Inertial frame
An inertial frame is a point of view from which an object with no forces acting on it moves in a straight line at a constant speed. A train carriage rolling smoothly is close to one; a carriage going round a bend is not, because objects inside it slide without anyone pushing them. All inertial frames are equivalent in the sense that the laws of physics take the same form in every one of them, a principle Galileo stated long before Einstein used it. The concept is what makes the phrase "at rest" meaningless on its own, since rest is only ever rest relative to some chosen frame.
Why it matters
Almost every confusing result in relativity dissolves once you ask which frame the measurement was made in, so this is the term to get right before going further.
Also written: inertial frames, inertial frame of reference, reference frame
Where it comes up
Read about Inertial frame on Wikipedia
Sources
- Inertial frame of reference English Wikipedia
- Galilean invariance English Wikipedia
- Inertial Frames Stanford Encyclopedia of Philosophy
