It’s one of the fundamental immutable laws of nature - the second law of thermodynamics, which
states that overall entropy of any isolated system always increases or remains constant over time.
Entropy, in a simplified sense, is a measure of the disorder or randomness in a system. For
example, in general, a highly ordered and organized system, like a crystal,
has low entropy, while a largely random and disordered system, like a gas, has high entropy.
It seemingly appears, however, that living things don’t follow this rule. Life, in general,
is characterized by high levels of organization and complexity. Organisms are highly ordered
and structured, and they actively maintain their internal organization to perform various
processes to sustain life. This seems to be in contradiction to the second law of thermodynamics,
which suggests that entropy, hence disorder should always increase.
So the question is, how could life have come to be and continue to exist in a universe where
everything as whole has increasing entropy? How can complexity and order arise in a universe
with increasing disorder? Stay tuned because I’m going to explain that coming up right now.
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Ok, so we are really asking two questions in this video. One is,
how can life thrive in a universe with increasing entropy since living organisms
appear to decrease their internal entropy, and the second question is how can life
arise in the first place in a universe with increasing entropy because wouldn’t
living things require order and organization which would necessitate decreasing entropy?
Let me first answer the first question because I think that’s a fairly simple one, how can life
thrive in a universe with increasing entropy? To answer this, let me first better define what
the second law of thermodynamics actually says. It deals with the direction of natural processes.
The simplest way to state it is that the total entropy of an isolated system can never decrease.
Let's break down this statement. An isolated system is one where energy and mass cannot come
in nor leak out. It is a closed system that is not affected by anything outside it. In reality,
it’s challenging to have a perfectly isolated system, but for the sake of understanding entropy,
this assumption needs to be understood. What is entropy? Entropy quantifies the number
of ways in which the microscopic components of a system can be arranged while still producing the
same macroscopic behavior. So for example, a cubic meter of gas which is composed of moving atoms is
going to have higher entropy than a cubic meter of crystalline solid which is composed of atoms in a
tight configuration. There are simply more ways that the atoms of a gas can be arranged within
the same volume than the atoms of a crystal. Keep this more precise definition in mind. But
for the sake of intuition and simplification, entropy is more commonly described as a measure
of the disorder or randomness in a system. In general, a highly ordered state has low entropy,
while a highly disordered state has high entropy. Lastly, what the second law implies is that
spontaneous processes tend to move towards states of higher entropy. It’s important
to point out that we are talking about the TOTAL entropy of an isolated system.
This means that it is possible for a system to undergo a decrease in entropy locally,
but this must be compensated by an even greater increase in entropy elsewhere in
the isolated system, or its surroundings. So for example, the refrigerator in your
home decreases the entropy of the food stored inside it by cooling everything down. However,
the refrigerator gives off heat from the evaporator coils which heats the kitchen.
The heating effect outside the refrigerator is greater than the cooling that takes place inside.
So if we considered your kitchen as the isolated system which contains the refrigerator inside,
we can conclude that the kitchen overall has an increasing entropy.
So the refrigerator by itself is not an isolated system.
What does this have to do life? I’m glad you asked because the analogy of the
refrigerator applies to living things. Like the refrigerator is not an isolated system,
living things are not isolated systems either. Take human beings for example. We consume low
entropy food, and turn it into high entropy waste in the form of heat and solid and liquid waste
products. So for example, the human body produces the heat equivalent of a 75 watt light bulb. This
radiation is part of the high entropy waste that we release from our low entropy food we eat. While
inside our bodies, we are decreasing entropy as we create well-ordered structures, new cells,
and other living processes, this happens at the expense of giving off higher entropy waste.
Similarly, plants consume low entropy ultraviolet light from the sun, and turn it into high entropy
heat energy in the process of photosynthesis. And during this process, it decreases its internal entropy as the process
of photosynthesis creates sugars, and parts of its internal structures. But the plant consumes
a lot more energy than it produces in the form of sugar. That energy is in the form of high
entropy heat energy. So the overall entropy of the plant and its surroundings still increases.
The plant by itself is not an isolated system.
Living things excel at taking in energy from outside their own system or body and using it
to decrease their own entropy. But by doing so, they are not decreasing ‘total’ entropy in the
larger system, they are just decreasing it locally within their own open system.
This is how life proceeds without violating the second law. Entropy inside the living organism
decreases at the expense of dumping even more entropy into its surroundings. Total entropy
always increases. -----
While we are on the subject of entropy of living organisms, let me address another question that
sometimes comes up in the same conversation. Some argue that the law of entropy violates
evolution, because organisms tend to evolve into more complex species.
Since more advanced creatures decrease their entropy by creating more internal structures and
processes, not increase it, some argue evolution should be excluded because of the 2nd law. The
history of life on earth certainly seems to indicate that more complex living things have
evolved over time from simpler organisms. While it would be a mistake to say that complexity is
a goal of evolution, (it is not), it is true that evolution appears to increase order.
For example, mammals are more ordered than microscopic multi-cellular organisms,
which are more ordered than single-celled organisms, which are more ordered than
a mixture of organic chemicals, and so on. So if order is supposed to decrease over time,
doesn’t evolution violate entropy laws? The answer is no. The reason
is that Earth is not a closed system. It is continuously bathed in energy from the
sun. And it is this energy that drives evolution. So plants take in sunlight to produce sugars.
