How to catch a gravity wave | Science News for Students

How to catch a gravity wave

The hunt to find and record this mystery radiation has taken more than a half-century
Feb 11, 2016 — 10:33 am EST
gravity waves

An artist’s idea of the gravity waves (golden reflections) emerging from the merging of two black holes (dark orbs). Scientists have just reported for the first time the detection of gravity waves from such a collision.

Henze, NASA

A January 2016 email caught me off guard. Peter Saulson, a physicist at Syracuse University in New York, was sharing a secret. In short order, he said, data would be formally published showing that gravity waves had finally been detected.

Such rumbles emanate from the most violent events in the universe. And in September 2015, two gravity-wave observatories — or detectors — had picked up the tremors. The waves had been unleashed around 1.3 billion years ago by the ferocious encounter of two massive black holes as they merged in a fateful embrace. I had been hoping for such news ever since I published Einstein’s Unfinished Symphony, 16 years ago. My book described astronomy’s most cutting-edge startup: gravity-wave astronomy.

In November 1915, Einstein had unveiled a new way of looking at the workings of gravity throughout the universe. The mathematics underlying that new view of gravity is known as general relativity. The next year, Einstein realized that this new theory also predicted the existence of gravitational waves — now commonly called gravity waves.

But getting a working detector to find them would take decades of blood, sweat and frustration. Now, almost exactly 100 years after Einstein’s first paper on gravity waves, success has emerged. It was as if, Saulson mused, “those black holes were waiting for that moment.”

What Einstein had predicted

The movement of electric charges up and down an antenna create radio waves, a type of electromagnetic radiation.  In much the same way, Einstein reasoned, waves of gravitational radiation must emerge when masses move about. But these waves would not travel through space the way light does, he noted in papers published in 1916 and 1918. These waves instead would ripple as quakes in the very framework of space and time — or spacetime.

gravity waveAlternately stretching and squeezing space, the wave at the site of the clashing black holes would have stretched a 1.8-meter (6-foot) man to 3.7 meters. Within a millisecond, it would have squeezed him down to 0.9 meter (3 feet), before stretching him out once again.

Einstein never imagined such outrageous sources for his waves. Back in the 1910s, the universe seemed relatively quiet. So Einstein pictured these waves rippling out from two stars, say, as they simply orbited one another. He and others knew the spacetime ripples from such events would be feeble. Certainly, they would be too weak to bother looking for.

Many scientists even wondered if such waves existed.

The two sides argued back and forth about this for decades. Until the late 1950s, anyway. Then, a young Joseph Weber at the University of Maryland in College Park decided it was time to settle the question. He’d build a gravity-wave detector.

How to catch a wave

For his device, Weber started with a large cylinder of aluminum. The solid bar was roughly the size of a home’s 50-gallon hot-water tank. He surrounded this with sensors, figuring that a passing wave would cause the bar to resonate like a bell. The sensors would convert that shaking into electrical signals that would be recorded on a paper chart. By using two detectors separated by hundreds of kilometers, scientists could identify local noises (because they would show up on only one detector). Any such local effects on the aluminum bar would be ignored.

Joseph WeberBy 1969, Weber grandly proclaimed at a science meeting that he had simultaneously recorded a signal on two bars. One was on his Maryland campus. The other detector was some 1,125 kilometers (700 miles) away at Argonne National Laboratory, west of Chicago.

Scientists at the meeting greeted his announcement with applause. Newspapers heralded it as perhaps the most important physics achievement in a half-century. The next year, Weber declared that his signals came from the center of the Milky Way galaxy. Perhaps they came from some supernova going off, he said, or from neutron stars (which had just recently been discovered).  Very quickly, other physics groups rushed to build their own detectors.

But they detected no waves. None at all.

Still, the various teams didn’t give up. By the 1980s, groups in various countries created even bigger bar detectors. Scientists also tweaked their designs. Their hope was that these would be more sensitive.

Still, no waves showed up.

Until his death in 2000, Weber insisted his detectors had recorded gravity waves. Today, physicists suspect Weber’s system merely picked up some natural noises coming from within his bars.

But even as Weber began work on his bars, a totally new technique for detecting gravity waves was being imagined. This method is known as laser interferometry (IN-tur-feh-ROM-eh-tree). In 1962, two Soviet scientists, Mikhail Gertsenshtein and V. I. Pustovoit, first described a way to use it to detect gravity waves. But no one outside Russia was aware of it. Weber, too, briefly thought of the technique. Then, in 1966, Rainer Weiss at the Massachusetts Institute of Technology, in Cambridge, also independently came up with the scheme.

The laser bounce

Weiss, who conducted lab experiments on gravity, was at one point asked to teach a course on general relativity. “I couldn’t admit that I didn’t know it,” he recalled. “I was just one exercise ahead of my students.” As part of the course he created, Weiss asked students to imagine how the distance between three massive objects, arranged to form the letter L (one at the corner, the others at each end), would change as a gravity wave passed by. Weiss knew that a gravity wave compresses space in one direction (say, north/south), while it expands it in the other (east/west). A millisecond later, as the wave passed by, the effect would reverse.

interferometerWhile thinking about the problem, Weiss realized it would be a good experiment. He could continually bounce laser beams between the masses. Then he’d make the beams eventually recombine (or optically “interfere” with one another). This should measure any jiggles in the masses if a gravity wave passed by. In other words, it would serve as a gravity-wave detector!

