Tiny Black Holes

Black holes are among the oddest objects believed to populate the universe. Long sought, they have recently been "found" lurking in the centers of galaxies, tipping the scales at a million to a billion times the mass of the sun. Now, just to stir things up, enticing hints have come to the fore that tiny black holes may exist, and if so, they may hold the key to testing whether the universe has more to offer than the three familiar spatial dimensions of length, width, and height.

Tiny in this context means something around a billionth of a billionth of the mass of the sun—a couple billion tons, or the mass of a small asteroid. A black hole of this mass would be about the size of an atomic nucleus. Physicists have speculated that, when the universe was very young and hot, copious numbers of miniature black holes may have been produced. (To our knowledge, tiny black holes cannot form today.)

Alive and well?

But would these "primordial" black holes still exist, roughly 14 billion years after the big bang? Surprisingly, the answer depends on the number of spatial dimensions in the universe. And the reason for that—bear with us—has something to do with the fact that black holes are not, strictly speaking, black.

Stephen Hawking famously discovered that black holes can lose mass by emitting elementary particles. (Quantum mechanics allows matter/antimatter pairs to spontaneously form and quickly disappear just outside a black hole's event horizon. If one particle falls in and the other flies away, it looks to a distant observer like the black hole has emitted a particle.) "Hawking radiation" should cause a black hole to shrink and eventually evaporate. This process is basically irrelevant for black holes the mass of our sun or larger, but it's vital for their minute cousins.

Very tiny primordial black holes—perhaps as small as a pound or two—may have been able to survive to today.

In Einstein's theory of general relativity, which describes a universe with three dimensions of space (plus one of time), Hawking radiation would have caused all primordial black holes smaller than a few hundred million tons to evaporate by now. That could change, though, if the universe has more than three spatial dimensions.

The idea of extra dimensions grew out of string theory—which needs them to explain how the strings vibrate—but it has taken on a life of its own. Whereas string theorists traditionally assume that extra dimensions are wrapped up so tight that they don't affect anything besides strings, other physicists have begun to entertain the notion that one or more of the extra dimensions may be large enough to affect physics on scales we can measure.

A matter of dimension

One thought is that everything we know might be confined to a three-dimensional membrane floating, like a strand of seaweed in the ocean, in a larger universe that actually has four spatial dimensions. Physicists Lisa Randall of Harvard and Raman Sundrum of Johns Hopkins University have turned this "braneworld" concept into a specific model that they and others are examining as a possible alternative to Einstein's general relativity.

In the Randall-Sundrum braneworld model, the fourth dimension of space changes how gravity operates on small scales, which changes the rate at which small black holes form and evaporate. The upshot is that very tiny primordial black holes—perhaps as small as a pound or two—may have been able to survive to today and may even constitute some of the exotic dark matter in the universe.

The question of whether tiny primordial black holes still exist therefore represents a clear distinction between general relativity and the Randall-Sundrum model. It could provide a crucial way to distinguish between these competing theories of the universe—if only we could figure out how to look for primordial black holes.

Finding evidence

Well, we have developed a way to do just that. While studying how the gravity from a black hole bends light rays, we recently discovered that a primordial black hole would create a rippling interference pattern in a passing light wave, much in the same way that a rock in a stream impedes a passing water wave.

A close relationship exists between the mass of a black hole and the wavelength of light it can affect. If the Randall-Sundrum model is correct, tiny primordial black holes could exist and produce interference patterns in light from the short-wavelength, gamma-ray portion of the electromagnetic spectrum. By contrast, if general relativity is right, no primordial black holes below a few hundred million tons should remain, and hence no interference patterns should appear in gamma-ray light.

Your neighbors could have a pet black hole, and you might not realize it.

After calculating the interference effect, we scratched our heads and asked each other, Is there any hope of measuring it? We knew that tremendous natural explosions in deep space called gamma-ray bursts produce light of the right wavelength. The question was whether any telescope today could measure this light.

It turns out that the perfect telescope is on the way: the Gamma-ray Large Area Space Telescope (GLAST), which is scheduled to be launched on a spacecraft in August 2007. A joint effort among NASA, the U.S. Department of Energy, and institutions in France, Germany, Japan, Italy, and Sweden, GLAST will be exquisitely sensitive to high-energy gamma rays—and able to measure interference effects from any primordial braneworld black holes.

In the neighborhood?

Because it seemed relevant for GLAST—not to mention fun—we tried to figure out where the nearest primordial braneworld black holes might be. We were startled to realize that they would be right in our backyard, astronomically speaking: If they make up 1 percent of the dark matter in the universe, thousands of tiny black holes may lie within our solar system! Our immediate reaction was, "No, that can't be. If black holes exist in the solar system, surely we would know."

Actually, maybe not. The gravity from these black holes is not very strong; you could add a few thousand of them to the solar system without really changing the planets' orbits. You would need to get within 12 feet of one of these black holes to feel as much gravity as you normally feel here on Earth. So your neighbors could have a pet black hole, and you might not realize it. Not that they could hold onto it: as far as we know, miniature black holes would not experience the atomic forces that make matter solid, so they would pass right through the planet.

An enormous idea

So we come away with the incredible thought that the solar system—and the rest of the universe—might be filled with tiny black holes, each carrying the signature of the fourth dimension. And in the next year or two, GLAST should make it possible to look for them. It is worth noting that if we don't see the characteristic interference patterns right away, it doesn't automatically mean braneworld gravity is wrong; it could just mean primordial black holes are rare.

If we see even one case of interference, though, that's when the fun begins. We would immediately know that tiny black holes exist. We would need to analyze the data carefully before drawing firm conclusions about gravity. But we would dive in with gusto, knowing that we were on the trail of something physically small but philosophically enormous. As large, perhaps, as a whole new dimension of space.