100 Physics Ideas in 100 Words

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Quantum Mechanics

Up to the turn of the 20th century, classical physics had increasing success in explaining the world around us. Some physicists even declared that they understood everything and that applying the known laws would solve all outstanding problems. But once their fellow scientists developed techniques to probe matter on the atomic scale, they saw that at that level the known laws were inadequate. Quantum physics was born.

The word “quantum” means a quantity or amount. The idea in quantum physics is that when two objects interact, energy is parcelled up in fixed quantities. In-between quantities are not allowed. Classical physics says that any value of energy is possible; quantum physics says no. It’s a bit like ...