An unforgiveable tension between stability and sensitivity
“[It is] an unforgivable tension between stability and sensitivity”, says Janna Levin of the 30-odd year scientific endeavour to build the instrument that would detect gravitational waves produced a billion light years away from us.
In her fascinating and lyrical book, Black Hole Blues and Other Songs from Outer Space, Jana Levin writes about the astronomical human endeavour to detect gravitational waves, as evidence to support Einstein’s model of how the universe works. Lighter stars like our sun will die as white dwarfs (spheres of matter as large as the earth, comprised of densely packed electrons that resist a total collapse). Heavier stars die out and completely collapse into black holes. When two black holes collide, a large amount of energy is released. This energy peaks in the final seconds of collapse. It is not released as light or heat as we would expect it to, but as a gravitational wave. “While astronomers voraciously collected the sky’s light into telescopes”, Levin says, the LIGO team set out to record the sounds of the universe, opening up a whole new set of data for us to understand the universe by.
When two black holes collide, a large amount of energy is released. This energy peaks in the final seconds of collapse. It is not released as light or heat as we would expect it to, but as a gravitational wave.
Gravitational waves were detected by the Laser Interferometer Gravitational Wave Observatory (LIGO) exactly 100 years after Einstein first presented his paper, ‘On the General Theory of Relativity’. The first gravitational waves we ever heard, exactly confirming Einstein’s hypothesis, were produced by ”two black holes, orbiting each other at close to the speed of light, and finally crashing together to merge into a single black hole that shudders a bit before settling down.” (Source)
The idea that drives LIGO (the gravitational waves detector), is that light takes a specific amount of time to travel a tube that is 4 kms long. If that tube is stretched, the light will take longer. If the tube is shortened, light will reach sooner.
Gravitational waves cause tubes and space (and yes, the cells in our body) to stretch and shrink. Gravitational force is one of the weakest known forces. “The gravitational pull of the entire earth is easily resisted by mere human muscle – we can jump.” LIGO is sensitive enough to detect and measure a change in tube length that is 1,000 times smaller than a proton.
Equally, LIGO has to be made insensitive to numerous sources of noise, like: Traffic, farming activities, ocean waves, winds, effects of the sun & the moon, seismic noise, thermal noise from the vibration of atoms in the mirrors, beam jitter and much more. This is all so complex that gravitational science could well be the new rocket science, especially given how mainstream rocket science has become (thanks, Elon). Hours before the first ever detection in 2015, scientists at LIGO were driving alongside the tubes in a noisy jeep, banging on the walls to see how the instrument held up to that commotion.
LIGO has two 4 kilometres long tubes, called arms, that meet at right angles. A laser source sends light from the intersection point, down each of the arms. At the end of each arm is a mirror. The mirror reflects the light back to the intersection of the arms, where there is an interferometer. If the length of the arms are unaltered, light will cancel out on one side, and add up perfectly on the other side. A passing gravitational wave contorts the length of the arms, and this change is captured and recorded as a wave form. An identical detector is placed in two cities hundreds of kilometres apart, to validate the results.
It has also been compared to a glissando, that thing when a rock star drags fingers from left to right across the piano keys with great flourish – rather appropriate for such a cosmic event.
A gravitational wave sounds joyful, like the chirp of a bird or like Mari Kondo’s ‘ting’, her vocal reaction to a spark of joy. It has also been compared to a glissando, that thing when a rock star drags fingers from left to right across the piano keys with great flourish – rather appropriate for such a cosmic event.
A large wave from a pair of colliding black holes, one billion light years away, glides across the skies. It washes over neighbouring galaxies, before entering the Milky Way, and then our own solar system, crossing Pluto, Neptune, Saturn before reaching us and ringing the LIGO detectors.
The aim of the entire 30+ years project, with thousands of scientists from across the world and hundreds of millions in funding, is to keep the machine stable when that big wave sweeps by, sensitive to the tiny perturbance caused by the wave, and yet be insensitive to all the noise around. The perpetual struggle to balance stability & sensitivity beautifully sums up human existence – isn’t this what all our lives are made up of?
The perpetual struggle to balance stability & sensitivity beautifully sums up human existence – isn’t this what all our lives are made up of?
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