A Short Guide to the Language of Relativity
The Grammar of Spacetime
This essay is just a placeholder. I am currently working on the individual entries. Sorry about that!
When we talk about relativity, we often borrow everyday words like time, space, event, frame, now, and give them new, sharper meanings. But if we keep hearing those familiar words in their everyday sense, we can’t follow what the theory is really telling us. The result is that many popular discussions of the “block universe” argument, even among philosophers, end up tangled in misunderstandings that trace back to language.
This short guide sets out the basic vocabulary and ideas that run through the Block Universe Series. It isn’t a physics lesson so much as a translation exercise: a way of seeing how relativity reshapes a few key concepts we thought we already knew. It can be read straight through as a primer of some basic relativity ideas, but it is by no means exhaustive. It’s been structured as a series of short sections, some are mini essays, while others just extended glossary entries. This is to fulfill its main function as an explainer to the main essays on the block universe. Most link to other essays that show how these ideas play out in detail.
Think of this as the grammar of spacetime. The framework that lets us speak clearly about what relativity does (and doesn’t) say about time, simultaneity, and reality.
Event. A point in spacetime (where and when something happens).
Reference frame. A choice of coordinates for referring to places, times, and events.
c. The velocity of light.
Relativity of Simultaneity. Which events are simultaneous is not an absolute fact, but depends upon the reference frame being used to describe them.
World‑slice. The set of events an observer calls ‘simultaneous’ with a chosen local event.
Light cone. The set of events reachable by light signals from a given event (future cone) or which could have sent light to it (past cone).
Elsewhere. The region of spacetime outside the light cones.
Timelike past/future. Events lying inside our past/future light cones, which we can influence or be influenced by.
Framelike past/future. Events lying outside our past/future light cones, with which we can have no causal connection.
Predetermination. The belief that the future is already fixed or “exists in advance.” Unlike determinism, it is a metaphysical claim about fate, not a physical claim about causation.
Spacetime Diagram. Essentially a distance/time graph like the ones we learn in school, but with time going up the page instead of to the right, and with features drawn from relativity theory.