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In certain regions of the world, summer heat can sometimes become unbearable. It feels as if the fiery sun is shooting its harsh scorching rays right through our bodies. We perspire profusely and run for shade. But people do manage to live in such heat. 

On the other hand, where there is no warmth around, and air is bitter cold in the open, we shiver and suffer frostbite and worse. People do manage to live through bone-chilling winter, too. Yet, though there are a few settlements in the Sahara, there aren’t any at the poles—where there is perennial ice and sheets of snow, where all is bleak and no vegetation will grow. 

We realize the importance of heat for our survival. We also need heat to cook our foods. Among the sensations we feel, warmth is most closely linked to comfort and coziness. We speak of a warm person and of warm hospitality. Poet John Keats speaks of love as “for ever warm and still to be enjoy’d.” It is supremely satisfying to warm ourselves in front of the fireplace on a wintry night. 

The human body needs heat to carry on its normal activities. Heat is at the very basis of life. This has been recognized since ancient times, and gave rise to the notion of innate heat and inborn fire as the root of all life. Heat was taken as playing a role in eating and in breathing, and in reproduction, too, as when we speak of an animal in heat. 

We are not content with the experience of heat and cold. We want to know what exactly is the source of these sensations. Physics has probed into the roots of this perceived reality also. 

The hot water from the shower does not give out light. The shining phosphorescent paint may be cool when we touch it. The hot slab of rock does not emit light. We feel no warmth from the full moon that shines so bright in the sky. All this might suggest that heat and light are intrinsically different. But then, the same sun that gives us light gives us heat also. The same fire that is so hot lights up the room, too. The candle flame is both bright and burning. From these it is reasonable to conclude that heat and light are intimately related. 

When we get into a car that had been closed and standing for long in the sun, we feel as if we are entering an oven. What we experience is radiant heat: energy in the form of electromagnetic waves emanating from the surfaces within the vehicle. 

The temperature of a glowing coil on the electric oven is very high, while that of the ice cream is quite low. The nurse takes the patient’s temperature, and the weatherman reports the day’s high and low temperatures. It is common to speak of temperatures, even if one is not very clear about what exactly this means. A widespread misconception is that temperature is a measure of “how much heat” a body contains. 

The temperature of a body refers to its thermal condition—i.e., the condition that determines the direction of heat flow to or away from the body. In other words, temperature is a state relative to others. It is not unlike water pressure that determines the direction of flow, always from regions of higher to regions of lower pressure. 

When we touch a body and it feels hot, the body’s temperature is higher than our own: Heat energy flows from the hot body into ours. When the body feels cold, heat flows away from our body: We say that the body’s temperature is lower than ours. In fact, it is this flow of energy that constitutes heat. There is a universal law of nature by which heat energy always flows from bodies at higher temperatures to bodies at lower temperatures. 

We say this is a law because this is how nature behaves. Perhaps this need not have been so. Why should not heat energy flow from colder to hotter bodies? Consider wealth, for example. Does it always flow from the richer to the poorer classes? 

In the world of perceived reality, some bodies are hot, some hotter still; some bodies are cold, some colder still. Like other measurable entities, there is a wide range of temperatures. At one extreme, we may get very close to absolute zero, and at the other extreme, temperatures of the order of several hundred million degrees are known to exist in the core of stars. 

Chemical reactions that change substances from one kind to another generally occur in the temperature range between the commonly experienced to about 1,200 degrees. Of these, the biochemical reactions sustaining our lives occur in the very narrow range of some 30 to 40 degrees Celsius. It is remarkable that in a world where the range of possible temperatures is so stupendous, such a narrow band is being maintained for such long intervals of time to keep biological functions going. Aside from intricate physiological subtleties, this is also made possible by extremely sensitive environmental factors that have been brought into play over eons. What we may not always realize is that, in principle, this balance could be upset without the slightest warning. 

The snow melts on mountain peaks and causes the majestic flow of rivers. Over the ages, glaciers—mighty masses of ice—had moved and chiseled the topography of land. Bits of ice we drop into a glass melt and cool the beverage. Deep in the bowels of the earth, rocks and metals are present, not as hard and solid chunks, but in molten liquid forms that sometimes gush out through volcanic apertures in eruptions. Heat changes solids to liquids. Heat is involved in any change of state. When water evaporates from lakes and oceans, when dew drops are formed on leaves, when snow melts and the pond freezes, we have change of state also. 

Our common experiences are within a narrow range: mostly the extremes of heat and cold between which our weather conditions swing. We may be subjected to very bitter wintry cold, and those who venture into the polar regions for exploratory thrills may come upon colder air still. Or again, there is the scorching sun of sandy Sahara. There are considerably hotter states in the stars and much colder ambiance as in the remote wilderness of the planet Pluto. 

