Paul Dirac
Paul Dirac OM FRS | |
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Born | Paul Adrien Maurice Dirac 8 August 1902 Bristol, England |
Died | 20 October 1984 , U.S. | (aged 82)
Nationality | British |
Education | |
Known for |
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Spouse |
Margit Wigner (m. 1937) |
Children | 2 |
Awards |
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Scientific career | |
Fields | Theoretical physics, mathematical physics |
Institutions | |
Ralph Fowler | |
Doctoral students |
Part of a series of articles about |
Quantum mechanics |
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Special relativity |
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Paul Adrien Maurice Dirac
Dirac made fundamental contributions to the early development of both
Dirac's contributions were not only restricted to quantum mechanics. He contributed to the Tube Alloys project, the British programme to research and construct atomic bombs during World War II.[20][21] Furthermore, Dirac made fundamental contributions to the process of uranium enrichment and the gas centrifuge,[22][23][24][21] and whose work was deemed to be "probably the most important theoretical result in centrifuge technology".[25] He also contributed to cosmology, putting forth his large numbers hypothesis.[22][26][27][28][29] Dirac is also seen as having anticipated string theory well before its inception, with his work on the Dirac membrane and Dirac–Born–Infeld action, amongst other contributions.[30][31]
Dirac was regarded by his friends and colleagues as unusual in character. In a 1926 letter to Paul Ehrenfest, Albert Einstein wrote of a Dirac paper, "I am toiling over Dirac. This balancing on the dizzying path between genius and madness is awful." In another letter concerning the Compton effect he wrote, "I don't understand the details of Dirac at all."[32] In 1987, Abdus Salam stated that "Dirac was undoubtedly one of the greatest physicists of this or any century . . . No man except Einstein has had such a decisive influence, in so short a time, on the course of physics in this century."[33] In 1995, Stephen Hawking stated that "Dirac has done more than anyone this century, with the exception of Einstein, to advance physics and change our picture of the universe".[34] Antonino Zichichi asserted that Dirac had a greater impact on modern physics than Einstein.[14] His achievements are said to be on par with Sir Isaac Newton, James Clerk Maxwell and Einstein.[35][36][37]
Personal life
Early years
Paul Adrien Maurice Dirac was born at his parents' home in Bristol, England, on 8 August 1902,[38] and grew up in the Bishopston area of the city.[39] His father, Charles Adrien Ladislas Dirac, was an immigrant from Saint-Maurice, Switzerland, of French descent,[6] who worked in Bristol as a French teacher. His mother, Florence Hannah Dirac, née Holten, was born to a Cornish Methodist family in Liskeard, Cornwall.[40][41] She was named after Florence Nightingale by her father, a ship's captain, who had met Nightingale while he was a soldier during the Crimean war.[42] His mother moved to Bristol as a young woman, where she worked as a librarian at the Bristol Central Library; despite this she still considered her identity to be Cornish rather than English.[43] Paul had a younger sister, Béatrice Isabelle Marguerite, known as Betty, and an older brother, Reginald Charles Félix, known as Felix,[44][45] who died by suicide in March 1925.[46] Dirac later recalled: "My parents were terribly distressed. I didn't know they cared so much ... I never knew that parents were supposed to care for their children, but from then on I knew."[47]
Charles and the children were officially Swiss nationals until they became
Education
Dirac was educated first at Bishop Road Primary School[52] and then at the all-boys Merchant Venturers' Technical College (later Cotham School), where his father was a French teacher.[53] The school was an institution attached to the University of Bristol, which shared grounds and staff.[54] It emphasised technical subjects like bricklaying, shoemaking and metalwork, and modern languages.[55] This was unusual at a time when secondary education in Britain was still dedicated largely to the classics, and something for which Dirac would later express his gratitude.[54]
Dirac studied
In 1923, Dirac graduated, once again with first class honours, and received a £140 scholarship from the
Family
In 1937, Dirac married[65] Margit Wigner, a sister of physicist Eugene Wigner[66] and a divorcee.[67] Dirac raised Margit's two children, Judith and Gabriel, as if they were his own.[68] Paul and Margit Dirac also had two daughters together, Mary Elizabeth and Florence Monica.[69]
