When people hear about relativity, they usually think about spaceships traveling at speed of light, and the strange properties of spacetime such as time dilation or length contraction. These effects actually also occur right in your car. This has nothing to do with the yet to appear Doc Brown’s DeLorean, but the physics behind how your car is working is pretty interesting.

Everything happens in the car’s lead-acid battery: about 80 percent of its voltage comes from relativity, according to a paper in Physical review Letters, January 7. The relativistic effects are coming from fast-moving electrons in the lead atom. When it comes to atoms, the heavier the nucleus, the faster the inner electrons move around it. For an element as heavy as lead, the speed of the orbiting electrons has to approach that of light for them to counter the attraction of their large nuclei.

Pekka Pyykkö of the University of Helsinki in Finland, Rajeev Ahuja of Uppsala University in Sweden and their colleagues 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.

The lead-acid battery is 150 years old, and though scientists expected relativistic effects, the team was the first to derive a theoretical model from fundamental physics principles, and to show that these effects are actually dominant.

In other words, without relativity, these batteries would have never worked out, and you would probably be riding a bike.

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