Door 1 removed a comfortable assumption:
There is no unique universe-wide present.
That immediately creates a problem. If the universe is not a three-dimensional stage that updates everywhere at the same cosmic instant, what is the more accurate picture? Relativity gives us spacetime.
1. Start with an event, not a moment
Physics can describe something happening by specifying:
- where it happens
- when it happens
That combination is a spacetime event. For example:
That is one event. A second event might be:
In ordinary thinking we tend to imagine space as the container and time as a separate clock running outside it. Relativity instead treats the spatial and temporal coordinates as parts of one four-dimensional description: x, y, z, t.
The exact coordinate values depend on the observer's reference frame. The event itself does not.
Observers may disagree about the coordinates assigned to an event without disagreeing that the event occurred.
2. An object traces a worldline
Now imagine following you through your entire physical life. At each instant, your body occupies some event in spacetime. Connect those events and you get a worldline.
A person standing still relative to a chosen spatial coordinate system has a nearly vertical worldline on a simple spacetime diagram. A spaceship moving through space has a tilted worldline. An accelerating spaceship has a curved worldline.
The important point is:
A worldline is not a set of possible paths. It is the physical history of an object through spacetime.
Later we will introduce branching possibilities. Do not confuse those with worldlines.
3. Different observers use different coordinates
Suppose Earth and a passing spacecraft observe the same sequence of events. They can disagree about spatial distances, elapsed coordinate times, and which distant events are simultaneous, but they can still refer to the same physical events.
This is analogous to two map systems assigning different coordinates to the same landmark. The coordinate labels differ. The landmark does not disappear.
Relativity is more subtle than ordinary map projection because the observers can also disagree about elapsed time and simultaneity, but the conceptual lesson is useful:
Coordinates describe relations from a frame. They are not the events themselves.
4. What remains objective?
Door 1 may have made relativity sound almost anarchic. It is not. Relativity preserves powerful observer-independent structure.
For example, observers agree about whether two sufficiently separated events can be causally connected. They also agree on the spacetime interval between events, even though they may split that interval differently into space and time coordinates. That invariant structure is what lets relativity remain physics rather than perspective alone.
We will use it in Door 3.
5. Your own clock still matters
Along your worldline, a clock traveling with you measures proper time. This is the time accumulated by the physical system following that particular trajectory. Different paths through spacetime can accumulate different amounts of proper time.
That is why the traveling twin and the Earthbound twin in the famous twin scenario can physically age by different amounts. We will make this precise in Door 4. For now, hold onto this:
There is no universal master clock, but there are perfectly real local clocks along physical worldlines.
6. The shift in picture
The naive model looks like this:
UNIVERSE AT t1
↓
UNIVERSE AT t2
↓
UNIVERSE AT t3
as though one master clock keeps replacing the entire state of the universe. The spacetime picture asks you to think differently:
spacetime contains events
+
objects trace worldlines
+
observers assign coordinates
+
causal structure relates events
Notice what we have not concluded. We have not said that every event is equally real, the future is fixed, consciousness sees all of spacetime, nothing changes, or time is imaginary.
Those are separate questions. For now, spacetime is simply the geometry that lets relativity describe events without requiring absolute space and absolute time.
7. The question Door 2 leaves us with
If objects trace worldlines through spacetime, are all directions through spacetime physically available? No. The speed of light imposes structure.
From any event, only certain other events can physically influence it or be influenced by it. That structure is the light cone.
Doorway to Door 3
Spacetime tells us how to organize events. Now we need to know which events can physically affect which others. That takes us to Door 3: Light Cones and Causality.