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SP2171 Lecture
Symmetries in the Physical Laws (Lecture 2)
Kuldip Singh
22 February 2002
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James Clerk Maxwell (1831-1879)
Wave Nature of light
Maxwells equations are not Covariant with
respect to Galilean Transformations
Maxwells Equations in free space
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Electromagnetic Waves and the Luminiferous Ether

• ? The wave nature of light had been recognized
  since the work
• of Young and Fresnel at the beginning of the
  nineteenth century.
• Since all known waves were transmitted by some
  medium
• sound waves by air, surface waves on water
  by water
• Physicist tried to identify the medium that
  transmits light wave.
• - They called it luminiferous ether or ether
  for short.

Whatever difficulty we may have in forming
consistent idea of the ether, there can be no
doubt that the interplanetary and interstellar
spaces are not empty, but are occupied by a
material substance or body which is certainly the
largest, and probably the most uniform body of
which we have any knowledge .

Maxwell
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The Michelson Morley Experiment
What is the speed of Earth relative to ether?
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So what is the speed of Earth relative to ether?
Results of the Experiment No shift in
fringes observed! ? There is no ether wind
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Albert Einstein (1879-1955)
? Galilean Transformations need revision!
Harpos frame
Oprahs frame
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• Implications
• Length Contraction
• Time Dilation

H.A. Lorentz (1853-1928)
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The first experiment shows that distances
transverse to the moving observers are the same
to each other. The protruding nails of each ruler
scratch the other as they pass. The symmetry of
the situation demands that the distances d and d
be equal
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Lightning strikes the front and back of the
moving carriage at the same time according to
Harpo. Oprah disagrees. Because she is moving to
the right, she sees the forward flash occur
before the rear flash.
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Simultaneity
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Third Experiment - Time Dilation
Time between tick and tock according to Oprah
Time between tick and tock as observed by Harpo
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Fourth Experiment - Length Contraction
Oprahs Clock as seen by Harpo
But
Oprahs Clock as seen by Harpo
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The pole and the barn both have proper length L.
A runner who moves at nearly the speed of light
carries the pole through the barn. In the rest
frame of the barn the pole is observed to be of
contracted length and should therefore fit
inside the barn. According to the runner, the
barn is moving and thus appears contracted both
ends must protrude for a definite interval.
Question Does the pole fit inside the barn, or
doesnt it?
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Oprah boards a spacecraft that travels close to
the speed of light, to visit a nearby star. The
spacecraft makes a round trip and comes back to
earth. To her twin brother Harpo, Oprahs clock
runs more slowly hence he would expect to find
the sister younger after the trip. According to
Oprah, Harpo was moving relative to her
spacecraft and argues that Harpo should be the
younger of the two, upon meeting.
There can only be one correct answer! Whos right?
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Thats all folks!