20th century

=== Physics ===
* New areas of [[physics]], like [[special relativity]], [[general relativity]], and [[quantum mechanics]], were developed during the first half of the century. In the process, the internal structure of [[atom]]s came to be clearly understood, followed by the discovery of [[elementary particle]]s.
* It was found that all the known [[force]]s can be traced to only four [[fundamental interaction]]s. It was discovered further that two forces, [[electromagnetism]] and [[weak interaction]], can be merged in the [[electroweak interaction]], leaving only three different fundamental interactions.
* Discovery of [[nuclear reaction]]s, in particular [[nuclear fusion]], finally revealed the source of [[sun|solar]] energy.
* [[Radiocarbon dating]] was invented, and became a powerful technique for determining the age of [[Prehistory|prehistoric]] animals and plants as well as historical objects.