Physicists have tried to create temperatures much colder than that to find out what happens to matter and motion in such conditions. A remarkable thing one notices when the temperature falls down considerably is that air becomes liquid. Prior to the 1870s, the very idea would have seemed ridiculous: a glass of liquid air like a cup of tea, except that it would not be hot, but cold. But in 1877, physicists succeeded in getting some oxygen and nitrogen in the liquid state. So began exploration of phenomena in extremely cold conditions: cryogenics. 

As often happens, curiosity—in this case, to know how the world behaves at very low temperatures—not only unraveled other roots of perceived reality, but also opened up new industries and technological possibilities. Liquid oxygen is used in the steel industry and as rocket fuel, liquid nitrogen in refrigeration, liquid argon in the electronics industry. Other peculiarities of liquid helium, such as superfluidity and superconductivity, promise yet other applications. 

Thus, the study of heat brought us to the study of cold, and the study of cold has come to serve us in rocket propulsion and supercomputers, among other things.

In most instances, human beings want to reach the highest there is: to climb the tallest mountain, to jump the highest hurdle, to attain the noblest goal. But there is a context in which human beings have tried to reach the ultimate at the lower extreme: to reach the lowest possible temperature. At one time, it was thought that some day we would hit rock bottom, the zero on the absolute scale. It is good that theory and experiments go hand in hand, each guiding the other. It was recognized through theoretical analysis that this would be a wild-goose chase: for, in principle, absolute zero is as unreachable as absolute rest. Human ingenuity may lead us to as close as we wish to that ultimate state of extreme coldness, but this cannot be reached. Indeed, we have managed to get temperatures of the order of a millionth part of a degree above absolute zero. 

Unlike the last tick of time, which one cannot even conceive, absolute zero temperature is a perfectly valid concept. It is very much there, but simply beyond the grasp of nature. It is like the point that begins a line, surely present, but untouchable as a separate entity when we approach it step by slow step on the line. 

As Ilya Prigogine reminded us, a recurring theme in 20th century physics is the impossibility of accomplishing certain things—of finding absolute rest, of measuring with a hundred percent precision, of reaching the absolute zero. On the logical plane, it sounds ironic that with the gathering of more knowledge and understanding, we are discovering more of what we cannot do. On the plane of wisdom, however, this is a revelation: that the more we learn, the more we come to realize our finiteness and our limitations. This is not an indictment of the scientific enterprise. Rather, it reflects science’s potency not only to unravel the roots of perceived reality, but also to reveal the scope and constraints of human intellect and ingenuity.

Rubbing causes heating, even sparks and fire. There is some intimate relation between heat and motion. The elaboration of the relationships between the mechanical energy of molecules and the thermal properties of bodies is the kinetic theory of heat. Because of their incessant motion, atoms and molecules possess kinetic energy. Temperature is a measure of the average kinetic energy of its atomic-molecular constituents. Any transfer of this atomic/molecular kinetic energy constitutes heat.

Heat is a form of energy, and so is motion. We measure heat energy in calories and motional (kinetic) energy in joules. When heat is transformed into mechanical energy or mechanical energy into heat, precise equivalent amounts result. Energy doesn’t disappear. 

The equivalence and preservation of the total amount when transformations occur is the principle of energy-conservation in physics or the first law of thermodynamics.

From pulley and lever to windmill and plough, many gadgets serve civilization. The model gadget is the heat engine—a device to transform heat into motion. The steam locomotive is a prime example: Take in an amount of heat and convert as much as possible into locomotion. The fraction of the input energy successfully transformed into mechanical energy measures the device’s efficiency. In a 30 percent efficient engine, three-tenths of the heat becomes mechanical work. The rest is dissipated away. Every heat engine has a source to absorb heat from, and a sink into which the unused part is cast. In all heat engines, there is some heat lost beyond recovery. More efficient engines lose less. 

Making a 100 percent efficient engine is like paying for a car what the manufacturer spent to construct it. This is impossible: The car has to be transported, the dealer compensated, taxes paid. Only a part of what we pay is for the car proper. 

Likewise, of the heat energy gobbled up by the engine, only a part becomes mechanical work. The efficiency of an engine is always less than 100 percent—another limitation by nature on what we can achieve. The impossibility of a 100 percent efficient engine constitutes the second law of thermodynamics. As Shakespeare said, “No perfection is so absolute / That some impurity doth not pollute.”

The second law of thermodynamics is one of the fundamental laws at the root of perceived reality. It is responsible for the eventual degradation of all forms of energy into heat. Much of the tangible matter in the universe is being continuously turned to radiant energy in the core of stars, and is dissipated away in empty space, here and there by stray planets. 

The conclusion is ominous. Eventually, the universe will turn into ultimate chaos in which all bodies will attain the same temperature, and there will be no more phenomena.  This will be the final heat-death, much worse than earthquakes or volcanic eruptions. This will be death to all, including mighty stars and shining sun. This is also cold scientific pessimism of the rational kind. 

