Key idea: The second law of thermodynamics only forbids total entropy from falling in an isolated system, and Earth is not isolated: sunlight flows in, heat flows out, and that flow pays for every snowflake, seedling, and growing child.
Why people ask this question
One argument against evolution sounds like it comes straight from physics. It goes like this. The second law of thermodynamics says that things always become more disordered over time. Evolution says living things became more complex and organized over time. So evolution must break a law of physics.
The argument is worth taking seriously, because the second law is one of the most reliable rules in all of science. But the argument quotes the law wrongly. Once you state the law the way physicists actually use it, the problem disappears.
What the second law really says
The word “entropy” was coined by the German physicist Rudolf Clausius in 1865. In the same work he summed up the idea in a famous line: the entropy of the universe tends toward a maximum.
Entropy is often described as “disorder”. That is a handy shortcut, but a more accurate picture is this: entropy measures how spread out energy is, or how many different ways the parts of a system can be arranged. Energy naturally spreads out. A hot cup of tea cools down. A drop of ink spreads through a glass of water. You never see the reverse happen by itself.
The key detail is where the law applies. Physicists sort systems into three kinds:
| Kind of system | What can cross its boundary | Example |
|---|---|---|
| Isolated | Nothing: no energy, no matter | A perfectly sealed and insulated box (an ideal that is only ever approximated) |
| Closed | Energy, but not matter | A sealed jar of water warming in the sunlight |
| Open | Both energy and matter | A plant, an animal, a city, a planet’s surface |
The second law says that the total entropy of an isolated system never goes down. It does not say that entropy can never go down anywhere. In an open or closed system, entropy can fall in one place, as long as it rises by a larger amount somewhere else.
You can see this every day. A refrigerator makes its inside colder and more ordered. It does not break the second law, because it is plugged into the wall and pumps even more heat out of the back into your kitchen. Unplug it, and the inside slowly warms up.
Earth is not a sealed box
Now ask the important question. Is Earth an isolated system?
No. Earth bathes in sunlight every second. According to NASA, the energy Earth receives from the Sun is balanced by an equal amount of energy radiating back into space as infrared heat. Energy in, energy out.
But the energy that leaves is not the same kind as the energy that arrives. Sunlight comes from the Sun’s surface, which NASA puts at about 5,500 °C. The heat Earth sends to space leaves at a much lower temperature. Seen from space, NASA says, Earth’s effective temperature is about -20 °C.
That difference matters. The same amount of energy carries far more entropy when it leaves as low-temperature heat than when it arrives as high-temperature sunlight. So every day, Earth takes in energy in a concentrated, useful form and throws it away in a spread-out, less useful form. In the process, Earth sends a huge amount of entropy out into space.
That steady flow is like the electricity running the refrigerator. It leaves plenty of room for order to build up in some places on Earth, while the total entropy of the Sun, Earth, and space keeps rising.
Everyday examples of order increasing
If local order could never increase, a lot of ordinary things would be impossible. Here are three.
Snowflakes
In 1611, the astronomer Johannes Kepler wrote a short essay called The Six-Cornered Snowflake. He wanted to know why snow crystals are always six-sided. In January 1885, a young farmer named Wilson Bentley, of Jericho, Vermont, became the first person to photograph a single snow crystal. Over his life he photographed thousands of them.
A snowflake is beautifully ordered. Water molecules, once moving freely in the air as vapour, lock into a neat six-sided pattern. Does that break the second law? No. When water freezes, it releases heat into the surrounding air. That heat raises the entropy of the air by more than the crystal lowers its own. The total still goes up. Nobody designs each snowflake. Ordinary physics, with heat flowing out, produces the pattern.
A growing baby
Every person started as a single fertilized egg cell. About nine months later, that cell has become a baby made of trillions of cells, arranged into a beating heart, working lungs, and a brain.
That is an enormous increase in local order. It happens millions of times a year, and nobody thinks it breaks physics. The reason is simple. The growing baby is an open system. It takes in energy and building materials from food, through its mother, and it gives off heat and waste. The order inside the body is paid for by a larger increase in entropy outside it.
If the second law really meant “order can never increase”, then the growth of every child would be impossible. An argument that proves too much proves nothing.
A seed becoming a tree
A plant takes in carbon dioxide from the air and water from the soil. Using sunlight, it builds those simple molecules into sugars, wood, leaves, and seeds. That process is called photosynthesis. It turns spread-out ingredients into highly organized structures, and it runs directly on energy from the Sun. Nearly every food chain on Earth starts here.
So how does evolution fit?
Evolution happens to living things, and living things are open systems. Plants capture sunlight. Animals eat plants, or eat animals that ate plants. Every living thing gives off heat as it lives.
Over many generations, natural selection keeps inherited variations that help organisms survive and reproduce. New variation comes from mutation. Neither process needs entropy to fall overall. They only need living things to keep using energy, which they always do.
In 2008, the physicist Daniel Styer of Oberlin College did a rough calculation in the American Journal of Physics. He estimated how much the entropy of living things would need to fall to account for evolution, and compared that with the entropy Earth sends to space because of sunlight. The entropy decrease needed for evolution turned out to be tiny in comparison. His conclusion was that there is no conflict between evolution and the second law.
”But raw energy destroys things”
A careful reader might push back. Sunlight fades paint and cracks car tyres. Energy alone does not build anything. Doesn’t that put the problem back?
This is a fair point, and it is half right. Energy alone is not enough. You also need a way to capture and use it. A fire releases energy but builds nothing. A plant, with its photosynthesis machinery, uses the same sunlight to grow.
That is why evolution is not just “energy plus time”. It is energy plus a process: living things that reproduce, offspring that vary, and an environment that filters which variants spread. Those are the ingredients described in what is evolution. The energy supply makes the process physically possible. Reproduction, variation, and selection do the building.
So the second law is not the obstacle. The real scientific questions are about mechanism, and those are answered by biology, not by a misreading of physics.
What about the origin of life?
Sometimes the thermodynamics argument is aimed at the very first living cell rather than at evolution. That is a separate question. The same physics applies, since early Earth also had sunlight, volcanic heat, and chemical energy. But how the first self-copying system arose is still an open research question, covered on its own page: does evolution explain the origin of life?
The best one-sentence answer
If you want a careful short answer, use this:
“The second law says total entropy cannot fall in an isolated system, but Earth constantly takes in sunlight and gives off heat, so living things can build order while total entropy still rises.”
That sentence matches how physicists actually state the law.
Where to go next
To see how order builds up in populations, read natural selection. For the basic ingredients of evolution, read what is evolution. For more objections like this one, go back to common misconceptions.