Modern · Module 10
One postulate — the speed of light is the same for everyone — and absolute time collapses. Nothing here requires mathematics beyond algebra, which makes the strangeness harder to dismiss.
By the end of this module
You can reason correctly about simultaneity, and you understand what E = mc² is actually claiming.
What do I have to give up to keep the speed of light constant?
Albert Einstein
Readable with high-school algebra. No references, no citations — just an argument.
Whose clock is really running slow?
Veritasium
The one-way speed of light problem — a genuinely unsettling foundational point most courses skip.
What quantity do all observers agree on?
Leonard Susskind, Stanford
What is E = mc² actually saying?
How do I write physics so the frame never appears?
Check yourself
Every answer comes with the reasoning, not just a verdict. Getting one wrong and reading why is the point.
01Two events simultaneous in frame A are, in frame B moving relative to A:
02E = mc² for a stationary object says that:
03The spacetime interval s² = (ct)² − x² is: