Transcript Lecture 7
W. G. SCOTT RAL/PPD 24 Jan 2006 PUBLIC UNDERSTANDING OF SCIENCE PPARC encourages us to spend 1% of our research grant funding on “outreach” activities, promoting understanding, appreciation and awareness of our science to the general public: Targeting 11-16 yr olds in schools etc. can help attract more young people into a scientific career. (e.g. RAL MasterClass 14-16 Mar 2007). Helps produce highly qualified researchers with advanced skills needed by academia, industry and commerce. Helps ensure that the UKs front line contribution in this science is recognised by opinion formers, decision-takers/politicians. Taxpayers can have an appreciation and awareness of the research being funded on their behalf. THE STANDARD MODEL: fermion kinetic term yukawa term (schematic) higgs kinetic term L Tr F 2 | |2 2 2 4 gauge | | | | “kinetic” term V ( ) “higgs potential” where: F [ , ] igA field-strength tensor “covariant derivative” vector potential THE HIGGS FIELD: 0 v v2 1 4 1 2 F G 250 GeV quarks leptons W,Z etc. gives masses to the pointlike particles : L m g v R m g v refraction?. L …but not to the neutron/proton! PROTON NEUTRON u=“UP” (+2/3) d=“DOWN” (-1/3) QUARKS ELECTRO-MAGNETIC INTERACTION e e e see “QED: the strange theory of light and matter” e A FEYNMAN DIAGRAM QUARK CONFINEMENT The “Strong” Force Quark Antiquark “CHROMOSTATICS” “CHROMOSTATICS” THE “COLOUR” FORCE QCD quarks 3 colours gluons 8 colours WEAK INTERACTION u e- W+ d e NEUTRAL CURRENTS (1973) ELECTRO-WEAK e Z e 0 THE “FORCE” PARTICLES g1 g2 g3 W- g4 g5 g6 Z0 W+ γ g7 g8 CERN THE “MATTER” PARTICLES 3 2 e 1 Q -1 0 b t s c d u -1/3 +2/3 ELECTROWEAK TGCs w+ w- z0 e- e+ see M. Gardner “The Ambidextrous Universe” WHAT IS THE DIFFERENCE BETWEEN A LEFT AND A RIGHT HABD? • RH IS BIGGER THAN LH ??? His right hand IS much bigger! WHAT IS THE DIFFERENCE BETWEEN A LEFT AND A RIGHT HAND? • RH IS BIGGER THAN LH ???. • THUMBS ON OPPOSITE EDGES ? • OR PALMS ON OPPOSITE SIDES ? LH AND RH ARE MIRROR REFLECTNS!! WHY DOES A MIRROR REVERSE LEFT AND RIGHT BUT NOT UP AND DOWN? • A CONFUSING QUESTION !!! • SURELY IT REVERSES FRONT AND BACK ? • BUT IT DOES SEEM TO CHANGE RH TO LH! CLOSE TO THE HEART OF THE PROBLEM: How would you communicate left and right over the phone? IT’S A RIGHT-HANDER’S WORLD SPIRAL STAIRCASE BUILT FOR DEFENDER WIELDING SWORD IN RIGHT HAND IN THE POPULATION AT LARGE: APPROX. 90% RIGHT-HANDED APPROX. 10% LEFT- HANDED LATIN: DEXTRA = RIGHT (cf “DEXTROUS”) SINISTRA = LEFT (cf “SINISTER”) Mirror Twins MORE ABSTRACTLY RIGHT HANDED LEFT HANDED COORDINATE SYSTEM COORDINATE SYSTEM z z y x x y MORE ABSTRACTLY RIGHT HANDED LEFT HANDED COORDINATE SYSTEM COORDINATE SYSTEM z z y x z y y x x Left Right A molecule that, like your hands, cannot be made to look like its mirror-image by rotating it or displacing it, is called chiral. This group of atoms is IN FRONT here, but goes BEHIND in the rotating reflection OPTICAL ACTIVITY OPTICALLY ACTIVE SUBSTANCES ROTATE THE PLANE OF POLARISATION OF POLARISED LIGHT Double Refraction (in a Calcite Crystal) Louis Pasteur Pateur felt that handedness could only be distinguished/separated by living things. The DNA structure is a double helix looking something like a twisted ladder. The two cross-linked helices are always right-handed, twisting in the same direction as a normal screw. Names for clockwise and anticlockwise helices: Away from your eye Away from your eye “Perhaps looking glass milk isn’t good to drink” - Lewis Carroll, Alice through the looking glass ALICE WAS RIGHT !!! NOW FOR SOME A/S PHYSICS Fleming’s Left Hand Motor Rule: IS THE PROBLEM SOLVED ? We DO know the direction of the curremt + - The positive terminal is connected to the central carbon rod The negative terminal is connected to the zinc casing Conventional Current flows from positive to negative (opposite to electron flow) But do we REALLY know the Magnetic Field Direction? Yes, here on Earth the North Pole of a magnet points North! Problem Solved !? But you can’t be sure it is the same on another planet? Even the Earth’s field reverses sometimes We could set up our magnetic field using a current? RIGHT HAND GRIP RULE: Current But applying a right hand rule followed by a left hand rule is equivalent to no handed rule at all. i.e. The wrong hand used for the grip rule followed by the wrong hand used for the motor rule produces the same predicted motion Magnetic field DEFINING LEFT AND RIGHT ABSOLUTELY WITH ELECTRO-MAGNETIC FORCES IS IMPOSSIBLE !! FLEMINGS LEFT-HAND RULE MERELY SWAPS THE LEFT/RIGHT PROBLEM FOR A NORTH/SOUTH PROBLEM WITH MAGNETS ! DEFINING LEFT AND RIGHT ASOLUTELY WITH ELECTRO-MAGNETIC FORCES IS IMPOSSIBLE !! FLEMINGS LEFT-HAND RULE MERELY SWAPS THE LEFT/RIGHT PROBLEM FOR A NORTH/SOUTH PROBLEM WITH MAGNETS ! MOST PHYSICISTS ASSUMED THAT AN ABSOLUTE DEFINITION OF LEFT AND RIGHT WAS COMPLETELY IMPOSSIBLE !! BUT IN 1956 THERE WAS THE SHOCKING DISCOVERY OF `PARITY VIOLATION’ BY THE `WEAK FORCE’ IN NUCLEAR BETA DECAY: Beta Decay neutron Beta Decay proton electron anti-neutrino Feynman diagram for Beta Decay W and W are the so - called `weak bosons' of the weak interactio n which mediate and decay NEUTRONS AND PROTONS IN NUCLEUS N NET ROTATION = “CURRENT” NUCLEUS BEHAVES LIKE S SMALL BAR MAGNET Madame Wu DISCOVERED “PARITY VIOLATION” Anti-neutrinos e Co Magnetic field Electrons e 60 Cobalt-60 experiment solves the L/R problem! Mirror Experiment electrons (e) Magnetic field anti-neutrinos Original Experiment anti-neutrinos Magneti c field electrons (e-) Neutrinos and Anti-neutrinos The weak interaction is a fundamental source of Left-Right asymmetry in nature Proving a neutrino is left-handed (Goldhaber’s Experiment) An Eu atom takes in an electron which reacts with a proton to form a neutron. Both a neutrino and a gamma ray are released. The “handed-ness” of the gamma ray and the neutrino are the same due to angular momentum because they are emitted in opposite directions. The photon in the gamma ray emitted was shown to be left-handed (using compton scattering), proving that the neutrino is also left-handed. + Analyse polarisation of photon in compton scattering Co-60 expt. doesn’t solve matter vs. anti-matter! Anti-Mirror Experiment neutrinos Magnetic field positrons (e+) Original Experiment anti-neutrinos Magneti c field electrons (e-) WHAT IF HE PUTS OUT HIS LEFT HAND ? Nowadays of course . . . we measure CP violation! Summary ₪ Left & right hands are mirror reflections ₪ Mirrors invert front and back ₪ L/R convention not verbally communicanle ₪ Left and right symmetry nearly perfect but broken by the weak interaction ₪ The Cobalt-60 experiment can distinguish left and right (and so north and south poles) ₪ Neutrinos are left handed and antineutrinos are right handed ₪ Aliens made of antimatter will get left and right wrong – so don’t get too close!!!!. z y x