehydrogen nucleus was a particle with a positive charge, which he named theproton. By then,Frederick Soddy's researches of radioactive elements, and experiments of J. J. Thomson andF.W. Aston conclusively demonstrated existence ofisotopes, whose nuclei have different masses in spite of identical atomic numbers. It prompted Rutherford to conjecture that all nuclei other than hydrogen contain chargeless particles, which he named theneutron.Evidences that atomic nuclei consist of some smaller particles (now callednucleons) grew; it became obvious that, while protons repulse each otherelectrostatically, nucleons attract each other by some new force (nuclear force). It culminated in proofs ofnuclear fission in 1939 byLise Meitner (based on experiments byOtto Hahn), andnuclear fusion byHans Bethe in that same year. Those discoveries gave rise to an active industry of generating one atom from another, even rendering possible (although it will probably never be profitable) thetransmutation of lead into gold; and, those same discoveries also led to the development ofnuclear weapons.
Further understanding of atomic and nuclear structures became impossible without improving the knowledge about the essence of particles. Experiments and improved theories (such asErwin Schrödinger's "electron waves") gradually revealed that there isno fundamental difference between particles andwaves. For example, electromagnetic waves were reformulated in terms of particles calledphotons. It also revealed that physical objects do not change their parameters, such astotal energy,position andmomentum, ascontinuous functions oftime, as it was thought of in classical physics: seeatomic electron transition for example.
Another crucial discovery wasidentical particles or, more generally, quantumparticle statistics. It was established that all electrons are identical: although two or more electrons can exist simultaneously that have different parameters, but they do not keep separate, distinguishable histories. This also applies to protons, neutrons, and (with certain differences) to photons as well. It suggested that there is a limited number of sorts of smallest particles in theuniverse.
Thespin–statistics theorem established that any particle in ourspacetime may be either aboson (that means its statistics isBose–Einstein) or afermion (that means its statistics isFermi–Dirac). It was later found that all fundamental bosons transmit forces, like the photon that transmits light. Some of non-fundamental bosons (namely,mesons) also may transmit forces (seebelow), although non-fundamental ones. Fermions are particles "like electrons and nucleons" and generally comprise the matter. Note that any subatomic or atomic particle composed ofeventotal number of fermions (such as protons, neutrons, and electrons) is a boson, so a boson is not necessarily a force transmitter and perfectly can be an ordinary material particle.
Thespin is the quantity that distinguishes bosons and fermions. Practically it appears as an intrinsicangular momentum of a particle, that is unrelated to itsmotion but is linked with some other features like amagnetic dipole. Theoretically it is explained from different typesrepresentations of symmetry groups, namelytensor representations (including vectors and scalars) for bosons with their integer (inħ) spins, andspinor representations for fermions with theirhalf-integer spins.
Improved understanding of the world of particles prompted physicists to make bold predictions, such asDirac'spositron in 1928 (founded on theDirac Sea model) andPauli'sneutrino in 1930 (founded on conservation of energy and angular momentum inbeta decay). Both were later confirmed.
This culminated in the formulation of ideas of aquantum field theory. The first (and the only mathematically complete) of these theories,quantum electrodynamics, allowed to explain thoroughly the structure of atoms, including thePeriodic Table andatomic spectra. Ideas ofquantum mechanics and quantum field theory were applied to nuclear physics too. For example,α decay was explained as aquantum tunneling through nuclear potential, nucleons' fermionic statistics explained thenucleon pairing, andHideki Yukawa proposed certainvirtual particles (now knows asπ-mesons) as an explanation of the nuclear force.
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Development ofnuclear models (such as theliquid-drop model andnuclear shell model) made prediction of properties ofnuclides possible. No existing model of nucleon–nucleon interaction cananalytically compute something more complex than4
He based on principles of quantum mechanics, though (note that complete computation ofelectron shells in atoms is also impossible as yet).
The most developed branch of nuclear physics in 1940s was studies related tonuclear fission due to its military significance. The main focus of fission-related problems is interaction of atomic nuclei with neutrons: a process that occurs in afission bomb and anuclear fission reactor. It gradually drifted away from the rest of subatomic physics and virtually became thenuclear engineering. The first synthesisedtransuranium elements were also obtained in this context, throughneutron capture and subsequentβ− decay.
Theelements beyond fermium cannot be produced in this way. To make a nuclide with more than 100 protons per nucleus one has to use an inventory and methods of particle physics (see details below), namely to accelerate and collide atomic nuclei. Production of progressively heavier synthetic elements continued into 21st century as a branch of nuclear physics, but only for scientific purposes.
