Observer (special relativity)

Observer (in special relativity) means a person or thing that watches or notices something. In special relativity, observers have to consider how different people moving at different speeds will view the same thing. For example, if two people are moving in different directions at different speeds, they will both see each other moving but the person who is moving faster will see the person who is moving slower as moving slower than they actually are. That's because time passes more slowly for the person who is moving faster, meaning that things seem to move more slowly for them.