Alexandru Proca
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Alexandru Proca (16 October 1897 – 13 December 1955) was a
Romania Romania ( ; ro, România ) is a country located at the crossroads of Central, Eastern, and Southeastern Europe. It borders Bulgaria to the south, Ukraine to the north, Hungary to the west, Serbia to the southwest, Moldova to the east, and ...
n physicist who studied and worked in
France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of overseas regions and territories in the Americas and the Atlantic, Pacific and Indian Oceans. Its metropolitan area ...
. He developed the vector
meson In particle physics, a meson ( or ) is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, ...
theory of
nuclear force The nuclear force (or nucleon–nucleon interaction, residual strong force, or, historically, strong nuclear force) is a force that acts between the protons and neutrons of atoms. Neutrons and protons, both nucleons, are affected by the nucle ...
s and the relativistic quantum field equations that bear his name ( Proca's equations) for the massive, vector spin-1 mesons.


Biography

He was born in
Bucharest Bucharest ( , ; ro, București ) is the capital and largest city of Romania, as well as its cultural, industrial, and financial centre. It is located in the southeast of the country, on the banks of the Dâmbovița River, less than north of ...
, the son of a civil engineer. He was one of the eminent students at the Gheorghe Lazăr High School and Politehnica University in Bucharest. With a very strong interest in theoretical physics, he went to Paris where he graduated in Science from the
Paris-Sorbonne University Paris-Sorbonne University (also known as Paris IV; french: Université Paris-Sorbonne, Paris IV) was a public research university in Paris, France, active from 1971 to 2017. It was the main inheritor of the Faculty of Humanities of the Universit ...
, receiving from the hand of
Marie Curie Marie Salomea Skłodowska–Curie ( , , ; born Maria Salomea Skłodowska, ; 7 November 1867 – 4 July 1934) was a Polish and naturalized-French physicist and chemist who conducted pioneering research on radioactivity. She was the first ...
his diploma of
Bachelor of Science A Bachelor of Science (BS, BSc, SB, or ScB; from the Latin ') is a bachelor's degree awarded for programs that generally last three to five years. The first university to admit a student to the degree of Bachelor of Science was the University o ...
degree. After that he was employed as a researcher/physicist at the Radium Institute in Paris in 1925. Proca became a French citizen in 1931. He carried out Ph.D. studies in theoretical physics under the supervision of Nobel laureate
Louis de Broglie Louis Victor Pierre Raymond, 7th Duc de Broglie (, also , or ; 15 August 1892 – 19 March 1987) was a French physicist and aristocrat who made groundbreaking contributions to Old quantum theory, quantum theory. In his 1924 PhD thesis, he pos ...
. In 1933 he defended successfully his
Ph.D. A Doctor of Philosophy (PhD, Ph.D., or DPhil; Latin: or ') is the most common degree at the highest academic level awarded following a course of study. PhDs are awarded for programs across the whole breadth of academic fields. Because it is ...
thesis entitled ''"On the relativistic theory of Dirac's electron"'' in front of an examination committee chaired by the Nobel laureate Jean Perrin. In 1939 he was invited to the
Solvay Conference The Solvay Conferences (french: Conseils Solvay) have been devoted to outstanding preeminent open problems in both physics and chemistry. They began with the historic invitation-only 1911 Solvay Conference on Physics, considered a turning point i ...
, which did not take place because of the outbreak of
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
. During the war he was for a short time a senior engineer at
Radio France Radio France is the French national public radio broadcaster. Stations Radio France offers seven national networks: * France Inter — Radio France's " generalist" station, featuring entertaining and informative talk mixed with a wide variety o ...
. In 1943 he made a brief stay in
Portugal Portugal, officially the Portuguese Republic ( pt, República Portuguesa, links=yes ), is a country whose mainland is located on the Iberian Peninsula of Southwestern Europe, and whose territory also includes the Atlantic archipelagos of ...
, where (replacing Guido Beck) he guided the seminar on Theoretical Physics, organized by Ruy Luís Gomes at the Center for Mathematical Studies at the University of Porto. From 1943 to 1945 he was in the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the European mainland, continental mainland. It comprises England, Scotlan ...
, at the invitation of the
Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...
and the British Admiralty, in order to assist in the war effort. Afterward he went back to Paris, where he led a seminar on elementary particle physics. He sought to get a chair at the
Sorbonne Sorbonne may refer to: * Sorbonne (building), historic building in Paris, which housed the University of Paris and is now shared among multiple universities. *the University of Paris (c. 1150 – 1970) *one of its components or linked institution, ...
or at the
Collège de France The Collège de France (), formerly known as the ''Collège Royal'' or as the ''Collège impérial'' founded in 1530 by François I, is a higher education and research establishment ('' grand établissement'') in France. It is located in Paris n ...
, but was unsuccessful. From 1950 he organized a colloquium in theoretical physics for the CNRS with Pierre Auger, while in 1951 he was the French representative at the
International Union of Pure and Applied Physics The International Union of Pure and Applied Physics (IUPAP ) is an international non-governmental organization whose mission is to assist in the worldwide development of physics, to foster international cooperation in physics, and to help in the ...
. In 1937 Proca was elected corresponding member of the
Romanian Academy of Sciences The Romanian Academy of Sciences was an institution established in Romania by a group of 26 scientists, dissatisfied with the imperfect organization of the Scientific Section of the Romanian Academy, which was left in the background, with only 12 ...
, while in 1990 he was elected post-mortem honorary member of the
Romanian Academy The Romanian Academy ( ro, Academia Română ) is a cultural forum founded in Bucharest, Romania, in 1866. It covers the scientific, artistic and literary domains. The academy has 181 active members who are elected for life. According to its byl ...
. He died in Paris in 1955 after a two-year battle with laryngeal cancer.


