Relativity in Everyday Matter

Relativity is usually associated with spacecraft and particle colliders. But inside heavy atoms, electrons move fast enough for it to matter, and the consequences are sitting on your finger and under the hood of your car.

Fast Electrons

The more protons in an atomic nucleus, the harder it pulls on the innermost electrons and the faster they must move. In heavy elements such as gold, mercury, and lead, those electrons travel at more than half the speed of light. At that speed special relativity makes a real difference: the innermost orbits shrink and bind more tightly, and the outer electrons rearrange themselves in response.

The Color of Gold

Most metals are silvery because they reflect all colors of visible light about equally. In gold, relativity narrows the energy gap between two sets of electrons just enough that the metal absorbs blue light. What is reflected, white light with the blue removed, looks yellow. A calculation of gold that leaves out relativity predicts a metal the color of silver.

Liquid Mercury

Mercury sits next to gold in the periodic table and is the only metal that is liquid at room temperature. The same relativistic tightening holds mercury's outer electrons so close that neighboring atoms bond to each other only weakly. Computer simulations indicate that without relativity mercury would melt at a temperature more than a hundred degrees higher, and thermometers would have needed a different history.

The Car Battery

A lead-acid battery delivers about 2.1 volts per cell, and six cells make the familiar 12-volt battery. In 2011 a team of researchers calculated where that voltage comes from. With relativity switched off in the calculation, the cell produced only a fraction of a volt. About 1.7 of the 2.1 volts in each cell, roughly 80 percent, is a relativistic effect on the electrons in lead. As the researchers put it, cars start because of relativity.

Electron Microscopes

An electron microscope forms images with a beam of electrons accelerated through hundreds of thousands of volts. At 300,000 volts the electrons reach about 78 percent of the speed of light. Their wavelength, which sets the detail the microscope can resolve, has to be calculated with relativity or the instrument's focus will be wrong.

How Direct Is the Link?

These are not inventions that came from Einstein's work. They are things that were always true, and that special relativity finally explained. They belong here because they answer a common question: does relativity matter in ordinary life? It does. It is in the wedding ring.