Great Moments in Science History

There are many great moments in science history that a good science teacher would know about, like Galileo’s discovery of his Law of Falling Bodies, or Faraday’s discovery of Electromagnetic Induction.

But it’s another thing to be alive when something truly phenomenal is discovered.

I can think of three great moments that have galvanized the physics community in the last quarter century, and they are a) the discovery that the expansion of space is accelerating, b) the detection of the Higg’s Boson, and c) the detection of a gravity wave.

This last moment, the direct detection of a gravity wave, was just announced, and has the whole world in a proverbial tizzy of excitement.

Two days ago, a news conference was held at the Louisiana site of the Laser Interferometer Gravitational-Wave Observatory (LIGO) to announce that a single gravity wave event was detected in September 2015, thereby verifying beyond doubt that Einstein was right (again) in predicting – nearly a century ago – the existence of gravity waves.

LIGO, a joint venture of Caltech and MIT, is actually two gravitational wave observatories –  one in Livingston, LA, and the other in Hanford, WA – separated by almost 1900 miles.

Two observatories are needed, because gravity waves are exceedingly weak, and so the detection mechanism must be ultrasensitive.  As a result, local seismic disturbances, even including a truck driving near one of the facilities, can cause a false trigger.  The only way to know if a signal detected at one LIGO location is a gravity wave coming from a distance part of space is if the other LIGO location detects it simultaneously.  Since gravity waves travel at the speed of light, the 1900 mile separation between the two LIGO observatories is trivial, and is easily corrected for.

Because gravity waves are quadrupole in nature, each observatory consists of two laser beams that perfectly destructively interfere with each other at right angles.  If one beam is stretched in the horizontal direction by a gravity wave, its right angle counterpart beam will be compressed by the gravity wave in the horizontal direction, thereby leading to an extremely short blip of partially constructive interference.

LIGO scientists then take this blip and transduce it into an stretched out audio signal that we humans can here.

The variation in two laser beams’ distances leading to the blip is extremely small, on the order of one millionth of a femtometer.  Because of the separation of the two observatories on Earth, the LIGO scientists can use geometry to approximately determine the angular location and distance of the gravitation wave source.

The signal detected was estimated to be about 1.3 billion light years away, and is thought to have been caused by the spiraling merger of two relatively small black holes.

To listen to the audio signal of the gravity wave detected by LIGO, click here.  To learn about gravity waves, their signal strengths and their quadrupole nature, click on the image above.


Author: Bob Mahoney

Physics teacher

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