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Put your right hand on your right temple, and your left on your left temple. Now gently squeeze; don't let up.
Okay. Now you are ready for a conversation with Arlie Petters, the energetic, broadly smiling man in the striped short-sleeve shirt and comfortable brown slacks who has come to meet you at a Barnes & Noble café. Many topics will come up. He will wonder whether amoebas could join the conversation. He will suggest searching the solar system for tiny black holes as a practical business enterprise. And he will say things like, "I don't see why reality should contain only three spatial dimensions." Again with the beaming, cheerful smile. "Do you?"
Now that he mentions it, maybe there are no good reasons for limiting reality to three spatial dimensions, but you are rather used to it that way, and … it's a good thing you didn't order coffee, because you could not pick up the cup: You need both hands to hold your skull together.
Perhaps Petters, a Duke professor of mathematics and physics, smiles so much because once he decided that he may have an entire extra physical dimension to work with, the usual limitations that make the rest of us so grumpy stopped pinching quite so tight. An additional dimension might enable him to be two places at once—saving money on daycare, perhaps, or at least making it easier to pick a child up there; or it might offer limitless extra time, or … space, or … something.
But maybe Petters is so cheerful because, if everything goes just right with a NASA satellite scheduled to launch any day now, it's possible that his name will be forever linked to the physical evidence that disproves Einstein's general theory of relativity. Or not exactly disproves Einstein's brilliant conflating of space, time, energy, and matter. "I would say that he missed something," Petters says. Something that, if it turns out to be true, "would give us a complete philosophical shift in our understanding of the physical world,"by proving that the physical universe has four, not three, spatial dimensions, making ours, when you include time, a five-dimensional universe. "It's a very exciting idea," Petters says.
He opens a clasp envelope and produces a sheaf of scrawled notes on lined paper, including a simple graph: an X- and a Y-axis, on which a straight dashed line angles down from left to right, with a sine wave superimposed. "The telltale wiggle," he calls it.
Here's the idea. The Gamma-ray Large Area Space Telescope (GLAST), scheduled to be launched into Earth orbit by NASA in February, will spend its time looking at gamma rays, the most energetic form of light there is—billions of times more energetic than the waves our eyes can perceive; millions of times stronger than even X-rays. The result should be new information about things like pulsars and supernovae, the kind of unimaginably massive energy sources that emit gamma rays and exist at the very edge of our current understanding of physics.
But with the new telescope, Petters and his colleague in this project, Charles Keeton, a Rutgers University astronomer, saw an opportunity to go even further. That graph that Petters produced, which he calls a "back of the envelope calculation," resulted from a flurry of e-mail messages between the two a couple of years ago when they heard about the telescope. The graph represents how gamma rays would bend—the "wiggle" in the graph—if they happened to pass a tiny but massive object called a braneworld black hole.
A black hole—not the braneworld kind, but the kind that most of us have heard of, even if we still don't quite comprehend what it is—is a massive object like a star or many stars that has collapsed into an unimaginably small and dense space with a gravitational pull so strong that even light cannot escape it. Einstein's theories predicted the existence of black holes, since verified by scientists. A braneworld black hole is a special kind of black hole. It's tiny, the size of an atomic nucleus or smaller, but has the mass of an asteroid. For now, its existence is theoretical. Proof will come only if a specific variant of the string theory of gravitation, which disagrees with Einstein's theory, turns out to be true.
But ignore that for a moment. For now, just keep in mind that Petters and Keeton want to look for the wiggle they predict they will find in the gamma-ray graph if the gamma rays happen to pass by one of those braneworld black holes. These wiggles are the subject of Petters' research.
They're caused by gravitational lensing, a process by which light (of any electromagnetic wavelength) is bent in the warped conditions of space and time that occur near massive objects like planets, stars, or black holes. Einstein predicted this phenomenon, and it was first observed during a 1919 total eclipse of the sun, when background stars viewed directly past the darkened sun appeared slightly out of position. The sun's mass had actually bent the rays of light from those distant stars. The phenomenon was regarded as a brilliant proof of the warping of space and time described in Einstein's general theory of relativity. Such lensing, now better understood, thanks in part to Petters, can produce not only bent but also multiple images of distant objects. What's more, objects with certain masses affect light of specific wavelengths according to specific signatures. Subjected to mathematical analysis, these signature bends yield secrets about the objects that cause them.
"Imagine dropping a pebble into a pond," Petters says. The pebble generates waves, with peaks and valleys: Big rock, big waves; tiny pebble, smaller waves. That is, an object massive enough to be a gravitational lens leaves a signature pattern affecting a specific wavelength of light. And those tiny black holes predicted by the braneworld theory would produce a wiggle in the specific electromagnetic range that the GLAST will be measuring, once it's in orbit.
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