The second law of thermodynamics states that the entropy of a closed system always increases over time - or, in layman's terms, all things decay. It is one of the four fundamental laws of thermodynamics, and the most important for understanding the future energy state of the universe.
- About the Laws of Thermodynamics -
The fundamental laws of thermodynamics are basic scientific principles about thermal energy and heat (the transfer of thermal energy), and are among the most important classical laws in the study of physics and chemistry. Three laws were originally formulated by the nineteenth century; in the twentieth, a fourth was added on the basis of a more sophisticated understanding of temperature. This final law is known as the Zeroth Law, reflecting the fact that it seems more fundamental than the other three but that "First" was already taken. The Zeroth Law is simply a logical formulation: that if Substance A is in thermal equilibrium (there is no transfer of heat) with Substance B and Substance B is in equilibrium with Substance C, then Substance A and Substance C are also in equilibrium.
The other laws are older and state fundamental principles of thermal energy. The First Law of Thermodynamics states that energy can be neither created nor destroyed: it can simply change form. This is also known as the law of conservation of energy. The Third Law of Thermodynamics states that as temperatures approach the coldest possible (absolute zero), entropy will decline to a minimum level (although it cannot be zero, just as a substance is never at absolute zero).
- The Second Law of Thermodynamics -
The Second Law of Thermodynamics bridges the gap between these two, and states that over time, the entropy in a closed system will increase over time. In essence, it observes that two systems with a difference in temperature (average thermal energy) will tend to equalize at a new equilibrium point, that we can take advantage of this heat transfer to do work. However, as the thermal energy equalizes, the capacity to do new work through heat transfer declines. Eventually, the two systems reach the same temperature, at which point no further work is possible. This does not mean that they are necessarily cold or hot: they have simply equalized in temperature. Still, the result is known as "heat death": because no heat transfer is possible any longer, no more work can be done within the system.
There are several important applications and consequences of this principle. First, and most importantly for our present understanding, it means that there can be no such thing as a perpetual motion machine, or a system which can continue performing work forever without any new energy in the form of chemical fuel, thermal energy, etc. In order for work to continue, there must be some sort of thermal difference to exploit - or entropy will increase to the point where no further work is possible.
On Earth, some sources of power do seem inexhaustible - and, at least for the moment, entropy on Earth alone is not necessarily increasing. However, this is because we have an external source of energy constantly available: the Sun. Taken as a whole, if the solar system is not receiving new energy from elsewhere, it will eventually run out of thermal energy. We know that the Sun will eventually stop producing this energy for us - although long before it literally loses the capacity to do so, it will run out of hydrogen in its core, explode outwards into a gas giant, and then collapse inward into a glowing cinder known as a white dwarf.
- Future of the Universe -
Most of the consequences which we encounter on a daily basis are small-scale, and do not involve truly closed systems: we require food to continue producing energy within our bodies, we need sunlight to continue growing plants, and so on. However, at one ultimate level - the universe as a whole - nature can be said to exist within what we believe is a closed system. This holds important and disturbing consequences for the future of life and the universe (although we will not have to face those consequences for trillions of years).
The universe is a closed system, and within that system there are a wide number of heat transfers going on. Over an extremely large period of time, however, the total entropy will increase and the total amount of potential heat transfer will decrease: stars will burn out, hydrogen fuel will be used up, white dwarfs will cool. Eventually, and inevitably barring some intervention from outside of the universe, this process will continue until there is no further heat transfer possible, and the universe will be said to have reached maximum entropy. This eventual end-state is referred to as the "heat death of the universe," although in fact the end temperature will be fairly cold. At that time, everything must have died: there will be no more stars, no more light, no more organic life forms, in fact no more transfers of energy in any form whatever. The universe will simply grow cold and dark.
Currently, observations indicate that the expansion of the universe is accelerating, and that it will continue until the end of the universe. If so, many physicists believe that because the total amount of energy in the universe is constant, eventually it will become dispersed over such vast distances that no further work is possible (and temperatures will approach absolute zero), even though maximum entropy has not been reached. This scenario, known as the "Big Freeze" theory, is subtly different than the heat-death scenario envisioned by the Second Law of Thermodynamics, although the consequence - a dark, cold universe - is essentially the same.