How fast does electromagnetic radiation travel through a vacuum?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — electromagnetic radiation Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

Electromagnetic radiation, regardless of its type, travels through a vacuum at a constant, universal speed known as the speed of light. This speed is defined as exactly 299,792,458 metres per second (approximately 186,282 miles per second).123

This velocity serves as a fundamental constant of nature and acts as the upper limit for the propagation speed of signals and material particles in the universe. Unlike mechanical waves, such as sound, which require a medium to travel, electromagnetic waves consist of propagating oscillations in electric and magnetic fields that maintain this constant speed even in empty space. Regardless of the motion of the observer or the source of the radiation, measurements will consistently yield this same universal value.2435