I.

Introduction

Ether (physics and astronomy), substance once thought to fill all space, but now known not to exist. Scientists of the late 19th and early 20th centuries believed that the ether was the medium, or substance, that allowed light to travel through space. The theory of relativity of German American physicist Albert Einstein showed that light did not need a medium through which to travel, so belief in the existence of the ether was abandoned. See also Quantum Theory.