Is this what we want to hear after centuries of penetrating physics? To have removed all the mystery from planets and stars was bad enough. To say that rainbows are just consequences of the laws of refraction is prosaic enough. To speak of comets as minor masses following very elongated elliptical orbits is odd enough. But to be told that someday the sun will fade away and the stars cease shining, and all will be degraded to lifeless uniformity: Well, this is the most unkindest cut of all. 

Poets and philosophers of ages past had surmised similar endings, too. John Milton had written in his Paradise Lost: “Day and night, / Seed-time and harvest, heat and hoary frost / Shall hold their course, till fire purge all / things new.”

But that was poetry, not to be taken literally. This is science, frighteningly closer to the truth, or so some feel. Fortunately, there is so much matter and so much space that thermal dissipation can go on for very long without affecting our plans of work, and even our dreams for the future. If ours were a smaller universe, things might be different. But still, there is something unpleasant in this whole idea of cosmic mortality. 

Philosophers and physicists have written on this theme. Some have persuasively argued for less drastic visions of the second law of thermodynamics. New interpretations are emerging to assure us that the heat-death is by no means a logical consequence of serious physics. 

We have often been struck by the order in the world, impressed by its harmony and enchanted by its beauty. Yet, just as underneath all the civility and polite exchanges there often reign raw passions and basic instincts, behind all the laws and regulations governing the phenomenal world, there is an irrepressible tendency toward disorder. Every change that spontaneously occurs moves the world closer to a state of total mix-up. The goal of the universe is supreme chaos. Such is the finding of thermodynamics. 

What does one mean by chaos in this context? The notion is simple: It refers to a more homogenous mixture of the component parts of a system. When we shuffle an ordered sequence of cards, we create greater chaos. When we stir coffee into which sugar has been poured, we are increasing the disorder. When dust spreads out more uniformly in the room, there is an increase in disorder, too. 

The world of physics measures the degree of disorder in a system in terms of entropy. When a physicist says that the entropy of a system has increased, all it means is that there is now more mixing up of the components of the system. No matter what happens, the net effect whenever heat is involved is always to increase the entropy of the system. 

Heat is a consequence of the disorderly motions of atoms and molecules. It is the randomness of the kinetic energy that distinguishes heat from mechanical energy. If a million molecules traveled all in a straight line with the same speed, their combined energy would not constitute heat. The molecules of a gas move every which way, they collide and rebound, they are like a swarm of brainless bees, bouncing off one another again and again, thoroughly mixing all the time. That is why all the oxygen molecules don’t accumulate in one corner of the room, nor nitrogen ones in another. Continuous random mixing prevents people from suffocating. 

This, then, is the fate of the world: to get thoroughly mixed up. The goal of the universe is to reach the state of maximum entropy. There has been a cosmic leveling principle working silently since the dawn of time, infinitely more drastic than what the most ardent communist could hope for on the economic plane. 

Entropy decrease would imply an increase in order, organization from disorganization, structure from mess. When a child collects the wet sand on the beach and builds a castle, a pattern emerges from the random splash of the sand that nature had strewn. The child has decreased the entropy around. When we rearrange books in the library, we are decreasing entropy. When the molecules in the food we eat are combined to form more complex ones in our bodies, there is also decrease in entropy. 

Life may be looked upon from many perspectives. One is to recognize it as a process in which there is a continuous diminution in entropy, an incessant struggle against tendencies to dissipate. The persistence of life depends on the transformation of simpler molecules into complex ones. When the organism ceases to live, decay sets in, entropy increase takes over, and the march toward the ultimate annihilation continues unhampered. 

We may look at the situation by means of an analogy. Imagine a plank with several pegs attached to it that is sliding down a hill. We may compare this to the universe whose entropy is increasing as a whole, the pegs corresponding to its various parts. Now picture some ants here and there on the plank that are crawling upward. These represent living organisms, the upward climbing ants reflect entropy decrease while the plank as a whole is moving downhill. 

The food we consume and the oxygen we breathe serve two purposes. They furnish us with the chemical constituents and the energy needed for our activities. But much of it is given back to the environment. It does not stay within the body. Food and oxygen also decrease entropy. The energy that is returned to the environment is in a high entropy form: heat radiation and discarded food. Food brings in complex molecules, which are low-entropy systems and metabolic activity decreases it further. That is why we cannot simply live on fire or light or electricity. We need low-entropy energy packages, which is what food is. 

Over the eons, plants have learned to construct low-entropy molecules by the complex process of photosynthesis. Sunlight brings in energy at a low entropy level. Plants utilize low-entropy sunlight to combine oxygen, carbon dioxide, and other elements to form the basic molecules of all the food that sustains humankind. 

This is the secret of life on earth: the silent photosynthetic mode by which low-entropy structures are formed from sunlight. How could anyone have uncovered this complex and utterly fascinating root of perceived reality without the laborious reductionist efforts of classical science?