On Star Trek, Captains Kirk and Picard often had to contend with their Romulan adversaries who possessed a “cloaking device” that rendered their ships invisible. Back on Earth, cop shows on TV often show suspects in an interrogation room with an interviewer while others outside watch behind a two-way mirror that seems like a regular mirror to those inside. It’s a way to observe without being seen yourself.
Physicists call such cloaking devices and two-way mirrors “nonreciprocal,” meaning that information or energy or light can flow in one direction, but not in another. In other words, with such a device, “I can see you, but you can’t see me.”
You can already buy windows that block sunlight, but blocking other kinds of electromagnetic radiation is much harder to do. Your cell phone, for example, operates with microwaves, and you probably would like to keep others from eavesdropping on your conversation
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