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2017 Physics Nobel Prize - Capturing Gravitational Waves

 2017 Physics Nobel Prize - Capturing Gravitational Waves

Gravitational Waves Captured by LIGO

Who? Rainer Weiss, Barry C. Barish, and Kip S. Thorne - LIGO/VIRGO Collarboration

What? Observation of gravitational waves for the first time using LIGO (Laser Interferometer Gravitational-Wave Observatory.

Where? Two locations in the US - Hanford and Livingston. (See figure 1)

Figure 1: LIGO in the US


When? 14 September 2015

HOW? 
  • The scientists captured gravitational waves by using an interferometer. The LIGO interferometer is a more glamorous interferometer than the original Michelson interferometer. It works through using light waves to measure gravitational interference (i.e. waves).
  • First, one needs to understand the parts of an interferometer. The LIGO interferometer (and most) is shaped as an L. It has two 4 km vacuum tunnel arms with a mirror at each end. At the center of the arms, there is a beam splitter. There is also a laser parallel to the beam splitter and a detector. To increase sensibility, there are also two more mirrors, each one in the arms of the interferometer and in between the end mirror and the beam splitter. These spaces are called Fabry Perot Cavities. Lastly, the LIGO interferometer has a power recycling mirror in between the laser and beam splitter. (See figure 2)
Figure 2: LIGO
  • The LIGO interferometer captured gravitational waves from colliding black holes. (See figure 3 for further explanation)
Figure 3

  • Before discussing how the waves are measured, one needs to understand the difference between constructive and destructive wave interference. Constructive interference is when the peaks of two waves intersect and create a larger wave. Destructive interference, what is put in place to occur before gravitational interference on the light waves (from laser beam) in using LIGO, is when the the peak of one wave meats the valley of an identical wave and they cancel each other out. (See figure 4)
Figure 4
  • Thus, how does LIGO work: A laser shoots light into the beam splitter and the light splits, shooting off in opposite directions toward both mirrors. The two light waves are then reflected by the mirror at the end of each tunnel and travel back toward each other in the middle. Scientists have manipulated these waves (without gravitational interference) to have a destructive interference when they meet back up. Thus, when a new wave is created, scientists know it is due to gravitational interference. 
  • Gravitational interference will alter light waves: It will cause light waves to change size simultaneously. (One wave will contract the other will expand). Thus, when they meet again, they will not cancel each other. 
  • This interference is captured as gravitational waves. 
  • Additional information: The Fabry Perot Cavities and the Power Recycling mirror are necessary for the interferometer to have enough power to capture the gravitational waves. They are both essentially the adding of more mirrors = more reflecting = greater distance light is traveling and thus results are (more) accurate and conclusive. 
  • What's Next? Figuring out the message of these gravitational waves. 

Sources:

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