strong

Forge of Heaven

Our understanding of atoms as being made of smaller subatomic particles began with the discovery of the electron in the late 1800s. After we learned that electrons were negatively charged, and that removing electrons from an atom left it positively charged, it was thought that atoms must be held together by electromagnetic forces. There were several proposed models, but one of the most popular was known as the plum pudding model. This proposed that negatively charged electrons were held in a positively-charged atom like plums in a pudding. But in the early 1900s Ernest Rutherford scattered alpha particles off thin layers of gold foil and found that atoms consisted of dense, positively charged nucleus surrounded by electrons. It was soon found that atomic nuclei consisted of positively charged protons as well as neutrons that had no charge. While it was clear that nuclei and electrons were held together by electromagnetic forces, we had no idea what held nuclei together. What could possibly hold protons so close to each other, given the immense repulsive force due to their charges? Read more

electromagnetism

Dance of the Hag

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Electromagnetism can produce a force between charges or magnets, but it is much more than a simple force.
gravity

Cradle to Grave

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We often speak of gravity as a force. More accurately it is a feature of spacetime. Even more accurately, we don't know what it is.
four

The Four Horsemen

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There are four fundamental forces in the universe, and each of them plays a crucial role in astrophysics.
Credit: Arne Schwettmann

Snap To

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The Stern-Gerlach effect is strangely counterintuitive, but we can use it to study the magnetic fields of stars.
Credit: NASA / Jet Propulsion Lab / U.S. Geological Survey

Wrong Way Triton

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Triton is Neptune's largest moon, and the largest moon it the solar system to orbit backwards.