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The Michelson interferometer

The Michelson interferometer. Alex Pinder. Intro. A very simple experiment Gave a very unexpected result: light moves at 3×10 8 m/s always Result: Einstein’s special relativity

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The Michelson interferometer

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  1. The Michelson interferometer Alex Pinder Physics Masterclass

  2. Intro • A very simple experiment • Gave a very unexpected result: light moves at 3×108 m/s always • Result: Einstein’s special relativity • Consequences: time moves differently for different people, things look shorterwhen they move past you at high speed Physics Masterclass

  3. First look Physics Masterclass

  4. The one you’ll use! Physics Masterclass

  5. What you’ll see… Fringes! Physics Masterclass

  6. Why fringes? Path 1 Path 2 Light from two paths interferes destructively at your eye  dark fringe Light from two paths interferes constructively at your eye  bright fringe Physics Masterclass

  7. Michelson-Morley experiment (1887) • Used Michelson interferometer: showed light doesn’t 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 Physics Masterclass

  8. 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 3×108 m/s LIGHT Sees same light catching up at 3×108 m/s Physics Masterclass

  9. 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 Physics Masterclass

  10. Interferometers again:ATLAS experiment • A different sort of interferometer being used to align the inner particle tracker • As useful today as it was in 1887… Physics Masterclass

  11. Your turn! • There are several Michelson interferometers in the DWB • Don’t be afraid: they won’t 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 Physics Masterclass

  12. Part you rotate 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 Physics Masterclass

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