system
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human
ELI5 Bose-Einstein Condensate

What is it?
gpt
All particles have a property called spin. It is a fundamental property of particles, just like mass. This spin is either integer or half-integer. Particles with integer spin are called Bosons, while particles with half-integer spin are called Fermions. Electrons, protons and neutrons are all Fermions with spin 1/2. Composite systems like atoms can also be either Fermions or Bosons, depending on how many  electrons, protons and neutrons they have. 

Fermions and Bosons behave very differently: 

Think of the energy levels of a quantum system as a ladder, where each rung represents a different quantum state. If you want to distribute a given number of Fermions on that ladder, then you cannot put more than one Fermion on one rung.  This is the so called Pauli-exclusion principle.

If, on the other hand, you want to distribute Bosons on that ladder then you can put as many Bosons on each rung as you like, since the Pauli-exclusion principle concerns only Fermions. 

Now, let's suppose you have some number (say 1000) of Bosons and you have distributed them on the ladder in some way (83 on the first rung, 78 on the second, 63 on the third etc). When you cool the system down enough then something strange will happen: all particles will start to amass on the lowest rung. While the fraction of particles in the lowest rung is at the beginning of the decrease of temperature relatively low, when the temperature goes to zero almost all of your 1000 particles will be in the lowest rung, i.e. lowest state. At T=0, all 1000 particles are in the lowest state.

This process, where Bosons tend to collect themselves in the lowest energy state with decreasing temperature, is called Bose-Einstein *condensation*. It's called *condensation* because it's akin to real condensation: when you cool a gas in a closed box it will condensate on the surfaces, when you cool a gas of Bosons it will condensate into the lowest energy state.