Heat is feared and undesirable in modern electronics. As processors and memories keep getting smaller, waste heat is becoming a bigger problem. But what if the heat is used correctly to calculate me? A number of researchers have now shown that it is possible. This opens the door to thermal computers that run on body heat or other waste heat sources.
Heat flows from hot to cold. This means that heat flows are constantly occurring. Where electricity consists of electrons, heat consists of elementary vibrations that form a kind of pseudoparticles: phonons. A metal rod then behaves, for example, as a heat conductor (substances that conduct electricity well are often also good heat conductors). This flow behavior is precisely predictable and you can calculate with predictable, manipulable processes. So in principle it is also possible to calculate with heat.
What further increases the possibilities is that materials can conduct heat in very different ways. For example, if you stack layers of heat conductor and heat insulator, the material will mainly conduct heat towards the sides. You can also think of heat transistors: a switch that when it gets warmer, enlarges an insulating space or makes it disappear. Compared to electronics these will be pretty big and very slow things, electricity moves at almost the speed of light and heat at only centimeters per second, but basically you can build a computer-like thing with this. Yuki Sato of Harvard, one of the authors, says that heat flows can in principle be manipulated just like electricity. In his new research he studies metamaterials, the thermal conductivity of which can be made very complex.
Sato and colleague Supradeep Narayana did just that. Their simplest demonstration is a thermal shield: a themed component that excludes heat from a specific area. This part consists of a large cylinder, the size of a large battery, consisting of forty concentric layers of natural rubber and boron nitride doped silicone rubber. The pair experimented with the part in thermally conductive gel, in which a temperature gradient was present. Without this heat shield, the heat would have passed from hot to cold. They also built a concentrator that sucked up the heat flow. The most interesting application was the inverter, which reversed the direction of the heat flow up to 180 degrees.
Calculate with heat
Continuing from this beginning to a circuit that can actually perform calculations is a big step. But not an impossible step, says Sato. There are already materials whose thermal conductivity depends on the temperature. If such materials are used in the inverter, the heat flow would only pass if the environment is warm enough. The basis for thermal computers, according to Sato. A thermal computer is Turing-complete, so it could be used for the same things as an electric computer (although it will be much slower). One advantage of a thermal computer is that it directly uses waste heat for calculations and could thus save energy.
Heat computer robust, but very unwieldy and slow
However, this advantage is extremely limited: heat flows are the result of a entropic process, in contrast to electrons, the phonons move crisscross through each other, so much slower and more unwieldy than electric currents (which are generated by a voltage difference), although solid-state physicists already work with phonons and surfaceplasmons (vibrations in electron plasma) at the nanoscale. These sometimes move at very high speeds. We also don't have the benefits of decades of electrical product development yet. In principle, an economical electrical circuit combined with, for example, a thermocouple that taps the heat difference and converts it into electricity, can therefore far exceed the performance of the thermal computer, also in terms of energy consumption. There are two areas of application where thermal computers can be of interest. Firstly, where strong electrical interference fields are present that cause electronic circuits to burn and secondly where large amounts of ionizing radiation are present. In other words: space travel, nuclear power plants and backup systems after an EMP attack. Thermal computers can also be interesting for sensors.
However, there are quite a few practical hurdles to be overcome. Unlike electricity, which can be transferred by conduction, induction (via a magnetic field) and by vacuum-moving electrons, heat can be transferred through three processes: conduction, flow, and radiation. It will still require the necessary artifice to rule out undesired heat transport taking place.