Margit, known as Manci, had visited her brother in 1934 in Princeton, New Jersey, from their native Hungary and, while at dinner at the Annex Restaurant, met the "lonely-looking man at the next table". This account from a Korean physicist, Y. S. Kim, who met and was influenced by Dirac, also says: "It is quite fortunate for the physics community that Manci took good care of our respected Paul A. M. Dirac. Dirac published eleven papers during the period 1939–46. Dirac was able to maintain his normal research productivity only because Manci was in charge of everything else".[70]
Personality
Dirac was known among his colleagues for his precise and taciturn nature. His colleagues in Cambridge jokingly defined a unit called a "dirac", which was one word per hour.[71] When Niels Bohr complained that he did not know how to finish a sentence in a scientific article he was writing, Dirac replied, "I was taught at school never to start a sentence without knowing the end of it."[72] He criticised the physicist J. Robert Oppenheimer's interest in poetry: "The aim of science is to make difficult things understandable in a simpler way; the aim of poetry is to state simple things in an incomprehensible way. The two are incompatible."[73]
Dirac himself wrote in his diary during his postgraduate years that he concentrated solely on his research, and stopped only on Sunday when he took long strolls alone.[74]
An anecdote recounted in a review of the 2009 biography tells of Werner Heisenberg and Dirac sailing on an ocean liner to a conference in Japan in August 1929. "Both still in their twenties, and unmarried, they made an odd couple. Heisenberg was a ladies' man who constantly flirted and danced, while Dirac—'an Edwardian geek', as biographer Graham Farmelo puts it—suffered agonies if forced into any kind of socializing or small talk. 'Why do you dance?' Dirac asked his companion. 'When there are nice girls, it is a pleasure,' Heisenberg replied. Dirac pondered this notion, then blurted out: 'But, Heisenberg, how do you know beforehand that the girls are nice?'"[75]
Margit Dirac told both George Gamow and Anton Capri in the 1960s that her husband had said to a house visitor, "Allow me to present Wigner's sister, who is now my wife."[76][77]
Another story told of Dirac is that when he first met the young Richard Feynman at a conference, he said after a long silence, "I have an equation. Do you have one too?"[78]
After he presented a lecture at a conference, one colleague raised his hand and said: "I don't understand the equation on the top-right-hand corner of the blackboard". After a long silence, the moderator asked Dirac if he wanted to answer the question, to which Dirac replied: "That was not a question, it was a comment."[79][80]
Dirac was also noted for his personal modesty. He called the equation for the time evolution of a quantum-mechanical operator, which he was the first to write down, the "Heisenberg equation of motion". Most physicists speak of Fermi–Dirac statistics for half-integer-spin particles and Bose–Einstein statistics for integer-spin particles. While lecturing later in life, Dirac always insisted on calling the former "Fermi statistics". He referred to the latter as "Bose statistics" for reasons, he explained, of "symmetry".[81]
Views on religion
Heisenberg recollected a conversation among young participants at the 1927 Solvay Conference about Einstein and Planck's views on religion between Wolfgang Pauli, Heisenberg and Dirac. Dirac's contribution was a criticism of the political purpose of religion, which Bohr regarded as quite lucid when hearing it from Heisenberg later.[82] Among other things, Heisenberg imagined that Dirac might say:
I don't know why we are discussing religion. If we are honest—and scientists have to be—we must admit that religion is a jumble of false assertions, with no basis in reality. The very idea of God is a product of the human imagination. It is quite understandable why primitive people, who were so much more exposed to the overpowering forces of nature than we are today, should have personified these forces in fear and trembling. But nowadays, when we understand so many natural processes, we have no need for such solutions. I can't for the life of me see how the postulate of an Almighty God helps us in any way. What I do see is that this assumption leads to such unproductive questions as to why God allows so much misery and injustice, the exploitation of the poor by the rich, and all the other horrors He might have prevented. If religion is still being taught, it is by no means because its ideas still convince us, but simply because some of us want to keep the lower classes quiet. Quiet people are much easier to govern than clamorous and dissatisfied ones. They are also much easier to exploit. Religion is a kind of opium that allows a nation to lull itself into wishful dreams and so forget the injustices that are being perpetrated against the people. Hence the close alliance between those two great political forces, the State and the Church. Both need the illusion that a kindly God rewards—in heaven if not on earth—all those who have not risen up against injustice, who have done their duty quietly and uncomplainingly. That is precisely why the honest assertion that God is a mere product of the human imagination is branded as the worst of all mortal sins.[83]
