March 2009Physics Masterclass1 The Michelson interferometer Alex Pinder

March 2009Physics Masterclass2 Intro A very simple experiment Gave a very unexpected result: light moves at 3×10 8 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

March 2009Physics Masterclass3 First look

March 2009Physics Masterclass4 The one you’ll use!

March 2009Physics Masterclass5 What you’ll see… Fringes!

March 2009Physics Masterclass6 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

March 2009Physics Masterclass7 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 c c + v c - v Actual size of interferometer = 11 m

March 2009Physics Masterclass8 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 3×10 8 m/s Sees same light catching up at 3×10 8 m/s LIGHT

March 2009Physics Masterclass9 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

March 2009Physics Masterclass10 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…

March 2009Physics Masterclass11 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

March 2009Physics Masterclass12 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