Relativity of simultaneity
Two events that happen at the same moment according to one observer can happen at different moments according to another observer moving relative to the first. This is not a problem with clocks or signal delays; it follows directly from the speed of light being the same for everyone. There is no universal present moment that all observers share, only each observer's own slicing of spacetime into "now". Observers still agree on the order of any two events that could influence each other, so cause never comes after effect.
Why it matters
It is the point where relativity stops being a correction to classical physics and starts contradicting ordinary intuition about time.
Also written: simultaneity
Where it comes up
Read about Relativity of simultaneity on Wikipedia
Sources
- Relativity of simultaneity English Wikipedia
- Conventionality of Simultaneity Stanford Encyclopedia of PhilosophyOn which parts of simultaneity are physical fact and which are convention.
- Time dilation English Wikipedia