Heisenberg's view was tolerant. Pauli, raised as a Catholic, had kept silent after some initial remarks, but when finally he was asked for his opinion, said: "Well, our friend Dirac has got a religion and its guiding principle is 'There is no God, and Paul Dirac is His prophet.'" Everybody, including Dirac, burst into laughter.[84][85]
Later in life, Dirac wrote an article mentioning God that appeared in the May 1963 edition of Scientific American, Dirac wrote:
It seems to be one of the fundamental features of nature that fundamental
mathematical theory of great beauty and power, needing quite a high standard of mathematics for one to understand it. You may wonder: Why is nature constructed along these lines? One can only answer that our present knowledge seems to show that nature is so constructed. We simply have to accept it. One could perhaps describe the situation by saying that God is a mathematician of a very high order, and He used very advanced mathematics in constructing the universe. Our feeble attempts at mathematics enable us to understand a bit of the universe, and as we proceed to develop higher and higher mathematics we can hope to understand the universe better.[86]
In 1971, at a conference meeting, Dirac expressed his views on the existence of God.[87] Dirac explained that the existence of God could be justified only if an improbable event were to have taken place in the past:
It could be that it is extremely difficult to start life. It might be that it is so difficult to start a life that it has happened only once among all the planets... Let us consider, just as a conjecture, that the chance of life starting when we have got suitable physical conditions is 10−100. I don't have any logical reason for proposing this figure, I just want you to consider it as a possibility. Under those conditions ... it is almost certain that life would not have started. And I feel that under those conditions it will be necessary to assume the existence of a god to start off life. I would like, therefore, to set up this connection between the existence of a god and the physical laws: if physical laws are such that to start off life involves an excessively small chance so that it will not be reasonable to suppose that life would have started just by blind chance, then there must be a god, and such a god would probably be showing his influence in the quantum jumps which are taking place later on. On the other hand, if life can start very easily and does not need any divine influence, then I will say that there is no god.[88]
Dirac did not commit himself to any definite view, but he described the possibilities for scientifically answering the question of God.[88]
Career
Dirac established the most general theory of quantum mechanics and discovered the relativistic equation for the electron, which now bears his name. The remarkable notion of an antiparticle to each fermion particle – e.g. the positron as antiparticle to the electron – stems from his equation. He was the first to develop quantum field theory, which underlies all theoretical work on sub-atomic or "elementary" particles today, work that is fundamental to our understanding of the forces of nature. He proposed and investigated the concept of a magnetic monopole, an object not yet known empirically, as a means of bringing even greater symmetry to James Clerk Maxwell's equations of electromagnetism. He is credited as being the one to create quantum field theory, alongside creating quantum electrodynamics and coining the term.[10][12] Dirac also coined the terms "fermion" and "boson".[89]
Throughout his career, Dirac was motivated by the principles of mathematical beauty,[90] with Peter Goddard stating that "Dirac cited mathematical beauty as the ultimate criterion for selecting the way forward in theoretical physics".[91] Dirac was recognised for being mathematically gifted, as during his time in university, academics had affirmed that Dirac had an "ability of the highest order in mathematical physics",[92] with Ebenezer Cunningham stating that Dirac was "quite the most original student I have met in the subject of mathematical physics".[93] Therefore, Dirac was known for his "astounding physical intuition combined with the ability to invent new mathematics to create new physics".[16] During his career, Dirac made numerous important contributions to mathematical subjects, including the Dirac delta function, Dirac algebra and the Dirac operator.
Quantum theory
Dirac's first step into a new quantum theory was taken late in September 1925.
