Gravity: Making Waves

First off, forget the apple.

One probably didn’t really fall on the head of Sir Isaac Newton in 1665, knocking loose enlightenment about the nature of falling bodies. And while you’re at it, forget what you learned about gravity in school. That’s not how it really works. But don’t take our word for it. Let the main contenders in the history of gravitational theory duke it out themselves.

Round 1: Newton  

“Gravity really does exist,” Newton stated in 1687. “[It] acts according to the laws which we have explained, and abundantly serves to account for all the motions of the celestial bodies.” Before Newton, no one had heard of gravity, let alone the concept of a universal law.

Cambridge University, where Newton studied, was closed due to plague in 1665. Finding respite at his childhood home, the 23-year-old plunged into months of feverish mathematical brainstorming. This, plus a dubious apple descent in the back orchard, laid the foundation for his masterwork  Philosophiae Naturalis Principia Mathematica. In Principia, Newton described gravity as an ever-present force, a tug that all objects exert on nearby objects. The more mass an object has, the stronger its tug. Increasing the distance between two objects weakens the attraction.

Principia’s mathematical explanations of these relationships were simple and extremely handy. With his equations, Newton was able to explain for the first time why the Moon stays in orbit around Earth. To this day, we use Newton’s math to predict the trajectory of a softball toss or of astronauts landing on the Moon. In fact, all everyday observations of gravity on Earth and in the heavens can be explained quite precisely with Newton’s theory.

Okay, we buy it. But how does it work?

Hello?

Silence from Newton’s corner of the ring.

The truth is, Newton could describe gravity, but he didn’t know how it worked. “Gravity must be caused by an agent acting constantly according to certain laws,” he admitted. “But whether this agent be material or immaterial, I have left to the consideration of my readers.”

For 300 years, nobody truly considered what that agent might be. Maybe any possible contenders were intimidated by Newton’s genius. The man invented calculus, for Pete’s sake.

Ding. Round 2: Einstein

Apparently Albert Einstein wasn’t intimidated. He even apologized. “Newton, forgive me,” he wrote in his memoirs. “You found the only way which, in your age, was just about possible for a man of highest thought and creative power.”

In 1915, after eight years of sorting his thoughts, Einstein had dreamed up (literally–he had no experimental precursors) an agent that caused gravity. And it wasn’t simply a force. According to his theory of General Relativity, gravity is much weirder: a natural consequence of a mass’s influence on space.

Einstein agreed with Newton that space had dimension: width, length, and height. Space might be filled with matter, or it might not. But Newton didn’t believe that space was affected by the objects in it. Einstein did. He theorized that a mass can prod space plenty. It can warp it, bend it, push it, or pull it. Gravity was just a natural outcome of a mass’s existence in space (Einstein had, with his 1905 Special Theory of Relativity, added time as a fourth dimension to space, calling the result space-time. Large masses can also warp time by speeding it up or slowing it down).

You can visualize Einstein’s gravity warp by stepping on a trampoline. Your mass causes a depression in the stretchy fabric of space. Roll a ball past the warp at your feet and it’ll curve toward your mass. The heavier you are, the more you bend space. Look at the edges of the trampoline--the warp lessens farther away from your mass. Thus, the same Newtonian relationships are explained (and predicted mathematically with better precision), yet through a different lens of warped space. Take that, Newton, says Einstein. With regrets.

Einstein’s theory also triumphantly punched a hole in Newton’s logic. If, as Newton claimed, gravity was a constant, instantaneous force, the information about a sudden change of mass would have to be somehow communicated across the entire universe at once. This made little sense to Einstein. By his reasoning, if the Sun disappeared suddenly, the signal for the planets to stop orbiting would logically have to take some travel time. And it would definitely take longer to arrive at Pluto than it would Mars. Nothing universally instant about that at all.

What did Einstein propose as the missing agent of communication? Enter, again, his very useful space warp. Much like a stone thrown into a pond, a change in mass will cause a ripple in space that travels out from its source in all directions at light speed. As it moves along, the ripple squeezes and stretches space. We call such a disturbance a gravitational wave.

