March 2009 Physics Masterclass 1

The Michelson interferometer

Alex Pinder

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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 shorter when they move past you at high speed

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First look

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The one you’ll use!

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What you’ll see…

Fringes!

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Why fringes?

Light from two paths interferes constructively at your eye bright fringe

Light from two paths interferes destructively at your eye dark fringe

Path 1

Path 2

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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

“Aether” flow

Speed v

c cc + v

c - v

Actual size of interferometer = 11 m

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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 3×108 m/s

Sees same light catching up at 3×108 m/s

LIGHT

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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 again:ATLAS 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

• 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

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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!

Part you rotate

The fixed line

The tickmarks