The third important stream in nuclear physics are researches related tonuclear fusion. This is related tothermonuclear weapons (and conceived peacefulthermonuclear energy), as well as toastrophysical researches, such asstellar nucleosynthesis andBig Bang nucleosynthesis.
In the 1950s, with development ofparticle accelerators and studies ofcosmic rays,inelastic scattering experiments on protons (and other atomic nuclei) with energies about hundreds ofMeVs became affordable. They created some short-livedresonance "particles", but alsohyperons andK-mesons with unusually long lifetime. The cause of the latter was found in a new quasi-conserved quantity, namedstrangeness, that is conserved in all circumstances except for theweak interaction. The strangeness of heavy particles and theμ-lepton were first two signs of what is now known as thesecond generation of fundamental particles.
The weak interaction revealed soon yet another mystery. In 1957Chien-Shiung Wu proved that it does notconserve parity. In other words, the mirror symmetry was disproved as a fundamentalsymmetry law.
Throughout the 1950s and 1960s, improvements in particle accelerators andparticle detectors led to a bewildering variety of particles found in high-energy experiments. The termelementary particle came to refer to dozens of particles, most of themunstable. It prompted Wolfgang Pauli's remark: "Had I foreseen this, I would have gone into botany". The entire collection was nicknamed the "particle zoo". It became evident that some smaller constituents, yet invisible, formmesons andbaryons that counted most of then-known particles.
The interaction of these particles byscattering anddecay provided a key to new fundamental quantum theories.Murray Gell-Mann andYuval Ne'eman brought some order to mesons and baryons, the most numerous classes of particles, by classifying them according to certain qualities. It began with what Gell-Mann referred to as the "Eightfold Way", but proceeding into several different "octets" and "decuplets" which could predict new particles, most famously theΩ−
, which was detected atBrookhaven National Laboratory in 1964, and which gave rise to thequark model of hadron composition. While thequark model at first seemed inadequate to describestrong nuclear forces, allowing the temporary rise of competing theories such as theS-matrix theory, the establishment ofquantum chromodynamics in the 1970s finalized a set of fundamental and exchange particles (Kragh 1999). It postulated the fundamentalstrong interaction, experienced by quarks and mediated bygluons. These particles were proposed as a building material for hadrons (seehadronization). This theory is unusual because individual (free) quarks cannot be observed (seecolor confinement), unlike the situation with composite atoms where electrons and nuclei can be isolated by transferringionization energy to the atom.
Then, the old, broaddenotation of the termelementary particle was deprecated and a replacement termsubatomic particle covered all the "zoo", with its hyponym "hadron" referring to composite particles directly explained by the quark model. The designation of an "elementary" (or "fundamental") particle was reserved forleptons, quarks, theirantiparticles, andquanta of fundamental interactions (see below) only.
Because the quantum field theory (seeabove) postulates no difference between particles andinteractions, classification of elementary particles allowed also to classify interactions andfields.
Now a large number of particles and (non-fundamental) interactions is explained as combinations of a (relatively) small number of fundamental substances, thought to befundamental interactions (incarnated in fundamentalbosons), quarks (including antiparticles), andleptons (including antiparticles). As the theory distinguishedseveral fundamental interactions, it became possible to see which elementary particles participate in which interaction. Namely:
The next step was a reduction in number of fundamental interactions, envisaged by early 20th century physicists as the "united field theory". The first successful modernunified theory was theelectroweak theory, developed byAbdus Salam,Steven Weinberg and, subsequently,Sheldon Glashow. This development culminated in the completion of the theory called the Standard Model in the 1970s, that included also the strong interaction, thus covering three fundamental forces. After the discovery, made atCERN, of the existence ofneutral weak currents,[7][8][9][10] mediated by theZ boson foreseen in the standard model, the physicists Salam, Glashow and Weinberg received the 1979Nobel Prize in Physics for their electroweak theory.[11]The discovery of theweak gauge bosons (quanta of theweak interaction) through the 1980s, and the verification of their properties through the 1990s is considered to be an age of consolidation in particle physics.
