Fysiikan historia kevät 2011 Luento 13

History of Nuclear Physics The proton- electron model In 1914 it was known that nucleus is the seat of all radioactive processes. The composition of the nucleus was, however, still unclear. Rutherford: The hydrogen nucleus in the positive electron. This nucleus was called generally the H- particle, the name `proton game later. To make atomic weights and charges to match, it was natural to assume that there were electrons in nucleus (neutrons were unkown). The helium nucleus was thought to consist of 4 H- particles and 2 electrons. The nucleus was thought to be bound together by an intensive electromagnetic force. What else could it be; electromagnetic force and gravity were the only forces known at that time.

Binding energy In 1905 Einstein suggested that a particle of a mass m has internal energy E = mc 2. This was a new form of energy, but Einstein was sceptical about the possibility of measuring it: for the moment there is no hope whatsoever. Planck pointed out a consequence: a bound system should weight less than its constituents. (The binding energy, ie the energy associated with the interactions of the constituents, is negative.) The binding energy between electrons and H- particles due to electromagnetic interactions was estimated to be much too small to match the data. In 1932 James Chadwick (1891-1974) found the neutron. It soon came clear that the nucleus is made of protons and neutrons. This led to more correct values of the binding energies. Discovery of the neutron In 1930 a puzzling reaction was detected: 9 12 He+ Be C + high energy neutral radiation 4 Chadwick was able to show that the high energy radiation consisted of neutral particles with a mass 1.007 times the proton mass. He called the neutral particle the neutron. He believed the the neutron is a bound state of a proton and an electron, as Rutherford had suggested. It took two years to understand that the neutron is as elementary particle as the proton.

Induced radioactivity French Irene (1897-1956) and Frederic Joliot- Curie (1900-1958) irradiated aluminium foil with α- particles. They saw an amazing thing: the positron radiation from the target did not end immediately when the α- source was removed. They had produced arti`icially a new radioactive isotope that decayed via β + emission: Five weeks later Italian Enrico Fermi (1901-1954) published a paper on radioactivity induced by neutron bombardment. He became soon the world s leading expert in neutron physics.

The energy spectrum of beta decay In the `irst experiments seemed to show that electrons emitted in beta decay have discrete energies. In some measurements the spectrum a broad band was seen instead of sharp lines. In 1914 James Chadwick found that the energy spectrum is actually continuos. The problem was that in the reaction (A,Z)- >(A,Z+1) + e- the electron should have a well de`ined energy determined by the masses of the mother and daughter nuclei. People solved the discrepancy by speculating that what is measured is not the primary electron but some secondary effect. In 1927 this was shown wrong. Bohr suggested non- conservation of energy in beta decay. He connected this problem with the problem of the structure of the nucleus (neutron was not discovered yet).

In 1930 German- Swiss Wolfgang Pauli (1900-1958) suggested a totally new solution: In beta decay another spin- 1/2 particle neutral and very light is emitted, a neutrino. The energy is devided between the electron and the neutrino in all possible ways, therefore the spectrum is continuous From his famous letter to a gathering of physicists in Tübingen: Dear radioactive ladies and gentlemen,... I have hit upon a desperate remedy to save the law of conservation of energy. But so far I do not dare to publish anything about this idea, and trustfully turn @irst to you, dear radioactive ones, with the question of how likely it is to @ind experimental evidence for such a neutron I admit that my remedy may seem almost improbable because one probably would have seen those neutrons, if they exist, for a long time. But nothing ventured, nothing gained Pauli giving a lecture. Pauli and Bohr making an experiment with a Tippe Top in 1951.

Theories of nuclear physics In 1934 Enrico Fermi (1901-1954) presented a `ield theoretical model for the beta decay, so called four fermion model: n e n p + e + ν e p ν e Never before it was considered theories were new massive particles can be created in interaction (like photons). This was a new type of interaction a new force - called weak interaction. The Fermi theory and its successful applications con`irmed the proton- neutron picture of the nucleus. Nuclear electrons were not needed any more: the beta decay electrons (and positrons) are created at the instant of decay. Fermi was both an experimentalist and a theorist.

