The supreme task of the physicist is to arrive at those universal elementary laws from which the cosmos can be built up by pure deduction.— Albert Einstein, 1918
The Idea in Plain English
One Postulate Is Enough
Einstein's first postulate — "the laws of physics are the same in all inertial frames" — is a deep symmetry principle. His second postulate — "light travels at the same speed for everyone" — is an observation about a specific thing. This paper proves the second one follows automatically from the first.
A Single Number Sorts All Possible Universes
When you demand that the laws of physics look the same from any moving frame, you get a family of possible universes, each labelled by a single number κ. The algebra then sorts them. Two possibilities are eliminated on purely mathematical grounds. Only one works.
The Algebra Demands a Speed Limit
In the one surviving universe (κ > 0), the transformation equations set their own internal speed scale: V = 1/√κ. No one chose this number. It emerged from the symmetry requirements alone. You need experiment only to measure what this speed is in practice — not to prove it must exist.
Five Chapters
The full argument, made accessible.
The Assumption That Wasn't Needed
Einstein built Special Relativity on two postulates. Only one was necessary. Discover which one was redundant — and why.
Three Possible Universes
A single postulate generates a family of possible universes. A single number — κ — separates them. Two are eliminated. One survives.
The Killing Form
An 1888 mathematical tool becomes the key to proving which universe we live in. Seventeen years before Einstein, Wilhelm Killing built the instrument that unlocks the proof.
What Experiment Gives Us
The algebra demands that a universal speed exists. Experiment measures it. These are two very different things — and confusing them cost physics a century.
The Interactive Lab
Turn the κ dial yourself. Watch causality appear. Slide the velocity and see time dilate. No physics degree required.
"Experiment is needed to measure that speed, not to establish its existence. Einstein needed one postulate, not two."
— Emad Mostaque, One Postulate, 2026