Most of the machines we encounter in everyday life are one-way devices. Kitchen appliances turn bread into toast and cabbage into cole slaw, but they cannot perform the opposite transformations. Even machines that claim to be reversible adopt a very shallow notion of what it means to go backwards. The drill in my tool box has settings marked "forward" and "reverse," but no matter which I choose, I cannot undrill a hole. The gearshift in my car also has a position labeled R, but when I back up, the engine keeps turning in the same direction; if the car were truly reversed, it would suck in pollution through the tailpipe, converting it into gasoline and air.

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Computers, too, are mostly irreversible machines. When a program is running, there's no way to turn it around and have it step through the same instructions in the opposite order. Even if that were feasible, the backward-running program would not uncompute an answer. Some of the individual instructions would also have to be reversed, or inverted; for example, undoing addition requires subtraction. For computers of the current generation, such reversals of logic and arithmetic are simply not possible.

In the case of toasters and gasoline engines, full reversibility is too much to ask, but no fundamental law forbids reversible computing. The theoretical possibility of a digital computer that can run forward and backward with equal ease has been recognized for more than 30 years. A few silicon prototypes have been built and shown to work. Still, up to now, reversible computing has been a toy technology, more of interest to theorists than to engineers. Only a small community of devotees believed it would ever be anything more.

That attitude is changing now. The reason is that reversible computing holds out the promise of dramatically lower power consumption, which is becoming an urgent need. Also, any computer based on quantum technology will necessarily be a reversible machine. As a result, forward-thinking chip designers are thinking backwards too. It's not too soon to ask what it might be like to have a reversible computer on your desk, or to write programs that can run in either direction.