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Quantum computers, quantum cryptography, and quantum (insert name here) are often in the news these days. Articles about them inevitably refer to entanglement, a property of quantum physics that makes all these magical devices possible.

Einstein called entanglement “spooky action at a distance,” a name that has stuck and become increasingly popular. Beyond just building better quantum computers, understanding and harnessing entanglement is also useful in other ways.

For example, it can be used to make more accurate measurements of gravitational waves, and to better understand the properties of exotic materials. It also subtly shows up in other places: I have been studying how atoms bumping into each other become entangled, to understand how this affects the accuracy of atomic clocks.

But what is entanglement? Is there some way to understand this “spooky” phenomenon? I will try to explain it by bringing together two notions from physics: conservation laws and quantum superpositions.

Conservation laws

Conservation laws are some of the deepest and most pervasive concepts in all of physics. The law of conservation of energy states that the total amount of energy in an isolated system remains fixed (although it can be converted from electrical energy to mechanical energy to heat, and so on). This law underlies the workings of all of our machines, whether they are steam engines or electric cars. Conservation laws are a kind of accounting statement: you can exchange bits of energy around, but the total amount has to stay the same.

Conservation of momentum (momentum being mass times velocity) is the reason why, when two ice skaters with different masses push off from each other, the lighter one moves away faster than the heavier. This law also underlies the famous dictum that “every action has an equal and opposite reaction.” Conservation of angular momentum is why — going back to ice skaters again — a whirling figure skater can spin faster by drawing her arms closer to her body.