Spin is a fundamental physical quantity that describes a specific type of angular momentum. Take the motion of the Earth as an example, the spin of the Earth characterizes the angular momentum ...

Nothing can exist without energy. Nothing is possible without energy. Despite being invincible and intangible energy can be stored. Such a fundamental physical quantity has no rigorous definition.

The universe has a number of bizarre properties that are difficult to understand with everyday experience. For example, the matter we know, consisting of elementary and composite particles building ...

If there’s one thing physicists can’t abide, it’s ambiguity. Precision is everything. But the definition of one fundamental physical quantity—the ampere, the unit of electric current—falls somewhat ...