In the 17th century Newton advanced our understanding of the cosmos through his clarification of the concept of force and through his discovery of the law of universal gravitation. In the 18th and 19th centuries, the important advances were in cla rification of the concept of energy and in the discovery of the laws of thermodynamics. The industrial revolution provided the impetus for scientists to study the relations between work, energy, and heat and to develop ever more sophisticated "he at-engines" to power the mines and factories. In the 19th century scientists began to understand the atomic nature of matter, and this led to further understanding of thermal properties of matter. To understand some of these discoveries, we need a few b asics.

An object in motion can do considerable damage if it is suddenly brought to rest. Think about a car hitting a brick wall! The amount of kinetic energy possessed by an object of mass m travelling with speed v is

K = mv2/2

(The word "kinetic" comes from the Greek kine meaning motion. ) An object moving twice as fast has four times as much energy.

Any material consists of atoms and molecules that are in continual, random motion. In a gas, the molecules move freely, interacting with each other briefly during collisions. In solids the molecules are locked in a lattice, and vibrate about a relatively fixed position. As the temperature is increased, the motions increase. In a gas, the molecules would cease to move at a temperature called absolute zero, (about -273° C.) (No real gas exists at this temperature; all gases condense into liquids or solids before this temperature is reached.) In air at room temperature, the molecules have an average speed of about 1000 mph! Temperature is actually a measure of the amount of energy possessed by each molecule.

The heat engines built by engineers such as James Watt in the 18th century are devices that extract some of the thermal energy and convert it to other, more practical, forms of energy. The science of thermodynamics was driven by a desire to build more efficient heat engines. (A modern example of such an engine is an automobile engine.)

Four laws of thermodynamics have been identified. The fourth (which is called the zeroth law) is not needed for our purposes here.

  1. The energy of a system may be changed by adding heat to it or doing work on it. Equivalently, a system may do work at the expense of its internal, thermal, energy.
  2. Any closed system tends toward increasing randomness (its entropy tends to increase). This explains why the efficiency of a heat engine is limited, and why a refrigerator puts more heat into the kitchen than it takes out of the food inside it .
  3. The only way to avoid the consequences of the 2nd law is to have a system at absolute zero. But that is impossible! (The entire universe is currently at an average temperature of 3° above absolute zero.)

These laws have profound effects on the way we view the universe. They explain why a star evolves into a burnt cinder, providing heat and light on its journey into oblivion. Our Sun will run out of its atomic fuel in another few billion years, and will expand to engulf the Earth. Our future descendents, if any remain, will have to travel to another star system if they are to survive.

But more importantly, perhaps, the laws suggest that the entire universe undergoes an evolution from a hot dense beginning to one of two equally appalling ends: either the universe will expand forever, its stars winking out, leaving a cold , bleak environment of dead stars and an ever-thinning sea of radiation (the so-called heat death); or the universe will recollapse into a dense conflagration mimicking the original big bang.

This picture of an evolving universe dominated by the passage of time was profoundly different from the unchanging universe of Aristotle or even the clockwork universe of Newton, which, once set in motion by some superior intellect, would run forever.

In the years following Newton's discoveries, astronomers learned that the solar system is in the galactic suburbs, and that our galaxy is but one of billions. Now we must also adapt to the notion that we live in a random time in the history of the universe, and face an uncertain and (for the moment) unpredictable future.