Heisenberg s theory of strong interactions In 1932 Heisenberg presented a quantum mechanical model for proton- neutron nucleus. Assumed still that the neutron is a kind of bound state of proton and electron. The electron carries a strong binding force that keeps nucleons together in the nucleus. Introduce the concept of isospin: the proton and the neutron are +1/2 and - 1/2 isospin states. In 1934 Japanese Hideki Yukawa presented another theory, where strong force was carried by a speci`ic heavy particle, the U- quanta (pion). The model could explain the short range of the strong force.

The birth of accelerator- based nuclear physics In earlier studies of atom and nuclear structure radioactive isotopes were used as the source of probe particles (usually α s). The energy of the probes was then limited to the typical nuclear physics scale of some MeV s. In 1932 John Cockcroft (1897-1967) and Ernest Walton (1903-1995) took in use the `irst particle accelerator, where protons were accelerated to the energies of hundres of MeV s. They produced the nuclear transformation Li 7 + p - > 2α + 14.3 MeV. Measuring the kinetic energy of α s and the masses known they could test Einstein s formula E = mc 2 within errors. This was the `irst test of it. The Cockcroft- Walton Accelerator was based on a voltage multiplier that used an intricate stack of capacitors connected by rectifying diodes as switches.

Nuclear ìssion Fermi got the Nobel prize in 1937 for demonstrating the existence of new radioactive elements produced by neutron irradiation. He had bombarded eg. uranium (Z=92) with neutrons. In 1938 Germans Otto Hahn (1879-1968) and Fritz Strassmann (1902-1980) started a careful radiochemical analysis of the elements produced in neutron- U collisions. They staggering result was that among the products they identiìed three isotopes of barium, which have Z=56. An Austrian Lise Meitner (1878-1968), who had for a long time worked in the group but who now lived in exile for Nazis in Stockholm, together with her nephew Otto Frisch (1904-1979) gave a physical explanation to the discovery, ìssion of the uranium. Large kinetic energy Fast neutrons This started on intensive study of ìssion everywhere. Niels Bohr gave his last scientiìc contribution in studying the ìssion process with Archibal Wheeler.

It was immediately understood that a chain reaction is possible due to the fast neutrons. A new energy source and also a new weapon, atomic bomb, were seen available. During the II World War, a great number of top physicist worked in the secret Manhattan project in Los Alamos to produce an atomic bomb, which was then used in Hiroshima and Nagasaki. Einstein and Leo Szilard, who made Einstein to sign a letter to president Franklin Roosevelt 1939 about the possibility of atomic bomb. Physics lost its innocence.

History of particle physics The history of particle physics as an independent research `ield started with new accelerators after the II World War. Also new detection techniques were developed. The most important detector types include: Cloud chamber (Charles Wilson, 1913). In supersaturated vapor ionizating particles causes creation of clouds on their track. (Sumukammio) Bubble chamber (Donald Glaser, 1952). Transparent liquid heated just below the boiling point. When the pressure is suddenly decreased, charged particles form an ionization track around which the liquid vaporizes and forms microscopic bubbles. (Kuplakammio) Wire chamber (Charles Charpak, 1968). A metal tube `illed with a suitable gas mixture; walls grounded and a set of wires in high voltage. Ionizing radiation causes short cuts in the gas and electric current in wires. (Lankakammio) Scintillators. Materials (some crystals, plastics, liquids, gases) that absorb energy from ionizing particles and re- emit (scintillate) it, usually as visible light. The light is observed usually with photomultipliers or photodiodes. (Tuikeilmaisin) Cherenkov detectors. A particle passing through a material at a velocity greater than the velocity of light in that material emits light (Pavel Cherenkov, 1934). The light is observed with photomultipliers. (Tsherenkov- ilmaisin)

The particles known before 1950 were the electron, proton, neutron, positron, muon, kaon. Positron, muon and pion were discovered from cosmic rays. With accelerators many new particles were soon discovered.

