Device transmits radio waves with almost no power – without violating the laws of physics

A new ultra-low-power method of communication at first glance seems to violate the laws of physics. It is possible to wirelessly transmit information simply by opening and closing a switch that connects a resistor to an antenna. No need to send power to the antenna.

Our system, combined with techniques for harvesting energy from the environment, could lead to all manner of devices that transmit data, including tiny sensors and implanted medical devices, without needing batteries or other power sources. These include sensors for smart agriculture, electronics implanted in the body that never need battery changes, better contactless credit cards and maybe even new ways for satellites to communicate.

Apart from the energy needed to flip the switch, no other energy is needed to transmit the information. In our case, the switch is a transistor, an electrically controlled switch with no moving parts that consumes a minuscule amount of power.

In the simplest form of ordinary radio, a switch connects and disconnects a strong electrical signal source – perhaps an oscillator that produces a sine wave fluctuating 2 billion times per second – to the transmit antenna. When the signal source is connected, the antenna produces a radio wave, denoting a 1. When the switch is disconnected, there is no radio wave, indicating a 0.

What we showed is that a powered signal source is not needed. Instead, random thermal noise, present in all electrically conductive materials because of the heat-driven motion of electrons, can take the place of the signal driving the antenna.

No free lunch

We are electrical engineers who research wireless systems. During the peer review of our paper about this research, published recently in Proceedings of the National Academy of Sciences, reviewers asked us to explain why the method did not violate the second law of thermodynamics, the main law of physics that explains why perpetual motion machines are not possible.

Perpetual motion machines are theoretical machines that can work indefinitely without requiring energy from any external source. The reviewers worried that if it were possible to send and receive information with no powered components, and with both the transmitter and receiver at the same temperature, that would mean that you could create a perpetual motion machine. Because this is impossible, it would imply that there was something wrong with our work or our understanding of it.

A graphic in the top half showing a horizontal cylinder on the left with a pipe extending to the right with a 90-degree bend upward connecting to an upside-down triangle with pairs of curved lines on either side, and in the bottom half the same but disconnected

Electrons that naturally move around inside a room-temperature resistor affect electrons in a connected antenna, which causes the antenna to generate radio waves. Connecting and disconnecting the antenna produces the ones and zeros of a binary signal.
Zerina Kapetanovic, CC BY-ND

One way the second law can be stated is that heat will flow spontaneously only from hotter objects to colder objects. The wireless signals from our transmitter transport heat. If there were a spontaneous flow…

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