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Einstein’s Legacy

Einstein’s Legacy. Physics from 1905- 2005 Roger Barlow The University of Manchester. Einstein: the person. Physics in 1905. Science triumphant Physics basically solved just a few small problems. 1905: The ‘Annus Mirabilis’. 3 papers, each a major breakthrough Brownian Motion

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Einstein’s Legacy

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  1. Einstein’s Legacy Physics from 1905- 2005 Roger Barlow The University of Manchester

  2. Einstein: the person Manchester ACES lecture

  3. Physics in 1905 Science triumphant Physics basically solved just a few small problems Manchester ACES lecture

  4. 1905: The ‘Annus Mirabilis’ 3 papers, each a major breakthrough • Brownian Motion • Photoelectric Effect • Relativity Manchester ACES lecture

  5. Infinitely Divisible Like angles, lengths… Elegant Mathematically neat Favoured by Physicists Made of ‘atoms’ Like forests, people… No hard justification Mathematically messy Favoured by Chemists 1: What is the Nature of Matter? Manchester ACES lecture

  6. The Question • If matter is made of atoms, how small are they? How many atoms of (say) Hydrogen go to make 1 gram of Hydrogen Manchester ACES lecture

  7. The Answer (Einstein) • There are close to 600000000000000000000000 atoms in a gram (of Hydrogen) • This puts the atomic theory on a sound footing. We know the size of atoms. Manchester ACES lecture

  8. How did he get there? • Atoms are in constant motion. • They all have the same (average) energy – which depends on temperature • Atoms are invisible, but pollen grains are not (Brownian Motion) Manchester ACES lecture

  9. Einstein’s Insight The speed/energy of the pollen can be measured indirectly by the rate at which it diffuses through the water ‘Random walk’ Distance time Measurement possible. Problem solved Manchester ACES lecture

  10. Newton: A stream of particles Huygens: Light is waves 2: What is light Manchester ACES lecture

  11. How it went • Newton’s theory predominated (Newton was Newton) • Young and Fresnel showed that light has wavelike properties • Newton’s theory dead. Waves Triumphant • Along comes Einstein Manchester ACES lecture

  12. Light according to Einstein Manchester ACES lecture

  13. The Detail Light is waves but comes in packets • The packet size is give by E=hf Energy E depends on frequency f • Explains, and predicts detail of, Photoelectric effect (solar panels, digital cameras) Manchester ACES lecture

  14. 3: Space and time Discovery of the Theory of Relativity “On the Electrodynamics of Moving Bodies” Manchester ACES lecture

  15. The Puzzle Uniform motion is undetectable (surely?) Checks out fine for mechanics Doesn’t work for Electricity and Magnetism Manchester ACES lecture

  16. Electrostatics: Coulomb’s Law Like charges repel Magnetism Ampere’s Law Like currents attract + + + + Manchester ACES lecture

  17. Clarification Common sense suggests different speeds for light, depending on motion. But electromagnetism says it should be the same If this is sorted, all the other difficulties are also sorted Manchester ACES lecture

  18. Revised question • How can the velocity of light not depend on motion? v c c c+v=c??? Manchester ACES lecture

  19. Observers differ, but… 1 Some things they must agree on (number of wheels, number of windows…) 2 Some things they will disagree on (how loud it is, angular size…) This disagreement is not just subjective. It has rules. Einstein said: lengths and times and masses belong in group 2 not group 1. The speed of light is in group 1 not group 2. He showed this could be done consistently, and found the rules relating observations. Manchester ACES lecture

  20. Why had nobody noticed earlier? The differences between relativity and ‘common sense’ are proportional to v2/c2 Even for fast trains, v<<c so the differences are unmeasurable Manchester ACES lecture

  21. So 1905 produced Light behaves like waves and like particles New ideas of space and time, based on measurement Convincing evidence for the atomic theory What happened next? Manchester ACES lecture

  22. 3: Space and time • Extend to more of mechanics. E=mc2 in 1907 • Extend to acceleration and gravity with General Relativity (1915) • Fashionable in the 1920’s – but esoteric. Relativity only becomes important at speeds approaching 186,000 mps. Academic? Manchester ACES lecture

  23. Particle Accelerators are everywhere Machines today accelerate electrons and protons to near lightspeed. Relativity is engineering Manchester ACES lecture

  24. General Relativity is useful too • Clocks run at different speeds depending on gravity. • Needed for GSM systems • Drives the expanding and exploding universe, black holes… Manchester ACES lecture

  25. 1: Beyond the atom Atomic theory accepted Nature of atom probed (Bohr, Rutherford). Made of protons and neutrons and electrons. What are protons and neutrons made from? Quarls What are quarks made of (if anything?) How? Why do they behave as they do? Manchester ACES lecture

  26. 2: Light waves are particles Next step: particles can behave as waves. Electrons are E=hf wave-packets too Led to quantum theory, indeterminacy (Schrödinger, Heisenberg, Dirac), modern chemistry and semiconductors Manchester ACES lecture

  27. General Relativity 1905 Astrophysics Relativity Accelerators Photons 2005 Atoms Wave Mechanics Medicine QuantumMechanics Atomic Physics Modern Chemistry Semiconductors Nuclear Physics Computers Lasers Particle Physics Nuclear Power Manchester ACES lecture

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