Animals eat plants. Meat-eating animals eat the plant-eating animals,
and this cycle goes on. Over long periods of time these species change due to changes in
their environment and other factors that drive natural selection. This is what evolution is.
But almost all the energy that drives evolution ultimately comes from the sun.
The entropy of the sun is increasing tremendously as it is converting low
entropy matter into high entropy energy. In fact, that entropy increase is orders
of magnitude, far far bigger than any increasing order due to evolution on earth.
So overall entropy of the solar system, of which the earth is a small part, is still increasing.
But we still need to answer the question of how life could have
begun on earth in the first place because as I alluded to earlier,
even the simplest self-replicating organism would be more ordered than a soup of chemicals.
First, I want to preface my answer with the fact that science has not currently
definitively answered this question. It is an area of active research by
origin-of-life scientists. A lot of pieces of the puzzle have been found,
but the puzzle has not been solved yet. But let me tell you a few things about what
we do know, as well as a hypothesis about how it could have happened.
As I said earlier, the TOTAL entropy of a system has to increase with time. But this
doesn’t mean the entropy of every single piece within the system has to increase with time.
In fact, if we can look at our solar system as being essentially an isolated system,
as orderly things arise in one place, then somewhere else, disorder will have
happened to more than compensate for this. So for example, 4.5 billion years ago,
diffuse dust and gases collapsed via gravitation to form a more ordered star
that is our sun and planets, including the earth. But this gravitational collapse of the planets
resulted in the release of heat from friction. Gravitational potential energy of the dust and
gases was converted to photon energy and radiated away.
So even though the matter collapsed to a more dense and ordered configuration of a planet,
it resulted in an overall entropy increase carried by the photons which radiated away.
Similarly the gravitational attraction that resulted in the formation of the sun,
resulted in igniting fusion within its core which continues to release huge amounts of energy.
The sun is turning low entropy matter into high entropy heat and radiation, more than compensating
for the order that arose from gravitational collapse of the diffuse gases that it’s made of.
Overall entropy always increased. Similarly, on earth the second law
was not violated when various chemicals and elements naturally combined to form larger more
complex molecules. Analogous to the way living things can create more order within themselves,
molecular systems can also become more ordered spontaneously. But
this results in higher entropy elsewhere. For example, crystals are highly ordered,
and form all by themselves. They go from a high-entropy state to a low-entropy one,
but they give off heat in the process, which increases overall entropy of the surroundings.
Conversely, when they melt, for example when ice crystals melt, they absorb heat, decreasing the
entropy of the environment, but increasing the entropy of the molecules by making them more
disordered. They go from a solid to a liquid. In this way the spontaneous interaction of
the molecules present in the early earth's atmosphere formed more complex organic molecules.
Any energy needed to drive the formation of these molecules came from either the sun’s radiation,
electrical discharges in the form of lightening, or heat from the cooling earth.
Such organic molecules have not only been found on earth but also in interstellar gases and
throughout the galaxy. So their formation is not unusual. These are the precursors
to more complex biological molecules. And similarly, the second law need not
be violated if such complex biological molecules combine to form the primordial
structures required for the precursors of life. Again, the full mechanism of how this happened
has not been determined yet. But I hope I have given you a sense of how entropy can
drive complexity and order in an environment where the total entropy is always increasing.
In fact there is a school of thought that low entropy from stars like the sun is in some ways
actually driving the universe towards life. Why? Because living things are very good at
causing disorder around them. They increase overall entropy of the earth, for example.
Taking this idea further, American Physicist Jeremy England has developed a hypothesis
of the physics of the origins of life, called "dissipation-driven adaptation."
He theorizes that random groups of molecules can self-organize to more
efficiently absorb and dissipate heat from the environment. His hypothesis states that
such self-organizing systems are an inherent part of the physical world.
He has developed a mathematical formula, based on established physics, showing that when a group of
molecules is driven by an external energy source, like the sun, and surrounded by a heat bath,
like the earth’s ocean or atmosphere, it will tend to restructure itself to dissipate more energy.
In other words, under certain conditions like that of earth,
matter self-organizes in a way that it begins to have some of the traits associated with life.
He uses the analogy of how a whirlpool spontaneously forms in a fluid in order
to dissipate the energy in the fluid. Tornadoes form this way. He says
similar spontaneous restructuring of molecules can result in order to
better dissipate energy from the environment. He says that he can show from his formula that
the more likely evolutionary outcomes are going to be the ones that absorbed and dissipated
more energy from the environment. He even argues that reproduction,
the main process that drives the evolution of life on Earth, is a mechanism by which a system
might dissipate an increasing amount of energy over time. He says a great way to dissipate more
is to make more copies of yourself. I want to reiterate that a lot of
work remains to prove this theory, but it’s intriguing because if it’s correct,
it would offer a direct link between the laws of thermodynamics, particularly entropy,
and the formation of life. It would show that the underlying principle driving the
whole process of life is essentially energy absorption and dissipation.
We know for example that a plant, is much better at capturing and dissipating solar
energy through itself than a random bunch of carbon-based molecules that it is made of.
And we also know that animals are good at creating higher entropy from
lower entropy plant food that they eat. Maybe the history of life on the Earth
and the history of evolution as a whole, can be viewed as the history of a perhaps one
lonely planet that learned to capture solar energy in the most efficient way possible,
through the creation of complex living things in order to increase the entropy of the universe.
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