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How Light Travel Time Affects What We See

-- | October 26, 2012

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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