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Chapter 31

Chapter 31. Physics in the 21 st Century May 6 th , 2013. Advances in Physics. Toward the end of the 19 th century, some physicists thought all the laws of physics had been discovered All that was left was to measure constants more accurately

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Chapter 31

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  1. Chapter 31 Physics in the 21stCentury May 6th, 2013

  2. Advances in Physics • Toward the end of the 19th century, some physicists thought all the laws of physics had been discovered • All that was left was to measure constants more accurately • Then the discoveries of quantum physics and relativity led to major advances during the 20th century • Currently, physics continues to be a very active research field

  3. Current Areas of Research in Phyics • Elementary particles • How protons, neutrons, and other subatomic particles are assembled • Biophysics • Photosynthesis, super-resolution imaging of living cells and tissues, systems biology, nanobiology, geomicrobiology, biomineralization • Medical physics • Diagnostics, imaging, robotic surgery, laser surgery • Astrophysics and Cosmology • Origin and fate of the universe • Condensed matter physics • New materials: graphene, topological insulators, superconductors, quantum computing.

  4. Cosmic Rays • Particles that arrive from space are called cosmic rays • Cosmic rays include protons (~89%), alpha particles (~10%), nuclei of elements more massive than He and other particles • It is believed that some cosmic rays are created when stars collide • Cosmic rays can also be created when a massive star uses up its fuel • The star collapses and triggers a supernova

  5. Supernova 1987A

  6. Cosmic Rays, cont. • Typical cosmic ray energies are 100 MeV • Some particles with energies above 1020 eV have been detected • New, more sensitive detectors continue to be built • The goal is a better understanding of cosmic rays and other particles that arrive from outer space • Some detectors are built specifically to find neutrinos • Neutrinos are generated in the fusion reactions in the Sun and in supernova explosions

  7. Super-Kamiokande Neutrino Detector

  8. Dirac’s Quantum Physics • P. A. M. Dirac formulated a theory of quantum mechanics that combined the quantum theory of Schrödinger and Heisenberg with the postulates of special relativity • Two important results were: • The prediction of the electron spin • The prediction of a completely new particle having the same mass as the electron and an electric charge of the same magnitude as an electron but with opposite sign, the positron.

  9. Antimatter • The positron is an example of antimatter • Antimatter particles have the same mass but opposite charge as the corresponding particles of matter • Other antimatter particles exist • An overbar is generally used to denote an antiparticle • The positron is denoted by e+ to distinguish it from the electron e-

  10. Some Properties of Particles

  11. Particle Annihilation, Example • An electron and a positron can undergo a reaction in which each particle is annihilated • Two photons will be formed • Total energy will still be conserved and includes the kinetic energies of the particles as well as the rest mass of the electron and positron

  12. Conservation Rules • All particle reactions conserve energy, momentum, and charge • These conservation laws are the same ones that were encountered in classical physics • Other conservation laws will be involved that go beyond the classical laws of physics

  13. Stability of Antimatter • Antimatter is stable if is it kept away from regular matter • Antiprotons and positrons can be brought together to form antihydrogen • It is difficult to produce large amounts of antimatter because it is difficult to contain it far enough away from regular matter to prevent annihilation • All known methods of creating antiparticles require more energy than the rest masses of the antiparticles • Thus antimatter is not a practical energy source (sorry Jean Luc Picard!)

  14. Richard Feynman • 1918 – 1988 • One of the developers of quantum electrodynamics (QED) • Worked in many areas of physics • Manhattan project • Liquid helium

  15. Quantum Electrodynamics • Quantum electrodynamics combines electromagnetism and Maxwell’s equations with quantum mechanics • According to QED, photons are the elementary quanta of electric and magnetic fields • They play a role in all electric and magnetic forces

  16. Forces Between Charged Particles • The classical view (A) is that the electric force on q2 is due to the electric field produced by q1 • The QED (B) picture explains the interaction by the exchange of photons between the particles • Photons mediate the electric force

