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Thought Experiments                        John AshmeadSunday, April 1, 2012
“I will a little tink”                        – Well-known SwabianSunday, April 1, 2012
Definition                  Go back to Pre-Socratics; long history                  Range from ‘scientific parable’ to      ...
Galileo  Salviati: If we take two bodies whose natural speeds are different, it is  clear that on uniting the two, the mor...
Einstein                  Chasing light                  Equivalence principle                  Clock in a boxSunday, Apri...
Young Einstein         "...a paradox upon which I had already hit at the age of         sixteen: If I pursue a beam of lig...
Chasing the light                 http://www.pitt.edu/~jdnorton/Goodies/                       Chasing_the_light/index.htm...
Equivalence principle                        You can’t tell the                        difference between                 ...
Clock in a                           box                        Open the shutter, briefly.                        Photon es...
It was a real shock for Bohr...who, at first,                        could not think of a solution. For the entire         ...
Chain of uncertainties                        But to weigh the box we                        must measure its height      ...
Other thought experiments                  Einstein Rosen Podolsky                  Maxwell’s demon                  Newto...
Scientific methods                  Observation                  Experiment                  Computer simulations          ...
Relativity & madness                              During the voyage, an around-the-globe                              sail...
References                  Einstein - The Meaning of Relativity                  Ohanian - Einstein’s Mistakes           ...
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Thought experiments

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Role of thought experiments in physics: Galileo, Einstein, others. What they are good for, how they evolve into real experiments.

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Transcript of "Thought experiments"

  1. 1. Thought Experiments John AshmeadSunday, April 1, 2012
  2. 2. “I will a little tink” – Well-known SwabianSunday, April 1, 2012
  3. 3. Definition Go back to Pre-Socratics; long history Range from ‘scientific parable’ to ‘experimental plan’ Used to illustrate, to attack, to support, & simply to understandSunday, April 1, 2012
  4. 4. Galileo Salviati: If we take two bodies whose natural speeds are different, it is clear that on uniting the two, the more rapid one will be partly retarded by the slower, and the slower will be somewhat hastened by the swifter. Do you not agree with me in this opinion? Simplicio: You are unquestionably right. Salviati: But if this is true, and if a large stone moves with a speed of, say, eight, while a smaller stone moves with a speed of four, then when they are united, the system will move with a speed of less than eight. Yet the two stones tied together make a stone larger than that which before moved with a speed of eight: hence the heavier body now moves with less speed than the lighter, an effect which is contrary to your supposition. Thus you see how, from the assumption that the heavier body moves faster than the lighter one, I can infer that the heavier body moves more slowly…Sunday, April 1, 2012
  5. 5. Einstein Chasing light Equivalence principle Clock in a boxSunday, April 1, 2012
  6. 6. Young Einstein "...a paradox upon which I had already hit at the age of sixteen: If I pursue a beam of light with the velocity c (velocity of light in a vacuum), I should observe such a beam of light as an electromagnetic field at rest though spatially oscillating. … From the very beginning it appeared to me intuitively clear that, judged from the standpoint of such an observer, everything would have to happen according to the same laws as for an observer who, relative to the earth, was at rest. … One sees in this paradox the germ of the special relativity theory is already contained."Sunday, April 1, 2012
  7. 7. Chasing the light http://www.pitt.edu/~jdnorton/Goodies/ Chasing_the_light/index.htmlSunday, April 1, 2012
  8. 8. Equivalence principle You can’t tell the difference between gravity & acceleration With conservation of energy, the foundation of general relativity Implies red-shift, curvature of light by gravity, time dilation, …Sunday, April 1, 2012
  9. 9. Clock in a box Open the shutter, briefly. Photon escapes with known uncertainty in time. Now, weigh the box. Now we know the energy! But this violates the energy/time uncertaintySunday, April 1, 2012 relation
  10. 10. It was a real shock for Bohr...who, at first, could not think of a solution. For the entire evening he was extremely agitated, and he continued passing from one scientist to another, seeking to persuade them that it could not be the case, that it would have been the end of physics if Einstein were right; but he couldnt come up with any way to resolve the paradox. I will never forget the image of the two antagonists as they left the club: Einstein, with his tall and commanding figure, who walked tranquilly, with a mildly ironic smile, and Bohr who trotted along beside him, full of excitement… The morning after saw the triumph of Bohr.Sunday, April 1, 2012
  11. 11. Chain of uncertainties But to weigh the box we must measure its height But to do this, we have to apply an impulse And therefore there is an uncertainty in its height And, therefore (by general relativity) in its time!Sunday, April 1, 2012
  12. 12. Other thought experiments Einstein Rosen Podolsky Maxwell’s demon Newton’s Apple Black hole information paradox …Sunday, April 1, 2012
  13. 13. Scientific methods Observation Experiment Computer simulations Big dataSunday, April 1, 2012
  14. 14. Relativity & madness During the voyage, an around-the-globe sailboat race, Crowhurst had been reading Einsteins book "Relativity: The Special and the General Theory." [constancy of the speed of light] "is in reality neither a supposition nor a hypothesis about the physical nature of light, but a stipulation which I can make June 24th , 1969 of my own free will in order to arrive at Donald Crowhurst a definition of simultaneity." threw himself off his trimaran in despair "You cant do THAT!" Crowhurst, an at relativity electrical engineer, protested to his journal. "I thought, the swindler. " From there he descended into madness.Sunday, April 1, 2012
  15. 15. References Einstein - The Meaning of Relativity Ohanian - Einstein’s Mistakes Isaacson - Einstein Sorenson - Thought Experiments Usual googling…Sunday, April 1, 2012
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