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Light takes time to reach us, so distant views carry older information.
A light-year is the distance light travels in one year. If a star is 100 light-years away, the light reaching us now began its journey roughly 100 years ago. We are seeing the star as it was when that light left.
This is a limit on the arrival of information, not a delay caused by the telescope. A more powerful telescope may collect fainter light, but it cannot make newly emitted light arrive instantly. Distance and look-back time are closely related in this nearby-star example.
If light leaves a star 20 light-years away this year, it takes about 20 years to reach us.
At cosmological distances, the expansion of the universe makes the relation between distance and travel time more complicated.
Astronomy 2e — Consequences of Light Travel Time
OpenStax contributors · OpenStax, Rice University
CC BY 4.0. Rewritten as a self-contained explanation; the example and reflection are Vakataka additions. Written and edited with AI assistance. No source images reproduced.
Source edition: 2024-10-17T15:19:16Z
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