In 1915, Albert Einstein published what many consider to be his magnum opus: The General Theory of Relativity (GTR). In this work, Einstein forever changed the way we think about gravity and it's relation to space and time. But GTR makes predictions that go far beyond our familiar conception of gravity. Proven phenomena such as time dilation, gravitational lensing, frame dragging, and even black holes all have their theoretical origins in GTR. Indeed, despite being a century old Einstein's general theory of relativity continues to bare fruit. But there is a final implication of GTR that has yet to be directly detected; An implication that physicists may finally be on the verge of measuring. This final prediction is gravitational waves.Gravitational waves are essentially ripples in the geometry of spacetime. To create such a wave a mass must be accelerated through space in a particular way such that the quadrupole moment of the mass distribution varies. That is, in order to create ripples of expanding and contracting spacetime a mass must accelerate through space in a non-spherically symmetric and non-cylindrically symmetric fashion.
Unlike waves of the longitudinal form, gravitational waves are what's known as quadrupole waves. Their mode of propagation is the deformation of space itself. This causes objects in their path to expand and contract as the wave passes through them. And it is precisely this property that physicists are trying to detect. In fact, this is the sole objective of the Laser Interferometer Gravitational-Wave Observatory (LIGO).
Founded in 1992 and at a cost of $620 million, LIGO is the largest scientific experiment ever devised with the intent of directly detecting gravitational waves. LIGO is attempting to detect these waves by essentially passing two lasers of the same wavelength through each other but 90 degrees out of phase. Consequently, this creates destructive interference and nothing is observed. If, however, a gravitational wave were to pass through this destructive interference pattern the beams' wavelengths would be altered opposite to each other. This means that the two overlapping lasers would no longer exhibit perfect destructive interference and a signal should be detected.
This set up must be incredibly precise to detect the minuscule effects of gravitational waves. It is estimated that even the strongest sources of gravitational waves would only contract earth's space by a millionth of the width of a proton. Additionally, other disturbances that could affect the system must be carefully weeded out. It may be no surprise then that LIGO's initial run, from 2002-2010, produced no detections.Rather than being an indictment of GTR, LIGO's initial detection failure was likely due to insufficient sensitivity. As a result, LIGO shutdown operation for multiple years in order to expand it's capabilities. With improved detectors, the Advanced LIGO has increased it's sensitivity by a factor of 10. Operations began again on September 18th 2015 and it is likely just a matter of time before a direct detection will be made by LIGO.
If successfully detected, gravitational waves could open up an entirely new way of studying astronomical objects. Up until now, astrophysicists' only information regarding the universe has come in the form of light and matter. It is exciting to imagine gravity being added to that list.
Thus, physicists may be on the verge of directly verifying gravitational waves. This will, thereby, further validate General Relativity and may, in time, expand the tools available to astrophysicists.
Sources:
https://ligo.caltech.edu/
https://en.wikipedia.org/wiki/Gravitational_wave
https://en.wikipedia.org/wiki/General_relativity
https://en.wikipedia.org/wiki/LIGO#Advanced_LIGO
http://www.skyandtelescope.com/astronomy-news/advanced-ligo-switches-on-10142015/
4 points. very nice.
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