What you can see and what you can change
Light takes time to travel. This limits how soon we can learn about a distant event, and how soon anything we do can affect it. A light-cone diagram helps us explore both.
Seeing something after it happens
Sunlight takes about eight minutes to reach Earth. When we look at the Sun, we see it as it was eight minutes ago.
Suppose a solar flare happens at this moment, according to clocks in our frame of reference. We won’t see it for another eight minutes. A signal we send towards the Sun now would also take eight minutes to arrive, so it could not affect the flare’s beginning.
We could observe that event later, and potentially influence something at the Sun later still. But the flare happening there and our actions here, at this moment, are too far apart to affect one another.
Reading a light cone
The diagram places distance across the page and time vertically. Each point represents an event: something happening at a particular place and time. Your position at this moment is the centre.
Light follows diagonal lines. The scales are chosen so that these lines sit at 45°, marking how far a signal can travel in a given time.
They divide the diagram into three regions:
- Your past light cone: events whose influence could have reached you by now.
- Your future light cone: events that a signal sent now could reach in time to influence.
- Outside the cones: events that cannot affect your present moment, or be affected by it, because doing so would require a signal faster than light.
Light arriving directly from a distant source now traces the boundary of your past cone. Events inside that boundary could have reached you earlier.
What changes when you move?
We normally assume that distant events have a single, shared “now”. Relativity requires us to be more precise.
To assign times to distant events, imagine a network of clocks at rest relative to you. You synchronise them using light signals, accounting for the signals’ travel time. Events with matching clock readings count as simultaneous in your frame of reference.
A passing spacecraft can use its own network of clocks. Those clocks are moving relative to yours, and the two networks do not agree about which distant events are simultaneous.
This difference remains after both observers account for the time light takes to arrive. It concerns the times they assign to events, as well as what they see.
In the visualisation, changing the observer’s motion tilts the line labelled “now”. The solar flare can fall before or after that line, depending on the observer.
However, an event that could have caused your present situation remains in your past for both observers. An event you could influence remains in your future. Changing the frame of reference preserves these possible connections between cause and effect.
Explore the three events
The visualisation includes a probe firing its engine, a rover preparing to drill, and the solar flare.
The probe’s firing is already in your past. The rover’s drilling is far enough in your future that a cancellation signal could arrive beforehand. The flare lies outside both cones relative to your present moment.
Change the observer’s speed and watch the “now” line move across the flare. Notice that the probe and rover retain their positions in the past and future cones.
This diagram uses flat spacetime, without gravity or cosmic expansion.
Seeing and reaching across the universe
On much larger scales, the expansion of the universe adds another limitation.
We can receive ancient light from galaxies that are now so far away that a signal sent today will never reach them. Their light began its journey when they were much closer. Meanwhile, the space between us has continued to expand.
In the standard cosmological model, the observable universe extends roughly 46–47 billion light-years from us, measured using present-day distances. The region containing destinations a signal sent today could eventually reach extends only about 16–17 billion light-years.
That second limit assumes the universe’s accelerated expansion continues as the model predicts. It is called the cosmic event horizon.
Compare the two regions. The smaller one occupies only about 4–5% of the larger region’s volume. We can therefore observe the past of many galaxies whose future we can no longer influence.
These distances describe where those regions are today; they are not the travel times of the light we receive.