The Michelson interferometer

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The Michelson interferometer

• Alex Pinder

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Intro

• A very simple experiment
• Gave a very unexpected result light moves at
  3108 m/s always
• Result Einsteins special relativity
• Consequences time moves differently for
  different people, things look shorter when they
  move past you at high speed

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First look
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The one youll use!
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What youll see
Fringes!
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Why fringes?
Path 1
Path 2
Light from two paths interferes constructively at
your eye ? bright fringe
Light from two paths interferes destructively at
your eye ? dark fringe
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Michelson-Morley experiment (1887)

• Used Michelson interferometer showed light
  doesnt travel through anything (unlike sound)
• If light medium (aether) existed, fringe
  pattern would change with rotation of the
  apparatus

c
c
Actual size of interferometer 11 m
c v
Aether flow
c - v
Speed v
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BUT!!!

• The fringe pattern did not change N 0
• Speed of light is fixed
• But in a strange way

Sees light coming towards him at 3108 m/s
LIGHT
Sees same light catching up at 3108 m/s
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Gravity waves

• Ripples in space and time caused by violent
  events in the universe
• E.g. a rotating black hole

Could be measured by LISA, a giant Michelson
interferometer floating in space! Gravity waves
make interferometer arms expand and contract,
then observe the changing fringes Arms 5 million
km long, since changes will be tiny
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Interferometers againATLAS experiment

• A different sort of interferometer being used to
  align the inner particle tracker
• As useful today as it was in 1887

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Your turn!

• There are several Michelson interferometers in
  the DWB
• Dont be afraid they wont break!
• There is a question sheet
• Measure wavelength of green light Michelson is
  very sensitive (Q1 3)
• Then 8 more questions on the MM experiment and
  the talk

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Some hints

• Moving the mirror by half a wavelength shifts the
  round-trip path by one wavelength one fringe
• Distance you move the mirror measured with a
  micrometer
• One rotation moves the screw 0.5 mm (check!)
• 50 tickmarks per turn 0.01 mm per tickmark
• But the mirror moves only 1/5 the distance of the
  screw!

The fixed line
The tickmarks