ENGN1211 Lecture Notes - Lecture 3: Beam Splitter, Proof Mass, Gravitational Wave
Detecting gravitational waves from the ground and space
• Scientists observed two black holes circling each other at the speed of light until they
merge
• Gravitational waves- Einstein predicted- when huge masses move away very rapidly
waves will spread out across space- like listening to the universe
• New way of observing the universe
• Big eyes are needed to see in the dark- really big telescopes have been used
• Keck, Spritzer, JWST, GMT (20 billion dollar investment, at ANU, going to be put on
a mountain in Chile)
• Building giant ears for earth
• Waves that move as the speed of light at furniture space level
• Oscillate quartol motion- if going through particles would oscillate as an ellipse
• Taken 60 years detect it because they are so small 1 part in 10^23
• 10^-18m change in mirror
• Very large laser beam reflecting off mirror at right angles
• 4 stage pendulum system- suppression of noise and active isolation
• Thermodynamic mirror surface fluctuation
◦ Highly polished mirror- fused silica
◦ All vibrations of mirror are pushed into very narrow frequency range
• Short noise-
◦ When a laser beam goes into a beam splitter it doesn’t split perfectly- arms
aren’t identical
• The final check-
◦ Very high signal to noise ratio
◦ Instruments were 1000km apart so that notice interferences would not be the
same
• Mass converted to energy- 3x mass of sun- 1.3 billion light years away
• Change in density- change in gravitation pull e.g. cloud, truck
• Ligo- on earth (cant be measures lower than 10-15hz)
• LISA path finder- 5000000km arms,
◦ Don’t need measurement sensitivity as it has much longer arms
◦ Gold cubes are floating space
▪ Space craft floats around proof mass shielding it from sun radiation,
winds
◦ Space graft and proof mass need to stay exactly the same distance apart as
otherwise gravitation will change, laser will be pulled
◦ Thruster releases oil
• GRACE- hyperology mission
◦ Exquisite measurement of gravitation
◦ Shows movement of water, where ice is growing, shrinking, ground water, tide
◦ Two spacecraft, around 200km apart
◦ Very low orbit- much more sensitive to gravitational pull
◦ Speeds up when there is gravitational pull
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Document Summary
All vibrations of mirror are pushed into very narrow frequency range: short noise- When a laser beam goes into a beam splitter it doesn"t split perfectly- arms aren"t identical: the final check- same. Don"t need measurement sensitivity as it has much longer arms. Space craft floats around proof mass shielding it from sun radiation, winds. Space graft and proof mass need to stay exactly the same distance apart as otherwise gravitation will change, laser will be pulled. Shows movement of water, where ice is growing, shrinking, ground water, tide. Very low orbit- much more sensitive to gravitational pull. Change in operation of two space crafts- very accurately. 2nd one used laser - 50nm over 200km. Outgoing beam is antiparallel to the incoming beam, but translated by 60cm. The virtual intersection of these three orthogonal planes will be placed. They can measure 1000th of a proton > thought it must be useful somewhere else.