Kosmisen säteilyn oli löytänyt alun perin Victor Hess 1912. Hess halusi osoittaa, että kaikkialla ilmenevä radioaktiivinen säteily ei ole peräisin ilmasta vaan kallioperästä. Osoittaakseen tämän, hän teki mittauksia ilmapallolennoilla (yli 5000 m:n korkeuteen!). Aluksi säteilyn määrä pienenikin korkeuden funktiona, mutta sitten se alkoi yllättäen kasvaa. Avaruudesta tuli siis säteilyä, jota ilmakehä absorboi sitä vähemmän mitä korkeammalla oltiin. Hess lähdössä ilmapallolennolleen vakava- ilmeisten assistenttiensa saattelemana.

1920- luvun lopussa ja 1930- luvun alussa useat tutkijat totesivat kosmisessa säteilyssä olevan läpäisykykyisiä varattuja hiukkasia. 1934 Carl Anderson ja Seth Neddermeyer totesivat näiden massan olevan elektronin massaa suurempi mutta protonin massaa paljon pienempi. Monta nimeä: mesoni, mesotroni, barytroni, raskas elektroni, yukoni, μ- mesoni à myoni (μ) Carl Anderson

Accelerators In 1931 Ernest Lawrence and David Sloan built the ìrst cyclotron (1.3 MeV). A drawing in Lawrence s patent The ìrst syclotron of Lawrence, the diameter 12 cm. Cyclotrons were ìrst used in nuclear physics. At higher energies relativistic effect on kinematics had to be taken into account. This is done in synchrocyclotrons The ìrst new artiìal elementary particle produced in accelerators was neutral pion π 0, which was discovered in Berkeley s synchrocyclotron in 1950.

The next step were synchrotrons, in which the particles are accelerated in a ring of constant radius. The bene`it is that one does not need one big magnet but separate small magnets along the ring. Cosmotron, Brookhaven 1953 (3 GeV protons) Bevatron, Berkeley1954 (6 GeV protons) Alternating Gradient Synchrotron (AGS) Brookhaven 1960 (33 GeV protons) Proton Synchrotron (PS) CERN 1959 (28 GeV protons) Super Proton Synchrotron (SPS) CERN 1976 (400 GeV protons), used later as a collider of protons and antiprotons. Tevatron, Fermilab 1983 (1 TeV proton and antiproton collider) Large electron positron collider (LEP) CERN 1989 (electrons and positrons 45 GeV+45 GeV) LHC, CERN 2010 (7 TeV + 7 GeV protons) Etc. Linear colliders has the bene`it of no energy loss for synchrotron radiation and no need for bending magnets. Stanford linear accelerator at SLAC is the largest. Brookhaven CERN

Building of CERN started in 1954 SLAC linear accelerator Tevtron at Fermilab CMS detector at LHC Areal view of the European Synchrotron Radiation Facility, ESRF, in Grenoble.

Gargamelle bubble chamber at CERN A bubble chamber picture and its interpretation. The discovery of the particle. Carl Andersson close to his cloud chamber. A wire chamber under construction. The heavy water Cherenkov detector at the Sudbury Neutrino Observatory.

Qark model In the early 1960s the number of hadrons (particles with strong interactions) was already so large that particle physicists started to look for fundamental basic structure of matter. In 1964 American Murray Gell- Mann (b. 1929 ) and Russian George Zweig (b. 1937) proposed independently the quark model of hadrons. All hadrons are bound states of three quarks (baryons like the proton, neutron etc.) or bound states of a quark and an antiquark (mesons like the pions, kaons etc). The quark model was based on Gell- Mann s earlier discovery of an SU(3) symmetry among hadrons (Eightfold way model). In Gell- Mann s model hadrons nicely `it in the representations of the SU(3) symmetry group. The model predicted eg the existense of the particle.

