Transparent Aluminum Is Real. Star Trek Was Only Half Right

Summary

Metals are usually opaque and reflective because their loosely bound electrons form an “electron sea.” When light makes those electrons oscillate, they emit a wave that cancels the incoming light and reflects it. The same mobile electrons allow metals to conduct electricity and heat. A metal can become transparent when its electron sea is too thin to respond to light or its electrons cannot move freely.

The video describes four ways researchers have overcome metal opacity:

1. Make the metal extremely thin. In the 1850s, Michael Faraday studied gold leaf thin enough to let some light pass through. Viewed through it, the gold appeared green, while gold-silver alloys could appear blue-purple. Heating the leaf made it more transparent; Faraday suspected that gaps had formed in the metal. His experiments also produced an early sample of colloidal gold, which appears ruby-red in water. Modern energy-efficient windows use similarly thin silver layers to transmit visible light while reflecting infrared heat.

2. Use light that the metal’s electrons cannot follow. In 1933, Robert Wood showed that films of alkali metals—including potassium and cesium—could block visible light while transmitting ultraviolet light. These metals have relatively sparse electron seas and therefore lower plasma frequencies. Below a metal’s plasma frequency, it reflects light; above it, the electrons cannot respond quickly enough. Cesium’s transparency begins near the violet edge of visible light, so a thin film can look blue when viewed through it.

Alkali-metal compounds are also used in photomultiplier tubes, including those in the Super-Kamiokande neutrino detector. These films detect faint light, including flashes associated with neutrinos. Observations from the detector helped establish that neutrinos change type and have mass.

Wood’s work followed a famous false claim. In 1904, René Blondlot reported “N-rays,” which supposedly passed through materials including aluminum. Wood’s visit to Blondlot’s laboratory exposed the result as an observational error: Blondlot continued to report the expected pattern even after Wood removed the prism said to produce it.

1. Squeeze sodium until its electrons are trapped. Calculations by Yanming Ma and Artem Oganov predicted that extreme pressure could make sodium stop conducting and become transparent. Using a diamond anvil cell, Mikhail Eremets and colleagues compressed sodium to roughly two million atmospheres. The sodium changed crystal structure, lost its metallic shine, and became a yellowish, transparent solid.

At this pressure, electrons become trapped in spaces between sodium atoms, forming an electride. Because the electrons can neither flow freely nor respond to light, the material is insulating and transparent. A separate study reported that lithium also becomes less metallic under pressure.

1. Use an intense X-ray laser for an instant. In 2009, an Oxford-led team used the FLASH free-electron laser in Hamburg to strike aluminum with an extremely intense pulse of extreme-ultraviolet light. The pulse removed inner-shell electrons before the aluminum atoms could move, briefly allowing the remaining light through. The solid aluminum became nearly transparent at that wavelength for about 40 femtoseconds—but not to visible light. The material then heated into “warm dense matter,” a state relevant to giant planets and fusion research.

Related Materials and Technologies

The video distinguishes these short-lived or specialized effects from materials used in everyday products:

• ALON, or aluminum oxynitride, is a transparent, hard ceramic—not a metal. Developed from Army research and later commercialized, it can be used in armored windows, optics, and missile sensor domes. It is transparent because its electrons are locked in chemical bonds rather than moving freely.
• Indium tin oxide (ITO) is a transparent conducting coating used in many touchscreens and displays. Adding tin provides a small number of mobile electrons—enough to conduct electricity but not enough to reflect visible light strongly. Indium supplies are limited; the video notes the element’s reliance on zinc refining and its importance to the display industry.
• Other approaches, such as fine metal meshes, silver nanowires, and ultrathin metal films, involve trade-offs between transparency and electrical conductivity. A microwave-oven door illustrates how a metal mesh can block long-wavelength microwaves while allowing visible light to pass through its small openings.

Central Conclusion

Transparent aluminum exists in several senses, but the practical bullet-resistant material is a ceramic. The experiment with actual aluminum metal made it transparent only to extreme-ultraviolet light and only for an instant. In general, making a material transmit light requires limiting or eliminating the free electrons that make it metallic.

Researchers and Sources Featured

Researchers and Scientists

• Justus von Liebig
• Michael Faraday
• René Blondlot
• Robert Wood
• Takaaki Kajita
• Masatoshi Koshiba
• Eugene Wigner
• Hillard Huntington
• Yanming Ma
• Artem Oganov
• Mikhail Eremets
• Ivan Troyan
• Sergey Medvedev
• Vitali Prakapenka
• Justin Wark
• Bob Nagler
• Sam Vinko
• Gianluca Gregori
• James McCauley

Institutions, Publications, and Other Sources

• Royal Institution
• Royal Society
• Johns Hopkins University
• Jilin University
• Stony Brook University
• Max Planck Institute for Chemistry
• Advanced Photon Source
• FLASH free-electron laser
• Super-Kamiokande
• Nature
• Nature Physics
• United States Army Materials Research Center
• Raytheon
• Surmet Corporation
• United States Geological Survey
• Reuters