Measuring the Speed of Light

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 Dont

3
How We Do It

• Measure Travel Time of Laser Pulse

4
Oscilloscope Setup

• Pros
• Simple
• Straightforward
• Direct

• Cons
• Less Accurate
• More Time Consuming

5
Electronic Circuitry Setup

• Pros
• More Accurate
• Faster

• Cons
• More Complicated
• Requires Calibration
• Many Components

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
Amplitude Amplitude (mV) TAC (ns)
15 20.82 219.2
30 41.64 223.1
45 62.46 226.9
60 83.22 230.9
75 104.1 235.3
Amplitude Amplitude (mV) TAB (ns) TBC (ns) c (108 m/s)
15 22.14 10.5 208.7 2.981
30 44.28 14.3 208.8 2.98
45 66.42 18 208.9 2.979
60 88.56 22.1 208.8 2.98
75 110.7 27.5 207.8 2.994
(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 400mV40ns ? 10mV1nS

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 cd/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 lt 1
• Speed of light speed of propagation of
  Maxwells EM waves!
• We can now test predictions of relativity