Lecture 22 (Ch. 28.1-28.3)

Lecture 22 (Ch. 28.1-28.3) - Lecture 22 Quantum Physics...

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Lecture 22 Quantum Physics
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Classical Physics o What you’ve learned so far: Newton’s Laws Laws of Thermodynamics Electromagnetism (Maxwell’s Equations) Optics o Equivalent to everything known ~100 years ago Believed at the time to be very close to a complete theory of everything “All that remains to do in physics is to fill in the sixth decimal place” – A. Michelson (Nobel Laureate), 1894
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But the Universe is much more mysterious than classical physics would suggest… o How wrong they were… Michelson regretted that remark for the rest of his life! o Within about 25 years, physicists’ entire picture of the universe– and of “reality” itself - changed radically and fundamentally (E=mc 2 , quantum physics, big bang, etc.) o This upheaval was forced on us by many experiments: If you look closely enough at things that are very small, the world does not behave anything like the way classical physics predicts!
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Loose Ends… o At the beginning of the 20 th century, there was a short list of seemingly minor problems with classical physics: Search for the “ether” (medium through which EM waves propagate) Radioactivity (Chapter 30) Black-body radiation The photoelectric effect Line spectra of atoms o Note that all involve the interaction of radiation and matter o New experimental technology was being developed: Vacuum Electronic measuring devices Emulsions (film) o This new technology allowed much more accurate measurements … and the measurements disagreed completely with the theory! The more experiments done, the more disagreements were found
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…Which Unraveled Classical Physics… o In classical physics: Waves are continuous and particles are point-like It is possible to measure a system without disturbing it The physical parameters of a system (position, momentum, etc) have reality independent of whether we measure them or not The world behaves mechanically (like a machine ) o In quantum physics: Waves are lumpy and particles are smeared out Measuring a system means interacting with it – the system is inevitably affected by a measurement The physical parameters of a system have reality only when we measure them The world behaves according to the laws of probability (randomly)
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…And Made Today’s World Possible o Things impossible without Quantum Physics: Structure of atoms and molecules Chemistry DNA and metabolic reactions Nuclear energy and the Sun Metals, transistors and computers Superconductivity Magnetic disks Lasers and photocells CD’s TV remote controls Electron microscopes, CT scans, PET scans, MRI Cosmology and understanding our place in the universe
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Physics Circa 1900: Two Puzzles 1. Thermal Radiation The “Ultraviolet Catastrophe”: -- Prediction : The spectrum of
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This note was uploaded on 09/08/2008 for the course PHYS 3B taught by Professor Wu during the Spring '08 term at UC Irvine.

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Lecture 22 (Ch. 28.1-28.3) - Lecture 22 Quantum Physics...

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