How Do Gravity Wave Detectors Work?
The first mention of gravity waves as we know then was by Einstein in a 1916 follow-up to his work on Relativity. He predicted that minute changes in mass in space-time would cause a wave of gravity to emanate from the object and travel somewhat like a ripple on a pond (but in three dimensions), not unlike how the movement of electrical charges causes photons to be released. However, Einstein felt that the waves would be too small to detect, according to his original draft to the 1936 Physical Review entitled "Do Gravitational Waves Exist?" Indeed, the only objects currently in existence strong enough to expel lots of energy as well as dense enough to make gravity waves we can detect are black holes, neutron stars, and white dwarfs. Einstein felt his equations generalized too many first order approximations, which made the non-linear equations he worked with easier to handle. But because of a mistake in his work, he withdrew the paper and later revised it when he noticed that a cylindrical coordinate system resolved many of his qualms with the mathematics, but his viewpoint on the waves being too small remained (Andersen 43, Francis, Krauss 52-3).
But in 1974 Russel Hulse and Joseph Taylor looked at a binary pair of stars with one being a neutron star and the other a pulsar. They knew based on calculations that if gravity waves existed then energy would be robbed from both objects as they orbited each other. This loss of energy would cause them to fall closer to their barycenter, or the apparent center of their orbits, and increase their speed (for closer objects orbit faster). The scientists came up with the orbital path with gravity waves and without them and observed the binary for years. Sure enough the path predicted by the gravity waves was right. Hulse and Taylor’s work was so profound it won them the 1993 Nobel Prize in Physics (Andersen 43).
So why should we care for such an elusive but known target? Gravity waves may be the key to seeing beyond the cosmic microwave background, the moment light was able to pass through matter (about 380,000 years after the Big Bang). This was because of the high energies that the Universe was in which prevented light from being transmitted freely. But the movement of matter through space-time should have created gravity waves, which can indeed be detected regardless of the conditions prior to the cosmic microwave background. They could be the key to proving that inflation happened and thus the possibility of a multiverse (Andersen 42)! However, the smallest waves would be kilometers in length so we would need such a sized detector if we hope to see some waves (Francis).
That being said, one team has hunted for primordial gravity waves in a different way. Known as the Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP 2), the project worked for over 3 years in the cold of Amundsen-Scott South Pole Station in the Antarctica, which lies over 2,800 feet above sea level. This makes for optimal viewing conditions as air is thinner and less light is absorbed. The team was led by John Kovac as the Harvard Center for Astrophysics, the University of Minnesota, Stanford University, the California Institute of Technology, and JPL combined their resources as BICEP 2 scanned about 2% of the sky looking for B-modes. This is a phase of light which has had one of its two main components (the magnetic or B and the electric or E) stretched while the other is compressed, which leaves a pattern in the CMB by curling that light. If directly measured then such a curl would be indicative of the elusive gravity waves (Ritter, Castelvecchi, Moskowitz).
In March of 2014, the BICEP 2 team announced that they had found the signal they had been hunting for. But the science community was skeptical, especially when it was found that the dust from our galaxy was not taken into account. It too could have made those B-mode patterns and contaminated the data. After reviewing data of the CMB from the Planck spacecraft it was found that the BICEP 2 results had indeed been contaminated by the dust. Fortunately, the Planck data does give the team new and better places to look where the dust is not as prevalent (Cowen, Timmer 22 Sept. 2014).
The Laser Interferometry Gravitational Wave Observatory (LIGO) started its hunt in 2002, but only after years of development and design. It started as the brain child of Rainer Weiss, who read about a (later busted) confirmation of gravity waves using aluminum bars that resonated at far distances from each other. This got Weiss thinking about a sound method for measuring gravity waves, and what he came up with was laser interferometry in 1968 (more on that below). Weiss gained allies in the form of Kip Thorne and Ronald Drever and together they founded LIGO in 1984. Years went by as they built prototypes and gained over $100 million from several non-profit groups (Wolchover).
Costing $570 million to build, the detector utilizes a ground-based method for finding gravity waves that relies on laser interferometry. What exactly does that entail? Two beams are sent parallel to each other, one going away and the other reflected back towards the detector. The return beam will be purposefully out of phase based on how it is reflected back so that the detector will have two waves of opposite amplitude meet and cancel each other out for a net signal of zero. But if a gravity wave passes by then the return beam will not be perfectly out of phase and so the detector should then record a signal (Andersen 43, 45).
