ALBERT EINSTEIN never learned to drive. He thought it too complicated and in any case, he preferred walking. What he did not know—indeed, what no one knew until now—is that most cars would not work without the intervention of one of his most famous discoveries, the special theory of relativity.
Special relativity deals with physical extremes. It governs the behavior of subatomic particles zipping around powerful accelerators at close to the speed of light and its equations foresaw the conversion of mass into energy in nuclear bombs. A paper in Physical Review Letters, however, reports a more prosaic application. According to the calculations of Pekka Pyykko of the University of Helsinki and his colleagues, the familiar lead-acid battery that sits under a car's bonnet and provides the oomph to get the engine turning owes its ability to do so to special relativity.
The lead-acid battery is one of the triumphs of 19th-century technology. It was invented in 1860 and is still going strong. Superficially, its mechanism is well understood. Indeed, it is the stuff of high-school chemistry books. But Dr. Pyykko realized that there was a problem. In his view, when you dug deep enough into the battery's physical chemistry, that chemistry did not explain how it worked.
A lead-acid battery is a collection of cells, each of which contains two electrodes immersed in a strong solution of sulphuric acid. One of the electrodes is composed of metallic lead, the other of porous lead dioxide. In the parlance of chemists, metallic lead is electropositive. This means that when it reacts with the acid, it tends to lose some of its electrons. Lead dioxide, on the other hand, is highly electronegative, preferring to absorb electrons in chemical reactions. If a conductive wire is run between the two, electrons released by the lead will run through it towards the lead dioxide, generating an electrical current as they do so. The bigger the difference in the electropositivity and electronegativity of the materials that make up a battery's electrodes, the bigger the voltage it can deliver. In the case of lead and lead dioxide, this potential difference is just over two volts per cell.