Dirac's attention was drawn to a mysterious mathematical relationship, at first sight unintelligible, that Heisenberg had established. Several weeks later, back in Cambridge, Dirac suddenly recognised that this mathematical form had the same structure as the
Dirac was famously not bothered by issues of interpretation in quantum theory. In fact, in a paper published in a book in his honour, he wrote: "The interpretation of quantum mechanics has been dealt with by many authors, and I do not want to discuss it here. I want to deal with more fundamental things."[98] However, in 1964 he wrote a short article about the interpretation of quantum field theory when based on the Heisenberg picture of quantum theory; his primary point in the article was that the Schrodinger model does not work for this purpose.[99]
The Dirac equation
In 1928, building on 2×2 spin matrices which he purported to have discovered independently of
The necessity of
Dirac's The Principles of Quantum Mechanics, published in 1930, is a landmark in the history of science. It quickly became one of the standard textbooks on the subject and is still used today. In that book, Dirac incorporated the previous work of Werner Heisenberg on matrix mechanics and of Erwin Schrödinger on wave mechanics into a single mathematical formalism that associates measurable quantities to operators acting on the Hilbert space of vectors that describe the state of a physical system. The book also introduced the Dirac delta function. Following his 1939 article,[103] he also included the bra–ket notation in the third edition of his book,[104] thereby contributing to its universal use nowadays.
Magnetic monopoles
In 1931, Dirac proposed that the existence of a single magnetic monopole in the universe would suffice to explain the quantisation of electrical charge.[105] In 1975,[106] 1982[107] and 2009,[108][109][110] intriguing results suggested the possible detection of magnetic monopoles, but there is, to date, no direct evidence for their existence (see also Searches for magnetic monopoles).
Gravity
Dirac quantised the gravitational field.[40][111] His work laid the foundations for canonical quantum gravity.[112] In his 1959 lecture at the Lindau Meetings, Dirac discussed why gravitational waves have "physical significance".[113] Dirac predicted gravitational waves would have well defined energy density in 1964.[111] Dirac reintroduced the term "graviton" in a number of lectures in 1959, noting that the energy of the gravitational field should come in quanta.[114][115]
String theory
Dirac is seen as having anticipated string theory, with his work on the Dirac membrane and Dirac–Born–Infeld action, amongst other contributions.[30][31] He also developed a general theory of the quantum field with dynamical constraints,[116][117][30] which forms the basis of the gauge theories and superstring theories of today.[30][40][118]
University of Cambridge
Dirac was the
Dirac's
However, from his once rejected notes he managed to work on putting quantum electrodynamics on "logical foundations" based on
Weisskopf and French (FW) were the first to obtain the correct result for the Lamb shift and the anomalous magnetic moment of the electron. At first FW results did not agree with the incorrect but independent results of Feynman and Schwinger.[125] The 1963–1964 lectures Dirac gave on quantum field theory at Yeshiva University were published in 1966 as the Belfer Graduate School of Science, Monograph Series Number, 3.
Florida State University and University of Miami
After having relocated to Florida to be near his elder daughter, Mary, Dirac spent his last fourteen years of both life and physics research at the University of Miami in Coral Gables, Florida, and Florida State University in Tallahassee, Florida.
In the 1950s in his search for a better QED, Paul Dirac developed the Hamiltonian theory of constraints
The Hamiltonian of constrained systems is one of Dirac's many masterpieces. It is a powerful generalisation of Hamiltonian theory that remains valid for curved spacetime. The equations for the Hamiltonian involve only six degrees of freedom described by , for each point of the surface on which the state is considered. The (m = 0, 1, 2, 3) appear in the theory only through the variables , which occur as arbitrary coefficients in the equations of motion. There are four constraints or weak equations for each point of the surface = constant. Three of them form the four vector density in the surface. The fourth is a 3-dimensional scalar density in the surface HL ≈ 0; Hr ≈ 0 (r = 1, 2, 3)
In the late 1950s, he applied the Hamiltonian methods he had developed to cast Einstein's general relativity in Hamiltonian form[129][130] and to bring to a technical completion the quantisation problem of gravitation and bring it also closer to the rest of physics according to Salam and DeWitt. In 1959 he also gave an invited talk on "Energy of the Gravitational Field" at the New York Meeting of the American Physical Society.[131] In 1964 he published his Lectures on Quantum Mechanics (London: Academic) which deals with constrained dynamics of nonlinear dynamical systems including quantisation of curved spacetime. He also published a paper entitled "Quantization of the Gravitational Field" in the 1967 ICTP/IAEA Trieste Symposium on Contemporary Physics.
