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Oct152010

Physicists Gone Wild

General Relativity teaches us that the fabric of space and time is malleable; like a sheet of flexible rubber. When objects with mass and energy - like stars and planets - are around, the fabric stretches in such a way that the measure of distance around the objects gets distorted. General Relativity tells us that the gravitational force is simply a result of an illusion arising from such distortions of the fabric of spacetime. Depending on what astrophysical process is at work, the distortion realizes various beautiful patterns, dynamical and complex. One of the most interesting phenomena results from violent events, such as the collision of two black holes… The event creates a ripple in the curvature of spacetime, a gravitational wave that propagates out at the speed of light nudging everything in its way…

Gravitational waves are waves that stretch and squeeze distances across space in an oscillatory pattern - as they propagate through the universe. They go through most everything and travel immense distances - carrying within them information about violent events far far away. Unlike light, they can puncture through the primordial plasma of the universe; hence, detecting them would allow us to probe the first 100,000 years since the beginning of time, past the opaque plasma that obstructs our view (see my previous post on the cosmic microwave radiation)… detection of gravitational waves would allow us to verify the theory of General Relativity beyond anything achieved so far, directly confirming the existence of exotic objects such as black hole and cosmic strings. In short, these ripples of spacetime pack in them physical information about the universe and cosmology, information that is simply unavailable from any other source we have. We desperately need to "see" them!

In the 1970s, a first indirect evidence of gravitational waves was identified. Later the results were refined and confirmed. Two massive objects known as neutron stars can form a binary system - basically orbit around each other along a circle in an exceedingly dangerous dance. As they go around, they loose energy by emitting gravitational waves and spiral inward faster and faster for an eventual collision: imagine water flushing down the toilet… Astrophysicists were able to identify such binary systems and measure the period of the spiraling motion. Even though the gravitational waves could not be detected, the spin rate of the neutron stars was increasing precisely as predicted by General Relavity...

So, now that you're energized and romanticized about detecting gravitational waves, let's bring things down to reality… Think of a gravitational wave as a ripple on the surface of the ocean. As a water wave gets past you, you rise and fall with the passing disturbance. To detect gravitational waves, you need to measure the distance between two fixed points in space. Say you hold two fingers a few centimeters  apart and let a wave from some black  hole collision light-years away pass through them; you measure the distance between your fingers and you would see this distance stretch and squeeze as the wave passes through. Here's the catch: the effect from a typical gravitational wave is expected to change the distance between two fixed points by about the radius of a proton over a distance between the Sun and the Earth… How the heck you measure that??!! Well, unachievable goals have never stopped physicists from trying; and hence the title of this post. 

There are currently two huge gravitational wave detectors in operation. They are looking for ripples in the fabric of space as they pass by our neighborhood… they're called the LIGO detectors - Laser Interferometry Gravitational wave Observatory (I guess LIGWO does not sound right...). They are capable of measuring a change in length of about 1/1000 the radius of a proton over a distance 4 kilometers… the instrument is a marvel of technological precision and physics ingenuity… check out the accompanying really good video to learn more about LIGO.  

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Reader Comments (2)

how can a variable path length be detected if the distance traveled is changing within the gravitational field?, Doesn't light, also, "lens" around gravitational disturbances?; there by incorporating a similar change in displacement within the reference distance? and assuming G waves travel at c, also, how would this work out?

October 7, 2011 | Unregistered Commenterjack7watch

Think of measuring time difference instead: you bounce a light across two mirrors and you measure the time it takes to deduce the distance between the mirrors. In reality, experiments try to measure the DIFFERENCE in two distances, since that is easier.

October 7, 2011 | Registered CommenterVatche Sahakian

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