LIGO

Written by Alan Lightman

In 1915, Albert Einstein turned the crank of his highly mathematical theory of gravity called General Relativity to predict the existence of gravitational waves, which are oscillations of gravitational energy that move through space like waves on the ocean. Gravitational waves can be produced by asymmetrically exploding stars, colliding black holes, and any mass that quivers (including the shaking of your fist). A passing gravitational wave would cause space and time to oscillate, like wind chimes responding to a passing breeze. However, gravity is an extremely weak force compared to other forces, and Einstein and other physicists at the time doubted that human beings would ever be able to build a detector sensitive enough to measure such things, even from huge cataclysmic events in outer space. To be more precise, the gravitational force between two elemental units of mass, like two protons, is smaller than the electrical force between them by a factor of about:

1,000,000,000,000,000,000,000,000,000,000,000,000.

In the 1960s, physicist Joseph Weber decided that he could build a detector sensitive enough to register a passing gravitational wave. While he claimed to have succeeded, there was a consensus in the rest of the scientific community that Weber’s detector was not nearly sensitive enough to do what he claimed. However, Weber is regarded as a pioneer in the field, and his work stimulated other physicists to begin thinking about detecting gravitational waves. If such things could be detected, they would open a new window on the universe. Theoretically, many phenomena, like colliding black holes, do not emit any electromagnetic waves (like visible light, radio waves, X-rays etc.) but do emit gravitational waves and could be observed in that way. Such a sensitivity would be like a new sensory organ by which to observe our surroundings.

In the 1970s, experimental physicist Rai Weiss conceived the idea that a laser beam bouncing back and forth between two widely separated mirrors might be a feasible way of detecting gravitational waves. As the space between the mirrors is alternately stretched and compressed while the wave passes by, the time for the laser beam to bounce back and forth would be altered. If a good clock could measure that alteration, it would be a sign of a gravitational wave.

Weiss discussed his idea with theoretical physicist Kip Thorne, and the two became the leaders of a team of physicists dedicated to building a Laser Interferometer Gravitational-Wave Observatory, or LIGO for short.

Since it is not easy to measure the absolute time for a beam of laser light to travel back and forth between two mirrors, the design of LIGO involves a pair of laser beams, traveling at right angles to each other, in an L-shaped configuration, with mirrors at both ends of the L and in the middle. As a gravitational wave passes by, it compresses space in one direction and stretches it in the perpendicular direction, like the action of a tide. Thus the round trip travel time of one laser beam going in the first direction would be decreased, while that of the companion beam traveling in the second, perpendicular direction would be increased. A decrease or increase in travel time corresponds to a shift in the position of the crests and troughs of the wave. By carefully measuring the way the two companion beams overlap with each other, this shifting of crests and troughs becomes measurable. Still, extraordinarily sensitive equipment is needed, partly to eliminate all external disturbances. To succeed, LIGO had to be able to measure the movement of its mirrors, separated by several miles, to an accuracy of one billionth the width of an atom! That’s equivalent to measuring the distance to a nearby star to within the width of a human hair. From the time that Weiss and Thorne had their first discussions, it took 40 years before LIGO was sensitive enough to detect a gravitational wave. There are actually two LIGO detectors, one in Louisiana and one in Washington state. If both detectors begin shaking at the same time, in a coordinated way, that is evidence of a gravitational wave, not a nearby truck disturbing the apparatus.

At 09:51, Universal Coordinated Time, on September 14, 2015, LIGO made its first detection. From analyzing the intensity of the detected gravitational wave and its detailed oscillations with time, scientists eventually concluded that the event that created the wave was the collision of two black holes, each about 30 times the mass of the Sun, at a distance of 1.3 billion light years from Earth. In other words, the astronomical event that produced those waves occurred about 1.3 billion years ago. All of that time, the waves had been traveling through space, until they finally reached Earth and LIGO on September 14, 2015. Since then, LIGO has made over a hundred detections of separate gravitational wave event.

Physicists are now working on a space-based version of LIGO called LISA (Laser Interferometer Space Antenna), which would involve three spacecraft arranged in an equilateral triangle, with sides about 1.7 million miles long. LISA, currently scheduled to launch in 2037, should be considerably more sensitive than LIGO and should be able to detect not only gravitational waves from colliding black holes and other such cataclysmic events, but also gravitational waves produced by energy and matter fluctuations in the early universe, shedding light on how our cosmos began.