--- title: "The Postulates of Special Relativity and the Loss of Simultaneity" description: "All inertial frames share the same physical laws and vacuum light speed. Frames in relative motion can disagree about the simultaneity of separated events. Events connected by a causal signal keep the" 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: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # The Postulates of Special Relativity and the Loss of Simultaneity All inertial frames share the same physical laws and vacuum light speed. Frames in relative motion can disagree about the simultaneity of separated events. Events connected by a causal signal keep their order. 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 An inertial frame does not accelerate or rotate. Free particles, with no net force, remain at rest or move at constant velocity. Special relativity has two postulates: the physical laws are the same in all inertial frames, and vacuum light travels at speed c in every direction in each inertial frame, independently of source motion. Each frame uses clocks at rest in that frame, synchronized by assigning equal times to outward and return light travel. An event's time is its local clock reading, not when distant light reaches your eye. Correct signal-travel delays before comparing events. If flashes at unequal distances send light to a stationary detector together, the farther flash was emitted earlier. **Example.** An ideal train moves at constant speed toward its front. Its midpoint emits light in both directions through vacuum. Train clocks record both doors being lit together: the stationary doors are equally far away and both pulses travel at c. Platform clocks record the rear door being lit first. In that frame, the rear moves toward its pulse and the front moves away. These are door illumination times, not signal-arrival times at someone's eyes. *(drawing: The cells compare event times in the two frames. They do not draw the train or the light paths.)* Clocks synchronized at different positions on the train are not synchronized according to platform coordinates. The reverse also holds. Different inertial frames can therefore assign different times to separated events even when all clocks work correctly. There is no universal simultaneity shared by every inertial frame. Ordinary forward velocity addition would give a vacuum pulse platform speed c+v when its source moves at speed v. For v=0.1c, that rule gives 1.1c. This conflicts with the second postulate, which gives c. Ordinary addition approximates low-speed motion of material objects, but it does not transform vacuum light speed correctly. Order can reverse only for spacelike-separated events: light could not cross their separation within their time gap. Events simultaneous at different positions are spacelike; motion along their separation changes their assigned times. If a signal travelling at c or less connects one event to the other, every inertial frame agrees on their causal order. An experiment confined to a sealed inertial lab cannot identify absolute rest. Every inertial lab uses the same physical laws. Measuring vacuum light speed c therefore does not select a preferred frame. Comparing the lab with an outside object can still measure their relative motion. **Recap.** Inertial frames share physical laws and vacuum light speed c. They can disagree on separated events' simultaneity, but they agree on causal signal order. *Lesson, questions and study cards written for this learning dataset. OpenStax relativity chapters were consulted for facts.* ## Practice 8 questions on this page, each with its working shown. ## Optional connections - [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)