How to Hunt for Gravitational Waves [Slide Show]
Various experiments seek different versions of this highly sought-after phenomenon
![How to Hunt for Gravitational Waves [Slide Show]](https://static.scientificamerican.com/sciam/cache/file/402B0572-0968-44FE-8B4EE25437D7DEEF.jpg?w=590&h=393)
How to Hunt for Gravitational Waves [Slide Show]
- WHAT ARE GRAVITATIONAL WAVES? Albert Einstein first hypothesized gravitational waves—ripples in the fabric of spacetime—in 1916. When massive objects move around or collide with one another, they disturb the surrounding spacetime like a pebble tossed into a still pond. By studying these waves, astronomers hope to learn about the cosmic phenomena that create them, which are often “invisible” objects such as black holes that cannot be detected otherwise. Discovering gravitational waves directly is one of the 21st century’s great physics challenges, and will lead into a new era of astronomy. CREDIT: Swinburne Astronomy Productions
- TYPES OF GRAVITATIONAL WAVES Researchers hunt for three main categories of gravitational waves. Massive bodies moving or merging, such as two black holes or two neutron stars orbiting each other until they collide, as well as the explosion of a supernova, cause ripples strong enough to be measurable on Earth. There are also much older waves thought to have originated in the young universe. Lower frequency, harder-to-detect signals from black holes at the centers of galaxies could be discerned in space. Another way of discovering these waves is via the incredibly precise bursts of radio emission from spinning stars called pulsars. Finally, some researchers have their sights set on the original burst of primordial gravitational waves. These waves would create a unique pattern in the cosmic microwave background of ancient light that pervades the universe. CREDIT: NASA
- LIGO The Laser Interferometer Gravitational-Wave Observatory, shortened to LIGO, was built to find waves that propagate all the way to the ground on Earth. These waves come from the most powerful events in the universe--collisions of black holes or extremely dense neutron stars. Researchers think these events occur frequently throughout the universe and could provide a recurring signal of gravitational waves to detect. Twin facilities, one near Richland, Wash., and the other in Livingston, La., wait for gravitational waves caused by binary systems or supernova explosions. The two sites are located more than 3,000 kilometers apart to ensure Earth-based events that might cause a false gravitational wave–like signal, such as that caused by a microearthquake, are obvious; although such an event might occur at one site, it is highly unlikely another would happen simultaneously at the other. LIGO's initial run was from 2005 to 2007 with additional runs after enhancement until 2010. After improvements to increase sensitivity, Advanced LIGO began observations in September 2015. CREDIT: Caltech/MIT/LIGO Lab
- HOW LIGO WORKS Gravitational waves are thought to cause incredibly small but detectable distortions in the positions of objects in the spacetime through which they pass. LIGO's design takes advantage of this fact: Its two L-shaped detectors have three mirrors, one at the end of each leg and one at the point where the legs meet. The two legs are the same length. Lasers bounce off the mirrors back to a photodetector, and because the lasers travel the same distance their light waves should be identical--or in phase--when they meet at the center. If a gravitational wave passes through the area, however, one leg will become slightly longer whereas the other leg will get slightly shorter, causing the light traveling through each leg to be slightly--but measurably--out of phase. Such a mismatch creates an interference pattern when the two lasers meet that reveals the gravitational wave's presence. CREDIT: California Institute of Technology
- LISA PATHFINDER AND eLISA Land-based LIGO is powerful, but being grounded has its limitations. LIGO can only detect high-frequency waves. A space-based interferometer detector, similar to LIGO, could have test masses millions of kilometers apart, creating an experiment targeted to lower frequency, longer wavelength gravitational waves than LIGO's four-kilometer legs could find. The LISA (Laser Interferometer Space Antenna) Pathfinder spacecraft, which launched in 2015 and entered its optimal orbit in early 2016, is a proof-of-concept for such a space-based detector. It will track its free-flying orbiting test weights and demonstrate that the extremely minute measurements needed to detect gravitational waves will be possible. Whereas LISA Pathfinder itself cannot detect gravitational waves, it will pave the way for eLISA, a triangular arrangement of three satellites that would scan the sky for gravitational waves and could launch as soon as 2028. CREDIT: ESA-C.Carreau
- PULSAR TIMING ARRAYS Ground-based telescopes search for signs from pulsars that could provide evidence of gravitational waves passing through the universe. Similar to how a gravitational wave–crossing LIGO would alter the length of its legs, gravitational waves rippling along through space can cause the light we see from pulsars to fluctuate. Pulsars give off incredibly regular “pulses” of light but a gravitational wave that stretched the space the light traveled through could cause these pulses to take slightly more time to arrive. Researchers hypothesize that gravitational waves should be common throughout the universe, which would mean pulsars in certain locations should exhibit the same wave-induced fluctuations. Pulsar Timing Arrays, such as NANOGrav and the European Pulsar Timing Array, compile pulsar measurements to hone in on the effect of these background gravitational waves. CREDIT: ESO/L. Calcada
- BICEP2 Unlike LIGO and eLISA, the BICEP2 experiment at the South Pole seeks out a different kind of gravitational wave. The waves LIGO looks for are downright recent compared with the ones BICEP2 is designed to detect. Its focus is the primordial gravitational waves that were emitted during the birth of the universe, which impacted the leftover thermal radiation that permeates the universe called the cosmic microwave background (CMB). According to theory, tiny fluctuations in spacetime that arose from quantum mechanical effects should have stretched when the very young universe expanded rapidly. This stretching would have caused a special signature in the CMB that BICEP2 could detect. The team made “waves” in April 2015 by announcing it had discovered this signal, but follow-up studies showed that the scientists most likely saw contamination due to dust in the galaxy, not primordial waves. CREDIT: Steffen Richter, Harvard University