With this final blow, Einstein’s General Relativity explained everything Newton’s theory did (and some things it didn’t), and better. “I am fully satisfied,” Einstein said in 1919. “I do not doubt anymore the correctness of the whole system.”

In this round, victory for Einstein.

Ding. Round 3: The Next Wave

Einstein may have predicted gravitational waves, but he had little faith scientists would ever detect them. Gravitational waves squeeze and stretch space only a small amount. In fact, it’s ridiculously, horribly, almost impossibly small: a distance hundreds of millions of times smaller than that of an atom.

So far, Einstein has been right. It’s been eight decades since he introduced General Relativity, and a gravitational wave has not yet been detected. It wasn’t until 1974 that scientists even got close. That year two radio astronomers, Joseph Taylor and Russell Hulse, were analyzing a pair of neutron stars (superdense collapsed stars) that orbit each other. Hulse and Taylor realized that the orbits were speeding up at a rate Einstein predicted would occur if gravitational waves were indeed being generated by the system. The first indirect evidence of gravitational waves was in, but the waves themselves were not directly measured.

Although any object can generate gravitational waves, only extremely massive ones produce warps of space big enough to measure. Such gargantuan changes in mass are found only in space, such as orbiting neutron stars, colliding black holes, or supernovas. Researchers are now searching for waves emanating from these sources with one of the most precise scientific instruments ever made: LIGO, the Laser Interferometer Gravitational-wave Observatory. LIGO is gigantic, clever, and odd-looking, and it took more than $365 million and 30 years to develop. Its ability to measure infinitesimal distances could help put the “discovery” of gravitational waves on the front page of every newspaper at any momentand herald the next big round in our understanding of gravity.

Gravitational waves are energetic ripples of space traveling from massive objects in the cosmos. To measure one, do these things: 

1.  Get $365 million from the National Science Foundation.
2.  Find some very flat land, like in Louisiana.
3.  Chop two 4 km long roads in an L shape through a swampy, snake-infested forest.
4.  Accounting for the Earth’s curvature, pave the roads to within a few millimeters of absolutely straight.
5.  Top each road with a 4 km long, 1.2 m wide steel tube. 
6.  Fit the tubes with precision instruments that have taken six years to design and construct.
7.  Wait for a gravitational wave to hit. Could be in moments. Could be in decades. Adjust instruments in the meantime.
8.  When the wave finally hits, the instruments will detect the space of the steel tube expanding or contracting a distance of one-hundred-millionth the diameter of a hydrogen atom.  Just ignore any pesky sources of competing vibrations, like the logging across the street.

Common sense looks at this experiment and says: weird. And really, really hard. But the team at LIGO, the Laser Interferometer Gravitational-wave Observatory in Livingston, Louisiana, has so far managed to complete steps 1 through 7. While no gravitational waves have yet been found, the LIGO researchers are optimistic they’ll eventually get one reading every few days. The team even has their own upbeat Cajun slogan: “Laissez les bonnes ondes rouler!” (“Let the good waves roll!”)

Einstein predicted gravitational waves using General Relativity in 1916. These waves are emitted by any object undergoing rapid acceleration, but only gargantuan masses, like colliding black holes or exploding stars, produce waves LIGO can detect. As they travel, these ripples literally warp space; they shrink it in one direction and stretch it in another. The farther they roam, the fainter they get. By the time they reach Earth, their already tiny warp is barely measurable.

If LIGO regularly registers gravitational waves, it will more than vindicate Einstein. Analyzing gravitational information may allow astronomers to answer pressing questions about the cosmos’s biggest mysteries, among them black holes, dark matter, and the Big Bang. It may also reveal space objects entirely unknown to science.

