The Most Misunderstood Concept in Physics

What does the Earth actually get from the Sun? While most people answer 'energy,' this leads to a deeper mystery about what energy truly represents.

• The Earth receives a specific amount of energy from the Sun daily • The Earth radiates approximately the same amount of energy back into space • If energy in equals energy out, what is the Earth really receiving from the Sun?

The answer involves a concept that is poorly understood and rarely discussed, despite governing everything from molecular collisions to storms and the direction of time itself.

• Governs molecular collisions and weather patterns • Determines the evolution of the universe from beginning to end • May explain the direction of time and why life exists

In 1813, during the Napoleonic Wars, Sadi Carnot, a 17-year-old student and son of a French general, attempted to join the fight but was devastated when Paris fell after only a day of fighting.

• French steam engine designs were falling behind those of Britain • The best steam engines could only convert about 3% of thermal energy into useful mechanical work • Nations' industrial and military power depended on having superior steam engines

Through conversations with his father, a physicist, Carnot understood that improving heat engine efficiency could restore France's advantage and decided to study the problem scientifically.

Over three years of study, Carnot developed the concept of an ideal heat engine with no friction or losses, which led to fundamental insights about how heat engines work and their maximum possible efficiency.

Carnot's ideal engine consists of two metal bars (one hot, one cold) with a chamber containing air and a piston connected to a flywheel, where heat flows only through the bottom of the chamber.

• Hot bar contacts chamber: air expands with heat input, pushing piston up and turning flywheel • Hot bar removed: air expands further without heat, cooling to match cold bar temperature • Cold bar contacts chamber: piston pushes down, compressing air and transferring heat out • Cold bar removed: flywheel compresses gas, raising temperature back to initial state

The ideal engine is completely reversible; running it in reverse returns the system to its original state with no additional energy input, meaning nothing fundamentally changes during the process.

• Despite being fully reversible, the engine cannot achieve 100% efficiency • Energy efficiency depends only on the temperature difference between hot and cold sides • To reach 100% efficiency would require infinite temperature on hot side or absolute zero on cold side, both physically impossible

Real steam engines in Carnot's time could reach temperatures up to 160 degrees Celsius with theoretical maximum efficiency of 32%, but actual efficiency was only 3% due to friction, heat dissipation, and non-constant heat transfer.

• When energy spreads out over cylinder walls, flywheel axles, and into the environment, it becomes impossible to recover • This spreading process is irreversible • Energy is most usable when concentrated and less usable when dispersed

German physicist Rudolf Clausius developed a way to measure how spread out energy is, calling this quantity entropy, which increases as energy becomes more dispersed and less available for work.

• First law: The energy of the universe is constant • Second law: The entropy of the universe tends toward maximum • In practice: Energy spreads out over time in ways that are impossible to reverse

Most laws of physics work the same forwards and backwards in time, so how does entropy create a clear time dependence where energy always spreads out in one direction?

• Using a model of bars with energy packets randomly hopping between atoms • Heat flowing from cold to hot is not impossible, just extremely improbable • As system size increases (more atoms and energy packets), the probability of hot becoming hotter drops dramatically

With 8 atoms per bar, there's a 10.5% chance of energy concentration increasing; with 80 atoms, only 0.05%; in everyday solids with trillions of atoms, heat flowing cold-to-hot essentially never happens.

There's only one solved state but quintillions of random states, so making random turns inevitably moves toward disorder; similarly, random energy distribution moves toward spreading out.

Air conditioning moves energy from cold (house interior) to hot (outside), seemingly decreasing entropy by concentrating energy, which should be impossible according to the second law.

• Entropy decrease in the house is only possible by increasing entropy more elsewhere • Power plants release concentrated chemical energy from coal • Energy spreads through turbines, generators, wires, and waste heat throughout the system

Whatever entropy decrease is achieved in the house is more than offset by the larger entropy increase required at the power plant and throughout the energy distribution system.

