What a Wonderful World (19 page)

How much energy does the Earth trap from the Sun? The answer is zero. Think about it. On a hot day, out in the Sun, you sweat. By this means, you shed heat exactly as fast as your body absorbs it. If you did not, you would get ever hotter until eventually you keeled over from heat exhaustion. Similarly, the Earth radiates heat back into space at the rate it receives it from the Sun. If it did not, it would get hotter and hotter until its rocks turned to the consistency of honey.
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But, if the Earth is not gaining any net energy from the Sun,
what
is it gaining? After all, something is powering all activity, including biological activity, on Earth. The clue is to look beyond the
amount
of energy arriving from the Sun to the
quality
of that energy. The heat radiated by the Earth back into space turns out to be of a
poorer quality
than the heat that is intercepted by the Earth from the Sun. The Earth saps something from it. But what?

To answer this, it is necessary to know that the Earth is like a steam engine. In fact, as English chemist Peter Atkins says, ‘We are all steam engines.’
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Don’t be perturbed by this. A steam engine is, in essence, a very simple device. Basically, it consists of a container at a high temperature filled with steam. The steam pushes a movable wall in the container – a piston – against the outside air pressure. Having done this, the steam ends up
condensed
as water at a low temperature – the temperature of the surrounding air. That’s it.

Focus for a moment on the piston. When something moves against a force it is said to do work. That is what the piston does as it moves against the outside air pressure. Work is what
everything
on Earth is doing today. Your muscles do work each time you lift your foot against the force of gravity. The electrons in the current flowing through your computer do work as they move against the resistive force of atoms blocking their path. Without work there would be no activity. Everything in the world would just sit there, inert, inactive, for all eternity.

In the case of a steam engine, work is done by heat energy, which starts out at a high temperature and ends up at a low
temperature
. And it is exactly the same for the Earth. Work is done by heat energy that starts at a high temperature – the 5,500 °C characteristic of the Sun – and ends up at a low temperature – the 20 °C typical of the Earth’s surface. However, instead of driving a mere piston, this energy drives everything from the swirling of hurricanes to the swimming of fish to the biochemical reactions that keep your body at a liveable 37 °C.

Clearly, then, it is the
temperature
that makes the heat energy radiated by the Earth qualitatively different from the heat energy intercepted by the Earth.
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But why is a change in temperature associated with work? To answer this it is necessary to
understand
what heat and work
are
.

Heat is disordered motion. If you could see the molecules in steam you would see them flying about randomly like a swarm of angry bees. If you could see the atoms in a white-hot bar of iron, you would see them jiggling randomly about their fixed positions. Work, by contrast, is ordered motion. If a piston moves, or a muscle in your arm contracts, a large number of atoms move as one, in lockstep.

So here is what happens when steam in a container does work by pushing a piston. Molecules of steam drum on the piston like countless raindrops on a tin roof.
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Although each molecule has only a tiny pushing effect, together they have enough oomph to drive the bulk of the piston.

Now, temperature is a measure of the average speed of a body’s microscopic constituents such as atoms; whereas the atoms of a hot body are moving quickly, those of a cold one are moving more sluggishly. And each molecule of steam, in
imparting
its tiny pushing force to the piston, loses some of its energy of motion, some of its speed.
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So the price of doing work on the piston is a drop in the average speed of the molecules – in other words, a reduction in the steam’s temperature.

But there is a subtlety here. And it will bring us back to the case of the Earth and the Sun – and arguably one of the most profound insights in the whole of science: the second law of thermodynamics.

How much heat can be converted into
useful
work for the piston? Naively, you might think, all of it. After all, one of the basic laws of physics – actually, the first law of thermodynamics – states that energy cannot be created or destroyed, only converted from one form into another. For instance, electrical energy can be converted into light energy and heat energy in a light bulb; chemical energy can be converted into the energy of motion of muscles in your body, and so on. However, the law of
conservation
of energy – its more common name – tells us only what is possible in principle, not what is possible
in practice
.
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The problem in harnessing steam to drive a piston is that it uses
random
microscopic motion to drive
ordered
bulk motion. While some of the steam atoms are flying in exactly the direction
of motion of the piston, many are not, and some are even flying at right-angles to the piston’s direction, making them useless at pushing it. Clearly, not all the energy of motion of the molecules of the steam is
usable
, not all of it can be converted into the energy of motion of the piston. A steam engine, consequently, can never be 100 per cent efficient.
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This is in fact a statement of the second law of
thermodynamics
, as formulated in the nineteenth century by the British physicist Lord Kelvin: ‘Heat cannot be turned into work with 100 per cent efficiency.’ This is not a very exciting or insightful
version
of the law – certainly not one that hints at its all-conquering power and its profound implications for life, the Universe and everything. That requires understanding the concept of entropy.

Entropy, in broad-brush terms, is the degree of microscopic disorder of a system such as a container filled with steam. When an amount of heat,
Q
, is added to the system at a temperature,
T
, its entropy,
S
, increases by
Q/T
. If this seems baffling, there is some common sense behind it.

Temperature is a measure of how vigorously atoms are moving about. Take a low-temperature body. Adding heat to it is like sneezing in a quiet environment such as a library. It has a big effect – that is, there is a large increase in the disorder, or entropy. Contrast this with a high-temperature body. Adding exactly the same amount of heat as before is like sneezing in a bustling shopping street. It has little effect – that is, there is only a small increase in the disorder, or entropy.

