A guide
What is a worldline?
A worldline is the path an object traces through spacetime. In physics it is a diagram of motion. In Worldline Identity Theory it becomes something more: the complete four-dimensional structure of a conscious life, the object that a person is.
01The basic idea
A worldline is the path of an object through spacetime. It records every event that the object participates in: where it was, when it was there, and what happened to it along the way.
In a spacetime diagram, time is usually drawn vertically and space horizontally. A stationary object appears as a vertical line. A moving object is tilted. The faster something moves, the more tilted its worldline. Light travels at the maximum possible speed, so its worldline is at the steepest possible angle.
The worldline is not just a trajectory. It is the complete history of an object. In relativity, it is the fundamental way to represent an object, because it does not depend on a particular choice of 'now'.
02From worldline to four-dimensional object
In three-dimensional space, a worldline is a one-dimensional curve through four-dimensional spacetime. But the object itself is not a curve. A human being is a four-dimensional object: a kind of filled-in tube, with spatial extent at every moment and temporal extent from birth to death.
This four-dimensional object is what you are, on the block universe view. Your worldline is not a line drawn in empty space; it is the spacetime boundary and interior of your whole life. The shape of that object is what the theory takes as the basis of personal identity.
The spatial cross-sections of that object are the three-dimensional bodies you see in the mirror. But each cross-section is just a slice. The person is the whole four-dimensional structure.
03How to picture it
A worldline is easy to draw in two dimensions: one axis for time, one for space. Real spacetime has three dimensions of space and one of time, so a true picture is impossible. But the two-dimensional diagram is not a distortion of the idea; it is a projection of it.
Think of a photograph as a two-dimensional slice of a three-dimensional scene. The photograph is real, but it is not the scene. In the same way, a present moment is a three-dimensional slice of your four-dimensional life. It is real, but it is not the life.
The sandbox on the home page lets you rotate a four-dimensional object and see its three-dimensional cross-sections. This is not just a visual effect; it is the same operation that a physicist performs when describing a slice of spacetime relative to a given observer.
04From physics to identity
Once you see a person as a four-dimensional object, the classic questions of personal identity change shape. What makes the child and the adult the same person is no longer a mysterious thread connecting two moments. It is the continuity and structure of one four-dimensional object.
Worldline Identity Theory says that this object is the person. There is no soul, no Cartesian ego, no hidden subject behind the experiences. The complete conscious worldline is the subject.
This is not a theory of what consciousness is or how it arises. It is a theory of what the subject of a conscious life is, given that conscious lives exist. The answer is the worldline.
05Common questions
- What is a worldline in physics?
- A worldline is the path of an object through spacetime. It traces every event that the object occupies: its birth, its movement, and its death. In a spacetime diagram, a stationary object is a vertical line, while a moving object is tilted.
- Is a worldline a physical object?
- It depends on your interpretation. In the block universe, the worldline is the fundamental object: a four-dimensional structure that exists as a whole. On presentist views, it is a collection of past, present and future locations, only the present part of which is real.
- Why are worldlines useful?
- They make causation, relativity, and structure visible. A worldline shows the complete history of an object in one diagram, which is exactly what relativity needs, since simultaneity is not observer-independent.
- What is a light cone?
- A light cone is the set of all spacetime points that a flash of light could reach from a given event, or that could reach it. It divides spacetime into the past, future, and elsewhere. The elsewhere contains events that cannot causally affect or be affected by the event.
- How does Worldline Identity Theory use the idea?
- It extends the physical concept into metaphysics. A person is not a momentary subject moving through time. A person is the complete conscious worldline: the entire spacetime structure of a conscious life, taken as one object.