If your laptop computer can't keep up with your most urgent computational needs, cheer up. A physicist has calculated how to make PCs almost unimaginably faster--although to reach this theoretical speed limit, they'd have to be as dense as black holes.

Seth Lloyd, a physicist at the Massachusetts Institute of Technology, calculated the ultimate physical limits on the speed of a computer using the laws of thermodynamics, information theory, relativity, and quantum mechanics. For starters, he used information theory to show that a 1-kilogram, 1-liter laptop could store and process at most 10^{31} bits of information. (A nice-sized hard drive holds about 10^{11} bits.)

Then Lloyd figured out how quickly it could manipulate those bits. To do this, he invoked Heisenberg's Uncertainty Principle, which implies that the more energy a system has available, the faster it can flip bits. Lloyd's ultimate laptop would convert all of its 1-kilogram mass into energy via Einstein's famous equation E = mc^{2}, thus turning itself into a billion-degree blob of plasma. "This would present a packaging problem," Lloyd admits with a laugh. If this energetic lump could somehow be turned into a computer, it could perform 10^{51} operations per second, he reports in the 31 August issue of *Nature*. By comparison, today's planned peak performer will be able to do 10^{13} operations per second.

But processing speed is only half of the story. If you *really* want to speed up your computer, Lloyd says, you must also slash the time it takes to communicate with itself--that is, to transfer information back and forth. The trick, he says, is to squeeze the computer down to the most compact possible size. Lloyd shows that a computer made of the most compressed matter in the universe--a black hole--would calculate as fast as a plasma computer. It would also communicate in precisely the same time that previous calculations showed is necessary to flip a bit--the hallmark of the ideal computer. Coincidence? Maybe not. "Something really deep might be going on," Lloyd remarks.

At present, scientists have no idea how to turn a laptop into a black hole (Windows 98 jokes aside). But Raymond Laflamme, a physicist at the Los Alamos National Laboratory in New Mexico, says that just thinking about such extreme scenarios might illuminate mysteries such as black holes.