A more succinct and comprehensible definition might be something like this: Matter/energy may be altered (converted), but not created (from nothingness) nor destroyed (reduced to nothingness). The First Law teaches that matter/energy cannot spring forth from nothing without cause, nor can it simply vanish.

The internal energy is just a form of energy like the potential energy of an object at some height above the earth, or the kinetic energy of an object in motion. In the same way that potential energy can be converted to kinetic energy while conserving the total energy of the system, the internal energy of a thermodynamic system can be converted to either kinetic or potential energy. Like potential energy, the internal energy can be stored in the system. Notice, however, that heat and work can not be stored or conserved independently since they depend on the process. The first law of thermodynamics allows for many possible states of a system to exist, but only certain states are found to exist in nature. The second law of thermodynamics helps to explain this observation.

If a system is fully insulated from the outside environment, it is possible to have a change of state in which no heat is transferred into the system. Scientists refer to a process which does not involve heat transfer as an adiabatic process.

Implications of the First Law

The First Law, although not formally defined until the 19th century, helps make science possible. Science depends on the ability to identify cause-effect relationships. If matter/energy could spontaneously appear (and have effects on other matter/energy around it), scientists would never know whether a given observation was due to a rational cause, or to a spontaneous generation of matter or energy that was uncaused. Scientific conclusions would be on shaky ground. The Law of Causality is thus closely linked with the First Law of Thermodynamics.

The First Law also demands, if we accept it, one of two possibilities about the nature of the universe. One is that it has always existed, changing form perhaps but never having come from nothingness, or returning to the same. The other possibility is that it did not come from nothingness, but from a transcendant (that is, outside the universe) creator who is not subject to the laws within the universe.

Critics who have asked "if God created the universe, who created Him?" miss the following points. First, they have unconsciously granted to the Law of Causality the very property of self-existence (that is, an eternal, uncreated nature) that they are presuming God couldn’t have. A being who created the universe and the laws within it, who pre-existed them, would not be slave to those laws. And since the Law of Causality is a statement about relationships between multiple entities, the law could not even exist until one entity began the act of creating another one (at which point it would implicitly come to exist). Finally, most atheists who use this argument grant to the universe the exact property of self-existence that they deny God. They either deny the First Law of Thermodynamics and believe the universe came into existence from nothingness, or believe that it is itself self-existing. However, this latter position violates the unity principle – that a valid law of science that is found to apply anywhere, applies everywhere and to everything in the universe, including the universe as a whole.

The only position that appears to be consistent with the First Law of Thermodynamics, the unity principle and causality is that the universe was created by a self-existent external agent not subject to the laws operational in the universe it created.

Thermodynamics (theory and practice)