I've been watching a lot of 'The Walking Dead' recently and it's begun to bother me a bit because I'm not sure where the walkers (zombies) get their energy from. I have seen zombies which have been wandering around for years which are clearly still moving even if they haven't eaten anyone, and this makes very little sense to me. Whereas it's true that if you don't need to make your own heat, it helps your food last longer, it just seems a bit excessive. Therefore I thought I'd set down a few ideas here.
First of all, the laws of thermodynamics themselves:
0. (The "zeroth" law): If one object is at the same temperature as a second object and the second object is at the same temperature as a third, then the first object is at the same temperature as the third.
1. Energy can be neither created nor destroyed, only converted from one form to another.
2. The entropy of a closed system tends towards a maximum. Everything wears out and runs down, often in the form of heat equalising everywhere, so for instance a bath or a hot drink warms the room slightly as it cools to the slightly raised room temperature and a splash of water turns into ripples, which then smooth out and the water surface is flat, or an ice cube melts and cools the water it's in. Hence perpetual motion is impossible, even perpetual motion of the entire Universe - the heat death of the Universe.
3. The coldest possible temperature is -273.15 degrees C, but it can never be reached because it always takes the same amount of energy to halve the temperature of something.
Animals generally get their energy from food. Sugar, more specifically glucose, is either eaten directly or converted from other kinds of food (e.g. starch), then gradually oxidised as it is broken down, releasing energy as if it's burning. Most of this is done in animals via the Krebs Cycle, but a relatively small amount of energy is released by further breaking down the end product, lactic acid, in a process called glycolysis which doesn't need oxygen. There are also other processes around which do the same thing, e.g. yeast converting sugar to ethanol and some bacteria converting ethanol to acetic acid. Plants also get their energy from food in the same way, but unless they're carnivorous they create that food themselves using the energy from sunlight, and that food is either sugar or made from sugar.
There are also a few bacteria which get energy from food in a different way, such as by combining it with sulphur instead of oxygen and producing hydrogen sulphide. However, by far the most efficient way of getting energy from food is the Krebs cycle, and as far as I know any large living thing which can move its body around uses the Krebs cycle, although if it moves fast or hasn't got much oxygen available to it, it builds up an "oxygen debt" which must later be paid, such as by panting.
Zombies are therefore a problem. They don't breathe and blood does not circulate around their bodies. Therefore, their cells are not getting oxygen, and since the cells are also dead, the mitochondria, where the Krebs cycle happens, aren't working either. They are therefore not getting their energy from the Krebs cycle and if they are moving using the same mechanisms as living things generally, they aren't able to get enough energy from food to move themselves. They will run down eventually and their bodies will stop moving. They do eat, of course, but their digestive systems also need energy to work, e.g. to swallow food and secrete digestive juices.
However, not all chemical reactions are like that. Burning is a much simpler reaction which releases energy (e.g. in petrol engines and cookers). However, it does so by completely converting the substances from which it releases energy into oxides apart from the ones which can't be combined with oxygen, and it tends to do so very quickly. Hence zombies could possibly work like petrol engines or fires, but they would consume themselves rapidly in the process and crumble into ash if that was what they were doing. Not quite sure about this actually as I can imagine each cell in a zombie's body working like a little petrol engine or something, but then the carbon dioxide wouldn't be carried away and there would be no oxygen provided, so even then the fire would go out.
Having said that, matter is energy and contains a lot of energy. Not all energy is chemical, and chemistry only works to release relatively tiny amounts of energy from chemicals while leaving most of the atoms and molecules alone and unchanged, just rearranged. Atomic energy is different, and can be released in much larger amounts. This works because atomic nuclei, at the centre of atoms, are held together by something called the Strong Nuclear force. Like charges normally push each other apart, but this doesn't happen in atoms because there is a stronger force than electricity and magnetism holding them together. Atoms of different sizes need different amounts of energy to hold themselves together. If an atom is changed from one kind to another and the second kind needs less energy to hold it together, it will release some of that extra energy. This is what happens during radioactive decay.
If zombies are nuclear powered, they would have to be highly radioactive and as well as killing and attempting to eat people, they would give them cancer and radiation sickness even if they escaped the immediately fatal process of being eaten. I quite like the idea of radioactive zombies. This could also mean that if enough zombies got together in one place, they would engage in a runaway nuclear reaction rather than a controlled one such as in a nuclear reactor and explode like a nuclear weapon. This also appeals to me because it makes them even more dangerous.
Even nuclear power and atom bombs only release a tiny fraction of the energy present in matter. According to Einstein, the energy present in matter is equivalent to the mass of the matter multiplied by the square of the speed of light. In other words, E=mc^2. This means, for example, that there is enough energy in the average adult human corpse to provide all the energy used by the United States for thirty years. Hence zombies could conceivably be surviving by gradually converting all of their mass into energy, and they could keep going for millions of years that way. However, the only known way of doing that is by bringing matter and antimatter together. If a zombie contains wormholes to another Universe where the body of a corresponding zombie is made of antimatter rather than matter, that would be one way this could be happening, but it would have to happen extremely slowly or a single zombie body would probably release enough energy in one go to destroy the Earth completely. Now that really would be a zombie apocalypse.