History of subatomic physics

The idea that matter consists of smaller particles and that there exists a limited number of sorts of

; they can cease to exist and create (other) particles in result.

Increasingly small particles have been discovered and researched: they include

.

Early development

Main articles:

Atomic theory

The idea that all

, Gassendi, and Newton.

Those early ideas were founded through

) contradict to almost everything known in modern physics.

In the 19th century, John Dalton, through his work on stoichiometry, 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 word atomos, 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.

From atoms to nucleons

The state of electromagnetic theory

Throughout the 1800's scientists explored many phenomena of electricity and magnetism, culminating in an accurate theory by

^: 77

The Electron

The electron was discovered between 1879 and 1897 in works of William Crookes, Arthur Schuster, J. J. Thomson, and other physicists; its charge was carefully measured by Robert Andrews Millikan and Harvey Fletcher in their oil drop experiment of 1909. Physicists theorized that negatively 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.

Radioactivity

Studies of the "radioactivity", that soon revealed the phenomenon of radioactive decay, provided another argument against considering chemical elements as fundamental nature's elements. Despite these discoveries, the term atom stuck to Dalton's (chemical) atoms and now denotes the smallest particle of a chemical element, not something really indivisible.

Researching particles' interaction

Early 20th-century physicists knew only two

.

In 1909

.

This section needs expansion. You can help by adding to it. (January 2014)

Inside the atom

particles including the positron

were even discovered by using this device

By 1914, experiments by Ernest Rutherford, Henry Moseley, James Franck and Gustav 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 of

in terms of the arrangement of electrons in the mostly empty volume of atoms.

In 1918, Rutherford confirmed that the

. Evidences that atomic nuclei consist of some smaller particles (now called nucleons) grew; it became obvious that, while protons repulse each other electrostatically, nucleons attract each other by some new force (nuclear force). It culminated in proofs of nuclear fission in 1939 by Lise Meitner (based on experiments by Otto Hahn), and nuclear fusion by Hans 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) the transmutation of lead into gold; and, those same discoveries also led to the development of nuclear weapons.

Revelations of quantum mechanics

spherically symmetric

, but it is convenient to assign to "distinct" p-electrons these two-lobed shapes.

Further understanding of atomic and nuclear structures became impossible without improving the knowledge about the essence of particles. Experiments and improved theories (such as

for example.

Another crucial discovery was

.

The

even

total 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.

The

spins.

Improved understanding of the world of particles prompted physicists to make bold predictions, such as

). Both were later confirmed.

This culminated in the formulation of ideas of a

) as an explanation of the nuclear force.

From nuclides to nuclear engineering

Nuclear physics
Nucleus Nucleons p n Nuclear matter Nuclear force Nuclear structure Nuclear reaction
Models of the nucleus Liquid drop Nuclear shell model Interacting boson model Ab initio
Nuclides' classification Isotopes – equal Z Isobars – equal A Isotones – equal N N − Z Isomers – equal all the above Mirror nuclei – Z ↔ N Stable Magic Even/odd Halo Borromean
Nuclear stability Binding energy p–n ratio Drip line Island of stability Valley of stability Stable nuclide
Radioactive decay Alpha α Beta β 2β 0v β+ K/L capture Isomeric Gamma γ Internal conversion Spontaneous fission Cluster decay Neutron emission Proton emission Decay energy Decay chain Decay product Radiogenic nuclide
Nuclear fission Spontaneous Products pair breaking Photofission
Capturing processes electron 2× neutron s r proton p rp
High-energy processes Spallation by cosmic ray Photodisintegration
Nucleosynthesis and nuclear astrophysics Nuclear fusion Processes: Stellar Big Bang Supernova Nuclides: Primordial Cosmogenic Artificial
High-energy nuclear physics Quark–gluon plasma RHIC LHC
Scientists Alvarez Becquerel Bethe A. Bohr N. Bohr Chadwick Cockcroft Ir. Curie Fr. Curie Pi. Curie Skłodowska-Curie Davisson Fermi Hahn Jensen Lawrence Mayer Meitner Oliphant Oppenheimer Proca Purcell Rabi Rutherford Soddy Strassmann Świątecki Szilárd Teller Thomson Walton Wigner
Physics portal  Category
v t e

Development of

in atoms is also impossible as yet).

The most developed branch of nuclear physics in 1940s was studies related to

.

The

elements 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 to

.

Physics goes to high energies

Strange particles and mysteries of the weak interaction

In the 1950s, with development of

of fundamental particles.

The weak interaction revealed soon yet another mystery. In 1957 Chien-Shiung Wu proved that it does not conserve parity. In other words, the mirror symmetry was disproved as a fundamental symmetry law.

Throughout the 1950s and 1960s, improvements in particle accelerators and particle detectors led to a bewildering variety of particles found in high-energy experiments. The term elementary particle came to refer to dozens of particles, most of them unstable. 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, form mesons and baryons that counted most of then-known particles.

Deeper constituents of matter

The interaction of these particles by scattering and decay provided a key to new fundamental quantum theories. Murray Gell-Mann and Yuval 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 the
Ω⁻
, which was detected at Brookhaven National Laboratory in 1964, and which gave rise to the quark model of hadron composition. While the quark model at first seemed inadequate to describe strong nuclear forces, allowing the temporary rise of competing theories such as the S-matrix theory, the establishment of quantum chromodynamics in the 1970s finalized a set of fundamental and exchange particles (Kragh 1999). It postulated the fundamental strong interaction, experienced by quarks and mediated by gluons. These particles were proposed as a building material for hadrons (see hadronization). This theory is unusual because individual (free) quarks cannot be observed (see color confinement), unlike the situation with composite atoms where electrons and nuclei can be isolated by transferring ionization energy to the atom.

Then, the old, broad denotation of the term elementary particle was deprecated and a replacement term subatomic 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 for leptons, quarks, their antiparticles, and quanta of fundamental interactions (see below) only.

Quarks, leptons, and four fundamental forces

Because the quantum field theory (see above) postulates no difference between particles and interactions, classification of elementary particles allowed also to classify interactions and fields.

Now a large number of particles and (non-fundamental) interactions is explained as combinations of a (relatively) small number of fundamental substances, thought to be fundamental interactions (incarnated in fundamental bosons), quarks (including antiparticles), and leptons (including antiparticles). As the theory distinguished several fundamental interactions, it became possible to see which elementary particles participate in which interaction. Namely:

• All particles participate in gravitation.
• All charged elementary particles participate in electromagnetic interaction.
  • As a consequence, neutron participates in it with its magnetic dipole in spite of zero electric charge. This is because it is composed of charged quarks whose charges sum to zero.
• All fermions participate in the weak interaction.
• Quarks participate in the strong interaction, along gluons (its own quanta), but not leptons nor any fundamental bosons other than gluons.

The next step was a reduction in number of fundamental interactions, envisaged by early 20th century physicists as the "

The discovery of the weak gauge bosons (quanta of the weak 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, although supersymmetry and string theory were believed by many theorists to be a promising avenue forward. The Large 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, as David Gross has said, "the kind of acceptable scientific confusion that discovery eventually transcends."^[14]

The fourth fundamental force,

, is not yet integrated into particle physics in a consistent way.

Higgs boson

As of 2011, the

, remained the only particle of the Standard Model to be verified. On July 4, 2012, physicists working at CERN's , 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.

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.

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, Higgslike 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]

Notes

References

• Kragh, Helge (1999), Quantum Generations: A History of Physics in the Twentieth Century, Princeton: Princeton University Press.