How is Einstein’s theory of relativity responsible for the color of gold?

Precious metals are important for backing up our currency, making our jewelry, or as a sound investment for the future. One of the most universally-accepted valuable metals is gold, which is quite unique when you think about it. 

The attractive metal gold, in its actual form, is golden in color while silver, aluminum, tin, nickel etc are silvery in color. Interestingly, it is said that Einstein’s theory of relativity is directly responsible for the golden appearance of gold. Sounds strange?

Here’s an explanation: Normally, in the majority of elements, the electrons move around the nucleus at the speed of 200 kilometers per second. This is hardly 0.07% of the speed of light. Gold, having a high density, attracts high electric charge. Thus, the electrons move around its nucleus at a comparatively higher speed than that of most other metals. According to Einstein’s theory of relativity, time slows down for the fast moving electrons and they refract the light rays of different wavelength. This phenomenon results in giving the gold a ‘golden’ appearance.

Before we delve into this further, we also have to understand just what color actually is. Normal light or white light is actually a mixture of each color. Each of these colors has a wavelength of their own. When regular white lands on any item, it gets reflected, absorbed, and scattered. A certain color in the light may be absorbed by the item, while another will reflect off it. The reflected color will be the one that we see. The color that we perceive could also be different than the actual physical color of an object. Biophysical cues, such as being color-blind, or a certain viewing angle could be the cause of this change. 

If we look at the issue of metal colors in the light of simple chemistry, it would make sense for gold to also have the silvery appearance of other metals. However, as we can see, gold is very different from silvery metals. This is why we need to look at Einstein’s Special Relativity as well as quantum mechanics to explain this phenomenon. 

Quantum mechanics explains the color of gold by looking at how the electrons in an atom are sitting in their own discrete orbitals. For metals like silver, it requires an ultraviolet photon with high energy to kick one of these electrons to any higher orbital. This results in the lower-energy photons being reflected onto the silver, giving a mirror-like effect. The photons here are visible, hence the silvery appearance of this precious metal. 

Einstein’s relativity then explains the color of gold with regard to the size of its atoms. The electrons inside gold travel at more than half of the speed of light. According to the theory put forward by Einstein, such high speeds cause the electrons’ mass to increase. Hence the energy required to take them to a higher orbital is lower than that of silver. As a result, the blue, low-energy photons get absorbed, thus diminishing the silvery effect on the metal. With the bluish effect removed, we see a yellowish or golden color. 

Related posts: