LIGO Vacuum Systems and Gravitational Waves

LIGO is an acronym for Laser Interferometer Gravitational-wave Observatory. The purpose of LIGO is to detect gravitational waves. Albert Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity. Einstein’s mathematics showed that massive accelerating objects (such as neutron stars or black holes orbiting each other) would disrupt space-time in such a way that ‘waves’ of distorted space would radiate from the source. In the semiconductor industry, I am concerned with a final product on a microscopic, nanometer scale on microprocessor chips. LIGO is at the other far end of the curve dealing with massive object colliding in space.

The problem is that detecting gravitational waves is an extremely difficult task. The waves will cause distortions in space on earth that are shorter than the dimension of an atom’s nucleus. In fact the distortions are on the order of 1/10,000th of the diameter of an atomic nucleus. The detector has to be super sensitive and in a very quiet location. Vacuum technology plays a key role in this experiment.

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LIGO Livingston. Courtesy Caltech/MIT/LIGO Laboratory

LIGO consists of two interferometers, each with two 4 km (2.5 mile) long arms arranged in the shape of an “L”. Each chamber encloses 10,000 cubic meters of volume. One interferometer is located in Hanford, Washington and the other in Livingston, Louisiana. The reason for two is that the earth is a very active place with lots of human hustle and bustle. There are earth quakes and storms. So if the detectors both capture the same signal, then that is strong evidence that the signal is a gravitational wave.

When gravitational waves pass through the system, the distance between the end mirrors and the beam splitter lengthen in one arm and at the same time shorten in the other arm in such a way that the light waves from the two arms go in and out of phase with each other. When the light waves are in phase with each other, they add together constructively and produce a bright beam that illuminates the detectors. When they are out of phase, they cancel each other out and there is no signal. Thus, the gravitational waves from a major cosmic event, like the merger of two black holes, will cause the signal to flicker, as seen here

Gravitational waves sent out from a pair of colliding black holes have been converted to sound waves, as heard in this animation. On September 14, 2015, LIGO observed gravitational waves from the merger of two black holes, each about 30 times the mass of our sun. The incredibly powerful event, which released 50 times more energy than all the stars in the observable universe, lasted only fractions of a second.

In the first two runs of the animation, the sound-wave frequencies exactly match the frequencies of the gravitational waves. The second two runs of the animation play the sounds again at higher frequencies that better fit the human hearing range. The animation ends by playing the original frequencies again twice.

As the black holes spiral closer and closer in together, the frequency of the gravitational waves increases. Scientists call these sounds “chirps,” because some events that generate gravitation waves would sound like a bird’s chirp.

Audio Credit: Caltech/MIT/LIGO Lab

The lasers are operated in a vacuum level on the order of 10-9 torr. This ensures that there are no air currents causing distortion of the laser beams either through transmission of sound or thermal energy. Also it lessens the chance of particle movement in the vacuum system.

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Spiral welding a section of a vacuum tube. Courtesy Caltech/MIT/LIGO Laboratory

LIGO’s vacuum tubes were constructed of spiral-welded 3 mm thick 304L stainless steel. With its relatively low carbon content, 304L steel is resistant to corrosion, especially at the critical welded seams. The 1.2 m diameter beam tubes were created in 19 to 20 m-long segments, rolled into a tube with a continuous spiral weld. To prevent collapse, LIGO’s tubes are supported with stiffener rings that provide a significant layer of resistance to buckling under the extreme pressure of the atmosphere. The tubes must withstand these stresses for at least 20 years.

Evacuating the chambers took 40 days of constant pumping to evacuate them to their optimal operating pressure. In that time, turbomolecular pumps removed the bulk of the air in the tubes while the tubes themselves were heated to 150-170 degrees C for 30 days to drive out residual gases.

The gases that remain in the system are primarily H2 and water vapor. There are liquid nitrogen cryogenic panels in place to capture the stray water molecule and ion pumps to capture H2 gas. There is so much more technology involved in the LIGO detectors. I encourage you to visit the LIGO website. Although LIGO depends on extreme vacuum engineering, the vacuum technologies involved are explained in Understanding Modern Vacuum Technology.

NOTE: On June 1st, 2017, LIGO made their third detection of a gravitational wave event from the collapse of 32 solar mass black hole and a 19 solar mass black hole forming one large black hole of 49 solar masses. The means that two solar masses of material were transformed into energy by the collision.

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