Light and LIGO

“The concept of what we’re looking for is so important,” insists Rainer Weiss. “The fact that the effect is tiny is just our misfortune.” Weiss is a professor emeritus of physics at the Massachusetts Institute of Technology. MIT, along with the California Institute of Technology, launched the LIGO project in 1979 with funds from the National Science Foundation. The seemingly tireless Weiss is ostensibly LIGO’s granddaddy, having been a (not the, he insists) person who first conceived the “half-baked idea” of measuring infinitesimal warps of space using light beams traveling over long distances. This is precisely what an interferometer like LIGO does.

LIGO’s main attraction is its two 4 km long arms, labeled X and Y. Like the axes of a graph, the X arm is perpendicular to the Y arm. This orientation corresponds to the two directions in which a gravitational wave affects space. As a wave travels toward Earth (perpendicular to the arms, long the third dimension of a Z axis), it will shrink space along the X axis. For LIGO this means the space that is the X arm (and all the matter in it) will shorten a fraction. The space of the Y arm will, at the same moment, stretch in response to a gravitational wave. Then vice versa, again and again, many times per second.

Imagine LIGO were Manhattan, suggests Weiss: “Squeeze Manhattan from uptown to downtown, and expand it east to west. Bang-boom!” In a flash, a gravitational wave extends the distance from the East River to the Hudson. Traveling from river to river would take more time. It’s not that the landmarks themselves are moving. It’s that the distance between the landmarks is expanding. 

Researchers set up LIGO with its own landmarks, or “test masses,” between which the distance of space can be measured with traveling light. The test masses are a set of four highly polished mirrors. One is placed at the end of each arm, and the remaining two sit closer to the vertex.

Taking a Reading

To scan for a passing gravitational wave, a laser generator near the vertex first emits light into a beam splitter. The splitter divides the beam into two identical beams that race down each arm. To maximize the travel distance, the two beams bounce 100 times between the far and near mirrors. They eventually return together to the corner station to be analyzed by a photodetector.

If no gravitational wave occurred, both arms will have remained identical in length. So the two beams reach the light sensor at the same exact moment. But if a gravitational wave hit, one arm would shrink, producing a difference in the arrival times of the two racing light signals. (The shorter arm’s light beam will have spent a mere .0000000000000000000000001 fewer seconds in transit, taking the term “photo finish” to a whole new level.) This difference means the wavelengths of the two returning light waves are now out of sync. The photodetector registers this interference (thus, interferometer) and alerts the scientists of a positive reading.

No Noise, Please!

If only it were so easy. Many, many types of competing vibrations, or noise, can jostle the test masses enough to mask the effect of a true gravitational wave.

Loggers felling trees nearby cause noise. The crash of ocean waves produce noise. “Even the motion of the atoms inside the mirrors are making the mirrors move,” says Gabriela González, a physicist at nearby Louisiana State University.

Scientists have taken painstaking precautions to reduce the impact of noise on LIGO. The mirrors are suspended on a single thin metal wire to reduce the effects of forces other than gravity. To dampen competing vibrations, investigators constantly adjust the mirrors with the ultraprecise equivalent of a car suspension system.

Still, how do you detect a gravitational wave and not a rabbit jumping nearby? “That’s the $300 million question,” laughs González. One way is by checking if a suspected wave coincided with a disturbance registered by other instruments on site, which look for changes in ground motion, magnetic field, power line voltage, and other aspects. Another way, González explains, is by double-checking results with LIGO’s twina complete duplicate facility constructed in a barren scrub desert in Hanford, Washington. At 3,030 km away, it’s distant enough that seismic and other disturbances won’t affect both observatories simultaneously.

So Have They Found Anything at All?

Nope. LIGO started making preliminary runs in 2002, but it still hasn’t noticed its first gravitational wave. Weiss says the instruments are not yet at the level of sensitivity they need to be to detect waves easily. To get to that level, the team spends their days “commissioning”: calibrating devices, finding and solving glitches, and analyzing noisy squiggles of initial data.

