How Light Travel Time Affects What We See

Question: If the sun disappears it would take us 8 min. to realize it.  How in the world, what we are seeing in telescopes are still existing?  What we are seeing is light that has been traveling minutes, hours, thousand of years to reach our eyes, and possibly the objects, stars, galaxies what ever we are seeing is not existing anymore.  Could you please explain this?  — Ferminrest Omilyda

Answer:  You are right.  For information about the state of any object anywhere (even just a few inches away) we are limited by the speed at which that information travels to us, which is the speed of light.  Remember that we have a measure of distance which simply states that the distance to an object is the time that light takes to travel that distance in one year, or the “light year”.  For distant objects in the Universe that are many light years away from us we see them as they appeared when the light from those objects was emitted, which would be many light years ago.  What this means is that our observations of objects in the Universe are “delayed” relative to their actual state if we were standing right next to them.

Jeff Mangum

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2 Responses to How Light Travel Time Affects What We See

  1. Fermin Ospina says:

    Dear Mr. Jeff Mangum:

    Thanks for replying to my question, although I am exactly at the very same place where I was. Why ? because what we are seeing in real time it happend a second, a minute, years, light years away, therefore we are living on a planet that wonders and marvels on past events. In other words this is the most unsettling knowledge of what we call real. How can astrophysicists determine that the universe is 13.5 billion years if we don’t know if our sun still exist until 8 minutes had passed ?

    Truly yours,

    Fermin Ospina

  2. Jeff Mangum says:


    You are mixing a couple of different things up here. Information travels at the speed of light, which is a fundamental limitation on how we receive information. This is different than the techniques we use to determine the age of the Universe, which is done by measuring the motions of objects relative to each other. These motions turn-out to be a general expansion of objects away from each other. It is observed that objects further away from each other move faster, thus suggesting a common point in time when this expansion started. This is how we obtain an estimate of the age of the Universe.

    Jeff Mangum

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