Gravitational Waves

black holes colliding to create gravitational waves
When two supermassive black holes collide and merge, some of the excess energy from the event is broadcast as gravitational waves. These can be detected on Earth using very delicate instruments at the LIGO observatory. The SXS (Simulating eXtreme Spacetimes) Project

Gravitational waves are created as ripples in the fabric of space-time by energetic processes such as black hole collisions out in space. They were long thought to occur, but physicists didn't have sensitive-enough equipment to detect them. That all changed in 2016 when gravitational waves from the collision of two supermassive black holes were measured. It was a major discovery predicted by research done early in the 20th century by physicist Albert Einstein

Origin of Gravitational Waves

In 1916, Einstein was working on his theory of general relativity. One outgrowth of his work was a set of solutions to his formulas for general relativity (called his field equations) that allowed for gravitational waves. The problem was, nobody had ever detected any such thing. If they existed, they would be so incredibly weak that they would be virtually impossible to find, yet alone measure. Physicists spent much of the 20th Century devising ideas about detecting gravitational waves and looking for mechanisms in the universe that would create them. 

Figuring Out How to Find Gravitational Waves

One possible idea for the creation of gravitational waves was probed by the scientists Russel Hulse and Joseph H. Taylor. In 1974, they discovered a new type of pulsar, the dead, but rapidly spinning hulk of mass left over after the death of a massive star. The pulsar is actually a neutron star, a ball of neutrons crushed to the size of a small world, spinning rapidly and sending out pulses of radiation. Neutron stars are incredibly massive and presented the type of object with strong gravitational fields that might also be implicated in the creation of gravitational waves. The two men won the 1993 Nobel Prize in physics for their work, which drew largely upon Einstein's predictions using gravitational waves.

The idea behind searching for such waves is fairly simple: if they DO exist, then objects emitting them would lose gravitational energy. That loss of energy is indirectly detectable. By studying the orbits of binary neutron stars, the gradual decay within these orbits would require the existence of gravitational waves that would carry the energy away.

The Discovery of Gravitational Waves

To find such waves, physicists needed to build very sensitive detectors. In the U.S., they constructed the Laser Interferometry Gravitational Wave Observatory (LIGO). It unites data from two facilities, one in Hanford, Washington and the other in Livingston, Louisiana. Each one uses a laser beam attached to precision instruments to measure the "wiggle" of a gravitational wave as it passes by Earth. The lasers in each facility move along different arms of a four-kilometer-long vacuum chamber. If there are no gravitational waves affecting the laser light, the beams of light will be in complete phase with each other upon arriving at the detectors. If gravitational waves are present and have an effect on the laser beams, making them waver even 1/10,000th of a proton's width, then a phenomenon called " interference patterns" will result. They indicate the strength and timing of the waves. 

After years of testing, on February 11, 2016, physicists working at with the LIGO program announced that they had detected gravitational waves from a binary system of black holes colliding with each other several months earlier. The amazing thing is that LIGO was able to detect with microscopic precision behavior that happened light-years away. The level of precision was equivalent to measuring the distance to the nearest star with a margin of error less than the width of a human hair!  Since that time, more gravitational waves have been detected, also from the site of a black hole collision. 

What's Next for Gravitational Wave Science

The major reason for excitement over the detection of gravitational waves, other than yet another confirmation that Einstein's theory of relativity is correct, is that it provides an additional way of exploring the universe. Astronomers know as much as they do about the history of the universe today because they study objects in space with every tool available.Until the LIGO discoveries, their work has been confined to cosmic rays and light from objects in optical, ultraviolet, visible, radio, microwave, x-ray, and gamma-ray light. Just as the development of radio and other advanced telescopes allowed astronomers to look at the universe outside of the visual range of the electromagnetic spectrum, this advance potentially allows for whole new types of telescopes that will explore the history of the universe at an entirely new scale.

The Advanced LIGO observatory is a ground-based laser interferometer, so the next move in gravitational wave studies is to create a space-based gravitational wave observatory. The European Space Agency (ESA) launched and operated the LISA Pathfinder mission to test possibilities for future space-based gravitational wave detection.

Primordial Gravitational Waves

Though gravitational waves are allowed in theory by general relativity itself, one major reason physicists are interested in them is because of inflation theory, which didn't even exist back when Hulse and Taylor were doing their Nobel-winning neutron star research.

In the 1980s, the evidence for the Big Bang theory was quite extensive, but there were still questions that it could not sufficiently explain. In response, a group of particle physicists and cosmologists worked together to develop inflation theory. They suggested that the early, highly-compact universe would have contained many quantum fluctuations (that is, fluctuations or "quivers" on extremely small scales). A rapid expansion in the very early universe, which could be explained due to the outward pressure of spacetime itself, would have expanded those quantum fluctuations significantly.

One of the key predictions from inflation theory and the quantum fluctuations was that actions in the early universe would have produced gravitational waves. If this happened, then the study of those early disturbances would reveal more information about the early history of the cosmos. Future research and observations will probe that possibility.

Edited and updated by Carolyn Collins Petersen.