Detecting gravitational waves: Why did it take so long?

-February 26, 2016

Recently, the scientific community was rocked by the announcement of the detection of gravitational waves as predicted by Albert Einstein. In November 1915, Einstein presented a series of lectures before the Prussian Academy of Sciences on his General Theory of Relativity, in which he contended that space and time form a continuum that is influenced (distorted/warped) by anything possessing mass. The effect of that influence he declared is gravity.

Einstein’s final lecture ended with his introduction of a series of equation that replaced Newton's law of gravity known as Einstein's Field Equations. These ten equations describe the fundamental interaction of gravitation as a result of space time being influenced by mass. Gravitational waves, which Einstein proposed in 1916, are something of a corollary to his General Theory of Relativity.

The detection of gravitational waves was made by the LIGO (Laser Interferometer Gravitational Wave Observatory) facilities in Livingston, Louisiana and in Richland, Washington on 14 September 2015, at 9:50:45 universal time (4:50 a.m. in Louisiana and 2:50 a.m. in Washington).

Gravitational waves may be understood as ripples in the curvature of space time that propagate as waves, travelling outward from a source of gravitational influence. This curvature is caused by the presence of a large mass within the confines of a relatively small volume ratio such as a star or black hole. Generally speaking, the bigger the mass contained within a given volume of space, the greater will be the curvature of space time at the boundaries of its volume.

In layman's terms, when a gravitational wave passes an observer, that observer will find their space time immediacy distorted to some degree. LIGO researchers detected a gravitational wave that stretched space by 1 X 10-21, making the entire Earth expand and contract by 1/100,000 of a nanometer (one nanometer is one billionth of a meter equivalent to 3.937 X 10-8 inch). In the video below, Prof. Brian Greene from Columbia University explains the phenomena.

Why did it take 100 years from the time Einstein predicted the existence of gravitational waves as a consequence of his theory of general relativity to the recent announcement of their detection? Read on to appreciate what it takes to measure this elusive phenomena.

When I heard about the LIGO announcement, I was immediately curious as to what technologies were used in detecting gravitational waves. From my previous readings on space-time continuum, I knew gravitational waves are very difficult to detect and measure, owning to their extremely small amplitudes requiring very sensitive, state of the art detectors. Not only must these detectors be very sensitive, but they also must be able to cancel out other sources of noise that can easily overwhelm them.

The two LIGO facilities are located 3000 km (1864 mi.) apart and operated in unison as a single observatory. Having two separate facilities spaced far apart allows detected gravitational waves to be triangulated in order to determine their origin. The time interval between detected gravitational waves between the two LIGO facilities is on the order of 10 ms.

Each LIGO facility has two tunnels known as light storage arms that are 4 km (2.485 mi.) long. The arms are so long that the curvature of the Earth must be taken into account in designing the arms (there is a 1 m vertical drop over the 4 km length of each arm). The light storage arms are positioned at 90° angles to each other in an "L" type configuration. These arms are maintained within an ultra-high vacuum equivalent to one-trillionth of an "atmosphere" (10-9 torr) encapsulating a volume of 10,000 m3 (350,000 ft3) equivalent to the volume within 11 Boeing 747-400 commercial airliners. To put this into perspective, the amount of air that must be pumped out of these arms could inflate two-and-a-half million footballs, or 1.8 million soccer balls!

At the heart of LIGO’s gravitational wave detection system are state-of-the-art Michelson interferometers. A Michelson interferometer produces interference fringes by splitting a beam of light into two paths; one path has a fixed length of travel, the other path susceptible to length variations. When the reflected beams are brought back together, an interference pattern is created symptomatic of the length of travel difference between the two beams. The LICO detection system uses a pre-stabilized laser emitting a beam with a power of up to 200 W that passes through a beam splitter, which splits the beams into two paths, one for each of the light storage arms. The interferometers are used to determine the length distance between the two paths (gravitational waves will distort one of the laser beam paths). These interferometers can discern length changes on the order of 1/10,000th the width of a proton (a proton width is 2 × 10-14 m, the average diameter of a strand of human hair is 1 × 10-4 m).

This incredible measurement capability is the equivalent of measuring the distance to the nearest star to within the width of a human hair (Proxima Centauri, the closest star to us is 39.9 × 1015 m away). Given the cutting-edge technologies required to detect a passing gravitational wave as it distorts space, you can see why it took 100 years since Einstein predicted gravitational waves for us, mere mortals, to be able to detect them!

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