When Stars Collide

Enter Einstein's grand construct of gravitational wonders, and do not attempt to adjust your television set.

Astronomers observed the ripples in space-time caused by gravitational waves from two black holes colliding. (Artist's impression: LIGO)
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“WE'RE NOT ASTRONOMERS, YET. We’re still instrument builders. We’ve built a telescope, but we haven’t yet seen a star.”

So Warren W. Johnson, Louisiana State University physics professor, summed up the current state of gravitational wave astronomy—a science with maybe a billion dollars invested in equipment worldwide and more than three decades of effort, but no direct detection of gravitational waves…yet. This fall could see the first, and scores of astronomers, physicists, and mathematicians around the world are watching detectors—indeed, have staked their careers on the belief that gravitational waves will be detected very soon.

What are gravitational waves? What would they tell us about the universe? And who cares whether they’re detected?

Gravitational waves are ripples in space-time. That’s the usual explanation offered to a lay audience, likening gravitational waves to the expanding circular waves seen when you throw a pebble into a pond. Not a bad analogy for invoking an image of crests and troughs radiating out from a disturbance. But that vivid comparison, of course, begs the question: What is space-time?

Space-time is what we live in—the three dimensions of space in which we all go to school or work, mow the lawn, and watch TV, plus the fourth dimension of time, in which we measure how long we take to do these things or note when we start and stop. Every one of us is constantly traveling through space-time, as Earth carries us around the sun and as the arrow of time inexorably carries us away from our births and toward our deaths.

Perhaps you have to be a physicist to truly visualize space-time in full four-dimensional splendor being warped by a passing gravitational wave, but even children can grasp the concept surprisingly accurately by imagining a red-and-white-checked tablecloth spread for a summer picnic. The tablecloth itself represents space-time, compressed from four dimensions into two; indeed, the checks can be used to specify locations in good old-fashioned X/Y coordinates (“the potato salad is 20 checks east and 15 checks north of the edge of the picnic table”). Juice splatters that stained three white checks in one area are embedded in the fabric of the tablecloth.

Now watch carefully. If the tablecloth around the juice stains is pulled on the fabric’s bias, the square checks will elongate into diamond shapes, stretched in one direction and compressed in another. One pair of juice stains will move farther apart, and another pair will move closer together. The X/Y coordinates of all three juice stains on the tablecloth remain the same because the stains are firmly embedded in their checks; yet the distances between the three stains have changed because the tablecloth itself has been deformed.

That’s exactly what passing gravitational waves are believed to do: They do not disturb the placement of objects in four-dimensional space-time, but they change the distance between them by stretching space-time itself in one direction while compressing it in the perpendicular direction. That’s what Albert Einstein mathematically predicted in 1916 in his general theory of relativity.

Einstein’s theory grappled with changing gravitational fields, such as that of a massive star when it explodes and throws off most of its mass. Centuries earlier, Galileo, Kepler, and Newton had all derived equations that accurately described the behavior of gravity between ordinary objects and Earth or the mutual interaction among suns and planets in space—the equivalent of ants crawling across the checked tablecloth. But Einstein wondered exactly how objects could “sense” changes in other objects’ positions or masses across the vacuum of space. So he invented a new concept of gravity. He realized that gravity could be explained as a curvature of space-time.

Mathematic calculations show that a single object drifting in a straight line at an unchanging velocity would remain embedded in space-time, sitting at the bottom of its gravitational well, its gravitational field a static force. But an object accelerating—exploding or rotating asymmetrically—or two objects revolving around each other would cause disturbances in space-time, or gravitational waves, which would propagate outward in all directions. The more massive the object(s) and the faster the motions, the greater the deformation of space-time, and the stronger the disturbances. And if the right kinds of instruments could be built, those gravitational waves should be detectable.

About Trudy E. Bell

Trudy E. Bell, M.A. has been an editor for Scientific American, senior editor for IEEE Spectrum magazine, and senior writer for the University of California High-Performance AstroComputing Center. She is the author of a dozen books and more than 500 articles, 19 of which have won journalism prizes, including the 2006 David N. Schramm Award of the American Astronomical Society (won in part for her Air & Space/Smithsonian article “When Stars Collide.”) Reach her at trudyebell.com or t.e.bell@ieee.org.

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