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Measuring the Speed of Light

Measuring the Speed of Light. John Klumpp And Ainsley Niemkiewicz. Why Measure The Speed of Light?. Inherently Difficult Electrodynamics Theory of Relativity Actually, We Don’t. How We Do It. Measure Travel Time of Laser Pulse. Pros: Simple Straightforward Direct. Cons:

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Measuring the Speed of Light

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  1. Measuring the Speed of Light John Klumpp And Ainsley Niemkiewicz

  2. Why Measure The Speed of Light? • Inherently Difficult • Electrodynamics • Theory of Relativity • Actually, We Don’t

  3. How We Do It • Measure Travel Time of Laser Pulse

  4. Pros: Simple Straightforward Direct Cons: Less Accurate More Time Consuming Oscilloscope Setup

  5. Pros: More Accurate Faster Cons: More Complicated Requires Calibration Many Components Electronic Circuitry Setup

  6. Data • Total Round Trip Distance: DAC = 62.71m • Short Trip Distance: DAB = .486m • Measured Flight Path Distance: DBC = 62.71m - .486m = 62.224m

  7. Oscilloscope Data Total round trip flight time, including lag, TAC, with signal amplitude 138.8mV: TAB, TBC, calculation of SoL with signal amplitude 136.4mV: (TBC = TAC – TAB, SoL = DBC/TBC = 62.224/TBC)

  8. Electronic Circuitry Data Calibration: • Time Calibrator Set to 40nS • MCA Histogram Peaks at 2570mV and 2970mV • Conclusion: 400mV=40ns  10mV=1nS

  9. Measurement of TBC: • Short Trip Peak at 510mV ~TAB • Long Trip Peak at 2599mV ~TAC • TBC ~ 2599mV – 510mV = 2089mV • Applying Calibration Constant, TBC = 208.9nS • We thus have c=d/t = 62.224m/208.9nS =2.979 * 108m/S = c

  10. Data Analysis Oscilloscope: • Average of five measurements • Uncertainty comes from-distance measurement-correlating peak locations • C = (2.983±.05)*108m/s • % Error = .498%

  11. Data Analysis • Electronic Circuitry • Electronic time measurements • Computer analysis of data • Very precise time measurements • Uncertainty comes from:-distance measurement-Do we trust our equipment? • C = (2.978±.03)*108m/s • % Error = .664%

  12. Conclusion • Classical Experiment, New Technology • Accurate, Straightforward Method • %Error < 1% • Speed of light = speed of propagation of Maxwell’s EM waves! • We can now test predictions of relativity

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