The Four Laws at a Glance
| Law | In one sentence | Simple version |
|---|---|---|
| Zeroth | If A is in thermal equilibrium with C, and B is too, then A and B are in equilibrium with each other | This is why thermometers work |
| First | Energy cannot be created or destroyed, only converted | You can't win |
| Second | The total entropy (disorder) of a closed system never decreases | You can't break even |
| Third | Absolute zero (0 K = −273.15 °C) can never be reached | You can't leave the game |
First Law: Conservation of Energy
ΔU = Q − WThe change in a system's internal energy (ΔU) is the heat added to it (Q) minus the work it does (W).
In space: A rocket engine turns chemical energy into heat, and the nozzle turns that heat into the kinetic energy of the exhaust. Stars turn mass into energy (E = mc², see Einstein's Theory of Relativity), and this energy leaves the star as light.
Second Law: Entropy
Heat flows by itself only from hot to cold, never the other way round. Every real process increases the total entropy. No engine can turn heat completely into work. The best possible efficiency is the Carnot efficiency:
η_max = 1 − T_cold / T_hot(temperatures in kelvin)
In space: Spacecraft electronics produce waste heat that must be dumped into cold space. Life on Earth works because Earth takes in "low-entropy" sunlight (a few high-energy photons) and radiates "high-entropy" infrared light (many low-energy photons).
Third Law: Absolute Zero
You can get closer and closer to 0 K, but you can never reach it. Even deep space is not that cold: the leftover radiation from the Big Bang (the cosmic microwave background) keeps it at about 2.725 K.
Heat Transfer in Space
On Earth, heat moves in three ways. In the vacuum of space, almost only one of them works:
| Method | On Earth | In space |
|---|---|---|
| Conduction (through contact) | Yes | Only inside the spacecraft |
| Convection (through air or liquid flow) | Yes | No, because there is no air |
| Radiation (infrared light) | Yes | Yes, the only way to get rid of heat |
Stefan–Boltzmann Law
The power a body radiates rises steeply with temperature:
P = ε · σ · A · T⁴
σ = 5.670 × 10⁻⁸ W/(m²·K⁴)ε is the emissivity (0 to 1) and A is the surface area. If you double the temperature, the radiated power grows 16 times.
This is why the ISS has large white radiator panels, and why the James Webb Space Telescope has a five-layer sunshield that keeps its instruments below about 50 K.
Wien's Law: Why Stars Have Colours
The hotter a body is, the shorter the wavelength at which it shines brightest:
λ_max = 2.898 × 10⁻³ m·K / TFor the Sun (about 5,770 K), λ_max ≈ 500 nm, which is green-blue visible light. Hot stars look blue and cool stars look red, see Types of Stars in the Universe.
How Warm Is a Planet?
A planet's balance temperature, if it had no greenhouse effect:
T = T_star · √(R_star / (2d)) · (1 − A)^(1/4)Here d is the distance to the star and A is the albedo (the share of light that is reflected). For Earth (A ≈ 0.3), this gives about 255 K (−18 °C). The real average is about 15 °C. The difference of about 33 °C comes from the greenhouse effect.
Thermodynamics of Stars
- Hydrostatic equilibrium: In a star, the outward pressure of hot gas balances the inward pull of gravity.
- Strange behaviour: Bodies held together by gravity have a negative heat capacity. When a star loses energy, it contracts and gets hotter. This acts like a built-in thermostat that keeps stars stable for billions of years.
- Solar constant: At Earth's distance, about 1,361 W/m² of sunlight arrives.
The Fate of the Universe: Heat Death
If entropy keeps rising, the universe will one day reach the same temperature everywhere. Stars will have burned out, and no energy differences will be left to drive any process. This hypothetical end state is called heat death. It lies unimaginably far in the future (well beyond 10¹⁰⁰ years).