Thinking with light cones

Alice can only notice happenings in her light cone: this is her space-time, used to give the where and when of such happenings (known more formally as events). Alice inhabits her cone and only her cone.

That’s true for every imaginable Alice: no-one can step outside their light cone. Her knowledge of events is restricted to the light cone by the universal speed.

Alice, the observer, may record, as well as notice. But she can only do so in her space-time - she cannot step outside of it. Bob could do likewise, but only in his space-time: he is another local witness. So could Charlie. There are only local witnesses. Combine this constraint, which results from the universal speed, with the requirement that physics is universal(so the same for every witness), and you’re well on the way to understanding relativity. The last step is to figure out how Alice's records will appear to Charlie or to Bob, who aree watching from afar.

PoV practice with cones

Exploring the limits to what can be known and affected by each local witness is usefully done with a set of 3D printed cones. Direct physical manipulation complements verbal descriptions to explore what different local witnesses would notice, both within their cones and how inhabitants of another cone might report what they notice and record.

Tools for recording

The universal speed converts distance to time reliably — it’s called universal for a reason. You can record durations with a clock — and it turns out that you can also measure distances with a clock(hardware stores stock laser rulers, so these measuring devices are near-everyday).

So what’s needed is the best possible clock.