| MadSci Network: Physics |
Most particles which make up matter (protons, electrons, neutrons, and others) have antiparticles (antiprotons, antielectrons, and antineutrons) with the same mass but the opposite electric charge (protons are +, but antiprotons are -). Photons (the particles which make up light) are different: "anti-photons" are the same thing as photons.
Anti-matter interacts with anti-matter in exactly the same way that matter interacts with matter. Electromagnetism (light) is one such form of interaction, so "anti-light" acts on antimatter the same way light acts on matter. But as we said before, "anti-light" and regular light are the same thing!
Therefore, antimatter will be transparent if the corresponding matter is also transparent. An anti-glass-window will be transparent, but an anti-brick will not be.
This is a purely theoretical argument: no one has ever created enough anti-matter to be able to measure its bulk optical properties.
(Caveat for advanced readers: because of the charge sign difference, there will be a 180-degree phase shift in the light waves produced by an an oscillating charged anti-particle. But this shouldn't make any difference, since there will also be a 180-degree phase shift in the response of an anti-particle to an incoming electromagnetic wave.)
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