Light Cones
Every event in the universe has a region of past events that could have caused it, and a region of future events it could affect. Outside that region: forever unreachable.
What you're looking at
This is a spacetime diagram. The horizontal axis is space (one dimension of it). The vertical axis is time, going up. A single point on this diagram is an event — a specific place at a specific moment.
The two diagonal lines drawn through the event represent the speed of light. Anything moving slower than light traces a path steeper than these lines. Anything moving at light speed traces a path along them.
The yellow region above is your future light cone — every event you could possibly affect. The yellow region below is your past light cone — every event that could have affected you. Drag the orange dot to move the event.
The unreachable region
Move the third slider. The point that appears outside the cone is in what physicists call elsewhere — it's neither in your past nor your future. It exists, but you cannot causally interact with it. Light signals haven't had time to travel between you. No information can pass either direction.
"All that matters is how long the information takes to get to you."
For an event 100 light-years away, anything happening there now sits in your "elsewhere." It's not in your past — its light hasn't reached you. It's not in your future — you can't affect it. It's a region of reality that's real but causally disconnected from you.
Every observer in the universe has their own light cone, centered on themselves. The universe is a network of overlapping cones. Reality is local. There is no master view.