Bend it like Petters: He and his collaborator, Charles Keeton, aim to prove the existence of tiny black holes originating from an extra dimension by capturing the interference patterns created when gravity bends light, in the same way radio waves, green, above, are bent by the sun's gravity in a 2002 experiment

NASA

Okay, braneworld black holes. These currently exist only on the blackboards of scientists who believe in a certain variation of string theory. String theory is a cosmological theory that considers the tiniest building blocks of matter to be something like vibrating strings rather than particles. The mathematics of this complex model end up requiring additional dimensions for everything to work out, though in most versions, extra dimensions are treated as little more than convenient theoretical constructs.

But a variant of string theory developed by Lisa Randall of Harvard University and Raman Sundrum of the Johns Hopkins University posits the universe we perceive as a sort of three-dimensional membrane (hence "braneworld") floating in a multidimensional universe. Petters loves the pragmatic elegance: "In braneworld, what I like is that this fourth dimension extends to infinity. In other string-theory models, it's this tiny, curled-up dimension," kind of stuffed into an inexpressibly small space like the end of a fiddlehead fern. In those models, the fourth dimension doesn't affect anything except strings. "They're not letting it loose," he says.

As he tries to explain the added spatial dimension, he runs his fingers along the café tabletop. "Imagine we are beings that live only on this desk," he says. That is, we're two-dimensional beings, inhabiting this flat, two-dimensional space. "That's not a limit of our eyes or ears, that's a limit of our physical existence." His eyes grow large: "You can't get off this table." He cites the famous Victorian satire Flatland, a book about two-dimensional creatures who receive a visit from a sphere and are simply unable to comprehend its three-dimensionality. He thinks a minute, then takes an intellectual step backward, to first principles.

"There are two acts of faith that go into science," he says. The first is that "the physical world is understandable to the human mind." The second, that "you can model it mathematically—quantitatively."

Then we're back to life on the tabletop. "That first postulate, that physical reality is accessible, is not quite right—not all of it. Think of an amoeba," he says. "It's a tiny entity that's wiggling around on this desk," in this essentially two-dimensional space. "Now think of our conversation. It completely transcends that amoeba, because of its wiring." That is, it completely lacks the capacity to perceive us: Living on its tabletop, it's going to think the universe has the limits of its perceptions.

"Who are we to think we are any different than this amoeba in the full spectrum of reality?" Petters says. Just because we can't think of where that fourth spatial dimension would be, and lack the capacity to perceive it, doesn't mean it's not there.

Telltale wiggle: X-Y graph predicting how gamma rays would be affected when close enough to be bent by a braneworld black hole. Dashed line shows gamma rays unaffected by the black holes. Diagram courtesy of Arlie Petters and Charles Keeton

Charles Keeton, Petters' partner in the paper they published about that telltale wiggle, cites a common way to try to imagine this extra dimension: "People often draw a piece of paper standing vertically," he says. "The third dimension," poking outward from both sides of the two-dimensional paper, "is perpendicular to both dimensions. Braneworld would have a fourth dimension perpendicular to all three dimensions" that we now perceive.

That's about as good a description as anybody can come up with—we seem to be like those tabletop amoebas, doomed to our limited understanding of reality. Still, take for comfort these words by perhaps the most famous journalist of our generation, uttered when she, too, was trying to comprehend a five-dimensional reality: "It had height, length, breadth," she said, "and a couple of other things." Those are the words of Lois Lane, describing Superman's nemesis Mr. Mxyzptlk, who came from the fifth dimension, wore a derby hat, and could be forced to return to his five-dimensional space only if tricked by Superman into saying his own name backwards. Looking at Mr. Mxyzptlk's five-dimensional manifestation, Lane said, "made my head hurt."

But hard as it may be for two-dimensional comic-book characters, or even us three-dimensional types, to wrap our minds around spatial four-dimensionality, the point is that the braneworld model puts it there, and that Petters and Keeton have found a way to look for its signature through the data gathered by the GLAST satellite.

That takes us back to the tiny black holes of braneworld. In an Einsteinian universe, black holes of that size could be created only in the conditions present at the dawn of the universe, and any created then would have evaporated by now. But according to the braneworld universe, they would not have evaporated and so would still exist to put their signature juju on that gamma-ray vibration.

If you find the interference pattern in the gamma rays, you've found a tiny braneworld black hole. That tiny black hole doesn't fit in Einstein's equations. Ergo, if you find the braneworld black hole, you demonstrate that the braneworld theory, not Einstein's, is correct. And scientists will probably start spending a lot of time looking for ways to investigate a fourth spatial dimension.

Petters leans back in his chair. "The way I look at this is, remember when we thought the world was flat?" He shakes his head. "Think of the poor elementary-school kids who will have to learn geometry into the fourth dimension." He imagines possible consequences of a fourth spatial dimension, wondering, for example, whether people will instantly try to develop weapons based on the incredibly high energy with which particles will move between dimensions. On the other hand, he recalls that in Flatland, a three-dimensional being—like you—could put a finger right into the middle of a two-dimensional being—like a square—without penetrating its boundaries: You could poke its insides without going through its "skin." Similarly, a being from four spatial dimensions might be able to poke you in, say, the spleen—from the inside. "But that's like spooks, right?" Petters asks. "That's like ghosts. You can run with this metaphor." He smiles. "It's almost like science fiction."

Yes, like science fiction. But tiny black holes the size of atomic particles, four spatial dimensions, gravity bending light? That isn't like science fiction. That stuff is all in a day's work.

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