When the quark model was invented all know hadrons could be explained in terms of three quarks (up, down ans strange). The fourth quark (charm) was discovered in November 1974 independently by American Samuel Ting (b. 1936) in Brookhaven ja American Burt Richter (b. 1931) in SLAC. They found a new particle J/Ψ (psi), a bound state of charm quark (c) and antiquark. This was very important discovery as it con`irmed the prediction of the so called electroweak model (see later). November Revolution. Samuel Ting Burt Richter 3.1 GeV

In 1977 the group of Leon Lederman (b. 1922) found another new particle Υ (ypsilon), which turned out to be a bound state of `ifth quark, bottom quark (b) and anti- bottom. The rest energy of b- quark is about 5 GeV. In 1995 the sixth and so far the last quark type was found in Fermilab, the top quark (t). It is the heaviest particle found so far: its rest energy is 175 GeV. The top quark was discovered with this detector. An ypsilon decay event. Leon Lederman

The Standard Model of particle physics QED The mother of all the modern particle physics theories is Quantum Electrodynamics (QED). It is a quantum `ield theory that describes the electromagnetic interactions of particles. Quantum `ield theories are generalization of quantum mechanics to the situations where particles are created and annihilated. The QED was developed in the 1940s by Richard Feynman (American, 1918-1988), Freeman Dyson (Brittish, b. 1923), Julian Schwinger (American, 1918-1994), and Sin- Itiro Tomonaga (Japanese, 1906-1979). The Shelter Island Conference (1947) on the Foundations of Quantum Mechanics is considered as one of the most in`luential meetings after the World War II. W. Lamb, J.A. Wheeler (standing), A. Pais, R. Feynman, H.Feshbach, J. Schwinger. Willis Lamb s discovery of the Lamb shift in the hydrogen spectrum was one of the mysteries tackled.

Electroweak model The QED is technically so called gauge ìeld theory based on the Lie symmetry U(1). In order to ìnd a similar theory for other forces, Americans C. N. Yang (b. 1922) ja Robert Mills (1927-1999) generalized QED in 1954 to allow for more complicated interactions, in particularly strong interactions, and symmetries (the Yang- Mills theory). American Sheldon Galshow (b. 1932) suggested that electromagnetic and weak interactions could be described with a single Yang- Mills theory based on the symmetry SU(2) U(1). The problem was that the theory predicted the carriers of the weak force, the W and Z particles, to be massless like photon which they could not be as the weak force has a ìnite range. C N Yang Sheldon Glashow

In 1967 Steven Weinberg (b 1933) invented the way the mass problem could be solved by using so called spontaneous symmetry breaking. The spontaneous symmetry breaking had been earlier studied by Japanese- American Yoichiro Nambu (b. 1921), British Peter Higgs (b. 1929), and Belgians Robert Brout (b. 1928) ja Francois Englert (b. 1932). The spontaneous symmetry braeking: The dynamics is symmetric but in the vacuum state symmetry is broken. Yoichiro Nambu holding his 2008 Nobel medal. Peter Higgs

In 1967 Steven Weinberg and Pakistan Abdus Salam (1926-1996) presented the SU(2) U(1) electroweak theory. In it the spontaneous symmetry breaking was used to get masses for weak bosons via the so called Higgs mechanism. The model was generally accepted only after Dutch Gerald t Hooft (b. 1946) showed it renormalizable in 1971. Abdus Salam Steven Weinberg Gerald t Hooft

Quantum Cromo Dynamics QCD The quark model did not tell much about the strong force that keeps quarks together in hadrons. In particular, why quarks were never observed as free particles. There were also problems with Pauli s exclusion princible: For example, in omega particle there are three s quarks with their spins parallel, a state of identical fermions forbidden by the exclusion princible. American Oscar Greenberg introduced in 1964 a new quantum number called color charge, which separates quarks in hadrons. In 1972 Gell- Mann ja German Harald Fritzsch (b. 1943) showed that the color charge can be realized in a gauge `ield theory based on the SU(3) symmetry. This theory, the counterpart of the QED for strong interactions, is called quantum chromo dynamics (QCD). Fritzsch and Gell- Mann quite recently with an unknown party company.

The Standard Model The electroweak theory and QCD form together the Standard Model of particle physics. The carriers of the electroweak force, the W and Z bosons, were discovered at CERN in 1982 in the UA1 experiment led by Carlo Rubbia. The carriers of the strong force, gluons, were discovered at the German research center DESY in 1979. An event were a W boson was produced. Carlo Rubbia Particles of the standard model. The Higgs- like particle H was discovered at CERN in 2012.

