---
title: "The Postulates of Special Relativity and the Loss of Simultaneity"
description: "Taking the laws of physics and the speed of light to be the same for every inertial observer forces two events simultaneous for one observer to be separated in time for another. Nothing else is assume"
canonical: https://lightmysky.com/learn/science/the-postulates-of-special-relativity-and-the-loss-of-simultaneity-mt_kZqVQ18bbc
source: https://lightmysky.com/learn/science/the-postulates-of-special-relativity-and-the-loss-of-simultaneity-mt_kZqVQ18bbc.md
retrieved: 2026-09-12
---

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# The Postulates of Special Relativity and the Loss of Simultaneity

Taking the laws of physics and the speed of light to be the same for every inertial observer forces two events simultaneous for one observer to be separated in time for another. Nothing else is assumed, and everything else follows.

Subject: Science · Area: Quantum & Modern Physics · Ages 18 to 19
Page: https://lightmysky.com/learn/science/the-postulates-of-special-relativity-and-the-loss-of-simultaneity-mt_kZqVQ18bbc

## Ready when they can

- States both postulates and identifies what counts as an inertial frame
- Uses a light-signal argument to show that simultaneity depends on the observer
- Explains why a fixed speed of light is incompatible with adding velocities the ordinary way

## Lesson: Two rules that rebuilt time

Relativity rests on two postulates. The laws of physics read the same in every inertial frame, and light in vacuum travels at the same speed for every inertial observer. An inertial frame drifts at constant velocity with no spin or push, so free particles glide straight.

Flash a light at the middle of a moving train and riders see both ends lit together, yet the platform sees the rear lit first. Each frame finds the other clocks out of sync, so there is no shared now, and widely split events can even flip their order between frames.

**Example.** Ride the train and the light reaches both doors together because you sit still relative to them. Watch from the platform and the rear door rushes toward the flash while the front door flees it, so simultaneity belongs to the observer.

**Tip.** A fixed light speed bans everyday velocity addition, which would hand light a different speed per frame. Newton absolute time is replaced by frame based time, while the laws stay the same everywhere.

**Recap.** Same laws plus same light speed for all drifters means simultaneity depends on the observer.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Relative Motion](https://lightmysky.com/learn/science/relative-motion-mt_-OndzpVsrv)
- [Progressive Waves: Phase and Path Difference](https://lightmysky.com/learn/science/progressive-waves-phase-and-path-difference-mt_RotpycCNF8)

## Opens up

- [Time Dilation and Length Contraction](https://lightmysky.com/learn/science/time-dilation-and-length-contraction-mt_7tpKiFWeG0)