Local entropy can decrease, but only at the cost of greater entropy increase elsewhere; the total entropy of any closed system always increases.

If entropy constantly increases and everything decays and mixes, how can structure exist on Earth? How can hot regions stay separate from cold ones, and how does life survive?

• The Sun provides low entropy energy that is concentrated and bundled • This energy is more useful and more compact than the energy radiated back to space • The difference enables structure and life to exist

• Plants capture solar energy and convert it to sugars • Animals eat plants and use that energy to maintain bodies and move • Bigger animals eat smaller animals, continuing the cascade • At each step, energy becomes more spread out

For each high-energy photon received from the Sun, 20 lower-energy photons are emitted back to space; all activities on Earth (growth, storms, life) occur in the process of converting concentrated photons into dispersed thermal radiation.

Life itself may be a consequence of the second law of thermodynamics, offering a mechanism to accelerate the universe's natural tendency toward maximum entropy.

Seawater produces 30-680% more entropy when cyanobacteria and organic matter are present than when absent, demonstrating life's exceptional ability to convert low entropy to high entropy.

If there is a constant stream of concentrated energy, structures that dissipate that energy are favored, leading over time to increasingly effective energy dissipators and eventually to life itself.

Starting with random atoms and shining light on them for long enough could naturally produce life as an energy dissipation mechanism, as described by Jeremy England's hypothesis.

The total entropy of the universe increases with time, meaning it was lower entropy yesterday, even lower the day before, all the way back to the Big Bang.

Right after the Big Bang, entropy was at its lowest; this explains why entropy increases in one direction but doesn't explain why entropy was initially low.

• The early universe was hot, dense, and almost completely uniform • Everything was mixed together • Temperature varied by at most 0.001% everywhere

When gravity is considered, having matter spread out uniformly is an extremely unlikely state, making it low entropy; as the universe expanded and matter clumped into denser regions, enormous potential energy converted to kinetic energy.

As the universe expanded and cooled, useful energy was converted to heat through collisions, increasing entropy; over time, this process formed stars, planets, galaxies, and life while continuously increasing entropy.

• Early universe: around 10^88 Boltzmann constants of entropy • Current stars: 9.5 × 10^80 (much less than early universe) • Interstellar/intergalactic medium: ~10× more than stars but still less than early universe

In 1972, Jacob Bekenstein proposed that black holes have entropy proportional to their surface area, suggesting black holes represent a major source of entropy in the universe.

• Stephen Hawking proved black holes emit radiation and have temperature, confirming Bekenstein's proposal • The supermassive black hole at the Milky Way center has 10^91 Boltzmann constants of entropy • All black holes combined account for 3 × 10^104, nearly all the universe's entropy

The early universe had only about 0.000000000000003% of the entropy it has now, demonstrating how dramatically entropy has increased since the Big Bang.

• Planetary systems form from dust clouds • Galaxies merge and mix • Asteroids crash and spread • Stars die and disperse energy • Life flourishes and grows • All occur because entropy increases in one direction

We never see asteroids uncrash, planetary systems unmix, or energy reassemble; the clear difference between past and future comes from entropy always increasing, creating an arrow of time.

• In more than 10^100 years, after the last black hole evaporates, energy will be completely spread out • The universe reaches its most probable state • No way to distinguish time moving forwards or backwards • The arrow of time disappears

While entropy can seem like an awful thing leading inevitably to boredom and decay, high entropy doesn't guarantee high complexity and low entropy doesn't guarantee complexity either.

Both completely ordered states (low entropy) and completely disordered states (high entropy) have low complexity; complex structures emerge and thrive in the middle ground between order and disorder.

Pure tea or pure milk is uninteresting; as milk is poured in, beautiful patterns emerge at the boundary between the two; these patterns exist briefly before returning to featureless mixture.

Humanity exists in the middle of the entropy spectrum where complex structures and life thrive; we should use the low entropy available to us while we can before maximum entropy is reached.