The two pieces that make up LIGO are separate interferometers, one in Washington State while the other resides in Louisiana. Each arm of a detector has a laser that is send down the entire length (4 kilometers). The detector’s job is to see if changes in distance less than one part in 1020 between the laser beams occur by a gravity wave warping space-time. Unfortunately, the only events that generates waves strong enough for LIGO are when binary pulsars fall close to each other and neutron star collisions. Amazingly, these events would only produce a maximum signal height “less than the diameter of a proton.” In short, miniscule (Andersen 43, 45; Faesi, Francis).
LIGO ended its initial run in 2010 with no measurements of gravity waves, but a $200 million upgrade began shortly thereafter and ended in 2015. Now rebranded Advanced LIGO, it should be able to measure gravity waves from as far away as 500 million light-years away, increasing its possible detections by a factor of 100. And the upgrade definitely paid off, for on February 11, 2016, a hundred years after Einstein predicted their existence, LIGO captured evidence of gravity waves. According to Dr. David Reitze, the executive director at LIGO, a black hole merger from Sept. 14, 2015 produced enough of a disturbance for the detector to register but that the delay in announcing the result came from verification. They wanted to make sure it was not a glitch, a hack, or something else. But the 5-sigma result held through, meaning that scientists are 99.9999% confident in the result. A new age of astronomy has begun (Andersen 43, Francis, Freeman, Wolchover).
And it continues on. On June 15, 2016, LIGO scientists released the findings on the second observed measurement of gravity waves. These came from GW151226 (that is, Gravity Waves from Dec. 26, 2015), another black hole merger, and observations of it were made just 70 seconds after the gravity wave detection. The reason for the delay in the finding was so that scientists could achieve the 5 sigma confidence in their results, knowing that the observation is incredibly unlikely to be a fluke. The masses of the black holes (one about 8 solar masses and the other about 14) were much smaller than those in the first merger and thus made accurate measurements more challenging. The new black hole is likely 1.4 billion light-years away, is about 21 solar masses and radiated about a sun's worth of mass in gravitational energy (Timmer 15 Jun. 2016, Betz).
While not yet in existence, the ongoing drama of the ESA’s Laser Interferometer Space Antenna (LISA) will surely be worth it once realized. It has been in various stages of development for the past 30 years and likely will not be built and launched for 20 more years. The basic approach is simple enough: three spacecraft located at the L1 Lagrange point make up the points of an equilateral triangle with sides of 5 million kilometers. A laser is sent back and forth to each probe so that the constant distance is known. If a gravity wave passes by, the distance will be altered and detection possible (Andersen 43-4, Haynes).
2011 was a critical year for the development of LISA as a big test towards the full-scale version was successfully accomplished. The Optical Meteorology Subsystem was put under space-like conditions by being in a vacuum and under cold conditions yet managed to beat the required precision by 3 times the expected value. The test was conducted at Ottobrunn by the Astrium Ltd. (Plotner).
The team at Advanced Concepts over at NASA is led by Babak Saif (the interferometer engineer for the James Webb Space Telescope) and Mark Kasevich (professor of applied physics at Stanford University) is working on an alternate method. While nothing is set in stone with it yet, an atom interferometer could be a potential detector. Unlike the traditional interferometer, which uses mirrors and lenses, an atom interferometer makes use of quantum mechanics. Clouds of atoms are placed in superposition by lasers, allowing them to occupy the same places in space-time. In this case the lasers cause the atoms to achieve a state of nearly perfect rest, aka absolute zero, then another set of lasers causes the superposition to occur. 10 seconds after being put in this state the clouds are released from the lasers, supposedly back in their original state minus the changes in momentum they may have gone under. But if a gravity wave passed by while the clouds were in superposition then the atoms will have their state changed. Unlike a laser interferometer, this version will have a high level of accuracy down to the nanoscale and it does not need large distances (5000 times less, actually) for it to work unlike its counterparts (Andersen 44, Keesey).
Pulsar Timing Array
Gravity waves should be a result of a natural process, so why not use another one as a detection tool? Millisecond pulsars fit the bill. This subclass of the spinning neutron stars pulses thousands of times a second courtesy of an angular momentum transfer from a binary companion. They pulsate at such a regular interval that any deviation would potentially be caused by depressions in the fabric of space-time caused by gravity. But one would not be enough to show this. We would need a network of them to show specifically what was causing it and to see if others are impacted. NANOGrav and The European Pulsar Timing Array has found some possible upper limits as to what the amplitude of a gravitational wave could be but nothing concrete has been found yet. Even over a decade of observations by the Parkes Pulsar Timing Array didn't find anything. This was despite finding 24 pulsars with low activity being observed, which would allow for any significant changes like gravitational waves to be easily seen. Such seemingly null findings are in fact able to eliminate some models for gravitational wave production, so not all is lost (Keesey, Timmer 27 Sept. 2015).