And it had one great advantage over the bars. The bars could respond to only one frequency of spacetime quaking. This laser interferometer, by contrast, could register a range of frequencies. And that would increase the chance that it would notice a passing wave.

By 1972, Weiss had written a landmark report identifying all the fundamental sources of noise that could mask a signal in such a setup. His paper is still consulted today by gravity-wave researchers. From then on, Weiss spent much of his career building a laser-based gravity-wave detector — and figuring out how to keep noise from getting in the way of its recognizing a true signal.

By the 1980s, Weiss joined forces with one of the world’s top experts on gravity waves: Kip Thorne at the California Institute of Technology in Pasadena. Scottish physicist Ronald Drever, then at Caltech, also was part of the team.  Together they began to plan for the construction of two, giant detectors.

In order to detect a wave, they knew the laser’s light path in each “arm” of the L had to be kilometers long. The longer the arm, the more sensitive the detector. (By the time that newly discovered gravity wave hit Earth, it jiggled the masses by less than the width of an atomic particle!)

LIGOThe system came to be named LIGO, for Laser Interferometer Gravitational-wave Observatory. The National Science Foundation proposed funding it. But when many scientists heard about its staggering cost (it finally rose to nearly $300 million), they complained it was too much to risk on a gamble — an as-yet unproven technology.

Eventually, LIGO’s supporters convinced the U.S. Congress to fund the project.

Work to build the first LIGO detectors began in 1994. One is in Livingston, La., the other 3,060 kilometers (1,900 miles) northwest of there in Hanford, Wash. Both were up and running within seven years. Each detector sends its laser beams down a path 4 kilometers long.

Over the next few years, scientists tested and refined the system. What they learned let them install new and improved hardware between 2010 and 2015. This “advanced” LIGO started up last fall. And almost as soon as it began operating, bingo! It found a gravity wave.

Elsewhere around the globe, other detectors are joining LIGO’s quest. For instance, a European team has been operating a LIGO-like detector, known as VIRGO, outside Pisa, Italy. A smaller interferometer, GEO600, operates in Germany.

What’s the big deal?

LIGO’s finding of ripples from the merging of those two distant black holes is like Galileo’s first peek at the heavens through his telescope in 1609. Simply put, each allowed the world to witness the birth of a new astronomy.

supernovaGalileo’s spyglass uncovered everything from moons orbiting Jupiter to jagged mountains and craters on the moon. These simply amazed 17th century eyes. Now, gravity-wave astronomy is poised to expand the vision of 21st century explorers. Electromagnetic waves — be they visible light, radio, infrared or X-rays — are released by individual atoms and electrons. Such radiation reveals information about a distant celestial object — such as how hot or old it is, what it looks like and what it is made of.

Gravity waves share similarly useful — but much different — information. They will tell astronomers how massive objects move, twirl and collide throughout the universe. That’s especially helpful for objects too small to be seen directly, such as neutron stars and stellar black holes.

“We’ve now embarked on an era of exploring phenomena in the universe that are made from warped spacetime,” says Caltech’s Thorne. “I like to call it the warped side of the universe.”

After more than four long and turbulent decades, Weiss, now 83, has at last seen his experimental dream come true. Did he ever despair? “No,” he says. Whatever the outcome had turned out to be, “The problems were interesting, you enjoyed the people you were working with, and it was fun to do!”

GRAVITY WAVES Scientists with LIGO detail how they found the "smoking gun" that proved the existence of gravitational waves. CalTech

The first signal picked up by LIGO was a very fast crescendo. It lasted less than a second. When converted to audio, the wave started as a deep bass and headed toward the piano’s middle C. With that signal, LIGO scientists are beginning a new journey, now able to listen for the myriad events that await detection.

One day, this new seismograph for space tremors may be able to pick up the remnant rumble of the first nanosecond of creation. Those gravity waves would trace back to the awesome spacetime jolt of the Big Bang itself. Indeed, that now suggests, Einstein’s symphony may never be finished.

LIGO infographic

Marcia Bartusiak is a professor of science writing at MIT and the author of six books on astrophysics and the history of astronomy. They include Einstein’s Unfinished Symphony: Listening to the Sounds of Space-Time. (Joseph Henry Press, December 15, 2000, 266 pp.)