Scientific achievements

In 1929, Proca became the editor of the influential physics journal '' Les Annales de l'Institut Henri Poincaré''. Then, in 1934, he spent an entire year with
Erwin Schrödinger Erwin Rudolf Josef Alexander Schrödinger (, ; ; 12 August 1887 – 4 January 1961), sometimes written as or , was a Nobel Prize-winning Austrian physicist with Irish citizenship who developed a number of fundamental results in quantum theo ...
in
Berlin Berlin ( , ) is the capital and List of cities in Germany by population, largest city of Germany by both area and population. Its 3.7 million inhabitants make it the European Union's List of cities in the European Union by population within ci ...
, and visited for a few months with Nobel laureate
Niels Bohr Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922 ...
in Copenhagen where he also met
Werner Heisenberg Werner Karl Heisenberg () (5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the main pioneers of the theory of quantum mechanics. He published his work in 1925 in a breakthrough paper. In the subsequent serie ...
and
George Gamow George Gamow (March 4, 1904 – August 19, 1968), born Georgiy Antonovich Gamov ( uk, Георгій Антонович Гамов, russian: Георгий Антонович Гамов), was a Russian-born Soviet and American polymath, theoret ...
. Proca came to be known as one of the most influential Romanian theoretical physicists of the last century, having developed the vector meson theory of nuclear forces in 1936, ahead of the first reports of
Hideki Yukawa was a Japanese theoretical physicist and the first Japanese Nobel laureate for his prediction of the pi meson, or pion. Biography He was born as Hideki Ogawa in Tokyo and grew up in Kyoto with two older brothers, two older sisters, and two yo ...
, who employed Proca's equations for the vectorial mesonic field as a starting point. Yukawa subsequently received the Nobel Prize for an explanation of the nuclear forces by using a pi-mesonic field and predicting correctly the existence of the
pion In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more gene ...
, initially called a 'mesotron' by Yukawa. Pions being the lightest
meson In particle physics, a meson ( or ) is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, ...
s play a key role in explaining the properties of the
strong nuclear force The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called the ...
s in their lower energy range. Unlike the massive spin-1 bosons in Proca's equations, the pions predicted by Yukawa are spin-0 bosons that have associated only ''scalar'' fields. However, there exist also spin-1 mesons, such as those considered in Proca's equations. The spin-1 vector mesons considered by Proca in 1936—1941 have an odd parity, are involved in electroweak interactions, and have been observed in high-energy experiments only after 1960, whereas the pions predicted by Yukawa's theory were experimentally observed by Carl Anderson in 1937 with masses quite close in value to the 100 MeV predicted by Yukawa's theory of pi-mesons published in 1935; the latter theory considered only the massive scalar field as the cause of the nuclear forces, such as those that would be expected to be found in the field of a pi-meson. In the range of higher masses, vector mesons include also charm and
bottom quark The bottom quark or b quark, also known as the beauty quark, is a third-generation heavy quark with a charge of −  ''e''. All quarks are described in a similar way by electroweak and quantum chromodynamics, but the bottom quark has exce ...
s in their structure. The spectrum of heavy mesons is linked through radiative processes to the vector mesons, which are therefore playing important roles in meson spectroscopy. The light-quark vector mesons appear in nearly pure quantum states. Proca's equations are equations of motion of the Euler–Lagrange type which lead to the Lorenz gauge field conditions: \partial_\mu A^\mu=0 \!. In essence, Proca's equations are: :\Box A^\nu - \partial^\nu (\partial_\mu A^\mu) + m^2 A^\nu = j^\nu, where: ::\Box = \left(\frac\frac\right)-\nabla^2. Here A^\mu is the 4-potential, the operator \Box in front of this potential is the
D'Alembert operator In special relativity, electromagnetism and wave theory, the d'Alembert operator (denoted by a box: \Box), also called the d'Alembertian, wave operator, box operator or sometimes quabla operator (''cf''. nabla symbol) is the Laplace operator of Mi ...
, j^\nu is the current density, and the nabla operator (∇) squared is the Laplace operator, Δ. As this is a relativistic equation, Einstein's summation convention over repeated indices is assumed. The 4-potential A^\nu is the combination of the scalar potential \phi and the 3-vector potential A, derived from
Maxwell's equations Maxwell's equations, or Maxwell–Heaviside equations, are a set of coupled partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric circuits. ...
: ::A^\nu = \left(\frac, \mathbf\right) ::\mathbf = -\mathbf \phi - \frac ::\mathbf = \mathbf \times \mathbf. With a simplified notation they take the form: ::\partial_\mu(\partial^\mu A^\nu - \partial^\nu A^\mu)+ \left(\frac\right)^2 A^\nu=0. Proca's equations thus describe the field of a massive spin-1 particle of mass ''m'' with an associated field propagating at the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
''c'' in
Minkowski spacetime In mathematical physics, Minkowski space (or Minkowski spacetime) () is a combination of Three-dimensional space, three-dimensional Euclidean space and time into a four-dimensional manifold where the spacetime interval between any two Event (rel ...
; such a field is characterized by a real vector A resulting in a relativistic
Lagrangian density Lagrangian may refer to: Mathematics * Lagrangian function, used to solve constrained minimization problems in optimization theory; see Lagrange multiplier ** Lagrangian relaxation, the method of approximating a difficult constrained problem with ...
L. They may appear formally to resemble the
Klein–Gordon equation The Klein–Gordon equation (Klein–Fock–Gordon equation or sometimes Klein–Gordon–Fock equation) is a relativistic wave equation, related to the Schrödinger equation. It is second-order in space and time and manifestly Lorentz-covariant ...
: :: \frac \frac \psi - \nabla^2 \psi + \frac \psi = 0 , but the latter is a scalar, ''not a vector'', equation that was derived for relativistic ''electrons'', and thus it applies only to spin-1/2 fermions. Moreover, the solutions of the Klein–Gordon equation are relativistic
wavefunction A wave function in quantum physics is a mathematical description of the quantum state of an isolated quantum system. The wave function is a complex-valued probability amplitude, and the probabilities for the possible results of measurements ...
s that can be represented as quantum plane waves when the equation is written in natural units: : - \partial_t^2 \psi + \nabla^2 \psi = m^2 \psi; this scalar equation is only applicable to relativistic fermions which obey the energy-momentum relation in
Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...
's
special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates: # The laws ...
theory. Yukawa's intuition was based on such a scalar Klein–Gordon equation, and Nobel laureate
Wolfgang Pauli Wolfgang Ernst Pauli (; ; 25 April 1900 – 15 December 1958) was an Austrian theoretical physicist and one of the pioneers of quantum physics. In 1945, after having been nominated by Albert Einstein, Pauli received the Nobel Prize in Physics ...
wrote in 1941: ``...''Yukawa supposed the meson to have spin 1 in order to explain the spin dependence of the force between proton and neutron. The theory for this case has been given by Proca".''
Wolfgang Pauli Wolfgang Ernst Pauli (; ; 25 April 1900 – 15 December 1958) was an Austrian theoretical physicist and one of the pioneers of quantum physics. In 1945, after having been nominated by Albert Einstein, Pauli received the Nobel Prize in Physics ...
, ''
Reviews of Modern Physics ''Reviews of Modern Physics'' (abbreviated RMP) is a quarterly peer-reviewed scientific journal published by the American Physical Society. It was established in 1929 and the current editor-in-chief is Michael Thoennessen. The journal publishes r ...
''. 13 (1941) 213.