From September 1970 to January 1971, Dirac was a visiting professor at Florida State University in Tallahassee. During that time he was offered a permanent position there, which he accepted, becoming a full professor in 1972. Contemporary accounts of his time there describe it as happy except that he apparently found the summer heat oppressive and liked to escape from it to Cambridge.[132]
He would walk about a mile to work each day and was fond of swimming in one of the two nearby lakes (Silver Lake and Lost Lake), and was also more sociable than he had been at the University of Cambridge, where he mostly worked at home apart from giving classes and seminars. At Florida State, he would usually eat lunch with his colleagues before taking a nap.[133]
Dirac published over 60 papers in those last twelve years of his life, including a short book on general relativity.[134] His last paper (1984), entitled "The inadequacies of quantum field theory," contains his final judgment on quantum field theory: "These rules of renormalisation give surprisingly, excessively good agreement with experiments. Most physicists say that these working rules are, therefore, correct. I feel that is not an adequate reason. Just because the results happen to be in agreement with observation does not prove that one's theory is correct." The paper ends with the words: "I have spent many years searching for a Hamiltonian to bring into the theory and have not yet found it. I shall continue to work on it as long as I can and other people, I hope, will follow along such lines."[135]
Students
Amongst his many students[3][136] were Homi J. Bhabha,[1] Fred Hoyle, John Polkinghorne[5] and Freeman Dyson.[137] Polkinghorne recalls that Dirac "was once asked what was his fundamental belief. He strode to a blackboard and wrote that the laws of nature should be expressed in beautiful equations."[138]
Honours
Dirac shared the 1933 Nobel Prize for physics with Erwin Schrödinger "for the discovery of new productive forms of atomic theory".[18] Dirac was also awarded the Royal Medal in 1939 and both the Copley Medal and the Max Planck Medal in 1952. He was elected a Fellow of the Royal Society in 1930,[139][6] a member of the American Philosophical Society in 1938,[140] an Honorary Fellow of the American Physical Society in 1948, a member of the United States National Academy of Sciences in 1949,[141] a member of the American Academy of Arts and Sciences in 1950,[142] and an Honorary Fellow of the Institute of Physics, London in 1971. He received the inaugural J. Robert Oppenheimer Memorial Prize in 1969.[143][144] Dirac became a member of the Order of Merit in 1973, having previously turned down a knighthood as he did not want to be addressed by his first name.[75][145]
Death
In 1984, Dirac died in Tallahassee, Florida, and was buried at Tallahassee's Roselawn Cemetery.[146] Dirac's childhood home in Bishopston, Bristol is commemorated with a blue plaque,[147] and the nearby Dirac Road is named in recognition of his links with the city of Bristol. A commemorative stone was erected in a garden in Saint-Maurice, Switzerland, the town of origin of his father's family, on 1 August 1991. On 13 November 1995 a commemorative marker, made from Burlington green slate and inscribed with the Dirac equation, was unveiled in Westminster Abbey.[146][148] The Dean of Westminster, Edward Carpenter, had initially refused permission for the memorial, thinking Dirac to be anti-Christian, but was eventually (over a five-year period) persuaded to relent.[149]
Legacy
The influence and importance of Dirac's work have increased with the decades, and physicists use daily the concepts and equations that he developed.