In a steam engine, the steam is initially at a high temperature. The energy that leaves it to drive the piston therefore reduces the entropy of the steam – but only by a small amount (remember the sneeze in the bustling street). But the energy ends up in the
surrounding
area at a lower temperature. This boosts the entropy to the surroundings by a large amount (remember the sneeze in the library).
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In other words, the result of doing work on the piston is a net
increase
in the entropy of the system and its surroundings.

And this
always
happens. When work is done, the entropy of the Universe always increases. In fact, this is the definitive
statement
of the second law of thermodynamics, arguably the most far-reaching of all laws of physics. ‘The law that entropy always increases holds, I think, the supreme position among the laws of Nature,’ said British physicist Arthur Eddington. ‘If your theory is found to be against the second law of thermodynamics, I can give you no hope; there is nothing for it but to collapse in deepest humiliation.’
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In the case of the Earth, heat energy with a characteristic temperature of 5,778 Kelvin
10
is absorbed from the Sun, and heat energy with a temperature of about 300 Kelvin is re-radiated into space.
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This corresponds to an enormous net increase in entropy.
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It is the price the Universe pays for the bewildering multitude of work processes driven on the Earth.

Contrary to popular belief, the Earth does not have an energy crisis – it uses essentially no net energy from the Sun. What the Earth actually has is an
entropy crisis
. Once heat has done work, it becomes heat at a lower temperature, which means it is more disordered, lower-quality energy, with a more limited ability to do any more work. Recall, after all, that more work can be done by a steam engine only if the heat is expelled at an even lower temperature. However, practically, there is a rock-bottom limit set by the temperature of the surrounding air. Once heat is at this temperature, it is impossible to expel it at a lower temperature and so no further work can be done.

But, if entropy – disorder – always increases, how come we appear to live in a world of order? In particular, how do living things, which are structured and as far from disordered as it is possible to imagine, buck the trend of increasing entropy?

The answer is that the second law insists only that entropy increases
overall
. So, in the creation of a cell, heat is generated by all the chemical reactions needed to assemble the cell
membrane
and internal cellular machinery. That heat boosts the entropy of the surroundings far more than the assemblage of the cell reduces it. Life exports disorder to the Universe.

And, just as all processes on Earth are ultimately driven by the temperature difference between the Sun and the Earth,
all processes in the Universe
are ultimately driven by the temperature difference between the stars, which are hot, and empty space, which is cold.
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Remember that next time you are out on a
crystal-clear
night and gaze upwards at the stars.

But this still does not explain how life bucks the trend of
ever-growing
disorder. After all, whenever work happens
spontaneously
– for example, when a slate falls off a roof, smashing on the ground, creating heat energy and sound energy – entropy always increases. The answer is that life is clever. Think of a big weight hoisted on a pulley to a great height (the analogue of a high temperature). If the weight falls, it can do work – perhaps driving the hands of a grandfather clock. At the same time, heat is inevitably generated – through friction of the rope on the pulley, friction in the clock mechanism, and so on – boosting entropy. But, say, things have been set up so that, as the weight falls, it hoists into the air a smaller weight. That creates a little
order, reducing entropy slightly. Later, if the smaller weight falls, it can do work. But say things have also been set up so that, as the small weight falls, it hoists into the air an even smaller weight, creating a little more order, reducing entropy a little more.

This is how life feeds off heat energy, degrading it to even lower-quality heat energy while at the same time creating more order. Of course, it does not use weights and pulleys – it employs a whole series of other tricks – but the principle is the same. Plants, for instance, absorb sunlight and use it to create
energy-rich
chemicals, the equivalent of raising a small weight that can be used later to do work. In a bewildering cascade of other chemical processes, life wrings every drop of work it can from solar energy, finally discarding it as low-quality heat, the ultimate slag of the Universe.

Life creates order at the expense of a lot more disorder, which it exports to its surroundings. The Earth’s biosphere is an island of organisation in a cosmic sea of chaos. ‘Life is nature’s solution to the problem of preserving information
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despite the second law of thermodynamics,’ says Howard Resnikoff.
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Of course, when looked at globally, no process in the Universe defies the trend of remorselessly increasing disorder. In the
nineteenth
century, when this was first recognised by physicists, it deeply depressed them. After all, if entropy is continually increasing, it stands to reason that, sooner or later, the Universe will reach a state of maximum entropy. At this point, all heat in the Universe will be reduced to its lowest-grade state. There will be no temperature differences to drive any activity. In this state
of cosmic ennui, which the nineteenth-century German physicist Rudolf Clausius called Heat Death, all cosmic machinery will come to a juddering halt. The Universe, in the words of the poet T. S. Eliot, will end ‘not with a bang but with a whimper’.
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As far as we are aware, there is no way the Universe can escape such a miserable fate. An interesting question therefore is: how close is the Universe to Heat Death? The answer is: much closer than you might imagine. Although it might seem that countless stars pumping out random starlight across the length and breadth of the Universe account for its disorder, this is an illusion. Most of the disorder in the Universe is in fact tied up in the afterglow of the fireball of the big bang. Incredibly, 13.8 billion years after the beginning of time, this cosmic
background
radiation still permeates every pore of the Universe. Greatly cooled by the expansion of the Universe over the past 13.8 billion years, it now appears as far infrared, a type of light invisible to the naked eye. Whereas starlight accounts for a mere 0.1 per cent of all the photons, or particles of light, in the
Universe
, the cosmic background radiation accounts for a whopping 99.9 per cent.