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A121O Addenda to Class on Light Bob Dickman, NRAO 520 Edgemont Rd., Room 311A 434.296.0288

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The “Four” Forces of Nature: Ranked by Strength (Strongest to Weakest) Strong force (Rutherford et al.) Electromagnetism (Maxwell) Weak force (Fermi et al.) Gravity (Newton; Einstein) All forces have “charges” which are the sources of the fields and which determine their strength, and field carriers whose exchange by the charges “causes” the forces

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The “Four” Forces of Nature: Historical The four fundamental forces were known by the 1930s – Gravity (Newton; Einstein) – Electromagnetism (Maxwell)* – Strong force (Rutherford et al.) Quantum world** – Weak force (Fermi et al.) Quantum world*, ** _____________________________________________________ * Unified into single force (“electroweak”) at high energies ** Range: Dimensions of an atomic nucleus (~10 -14 m) ** Character of force: Non-Abelian

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Fundamental Constituents of Matter strong, electromagnetic, weak electromagnetic, weak weak electromagnetic strong weak

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Unification Einstein searched fruitlessly for a simple mathematical extension of his theoretical formulation of gravity that would also naturally include electromagnetism: The 2 phenomena would be different manifestation of the same thing. He failed. Weinberg, Salaam, Glashow exploited quantum theory, vastly deeper experimental knowledge, and a far more sophisticated mathematical framework during the 1970s to unify electromagnetism and the weak force – At very high energies (attained 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 1 number needed to explain the masslessness of the photon and the masses of the W ± and Z particles Natural question: 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? Not so good, actually : – “Supersymmetry”: proposed to unify strong + electroweak: The simplest, most elegant form of the theory crashes and burns (predicts protons quite unstable – not seen); also, experiments with new LHC show no evidence of predicted “superpartner” particles – Supersymmetry + gravity: Done with (Super-) string theory. Beautiful, very very difficult mathematical framework. May not be testable experimentally; predicts multiple (~10 500 ) possible universes -- Why do we live in the one we live in?

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Structure of the Universe

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Content of the Universe

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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! 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, etc. Electromagnetic force: Vitally important for our civilization. Underlies much of human progress from the late 19 th century onward…. Electromagnetic radiation is the touchstone of nearly all observational astronomy! Strong interaction: Makes the stars shine (along with the weak interaction). Underlies nuclear fission, fusion. Weak interaction: Supernova explosions are driven by neutrinos Perhaps the most important gauge of the Universe and its future Watch the news tomorrow(?) morning for the 2011 Nobel prize in physics. It may be awarded to Harvard and Berkeley groups for discovery of the accelerating expansion of the Universe

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