The laws of thermodynamics are three principles describing the way energy moves within a system and is transferred between groups of objects.
The laws have their origins in studies of heat and machines in centuries past, but have since come to be fundamental statements on how the Universe works at a basic level.
There's no special order to the trio of principles, but today, they are generally listed as follows:
What is the first law of thermodynamics?
This one is often summarised as 'energy can't be created or destroyed, it can only change form', and it describes the conservation of energy. There is a little more to this rule, though.
The first law refers only to closed systems, where there's no energy being added or subtracted. Our entire Universe, for example, is a closed system where the energy it started with is conserved in some form or other.
What is the second law of thermodynamics?
The Universe's sum of energy might be endless and eternal according to the first law, but according to the second law its transformation is a one-way street into boringville.
Energy is effectively a description of change in a system. Some forms of energy can easily do what we call work; making things move, creating a variety of changes, preserving or even increasing some sort of structure or order. Sunlight's powerful radiation, for example, can change simple compounds into a diversity of living things. Without light, life would die, wither, and eventually crumble into cold dust.
But all transfers of energy create random fluctuations as jiggles we refer to as heat. Thanks to this unavoidable touch of randomness – or what's known as entropy – a closed system will always creep towards a uniformly disordered state.
You can shuffle in more energy to recreate some unique bits of order, but remember, the Universe isn't getting a fresh supply of useful energy, and in the end, everything will be the same old, useless heat filtering out into the icy depths of expanding space.
This inevitable slide into a chaotic state is the increase of entropy. Maximum entropy would occur when all the energy in the Universe is uniformly spread out. This is known as heat death (or, if you're a dramatic type, the Big Chill) and could be the ultimate fate of the Universe.
What is the third law of thermodynamics?
It's the law nobody talks much about, but is a pretty profound one, setting a hard edge to the limits of energy by connecting together rules on matter, heat, and order.
In technical terms, it says the entropy of a perfectly arranged lattice of atoms at zero degrees Kelvin is zero. But this hypothetical cold crystal can only ever exist in our imaginations, as such an absolute absence of heat is an impossibility in the real world.