Researchers have discovered a way to transmit information wirelessly without power, simply by opening an closing a switch that connects a resistor to an antenna. This effect does not violate any physics - it works because the random thermal noise signature of the transmitter changes when the resistor is connected or disconnected.
The researchers used an RTL-SDR with high gain horn antenna and low noise amplifiers to measure changes in the thermal noise signature of the transmitter.
They also compare their idea to backscatter devices, which are another form of passive RF communications that make use of ambient radio signals such as from TV transmitters. They note that their thermal noise approach has a lower data rate and range compared to backscatter, but their next goal is to try and improve this.