When Stars Collide
Enter Einstein's grand construct of gravitational wonders, and do not attempt to adjust your television set.
- By Trudy E. Bell
- Air & Space magazine, September 2005
Dana Berry/NASA GSFC
“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.