Programming the Universe: A Quantum Computer Scientist Takes on the Cosmos

by Seth Lloyd

While you run, the free energy in the sugar is converted into motion by your muscles. By the time you’re finished running, you’re hot: the free energy in the sugar has been converted into heat and work. The number of calories of heat and work exactly matches the calories of free energy in the apple’s sugar. In obedience to the first law of thermodynamics, the total amount of energy remains the same. (In obedience to the second law, the amount of information required to describe the extra jiggling of molecules in your hot muscles and sweaty skin is much greater than the amount of information required to describe the ordered chemical bonds in the apple’s sugar.) Unfortunately, to reverse this process is not so easy. If you wanted to convert the energy in heat, which has lots of invisible information (or entropy), back into energy in chemical bonds, which has much less entropy, you would have to do something with that extra information. As we will discuss, the problem of finding a place for the extra bits in heat puts fundamental limits on how well engines, humans, brains, DNA, and computers can function.