Artificial black holes: on the threshold of new physics
PASADENA, CA — For several decades now, there has been a fundamental problem with modern physics. The problem is actually an embarrassment of riches: we have not one, but two systems that describe the universe remarkably well. One is quantum mechanics, which describes the rich and subtle behavior of waves and particles. The other system, general relativity combines space and time into one continuum, providing us with the best description of the movement of the planets and the expansion of the universe.
Scientists have realized that to truly understand the universe, we've got to make these two systems work together, even merge into a single, more accurate depiction of reality. But the two systems have not given up their independent identities easily. The challenge has been to find conditions in the universe where both the effects of quantum mechanics and general relativity are significant and measurable.
For this to be the case, you've got to pack a whole lot of mass (as general relativity mainly relates to gravity), into an extremely tiny volume (where quantum effects become important). Where do you think those conditions might exist? Fortunately, the universe has provided us with such a natural laboratory for fundamental physics: black holes.
Black holes are gravitational relics of dead stars. They are, quite literally, bottomless pits in space and time that are capable of swallowing any amount of material. Everything a black hole swallows gets compressed into an unimaginably tiny central region called a singularity. According to our current knowledge, this singularity is infinitely dense, and infinitely small. And if you think that doesn't make any sense, you're not alone.
Scientists have long viewed the central region of a black hole with the same kind of suspicion that early mariners held for regions of the map that read "there be monsters." It just can't be right. If the only thing we can say about the singularity is that it doesn't make any sense, then there must be a problem with the way we're trying to understand it. So there, deep in the hearts of black holes, may lie the clues to how quantum mechanics and general relativity dovetail together under extreme conditions.
Unfortunately, there's an intrinsic problem with actually observing what conditions near the center of a black hole are like. Black holes, most famously, have the ability to suck light itself into their maws, effectively cutting themselves off from the rest of our universe. The "point of no return" for light as it nears a black hole is called the event horizon, as no event can ever be observed taking place beyond it. There seems to be no way to get any information about what is happening inside a black hole, as no kind of signal can ever come out.
Or can it? In the 1970's, the well-known physicist Stephen Hawking proposed a way in which black holes radiate energy away, eventually "evaporating" completely. Over time, the black hole gradually leaks away all its energy and disappears in a final burst of radiation.
The final death throes of a black hole, scientists suspect, might be very illuminating indeed. Exactly how a black hole dies and what sort of information is carried in the Hawking radiation could tell us quite a bit about what the center of a black hole is really like.
But there are two major problems with observing the last gasp of a black hole. For one thing, the nearest black holes we know of are light-years away, making accurate measurements of Hawking Radiation nearly impossible. Secondly, black holes take a huge amount of time to evaporate, the time being proportional to their mass. Even relatively small stellar black holes would take longer than the current age of the universe to dissipate, and the monster black holes in the middle of galaxies may be the last things to exist in our universe, taking ten thousand trillion trillion trillion trillion trillion trillion trillion trillion years to die away (sorry, I just have to do that sometimes. That is the actual estimate of how long a massive black hole will last).
So what do you do if you can't wait around that long? For the best chance to observe Hawking Radiation and evaporation, you'd want a black hole that was much closer than naturally occurring black holes, and much less massive. It's a common misconception that you have to have a huge amount of mass to create a black hole. Any amount of mass will do, as long as you cram it into a sufficiently small space. A super-massive black hole with the mass of a billion Suns might be the size of our Solar System, but the Earth could be a black hole too if you packed it into the volume of a marble. Even a person will do, although you'd have to cram them into the space occupied by a single electron.
This line of reasoning has led scientists to the inevitable: If we really want to observe black holes and how they radiate, we'll have to whip them up in our own laboratories. And that's exactly what we are on the threshold of being able to do. Now, there is no kind of technology with the ability to physically crush matter to black hole densities, but there's an easy away around that. Einstein showed us that matter and energy are equivalent, so you can also make a black hole by pushing a huge amount of energy into a tiny volume. For those kinds of experiments, there's an obvious choice: particle accelerators. And the next generation is just about to be unveiled.
Amazingly, scientists are becoming increasingly confident that they will be able to create black holes on demand, in quantity, using the new atom-smashers due to come online in the next five years. Some estimates suggest that the new Large Hadron Collider (LHC) at the European Center for Nuclear Research (CERN -the acronym is in French) will be able to create an average of one black hole each second. LHC will bombard protons and antiprotons together with such a force that the collision will create temperatures and energy densities not seen since the first trillionth of a second after the Big Bang. This should be enough to pop off numerous tiny black holes, with masses of just a few hundred protons. Black holes of this size will evaporate almost instantly, their existence detectable only by dying bursts of Hawking radiation.
What exactly are scientist looking for in the Hawking radiation? For one thing, it's a big mystery whether Hawking radiation contains any information about the particles that formed the black hole in the first place, or fell in later. Those particles had charge, spin, and other fundamental characteristics that may not have been erased by the black hole. Also, the exact manner in which a black hole dies may give us a view into higher dimensions in space. The most recent theories about the Big Bang and the earliest moments of our universe suggest that there were more than the four (three of space, one of time) we normally experience.
For some reason, the other dimensions didn't expand with our four, and remained "rolled up" at very small scales. These extra dimensions might still be important, and directly felt, in regions right around the central singularity of a black hole. In fact, these higher dimensions might solve the mystery of what a singularity really is. Instead of truly being an infinitively small and dense point, there might suddenly be a whole lot more room provided by extra dimensions that only act on tiny scales. Scientists already have ideas about how these dimensions would affect the temperature and intensity of Hawking radiation. Now all we have to do is power up the acclerator and put those theories to the test.
"But wait", I hear you say, "Has anyone considered that creating artificial black holes might not be the best idea?" The idea of creating black holes in the laboratory has to give one pause. I mean, how can anyone resist the urge to imagine future headlines like "Artificial Black Hole Escapes Laboratory, Eats Chicago" or some such thing? In reality, there is no risk posed by creating artificial black holes, at least not in the manner planned with the LHC. The black holes produced at CERN will be millions of times smaller than the nucleus of an atom; too small to swallow much of anything. And they'll only live for a tiny fraction of a second, too short a time to swallow anything around them even if they wanted to.
If it makes you feel any more comfortable, we're pretty sure that if the LHC can produce black holes, then so can cosmic rays, high-energy particles that smash into our atmosphere every day. There are probably a few tiny black holes forming and dying far above you right now. So I think we should all relax, fire up the Large Hadron Collider, and get ready for a view of the universe that we've never seen before.