A ligh cone is the possible path of travel of a photon. Since we live in the present, the light cone of a past event will necessarily be from a distant (in time as well as in space) event. So when we see something 20,000 light years away, all that time (20,000 years) has passed since the event took place and also we have no idea what has happened since because the light has not yet reached us. The object is no longer (now) where it was 20,000 years ago. Similarly what we do now won't be seen by someone 20,000 light years away for another 20,000 years. Therefore we cannot go back in time and change something 1000 years ago because the light cone emited back then is en route to somewhere else and constancy of events won't permit the change.
What misnoma5 said. I just want to add that the reason why we call it it a cone is if you plot time on the x axis and the distance light has traveled in that time on the y and z axes, the shape forms a cone because of the ever widening circle.
If two objects start out in different light cones, The only time they can sense each other is after the light cones intersect.
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A ligh cone is the possible path of travel of a photon. Since we live in the present, the light cone of a past event will necessarily be from a distant (in time as well as in space) event. So when we see something 20,000 light years away, all that time (20,000 years) has passed since the event took place and also we have no idea what has happened since because the light has not yet reached us. The object is no longer (now) where it was 20,000 years ago. Similarly what we do now won't be seen by someone 20,000 light years away for another 20,000 years. Therefore we cannot go back in time and change something 1000 years ago because the light cone emited back then is en route to somewhere else and constancy of events won't permit the change.
here's a useful link: http://www.atlasoftheuniverse.com/redshift.html
What misnoma5 said. I just want to add that the reason why we call it it a cone is if you plot time on the x axis and the distance light has traveled in that time on the y and z axes, the shape forms a cone because of the ever widening circle.
If two objects start out in different light cones, The only time they can sense each other is after the light cones intersect.