Observer (special relativity)

In special relativity, an observer is a frame of reference from which a set of objects or events are being measured. This usually means an inertial reference frame, also called an "inertial observer." Sometimes, it can refer to a more complex type of reference frame, like a Rindler frame, which is known as an "accelerating observer."

The way physicists use the word "observer" in special relativity is quite different from everyday language. A reference frame in this context covers a large area of space and time, so it doesn't make sense to say the observer has a single location. Also, an inertial observer cannot start moving at a constant speed later, and an accelerating observer cannot stop accelerating.

When scientists talk about an observer in special relativity, they are not necessarily thinking of a real person watching events. Instead, they are referring to a specific way of measuring and describing objects and events using mathematics. The interesting effects predicted by special relativity happen whether or not there is anyone actually there to see them.

History

Einstein used the word "observer" in his 1905 paper on special relativity to describe someone measuring events, like a person watching from a moving train. He talked about observers using tools like clocks and rulers to track what happens around them.

Later writers often use "observer" to mean the whole system of tools and coordinates used for measuring, not just the person. Some still keep Einstein's idea, focusing on the observer's movement and choices for describing events. Others suggest using teams of observers working together to share information.

"Observer" as a form of relative coordinates

In everyday language, we often describe where things are using words like "left" or "right," which depend on who is looking or where they stand. For example, something might be to your left when facing a door.

In physics, especially in special relativity, the term "observer" works in a similar way. It refers to a way of looking at the world from a certain point of view, called a reference frame. This isn't about a single person but about a whole frame of reference, like being on a ship with its own directions such as bow, aft, port, and starboard. An observer in special relativity helps us understand events from that frame, not from one specific spot.

Main article: Ozma Problem

Usage in other scientific disciplines

Further information: Observer effect (physics)

The word "observer" means different things in various sciences. In quantum mechanics and information theory, it relates to how we measure and understand things, like in the famous thought experiment Schrödinger's cat. In general relativity, an observer is usually someone or something making simple, local measurements, similar to how we normally use the word. In quantum mechanics, observing something is linked to taking a quantum measurement, and what we can measure is called an observable. These different meanings can sometimes cause confusion.