Hyökkäyksiä fysiikkaa vastaan Toisen maailmansodan jälkeen fysiikka kukoisti Yhdysvalloissa: Sotateollisuudesta vapautui runsaasti tutkimusvoimaa perustutkimukseen, mutta sotateollisuus Neuvostoliiton Sputnik 1956 johti voimavarojen lisäämiseen. Osa kylmää sotaa. Ilmapiiri muuttui 1960- luvun lopulla Fysiikan tutkimus kytkettiin mielissä sotateollisuuteen (suurin osa fyysikoista edelleen työskenteli niissä töissä, ja suuri osa rahoituksesta tuli sitä kautta). Ydinvoima alkoi menettää viattomuuttaan ja kiehtovuuttaan. Ydinvoiman ajateltiin edistävän ydinaseiden leviämistä. Luonnontieteiden ei nähty edistävän taloudellista kasvua: Perustutkimus on puhtaasti ja yksinkertaisesti ylellisyyttä Rahoitus väheni, tohtorituotanto laski merkittävästi. Myös Euroopassa Tutkimus kuitenkin edistyi hyvin.

1970- luvulla alkoi ilmestyä kirjoituksia fysiikan kriisistä. Tutkimuksen riippumattomuutta epäiltiin, kun katsottiin että tutkimus etenee joiden vaikutusvaltaisten tutkijoiden määräämiin suuntiin. Tutkimusta hallitsee järjestelmä. Euroopassa puhuttiin kapitalistisesta tieteestä, joka huipentui hiukkasfysiikkaan. Luonnontiede on organisoitunut sosiaalisesti väärällä tavalla ja sitä käytetään vääriin tarkoituksiin. Joidenkin mielestä tiede olisi hylättävä kokonaan. Pyrkii vain luonnon ja ihmisen alistamiseen. Fyysikot teollisuutta ja sotalaitosta palvelevia siuluttomia koneita. Thomas Kuhnin Tieteellisen vallankumouksen rakenne (1962) ruokki tarkoittamattaan fysiikan vastaisia asenteita. Kirjan oleellinen sanoma koski sitä, miten tiede edistyy paradigman muutosten kautta (esim Newtonin matemaattinen fysiikan teoria). Jotkut tulkitsivat häntä kuitenkinn niin, ettei fysiikka ollut sen tieteellisempää kuin esim. kirjallisuuskritiikki. Feuerbach päätteli, että fysiikka oli samalla tasolla kuin uskonto, myytit ja propaganda. 1980- ja 1990- luvulla syntyi sosiaalisen kontrukrivismin oppi, jonka mukaan tieteellinen maailmankuva ei perustu luontoon eikä ansainte sen perustavampaa asemaan kuin muutkaan maailmankuvat. Fysiikka on perimmiltään sosiaalinen ja kulttuurellinen rakenne.

Fysiikan loppu? Stephen Hawking kysyi virkaanastujaisesitelmäsässään 1980: Onko teoree)sen fysiikan loppu näkyvissä? Arveli loppun koi?avan 1900- luvun lopussa. Mitä loppu tarkoi?asi? KenFes luodaan täydellinen, risfriidaton ja yhtenäinen fysikaalisten vuorovaikutusten teoria, joka kuvaa kaikki mahdolliset havainnot. Tämä kuvasf pyrkimyksiä löytää lopullinen teoria ja tehdä sen jälkeen itsemurha. RedukFonismi: Kaikki luonnonimiöt voidaan palau?aa lopullisen teorian erikoistapauksiksi. Amer. Philip Andersson on vastustanut voimakkaasf tätä näkemystä. Hän oli myös keskeinen henkilö, kun USA pääp lope?aa suuren SSC- kiihdypmen rakentamisen 1990- luvun alussa. Mene?ääkö ihminen kiinnostuksensa fysiikan tutkimukseen ja/tai katsovatko valfot sen rahoi?amisen tarpee?omaksi ja tuo?ama?omaksi?