In 1975, Dirac gave a series of five lectures at the
Immediately after his death, two organisations of professional physicists established annual awards in Dirac's memory. The Institute of Physics, the United Kingdom's professional body for physicists, awards the Paul Dirac Medal for "outstanding contributions to theoretical (including mathematical and computational) physics".[151] The first three recipients were Stephen Hawking (1987), John Stewart Bell (1988), and Roger Penrose (1989). Since 1985, the International Centre for Theoretical Physics awards the Dirac Medal of the ICTP each year on Dirac's birthday (8 August).[152]
The Dirac-Hellman Award at Florida State University was endowed by Bruce P. Hellman in 1997 to reward outstanding work in theoretical physics by FSU researchers.[153] The Paul A.M. Dirac Science Library at Florida State University, which Manci opened in December 1989,[154] is named in his honour, and his papers are held there.[155] Outside is a statue of him by Gabriella Bollobás.[156] The street on which the National High Magnetic Field Laboratory in Innovation Park of Tallahassee, Florida, is located is named Paul Dirac Drive. As well as in his hometown of Bristol, there is also a road named after him, Dirac Place, in Didcot, Oxfordshire.[157] The Dirac-Higgs Science Centre in Bristol is also named in his honour.[158]
The BBC named a
Publications
- The Principles of Quantum Mechanics (1930): This book summarises the ideas of quantum mechanics using the modern formalism that was largely developed by Dirac himself. Towards the end of the book, he also discusses the relativistic theory of the electron (the Dirac equation), which was also pioneered by him. This work does not refer to any other writings then available on quantum mechanics.
- Lectures on Quantum Mechanics (1966): Much of this book deals with quantum mechanics in curved space-time.
- Lectures on Quantum Field Theory (1966): This book lays down the foundations of quantum field theory using the Hamiltonian formalism.
- Spinors in Hilbert Space (1974): This book based on lectures given in 1969 at the University of Miami deals with the basic aspects of spinors starting with a real Hilbert space formalism. Dirac concludes with the prophetic words "We have boson variables appearing automatically in a theory that starts with only fermion variables, provided the number of fermion variables is infinite. There must be such boson variables connected with electrons..."
- General Theory of Relativity (1975): This 69-page work summarises Einstein's general theory of relativity.
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- ^ a b "Dirac takes his place next to Isaac Newton". Florida State University. Archived from the original on 27 April 1997. Retrieved 4 April 2013.
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General sources
- Capri, Anton Z. (2007). Quips, Quotes, and Quanta: An Anecdotal History of Physics. Hackensack, New Jersey: World Scientific. OCLC 214286147.
- OCLC 13008048.
- ISBN 978-0-465-01827-7.]
- OCLC 11970045.
- OCLC 115992.
- OCLC 20013981.
- OCLC 532357.
- OCLC 28966591.
- OCLC 318585662.
Further reading
- Brown, Helen (24 January 2009). "The Strangest Man: The Hidden Life of Paul Dirac by Graham Farmelo – review [print version: The man behind the maths]". The Daily Telegraph (Review). p. 20. Archived from the original on 2 February 2009. Retrieved 11 April 2011..
- Gilder, Louisa (13 September 2009). "Quantum Leap – Review of 'The Strangest Man: The Hidden Life of Paul Dirac by Graham Farmelo'". The New York Times. Retrieved 11 April 2011. Review.
- Mukunda, N. (1987) "The life and work of P.A.M. Dirac", pages 260 to 282 in Recent Developments in Theoretical Physics, MR935624
External links
- Oral history interview transcript with P. A. M. Dirac on 1 April 1962, American Institute of Physics, Niels Bohr Library & Archives - Session I
- Oral history interview transcript with P. A. M. Dirac on 6 May 1963, American Institute of Physics, Niels Bohr Library & Archives - Session II
- Oral history interview transcript with P. A. M. Dirac on 7 Mary 1963, American Institute of Physics, Niels Bohr Library & Archives - Session III
- Oral history interview transcript with P. A. M. Dirac on 10 May 1963, American Institute of Physics, Niels Bohr Library & Archives - Session IV
- Oral history interview transcript with P. A. M. Dirac on 14 May 1963, American Institute of Physics, Niels Bohr Library & Archives - V
- Free online access to Dirac's classic 1920s papers from Royal Society's Proceedings A
- Annotated bibliography for Paul Dirac from the Alsos Digital Library for Nuclear Issues
- The Paul Dirac Collection at Florida State University
- The Papers of Professor Paul Dirac at Churchill Archives Centre