While accelerators have confirmed most aspects of the Standard Model by detecting expected particle interactions at various collision energies, no theory reconciling general relativity with the Standard Model has yet been found, althoughsupersymmetry andstring theory were believed by many theorists to be a promising avenue forward. TheLarge Hadron Collider, however, which began operating in 2008, has failed to find any evidence whatsoever that is supportive of supersymmetry and string theory,[12] and appears unlikely to do so, meaning "the current situation in fundamental theory is one of a serious lack of any new ideas at all."[13] This state of affairs should not be viewed as a crisis in physics, but rather, asDavid Gross has said, "the kind of acceptable scientific confusion that discovery eventually transcends."[14]
Gravitation, the fourth fundamental interaction, is not yet integrated into particle physics in a consistent way.
As of 2011, theHiggs boson, the quantum of a field that is thought to provide particles withrest masses, remained the only particle of the Standard Model to be verified.On July 4, 2012, physicists working at CERN'sLarge Hadron Collider announced that they had discovered a new subatomic particle greatly resembling the Higgs boson, a potential key to an understanding of why elementary particles have masses and indeed to the existence of diversity and life in the universe.[15]Rolf-Dieter Heuer, the director general of CERN, said that it was too soon to know for sure whether it is an entirely new massive particle – one of the heaviest subatomic particles yet – or, indeed, the elusive particle predicted by theStandard Model, the theory that has ruled physics for the last half-century.[15] It is unknown if this particle is an impostor, a single particle or even the first of many particles yet to be discovered. The latter possibilities are particularly exciting to physicists since they could point the way to new deeper ideas,beyond the Standard Model, about the nature of reality. For now, some physicists are calling it a "Higgslike" particle.[15]Joe Incandela, of theUniversity of California, Santa Barbara, said, "It's something that may, in the end, be one of the biggest observations of any new phenomena in our field in the last 30 or 40 years, going way back to the discovery of quarks, for example."[15] The groups operating the large detectors in the collider said that the likelihood that their signal was a result of a chance fluctuation was less than one chance in 3.5 million, so-called "five sigma," which is the gold standard in physics for a discovery.Michael Turner, a cosmologist at the University of Chicago and the chairman of the physics center board, said
This is a big moment for particle physics and a crossroads — will this be the high water mark or will it be the first of many discoveries that point us toward solving the really big questions that we have posed?
— Michael Turner, University of Chicago[15]
Confirmation of the Higgs boson or something very much like it would constitute a rendezvous with destiny for a generation of physicists who have believed the boson existed for half a century without ever seeing it. Further, it affirms a grand view of a universe ruled by simple and elegant and symmetrical laws, but in which everything interesting in it being a result of flaws or breaks in that symmetry.[15] According to the Standard Model, the Higgs boson is the only visible and particular manifestation of aninvisible force field that permeates space and imbues elementary particles that would otherwise be massless with mass. Without this Higgs field, or something like it, physicists say all the elementary forms of matter would zoom around at the speed of light; there would be neitheratoms nor life. The Higgs boson achieved a notoriety rare for abstract physics.[15] To the eternal dismay of his colleagues, Leon Lederman, the former director ofFermilab, called it the "God particle" in his book of the same name, later quipping that he had wanted to call it "the goddamn particle".[15] Professor Incandela also stated,
This boson is a very profound thing we have found. We're reaching into the fabric of the universe at a level we've never done before. We've kind of completed one particle's story [...] We're on the frontier now, on the edge of a new exploration. This could be the only part of the story that's left, or we could open a whole new realm of discovery.
— Joe Incandela, University of California[16]
Dr.Peter Higgs was one of six physicists, working in three independent groups, who in 1964 invented the notion of the cosmic molasses, or Higgs field. The others wereTom Kibble ofImperial College, London;Carl Hagen of theUniversity of Rochester;Gerald Guralnik ofBrown University; andFrançois Englert andRobert Brout, both ofUniversité Libre de Bruxelles.[15] One implication of their theory was that this Higgs field would produce its own quantum particle if hit hard enough by the right amount of energy. The particle would be fragile and fall apart within a millionth of a second in a dozen different ways depending upon its own mass. Unfortunately, the theory did not predict the particle mass making it difficult to find. The particle eluded researchers at a succession of particle accelerators.[15][better source needed]
Further experiments continued and in March 2013 it was tentatively confirmed that the newly discovered particle was a Higgs Boson.
Although they have never been seen, Higgs-like fields play an important role in theories of the universe and in string theory. Under certain conditions, according to the strange accounting of Einsteinian physics, they can become suffused with energy that exerts an antigravitational force. Such fields have been proposed as the source of an enormous burst of expansion, known as inflation, early in the universe and, possibly, as the secret of the dark energy that now seems to be speeding up the expansion of the universe.[15]
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