Lecture Four

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Lecture Four
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Attempts to Save Ether Hypothesis
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Status

• Michelson-Morley experiment shows
• null result.

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Attempts to Save Etherbefore Special Relativity

1. Contraction Hypothesis
2. Ether-Drag Hypothesis
3. Modify Electrodynamics

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Contraction Hypothesis

• Fitzgeral (1892)
• Lorentz
• Length is contracted in the direction of motion.

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DemolishedbyKennedy-Thorndikeexperiment (1932)
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Ether-Drag Hypothesis

• contradicted with
• Stellar aberration
• (Bradley 1727)
• Fizeau convection coefficient
• (Fresnel 1817, Fizeau 1857)

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Stellar Aberration
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Stellar Aberration
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Stellar Aberration
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Fizeau Experiment
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Fizeau Experiment

• velocity of light v in a medium of refractive
  index n moving with a velocity vw

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Modify Electrodynamics

• Emission Theories
• The velocity of a light wave is connected with
  the motion of the source rather than with an
  ether.

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Modify Electrodynamics

• Emission Theories
• The velocity of light is c relative to the
  original source.
• This velocity is independent of the state of the
  medium transmitting the light.

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Emission Theories

• are contradicted with
• de Sitter observations on binary stars
• Michelson-Morley experiment using
  extraterrestrial light source

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Conclusions

• The speed of light is the same in all inertial
  systems, independent of the relative motion of
  source and observer.
• A relativity principle applicable both to
  mechanics and to electrodynamics.

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Conclusions

• Galilean transformations must be replaced.
• The laws of mechanics which were consistent with
  Galilean transformations needs to be modified.

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Special Relativity

• Einstein 1905
• On the Electrodynamics of Moving Bodies

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Two Postulates

• Principle of Relativity
• Principle of Constancy of Speed of Light

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Principle of Relativity

• include all laws of physics
• no measurements to designate an inertial system
  as intrinsically stationary or moving

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Principle of Relativity

• no experiment entirely within an inertial system
  can tell the motion with respect to any other.

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Principle of Constancyof Speed of Light

• consistent with Michelson-Morley experiment

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Experimental Basis
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Program of the Theory

• nature of time
• transformation keeping velocity of light constant
• Lorentz transformation

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Einstein

• In his early life none of his elders recognized
  his genius.
• Until almost thirty he had never seen a real
  theoretical physicists, except in the mirror!

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Einstein

• statistical mechanics
• atomic nature of matter
• special relativity
• principle of equivalence
• light quanta hypothesis
• specific heat of solids

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Einstein 1905

• Quantum theory of light
• Brownian motion
• Special Relativity

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Einstein

• not in the mainstream
• nature of time
• abandoned Newtons universal time