Power Words

Word Find (click here to enlarge for printing)

Power Words

(for more about Power Words, click here)

Argonne National Laboratory   A federal laboratory owned by the U.S. Department of Energy, outside of Chicago, Ill. It was formally created on July 1, 1946. Today, its roughly 1,400 scientists and engineers (and 1,000 students) conduct research across a broad range of fields, from biology and physics to materials science, energy development and climate studies.

astronomy    The area of science that deals with celestial objects, space and the physical universe. People who work in this field are calledastronomers.

astrophysics   An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.

atom   The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

audio   Having to do with sound.

black hole  A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.

celestial   (in astronomy) Of or relating to the sky, or outer space.

electromagnetic radiation  Energy that travels as a wave, including forms of light. Electromagnetic radiation is typically classified by its wavelength. The spectrum of electromagnetic radiation ranges from radio waves to gamma rays. It also includes microwaves and visible light.

electron  A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

frequency  The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.

galaxy  A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.

gravity waves (also known as gravitational waves)    Ripples in the fabric of space that are produced when masses undergo sudden acceleration. Some are believed to have been unleashed during the Big Bang, when the universe got its explosive start.

gravity  Schools tend to teach that gravity is the force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity. But Einstein’s general theory of relativity redefined it, showing that gravity is not an ordinary force, but instead a property of space-time geometry. Gravity essentially can be viewed as a curve in spacetime, because as a body moves through space, it follows a curved path owing to the far greater mass of one or more objects in its vicinity.

interferometry   (in optics) A technique used to measure such things as the wavelength of electromagnetic radiation and distances. It works by dividing a beam of light into two. Then, at some distance, it recombines the two rays of light back together. This creates what’s known as interference. As the two waves overlap, they create a new wave, whose size and shape reflects the patterns of the initial waves.

Jupiter  (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).

laser   A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.

LIGO (short for Laser Interferometer Gravitational wave Observatory)   A system of two detectors, separated at a great geographical distance, that are used to register the presence of passing gravitational waves.

mass    A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

Milky Way   The galaxy in which Earth’s solar system resides.

milli      A prefix for fractional units of measurement, here referring to thousandths in the international metric system. For instance, a millisecond equals one-thousandth of a second.

moon  The natural satellite of any planet.

nano   A prefix indicating a billionth. In the metric system of measurements, it’s often used as an abbreviation to refer to objects that are a billionth of a meter long or in diameter.

National Science Foundation   The U.S. Congress created this independent federal agency in 1950 to promote the advancement of science; national health, prosperity and welfare; and the nation’s defense. This agency funds nearly one-fourth of all federally supported basic research in U.S. colleges and universities. In many fields such as mathematics, computer science and the social sciences, NSF is the major source of federal funding.

neutron star  The very dense corpse of what had once been a star having a mass four to eight times that of our sun. As the star died in a supernova explosion, its outer layers shot out into space. Its core then collapsed under its intense gravity, causing protons and electrons in its atoms to fuse into neutrons (hence the star’s name). Astronomers believe neutron stars form when large stars undergo a supernova but aren’t big massive enough to form a black hole. A single teaspoonful of a neutron star, on Earth, would weigh a billion tons.

optical    Having to do with vision or sight, with the fields of optics, or with visible light.

physics     The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study which emerged later, is a more accurate way of  explaining the motions and behavior of matter. A scientist who works in that field is known as a physicist.

radiation   (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

relativity    A theory developed by physicist Albert Einstein showing that neither space nor time are constant, but instead affected by one’s velocity and and the mass of things in your vicinity.

resonate   To reverberate, like a ringing bell, producing a clear tone or frequency of radiating energy.

seismometer (also known as a seismograph)   An instrument that detects and measures tremors (known as seismic waves) as they pass through Earth.

sensor   A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly.

Soviet Union, also known as the Union of Soviet Socialist Republics (or USSR)    A federation of republics that grew over time. It existed in various forms from 1922 to 1991, and at times stretched from the Baltic and Black Seas to the Pacific Ocean. Its area once was two-and-a-half times that of the United States. At its height, The USSR had 15 member states across 11 time zones: Armenia, Azerbaijan, Belorussia (now Belarus), Estonia, Georgia, Kazakhstan, Kirgiziya (now Kyrgyzstan), Latvia, Lithuania, Moldavia (now Moldova), Russia, Tajikistan, Turkmenistan, Ukraine and Uzbekistan. Its capital, Moscow, is still the capital of Russia. The term Soviet comes from a Russian word for council, assembly or harmony.

spacetime   A term made essential by Einstein’s theory of relativity, it describes a designation for some spot given in terms of its three-dimensional coordinates in space, along with a fourth coordinate corresponding to time.

star  The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

stellar   An adjective that means of or relating to stars.

supernova  (plural: supernovae or supernovas) A massive star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass.

theory  (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.

universe   The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).

wave  A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

wavelength  The distance between one peak and the next in a series of waves, or the distance between one trough and the next. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.


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B. P. Abbott et al. Observation of gravitational waves from a binary black hole mergerPhysical Review Letters. Published online February 11, 2016. doi: 10.1103/PhysRevLett.116.061102. 

Further Reading

S. Ornes. “‘Ruler’ to measure space.” Science News for Students. April 16, 2012.

S. Ornes. “Mapping the invisible.” Science News for Students. February 1, 2012.

S. Ornes. “Black hole mysteries.” Science News for Students. May 29, 2013.

Kids: Learn more about gravity waves here from NASA’s Space Place.

LIGO: Frequently asked questions about gravity waves here.

Chris Woodford. “Interferometers.” ExplainThatStuff. Updated as of November 22, 2015.

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