  17. Photons • Photons mediate all electric and magnetic forces • It is believed that all fundamental forces in nature are mediated by elementary particles • The photon is an elementary particle • It has zero rest mass • The photon is its own antiparticle • There is no antiphoton

  18. The Standard Model • The standard model describes the propertiesof fundamental particles and the interactionsbetween them • There are two classes of particles • Hadrons (baryons and mesons) are composed of quarks • protons and neutrons are hadrons, but there are many more • Leptons • electrons and positrons are leptons, but thee are many more

  19. Quarks • There are six different kinds of quarks • Each quark has a corresponding antiparticle • Quarks were first discovered in collision experiments involving protons • These experiments showed there were three point-like particles inside the proton

  20. Hadron Composition • Baryons • Hadrons composed of three quarks • Examples include the proton and neutron • Mesons • Hadrons composed of two quarks • A quark and an antiquark

  21. Hadron Composition charge (in usints of e) 0 -1 1 2/3 2/3 -2/3 -2/3 2/3 -1/3 -1/3 1/3 -1/3 1 1 2/3 2/3 1/3 1/3

  22. Properties of Some Baryons All baryons have charge 1, 0, or -1. Fractional charges are forbidden. To practice, calculate the charge of all baryons in this table.

  23. Properties of Some Mesons All mesons have charge 1, 0, or -1. To practice, calculate the charge of all mesons in this table.

  24. Some Properties of Quarks • The interactions between quarks determine the properties of hadrons • Quarks are charged and therefore interact via the electric force • They also interact via the strong force • The strong force holds the quarks together to form nucleons as well as holding the nucleons together to make nuclei

  25. Rules for Combining Quarks • Charges • The charge of any hadron can be found by adding up the charges on its constituent quarks • An isolated particle cannot have a fractional charge • This implies the quarks themselves cannot be isolated • Baryon number must be conserved in all particle reactions • Each quark has a baryon number of +1/3 • Each antiquark has a baryon number of -1/3

  26. Mass of a Quark • One way to measure the mass of a quark is through Newton’s Second Law or the corresponding result from quantum mechanics • Since quarks are confined in baryons or mesons, it is impossible to study how a free quark would respond to an applied force • Probing the motions of the quarks inside the baryon or the meson is used to deduce the mass based on the strong force • Analysis must also allow for other potential and kinetic energies • The masses of quarks are not known with the same precision as the masses other particles

  27. Leptons • There are six fundamental leptons plus their corresponding antiparticles • They group into three pairs • The electron is stable, but the muon and tau particles are not • Neutrinos can oscillate between types as they travel through space • Neutrinos have very small masses • Leptons are produced in many nuclear decay reactions

  28. Reactions with Leptons • Reactions involving leptons must satisfy conservation of lepton number • All leptons have a lepton number of +1 • All antileptons have a lepton number of -1 • Example: neutron decay • Must obey conservation of lepton number and baryon number

  29. Problem 31.14 Principle. All reactions with elementary particles must conserve baryon number, lepton number, and electric charge. Reaction (a) is forbidden. Baryon number is not conserved because the proton is a baryon but the positron and electron neutrino are both leptons. This reaction also does not conserve lepton number. Reaction (b) is forbidden. Before the reaction the baryon number is +1 whereas the final baryon number is zero, hence baryon number is not conserved. Lepton number is also not conserved. Reaction (c) is forbidden. Before the reaction the lepton number is zero but the final lepton number is +2, hence lepton number is not conserved.

  30. Problem 31.14 Principle. All reactions with elementary particles must conserve baryon number, lepton number, and electric charge. Reaction (d) is forbidden. Before the reaction the baryon number is +1, the lepton number is zero, and the charge is +e. After the reaction the baryon number is zero, the lepton number is +1, and the charge is zero, hence baryon number, lepton number, and charge are not conserved. Reaction (e) is forbidden. Before the reaction the baryon number is zero but the final baryon number is +1, hence baryon number is not conserved. What does it mean? Conservation laws constrain the kinds of elementary particle reactions that are allowed.

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