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Unformatted text preview: Building blocks of the universe Lecture 4
Outline: The quantum realm Quantum uncertainties Fundamental particles Fundamental forces Read sections S4.1 - S4.3 in textbook. Last time Kepler's laws of planetary motion Newton's version of Kepler's third law Determining masses of stars and planets Energy and orbits.
A Lecture Question
A probe is in orbit around another planet as shown below. If you want it to have a larger, more circular orbit, what must you do? Fire engines in the same/opposite direction of motion at A/B? What if you want to make it a smaller, more circular orbit? B Lecture Question
Fire engines in the same direction of motion at B. To make it a smaller, more circular orbit fire engines in opposite direction at A. The Quantum Realm Light behaves like particles (photons), Atoms consist mostly of empty space, Electrons in atoms are restricted to particular energies, The science of this realm is known as quantum mechanics. A B Surprising Quantum Discoveries Protons and neutrons are not truly fundamental--they are made of quarks, Antimatter can annihilate matter and produce pure energy, Just four forces govern all interactions: gravity, electromagnetic, strong, and weak, Particles can behave like waves, Quantum laws have astronomical consequences. Particle Accelerators Much of our knowledge about the quantum realm comes from particle accelerators. Smashing together high-energy particles produces showers of new particles. Properties of Particles Mass Charge (proton +1, electron -1) Spin Fermions and Bosons Physicists classify particles into two basic types, depending on their spin (measured in units of h/2!) Fermions have half-integer spin (1/2, 3/2, 5/2,...) Electrons, protons, neutrons Bosons have integer spin (0,1,2,...) Photons. Each type of subatomic particle has a certain amount of angular momentum, as if it were spinning on its axis Orientation of Spin Fermions with spin of 1/2 have two basic spin states: up and down. Fundamental Particles Quarks: fundamental particles Quarks and Leptons Six types of quarks: up, down, strange, charmed, top, and bottom Leptons are not made of quarks and also come in six types Electron, muon, tauon Electron neutrino, mu neutrino, tau neutrino Neutrinos are very light and uncharged. Protons and neutrons are made of quarks. Up quark (u) has charge +2/3. Down quark (d) has charge -1/3. Matter and Antimatter Matter and Antimatter Each particle has an antimatter counterpart, When a particle collides with its antimatter counterpart, they annihilate and become pure energy in accord with E = mc2. Energy of two photons can combine to create a particle and its antimatter counterpart (pair production). Four Forces of Nature Strong Force (holds nuclei together) Exchange particle: gluons Electromagnetic Force (holds electrons in atoms) Exchange particle: photons Weak force (mediates nuclear reactions) Exchange particle: weak bosons Gravity (holds large-scale structures together) Exchange particle: gravitons. Strength of Forces Inside nucleus: strong force is 100 times electromagnetic weak force is 10-5 times electromagnetic force gravity is 10-43 times electromagnetic force. Outside nucleus: Strong and weak forces are unimportant. The Uncertainty Principle The more we know about where a particle is located, the less we can know about its momentum, and conversely, the more we know about its momentum, the less we can know about its location. Position of a Particle In our everyday experience, a particle has a welldefined position at each moment in time. But in the quantum realm particles do not have welldefined positions. Electrons in Atoms In quantum mechanics an electron in an atom does not orbit in the usual sense. We can know only the probability of finding an electron at a particular spot. Electron Waves On atomic scales, an electron often behaves more like a wave with a well-defined momentum but a poorly defined position. Location and Momentum Energy and Time Uncertainty Uncertainty Planck's X in momentum = Constant (h) in location Uncertainty Uncertainty Planck's = X in time in energy Constant (h) What is the exclusion principle? Quantum States The quantum state of a particle specifies its location, momentum, orbital angular momentum, and spin to the extent allowed by the uncertainty principle. Exclusion Principle Two fermions of the same type cannot occupy the same quantum state at the same time. Exclusion in Atoms Two electrons, one with spin up and the other with spin down can occupy a single energy level. A third electron must go into another energy level. Virtual Particles Uncertainty principle (in energy & time) allows production of matter-antimatter particle pairs. But particles must annihilate in an undetectably short period of time. Vacuum Energy According to quantum mechanics, empty space (a vacuum) is actually full of virtual particle pairs popping in and out of existence. The combined energy of these pairs is called the vacuum energy. ...
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- Fall '07