# quantum tunneling

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A quantum mechanical effect in which particles have a finite probability of crossing an energy barrier, such as the energy needed to break a bond with another particle, even though the particle's energy is less than the energy barrier. Quantum tunneling has no counterpart in classical mechanics, in which a particle can never cross an energy barrier with a higher energy level than the particle has. The emission of alpha rays in radioactive decay is a case of quantum tunneling; though the alpha particles are strongly bound to the nucleus and don't have as much energy as the bond does, they still have a finite probability of escaping the nucleus. The design of transistors and many diodes makes use of this effect. See also radioactivity.

## QUIZZES

#### QUIZ YOURSELF ON PARENTHESES AND BRACKETS APLENTY!

Set some time apart to test your bracket symbol knowledge, and see if you can keep your parentheses, squares, curlies, and angles all straight!

Question 1 of 7

Let’s start with some etymology: What are the origins of the typographical word “bracket”?

First appeared around 1750, and is related to the French word “braguette” for the name of codpiece armor.

First appeared in 1610, based on the French word “baguette” for the long loaf of bread.

First appeared in 1555, and is related to the French word “raquette” for a netted bat.

TAKE THE QUIZ TO FIND OUT ## Words nearby quantum tunneling

quantum statistics, quantum sufficit, quantum system, quantum teleportation, quantum theory, quantum tunneling, Quantz, Quanzhou, Quao, Quaoar, Quapaw

The American Heritage® Science Dictionary
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