Notes


See also

* Euler–Lagrange equations of motion * Proca action *
Vector meson In high energy physics, a vector meson is a meson with total spin 1 and odd parity (usually noted as ). Vector mesons have been seen in experiments since the 1960s, and are well known for their spectroscopic pattern of masses. The vector meson ...
*
Klein–Gordon equation The Klein–Gordon equation (Klein–Fock–Gordon equation or sometimes Klein–Gordon–Fock equation) is a relativistic wave equation, related to the Schrödinger equation. It is second-order in space and time and manifestly Lorentz-covariant ...
* relativistic electron *
Special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates: # The laws ...
* Nuclear forces * Yukawa theory *
Pion In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more gene ...
s *
Meson In particle physics, a meson ( or ) is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, ...
s * Quarks


References

* *


External links


Brief History of IFIN-HH: Precursors Hon. Acad. Alexandru Proca (1897–1955)
and Acad. Prof. Dr.
Horia Hulubei Horia Hulubei (; 15 November 1896 – 22 November 1972) was a Romanian nuclear physicist, known for his contributions to the development of X-ray spectroscopy. Education and military service Born in Iași, he graduated in 1915 first in his clas ...
(1896–1972). {{DEFAULTSORT:Proca, Alexandru 1897 births 1955 deaths Scientists from Bucharest Gheorghe Lazăr National College (Bucharest) alumni Politehnica University of Bucharest alumni University of Paris alumni Members of the Romanian Academy elected posthumously Members of the Romanian Academy of Sciences Romanian emigrants to France Romanian nuclear physicists French nuclear physicists