# Explain why two observers in different inertial frames of reference will disagree on whether two spatially separated events are simultaneous, even if both observers correctly apply the laws of physics.

> IB DP Physics Higher Level (2023 syllabus) — A.5 Galilean and special relativity (HL only) · Explain · 4 marks

> A rocket travels at a speed of 0.8c relative to a space station. A light signal is emitted from the midpoint between two detectors that are at rest relative to the rocket. The signal reaches both detectors at the same time according to an observer on the rocket.

## Mark scheme (4 marks)

1. The postulate of special relativity states that the speed of light is the same (c) in all inertial frames of reference.
2. From the space station frame, the two detectors are moving (toward or away from the oncoming light), so light must travel different distances to reach each detector.
3. Since light still travels at c in the station frame but covers unequal distances to each detector, it arrives at the two detectors at different times according to the station observer.
4. This is a genuine (physical) disagreement: neither observer is wrong; simultaneity is relative and depends on the inertial frame, a direct consequence of the invariance of c.

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-two-observers-in-different-f50046ef) · Published by Druglandscape Ltd.