Expect the observatory to turn on for real in 2005. By 2010, next-generation equipment will be retrofitted into all existing LIGO facilities. This will improve the sensitivity by about 15 times to capture more, fainter, and different frequencies of gravitational signals. 

Most astrophysicists feel that LIGO’s payoffs will be worth the incredible effort it has taken to construct and operate it. “Imagine life before Galileo pointed a telescope at the sky,” suggests Janna Levin, a Columbia University cosmologist. “There’s no question. It could be that big.”

Why do all those massive, exploding outer-space events get all the fun? Why is it that only they can create a gravitational wave? The truth is, anything with an accelerating mass has a gravitational effect. Detecting it? Well, that’s another matter entirely.

Eyeball our lineup below to learn what sources produce these ripples in space, and why capturing such waves for the first time using the Laser Interferometer Gravitational-wave Observatory (LIGO) means so much to scientists.

You: “Every time you accelerate — say by jumping up and down — you’re generating gravitational waves,” says Rainer Weiss, Professor Emeritus of Physics at MIT. “There’s no doubt of it.” But just standing there won’t cut the mustard. To make a wave, your mass has to both move (have velocity) and have acceleration (change the rate of motion, direction, or both).

Still, don’t get your hopes up. No matter how fast you jump, sprint, or cartwheel, the resulting warp your waves make on space is so weak that it’s utterly unmeasurable — perhaps  100,000,000,000,000,000,000,000 times less so than the warp made by massive exploding space objects. And LIGO has a tough enough time measuring those.

Spinning Aircraft Carrier: Only enormous amounts of motion at  enormous speeds from enormous masses can produce a ripple that LIGO could detect. “To rival here on Earth the strength of gravitational waves from a supernova in the center of our galaxy,” suggests Mike Zucker, the head of LIGO’s Livingston facility, “you’d need to take an aircraft carrier and spin it, end over end, a thousand times a second.” Not very likely.

Atomic Bomb: This type of acceleration — that of billions of atomic nuclei splitting and spewing energy — might launch a space warp that LIGO could notice. Scientists actually considered testing early interferometers this way. Besides the more obvious concerns, one glitch was that an atomic detonation would have to be constrained so that the explosion wasn’t spherically symmetric. Weiss explains that equal motion in all directions does not produce gravitational waves. “The waves from all the different parts of a sphere would cancel each other out,” he says. “You need motion that’s nonspherical.” In the face of these challenges, the atomic bomb idea lost steam.

Space Sources: Now we’re talking. Mere Earth objects can’t match the gargantuan, asymmetrical motions of mass produced by space phenomena like supernovas, neutron stars, and black holes. Currently, everything we know about these objects comes from telescope observations of visible light waves, radio waves, and other electromagnetic radiation. But gravitational radiation is a completely different form of energy, emitted from the objects’ obscured cores. Such data will reveal unprecedented information about the mysterious interiors of space phenomena.

For LIGO, the most anticipated space sources of gravitational waves are the billions of binary star systems in our galaxy. These are pairs of stars, in various stages of life and death, that orbit one another. But gravitational-wave detectors will also be able to catch the emissions of evolving single stars as well as other space sources, such as supermassive black holes and the Big Bang.

• Supernovas: Stars have a finite lifetime in the millions to billions of years. Some very massive stars call it quits in a huge explosion called a supernova. If a nonspherical supernova occurs in our own galaxy, says Weiss, it will radiate gravitational waves in a colossal burst that LIGO could detect. “But this may happen once every 30 years,” he says. “Not such a nice source.”
• Neutron Stars: The collapsed remains of a supernova can develop into a neutron star. These rapidly spinning, 10 km wide stars are so dense that a teaspoon would tip the scales at a billion tons. Neutron stars often exist in binaries. As the two stars orbit one another, they shed gravitational waves. This continuous loss of energy causes them to spiral ever-closer toward one another. Eventually they coalesce violently, releasing a spurt of additional gravitational waves that LIGO should be able to catch about once yearly at its current sensitivity, says Weiss. 
• Pulsars: Pulsars are neutron stars that emit jets of electromagnetic energy that sweep past Earth during each revolution, like a lighthouse beam. Since the jet sweeps by extremely fast, we perceive it on Earth as an electromagnetic “pulse.” All pulsars have some sort of deformationan elongated overall shape or a surface bulge, for examplethat cause their spins to be asymmetric. “A pulsar thus will emit gravitational waves constantly as it rotates,” says Zucker.