Surprisingly, ordinary stars may be all we need to find evidence of gravity waves. Barry McKeman from the American Museum of Natural History in New York and his team have shown that it is possible for a star to absorb some of the gravitational energy the wave would supply so long as it was in tune with the right frequency of the wave, aka resonance. This would cause an output increase in terms of brightness that we can measure. Of course many things can cause a star to brighten up, so the study authors suggest that if a group of stars could be found then it would be possible for them all to be hit by the resonant wave and thus increase as a group. The best-case scenario for this would be around a black hole, for the stars would be close together and near a potentially large gravity wave maker (Choi, American Museum of Natural History).
American Museum of Natural History. “Finding Hints of Gravitational Waves in the Stars.” Astronomy.com. Kalmbach Publishing Co., 23 Sept. 2014. Web. 25 Sept. 2015.
Andersen, Ross D. “An Ear to the Big Bang.” Scientific American Oct. 2013: 42-6. Print.
Betz, Eric. "LIGO Detects a Second Set of Gravitational Waves." astronomy.com. Kalmbach Publishing Co., 15 Jun. 2016. Web. 20 Jul. 2016.
Castelvecchi, Davide. “Gravitation Waves: Here’s Everything You Need to Know.” HuffingtonPost.com. Huffington Post, 18 Mar. 2014. Web. 13 Oct. 2014.
Choi, Charles Q. “Einstein’s Gravity Waves Could Be Found With New Method.” Space.com Space.com, 24 Oct. 2014. Web. 25 Sept. 2015.
Cowen, Rob. “Gravitational Wave Discovery Called Into Question.” HuffingtonPost.com. Huffington Post, 19 Mar. 2014. Web. 16 Oct. 2014.
Faesi, Chris. “The Race to Detect Gravitational Waves: Pulsar Timing Arrays.” Astrobites.org. Astrobites, 30 Nov. 2012. Web. 23 Sept. 2015.
Freeman, David and Jacqueline Howard. "Scientists Prove Einstein Was Right After Detecting Gravitational Waves." HuffingtonPost.com. Huffington Post, 11 Feb. 2016. Web. 25 Mar. 2016.
Haynes, Korey. "LISA Tests Gravitational Wave Tech." Astronomy Jan 2016: 19. Print.
Keesey, Lori. “NASA Pursues Atom Optics to Detect the Imperceptible.” NASA.gov. NASA, 18 Oct. 2012. Web. 23 Sept. 2015.
Krauss, Lawrence M. "What Einstein Got Wrong." Scientific American Sept. 2015: 52-3. Print.
Moskowitz, Clara. “Multiverse Debate Heats Up In Wake of Gravitational Waves Findings.” HuffingtonPost.com. Huffington Post, 31 Mar. 2014. Web. 13 Oct. 2014
Plotner, Tammy. “LISA Pathfinder – Surfing Gravity Waves.” UniverseToday.com. Universe Today, 14 Nov. 2011. Web. 22 Sept. 2015.
Ritter, Malcom. “ ‘Cosmic Inflation’ Discovery Lends Key Support of Expanding Early Universe.” HuffingtonPost.com. Huffington Post, 17 Mar. 2014. Web. 11 Oct. 2014.
Timmer, John. “Gravitational Wave Evidence Disappears Into Dust.” arstechnica.com. Conde Nast, 22 Sept. 2014. Web. 17 Oct. 2014.
---. "Gravitational Waves Missing in Action in Latest Test." arstechnica.com. Conde Nast, 27 Sept. 2015. Web. 20 Jul. 2016.
---. "LIGO Data Includes At Least One More Black Hole Merger." arstechnica.com. Conde Nast, 15 Jun. 2015. Web. 20 Jul. 2016.
Wolchover, Natalie. "Gravitational Waves Discovered at Long Last." Quantamagazine.org. Simons Foundation, 11 Feb. 2016. Web. 25 Mar. 2016.
© 2016 Leonard Kelley
More by this Author
Figuring out how the moon was created is one of the deepest mysteries of archeological astronomy. We do have ideas though...
This space observatory got its roots in a hidden frontier of light, and now continues to make advances into the x-ray world.
An awfully subjective topic, but take a look and see if you find something you never knew existed. Our solar system has plenty to look at.