When students first learn about Einstein’s theory of relativity, they usually spend a lot of time conducting thought experiments on very fast moving trains and spacecraft. The effects seem equally abstract: time can appear to slow down, lengths can change and simultaneous events can suddenly go out of synch.

So it was with some surprise that we noticed a paper in Physical Review Letters claiming that relativity was also at work every time you start your car.

Actually, the claim isn’t quite as outrageous as it might sound. As other blogs point out, relativity plays an important role in determining the interactions of electrons orbiting heavy atoms. Basically, the bigger the nucleus, the faster the inner electrons move around it. As the electrons’ orbital speeds approach that of light, relativistic effects begin to kick in. For really heavy stuff like, say, lead, that can change the energy levels of the outermost electrons.

Lead is a major component of car batteries, which generate electricity through electrochemical reactions between lead compounds and sulphuric acid. Pekka Pyykkö of the University of Helsinki in Finland and his colleagues there and at Uppsala University in Sweden modelled chemical reactions and found that relativistic effects account for somewhere between 1.7-1.8 volts of a standard 2.11-volt lead-acid cell. In other words, it takes the theory of relativity to start your car.

Lead-acid technology is 150 years old, and likewise, the relativistic effects in heavy atoms have been known about for decades, so there’s no real shocker in any of this. But this is the first time anyone has ever bothered to do the maths and show that relativity really plays a major role in lead-acid batteries.

Of course, if there were no general relativity (i.e. gravity), getting your car to start would be the least of your problems.

Credit: Wikipedia