Understanding Electromagnetism, Forces, and Fields in Nature

ASTR 3420 The Forces of Nature Supplementary Slides February 7, 2017 Bob Dickman

Electromagnetism and Light The subject was slightly known, even in antiquity: Lodestones known to point toward the North Pole Amber ( Elektron ) When rubbed with fur, made sparks Electromagnets laws worked out early 19thcentury Maxwell Perfected the Theory of Electromagnetism: Electric and Magnetic Fields Different aspects of a more fundamental unified object: the Electromagnetic Field Maxwell s Theory Predicted: Electromagnetic Waves Exist transport of energy through space Predicted speed of these waves: Speed of light! All electromagnetic waves are produced by accelerating and decelerating electric charge This led to the development of radio in late 19thcentury

Forces and Fields Electromagnetism Led to the General Concept of Forces and Fields A field is a condition in space that is used to describe the force on particles Fields are produced by charges, and influence the charges Example: Electric charges exert forces on other charges The field carries or transmits the force The quantum world At small enough distances and small enough energies, the world is made of atoms, elementary particles, and field bosons. These are all discrete, granular objects, and they follow the rules of quantum mechanics, something very different from everyday experience Forces are carried by particles only in large groups, the particles behave as classical fields The sources of fields are also particles example: electrons The general term for field sources: Charges The general term for field carriers: Bosons Are there other forces/fields in Nature besides electromagnetism? YES! You know about gravity and electromagnetism, but there are others There are four fundamental forces in nature plus a Higgs field, which helps give mass to matter Gravity is still poorly understood quantum mechanically, though we understand it very well on the non-quantum level, thanks to Newton and Einstein

The Four Forces of Nature Ranked by Strength (Strongest to Weakest) 1. Strong force (First pioneered Rutherford et al.) 2. Electromagnetism ( Maxwell) 3. Weak force ( Fermi et al.) 4. Gravity ( Newton; then Einstein)

Fundamental Constituents of Matter (As Of 2012) 126 GEV 0 0 Higgs Higgs field electromagnetic strong, electromagnetic, weak strong, electromagnetic, weak strong weak weak weak electromagnetic, weak

Unification of Fields Einstein searched fruitlessly for a simple mathematical extension of his theoretical formulation of gravity that would also naturally include electromagnetism: Ideally, they would be different manifestations of the same thing. He failed. Weinberg, Salaam, Glashow exploited quantum theory, vastly deeper experimental knowledge, and a far more sophisticated theoretical framework during the 1970s to unify electromagnetism and the weak force At very high energies, when the Universe was between ~ < 10-36 and 10-12 seconds old, photons and the W , Z carriers of the weak force had no distinct identity instead there was only a single force mediated by 4 massless particles Only one number needed to explain the masslessness of the photon and the masses of the W and Z particles! Natural questions: Can one extend unification so that all 4 forces we perceive turn out to be manifestations of one, single force? We d also like our unified theory to explain of the various, seemingly arbitrary masses of the fundamental particles in a simple obvious way How are we doing with the program? Not so good, actually : Supersymmetry : proposed to unify strong + electroweak: The simplest, most elegant form of the theory crashes and burns (predicts protons to be quite unstable not seen); experiments with the LHC show no evidence of the superpartner particle partners required by the theory Supersymmetry + gravity: (Super-)string theory. Beautiful, but very, very difficult mathematical framework. May not be fully testable experimentally, predicts an enormous number of (~10500) possible universes. Some of the same experimental expectations as Supersymmetry place pressure on String Theory as well.

The Four Forces of Nature: Why care? On the largest scales, the Universe is neutral and the electromagnetic, strong and weak forces don t need to be invoked to explain human experience. So we don t need to understand them? Not! , we do. Gravity: It orders the Universe on the largest scales, explains the origin of the Universe, its expansion, the motions of the galaxies, the motions of the planets. Electromagnetic (EM) Force: Vitally important for our civilization. EM technologies underpin human technological progress from the late 19thcentury onward . AND Electromagnetic radiation is the touchstone of nearly all observational astronomy! Strong Force: Makes the stars shine (along with the weak interaction)! Underlies understanding of nuclear fission, fusion and the formation of the elements. Weak Force: Supernova explosions are driven by neutrinos The discovery that neutrinos have very small but real masses may offer clue to the next theoretical steps in particle theory.