For the first time, scientists were able to detect the existence of gravitational waves. These waves, or “ripples in space-time,” were first predicted in Einstein’s theory of general relativity. Over 100 years later, with the help of laser technology, the prediction has proven to be true.
The waves were detected by LIGO (Laser Interferometer Gravitational-Wave Observatory) — a massive experiment developed by Kip Thorne and Ronald Drever. LIGO consists of two Observatories; one in Livingston, LA and another in Richland, WA. The distance between the two sites help to gauge the source of the wave, by helping to measure discrepancies in arrival time. Both observatories consist of an L-shaped, high-powered vacuum system, measuring approximately 2.5 miles on each side. Each vacuum system can hold five interferometers. The interferometers have mirrors suspended on each corner of the “L” shape. A laser then emits a beam up to 200 W, which passes through an optical mode cleaner, before the beam is split at the L’s vertex. The beam then travels down the length of the leg. Each leg of the “L” contains a Fabry-Pérot Interferometer cavity, which consist of a transparent plate with two reflective surfaces, which store the beams and help to increase the path length.
In the event that a gravitational wave passes through the interferometer, the area’s space-time is altered. Depending on the length of the wave, and its source, this will cause a change in the length of the cavities. This length change in the beams will cause the existing light inside the system to become slightly out of phase with the light entering. This will then cause the cavity to become occasionally out of coherence, and the beams will vary in detuning becoming a measuring signal.
After numerous trips down the lengths to the mirrors and back, the beams leave the legs and meet at the split. The beams are kept out of phase so that when the legs are operating as normal, no light should be able to reach the photodiode. When a gravitational wave passes through, the legs of the interferometer shorten and lengthen, allowing light to reach the photodiode. This indicates the signal of a gravitational wave. Signals are then compared between the two observation sites to reduce the chance of unrelated noise creating a potential signal.
The initial detection of gravitational waves took place months before the rest of world received the news. On September 14, 2015, Marco Drago, a LIGO team member, was sitting in his Hanover, Germany office at the Max Planck Institute for Gravitational Physics. Here, Drago monitors one of four computer systems that displays data for any significant variations in signals detected by LIGO. While on a phone call, Drago received his daily notification on the status of LIGO, only to find that both structures detected “an event” or irregular reading.
Drago initially took the discovery with a grain of salt, and assumed the reading was artificial. In order to test the LIGO facilities, the team had developed a way to create a “false” signal, mostly to keep researchers alert for possible developments. To most LIGO team members, this was just another “blind signal injection.” Normally, the reading would have been simply noted, then verified later. What made this reading so different from others in the past, was the fact that due to some necessary system tune ups, the machines needed to conduct the injection were not currently operational. After checking in with other team members, and seeking possible false readings, such as an earthquake or other natural event, there was nothing to say that the signal detected was not the result of a gravitational wave. The next few months were spent running numerous analyses to verify that it was no fluke. By February, the verdict was out: LIGO discovered a true gravitational wave.
The wave detected by LIGO was the result of two massive black holes colliding, over one billion years ago. The impact of the two gravitational fields sent gravitational waves through the universe, eventually reaching Earth, in September. The wave is said to have stretched space by one part in 1021, causing Earth to grow and shrink by 1/100,000 of a millimeter. The detection of the waves simultaneously tests Einstein’s theories of both general relativity and gravity, as well as provides proof for the existence of black holes.
LIGO’s detection of a gravitational wave is an immensely notable discovery for the study of physics; one that would not be possible, were it not for the application of laser technology. The constantly growing application of lasers opens the door for further discoveries and innovations, in many fields. Laser Institute of America promotes this continuous growth and the safe use of laser applications. Visit www.lia.org to find out how to enhance your own laser safety knowledge.