People have always understood that something flows from hot objects to cold ones. We call that flow heat. In the eighteenth and early nineteenth centuries, scientists imagined that all bodies contained an invisible fluid which they called caloric. Caloric was assigned a variety of properties, some of which proved to be inconsistent with nature (e.g., it had weight and it could not be created nor destroyed). But its most important feature was that it flowed from hot bodies into cold ones. It was a very useful way to think about heat. Later we shall explain the flow of heat in terms more satisfactory to the modern ear; however, it will seldom be wrong to imagine caloric flowing from a hot body to a cold one.
The flow of heat is all-pervasive. It is active to some degree or another in everything. Heat flows constantly from your bloodstream to the air around you. The warmed air buoys off your body to warm the room you are in. If you leave the room, some small buoyancy-driven (or convective) motion of the air will continue because the walls can never be perfectly isothermal. Such processes go on in all plant and animal life and in the air around us. They occur throughout the earth, which is hot at its core and cooled around its surface. The only conceivable domain free from
heat flow would have to be isothermal and totally isolated from any other region. It would be “dead” in the fullest sense of the word — devoid of any process of any kind.
The overall driving force for these heat flow processes is the cooling (or leveling) of the thermal gradients within our universe. The heat flows that result from the cooling of the sun are the primary processes that we experience naturally. The conductive cooling of Earth’s center and the radiative cooling of the other stars are processes of secondary importance in our lives.
The life forms on our planet have necessarily evolved to match the magnitude of these energy flows. But while “natural man” is in balance with these heat flows, “technological man”1 has used his mind, his back, and his will to harness and control energy flows that are far more intense than those we experience naturally. To emphasize this point we suggest that the reader make an experiment.
