FROM ONE LIGHT ECHO TO A COLLISION
Relativity as measurement: clocks, light signals and four-momentum
Every claim of special relativity can be checked with a clock, a light pulse and a count. Start by dating a distant tick, follow a twin who turns around, then weigh a collision that every frame describes differently.
Clocks and signals · Chapters 1–5
What one observer can measure
Separate what you see from when it happened, set distant clocks with an echo, then let a twin travel and count the signals each one receives.
Start with one flash →Acceleration and collisions · Chapters 6–8
What every frame agrees on
Give an accelerating observer one time for each event, weigh collisions by invariant mass, then check a research paper’s figures yourself.
Start with radar time →Eight chapters, with examples and experiments
01 L1
Events, light signals and the delay in seeing
Why do we see an event later than it happens?
Read and try →02 L1
Setting distant clocks with a light echo
How can two distant clocks be set to the same time?
Read and try →03 L2
Why moving observers disagree about “at the same time”
Why do observers in relative motion disagree about which distant events are simultaneous?
Read and try →04 L2
Proper time is what a moving clock records
What does a moving clock measure, and why does it fall behind?
Read and try →05 L2
The twin trip, told by the signals each twin receives
If each twin sends a signal every year, how many does the other receive?
Read and try →06 L3
Radar time for an observer who accelerates
How can an accelerating observer give each distant event one time?
Read and try →07 L3
Four-momentum and the mass every frame agrees on
What does every frame agree on in a collision?
Read and try →08 L4
Reading Dolby and Gull on radar time and the twins
How do you check Dolby and Gull’s radar-time argument yourself?
Read and try →