• Black Holes: Black holes represent the end of the road for the most massive stars. After a star explodes as a supernova, its core can condense and become so compact that nothing, not even light, can escape its gravity. It is now a black hole. After an unlucky object disappears into its yawning cavern of gravity, the black hole’s “horizon”, or edge, wiggles as it settles down to a simple shape. This wiggle emits enormous amounts of gravitational radiation that LIGO could notice, says Zucker. Analyzing such waves could answer pressing questions about the evolution, size, and commonness of black holes. 

  Black holes can exist in binaries, too, either with another black hole or with a neutron star. LIGO could detect the gravitational waves purged by these pairs as they collide, just as in binary neutron stars. 

• Supermassive Black Holes: Behemoth black holes the mass of a million to a billion Suns may be lurking in the center of most of the Universe’s galaxies — including ours. “We’re not exactly sure how these black holes form and grow,” says astrophysical theorist Scott Hughes. “Gravitational waves offer a window into the early development of these structures.” Because gravitational radiation interferes little with matter standing in its way, waves from supermassive black holes are able to travel unblemished for billions of years from their long-gone beginning stages. (Unfortunately, LIGO itself probably won’t be able to detect waves at this frequency. To find out what instrument might, read LIGO’s Extended Family.)

• Big Bang: Scientists are hoping to use gravitational waves to peer into the earliest moments in the development of our Universe. The Big Bang is theorized to have begun around 14 billion years ago. The microscopically small Universe then expanded dramatically. In the first split seconds, the system spewed forth gravitational waves, and after about 400,000 years, electromagnetic waves.

  To this day we can still “see” the primeval Universe’s electromagnetic leftovers using radio telescopes. The prospect of LIGO seeing much, much further back into creation by detecting a constant gravitational “white noise” has scientists practically jumping out of their seats. “It would revolutionize the way we think about the Universe,” says Weiss. 

Who Knows What?: Physicists fully expect to detect gravitational waves from sources not mentioned in this Rogue’s Gallery. Some could be completely unknown to science, detectable by their gravity alone. “The rule has been,” says Weiss, “that when one opens a new channel to the Universe, there is usually a surprise in it. Why should the gravitational channel be deprived of this?” We can’t see any reason why not.

If you fly into Baton Rouge, and you have a window seat, you’ll notice a strange sight. The mosaic of pine forest near Livingston becomes oddly parted. A perfect L cuts through 8 km of foliage, with the long, skinny concrete arms of the Laser Interferometer Gravitational-wave Observatory visible in the clearing.

But if you fly over Hanford, Washington; Pisa, Italy; Hannover, Germany; or Tokyo, Japan, you’ll realize that giant L’s aren’t peculiar to Louisiana. The world actually has not one, but five large-scale gravitational-wave detectors. Each is devoted to the collective cause of detecting, for the first time, gravitational radiation streaming from neutron stars, black holes, and other giant, moving masses in space.

Gravity from the Ground

As landmark as LIGO is, its reach is limited. In fact, it can’t achieve its goals without a network of similar interferometers, instruments that use light waves to detect a gravitational wave’s minute warp on space. The network will be able to detect the same sources that LIGO does, thereby verifying its observations.

The other interferometers that share LIGO’s earthly burden — LIGO’s Hanford, Washington, facility, Italy’s VIRGO, Germany’s GEO, and TAMA in Japan — have so far undergone various “dress rehearsals” together to compare and contrast initial data. Real collective runs using all detectors will hopefully begin no later than 2006.

If three of the detectors notice the same gravitational signal at once, they will be able to pinpoint where the source lives in space, says Neil Cornish, a gravitational-wave researcher and astrophysicist at Montana State University. “Because gravitational waves travel at the speed of light, they’ll be slight delays in the arrival times at the different detectors around the world,” he explains. Researchers can easily calculate these differences to triangulate the source location in space.

Locating such sources is a key step in plotting a brand-new map of space using gravitational information. Researchers can then scan the spot thoroughly with conventional optical and radio telescopes to add even greater detail about these sources.

Gravity from Space

Plans are underway for another interferometer that will have a front-row seat to the cosmic objects that emit gravitational waves: a seat from space itself. Slated for launch in 2013, NASA’s Laser Interferometer Space Antenna (LISA) will boast the longest interferometer arms in the Universe--each 5 million km long!

As space provides a near-perfect vacuum, LISA doesn’t need steel tubes or concrete to encase its arms. Instead, a virtual L will be created using a trio of satellites that will orbit the Sun together in a fixed arrangement. Each satellite will have its own laser directed towards the other two. The travel time of the beams leaving the satellite will be compared to that of the beams arriving from the distant satellites. This will determine whether or not a gravitational wave has affected the “arm” length. 

LISA’s superior location and keen instrumentation won’t render ground-based interferometers useless, however. The detectors will complement one another by capturing waves at different points along the gravitational spectrum. Ground-based instruments are tuned to pick up the spastic, infrequent, high-frequency gravitational waves like those shed by binary objects ready to collide after billions of years of spiraling toward one another. LISA’s instrumentation, on the other hand, will capture the much slower, longer-lasting, lower-frequency waves launched during the early stages of these binaries’ spiraling-in process, thousands of years before they collide. Strong gravitational signals from these binary sources will pile up LISA’s detector with “an embarrassment of riches,” predicts Cornish.

Because LISA will have little interference from outside noise, the low-frequency waves should show up as obvious spikes in the signal output. In addition, LISA should be able to pick up gravitational information from supermassive black holes, which radiate waves at frequencies ground-based interferometers can’t register. “If we haven’t detected a gravitational wave within a day or two of LISA starting operation,” says Cornish, “we’ll know that something’s gone horribly wrong.”

Probing Creation

As ambitious as LISA is, the search for gravitational waves won’t end there. Fast-forward 30 years into the future. At that time, Cornish anticipates being heavily involved in an even-more-next-generation interferometer that he describes as “LISA on steroids”: the Big Bang Observatory. Along with LISA, this observatory is a key component of the Beyond Einstein project, a collection of missions aiming to answer fundamental questions about the structure and evolution of the Universe. The space-based Big Bang Observatory could offer scientists an unprecedented view of the very earliest moments of our Universe’s expansionone seen through gravitational waves.

Gravitational waves first started escaping from the Big Bang at about 10 “35 seconds after it began 14 billion years ago. It’s taken that long for the gravitational information to reach our cosmic doorstep, and to this day, waves from this event continue to pass by. Because gravitational waves are affected little by intervening space objects, these ancient signals are theorized to be intact, yet faint. They’ve had quite a journey, after all.

“We expect these gravitational waves from the Big Bang to be fairly weak,” explains Cornish, “so we have to up the sensitivity of the instrument.” This translates into two supersized interferometers for the Big Bang Observatory. Each will resemble LISA, but they will be equipped with lasers over a thousand times more powerful and they will have much larger mirrors, each a few meters wide.

The behemoth instrument is very much in the conceptual stages. Cornish is currently determining how the endless stream of competing signals from binary sources will be subtracted from the observatory’s anticipated readings in order to get the purest possible Big Bang signal.

With five ground-based interferometers in place and two space projects in the works, the science community is putting quite a lot of stock into a theoretical signal that hasn’t even been officially detected yet. But Cornish, like most physicists, isn’t worried. “The most spectacular discovery for gravitational-wave observatories would be an absence of waves,” he says. “That would be the most revolutionary discovery of science this century.”